Solidification Behavior of Additively Manufactured 17-4 Martensitic Precipitation-Hardenable Stainless Steels

The extreme solidification conditions found in the additive manufacturing (AM) of metallic alloys leads to significant compositional heterogeneity, non-equilibrium phase formation, and disparate microstructure that produce materials with properties nothing like those found in their wrought counterparts. Nowhere is this more apparent than in steels, specifically martensitic precipitation-hardenable (PH) stainless steels, where small variations in the solidification conditions can lead to changes in primary solidification phase (austenite or δ-ferrite), a transition from columnar to equiaxed grain morphology, and wildly different solid-state transformations caused by the cyclic heating/cooling cycles of the AM process. This work aims to understand the mechanisms of the solidification behavior of laser-powder-bed-fusion-produced (L-PBF) 17-4 PH stainless steels through the optimization of chemistry of powder feedstock and manipulation of processing parameters. Changes in gas atomization (N₂ versus Argon-atomization) and subsequent changes in chemistry are found to have profound effects on microstructure and mechanical properties. Variation in chemistry caused by gas atomization allows for the identification of some critical N₂ content that either stabilizes or prevents the presence of retained austenite in the as-built microstructure. The role of laser power and scan speed on solidification behavior is explored to gain greater control of the as-built microstructure. This understanding and control will produce either more efficient post-build thermal processing or improved mechanical performance and reliability of L-PBF-produced 17-4. The realization of L-PBF 17-4 PH stainless steel to produce materials alike to their wrought counterparts marks significant understanding in solidification behavior and allows for further implementation of additive manufacturing in the aerospace, petroleum, and chemical industries.