Variability in mechanical properties of additively manufactured 17-4 PH stainless steel produced by multiple vendors: insights for qualification

In applications where a combination of good strength and corrosion resistance is required, 17-4 precipitation hardenable (PH) stainless steel is a common material choice. This alloy is traditionally processed through a combination of casting, rolling, and machining. A variety of heat treatments are used to anneal and harden the material via precipitation strengthening. While additive manufacturing (AM) removes many geometric design constraints from these traditional forming processes, until recently, structures fabricated via laser powder bed fusion (L-PBF) were porous and contained undesirable columnar grain structures that contributed to unpredictable and anisotropic mechanical properties. Recent advances in L-PBF processing technology including improved gas flow, powder atomization, and print parameter optimization enable printing of high-quality 17-4 PH with properties that are comparable to traditionally processed material. However, it is yet to be determined whether these properties can be reliably reproduced across various machines and if machine-agnostic material property baselines can be established based on data available thus far. If baselines can be established, their implementation should allow for identification of machines that generate non-conforming material. In this work, we evaluate the consistency of mechanical properties in L-PBF 17-4 PH produced by six vendors with the ultimate goal of establishing mechanical property baselines, which is fundamental to modern qualification paradigms. We find non-conforming data from two vendors by examining anomalies in mechanical response (e.g., transformation induced plasticity effects) and demonstrate that typical sources of variation can be detected using qualification testing protocols. Ultimately, after standard solution annealing and heat treating, the microstructure and mechanical properties across vendors converged with very few, easily explainable exceptions. In particular, powder atomized in nitrogen promoted formation of retained austenite that lead to a yield point phenomenon in as-built conditions, and high surface roughness from as-built surfaces reduced the fatigue strength. However, with conventional post-processing heat treatments and surface polishing, AM 17-4 PH behaved comparably and consistently to conventionally processed material.