Effects of Atmosphere Oxygen Content on Microstructure and Mechanical Properties of TC4 Titanium Alloy Prepared by Laser Directed Energy Deposition

TC4 titanium alloy specimens were prepared by laser directed energy deposition in the inert atmosphere with different oxygen contents. The effects of oxygen content in the argon shielding gas on microstructure, fracture surface and mechanical properties of TC4 titanium alloy by laser directed energy deposition have been investigated by optical microstructure (OM), scanning electron microscopy (SEM), transmission electron microscope (TEM), microhardness testing and tensile testing. The results show that the oxidation degree of the samples surface increase, and the surface color of TC4 titanium alloy samples gradually change from silver white to yellow, blue and dark gray with the increasing the oxygen content of the atmosphere. XRD analysis shows that the types of oxides on the sample surface increase, and the thickness of the oxide layers also increase gradually. The microstructure of the specimens gradually changes from basket structure to acicular martensite alpha' structure, the laths are coarsened and the aspect ratios are reduced. The hardness and tensile strength at room temperature of TC4 titanium alloy samples increase gradually with the increase in oxygen content in the forming atmosphere. Due to the influence of metal lattice distortion and cooling rate, the tensile strength of the deposition samples increases from 920 MPa to 982 MPa and the elongation decreases from 12.4% to 10.9%.