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Phase transition, microstructural evolution and mechanical properties of Ti–6Al–4V and Ti–6.5Al–3.5Mo–1.5Zr–0.3Si joints brazed with Ti–Zr–Ni–Cu filler metal

Yang Zhenwen, Chen Yuhan, Niu Shiyu, Wang Ying, Han Yajing, Cai Xiaoqiang, Wang Dongpo

Archives of Civil and Mechanical Engineering

2020

Vol. 20, no. 3

353--367

Ti–6Al–4V (TC4) and Ti–6.5Al–3.5Mo–1.5Zr–0.3Si (TC11) joints were achieved via Ti–37.5Zr–15Ni–10Cu (wt%) filler metal when the brazing temperature was in the range from 950 °C (below β-transus) to 1040 °C (above β-transus) for 10–60 min. The role of brazing parameters in the microstructure evolution as well as mechanical properties of both base alloys and brazed joints was studied. The research analysis suggested that the typical interfacial microstructure was divided into five characteristic zones including reaction phases of α-Ti, β-Ti and (Ti, Zr)2CuNi. With the holding time prolonged, the sectionalized structure transformed into lamellar Widmanstätten structure in the brazing seam at the temperature of 950 °C, and the optimized shear strength reached 616 MPa at 950 °C for 60 min. Nevertheless, when increasing the brazing temperature to 1000 °C, the joint shear strength tended to be stable as the holding time exceeded 20 min due to the elemental homogenization, and the shear strength reached 627 MPa for the holding time of 20 min. Tensile test results showed that the mechanical properties of both TC4 and TC11 alloys were dramatically degraded at the heat treatment temperature of 1000 °C owning to the drastic grain coarsening and phase transition. Additionally, the plastic strain of TC4/TC11 joint brazed at 1000 °C for 20 min was 1.66%, while that of joint brazed at 950 °C for 60 min reached 2.01%. The variation in mechanical properties of base alloys as well as brazed joints under different thermal conditions revealed that the optimized temperature for brazing of titanium alloys was lower than β-transus with a long time.