Effects of ultrasonic peening treatment in three directions on grain refinement and anisotropy of cold metal transfer additive manufactured Ti-6Al-4V thin wall structure

In the cold metal transfer additive manufacturing process of Ti-6Al-4V thin wall structure, ultrasonic peening treatment (UPT) in three directions is proposed to refine the large columnar prior-β grains and secondary α grains, and to improve anisotropy in tensile properties. The experimental results showed that UPT in three directions applied to each weld right after arc extinguishing has a minor influence on the surface appearance, which shows no apparent plastic deformation, but has a great improvement in grain refinement. The changes in microstructure and dislocations of thin wall structure treated by UPT in three directions were observed. By comparing with those without UPT, the main causes for refinement of columnar prior-β and secondary α grains was explored, namely mechanical effects of ultrasonic at the temperature range of α’ dissolution temperature Tdiss – liquidus temperature Tl. Specimens with UPT have better properties, higher loads with the same indentation displacement in nano-indentation tests, an increase in ultimate tensile strength and a reduction in anisotropic percentage in tensile tests.