Effects of high deposition rate during cold metal transfer additive manufacturing on microstructure and properties of Ti-6Al-4V

Abstract

Cold metal transfer (CMT)-based wire-arc directed energy deposition (waDED) is a high-speed deposition process capable of manufacturing Ti-alloys with high production rates and lower costs. However, CMT-processing of Ti-alloys with tailored properties for aerospace applications demands a fundamental understanding of the microstructural evolution. The complex thermal cycles during CMT-based waDED of Ti-6Al-4V affect the phase transformation pathways, leading to heterogeneous microstructures and property variations within builds. The current study aims to investigate the effect of a new CMT high deposition rate strategy (4.3 kg/h) on those heterogeneities using multiscale characterisation techniques. The process characteristics and thermal profile of the CMT process decreased the size and volume fraction of the α-phase along with the size and aspect ratio of the parent β grains with increasing build height. The CMT process is effective in stirring the melt pool and limiting epitaxial growth of the β-phase. Further, the imposed thermal cycles induce nucleation and refinement of β parent grains. Chemical solute partitioning and variations in the α-lath thickness in the characteristic colony and basketweave structures are observed. The developed microstructures are associated to local three-variant clusters and grain boundary α-variant selection mechanisms, promoted by the diffusional nature of Ti-6Al-4V CMT. The capability to remove epitaxially grown β grains and promote three-variant clusters over α-colonies improves mechanical strength and reduces anisotropy. Our work provides advanced understanding of the microstructure evolution and its implications during CMT compared to other AM processes. It showcases the potential of CMT to become a viable processing route for manufacturing larger engineering parts for future aerospace applications and beyond.