A comprehensive literature review on laser powder bed fusion of Inconel superalloys 2022

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Additive Manufacturing 55 (2022) 102871
Available online 10 May 2022
2214-8604/© 2022 Elsevier B.V. All rights reserved.
A comprehensive literature review on laser powder bed fusion of 
Inconel superalloys 
Guilherme Maziero Volpato a,b,*, Ulrich Tetzlaff b, Márcio Celso Fredel a 
a Federal University of Santa Catarina, Department of Mechanical Engineering, Campus Universitário Reitor João David Ferreira Lima, 88040900, Florianópolis, Brazil 
b Technical University of Applied Sciences Ingolstadt, Faculty of Mechanical Engineering, Esplanade 10, 85049 Ingolstadt, Germany 
A R T I C L E I N F O 
Keywords: 
Inconel 718 
Inconel 625 
Laser powder bed fusion 
Selective laser melting 
Additive manufacturing 
A B S T R A C T 
Inconel superalloys are one of the main classes of materials with good potential for production using the additive 
manufacturing process of laser powder bed fusion (PBF-LB), which has a broad range of applications in the 
transportation and energy sectors. Thus, they have received considerable attention in the scientific literature, 
with researchers aiming to better understand their capabilities and limitations in recent decades. This review is 
based on around 340 publications spanning the past 14 years and highlights the most significant findings in the 
field along with the current challenges and research goals resulting from them. A contextualization is followed by 
a succinct microstructural review, which introduces the topics analyzed. The effects of the processing parame-
ters, heat treatment and machining on the materials and their composites are evaluated and the performance of 
materials is compared in terms of their mechanical, corrosion and tribological behavior. Publications are mapped 
according to their research focus in order to reveal the current research trends in the literature. 
1. Introduction 
Additive manufacturing (AM), popularly known as “3D printing”, 
encompasses a set of advanced manufacturing techniques that are able 
to produce tridimensional components from virtual models generated by 
computer-aided design (CAD) software. This technology, developed 
from rapid prototyping methods in the 1980 s [1], is based on the 
layer-by-layer deposition of material according to the geometry defined 
by a model, allowing extensive freedom in the design [2,3]. This capa-
bility is especially desirable in fields of engineering associated with a 
high degree of responsibility, such as aerospace, automotive and 
biomedical applications [4]. 
Several AM techniques have been focused on the production of metal 
components. Among these, laser powder bed fusion (PBF-LB) is associ-
ated with the small-batch production of highly valuable components 
with precisely controlled geometries [3]. It consists in a cyclical process, 
in which a laser source melts specific regions of a metallic powder bed 
and a coater adds a new powder layer on top of it, which becomes 
molten in the following step. Each layer corresponds to a slice of the CAD 
model, in which the regions to be molten constitute subsequent 
cross-sections of the final component geometry. 
The process, also known as selective laser melting (SLM), laser 
powder bed fusion (L-PBF), laser beam melting (LBM) or direct metal 
laser sintering (DMLS) [5], is a valuable tool for the manufacturing of 
parts that involve the use of materials whose properties present re-
strictions for classical manufacturing techniques, such as considerable 
Abbreviations: AM, Additive manufacturing; AMS, Aerospace Material Specifications; ASTM, ASTM International; CAD, Computer-aided design; CT, Computed 
tomography; DED, Directed energy deposition; DMLS, Direct metal laser sintering; EBSD, Electron backscatter diffraction; EDS, Energy-dispersive X-ray spectroscopy; 
FCC, Face-centered cubic crystal; HIP, Hot isostatic pressing; HT, Heat treatment; IN625, Inconel alloy 625; IN718, Inconel alloy 718; IN738LC, Inconel alloy 738LC; 
Laves, Hexagonal intermetallic phase with (Ni,Fe,Cr)2(Nb,Mo,Ti) stoichiometry; LBM, Laser beam melting; LED, Linear energy density; LM, Light microscopy; L-PBF, 
Laser powder bed fusion; PBF-EB, Powder bed fusion using an electron beam; PBF-LB, Powder bed fusion using a laser beam; SEM, Scanning electron microscopy; 
SLM, Selective laser melting; S-N, Stress-life curve; Sv, Maximum valley depth; TEM, Transmission electron microscopy; TMF, Thermomechanical fatigue; TTT, Time- 
temperature-transformation curve; VED, Volumetric energy density; XRD, X-ray diffraction; γ, Ni-rich matrix of Inconel superalloys; γ’, Cubic intermetallic phase 
with Ni3(Al,Ti) stoichiometry; γ’’, Tetragonal intermetallic phase with Ni3Nb stoichiometry; δ, Orthorhombic intermetallic phase with Ni3Nb stoichiometry; σ, 
Tetragonal intermetallic phase with CrFe stoichiometry. 
* Correspondence to: Ceramic & Composite Materials Research Laboratories, Department of Mechanical Engineering, Federal University of Santa Catarina, Flo-
rianópolis, Brazil. 
E-mail address: guilhermemvolpato@gmail.com (G.M. Volpato). 
Contents lists available at ScienceDirect 
Additive Manufacturing 
journal homepage: www.elsevier.com/locate/addma 
https://doi.org/10.1016/j.addma.2022.102871 
Received 9 March 2022; Received in revised form 25 April 2022; Accepted 2 May 2022 
mailto:guilhermemvolpato@gmail.com
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Additive Manufacturing 55 (2022) 102871
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work hardening, high hardness and low thermal conductivity [6]. One 
such case is that of nickel-based superalloys used to produce turbine 
blades, which are conventionally manufactured by either powder met-
allurgy or by a combination of casting and forging [7]. 
Nickel-based superalloys are materials that display excellent high- 
temperature behavior achieved through high strength allied to high 
fatigue, creep and corrosion resistance in temperatures up to around 
1000 ◦C [7]. One family of such materials, known by the Inconel 
trademark, has been at the center of an extensive research effort to 
enable the cost-effective production of superalloy components, with a 
high level of control and reproducibility, by PBF-LB in recent decades. 
Within the Inconel family, constituted of polycrystalline austenitic 
nickel-chromium superalloys, three materials are mainly studied ac-
cording to the PBF-LB literature. Their nominal chemical compositions 
are shown in Table 1: 
1. Inconel 718 (IN718), a γ’’ precipitation-hardened Ni-Cr wrought 
superalloy that, due to its outstanding high temperature properties, 
is currently the most widely used superalloy for aerospace applica-
tions worldwide both in market volume [8] and weight fraction of 
the components [9]; 
2. Inconel 625 (IN625), a Ni-Cr-Mo solid-solution-hardened wrought 
superalloy used in environments associated with severe corrosion 
and high temperatures, such as in marine and aerospace engineering, 
chemical processing and nuclear reactors; and 
3. Inconel 738LC (IN738LC), a γ’ precipitation-hardened Ni-Cr cast 
superalloy with low carbon content and high volume fraction of 
strengthening precipitates. 
Along with these materials, other superalloys have been studied 
together with PBF-LB production techniques to a lesser extent, such as 
Hastelloy X [10–17], CM247LC [18–22], Nimonic 263 [23], CMSX486 
[24], MAR-M247 [25] and K418 [26]. Within the Inconel family, 
Inconel 939 has recently been the focus of precursor studies [27–29]. 
Fig. 1 presents the annual number of scientific publications focused 
on the production of the three selected alloys through PBF-LB (articles 
and conference papers). The data was collected in the Web of Science 
platform (Clarivate Plc, United States), selecting the maximum data 
range up to Decemberto heat-affected zones, due to the fine 
equiaxed grain geometry of the former [75,230]. Lastly, post processing 
methods have been developed to both increase [203,251,302,316–318] 
and decrease [303] the superficial hardness of specimens, which tends to 
reduce with increasing subsurface depth [91,319]. 
An overview of the publications that focus on the measurement of 
hardness is found in Appendix C, Fig. 2 C. 
7.3. Fatigue 
The layer-wise deposition of PBF-LB produces Inconel components 
with an anisotropic microstructure, subsurface defects and surface ir-
regularities. These manufacturing artifacts, in turn, seem to affect the 
performance of the components under fatigue conditions. Authors have 
reported not only shorter fatigue lives [189,265] and higher crack 
growth rates [189,320] for as-built conditions compared to conven-
tional manufacturing processes but also the inexistence of fatigue limits 
for IN718 obtained through additive manufacturing [321]. This inferior 
performance, which seems to be more significant under high strain 
amplitudes [285] and temperatures [42], has been the subject of 
improvement attempts in various studies in the reviewed literature, and 
conditions comparable to conventional manufacturing routes have been 
achieved [234,276,322–325]. 
Several researchers have observed that samples built under different 
build orientations present different fatigue lives [134,170,286,287,322, 
326–331] and crack growth rates [169,268,271,332–335] under as-built 
conditions. There are, however, at least three factors which can influ-
ence this behavior: the direction of microstructural texturing [326,327, 
331], orientation-dependent surface defects [134,167,169,268,287,328, 
332,336,337] and the positioning of fatigue notches [134,322,327–329, 
332,335], all of which can be altered in different orientations. Given that 
machined specimens seem to not present significant differences in terms 
of fatigue life [322,336,337] and crack growth rates [320] regardless of 
microstructural texturing, surface irregularities appear to dominate the 
effect of build orientation on fatigue. This conclusion is supported by the 
report of changes in the threshold region of the Paris curve when 
comparing different build orientations [169,333]. Crack growth, how-
ever, seems to be independent of both build orientation [169,333,335] 
and microstructural isotropy [324,325] in the Paris regime. 
Microstructural features, on the other hand, have been reported to be 
less critical than the incidence of subsurface defects, such as pores, 
regarding a reduction in the fatigue life [337]. PBF-LB porosity, apart 
from being detrimental in terms of fatigue resistance in general [87,271, 
321,338], tends to cause embrittlement [339] and increase crack growth 
rates [268,339], so that the morphology of the pores seems to have a 
greater impact than the pore depth in relation to the surface of the 
component [340]. Although these effects are more pronounced the 
greater the size of critical individual pores [245,275,338,339,341], 
sub-critical porosity has also been reported to induce fatigue failure due 
to pore-pore and pore-surface interactions [341], the latter being espe-
cially relevant given the preferential presence of porosity [177,212,245, 
273], precipitates [283] and residual tensile stress [284] near PBF-LB 
free-surfaces due to hatch configurations. 
In order to reduce the incidence of these defects and overall anisot-
ropy, heat treatments and HIP have been commonly employed [169, 
G.M. Volpato et al. 
Additive Manufacturing 55 (2022) 102871
14
245,331]. In one study, HIP was found to increase the high cycle fatigue 
life in comparison with precipitation hardening alone [204], while other 
researchers have noted that the formation of high temperature twins 
[191,285] and the δ intermetallic phase [39,206] in the treatment may 
be the cause [39,42,265] of the observed reduction in the life of com-
ponents when subjected to fatigue after HIP [206,245,285,342]. None-
theless, another study focusing on the high cycle fatigue of homogenized 
specimens showed that fatigue life was improved by the pre-treatment 
despite substantial twinning [164]. 
However, it does not seem possible to control the incidence of surface 
defects applying heat treatments or HIP. Reportedly, these are more 
critical than subsurface defects in terms of fatigue resistance [245,321, 
337,343] and they have also been inversely correlated to fatigue life 
when considering surface roughness parameters such as maximum val-
ley depth (Sv) [76]. Although the negative impact of these defects on the 
fatigue properties [49,134,245,275,336,343] can be hindered by sur-
face finishing [87,169,234] and/or machining [173,204,234,245,322], 
the design of notches for fatigue testing is particularly affected by the 
characteristic surface irregularities of PBF-LB specimens [322,328,330, 
344]. Thus, notches that are printed with overhanging surfaces [134, 
327–330,335,345–347] not only present unmolten particles on their 
surface and the stepping effect [87,166,168,266], but nucleate cracks in 
an abnormal fashion when compared to wrought and cast specimens, 
which tend to grow transgranularly [170,190,324,325,334] and unaf-
fected by precipitate phases [190]. The poor quality of the overhanging 
surface induces crack nucleation out of the notch root for intermediate 
notch radii [330,346] (Fig. 8) especially under high cycle conditions 
[347], which strongly affects crack growth measurements. 
Thus, researchers have identified a competition behavior between 
crack initiation mechanisms for PBF-LB IN718 [134], which leads to two 
separate S-N curves according to the site of crack initiation within the 
microstructure. Cracks originated from superficial defects, where the 
nucleation sites seem to follow a preferential pattern [336] and depend 
on build orientation [134], tend to induce a shorter fatigue life than 
cracks nucleated in the interior of components, and thus the fatigue life 
can be considerably increased by preventing the former mechanism in 
favor of the latter [134,321,343]. 
