Analysis of formation and growth of the σ phase in additively manufactured functionally graded materials

This study focuses on the formation of the σ phase in three functionally graded material (FGM) systems made by additive manufacturing (AM): stainless steel 420 (SS420) to V to Ti–6Al–4V, Ti–6Al–4V to V to stainless steel 304 L (SS304L), and SS420 to V. Directly joining Ti–6Al–4V and stainless steel may result in the formation of brittle Fe–Ti intermetallics. This study investigates the potential use of V as an intermediate element between terminal alloys of Ti–6Al–4V and stainless steel. Experimental analysis of the elemental and phase composition revealed that different amounts of σ phase were present in the three FGM systems at locations with similar elemental compositions. Computational studies were performed to simulate the thermal history, phase transformation kinetics, and σ phase growth within these FGMs. The computations suggested that, at the conditions studied, the σ phase should nucleate faster and grow to a larger volume fraction in the SS420–V alloy than the SS304L–V alloy, contrasting with experimental observations. Instead, experimental analysis confirmed that the disparate growth of σ phase in the FGMs was due differences in cracking during fabrication, resulting in different amounts of time spent at the elevated temperatures conducive to σ phase growth in each of the samples.