This study provides a systematic investigation into the size and geometry-dependent microstructural evolution and cracking susceptibility of LPBF-fabricated Rene 41. By coupling experimental microstructural analysis with thermal modeling, this research uniquely identifies the relationship between geometry-specific thermal histories, carbide coarsening, and liquation cracking. Four different geometries with varying thickness were fabricated with the identical process parameters. It was found that the grain size and morphology are not affected by the part size. However, the thinner parts exhibited coarser sub-grain structures compared to the thicker ones. The crack formation was observed for the parts with cross-sections smaller than 1 mm, whereas thicker parts had high density without any defects. The cracks were observed in the interdendritic regions, suggesting that liquation cracking was the active micro-crack formation mechanism. The detailed microstructural analysis combined with a thermal finite element analysis proved that the heat extraction efficiency was lower for thinner parts causing a lower cooling rate and coarser carbides, making them more susceptible to constitutional liquation. Microhardness measurements were conducted for each geometry and correlated with the observed microstructural variations. The findings highlight the critical need for geometry-specific optimization of LPBF process parameters to mitigate cracking and achieve microstructural uniformity, offering valuable insights into the fabrication of complex, high-performance aerospace components.

The present study focuses on advancing one of the most popular AM techniques, namely, laser powder bed fusion (LPBF) technology, which has the ability to produce complex geometry parts with minimum material waste but continues to face challenges in minimizing the surface roughness. For this purpose, a novel hybrid manufacturing technology, which applies in a single setup (in-envelope) both LPBF technology and high-speed machining, was examined in this research for the fabrication of tensile specimens with three different surface finish conditions: as-built, hybrid (in-envelope machining) and post-machining (out-of-envelope) on Inconel® alloy 718, hereafter referred to as IN718. As the application of the IN718 alloy in service is typically specified in the precipitation-hardened condition, three different heat treatments were applied to the tensile specimens based on the most promising thermal cycles identified previously for room-temperature tensile properties by the authors. The as-built (AB) specimens had the highest average surface roughness (Ra) of 5.1 μm ± 1.6 μm, which was a significant improvement (five-fold) on the hybrid (1.0 μm ± 0.2 μm) and post-machined (0.8 μm ± 0.5 μm) surfaces. The influence of this surface roughness on the mechanical properties was studied both at ambient temperature and at 650 °C, which is close to the maximum service temperature of this alloy. Regardless of the surface conditions, the room-temperature mechanical properties of the as-fabricated IN718 specimens were within the range of properties reported for standard wrought IN718 in the annealed condition. Nonetheless, detailed examination of the strain localization behavior during tensile testing using digital image correlation showed that the IN718 specimens with AB surfaces exhibited lower ductility (global and local) relative to the hybrid and post-machined ones, most likely due to the higher surface roughness and near-surface porosity in the former. At 650 °C, even though the mechanical properties of all the heat-treated IN718 specimens surpassed the minimum specifications for the wrought precipitation-hardened IN718, the AB surface condition showed up to 4% lower strength and 33–50% lower ductility compared with the hybrid and PM surface conditions. Microfocus X-ray computed tomography (µXCT) of the fractured specimens revealed the presence of numerous open cracks on the AB surface and a predisposition for the near-surface pores to accelerate rupture, leading to premature failure at lower strains.

Equiatomic CoCrFeMnNi high entropy alloy (HEA) powder was processed by laser powder bed fusion (LPBF) additive manufacturing (AM). The properties of the spherical pre-alloyed CoCrFeMnNi powder were characterized and its processability using LPBF AM was systematically investigated through the volumetric energy density (VED) based on the surface roughness, defects (micro-cracks and porosity) and densification. After optimization, LPBF processing at a VED of 104 J/mm3 achieved highly dense and crack-free vertical and horizontal test specimens with a porosity fraction lower than 0.01

This study presents the successful development of a new high γ′ nickel-based superalloy, LW 4280, using laser powder bed fusion. Microstructural analysis revealed the absence of γ′ precipitates in the as-built condition. A designed sub-solvus heat treatment resulted in a bimodal distribution of γ′ precipitates. The heat-treated specimens displayed yield strength of 1038, 904, 690, 500, 333, 264, and 169 MPa, the ultimate tensile strength of 1455, 1055, 807, 621, 424, 329 and 205 MPa, and elongation of 26%, 26%, 17%, 13%, 24%, 19% and 30% at 21, 760, 871, 927, 982, 1038, and 1065 ⁰C, respectively. Stress rupture tests on heat-treated specimens displayed good rupture characteristics, with rupture times of 190, 205, and 42 h at 871, 927, and 981 °C for stresses of 276, 152, and 138 MPa, respectively. A fractographic analysis was conducted on both the tensile and stress-ruptured specimens to comprehend the failure mechanism.