An overview of the publications that focus on fatigue behavior is 
found in Appendix C, Fig. 3 C. 
7.4. Creep 
As a result of the rapid solidification regime generated by PBF-LB 
processing, components produced by the technique present a signifi-
cant grain refinement under as-built conditions, which directly degrades 
their creep resistance [238]. This is especially important in view of the 
fact that grain boundary sliding was identified as a dominant process for 
creep damage in these materials [272] and failure by intergranular 
cracking has been reported for all three reviewed superalloys [49,266, 
272,348,349] especially under critical conditions of low strain rates 
[272,350]. This is further aggravated by the intense segregation within 
additive manufactured microstructures [351], which induces the for-
mation of brittle intermetallic phases such as Laves and δ in grain 
boundaries. 
Laves is regarded as undesirable for creep resistance due to the 
nucleation of intergranular defects and high brittleness [8,49,348,350, 
352], but it has been reported that δ can act as a crack barrier [40,49], 
avoid grain boundary sliding by pinning, and impede microvoid coa-
lescence [53] despite its absence having been reported to increase creep 
resistance [8,41,43,222,352] and induce grain growth [352]. Thus, heat 
treatments are employed to promote, among other things, the solubili-
zation of these phases, which increases the volume fraction and di-
mensions of γ’ and γ’’ aging precipitates as well as the creep resistance 
[8,40,41,49,51,222,352]. The expected grain growth tends to be hin-
dered [49] and inhomogeneous[41]. 
The effect on the creep behavior of the addition of an HIP step in heat 
treatments, on the other hand, seems to be dependent on the presence of 
an intermediate homogenization step. Although some authors have 
found that HIP increases the creep life when employed before solution 
and aging [41], it has been reported that creep life is reduced if the 
technique precedes homogenization at 1100 ◦C together with the stan-
dard treatment [222]. One possible explanation for this is the fact that 
the hindering of grain boundary sliding produced by carbide growth 
[58,185,187,213] and precipitation [188] under a single precipitation 
hardening pre-treatment may be prevented by the dissolution [174] of 
the same carbides applying joint treatments, thus reducing creep life. 
A further mechanism to consider for the enhancement of creep 
resistance is the high anisotropy of PBF-LB specimens, which seems to be 
significant even after heat treatments [55,183,349]. Longer creep life, 
Fig. 8. – Abnormal crack initiation on notches with poor surface quality due to PBF-LB defects 
(reproduced from [330]). 
G.M. Volpato et al. 
Additive Manufacturing 55 (2022) 102871
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higher ductility and lower strain rates have been identified when load is 
applied in a direction parallel to the build direction [8,53,103,160,238, 
260,349], and this has been assigned to a coarser [349] and more 
elongated [103,349] microstructure. When a notch is present, however, 
the influence of the anisotropy seems to be the opposite and thus the 
same direction has been assigned to minimum creep life [159]. This may 
be associated with a report that, while creep specimens fail with crack 
growth between AM layers when built parallel to the build direction 
(Fig. 9a), failure occurs between melt pools when samples are built 
orthogonally to it (Fig. 9c) [53]. Also, some studies also have shown that 
energy density [71], laser focus height [103], oxidation [43], surface 
integrity [40] and ceramic reinforcements [353] have an influence on 
the high temperature degradation phenomenon. Changing the scan 
strategy can affect the creep behavior [103], but the use of multiple laser 
beams reportedly does to not have detrimental effects on it [160]. 
An overview of the publications that focus on creep behavior is found 
in Appendix C, Fig. 4 C. 
8. Corrosion behavior 
Due to the development of passivation layers at the surface, Inconel 
alloys often present high resistances against corrosion and oxidation. 
When processed by PBF-LB, however, these materials undergo micro-
structural and superficial changes that alter their corrosion behavior. 
Thus, lower corrosion rates have been reported for both as-build and 
heat-treated samples in comparison to their wrought counterparts 
[354]. 
The microstructural anisotropy of as-built PBF-LB specimens can 
induce corrosion anisotropy in IN718 and IN625 [161,163,355], so that 
lower corrosion resistance has been identified for surfaces normal to the 
build direction [355]. Because both porosity [355] and the precipitation 
of secondary phases such as carbides [216,356], δ [48,215,356] and 
Laves [44,357] have been reported to locally increase the corrosion 
susceptibility of these materials due to sensitization, this anisotropy has 
been related to the presence of these features on the specimen surface. 
Nonetheless, it has also been argued that these phases can act as 
nucleation sites for oxide passivation layers [358]. 
The corrosion behavior of PBF-LB Inconel components is also 
affected by its dendrite and grain geometry as well as the roughness. 
Considering that Cr2O3 and Al2O3 passivation layers have been reported 
to preferentially nucleate and stabilize at exposed grain boundaries 
[186,359] (Fig. 10), the grain refinement and consequent higher grain 
Fig. 9. – Schematic intergranular creep crack path mechanisms for IN718 in different build orientations 
(reproduced from [53]). 
Fig. 10. – Schematic cross-section of an oxidized surface of PBF-LB IN718 
(reproduced from [360]). 
G.M. Volpato et al. 
Additive Manufacturing 55 (2022) 102871
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boundary density of these materials tends to induce the formation of 
more nucleation sites and, consequently, higher corrosion resistances at 
both temperature and under higher temperature conditions [124,163, 
356]. Accordingly, the high degree of roughness of as-built PBF-LB 
specimens (Fig. 10) contributes to an under-estimation of the surface 
area in corrosion kinetics measurements [360], an effect that, together 
with corrosion susceptibility itself [361], can be controlled by 
electro-chemical polishing [216,253,303]. 
Some authors have reported that additional mechanisms can be 
applied to increase the corrosion resistance of PBF-LB Inconel 
superalloys. Surface peening [318,357], alloying with Re [162] and the 
addition of TiC [355] or Y2O3 nanoparticles [291] seem to be effective 
approaches to reducing the corrosion kinetics, but solution heat treat-
ments have been related with diminished oxidation resistance [362]. 
Nonetheless, the homogenization of the microstructure by the solution 
of segregates after heat treatment has been shown to be beneficial for the 
overall corrosion resistance [44,215,357], which is further increased by 
the generation of γ’ and γ’’ precipitates and stress release after 
controlled aging [216,363]. 
9. Composites 
Composites of PBF-LB can be produced by adding reinforcements to 
the powder bed either as distinct particles or by coating the Inconel 
powder with it. In the reviewed literature, the use of reinforcements 
such as TiC [73,123,125–127,188,242,299,318,355,358,364], WC [71, 
84,94,107,122,139,297,365], TiB2 [90], BN [308], Al2O3 [353], gra-
phene [174,300] and carbon nanotubes [38] have been reported. 
The addition of reinforcements generally increases the strength [38, 
127,174,188,293,297,299,308,364] and hardness [84,122,123,125, 
293,297,308] of a sample while decreasing its elongation [38,188,293, 
297,299,300] and suppressing grain growth [38,90,94,125,174,188, 
299,364]. The latter effect seems to be connected with a notable inhi-
bition of crack development [364]. In addition, this approach appears to 
enhance the wear resistance [84,94,122,123,125,308]. The elastic 
modulus can be reduced by the addition of TiC [299], while the presence 
of graphene [300], TiB2 [90] and in-situ Al2O3 [353] has been found to 
increase it. In one study, the latter also reduced creep rates while 
increasing creep life, an effect that was attributed to the control of 
dislocation glide by the reinforcements [353]. These effects, however, 
seem to be influenced by the reinforcement fraction and size, the addi-
tion of WC in fractions superior to 15 wt% causing a steep reduction in 
strength [297]. The refinement of particles and its deagglomeration can 
be achieved by the use of high energy densities [123,125]. 
The behavior of composites has also been shown to be dependent on 
heat treatment. With the addition of carbon nanotubes there was an 
Fig. 11. – Influence of interfacial diffusion layers on the fragmentation of WC particle reinforcements by laser-induced stresses 
(reproduced from [139]). 
Fig. 12. – Relative efficiency of post-processing methods for the control of 
roughness. UIT - ultrasonic impact treatment, USP - ultrasonic shot peening, SP 
- shot peening, BF - barrel finishing 
(reproduced from [316]). 
G.M. Volpato et al.Additive Manufacturing 55 (2022) 102871
17
increase in strength after precipitation hardening [38], but with the 
addition of TiC [188] or graphene [174] the strength decreased after 
heat treatment. This has been attributed to the suppression of γ’ and γ’’ 
precipitation by the carbon-rich reinforcements, where a diffusion shell 
develops by interdiffusion under high-temperature processing [84,90, 
107,122,139,174,188,297,365], depleting the matrix of the elements 
necessary for the precipitation of aging phases due to the nucleation of 
carbides [38,139,174,188] (Fig. 11a). On the other hand, the interfacial 
diffusion shell can act as a crack barrier [84,122], protecting against 
particle fragmentation due to laser incidence [139] (Fig. 11b), since 
cracks in the reinforcements result not only from matrix contraction but 
also interfacial porosity [94]. 
10. Tribological behavior 
The contact behavior of Inconel components produced by PBF-LB is 
highly influenced by the defects inherent to the additive manufacturing 
process, which affect both the microstructure, topography and the sur-
face quality of as-built materials. These defects, which can be measured 
using surface characterization techniques such as profilometry [87,134, 
166,167,241,316], focus variation microscopy [204,360], interferom-
etry [345,360] and confocal microscopy [61], can be both controlled 
and counterbalanced by careful control of the processing conditions or 
by adequate post-processing. 
In this regard, studies have shown that the control of parameters, 
such as scan speed [84], remelting [107], and linear [122,124,125] and 
volumetric [123] energy densities, can improve wear resistance while 
decreasing the coefficient of friction (COF) of dry-sliding contact until 
conditions are similar to those of conventionally cast materials [366]. 
This is especially critical regarding contour parameters, which are a key 
factor in the definition of the surface topography and roughness of 
produced materials [76,80,87]. Moreover, not only high energy den-
sities can change the wear mechanism from adhesive to abrasive wear 
[125], since some studies have shown that build orientations of 0◦ [158] 
and 45◦ [165] are optimum for wear resistance, although other have 
indicated no build direction dependance [366]. The temperature of the 
tribological contact has also been described as a factor with an influence 
on the wear behavior of these materials and both the wear loss and COF 
increase together with temperature while the mechanism of wear 
changes from abrasion to oxidation and/or delamination [367]. 
Besides the control of build parameters, improvement in wear 
resistance could also be achieved by applying appropriate heat treat-
ments, like the higher resistance to erosion induced by the in-situ su-
perficial precipitation of aging phases [302]. Although such phases are 
generally related to an improvement in the overall wear resistance, due 
to an increase in hardness and strength [217], γ’’ was reported to act as a 
third body abrasive together with δ under dry sliding, while Laves tends 
to be detached from a brittle aged surface [95]. Additionally, the wear 
rate of heat-treated specimens has been reported to present a consid-
erable spatial dependence, and regions near support structures exhibited 
higher wear resistance than those distant from them [310]. 
An increase of bulk hardness and, consequently, wear resistance 
could also be achieved by the addition of hard and homogeneously 
distributed [94] particle reinforcements, such as WC [84,94,107,122] 
and TiC [123,125] in the powder bed, which also seem to induce a 
change from severe abrasive contact to moderate adhesive wear [84]. In 
addition, an overall reduction in friction intensity has been achieved 
through the addition of intrinsically lubricating reinforcements such as 
hexagonal BN [308]. 
However, to avoid altering the optimal manufacturing conditions 
while simultaneously improving wear behavior, post-processes have 
been devised as an alternative to the aforementioned techniques. The 
plasma nitriding of PBF-LB IN718 has been reported to improve resis-
tance to both wear and oxidation of tested specimens, but it increased 
the already high degree of roughness due to the superficial growth of 
CrN [307]. Studies have shown that this roughness, a consequence of the 
layer-wise melting of powder, can be controlled by methods such as 
barrel finishing, shot peening, ultrasonic shot peening, ultrasonic impact 
[316] and ultrasonic cavitation [368]. Turning the surface has been 
reported to result in higher hardness and lower wear rates than vibratory 
and lapping finishing [369]. Fig. 12 shows the relative efficiency of some 
of these methods. 
11. Current challenges 
Materials of the Inconel family present a wide range of heat- 
resistance applications in the energy and transport industries. Their 
manufacturing through laser powder bed fusion has been an important 
field of research in the recent decades, given the potential of this AM 
technique for the production of tailored and optimized components. 
