Heat treatment response and characterization of Ti6Al4V + xMo produced by laser metal deposition

Abstract

During the laser metal deposition additive manufacturing processing of Ti6Al4V ELI alloy, the parts produced were exposed to high levels of thermal gradients, which resulted from rapid heating and cooling rates in the material. This had an adverse effect on the material properties, as tensile residual stresses were created in the parts and increased the strength while significantly reducing ductility. Additionally, the presence of columnar grains compromised the material properties because it resulted in inhomogeneous microstructures that exhibit anisotropy in parts. This study investigated the influence of β annealing temperatures on the microstructure of Ti6Al4V ELI alloy produced during laser metal deposition, and the Ti6Al4V ELI in-situ alloyed with varying molybdenum content. The observations made included a temperature driven phase transformation, which resulted in a change from columnar to equiaxed grains due to heat treatment of the Ti6Al4V ELI alloy, while the solidification structure of the alloy changed from planar to cellular due to the addition of Mo. The Ti6Al4V ELI alloy heat treated at 1000 °C reported a hardness profile of 204 ± 5 HV0.3, which was comparable to the reported hardness (206 ± 34 HV0.3) of the Ti6Al4V ELI in-situ alloyed with 10 mass percent Mo (10% Mo). This implies that the effects of the in-situ alloying of Ti6Al4V ELI with 10% Mo are comparable to the heat treatment of Ti6Al4V ELI alloy at a β annealing temperature of 1000 °C, in terms of stabilization of the β-phase.