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This study aimed to provide a comprehensive understanding of how aging treatments (namely, HT1 and HT2) affect the microstructure, cracking behavior, and crystallographic texture of IN939 fabricated by powder bed fusion-laser beam (PBF-LB) method. Although both aged samples demonstrated similar grain structure and recrystallization behavior according to the electron backscatter diffraction (EBSD) analysis, as well as the precipitation of bimodal γ′ phase and MC- and M23C6-type carbides, notable differences were observed in the size and morphology, particularly the γ′ phase. The HT1 sample displayed coarsened primary γ′ phase, with sizes reaching up to 2 μm and exhibiting varied morphologies, including irregular and cuboidal shapes. Additionally, this treatment led to the formation of some γ′-γ eutectic regions and plate-like η phase, along with the decomposition of MC-type carbides into M23C6-type carbides. In contrast, the HT2 sample displayed uniformly distributed spherical primary γ′ phase with sizes ranging from 70 to 120 nm, accompanied by very fine secondary γ′ phase. Furthermore, it was found that changes in both aged sample microstructures could result in the formation of strain-age cracks due to the γ′ phase formation and liquation cracks due to the partial remelting of lower melting point phases. The findings also revealed that with the application of aging treatments, the hardness of the as-fabricated sample (339.8 ± 3.4 HV) increased to 440.2 ± 5.6 HV and 508.1 ± 4.8 HV for the heat treatment of HT1 and HT2, respectively.

This study provides a comprehensive investigation into the effects of different scanning strategies on the material properties of IN939 fabricated using the PBF-LB process. The scanning strategies examined included alternating bi-directional scanning with rotation angles of 0°, 45°, 67°, and 90° between adjacent layers (named as shown), as well as alternating chessboard scanning with rotation angles of 67° and 90° (named as Q67° and Q90°). The results revealed that the 45° and 67° samples had the highest relative density, while the 0° and Q67° samples showed the highest average porosity. Moreover, various types of cracks, including solidification, solid-state, and oxide-induced cracks, were observed. Among the bi-directional scan samples, the 0° sample displayed the most extensive cracking and the highest σmax residual stress values in both XZ and XY planes. Conversely, the 45° and 67° samples exhibited fewer cracks. Notably, the lowest σmax residual stress in the XZ planes among the bi-directional scan samples was observed in the 67° sample. Additionally, microstructural analyses indicated differences in grain size and morphology, among the samples. Texture analysis indicated that the 0° and 90° samples exhibited strong cube textures, whereas the texture intensity weakened for the 45° and 67° samples. Moreover, the alternating chessboard scanning strategy led to rougher surfaces (higher Sa and Sz values) compared to the alternating bi-directional scanning strategy, regardless of the rotation angles. Furthermore, the microhardness values among the samples showed minimal variance, ranging between 321 ± 14 HV and 356± 7 HV.

The effect of various solution heat treatment temperatures (i.e., 1120, 1160, 1200 and 1240 °C) on the microstructure, grain morphology and crystallographic texture of IN939 fabricated by powder bed fusion-laser beam (PBF-LB) was investigated. Microstructural analyses showed that the high-temperature gradient and rapid solidification of the PBF-LB processing caused different resulting microstructures compared to conventionally produced counterparts. The melt pool morphologies and laser scanning paths were examined in the as-fabricated samples in the XZ- and XY-planes, respectively. After the application of solution heat treatment at 1120 °C, the as-fabricated PBF-LB initial microstructure was still apparent. For solution heat treatments of 1200 °C and above, the melt pool and scanning path morphologies disappeared and converted into a mixture of columnar grains in the XZ-plane and equiaxed grains in the XY-plane. On the other hand, large equiaxed grains were observed when the samples were solutionized at 1240 °C. Additionally, γ′ phase precipitated within the matrix after all solution heat treatment conditions, which led to increase in the microhardness values. According to electron backscatter diffraction (EBSD) analyses, both as-fabricated and solution heat-treated samples had intense texture with {001} plane normal parallel to the building direction. The first recrystallized grains began to appear when the samples were subjected to the solution heat treatment at 1160 °C and the fraction of the recrystallized grains increased with increasing temperature, as supported by kernel average misorientation (KAM) and grain spread orientation (GOS) analyses.

This study is focused on a comprehensive characterization of virgin and spatter IN939 powders and the effects of a certain amount of spatter powder on the part quality of IN939 fabricated by the L-PBF process. A brown tint coloration formed Al2O3 oxide, pores, a 124.4% increase in the average particle size, a 10.2% decrease in the powder circularity, and a 7.5% decrease in the powder aspect ratio were observed in the spatter powder. Additionally, higher average grain size and lower nanohardness were obtained for the spatter powder. In order to understand the effect of a certain amount of spatter powder on the part quality, 10 wt% spatter powder was mixed with the virgin powder. This addition was found to decrease the flowability of the powder. Moreover, this addition decreased relative density by around 0.3% and increased surface roughness by around 80.8% in the fabricated samples (termed as V and SV). On the other hand, there was no considerable microstructural, texture, microhardness, and nanohardness difference between V and SV samples, although the spatter powder addition caused a 30.2% increase in the average grain size of SV. The overall texture for both V and SV samples exhibit (001)//BD.

The recrystallization and grain growth behaviour of IN718 alloy additively manufactured by laser powder bed fusion (L-PBF) is presented herein. The effects of three different temperatures (1050, 1150 and 1250 °C) and holding times (15, 45 and 90 min) were investigated. The texture evolution of the samples was recorded via electron backscatter diffraction (EBSD). The as-built sample is composed of bowl-shaped melt pools, a chessboard-like grain pattern and has a cube texture {100}. Recrystallized grains were observed in the samples treated at 1150 °C for 15 min, as well as the samples treated for longer periods and at higher temperatures. Recrystallization was observed to start from high dislocation density regions, including the overlapping melt pools and the borders of the chessboard-like pattern. The initial cube texture transforms into a first-generation cube-twin texture {122} via a twinning-assisted recrystallization mechanism. Then, those recrystallization nuclei sweep through the high defect density matrix; during which almost no new twins are formed. The samples treated at 1250 °C are almost completely recrystallized, which forms a weaker cube texture and a stronger P-orientation {011}. However, the growth of recrystallized grains is very limited due to the presence of non-coherent precipitates.

Achieving good surface profile and low levels of porosity are prime challenges in the Laser-Powder Bed Fusion (L-PBF) additive manufacturing technique. In order to optimise these properties, post-processing is often required. However, the compression of powder spread on the build plate and re-melting of each build layer during the L-PBF process could address these challenges. In this study, the effect of different powder compression ratios and laser re-melting regimes on the density, microstructure morphology, surface profile and mechanical properties of L-PBF produced parts were investigated. Two different metal printers with same laser processing parameters were used to fabricate 10 × 10 × 10 mm3 stainless steel 316L samples. To examine the impact of compression ratio and layer re-melting, one set of samples was prepared with three different compression levels for each layer and a second set of samples either a single or double set of laser passes for each layer. The Volumetric Energy Density (VED) range examines was from 26.7 J/mm3 to 80 J/mm3. Density, hardness, elastic modulus, microstructure and surface profiles of the printed samples were characterised. A 3% increment in density and a 50% reduction in the surface roughness were achieved using a laser double pass over each layer. The results demonstrate, by applying different powder compression ratios onto the powder bed and by re-melting each layer, that the density, surface roughness and the elastic modulus of the produced samples can be improved.