Nonetheless, the manufacturing of Inconel components using AM tech-
nology is still limited by key factors that hinder its use in industrial series 
production. This review of 337 publications published in the last 14 
years enabled the developments in the field related to processing con-
ditions and material properties to be summarized. In this context, the 
following topics were identified as current challenges according to the 
literature, which currently hinder a broader adoption of the technology: 
1. Parameter restrictiveness – the production of Inconel superalloys by 
PBF-LB is heavily dependent on the correct choice of manufacturing 
parameters. The results of parametrization studies are, however, 
mostly empirical, and thus commercially standard parameters are 
restricted not only to a given alloy, but to a set of processing con-
ditions that vary according to the employed machine system and the 
production environment. Although the design of composite param-
eters, such as energy density, has been aimed at the standardization 
of manufacturing parameters, their validity as a predicting gauge is a 
subject of debate; 
2. Powder reuse – in order to lower the costs associated with PBF-LB, 
there is considerable interest in reusing the Inconel powder that 
was not consolidated in the production of previous components. This 
option, however, brings about uncertainty regarding changes in the 
chemical and rheological characteristics of the powder, and thus the 
properties of the built component might be compromised. Some 
authors have observed no changes in the properties after powder 
recycling, but there is evidence of the powder quality being reduced 
due to various phenomena during laser incidence; 
3. Microstructural changes – although the microstructure and the 
associated phases of Inconel superalloys are well described in the 
literature, the solidification and precipitation behavior of these al-
loys after the production by PBF-LB is changed, and intense anisot-
ropy, grain refinement, microsegregation and out-of-equilibrium 
solidification have been verified. Thus, certain phases tend to appear 
under conditions in which they are not expected according to con-
ventional transformation kinetics, which results in different micro-
structures and properties of the AM materials when compared to 
wrought and cast alloys; 
4. Heat treatment unpredictability – since the performance of Inconel 
alloys is highly dependent on precipitation hardening, these mate-
rials undergo a series of heat treatments to promote the nucleation 
and growth of the reinforcing phases. However, in view of the 
intense segregation of these materials after manufacturing by PBF- 
LB, these heat treatments also precipitate undesirable phases 
together with the aging reinforcements. There is thus a conflict 
G.M. Volpato et al.Additive Manufacturing 55 (2022) 102871
18
between precipitation hardening optimization and segregation con-
trol in these alloys when produced by the AM technique, and lower γ’ 
and γ’’ volume fractions might be desirable in order to avoid the 
growth of undesirable phases in its microstructure; 
5. Competition of optimization – although there are conditions under 
which certain properties of PBF-LB Inconel are optimized, they are 
often not optimal regarding other relevant properties. This can be 
verified when considering HIP and sample orientation, which leads 
to a competition between the optimization of monotonic and dy-
namic strength. HIP is mostly beneficial for fatigue strength due to 
the closing of metallurgical porosity, but the grain growth and 
dissolution of precipitates induced by the technique tend to reduce 
the monotonic strength. Similarly, while tensile strength seems to be 
optimum in samples built either parallel to or at a 45◦ degree in 
relation to the build plate, creep resistance is optimized in samples 
built perpendicularly to the build plate (90◦); 
6. δ-Ni3Nb – there is no consensus on the desirability of the δ phase to 
enhance creep resistance. The intermetallic phase is usually consid-
ered undesirable for tensile strength, but authors do not agree on 
whether its presence in the microstructure of PBF-LB Inconel is 
mostly beneficial or detrimental in terms of creep properties. Some 
argue that its presence at grain boundaries may inhibit grain 
boundary sliding while inducing local strengthening by the entrap-
ment of dislocations, but others emphasize the undesirability of the 
phase due to its brittleness, its behavior as a stress concentrator and 
the local entrapment of elements necessary for the precipitation of 
aging reinforcements; 
7. Novel physical phenomena – the manufacturing conditions to which 
Inconel alloys are submitted when produced using PBF-LB give rise 
to physical phenomena that are not present in conventional 
manufacturing methods. These phenomena alter the operation 
behavior of PBF-LB superalloys when in comparison to conventional 
materials, being present not only with regard to mechanical response 
but also when considering conditions that involve non-mechanical 
damage. Some particular mechanical phenomena that can be 
attributed to PBF-LB processing include specific types of anisotropy, 
high-temperature embrittlement, fatigue dependence on PBF-LB 
surface artifacts and microstructural texture, and the dependence 
of creep on phases derived from PBF-LB microsegregation. These are 
accompanied by reports of novel material behavior when samples 
are submitted to post-processing techniques such as heat treatment 
and machining, whose final quality hinges upon microstructural 
artifacts. Operation in corrosive and tribological environments also 
have been linked with new phenomena, which include the nucle-
ation of passivation layers according to PBF-LB surface features and 
the dependence of wear resistance on PBF-LB defects and micro-
structure. Thus, in order to properly predict material behavior, it is 
imperative that these physical phenomena are taken into account 
when designing superalloy components to be produced using PBF- 
LB; 
8. Restricted literature – despite the growing literature on the subject, 
the PBF-LB processing of Inconel superalloys is strongly focused on 
IN718 and, to a lesser extent, IN625. In this study, publications 
dealing with IN738LC were also reviewed, although they are rela-
tively small in number. Additionally, the application of the tech-
nology to IN939 appears to be in the initial stages of development. 
Thus, considering the number of alloys within this family, the liter-
ature is still restricted; 
9. Scalability of results – it was verified that, despite the extensive 
research that has been conducted in this field, there is still uncer-
tainty regarding how well the promising results of studies can be 
scaled to produce industrial and commercial components. This is 
emphasized by reports that the mechanical properties are dependent 
on sample size and geometry even at laboratory scale. 
12. Concluding remarks 
This comprehensive overview of past and ongoing research in the 
field of Inconel superalloys produced by PBF-LB highlights the great 
potential that the AM technique offers for the production of complex 
components with optimized and tailored properties. Nonetheless, due to 
the novelty of the field and the complex relations that control the 
properties and the performance of Inconel components, it is clear that 
various aspects of research still require further investigation. 
Many studies have been focused on the tensile and hardness prop-
erties of these alloys, but comparatively few describe their behavior 
under conditions closer to those of potential applications. The limited 
literature concerning creep, fatigue, thermomechanical fatigue (TMF) 
and creep-fatigue interactions indicates the need for more research on 
these topics, since Inconel superalloys often fail as a result of these 
phenomena. 
Despite several initiatives aimed at the standardization of the 
manufacturing of Inconel alloys by PBF-LB, it was noted that authors 
still employ not only various nomenclatures, mathematical notations 
and methods, but they also arrive at conclusions that are difficult to 
integrate in a broader reference frame. A widespread standardization of 
the approach to research in this field is therefore required and authors 
need to promote methods that facilitate correlations between studies 
and research projects carried out in different contexts. 
CRediT authorship contribution statement 
Fredel Márcio Celso: Writing – review & editing, Supervision, Re-
sources, Project administration, Funding acquisition, Conceptualization. 
Tetzlaff Ulrich: Writing – review & editing, Conceptualization, Funding 
acquisition, Project administration, Resources, Supervision. Volpato 
Guilherme Maziero: Writing – original draft, Investigation, Data 
curation, Conceptualization. 
Declaration of Competing Interest 
The authors declare that they have no known competing financial 
interests or personal relationships that could have appeared to influence 
the work reported in this paper. 
Acknowledgments 
The authors are grateful for the financial support provided by the 
German Academic Exchange Service (DAAD - Germany), by means of 
the project enGlobe of the Bavarian Center for Applied Research and 
Technology with Latin America (AWARE), on behalf of which this 
research was conducted. Mr. Volpato is grateful for the Masters’ schol-
arship granted by the Brazilian Coordination for the Improvement of 
Higher Education Personnel (CAPES - Brazil), and support from the 
Mechanical Engineering Postgraduate Program of the Federal University 
of Santa Catarina (POSMEC/UFSC). The helpful English revision by Dr. 
Wiese is gratefully acknowledged. 
G.M. Volpato et al. 
Additive Manufacturing 55 (2022) 102871
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Appendix A. Map of reviewed publications
Fig. 1A. Map of reviewed publications. Each number refers to the associated publication number in the References section, while each color corresponds to a su-
peralloy: blue for IN718, red for IN625 and green for IN738LC. The size of each method node (in gray) is parametrized by the number of publications that either 
employ or focus on the method itself. 
Appendix B. Parameter tables 
Table 1B, 2B and 3B show a selection of printing parameters employed to build bulk specimens of Inconel alloys 718, 625 and 738LC in the 
reviewed literature, respectively. Only studies that explicitly report the values of bulk laser power, scan speed, hatch distance and layer thickness are 
included, so that publications that either indicatecommercial parameter sets or fail to mention one or more of the aforementioned parameters are not 
referenced. Parameter studies, which change one or more parameters, are indicated with the word “variable” in the parameter subject to change. 
The tables are organized in ascending order of layer thickness, followed by laser power. 
G.M. Volpato et al. 
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Table 1B – 
Parameters employed in the reviewed literature to build bulk IN718 specimens. 
Laser 
power 
[W] 
Scan 
speed 
[mm/s] 
Hatch 
distance 
[μm] 
Layer 
thickness 
[μm] 
Reference number 
95 800 50 20 [207] 
170 417 70 20 [264] 
195 1200 90 20 [67,133,186,277,289] 
200 900 70 20 [48] 
280 450 30 20 [43] 
285 2500 50 20 [97] 
Variable Variable 100 20 [135] 
90 800 80 25 [308,309] 
180 850 150 25 [297] 
121 400 60 30 [125] 
175 620 120 30 [39,193] 
180 600 90 30 [280] 
180 600 105 30 [117,159,208,305,312, 
322,336] 
180 600 150 30 [106] 
190 1200 70 30 [269] 
195 1100 80 30 [36,181] 
200 900 120 30 [284] 
200 7000 60 30 [174] 
200 900 90 30 [281] 
200 7000 100 30 [242,299,300] 
200 900 120 30 [58,314] 
280 1200 70 30 [50,51] 
300 1300 120 30 [194] 
400 900 120 30 [68,196] 
200 Variable Variable 30 [200] 
Variable Variable 100 30 [131] 
Variable 875 Variable 30 [156] 
180 600 105 35 [180,211,212] 
Variable Variable 25 35 [130] 
100 560 60 40 [140] 
110 450 50 40 [84] 
200 1300 90 40 [140] 
225 680 90 40 [140] 
230 760 110 40 [201,302] 
230 760 110 40 [220] 
260 1000 110 40 [42] 
280 950 110 40 [367] 
285 1000 110 40 [52] 
285 960 80 40 [34,37,55,110,113,147, 
177,189,205,206,245, 
257,287,307,320,321, 
334,340,349,352,356] 
285 1000 110 40 [218,350] 
285 970 150 40 [165,354,366] 
300 1500 55 40 [292] 
300 1200 70 40 [248] 
300 1200 85 40 [203] 
350 1000 100 40 [146] 
350 700 110 40 [202] 
350 800 120 40 [60,195,215] 
350 1000 200 40 [187,214] 
400 7000 20 40 [45,59] 
400 7000 80 40 [41] 
Variable Variable 70 40 [119] 
Variable Variable 80 40 [105] 
Variable Variable 110 40 [168] 
Variable Variable Variable 40 [66,98,100] 
350 Variable 105 45 [301] 
100 85.7 160 50 [89] 
125 100 50 50 [93,94] 
130 400 50 50 [359] 
160 800 160 50 [54] 
200 1000 80 50 [154] 
200 200 180 50 [324,325] 
220 800 100 50 [353] 
240 800 50 50 [355] 
250 700 120 50 [49,175,261] 
250 760 120 50 [327,328] 
250 805 120 50 [345] 
275 805 120 50 [198,219] 
300 800 50 50 [107,298] 
Table 2B 
– Parameters employed in the reviewed literature to build bulk IN625 
specimens. 