In this research, two Ti-8Al-1V-1Mo compressor blades with approximately 20,000 h of service were repaired using electron beam wire-feed (EB-WF) additive manufacturing (AM) technology. Ti-6Al-4V wire was used as the feedstock to fabricate layer-by-layer a thin-walled structure on the worn blade. The repaired compressor blades were inspected to examine the residual stresses, distortion, microstructure, chemistry, and mechanical properties with special attention given to characterize the interfacial zone between the dissimilar titanium alloys. The distortion after the repair was limited to +/- 0.1 mm on the leading edge just adjacent to the interface, which shows the good potential of the developed method for repair applications. The different microstructural regions, consisting of transient, steady-state, dilution, and heat affected zones, were thoroughly characterized using optical microscopy and correlated with the microhardness. The average Vickers hardness in the blade and repair were measured as 338 +/- 6 Hv and 312 +/- 7 Hv, respectively. To characterize the tensile properties, miniature tensile test specimens were extracted from the repair interface. The yield strength, ultimate tensile strength, and elongation at break of the tested miniature specimens ranged between 869 and 906 MPa, 925-956 MPa, and 10-11 %, respectively. The tensile results were compared to similar literature studies and related standards. All specimens met the minimum requirement of AMS 4911 P wrought specifications, indicating the sound mechanical integrity of the repair. Fractography analysis showed that failure occurred in the repair section, which carries a high potential for maximizing the service life of the blade by allowing periodic repair and overhaul.

This study investigated the application of an in envelope additive/subtractive (LPBF) manufacturing method (Matsuura LUMEX-Avance-25) to fabricate IN718 benchmarking coupons. The coupons were then examined comprehensively for surface finish both with and without high-speed micro-machining. The microstructure of the manufactured IN718 coupons was investigated thoroughly in the as-fabricated condition and following three different standard and one non-standard post-processing heat treatments. As built coupons revealed columnar grain morphology mainly along the <100> direction with a cellular dendritic sub-grain structure and without any strengthening precipitates. Grain size, aspect ratio, and texture were maintained after each of the applied four heat treatments. Only one of the standard heat treatments resulted in the δ phase formation. The other three heat treatments effectively dissolved the Laves phase preventing the δ formation while promoting the formation of γ′/γ″ precipitates. Despite the observed differences in their microstructures, all of the heat treatments resulted in similar yield and ultimate tensile strength values that ranged between 1103–1205 MPa and 1347–1387 MPa, respectively. These values are above the minimum requirements of 1034 MPa and 1241 MPa for the wrought material. The non-standard heat treatment provided the highest elongation of 24.0 ± 0.1% amongst all the heat-treated specimens without a significant loss in strength, while the standard heat treatment for the wrought parts resulted in the lowest elongation of 18.3 ± 0.7% due to the presence of δ phase.

The present work investigates the effectiveness of two heat treatment cycles—solution treatment + aging (STA) and direct aging (DA)—on optimizing the microstructure and enhancing the mechanical properties of 18Ni-300 maraging steel (300 MS) produced by additive–subtractive hybrid manufacturing (ASHM). The STA treatment led to a fully martensitic microstructure with minor remnants of the cellular substructures associated with the solidification conditions in ASHM. DA resulted in some reverted austenite and partial dissolution of the cellular morphologies into shorter fragments. Despite the contrasting microstructures, the tensile strength and the macro- and micro-hardness were comparable between STA and DA conditions. By contrast, the potential for improving the ductility was higher with the DA heat treatment. This is attributed to the higher reverted austenite content in the samples treated by DA, i.e., up to a maximum of 13.4% compared to less than 3.0% in the STA samples. For the DA sample with the highest reverted austenite content of 13.4%, the highest local and global fracture strain values of 30.1 and 5.9 ± 0.6% were measured, while the respective values were 23.4 and 4.4 ± 0.1% for the corresponding STA sample. This work suggests that DA of 300 MS produced by ASHM is sufficient to achieve comparable hardness and tensile strength to STA, whilst maintaining reasonable ductility. Avoiding the solution treatment cycle, with its appreciably higher temperatures, could benefit the dimensional stability and surface quality that are important for ASHM of 300 MS parts.

