Improvement of microstructure and mechanical properties of TC4 titanium alloy GTAW based wire arc additive manufacturing by using interpass milling

Abstract

TC4 titanium alloy wire arc additive manufacturing (WAAM) is easy to produce coarse columnar β grain structure which affects mechanical properties. So in this paper, an innovative process of interpass milling assisted TC4 titanium alloy gas tungsten arc welding (GTAW) WAAM is proposed. The controlling interpass milling depth is used to improve the microstructure and mechanical properties of TC4 titanium alloy parts during the GTAW-WAAM. The results showed that the coarse columnar β grains and fine α phase were observed in the non-interpass milling part. When the interpass milling assistance GTAW-WAAM was employed, the coarse columnar β grains transformed into short columnar and equiaxed grains. With the increased of interpass milling depth, the average size of α phase was gradually refined to 13.5 μm below. The highest average microhardness in the top, middle and bottom areas obtained by using the interpass milling depth of 300 μm were 369 HV0.2, 371 HV0.2 and 371 HV0.2 respectively, which was increased by 32 HV0.2, 28 HV0.2 and 26 HV0.2 compared with non-interpass milling respectively. The horizontal and vertical ultimate tensile strength first increases and then decreases with the increase of interpass milling depth, reaching its maximum at a milling depth of 300 μm. The highest tensile strength of the horizontal and vertical directions obtained by using the interpass milling depth of 300 μm were 1022 MPa and 1042 MPa. The level of the ultimate tensile strength is related to the combined proportion of total average β grain size and total average < 15 μm α phase size, and the proportion of both is balanced and maximized to obtain the maximum ultimate tensile strength. The interpass milling depth of 300 μm satisfies both conditions, so its maximum horizontal and vertical ultimate tensile strength is achieved. Fracture type of tensile specimen was ductile fracture due to observing a lot of dimples on the fracture morphology. The above conclusions show that interpass milling is an aided process to effectively improve the microstructure and mechanical properties of TC4 titanium alloy GTAW wire arc additive manufacturing thin-walled parts, which has high research significance and practical value.