Laser 
power [W] 
Scan speed 
[mm/s] 
Hatch 
distance 
[μm] 
Layer 
thickness 
[μm] 
Reference 
number 
160 500 60 20 [229] 
185 900 70 20 [69] 
195 1200 90 20 [224,225,228, 
362] 
195 800 100 20 [46,56,116,235, 
246,258] 
274 1000 80 20 [263] 
Variable Variable Variable 20 [101,102,114, 
128,136,138] 
Variable Variable 90 20 [313] 
175 600 140 30 [155] 
200 400 80 30 [317,358] 
280 950 110 30 [158] 
200 Variable 80 30 [126,127,129] 
285 960 110 40 [268,339] 
300 1000 100 40 [169] 
1000 200 180 40 [96] 
50 1300 120 50 [99] 
80 25 100 50 [92] 
80 100 100 50 [90] 
80 1500 100 50 [227] 
200 600 80 50 [318] 
360 400 125 50 [38] 
Variable Variable Variable 50 [153] 
200 600 140 60 [252] 
253 500 100 60 [282] 
Variable 600 140 60 [288] 
Variable Variable Variable 100 [65] 
Table 1B – (continued ) 
Laser 
power 
[W] 
Scan 
speed 
[mm/s] 
Hatch 
distance 
[μm] 
Layer 
thickness 
[μm] 
Reference number 
300 800 130 50 [188] 
400 500 160 50 [162,271] 
700 1500 150 50 [85] 
6000 480 5000 50 [154] 
300 Variable 50 50 [71] 
Variable Variable 50 50 [73] 
Variable Variable 110 50 [82] 
Variable Variable Variable 50 [79,365] 
200 875 90 60 [217,241,369] 
212.5 850 90 60 [53,160] 
Variable Variable Variable 70 [172] 
200 200 70 75 [95] 
370 700 140 80 [104] 
2000 500 800 100 [154] 
370 700 130 Variable [338] 
60 Variable Variable Variable [86] 
Table 3B – 
Parameters employed in the reviewed literature to build bulk IN738LC 
specimens. 
Laser power 
[W] 
Scan speed 
[mm/s] 
Hatch 
distance [μm] 
Layer 
thickness [μm] 
Reference 
number 
135 1150 60 20 [237] 
270 950 90 30 [291] 
300 1000 90 30 [364] 
200 Variable Variable 30 [83] 
Variable Variable 90 30 [70] 
Variable Variable Variable 30 [61] 
Variable Variable Variable 40 [64,74] 
G.M. Volpato et al. 
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Appendix C. Maps of publications focused on mechanical behavior 
The figures of this appendix show the research focuses of publications classified in the “Tensile”, “Hardness”, “Fatigue” and “Creep” groups ac-
cording to Section 2, in order to better map the research efforts in regard to mechanical properties in the reviewed literature. A description of how to 
interpret each map is to be found in Appendix A. 
Fig. 1 C, which shows the publications associated with the “Tensile” group, divides publications according to the following categories: 
1. Heat treatment (HT): studies that measured the tensile behavior of samples that underwent different heat treatments; 
2. Anisotropy: studies that described the anisotropic tensile response of the samples produced; 
3. Elastic constants: studies that measured the anisotropic elastic constants of samples produced; 
4. Fracture toughness: studies that measured the fracture toughness of samples produced; 
5. Compression: studies that evaluated the monotonic mechanical behavior of samples under compressive stresses; 
6. Shear: studies that evaluated the monotonic mechanical behavior of samples under shear stresses; 
7. Impact: studies that evaluated the impact resistance of samples produced; 
8. High temperature: studies that evaluated monotonic mechanical behavior under high-temperature conditions; 
9. Parameters: studies where the influence of build parameters on the monotonic mechanical behavior of samples was considered; 
10. Alloying: studies that evaluated the monotonic mechanical behavior of materials submitted to alloying; 
11. Composites: studies that evaluated the monotonic mechanical behavior of composite materials. 
Fig. 1 C. Map of the publications associated with the “Tensile” group. 
Fig. 2 C, which is associated with the “Hardness” group, categorizes publications according to the following categories: 
G.M. Volpato et al. 
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1. Nanohardness: studies where the nanohardness of the materials was measured; 
2. Heat treatment (HT): studies that measured the hardness of samples that underwent different heat treatments; 
3. Distribution: studies that evaluated the spatial distribution of hardness within the microstructure or geometry of samples; 
4. Parameters: studies where the influence of build parameters on the hardness of samples was considered; 
5. Surface: studies that measured the surface hardness of samples; 
6. Composites: studies that evaluated the hardness of composite materials. 
Fig. 2 C. Map of the publications associated with the “Hardness” group. 
Fig. 3 C, a map of the publications of the “Fatigue” group, divides them into the following research focuses: 
1. Low cycle fatigue (LCF): studies that conducted fatigue tests in the low cycle fatigue regime; 
2. High cycle fatigue (HCF): studies that conducted fatigue tests in the high cycle fatigue regime; 
3. Very high cycle fatigue (VHCF): studies that conducted fatigue testsin the very high cycle fatigue regime; 
4. Isothermal fatigue (IF): studies that conducted fatigue tests under a constant temperature higher than that of room temperature; 
5. Thermomechanical fatigue (TMF): studies in which fatigue tests were conducted with simultaneous changes in temperature and mechanical 
strain; 
6. Creep-fatigue interaction (CFI): studies that evaluated the simultaneous contribution of fatigue and creep damages; 
7. Surface: studies that evaluated the influence of surface defects and quality on fatigue behavior; 
8. Fatigue crack propagation (FCP): studies that analyzed the fatigue crack initiation and growth of samples upon cyclic loading; 
9. Orientation: studies that evaluated the influence of build orientation on fatigue properties and behavior of samples; 
10. Defects: studies that evaluated the influence of internal defects, such as pores or inclusions, on the fatigue properties of samples; 
11. Microstructure: studies where the microstructure of samples was analyzed in order to account for fatigue results. 
G.M. Volpato et al. 
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Fig. 3 C. Map of the publications associated with the “Fatigue” group. 
Finally, Fig. 4 C, associated with the “Creep” group, divides publications according to the following categories: 
1. Heat treatment (HT): studies that evaluated the creep behavior of samples submitted to different heat treatments; 
2. Anisotropy: studies that described anisotropic creep response of the samples produced; 
3. Delta + : studies that, when evaluating the influence of the δ-Ni3Nb intermetallic phase on the creep behavior of samples, regard it as generally 
beneficial for creep properties; 
4. Delta -: studies that, when evaluating the influence of the δ-Ni3Nb intermetallic phase on the creep behavior of samples, regard it as generally 
detrimental for creep properties; 
5. Delta optimization: studies that, when evaluating the influence of the δ-Ni3Nb intermetallic phase on the creep behavior of samples, propose that a 
given incidence and/or distribution of the phase is desirable to optimize creep properties; 
6. Parameters: studies where the influence of build parameters on the creep behavior of samples was considered; 
7. Microstructure: studies where the microstructure of samples was analyzed in order to account for creep results. 
G.M. Volpato et al. 
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Fig. 4 C. Map of the publications associated with the “Creep” group. 
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grain structure and crystallographicof 2021. 
It can be seen from Fig. 1 that, although IN625 was the first Inconel 
superalloy to be produced using PBF-LB (2007), IN718 has gained 
incrementally more attention as the manufacturing technique was 
developed over the years, quickly overcoming it in terms of research 
volume. This trend continues up to the date of the publication of the 
present literature review, although IN738LC has also been the focus of 
studies since 2011. 
These publications focus mostly on the manufacturability through 
PBF-LB and on the behavior of Inconel superalloys under conditions that 
aim at simulating aspects of the application environment. Thus, they 
analyze not only static mechanical response but also high-temperature 
conditions, dynamic loading and environmental damage. 
Nonetheless, despite the appreciable number of studies that have 
been published in the field, there are still technological challenges and 
knowledge gaps that prevent the widespread use of the technology in the 
industrial production of Inconel alloys. The amount of data gathered 
under conditions near to operation is insufficient to predict the behavior 
of these materials with reliability, since it is not clear how much of the 
knowledge gathered from conventional manufacturing technologies can 
be applied in the case of PBF-LB. For instance, as it will be shown in the 
following pages, prior research has identified not only microstructural 
artifacts but also several phenomena that make the behavior of PBF-LB 
superalloys very different from that of conventionally produced mate-
rials in the same application. In addition, since the extent of these 
phenomena is restricted to precise manufacturing conditions, the 
generalization and scalability of results are particularly difficult, thus 
leading to the need for further research with broader scope as well as for 
synthetic overviews. 
In this context, the aim of this review was to gather, classify and 
analyze the results of publications in the field of Inconel superalloys 
processed by laser powder bed fusion, in order to provide a compre-
hensive overview of the experimental findings and developments within 
the theme from 2007 up to December of 2021. Only studies where pri-
marily an experimental approach was taken were included, and thus 
simulation results were only considered when experimentally applied. 
No studies focused solely on the development of machinery and 
manufacturing processes were considered, due to the lack of focus on the 
behavior of the materials considered herein. 
This paper builds on former reviews which addressed associated 
themes. These include a 2015 publication by Wang et al. that focused on 
the PBF-LB processing of Inconel 718 [30], a 2017 article by Karia et al. 
on the PBF-LB processing of Inconel superalloys [31], a 2019 paper by 
Tian et al. who reviewed PBF-LB applied to Inconel 625 [6], a 2019 
publication by Hosseini and Popovich which provided an overview of 
the mechanical behavior of AM Inconel 718 [32] and a 2021 article by 
Sanchez et al. focused on the PBF processing of nickel-based superalloys 
[33]. A total of 337 publications were reviewed in this study, 251 of 
which are focused on IN718 (74.5%), 69 on IN625 (20.5%) and 17 on 
IN738LC (5%). 
Table 1 – 
Nominal compositions of Inconel superalloys currently applied in PBF-LB [7]. 
Composition (w%) 
Cr Mo Ti Nb W Co Al C Ni 
IN718 19.0 3.0 0.9 5.1 – – 0.5 0.04 Balance 
IN625 21.5 9.0 0.2 3.6 – – 0.2 0.05 Balance 
IN738LC 16.0 1.75 3.4 0.9 2.6 8.5 3.4 0.11 Balance 
Fig. 1. – Number of annual publications focusing on the production of alloys 
718, 625 and 738LC using PBF-LB. 
G.M. Volpato et al. 
Additive Manufacturing 55 (2022) 102871
3
2. Outline of topics 
Due to the fact that the set of all reviewed publications presents a 
wide multifaceted nature, a number of categories were established to 
better classify them for analysis. After a general distinction based on 
alloy type (between IN718, IN625 and IN738LC), the publications were 
separated into several intersecting groups, which were defined as 
research subjects approached by each experimentation route, as follows: 
1. Parameters: studies that either achieved the optimization of one 
or more PBF-LB manufacturing parameters considering criteria 
such as build density, defect incidence and build rate, or deter-
mined their effects on material and surface properties; 
2. Heat treatment (HT): studies that submitted as-built superalloys 
to one or more heat treatments in order to analyze the perfor-
mance and microstructural changes of the materials; 
o Hot isostatic pressing (HIP): studies that employed hot isostatic 
pressing with aims similar to those of the HT studies; 
3. Machining: studies that evaluated machining processes applied to 
the materials, analyzing the effects and properties associated 
with either the tool or the part; 
4. Tensile: studies that employed tensile testing to evaluate the 
quasi-static mechanical behavior; 
5. Hardness: studies where the macro-, micro- or nanohardness of 
the materials were measured; 
6. Fatigue: studies conducted to evaluate either low or high cycle 
fatigue of the materials, as well as the crack growth behavior; 
7. Creep: studies carried out to access the tensile and/or compres-
sive creep behavior of the materials; 
8. Corrosion: studies in which the corrosion, oxidation and passiv-
ation mechanisms associated with the materials were 
investigated; 
9. Composites: studies reporting the production and/or analysis of 
metal matrix composites reinforced with various particles and/or 
fibers; 
10. Tribology: studies where the topography and/or the wear/fric-
tion behavior of the materials were addressed; 
11. Simulation: studies where simulation methods were employed to 
further analyze data acquired experimentally; and 
12. Microstructure: studies that evaluated microstructural features 
and changes related to the materials. 
Table 2 – 
Number of studies by research subject and method. 
Group Subgroup IN718 IN625 IN738LC 
Microstructure TEM 59 14 5 
SEM 196 49 15 
EDS 81 23 9 
LM 146 31 12 
EBSD 100 24 12 
XRD 73 20 5 
CT 26 5 5 
Parameters 85 25 8 
HT 104 20 6 
HIP 29 7 5 
Machining 18 7 0 
Tensile 95 28 9 
Hardness 73 17 2 
Fatigue 44 14 0 
Creep 28 3 3 
Corrosion 19 8 1 
Composites 22 6 1 
Tribology 23 4 0 
Simulation 31 8 4 
Total 251 69 17 
Fig. 2. – Connectivity between experimental methods in the reviewed literature related to the production of Inconel by PBF-LB. The numbers correspond to the 
number of studies encompassing the subjects at both ends of the associated edge. 
G.M. Volpato et al. 
Additive Manufacturing 55 (2022) 102871
4
Because a significant number of the studies involved microstructural 
analysis performed by different methods, this group was further divided, 
according to the characterization techniques employed, into the 
following subgroups: 
1. TEM: transmission electron microscopy and its associated 
techniques; 
2. SEM: scanning electron microscopy and its associated techniques; 
Table 3 – 
Phases present in PBF-LB Inconel superalloys. 
