Experimental and computational analysis of site-specific formation of phases in laser powder bed fusion 17–4 precipitate hardened stainless steel

Abstract

Additive manufacturing allows for the production of intricate geometries with reduced material waste. However, due to the complex thermal gyrations that are linked to the processing parameters and geometry, inconsistencies in microstructures and mechanical properties occur from build to build. These inconsistencies are exemplified in multi-phase alloys such as 17–4 precipitation hardened (PH) stainless steel where the formation of ferrite, austenite, and martensite is sensitive to the powder composition and thermal conditions. The work presented here investigates a concentric scan strategy’s impact on thermal conditions within laser powder bed fusion (PBFLB/M) processed 17–4 PH stainless steel, and the resulting phase formation and morphologies that occur. Through the combined use of computational materials science and experimental characterization, complex phase transformation routes were identified as being dependent on the site-specific thermal gyrations within the builds. Within the outer regions of the builds, a primarily δ-ferrite solidification microstructure was identified with two notable morphologies of austenite detected, allotriomorphic and Widmanstatten ¨ austenite. Within the center of the sample, however, a primarily austenite microstructure that nucleated from the prior δ-ferrite was identified. Small fractions of α-ferrite and/or martensite were potentially identified within the austenite grains. These results are rationalized using computational thermodynamics and kinetics in conjunction with proposals for pathways towards site-specific control of phase formation and morphology in PBF-LB/M stainless steels.