Strain hardening in twinning-induced plasticity stainless steel produced by laser powder bed fusion

Abstract

A modelling approach is presented to identify the deformation mechanisms of 316L stainless steel produced by laser powder bed fusion (LPBF). The approach incorporates the evolution of dislocations, forming a forest, and of twins, which develop a back-stress. The overall plasticity behaviour is described in terms of dislocation multiplication and annihilation progress with strain. The modelling is matched up with detailed electron microscopy observations; the combination of both demonstrates the deformation behaviour of LPBF builds is intrinsically different to that of wrought alloys. LPBFed samples undergo three stages of deformation, with the first developing twins, which formation quickly saturates; the second sees a dramatic increase in dislocation forest hardening, combined with dislocation recovery; and the third undergoes dynamic recrystallisation taking place around heavily twinned sections. Opposite to wrought alloys, LPBFed specimens decrease their density of statistically stored dislocations throughout deformation, and it is shown that this behaviour is replicated by other LPBFed metals, including high-entropy alloys. The intrinsic behavioural differences in LPBF plasticity is thought to be due to the presence of a residual stress; this promotes dislocation recovery from the onset of deformation.