This study reports the successful crack-free fabrication of the non-weldable high γʹ Ni-superalloy IN 738 by laser powder bed fusion. The as-fabricated texture was composed of columnar grains with preferred orientation along <100> direction. Scanning electron microscopy and X-ray diffraction analysis revealed the presence of M(Ti, Ta, W, Mo, Nb)C and M(Cr, Mo, W)23C6 carbides along the grain boundaries. A heat treatment used for IN 738 castings yielded a bimodal distribution of γʹ precipitates with a primary γʹ volume of 25% and size of ∼226 nm and secondary γʹ with a volume of 43% and a size of ∼88 nm. The carbides observed in the as-built condition were maintained. The heat treatment increased the hardness from 408 HV to 487 HV. The specimen exhibited an excellent room temperature yield strength, ultimate tensile strength, and elongation of 1010 MPa, 1444 MPa, and 13%, respectively. The coupons showed yield strength of 560 MPa and 388 MPa, the ultimate tensile strength of 765 MPa and 538 MPa, and an elongation of 17% and 14% at 850 °C and 927 °C, respectively. Finally, fracture analysis was used to better understand fracture behavior.

Electron beam wire fed (EB-WF) additive manufacturing (AM) can be utilised for cost-effective part repair in the aerospace industry and, especially for titanium alloys, the vacuum processing effectively mitigates high temperature contamination for enabling reliable high-performance. This research advanced EB-WF additive processing and simulation for depositing Ti-6Al-4V thin walls (3 mm in thickness) that emulates repair of damaged fan and compressor blades. The main focus of this research was to understand the effect of the initial substrate microstructure and residual stress profile on the final deposit properties. The results revealed that the EB-WF additive repair process for depositing Ti-6Al-4V yielded minimal distortion (< 350 mu m) and residual stresses (< 150 MPa (0.15 sigma(ys))). The initial residual stress states of the substrate were found to have a negligible effect on the final residual stress profiles, from the stress relaxation effect during EB-WF AM. Although significant variance in the microstructure for each substrate condition was present after deposition, their mechanical properties were similar. Deposited test specimens had tensile yield and ultimate strength values ranging between 800-830 MPa and 860-880 MPa, respectively. The similar mechanical properties of the interface were correlated with the microstructural features such as layer bands and titanium alpha (alpha) colonies.

In the present study, dense and crack-free Hastelloy X (HX) thin cylindrical struts with diameters ranging from 0.25 mm to 2 mm were fabricated through laser powder bed fusion (LPBF). Their crystallographic texture was found to transition from a < 110 > || building direction (BD) fiber texture in the 2 mm strut, to a single crystal in the 0.25 mm strut with < 110 > || BD. The size of the cellular sub-grain microstructure increased as the strut diameter decreased and was linked with an inverse trend in microhardness, which was the highest in the 2 mm strut due to a smaller cell size. Lower hardness measurements in the regions close to the strut edges was also related to larger cell size observed in these regions. From tensile testing, elongation to fracture values were measured in the range of 35-50%. Deformation twinning and lattice rotation after testing were observed. Differences in strength from between different struts of different diameter are also discussed. Finally, fractography analysis further confirmed the ductile fracture behavior. (c) 2022 Elsevier B.V. All rights reserved.

In the present study, solid solution strengthened Ni-based superalloy Hastelloy X (HX) parts were fabricated using the laser powder bed fusion (LPBF) additive manufacturing process with the intent of developing a strong crystallographic texture. Their tensile properties at room temperature were investigated along the < 100 > , < 110 > , and < 111 > crystallographic orientations. Tensile behavior was found to be highly dependent on the crystallographic orientation and present unique combinations of strength and ductility when compared with other LPBF-HX counterparts. EBSD (Electron backscatter diffraction) analysis after fracture revealed deformation twinning in the < 110 > and < 111 > samples, but not in the < 100 > orientation. It was observed that crystallographic orientation had a great impact on the effective stacking fault energies. Critical stress for deformation twinning was also observed to be crystallographic orientation-dependent. It was close, if not below the yield strength (YS) for < 110 > and < 111 >, while it was well above the ultimate tensile strength (UTS) of the < 100 > orientation. The YS value of < 111 > (807 +/- 28 MPa) was higher than that of < 100 > (693 +/- 8 MPa) and < 110 > (648 +/- 13 MPa). The results suggest that deformation twinning can occur in solid solution strengthened Ni-base superalloys at room temperature, and their formation does not mandatorily require the presence of gamma' precipitates or thermally assisted mechanisms. In contrast to the < 100 > and < 111 > orientations, rotation of the lattice after deformation was found for the < 110 > . (C) 2021 Published by Elsevier B.V.