Table 4 – 
Undesirable phases present in PBF-LB Inconel superalloys. 
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5
3. EDS: energy-dispersive X-ray spectroscopy and its associated 
techniques; 
4. LM: light microscopy and its associated techniques; 
5. EBSD: electron back-scattered diffraction and its associated 
techniques; 
6. XRD: X-ray diffraction and its associated techniques; and 
7. CT: computed tomography and its associated techniques. 
After these categories had been established, the publications iden-
tified for inclusion in the study were allocated to the appropriate group/ 
subgroup. Most studies could be placed in two or more of these cate-
gories and were thus placed in more than one group. The results of the 
literature classification are shown in Table 2, which shows the number 
of studies associated with each research subject category and organizes 
them by superalloy type. 
It could be noted from the reviewed literature that some subjects 
were more thoroughly investigated than others. A greater number of 
authors have focused on the topics of heat-treatment, quasi-static me-
chanical behavior, hardness and parameter optimization. A considerable 
number of publications report microstructural analysis, mostly 
employing SEM and LM. In contrast, the topics of machining, corrosion 
and creep behavior are less frequently investigated. 
The connectivity between the categories, i.e., how many studies 
focus on two of these categories simultaneously, is shown in Fig. 2, with 
the exception of the “Microstructure” and “Simulation” groups. Every 
group is represented with a circle whose diameter is proportional to the 
number of studies which were focused on that subject. These circles are 
connected with lines and their width is proportional to the number of 
studies that have both groups as subjects simultaneously. 
Following the observations gained from Table 2, Fig. 2 indicates that 
the topics of heat-treatment, quasi-static mechanical behavior, hardness 
and parameter optimization are not only the most represented subjects 
in the reviewed literature but also the most inter-connected, and the 
interaction between heat treatment and quasi-static mechanical 
behavior is the most common. On the other hand, a few gaps could be 
noted, for instance, no reviewed studies were focused on the interactions 
between corrosion and fatigue, composites and fatigue, corrosion and 
HIP, composites and HIP or creep and tribology. 
A full overview of the studies analyzed in this review can be seen in 
the graph of Appendix A, in which each individual publication is map-
ped to show its associated research subject categories. Each number in 
the Appendix refers to the associated publication number in the Refer-
ences section, while each color corresponds to a superalloy: blue for 
IN718, red for IN625 and green for IN738LC. The size of each method 
node (in gray) is parametrized by the number of publications that either 
employ or focus on the method itself. 
3. Microstructure 
The microstructure of Inconel superalloys can be described as a face 
centered cubic (FCC) γ matrix reinforced by several strengthening pre-
cipitates, solid solution elements and carbides. This kind of micro-
structure is associated with Inconel regardless of the manufacturing 
technique employed. Some phases provide the alloys with their char-
acteristic properties and strength (Table 3) while others are undesirable 
and avoided due to performance deterioration (Table 4). 
Among the microstructural strengthening mechanisms, the precipi-
tation of metastable γ’ and γ’’ phases coherent with the γ matrix brings 
the most substantial strengthening to some of these superalloys [34]. A 
high volume fraction of these phases is associated with high yield and 
ultimate tensile stress at high temperatures, as well as high hardness and 
creep rupture strength [7]. The precipitation of γ’ is increased by the 
addition of Ti and Al, but the presence of Nb increases the development 
of γ’’. This effect is predominant in two of the three Inconel alloys 
commonly considered in studies on PBF-LB. Thus, while IN718 presents 
nano-scale precipitation of γ’’ as its main strengthening mechanism due 
to the high Nb content of the alloy, IN738LC has a microstructure with a 
high volume fraction of γ’ when compared to most other superalloys due 
to its high Ti contents. 
A second strengthening mechanism is the solid-solution due to the 
presence of atoms with slight atomic radius mismatch with Ni in the γ 
matrix. These atoms induce a distortion of the crystal lattice together 
with the development of internal stresses associated with alloy 
strengthening. Because this expansion is dependent on the degree of 
atomic radius mismatch, larger solid-solution atoms such as W and Mo 
increase the extent of this mechanism. Thus, it is predominant within the 
microstructure of IN625, a Mo-rich superalloy. 
A third strengthening mechanism is the formation of carbides, which 
are found in various shapes and compositions within the microstructure 
of Inconel superalloys. Relevant to the three alloys commonly studied 
with PBF-LB are the MC, M23C6 and M6C carbides, in which M stands for 
atoms such as Ti, Cr and Mo. These carbides have been associated with 
the inhibition of grain growth and grain boundary sliding [35] while not 
presenting deleterious levels of stress accumulation due to crystallo-
graphic coherency with the γ matrix [36], being thus desirable up to a 
certain amount for the mechanical performance of polycrystalline 
superalloys. 
On the other hand, the microstructure of PBF-LB Inconel alloys might 
also include a number of phases which are undesirable, since their 
presence deteriorates the mechanical behavior of the material due to 
topologically close-packed crystal structures. These include the Laves 
and δ phases, which are often described in the PBF-LB literature. Further 
detrimental phases that are common in conventional Inconel materials 
were not described in the reviewed literature. This is the case of the 
tetragonal σ CrFe phase, despite its significance with regard to the 
general microstructure of Inconel. 
The Laves phase corresponds to irregularly shaped, bulky and brittle 
structures usually in the grain boundaries and interdendritic areas of 
IN718 [37] and IN625 [38], being developed from Ti and Nb segrega-
tion upon solidification [39]. It provides preferential sites for the 
nucleation of microvoids and cracks [36,40–42] at the same rate that 
depletes the matrix from elements needed for γ’’ precipitation [41,43] 
and locally reduces γ corrosion resistance [44], thus being mechanically 
detrimental for the superalloy. 
The δ phase is an acicular phase that has been shown to segregate in 
Nb-rich interdendritic regions [34,45–47]. Although studies suggest that 
moderate amounts of this phase can improve high temperature me-
chanical behavior through the inhibition of grain growth, microvoid 
coalescence and grain boundary sliding due to pinning [40,48–53], it is 
argued that these effects can be better achieved by nano-sized pre-
cipitates and the PBF-LB dislocation cell structure itself [54,55]. More-
over, because this phase has also been associated with a reduction in 
fracture toughness, high-temperature ductility, fatigue life and corro-
sion resistance in intergranular zones [36,42,45,46,50,55–60], it is also 
regarded as undesirable in terms of material performance. 
4. Manufacturing parameters 
Additive manufacturing technologies are highly dependent on 
careful control of the manufacturing parameters, which influence both 
the quality and performance of the components. For this reason, a 
considerable fraction of the PBF-LB literature is focused on analyzing the 
influence of various parameters, either independently or in relation to 
each other, with a view to their optimization. These so-called “para-
metric studies” are based on the arrangement of various PBF-LB build 
conditions with different parameter sets, each of which is subsequently 
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Direct metal laser sintered (DMLS) process to develop Inconel 718 alloy for 
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10.1016/j.ijleo.2019.163735. 
[355] H. Zhang, D. Gu, L. Xi, H. Zhang, M. Xia, C. Ma, Anisotropic corrosion resistance 
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J. Mater. Sci. Technol. 35 (2019) 1128–1136, https://doi.org/10.1016/j. 
jmst.2018.12.020. 
[356] Y.W. Luo, B. Zhang, Z.M. Song, C.P. Li, G.F. Chen, G.P. Zhang, A comparative 
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10.1557/jmr.2020.98. 
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selective laser melted in718 alloy upon post heat treatment and shot peening, 
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superalloy parts: high-temperature oxidation property and its mechanisms, Opt. 
Laser Technol. 62 (2014) 161–171, https://doi.org/10.1016/j. 
optlastec.2014.03.008. 
[360] T. Sanviemvongsak, D. Monceau, B. Macquaire, High temperature oxidation of IN 
718 manufactured by laser beam melting and electron beam melting: effect of 
surface topography, Corros. Sci. 141 (2018) 127–145, https://doi.org/10.1016/j. 
corsci.2018.07.005. 
[361] H. Zhang, D. Gu, C. Ma, M. Xia, M. Guo, Surface wettability and 
superhydrophobic characteristics of Ni-based nanocomposites fabricated by 
selective laser melting, Appl. Surf. Sci. 476 (2019) 151–160, https://doi.org/ 
10.1016/j.apsusc.2019.01.060. 
[362] S. Parizia, G. Marchese, M. Rashidi, M. Lorusso, E. Hryha, D. Manfredi, 
S. Biamino, Effect of heat treatment on microstructure and oxidation properties of 
Inconel 625 processed by LPBF, J. Alloy. Compd. 846 (2020), 156418, https:// 
doi.org/10.1016/j.jallcom.2020.156418. 
[363] X. Liu, R. Zhang, S. Feng, H. Li, Y. Wang, J. Gong, H. Sun, Effect of aging 
treatment on corrosion resistance of Inconel 718 fabricated by selective laser 
melting, IOP Conf. Ser. Mater. Sci. Eng. 772 (2020), https://doi.org/10.1088/ 
1757-899X/772/1/012075. 
[364] W. Zhou, G. Zhu, R. Wang, C. Yang, Y. Tian, L. Zhang, A. Dong, D. Wang, D. Shu, 
B. Sun, Inhibition of cracking by grain boundary modification in a non-weldable 
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induced by inoculants during the selective laser melting of IN718, Addit. Manuf. 
21 (2018) 465–471, https://doi.org/10.1016/j.addma.2018.02.018. 
[366] R. Krishnan, M. Naik, D.G. Thakur, V. Dillibabu, Experimental investigation on 
wear behavior of additively manufactured components of IN718 by DMLS 
process, J. Fail. Anal. Prev. 20 (2020) 1697–1703, https://doi.org/10.1007/ 
s11668-020-00978-8. 
[367] C.S.S., M. Arivarasu, T.R. Prabhu, High temperature dry sliding wear behaviour of 
laser powder bed fused Inconel 718, Addit. Manuf. 34 (2020), 101279, https:// 
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	A comprehensive literature review on laser powder bed fusion of Inconel superalloys
	1 Introduction
	2 Outline of topics
	3 Microstructure
	4 Manufacturing parameters
	4.1 Laser power
	4.2 Scan speed
	4.3 Hatch parameters
	4.4 Layer thickness
	4.5 Linear and volumetric energy densities
	4.6 Additional parameters
	5 Heat treatments
	5.1 IN718
	5.2 IN625
	5.3 IN738LC
	6 Machining
	7 Mechanical behavior
	7.1 Tensile strength
	7.2 Hardness
	7.3 Fatigue
	7.4 Creep
	8 Corrosion behavior
	9 Composites
	10 Tribological behavior
	11 Current challenges
	12 Concluding remarks
	CRediT authorship contribution statement
	Declaration of Competing Interest
	Acknowledgments
	Appendix A Map of reviewed publications
	Appendix B Parameter tables
	Appendix C Maps of publications focused on mechanical behavior
	ReferencesThe main build parameters evaluated in PBF-LB studies on Inconel 
superalloys are the laser power, scan speed, hatch distance and layer 
thickness. These are all machine-specific parameters, which makes them 
easy to change from sample to sample. They will be described in more 
detail below. 
G.M. Volpato et al. 
Additive Manufacturing 55 (2022) 102871
6
However, because the number of combinations of such parameters 
can be considerably large and the parameters themselves are highly 
sensitive to each other, such studies tend to be lengthy and complex. 
Therefore, several methods have been developed to speed up these 
analytical processes, such as the evaluation of combined parameters 
(mathematical parameters aimed at evaluating various build parameters 
simultaneously) or the use of statistical design of experiment (DoE) 
methodologies. 
Various other parameters have also been evaluated in the literature, 
either individually, together with the main parameters or in relation to 
each other. These include the build gas, support structures, powder 
variables, laser beam geometry and path, and build orientation, which 
are detailed in Section 4.6. 
A concise exposition of the parameters employed to build bulk 
Inconel samples using PBF-LB is shown in Appendix B, which brings a 
series of tables with the employed values of laser power, scan speed, 
hatch distance and layer thickness. 
4.1. Laser power 
The laser power is the main parameter that controls the transference 
of energy from the PBF-LB system to the process powder, aimed at its 
melting in a controlled and homogeneous fashion to produce dense bulk 
components. 
Increasing laser power is often associated with easier melting of the 
precursor powders and therefore a better densification of as-built com-
ponents [61–64]. However, studies have shown that an indiscriminate 
increase in this parameter leads to the formation of porosity in the final 
pieces due to the development of keyhole pores and strong convective 
flows in the molten material [62,65]. Therefore, maximum material 
density is expected by determining an optimum intermediate value of 
bulk laser power, which avoids these disadvantages [62,66–71]. This 
optimization is also an important method of reducing the overall cost of 
the production process, which is considerably increased by the use of 
high laser power [63]. 
