Effect of Multiple Thermal Cycles on Microstructure and Mechanical Properties of Cu Modified Ti64 Thin Wall Fabricated by Wire-Arc Directed Energy Deposition

Abstract

This study investigated the effect of thermal cycles on Cu-modified Ti64 thin-walled components deposited using the wire-arc directed energy deposition (wire-arc DED) process. For the samples before and after experiencing thermal cycles, it was found that both microstructures consisted of prior β, grain boundary α (GB α), and basketweave structures containing α+β lamellae. Thermal cycles realized the refinement of α laths, the coarsening of prior β grains and β laths, while the size and morphology of continuously distributed GB α remained unchanged. The residual β content was increased after thermal cycles. Compared with the heat-treated sample with nanoscale Ti₂Cu formed, short residence time in high temperature caused by the rapid cooling rate of thermal cycles restricted Ti₂Cu formation. No formation of brittle Ti₂Cu means that only grain refinement strengthening and solid-solution strengthening matter. The yield strength increased from 809.9 to 910.85 MPa (12.46% increase). Among them, the main contribution from solid solution strengthening (~ 51 MPa) was due to the elemental redistribution effect between α and β phases caused by thermal cycles through quantitative analysis. The ultimate tensile strength increased from 918.5 to 974.22 MPa (6.1% increase), while fracture elongation increased from 6.78 to 10.66% (57.23% increase). Grain refinement of α laths, the promoted α′ martensite decomposition, decreased aspect ratio, decreased Schmid factor, and local misorientation change of α laths are the main factors in improved ductility. Additionally, although the fracture modes of the samples in the top and middle regions are both brittle–ductile mixed fracture mode, the thermal cycles still contributed to an improvement in tensile ductility.