In the process of metal additive manufacturing, due to the rapid melting and solidification of the material, large temperature gradient and cooling rate are generated in the molten pool, leading to the formation of thick columnar crystalline structure inside the material (for example, the additive manufacturing structure of titanium alloy wire usually presents the characteristics of thick columnar crystalline structure). These columnar crystals not only result in the anisotropy of additive manufacturing materials, but also lead to the low ductility, damage tolerance and fatigue properties of additive manufacturing materials. Therefore, how to improve the microstructure of metal deposition and obtain small-size equiaxed grain microstructure by simple and easy process is the key technology to be solved urgently in the popularization and application of metal additive manufacturing technology.
High power ultrasonic energy field has been widely used in the field of casting and welding for grain refinement, mainly due to the acoustic cavitation and acoustic flow effect, as well as the homogenization temperature field and solute field caused by ultrasonic during the solidification process of metal melt. Based on this, the application of ultrasonic energy field in metal additive manufacturing becomes a possibility.
The proposed ultrasonic energy field assisted forming technique can control the microstructure and properties of metal components by acting on molten pool. In addition, the ultrasonic wave can also impact aging on the metal deposition layer at the same time, realize the on-line control of the residual stress of metal components, and reduce the tendency of deformation and cracking of large complex metal components. It can be seen that this technology has a wide application prospect in the high-energy beam direct deposition additive manufacturing of large metal components.
The results show that the introduction of ultrasonic vibration has a positive effect on the solidification structure during additive manufacturing process, which can effectively inhibit the growth of columnar grains, weaken texture and promote the formation of equiaxed grains. The proposed method provides a suitable technique for solving the problem of coarse columnar crystal structure in additive manufacturing of large metal components and can be used for on-line regulation of microstructure and properties in additive manufacturing of large metal components.