It has been evidenced that a high laser power also increases the di-
mensions of the melt pool generated by heating [62,66,72–78], which, 
in turn, has been associated [74] with microstructural coarsening [79], 
reduced surface roughness [76,80] and a higher incidence of micro-
cracking [64,74]. Since microhardness within the melt pool has been 
shown to be independent of the laser power [75], the latter can be 
attributed to increased temperature gradients after laser melting, which 
induce residual stress in the solidified material [73,81]. This also con-
trols the degree to which layers of deposited material undergo remelting 
when subsequent layers are added, which is more pronounced in the 
case of deeper melt pools [72,73]. 
4.2. Scan speed 
The scan speed describes how fast the laser beam travels through the 
surface of the powder bed, thus being a critical factor for the definition 
of the build rate of PBF-LB components. Moreover, it controls the time 
during which each part of the powder bed is submitted to the laser 
power, having an influence on the energy available for its melting. 
Although high scan speeds contribute to high build rates [63], this 
condition can also lead to lack of fusion porosity [63–65,82–84] and 
defects such as balling [66,72,85] in as-built materials. These effects, in 
turn, have been described to be a consequence of the high shear stresses 
within the melt pool under such conditions [69]. On the other hand, a 
low scan speed can increase the amount of energy per unit of time in the 
melt pool, thus increasing its dimensions [66,73,75,77,78,83,86–88] 
and inducing both the keyhole phenomenon [61,66,72,77] and spat-
tering due to turbulent melting [87]. Nonetheless, in various studies 
focused on building samples with considerably low [89–96] or high [64, 
85,97] scan speeds, these effects were ameliorated by adjusting other 
parameters. 
It has been verified that both extremes of this parameter increase 
roughness in relation to optimum values [61,66]. Some authors argue 
that this effect is more pronounced with high scan speeds [66,76], 
whereas others have found the opposite scenario [87]. Nonetheless, 
Fig. 3. – Hatch parameters: (a) hatch distance, (b) layer strategies and (c) layer rotation. 
G.M. Volpato et al. 
Additive Manufacturing 55 (2022) 102871
7
although scan speed has been described as more significant than laser 
power in terms of roughness control [66], it has also been reported that 
this parameter has a negligible influence on the same property [75]. In 
addition, some authors have found that the scan speed has a major 
impact not only in controlling the dimensions of components but also 
the volume fraction of aging precipitates [98]. 
High crack density upon solidification has been attributed to low 
scan speeds [61,64,83], as this condition increases the maximum tem-
perature within the powder bed [73] and, consequently, the residual 
stresses in the solidified melt pool [61,86]. 
4.3. Hatch parameters 
Hatch parameters define the path taken by the laser beam within a 
layer, as well as how this path changes from layer to layer. The three 
main aspects of this parameters, which are schematically represented in 
Fig. 3, are as follows: 
1. Hatch distance - the space between adjacent paths of the laser beam, 
which can account for the degree of beam overlap; 
2. Layer strategy - the geometry that the laser paths take within a same 
layer; and 
3. Layer rotation - the rotation of the layer strategy after each new 
layer. 
The hatch distance defines the space between two passes of the laser 
beam (Fig. 3a) and thus controls the way in which the surface of the 
powder bed is heated. A large distance can lead to a lack of contact 
between molten regions of the powder bed, whereas low distances can 
result in excess energy input. Therefore, an optimal distance needs to be 
identified to ensure the final quality of the build. Optimum hatch dis-
tances associated with minimum porosity have been established for 
IN718 [67], IN625 [99] and IN738LC [61]. Optimum values for mini-
mum roughness have also been determined for IN738LC [61]. Some 
authors have shown that increasing the hatch distance leads to greater 
porosity in as-built specimens [38,61,69,74,86,99], although it has also 
been associated with reduced crack incidence [61,64,74], cooling rates 
of the powder bed, asymmetry of the melt pools [86] and hardness [69]. 
Thus, this parameter has been considered a dominant factor for the 
definition of both the above-mentioned factors [98] and the porosity 
[100]. In one study on the IN738LC melt pool, asymmetry was analyzed 
by verifying a micro-humping phenomenon in pools when the hatch 
distance was too small [61]. 
The layer strategy has a strong influence on the final microstructure 
and surface topography of the components, and the most common ge-
ometries are shown in Fig. 3b. They can be designed considering either 
unidirectional or bidirectional scanning, the latter being reported to 
induce bimodality in melt pool sizes as well as a loss of melt pool cross- 
section symmetry due to the dynamic distribution of the powder bed 
heating[101,102]. It has been shown that the continuous strategy tends 
to develop higher texture, anisotropy [103] and geometry-induced re-
sidual stresses [104] than the alternatives, which induce better me-
chanical properties and creep resistance in specific sample orientations 
[53,103]. One study has shown that this strategy can be used to tailor 
the crystallographic texture from polycrystalline up to single-crystal-like 
microstructures [105]. The island strategy (also called “chessboard” by 
some authors) is associated with the highest isotropy and the lowest 
residual stresses of all such geometries [104], and the size of the islands 
seems to be proportional to the relative density of the component [106]. 
This may be related to a reported superior wear behavior in comparison 
to other strategies [107]. Strip geometries can provide intermediate 
isotropy results and have been shown to lead to less texture when a 
longer hatch length is applied [108]. In addition, different strategies on 
how the contour region of samples is exposed to the laser radiation have 
also been reported to have major effects on the surface quality of pro-
duced components, which is connected to performance under fatigue 
and tribology applications [76,80,87]. 
Layer rotation (Fig. 3c) can be applied to any of the layer strategies, 
leading to different results. If no rotation is applied, the component is 
expected to have more heterogeneity, residual stress and shrinkage, as 
the laser beam is always focused on the same region of the powder bed 
[109–111]. This produces a bimodal grain structure [67,112], which 
was also verified when 90◦ rotation was employed [67]. However, 90◦
rotation seems to reduce the tensile and fatigue strengths when 
compared to non-rotated samples [113], which may be associated with 
the reduced residual stress [111] and microstructural columnarization 
[112] observed under this condition. A rotation of 67◦ is often applied in 
order to change the path of the laser beam for every new layer, thus 
avoiding grain growth [114] and bimodality [67] while also diminishing 
the degree of texture [109,115] and facilitating machining [116]. 
Nonetheless, a preferential alignment to the build direction is 
expected in as-built samples due to the orientation of the thermal 
gradient during build-up [67,105,115,117–119], despite the careful 
control of parameters being able to tailor the preferential texture after 
processing [105]. 
Fig. 4. – Schematic representation of the effects of increasing and decreasing linear and volumetric energy densities as reported in the reviewed literature. The 
properties are arranged moving toward their maximization. 
G.M. Volpato et al. 
Additive Manufacturing 55 (2022) 102871
8
4.4. Layer thickness 
The powder layer thickness defines the height of each subsequent 
powder layer to be turned molten by the laser beam, thus controlling the 
build rate and, consequently, the total manufacturing time of individual 
components. Thus, because this total time is one of the main constraints 
of additive manufacturing technologies for industrial applications, 
increasing the layer thickness could be an effective way to speed up the 
build process and increase the cost efficiency. However, it has been 
shown for IN718 that higher layer thickness is associated with detri-
mental changes to the properties of the final component, such as reduced 
dimensional accuracy [120] and lower mechanical properties due to 
lower yield strength [97,120], ultimate tensile strength [97,120], 
Young’s modulus [97], Vickers microhardness [86,97] and impact 
toughness [120]. Additionally, it has been shown that an increase in 
layer thickness induces lower cooling rates within the powder bed as 
well as a larger dendritic arm spacing in the final microstructure [86], 
despite the parameter having marginal importance for the absorptivity 
of the laser power [121]. 
4.5. Linear and volumetric energy densities 
The linear and volumetric energy densities (LED and VED, respec-
tively) are combined parameters, developed for the joint evaluation of 
the effects of the aforementioned individual parameters. They are 
defined, respectively, by Eqs. 1 and 2, which are represented by various 
notations in the literature: 
LED =
Laser power
Scan speed
[
J
mm
]
(1) 
VED =
Laser power
Scan speed × Hatch distance × Layer thickness
[
J
mm3
]
(2) 
In parametric studies, the control of the LED and the VED has led to 
similar effects on the as-built specimens, despite the difference between 
the two parameters. A summary of these effects can be found in Fig. 4. 
Because these parameters describe the amount of energy required to 
melt the powder bed, it is commonly acknowledged that applying high 
values leads to better overall densification of the material [85,122–130], 
which results in greater mechanical stability and, consequently, higher 
stiffness and hardness [71,123–126,128,130,131]. Nevertheless, this 
effect has been shown to occur only in specific processing windows, and 
both density [61,64,65,67,68,71,132,133] and hardness [67,71,122, 
133] eventually decrease once the energy density employed becomes 
too high. 
The fact that density and hardness are optimized at intermediate 
values of LED and VED is also verified considering various other prop-
erties of the as-built specimens, such as yield stress [61,81], ultimate 
tensile stress [81], elongation [61,81] and fatigue life [134]. This could 
be associated with the development of porosity at both extremes of 
energy density, that is, low values induce irregular lack-of-fusion 
porosity due to insufficient melting of the powder bed [63,67,68,131, 
132,134–136] while excessively high values lead to smaller round pores, 
attributed to turbulence of the melt pool, gas entrapment and keyhole 
formation [61,63,67,132,136]. 
Other phenomena have been verified to behave proportionally to 
these parameters. Due to larger energy input, higher energy density 
values result in hotter, wider and deeper melt pools [63,72,74,78,125, 
135,137], which tend to present a higher degree of overlapping [138]. 
The pool dimensions seem to grow linearly with the energy density [63, 
78]. This also influences the microstructure of as-built specimens, which 
tend to present equiaxed grains when low energy density values are 
applied and columnar microstructures when higher values are used 
[126,127,130,131,135]. The diameter of the columnar dendrites has 
been reported to be inversely proportional to VED [130]. 
Oxidation resistance [124,129], susceptibility to creep [71] and 
crack density [61,74,134] have also been reported to increase with 
higher energy density. In the case of crack density, this may be related to 
the reportedly higher residual stresses [81] and segregation [130] under 
this condition. When considering porosity-nucleated cracking, however, 
the tendency seems to be the opposite [136]. 
In addition, the dimensions of reinforcement particles within the 
microstructure are refined, their interfacial roughness decreases and 
their volume fraction reduces with higher energy density, which also 
leads to higher diffusion and reactivity between the matrix and the 
reinforcement particles within composites [122,125]. Increasing the 
energy density further can induce the fragmentation of reinforcement 
particles [139]. Both the Poisson’s ratio [128] and superficial roughness 
[65,87,133] of the samples appear to be minimized on applying higher 
LED and VED values, the latter up to a threshold [61,133]. Accordingly, 
both the wear rate and the coefficient of friction seem to be reduced 
[122,124–126,130], although there is evidence of optimum values for 
their minimization [122]. 
Despite the widespread use of theseparameters as representative 
factors in parametric studies, the degree of their validity is subject of 
controversy. Some authors report the statistical significance of com-
bined parameters as factors of influence for the measured properties 
[133] while others argue that they cannot be used to describe the 
experimental settings [87,100]. This inadequacy is clear in specific 
studies, in which different parameter combinations achieve as-built 
microstructures with different characteristics in terms of melt pool ge-
ometry, hardness, fracture mechanism and thermomechanical fatigue 
life while still having the same VED [49,119]. 
4.6. Additional parameters 
In addition to the main PBF-LB build parameters and their combi-
nations, the effects of other factors have been studied and the results 
reported in the literature. These include, but are not limited to, the build 
gas, support structures, powder variables, laser beam geometry and 
path, and build orientation. 
Due to the superficial oxidation induced by the melting of the 
powder bed, inert gases are a common processing atmosphere in PBF-LB 
systems [140–142]. In a comparative study with N2, Ar has been shown 
to induce grain refinement and larger melt pools in IN718 as-built 
samples, leading to a higher overall strength and ductility [143] 
despite greater roughness [144]. The use of He as a shielding gas for 
IN625 reduced the spattering intensity in comparison to Ar [137]. 
However, specimens built under vacuum conditions presented a greater 
γ’ stability, as well as a diminished incidence of the δ phase and larger, 
more anisotropic, grains compared with the use of Ar, which resulted in 
greater creep resistance [43]. 