This study reports the crack-free fabrication of a new high gamma prime precipitation hardened Ni-based superalloy by Laser Powder Bed Fusion (LPBF) process. In the as-built condition, scanning electron microscopy and electron backscattered diffraction analysis indicated the presence of elongated columnar grains with an orientation along (1 0 0) direction. Following the sub-solvus heat treatment cycle determined using Thermo-Calc software, crack-free microstructure with columnar grains was observed along (1 0 0) orientation. High-resolution scanning electron microscope analysis revealed a bimodal distribution of gamma prime precipitates and a uniform distribution of carbide precipitates in the heat treated samples. As a result of heat treatment, hardness increased from 365 HV to 508 HV. The heat treated samples also displayed excellent yield strength, ultimate tensile strength, and elongation of 939 MPa, 1368 MPa, and 23% respectively at room temperature. Mechanical testing at elevated temperatures confirmed the yield strength, tensile strength, and elongation reached 80 MPa, 106 MPa, and 34% respectively at 1121 degrees C. The stress rupture test of the heat-treated samples revealed good creep properties with rupture times of 559 h and 75 h, respectively, at 927 degrees C and 982 degrees C, for stresses of 152 MPa and 138 MPa. (c) 2021 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Metastable β-titanium alloys are attractive for their high strength-to-density ratio, good hardenability, excellent fatigue behavior, and corrosion resistance. Among these alloys, β-21S, with a composition of Ti–15Mo–3Nb–3Al–0.2Si (wt%), is known to offer improved elevated temperature strength, creep resistance, thermal stability, and oxidation resistance. In this study, laser powder bed fusion (PBF-LB) of β-21S and the effect of post-PBF-LB heat treatment were investigated to understand the relationship between the microstructure and the mechanical properties. The as-built (AB) alloy is primarily composed of β-phase, with columnar grains oriented along the build direction. The alloy AB presented a microhardness of 278 HV, a yield strength (YS) of 917 MPa, an ultimate tensile strength (UTS) of 946 MPa, and a ductility of 25.3% at room temperature (RT). Such properties are comparable to β-21S in solution treatment (ST) condition. Solution treatment and aging (STA) of the alloy precipitated the α-phase, increasing the microhardness to 380 HV, YS to 1281 MPa and UTS to 1348 MPa, while reducing the ductility to 6.5% at RT. The STA alloy presented a YS of 827 MPa, UTS of 923 MPa, and a ductility of 7.7%, at 450 °C. The thermal treatment applied to PBF-LB β-21S had a similar effect compared to β-21S fabricated by non-AM techniques. The properties obtained demonstrate that β-21S is a potential candidate for AM.

The microstructure and elevated temperature mechanical properties of a precipitation hardenable Nickel based superalloy, Rene 41, fabricated by laser powder bed fusion followed by two different heat treatment regimes were studied. The as-built (AB) microstructure consists of γ columnar grains aligned in <100> direction. No γ′ precipitates were observed in the AB condition. Following to a sub-solvus solutionizing and aging heat treatment, the AB grain morphology was maintained. The development of fine γ′ precipitates within the grains along with the discrete carbide particles on the grain boundaries occurred. Heat treatment above the solvus temperature of γ’ resulted in the formation of a random equiaxed grain morphology. The γ′ and carbide precipitation was also observed in this heat treatment regime, but their distribution and morphology were different. Uniaxial tensile tests were conducted at 760 °C. Average yield strength values for AB, sub-solvus and super-solvus heat treated alloys were 879, 824 and 855 MPa respectively. The three tested conditions showed similar strength values that are comparable with a wrought alloy at the testing temperature. However, the elongation and deformation behaviors were different for each condition. Sub-solvus heat treatment lead to the highest elongation at break with 22% whereas super-solvus heat treatment resulted in the highest work hardening rate during deformation.

In the present study, single-crystalline-like bulk stainless steel (SS316L) specimens with a {110} <001> Goss texture were produced by laser powder bed fusion (LPBF). The tensile behaviours of the LPBF-fabricated SS316L along the <100>, <110> and <111> crystallographic directions were systematically investigated. The samples along the three crystallographic directions enabled a broader strength-ductility paradigm of LPBF-fabricated SS316L and exhibited a superior strength-ductility synergy over their traditionally manufactured counterparts. The tensile responses of the SS316L samples were highly dependent on their crystallographic orientations. The <111> orientated samples exhibited higher yield strength (YS) than those of the <100> and <110> orientated samples, which was mainly attributed to the lower Schmid factors of the <111> grains along their tensile axes (TAs). The dominant deformation mechanisms were found to be dislocation slip and deformation twinning for the <100> and <111> orientated samples, respectively. For <110> orientated samples, significant deformation twinning as well as evident lattice rotation were observed simultaneously. The higher tendency towards deformation twinning of the <110> and <111> orientated samples arose from the larger separations between the partial dislocations in these samples, which reduced the effective stacking fault energies as well as the critical stresses for deformation twinning significantly. Due to the higher propensity towards deformation twinning, the <110> and <111> orientated samples showed better ductility over the <100> orientated samples by facilitating twinning-induced plasticity (TWIP) effect. Furthermore, the lattice rotation of the <110> samples during tension featured a modest TWIP effect which enabled a more prolonged strain hardening rate uphill than that of the <111> samples, resulting in a superior ductility with a total elongation (TE) of ~100 %.

