Comparative study on the microstructure and mechanical properties of the laser welded joints of additive manufactured and forged TC11 titanium alloy

Additive manufacturing of large-size components for aerospace applications is always limited by processing efficiency and the maximum machining dimension of the equipment. Therefore, it is expected that a hybrid manufacturing technology combining additive manufacturing and welding will tackle this problem. In this paper, laser welding tests of additive and forged TC11 titanium alloy were carried out. The weldability of additive TC11 titanium alloy (TC11-AM), of "TC11-AM/forged TC11 (TC11-R)" joint and of TC11-R/TC11-R joint were studied.The microstructure of welded joints, hardness and tensile tests were conducted, and the fracture morphologies of tensile specimens were observed. The results show that no obvious porosity was found in the weld zone of TC11-R/TC11-R joint, TC11-AM/TC11-R joint, or TC11-AM/TC11-AM joint. Due to the large temperature gradient and fast cooling rate of the molten pool, the microstructure of the weld was coarse columnar grain with α' martensite phase inside. The tensile strengths of the three joints mentioned above were about 1575, 1687 MPa and 1593 MPa, respectively. The cross sectional microhardness of TC11-R/TC11-R and TC11-AM/TC11-AM joints shows Gaussian distribution, and the variation range ΔHV of hardness values was about 445 HV ± 31 HV and 424 HV ± 6 HV, respectively. Due to the notable difference in the microstructures at different zones of the joint, the microhardness of TC11-AM/TC11-R joint showed a ladder-shaped distribution with higher hardness in TC11-AM side and lower hardness in TC11-R side, and the range of hardness was about 432 HV ± 21 HV. By comparing the tensile strength of the joints, it is found that TC11-AM/TC11-R joint and TC11-AM/TC11-AM joint are better in terms of weldability.