Effect Of Ultrasonic Impact On The Properties Of Arc Additive Manufacturing Of Titanium Alloy

Wire Arc Additive Manufacture(WAAM) is one of excellent techniques to manufacture large scale structures of titanium alloy. However, coarse columnar grains easily form during WAAM manufacturing, which leads to poor mechanical properties. In order to solve this problem,Ultrasonic Impact Treatment(UIT) were introduced in WAAM in this study. The effects of UIT parameters on the microstructure and mechanical properties of WAAMed TC4 titanium alloy were studied, and the relevant mechanisms were discussed. The main results are listed as follows.It was found that UIT avoided the formation of coarse columnar grain, and transformed it to fine equiaxed grains. Meanwhile, it increased the average width of deposited layers. Increasing the processing speed or decreasing the acting number increased the average diameter of equiaxed grains and decreased the average width of deposited layers. When the UIT amplitude is 40%, the average diameter of equiaxed grains was the minimum, and the average width of deposited layers was the maximum. In this study, the average diameter of equiaxed grains reached the minimum(0.80mm), and the average width of deposited layers reached the maximum(9.9mm) under the optimized parameters(amplitude is 40%, speed is 3mm/s and acting number is 4).The UIT acting promoted the multiplication and migration of the dislocations in the deposited layers. It broke the grains, and increased the nuc leus number, which changed the nucleating environment and increased deformation energy. It then avoided the formation of coarse columnar grains caused by the epitaxial growth during WAAM without UIT, and changed them to be equiaxed grains by recovery, recr ystallization and phase transformation induced by the thermal effect of subsequent deposited layers. Decreasing the processing speed, increasing the acting number and using the amplitude around 40% all increased the UIT energy on deposited layers, and then refined the average diameter of equaxied grains by strengthening the dislocation multiplication and migration and increasing the nucleus.Because the microstructure difference in the direction was decreased by the transformation in grain morphology, the a nisotropy percentage of UITed samples can be decreased from the 12.5% of the sample without UIT to the 1.5%. Where, the tensile strength at vertical direction of UITed sample increased from the 892.5MPa of the sample without UIT to the 953.6MPa, which is only 14.8MPa lower than that at horizontal direction.