Support structures have been found to reduce the overall residual 
stress of components [145], this effect being more pronounced under 
conditions where the substrates present either low thermal conductivity, 
pre-heating or pre-compression [146]. Additionally, although the effects 
of these structures on the local microstructure and hardness are not 
strong, they have been shown to become more pronounced as the 
structures become taller and narrower [147]. 
The reuse of the powder is of great interest with regard to the eco-
nomic viability of PBF-LB processing, due to the high percentage of 
waste and high cost associated with these types of consumables. This is 
especially true for gas-atomized powders, which have been shown to 
produce denser specimens than water-atomized powders [148]. This 
effect can be attributed to the greater regularity in the morphology of 
gas-atomized powders, especially under high humidity conditions 
[149]. However, the potential for the reuse of powders is related to 
changes in its particle diameter and composition by oxidation [140], 
particularly in the accumulation of fines after sieving, which seems to 
affect both the flowability of the powder bed [150] and the primary 
dendrite arm spacing of as-built specimens [141]. These effects are 
mostly associated with powder that undergoes spattering during the 
G.M. Volpato et al. 
Additive Manufacturing 55 (2022) 102871
9
PBF-LB processing, other types tending to be resistant to the additive 
manufacturing process [142]. Nevertheless, the extent of the impact of 
such changes on the final components is still a subject of research, as 
authors have reported no changes in the mechanical behavior of speci-
mens subjected to cycles of reuse (up to 14 times) [151]. 
Besides the main PBF-LB parameters, the geometry and path of the 
laser beam have been considered as promising parameters for the opti-
mization of the density and microstructure in the processing of Inconel. 
This can be verified by the development of skywriting [132] and circular 
oscillating beam scans [152], both of which tend to reduce 
micro-porosity. The latter has also been associated with a reduction in 
the microsegregation and the enhancement of texture. Higher anisot-
ropy has also been achieved by using a defocused laser beam, which 
seems to produce larger grains and, consequently, improved creep 
resistance [103]. In addition, the beam profile can be altered so as to 
depart from a Gaussian geometry and achieve lower local maximum 
temperatures, thus reducing keyhole formation, increasing process sta-
bility, reducing spattering, increasing build rates and influencing crack 
density [83,153]. If applied with very high power, non-Gaussian beam 
profiles have also been reported to considerably increase grain size and 
crystallographic texture [154]. 
Various effects of the build orientation on the behavior of PBF-LB 
Inconel components have been studied in the reviewed literature, 
which usually affects the directional dependence of certain properties of 
these superalloys. It has been reported that lower strength, hardness and 
stiffness are to be expected in directions normal to the additive manu-
factured layers [68,104,155–157], which may be related to reports of a 
significant effect of orientation on the number of pores [150] and grain 
size in as-built [131,158] and heat-treated [53] components. A similar 
dependence has been verified for creep life [53,159,160], corrosion 
resistance [161–163], heat treatment efficiency [164] and wear resis-
tance [158,165]. In addition, the overhanging surfaces produced by 
certain build orientations have been shown to present substantial 
roughness, which is attributed to a lack of support from the powder bed 
during the building process and to the sticking of spattered powder [87, 
134,158,161,166–168]. This, in turn, is especially critical in view of 
fatigue properties [134], as it has been shown that the defects present at 
overhanging surfaces can induce severe changes in fatigue life and crack 
growth behavior [134,169] as well as in contact fatigue [170]. 
5. Heat treatments 
Inconel superalloys are heat treated to increase the mechanical 
properties at both high temperatures and under room-temperature 
conditions. This is normally achieved by a precipitation hardening 
heat treatment, which is based on a solution annealing step followed by 
an aging sequence. While the high-temperature solution treatment aims 
to produce a homogeneous, stress-relieved, isotropic microstructure free 
of precipitates, aging takes this microstructure as a basis for the 
precipitation of hard γ’ and γ’’ phases within the coarsened grains, 
which provides an alloy with high mechanical and thermal performance. 
When produced by PBF-LB, however, the initial microstructure of 
these alloys is radically different from both wrought and cast compo-
nents, thus needing tailored thermal post-processing to ensure the 
satisfactory performance of the components. 
5.1. IN718 
The conventional heat treatments applied to IN718 are dependent on 
the manufacturing route of each component. As an example, while 
wrought IN718 components are usually submitted to the AMS 5662 heat 
treatment (solution annealing at 980 ◦C for 1 h followed by double aging 
at 720 ◦C for 8 h and 620 ◦C for 8 h), the sequence is preceded by a 
homogenization step (1080 ◦C for 1 h) for casting specimens, in accor-
dance to AMS 5383 [171]. This change is aimed at reducing the segre-
gation in the irregular microstructure of cast materials. 
Accordingly, the recently published standard ASTM F3301 describes 
a heat treatment aimed at PBF-LB IN718 that adds stress release 
(1065 ◦C for 1.5 h) and HIP (1120 ◦C for 4 h) steps before the applica-
tion of the standard AMS 5662, to address artifacts produced during the 
additive manufacturing process. A similar approach to PBF-LB IN718 is 
described in the reviewed literature, where various methods are applied 
to adjust the as-built microstructure to industry standards. 
Because the microstructure ofPBF-LB IN718 presents severe 
anisotropy, residual stress and segregation together with a fair number 
of defects, various authors have employed high-temperature steps 
before the precipitation hardening heat treatment in order to mitigate 
these aspects by recrystallization. These include homogenization treat-
ments and also the use of HIP or a high-temperature solution. 
In addition to the various typical recrystallization effects that have 
been reported for all three methodologies (Table 5), further results show 
that although the pre-treatments tend to reduce the mechanical strength 
and increase ductility, better mechanical properties are achieved after 
the full heat treatment in comparison with precipitation hardening alone 
[50,89,144,172–175]. One reason for this improvement may be the 
reported increase in the volume fractions of γ’ [176] and γ’’ [8] after 
treatments preceded by homogenization and high temperature solution, 
respectively, which has been related to the dissolution of segregates rich 
in elements for the precipitation of these phases [174,177]. Nonetheless, 
other studies revealed no significant changes in the static mechanical 
properties when adding homogenization [42,178–180] or HIP [180] 
before the precipitation hardening treatment. 
HIP is usually employed to reduce the metallurgical porosity of PBF- 
LB specimens [41,144,172,177,193,208–210], so that lack-of-fusion 
defects tend not to be as affected by the technique [211]. The temper-
ature of the treatment (usually higher than 1050 ◦C [193]) has been 
reported to have a greater effect than the pressure on the final densifi-
cation [193]. This may be associated with the precipitation of δ [39,212] 
due to the high-temperature dissolution of γ’’ [213] or Laves [43,49, 
159] phases followed by slow cooling, which could influence the re-
ported effects of the treatment on mechanical properties such as fatigue 
and creep resistance. Nonetheless, when succeeded by a solution heat 
treatment, HIP has also been reported to induce the dissolution of δ 
[190]. 
By changing the homogenization and solution temperature, re-
searchers have found that 1080 ◦C provides optimum mechanical 
strength [51,52,195,196], 1065 ◦C presents optimum solutioning effi-
ciency without a significant loss in mechanical properties [186] and the 
standard temperature of 980 ◦C provides maximum hardness [48]. 
Moreover, high treatment temperatures have been reported to induce a 
higher incidence of twinning and higher grain boundary angles [51,214] 
as well as improved tribological behavior [95] and corrosion resistance 
[215], although higher pitting has also been described [216]. Higher 
aging temperatures have been observed to reduce hardness through the 
coarsening of aging precipitates and the transformation of γ’’ into δ 
Table 5 – 
Reported effects of precipitation hardening pre-treatments applied to IN718. 
Homogenization HIP High-temperature 
solution 
Reduced 
segregation 
[42,55,176,179, 
181] 
[175,182,183] [8,41,50,57,174, 
184–188] 
Grain growth [164,179,181,189] [182, 
190–194] 
[41,186,187, 
194–196] 
Reduced 
anisotropy 
[164,189,197] [182] [188] 
Reduced strength – [39,189,191, 
198] 
[174,184,186,196, 
199] 
Reduced 
hardness 
[200–203] [194,204] [48,187,195] 
Increased 
elongation 
[178,200,201] [39,198] [184,196] 
Stress relief [171,200,205] [206] [36,185,207] 
G.M. Volpato et al. 
Additive Manufacturing 55 (2022) 102871
10
[60], which reduce the wear resistance [217]. 
Increasing the solution time induced recrystallization effects similar 
to those in Table 5 [104,185], such as grain [104,186,218] and phase 
[57] growth together with a reduction in the anisotropy [104], strength 
[104,215] and hardness [57,104,218]. The amount of δ has been 
described as directly proportional to solution time [52,218]. By 
increasing the duration of the aging treatment, on the other hand, the 
coarsening of γ’’ [67] has been reported together with the precipitation 
of δ [60,67,219], which induced a reduction in bulk hardness [195]. 
However, on decreasing the same parameter, authors have succeeded in 
increasing the ductility [220]. 
Some studies focused on the use of a single aging step instead of the 
standard double aging have been reported [40,54,67,184,221–223]. 
Although authors argue that this change does not improve the me-
chanical behavior of the precipitation hardened alloys when compared 
to those subjected to the traditional double aging procedure [184], two 
studies on novel heat treatments based on long single aging steps indi-
cated that, in relation to the standard, they greatly improve the elon-
gation to failure results while retaining [223] or improving [54] the 
strength to levels beyond that of wrought specimens. 
5.2. IN625 
As with IN718, the grain morphology of IN625 can be altered by 
Fig. 5. – Schematic microstructures for PBF-LB Inconel 625 under as-build, direct aged, solutioned and solutioned + aged conditions 
(reproduced from [224]). 
G.M. Volpato et al. 
Additive Manufacturing 55 (2022) 102871
11
solution annealing heat treatments, which can be followed by an aging 
sequence for precipitation hardening. Fig. 5 provides a scheme repre-
senting the microstructures of PBF-LB IN625 samples as-built and after 
various heat treatments, which correspond to the majority of results 
reported in the reviewed literature for standard conditions [224]. Apart 
from these cases, however, studies have been mostly focused on the 
themes of annealing parameters, effects of HIP and the precipitation of 
the deleterious δ phase. 
Regarding the solution annealing treatment, most studies were 
focused on analyzing the effect of temperature on the structure and 
properties of the PBF-LB superalloy. It has been reported that higher 
solution temperatures tend to reduce the anisotropy [225–227], 
strength, stiffness [225] and residual stresses [92,227] of treated spec-
imens, which may be related to grain growth [225–227], twinning and 
the dissolution of hard precipitates [227] under such conditions. The 
Laves phase has been reported to dissolve at temperatures higher than 
1040 ◦C [38] while δ preferentially precipitates at temperatures be-
tween 800 ◦C and 900 ◦C [228] and dissolves at temperatures higher 
than 1000 ◦C [229], the latter being associated with a decrease in 
hardness [230]. This has also been verified in a study focused on the 
effect of annealing time on the behavior of the material, where a cor-
relation between the treatment time, dissolution of intercellular pre-
cipitates and decrease in dislocation density was described [231]. 
HIP has been commonly employed for the densification of PBF-LB 
IN625 specimens [155,232,233], although its use is restricted to 
metallurgical pores with no connection to the material surface [232]. 
This treatment has been shown to improve fatigue life [233,234], but it 
greatly reduces the mechanical strength when compared to full 
heat-treated specimens [35,226,234], which can be considered as a 
consequence of the dissolution of hard phases and partial recrystalliza-
tion [35]. 
In a number of studies reported in the reviewed literature it was 
found that an industry-recommended stress-relief heat treatment for 
IN625 (870 ◦C for 1 h) promotes the formation of a significant amount 
of the incoherent δ phase in IN625 PBF-LB specimens [46,56,235,236], 
which is expected only after much longer times for wrought alloys at this 
temperature [46]. It has been further shown that the TTT curve for the 
precipitationof this phase is indeed moved to lower times in PBF-LB 
processing [46] (Fig. 6), which evidences the easier precipitation of δ 
under these conditions. This phase can be dissolved by subsequent ho-
mogenization at 1150 ◦C for 1 h [236]. 
5.3. IN738LC 
The standard heat treatment for IN738LC is a combination of solu-
tion annealing at 1120 ◦C for 2 h followed by a single aging step at 
850 ◦C for 24 h [237]. This treatment can be preceded by HIP, the in-
fluence of which is highly dependent on the applied temperature. Some 
studies have shown that HIP at 1000 ◦C [237], 1120 ◦C [64] and 
1180 ◦C [238] was not able to change the grain morphology or affect the 
mechanical properties, but a higher temperature of 1210 ◦C produced a 
different grain configuration due to widespread recrystallization, which 
coarsened the γ’ precipitates and reduced the dislocation density and 
overall stiffness [237]. This can also be achieved by adding extra 
stress-relief [239] or recrystallization [115] steps prior to applying the 
standard heat treatment, which have been reported to reduce the 
anisotropy and texture of as-built materials. Both results can be associ-
ated with the γ’ solvus temperature for IN738LC, reported as approxi-
mately 1150 ◦C [237]. 
