Microstructures, properties, and applications of laser sintered 17‐4PH stainless steel

Abstract

The effects of hot isostatic pressing (HIP) on the densification, mechanical properties, and microstructures of laser‐powder bed fusion (L‐PBF) 17‐4 PH stainless steel parts were studied using gas‐ and water‐atomized powders. The % theoretical density, ultimate tensile strength, yield strength, elongation, and hardness of as‐printed and HIP‐ed L‐PBF parts were sensitive to energy density and starting powder shape, size, and type. At low‐energy densities of 64 and 80 J/mm3, densification was significant for water‐atomized L‐PBF parts when subjected to HIP treatment and density increased from 90% to 97%. For all the energy densities, the gas‐atomized L‐PBF parts after the HIP treatment showed significantly higher tensile strength, yield strength, and hardness when compared to water‐atomized L‐PBF parts properties. At low‐energy densities of 64 and 80 J/mm3, long columnar grains in the as‐printed L‐PBF parts did not change significantly after the HIP treatment whereas the columnar grains present in as‐printed gas‐atomized L‐PBF parts completely disappeared when subjected to HIP treatment. However, at high‐energy densities of 84 and 104 J/mm3, the columnar grains in as‐printed L‐PBF gas‐ and water‐atomized L‐PBF parts were changed to equiaxed grains and showed a higher level of homogenization when subjected to HIP treatment. This variation in grains and grain size had significantly affected the yield strength and elongation of HIP‐treated gas‐ and water‐atomized L‐PBF parts.

Material-process-property relationships of 17-4 stainless steel fabricated by laser-powder bed fusion followed by hot isostatic pressing.

“…A similar increase in density due to HIP treatment was observed in previous studies for L-PBF parts fabricated from other compositions [15]- [24]. Further, the densities of gas-(D50=13 µm) and wateratomized (D50=43 µm) L-PBF parts obtained from the current study is higher than densities of L-PBF parts fabricated from the same powders when they were post-processed using a different HIP cycle [69]. The HIP cycle used in the current study had an additional step in the cycle (Figure 3.2), at a solutionizing temperature of 1050 0 C and 70 MPa pressure for 20 minutes which could have accelerated the densification of the L-PBF parts [69].…”

“…The fractional increase in ultimate tensile strength due to HIP treatment is significant for as-printed wateratomized L-PBF parts when compared to gas-atomized L-PBF parts (Figure 3.3). Further, the ultimate tensile strength of gas (D50=13µm) and water-atomized (D50=43µm) L-PBF parts after HIP treatment used in the current study is higher than the tensile strength of the L-PBF parts obtained after the HIP cycle used in our previous study [69].…”

“…In this regard, the present study was performed to understand the effect of the HIP on the densification, mechanical properties and microstructures of 17-4 PH stainless steel gas and water-atomized as-printed L-PBF parts when they are fabricated at various energy densities. The current study builds from our initial work [69] on the effect of HIP on gas and water-atomized L-PBF parts.…”

“…stainless steel is used in many applications that take advantage of its high mechanical properties in combination with its excellent corrosion resistance [26], [84], [90], [101], [104], [107]. Several research studies have been carried in our group as well as literature to fabricate defect-free L-PBF parts from 17-4 PH stainless steel and enhance the resulting mechanical properties using the post-processing operations such as hot isostatic pressing (HIP) and heat-treatment [2], [3], [6], [8], [15], [32], [50], [51], [56]- [69].…”

“…The enhanced densification, reduction in porosity, increased martensite phase content, as well as changes in grain size and shape resulted in an increase in mechanical properties for 17-4 PH stainless steel L-PBF parts when they were HIP treated at various conditions [69], [108]. These results are consistent with previous powder metallurgy studies on stainless steel, magnesium, aluminum alloys on the effects of post-processing on microstructural features such as phase variation and grain refinement [16], [18]- [20], [24], [109]- [111].…”