Fabrication of γ′ precipitation strengthened nickel-based superalloys via laser powder bed fusion still remains a challenge. In this study, Rene 77, a high γ′ containing superalloy that is considered as difficult to weld, was processed by laser powder bed fusion. Crack-free parts with high density were fabricated without any compositional modifications or preheating of the built plate. This defect-free structure was maintained upon solutionizing and aging heat treatment. The microstructure of the samples has been characterized in detail following the fabrication and after the heat treatment. Scanning electron microscopy analysis revealed that the as-built microstructure consists of columnar grains mainly aligned in the <100> direction along with extremely fine γ′ precipitates and spherical cell boundary carbides. The grain structure and texture were unaffected by the applied heat treatment due to the pinning effect exerted by the carbide particles. Development of a bimodal γ′ distribution including cuboidal primary and spherical secondary precipitates was observed in the heat-treated sample. Additional carbide formation as a discontinuous grain boundary film was also seen. Tensile deformation behavior for both conditions was also tested at room temperature and 810 °C. Measured strength values for all test conditions were higher compared to a wrought and heat-treated alloy tested at room temperature. The as-built sample showed hardening and loss of ductility during elevated temperature testing due to γ’ precipitation at the test temperature. The microstructure of the heat-treated sample was not altered during testing at 810 °C. However, improved elongation behavior and transition in fracture mode from cleavage to ductile fracture were observed due to microtwin formation at elevated temperatures.

In this study, dense and crack-free specimens were fabricated from the solid-solution strengthened nickel-based superalloy Hastelloy X (HX) through laser powder bed fusion (LPBF) in both horizontal and vertical build orientations. Samples were subjected to one of three conditions: as-built (AB), hot isostatic pressing treatment (HIP), and conventional heat treatment (HT). Tensile testing was carried out for every orientation and condition at room temperature (RT) and an elevated temperature of 750 degrees C (ET). AB samples showed columnar grains and a partial fiber texture aligned with the build direction, which is reduced after HIP and HT, where partial recrystallization occurred. Mo and Cr carbides, as well as Al-O enriched precipitates were found in the RT microstructures, and were seen to increase in size after ET exposure. AB samples displayed high strength (up to 924 +/- 10 MPa ultimate tensile strength), while HIP and HT samples maintained strength values close to a reference HX wrought alloy. Although lower ductility was observed for the AB samples when testing at ET, the elongation was improved after HIP and HT. Strength and elongation comparable to that of the wrought alloy at RT can be obtained by performing just HT, without the need of HIPing.

Ti-6Al-4V and Ti-6Al-7Nb were manufactured with laser powder bed fusion (LPBF). Microstructural comparison study between Ti-6Al-4V and Ti6Al-6Nb was used to understand processability similarities between two different titanium alloys. Quantitative similarities between two alloys revealed that Ti-6Al-4V processing parameters can be used for optimization of Ti-6Al-7Nb. The microstructure, processing, properties relationship and the influence of heat treatments were investigated for Ti-6Al-7Nb. The as-built microstructure was composed of a columnar prior beta grains with fine acicular alpha' martensite resulting in a yield strength of 1082 MPa and an ultimate tensile strength of 1160 MPa with an elongation of 9.7%. Solutionizing at 1055 degrees C and aging at 540 degrees C completely transformed the columnar structure of the prior beta grains to equiaxed via phase transformation and grain growth, solutionized the alpha' martensite into beta and then created a fine lamellar alpha + beta structure with air cooling. The resultant microstructure had reduced strength and hardness but increased ductility. The reduction in yield (871 MPa) and ultimate tensile (940 MPa) strength would be positive to minimize stress shielding of orthopedic implants. The improved elongation of 11.5% meets the requirements for biomedical applications which stipulates an elongation of at least 10% according to the ISO 5832-3 Standard.