Another approach to altering the mechanical behavior is the inclu-
sion of an extra aging step after the solution step of the standard heat 
treatment. This addition, when allied with HIP, was able to produce 
better mechanical properties than those achieved solely by applying the 
standard heat treatment [70]. Nonetheless, authors suggest that PBF-LB 
in-situ precipitation may be a viable route for the production of IN738LC 
Fig. 6. – Displacement of TTT curve for precipitation of δ phase in PBF-LB 
Inconel 625 
(reproduced from [46]). 
Fig. 7. – Directional dependence of machining in Inconel alloys produced by PBF-LB 
(reproduced from [248]). 
G.M. Volpato et al. 
Additive Manufacturing 55 (2022) 102871
12
without heat-treatments, given the intense segregation of the additive 
process [240]. 
6. Machining 
Inconel components manufactured by PBF-LB are usually submitted 
to machining, which increases the dimensional precision and reduces 
the surface roughness [91,241–243] and the fraction of superficial de-
fects [244] of samples. This post-processing technique is especially 
important for satisfactory fatigue behavior [204,234,245]. 
However, the machinability of these components is strongly affected 
by the microstructure of the materials. Because as-built PBF-LB speci-
mens present substantial texture and anisotropy, the machining condi-
tions and results are dependent on the direction of the material removal 
in relation to PBF-LB coordinates [116,203,246–249]. In this regard, 
better surface quality is expected when machining with the cutting 
speed along the scanning direction instead of the build direction [248] 
(Fig. 7). The final quality is further affected by the defects present in 
PBF-LB components [203,249], which, on the other hand, ease the 
removal of material, reduce residual stress in the part and reduce tool 
wear when compared to wrought materials [203,247,249,250]. Thus, 
although defects can be reduced by applying HIP, this technique hinders 
the machinability [249], which is also verified after the addition of 
ceramic reinforcements in the microstructure [242]. 
In addition, dry machining has been reported to produce better 
roughness and fatigue life results than emulsion [173], and better creep 
resistance was reported for samples machined by electro discharge 
machining than by conventional computerized numerical control sys-
tems [40]. Methodologies have been proposed for both roughening 
[251] and polishing [161,252–254] of PBF-LB Inconel internal surfaces, 
as well as for the machining [255,256] and dissolution [257] of support 
structures. 
7. Mechanical behavior 
7.1. Tensile strength 
The quasi-static mechanical behavior of PBF-LB Inconel superalloys 
differs from conventionally manufactured materials due to differences in 
the microstructure, anisotropy and defects caused by the additive 
manufacturing processing. This leads to a high variability in the material 
properties [258,259], especially in the case of precipitation-hardened 
alloys [259]. It has been reported that although as-built PBF-LB sam-
ples present superior strength in comparison to cast materials [64,70, 
178,243,260,261], lower stiffness, strength and ductility are expected in 
comparison to standard wrought superalloys [35,82,96,103,178,189, 
226,232,262–266]. The ductility of additive manufacturing samples is 
further affected by the fact that PBF-LB Inconel superalloys present a 
strong embrittlement under high-temperature conditions, which has 
been attributed to intense segregation induced by the laser processing 
[35,103,201,262,266,267]. 
Nonetheless, the application of appropriate heat treatments can in-
crease the mechanical properties of additive-manufactured superalloys 
by ameliorating detrimental microstructural artifacts such as undesir-
able precipitation and anisotropy. With this approach, strength com-
parable [45,50,175,178,179,184,189,191,264] or superior [42,51,59, 
65,89,104,181,182,259,268,269] to wrought materials has been ach-
ieved for heat-treated PBF-LB Inconel superalloys. Fracture and impact 
toughness of heat-treated PBF-LB IN625 have been shown to be similar 
or superior to conventionally produced materials [259], despite IN718 
presenting impact toughness inferior to the wrought superalloy [182]. 
These heat treatments not only induce stress relief, recrystallization and 
precipitation of aging reinforcements [35,38,39,64,70,174,180,184, 
188,191,195,199,224,226,234,263,270,271], but they have also been 
associated with changes in the mechanisms of deformation responsible 
for the mechanical behavior of these materials at both high temperatures 
and under room-temperature conditions [237,272]. 
However, due to the crystallographic texture of materials produced 
by additive manufacturing, their mechanical behavior tends to be 
anisotropic. Although heat treatments such as HIP and pre-aging heating 
cycles contribute to the isotropy of heat-treated samples [169,171,182, 
225,234,273], and separate homogenization and solution steps seem to 
present no benefit in relation to a single heating [44,178,180,269], 
direction-dependent mechanical responses, even after heat treatments, 
are well documented in the reviewed literature [274]. 
This directional dependence of the mechanical behavior of PBF-LB 
materials is usually analyzed, for geometries produced under various 
build directions, parallel to the PBF-LB cartesian coordinates. Thus, 
several authors have reported that when stress is applied orthogonally to 
the build plate (parallel to the build direction), all three reviewed PBF- 
LB Inconel superalloys tend to present minimum relative values of 
stiffness [54,89,191,221,238,260,275,276], yield [45,54,68,89,99,120, 
155–157,159,171,172,179,189,191,196,221,234,260,273,276–281] 
and ultimate tensile strength [35,45,54,68,89,120,155,156,159,171, 
172,179,191,196,225,226,234,260,267,273,276,277,279–282] 
together with maximum relative ductility [45,68,89,120,156,159,171, 
191,196,198,225,260,273,280–283]. IN718 presented maximum re-
sidual stress [284] and minimum deformation anisotropy [157] when 
printed under these conditions. This behavior has also been identified 
after heat treatment and under high temperature conditions [45,89,159, 
260], although the degree to which the mechanical strength is lost has 
been reported to be independent of sample orientation [277] and the 
machining of heat-treated samples seems to strongly reduce overall 
mechanical anisotropy [273]. 
On the other hand,the relative maximum of directional mechanical 
strength has been commonly identified under conditions in which the 
materials are stressed at a 45◦ angle in relation to the build plate. This 
direction has been reported to present either comparable [54,89,143, 
159,283] or superior [35,89,104,191,226,267,271,277,285] properties 
compared with those associated with loads applied parallel to the build 
plate (orthogonal to the build direction). The authors of a study carried 
out to compare the mechanical properties of samples under incremental 
build angles arrived at the same conclusions [163]. Similar tendencies 
can be verified through the observations that mechanical properties are 
enhanced when specimens are loaded in directions not parallel to the 
PBF-LB scan vectors [65,89,278] and that the shear properties are 
likewise dependent on the build direction [286,287]. 
This anisotropic mechanical behavior has been associated with the 
incidence, distribution and geometry of microstructural elements such 
as intergranular precipitates and defects within the columnar micro-
structure of materials produced by AM [45,163,268,288,289], and thus 
it has been argued that it is the most significant factor related to stiffness 
[198,261] and ductility [198]. A similar tendency can be verified from 
the finding that the strength of PBF-LB IN718 tends to decrease along the 
build direction, which was attributed to changes in both the precipitate 
density and the grain size with build height [290]. The mechanical 
properties resulting from these microstructural elements can be 
controlled by the appropriate choice of processing parameters [81,89, 
99,113,120,143,192,261,288,291–293]. The processing conditions can 
also affect the mechanical behavior of the components, as revealed by 
studies on the use of multiple laser beams [270], powder recycling [292] 
and varying the specimen size [283,290]. 
Another mechanism employed to alter the mechanical behavior of 
PBF-LB Inconel superalloys is the use of the additive manufacturing 
system to change the chemical composition and microstructure of the 
base superalloy, either through the production of composites or by 
alloying. These methods tend to increase the overall entropy of the 
system, creating barriers to the dislocation motion and thus restrictively 
increasing the strength while reducing the ductility. This has been 
verified not only through the alloying of IN718 with Re [294] and Al 
[295,296], but from the in-situ production of various composites [38, 
174,188,199,297–300]. In contrast, alloying with Cu tends to increase 
G.M. Volpato et al. 
Additive Manufacturing 55 (2022) 102871
13
the porosity and reduce the strength [301]. A similar effect can be 
observed from the influence of post-processes on the incidence of 
work-hardening of the material, and thus the superficial erosion pro-
moted by techniques such as sandblasting is expected to reduce the 
ductility [302] while stress-relief techniques tend to increase it [303]. 
An overview of the publications that focus on monotonic tensile 
behavior is found in Appendix C, Fig. 1 C. 
7.2. Hardness 
With regard to conventional Inconel components, high hardness can 
be achieved by the precipitation of coherent phases after heat treatment, 
solid solution hardening, grain refinement and, in some cases, work 
hardening. Although these effects can also be observed in materials 
produced by PBF-LB, the high solidification rates achieved via the 
manufacturing route produce considerable microstructural refinement 
[93,106,122,186,195,229,261,293,304,305]. Thus, due to the 
Hall-Petch effect allied to intense segregation, as-built components tend 
to be harder than those produced not only by conventional methods 
[220,263,306,307], but directly by other AM techniques such as 
directed energy deposition (DED) [69] and powder bed fusion by elec-
tron beam (PBF-EB) [227]. 
Because this refined microstructure is already present in as-built 
materials, the hardness of Inconel components produced by PBF-LB 
can be tailored both during the primary manufacturing process and 
during post-processing steps. The introduction of composite re-
inforcements to the powder bed is a reliable alternative to increase 
hardness [90,122–125,293,297,308], but several authors have reported 
that an efficient choice of processing parameters can lead to optimum 
hardness of as-built components [65,91,122,124,125,133,136,261,293, 
309]. On the other hand, other researchers have found that the behavior 
of this property is independent of specific parameters [106,172,227]. 
Regardless of these scenarios, the hardness of the as-built refined 
microstructure can be further increased by precipitation hardening heat 
treatments [49,172,173,201,261,271,310], where the basic steps of 
solution and aging tend to decrease [185,202,224,302] and increase 
[57,58,67,171,195,202,237,302,311] the hardness, respectively. These 
treatments tend to increase the grain size and dissolve hard phases such 
as δ [49,202,203,227,229] and carbides [122,293] in the solution step, 
but they also lead to a microstructure with greater isotropy [104,179, 
310] and, through the dissolution of hard intermetallic phases, more 
elements for the precipitation of aging phases [67,88,312], thus 
increasing the hardness. Therefore, although brittle phases such as δ and 
Laves present high hardness when considered individually, their disso-
lution increases the volume fraction of aging precipitates within a 
heat-treated microstructure. This can be verified through a comparison 
of the results obtained employing HIP since, although the dissolution of 
δ lead to lower hardness [175], precipitation-hardened samples have 
increased hardness when the heat treatment is preceded by HIP [172, 
175]. Conversely, as-built samples submitted to annealing but no aging 
show an increase in hardness [229,263] due to δ precipitation [229]. 
These heat treatments have, therefore, a considerable influence on the 
hardness behavior of Inconel samples produced by PBF-LB. After heat 
treatment, samples produced by PBF-EB are able to achieve comparable 
hardness to those produced by PBF-LB [204], which, in turn, are not as 
hard as those produced by DED [263]. 
Furthermore, heat treatments also homogenize the spatial distribu-
tion of hardness in PBF-LB samples [82,104,179,264], which seems to be 
to some degree dependent on location after the building process. The 
hardness of individual sections has often been described to be uniformly 
distributed regardless of position [147,200,264,309,310] but sections 
cut parallel to the build plane (i.e., the surface of the powder bed) have 
been reported to be less hard than sections cut orthogonally to it [104, 
179,312,313]. Nonetheless, studies show that when vertically 
sectioning tall samples, there is a heterogeneous hardness distribution 
due to differential precipitation of aging and intermetallic phases, which 
was not present in samples built horizontally [185,314] and may be 
correlated to similar results regarding mechanical strength [290]. This 
indicates a possible dependence of the distribution of section hardness 
on the build direction [248], which may or may not be significant [171]. 
Regarding surface hardness, in one study it was observed that the top 
surface of as-built components seems to present the highest degree of 
hardness and the bottom surface the lowest [315]. This is in agreement 
with a report of low hardness at the interface between the component 
and the build substrate [75] and thus the lack of support structures has 
also been associated with higher overall hardness [147]. The maximum 
top hardness can also be linked to the higher hardness reported in 
molten regions in comparison