On the microstructure and texture evolution in 17-4 PH stainless steel during laser powder bed fusion: Towards textural design

M. S. Moyle, N. Haghdadi, X.Z. Liao, S.P. Ringer, S. Primig
Journal of Materials Science & Technology 117 (2022) 183-195. (OPEN ACCESS)

17-4 PH stainless steel is favoured in industries where simultaneous high strength and corrosion resistance are required. Its good weldability explains why this steel is now also chosen for production of complex parts by laser powder bed fusion (LPBF). The microstructural evolution of this alloy during LPBF is of great intrigue, with different published works reporting somewhat ambiguous results on austenitic, martensitic, or ferritic matrix microstructures in the as-printed conditions. Thus, further work evaluating the phase composition and distribution of 17-4 PH samples in this state is required to inform future microstructural design in this alloy through LPBF.

The textures observed within 17-4 PH after LPBF are more widely applicable, with many other cubic alloy systems showing similar textures after similar processing. However, the mechanisms by which these textures form warrant additional research and discussion. A more complete understanding of these mechanisms can likewise be used to inform microstructural engineering decisions. Having a precise control over texture could lead to the LPBF production of texture sensitive property gradients through specialised AM designs.

The current study investigates the effects of laser power and scanning pattern in LPBF on the microstructure and texture of as-printed 17-4 PH builds. We show that the as printed 17-4 PH microstructures investigated are largely δ-ferritic with a small volume fraction of austenite, which is retained more strongly around melt pool boundaries. We also show that the total volume fraction of austenite can be increased by reducing the time between adjacent passes of the laser during fabrication. Increasing laser power has the effect of increasing grain size, epitaxial grain growth, and the overall intensity of texture within the builds. We observe and propose a mechanism for the formation of “mosaic”-like grain structures, which have previously been observed in literature but not explained.