Grain boundary engineering during the laser powder bed fusion of TiC/316L stainless steel composites: New mechanism for forming TiC-induced special grain boundaries

In this work, we demonstrate a novel machining solution for controlling microstructures with special grain boundaries and nanotwins in composites during laser powder bed fusion (LPBF) that is based on a composite design of TiC/316L stainless steel (TiC/316LSS). Moreover, a microscopic mechanism is proposed. Gradient element segregation toward the amorphous TiC interface induces a low stacking fault energy (SFE), driven by the residual stress generated during LPBF thermal cycling, and 9R formed at the TiC interface with a low SFE and transformed to nanotwins. The migration of the incoherent twin boundary regenerates a high proportion of special grain boundaries. Based on the mechanistic analysis, the evolution of the microstructure, including TiC nanoparticle agglomeration, grain refinement and special grain boundaries in the microstructure, is well explained by optimizing the laser power, scanning speed and hatch spacing. TiC/316LSS composite with improved characteristics are observed, including a local residual stress decrease and grain refinement, which are notably superior to those of conventional thermomechanically treated 316LSS. The composite possesses a high proportion of special grain boundaries and nanotwins that maintain wall strengthening via an ultrafine dislocation cell structure produced during LPBF. Microstructural control is realized in additive manufacturing (AM) by predictably adjusting the process parameters. Thus, additive manufacturing grain boundary engineering (AM-GBE) shows great potential due to growth twinning and special grain boundary regeneration by optimizing the LPBF parameters and segregation-induced design.