Twin-related Grain boundary engineering (GBE) is accomplished by applying specific thermo-mechanical treatments, mainly on low-stacking fault energy face centered cubic (FCC) materials, e.g., copper, nickel alloys and stainless steels. The aim is to create a high content of coincidence site lattice (CSL) boundaries, so as to improve the resistance against different types of materials degradation, including corrosion, cracking and embrittlement. The conventional GBE approach typically begins with the introduction of a small amount of deformation (typically ~5% strain), followed by annealing to generate CSL boundaries. Subsequently, these CSL boundaries break up the random boundary network, forming twin related domains (TRD). However, since most of the deformation is mechanically induced, conventional GBE method is not readily applied to bulk, three-dimensional parts or components with intricate geometries.

 

Additive manufacturing (AM), e.g., laser powder bed fusion (LPBF) and laser direct energy deposition (DED), offers pathways for manufacturing bulk, near-net-shape parts with intricate geometries. However, high density of twin boundaries in the as-solidified microstructures of AM-produced 316L stainless steel is generally very limited. Current efforts to achieve GBE in AM 316L stainless steel (SS) mainly focus on inducing recrystallization in the post-AM condition, similar to the conventional GBE processing methods. Only a few studies have successfully introduced moderate number of twins into the as-solidified microstructures in situ during AM.

 

This review will investigate the mechanisms of twin formation during rapid solidification in AM, including particle-induced twins, icosahedral short-range order (ISRO)-induced twins, and massive phase transformation-induced twins. Additionally, this review will cover twin evolution during post-heat treatment, focusing on twin interactions and the formation of twin-related domains.