Numerical simulation of heat and mass transfer in novel stepped structure friction stir welding of Ti/Al dissimilar alloys

Friction stir welding (FSW) is one of the most suitable joining methods for Ti/Al dissimilar welding. Understanding and optimizing the heat and mass transfer behavior of Ti/Al dissimilar FSW is crucial for enhancing the mechanical properties of the joints. In this work, a novel stepped structure friction stir welding (SFSW) was proposed to optimize heat and mass transfer for simultaneously improving mechanical properties and welding efficiency. A validated multi-phase numerical model for SFSW of Ti/Al dissimilar alloy was developed using CFD in cooperation with the VOF method. Through numerical modelling and experimental analysis, the influence mechanism of the novel stepped structure on the heat and mass transfer, as well as the resulting microstructure at the weld root was revealed. The stepped structure leads to an increase in temperature at the weld root by approximately 17 K, significantly improving the heat and mass transfer behavior at the weld root. The SFSW approach significantly raises the temperature and strain rate, promoting metallurgical bonding in the Ti/Al interface. It is concluded that the temperature rise caused by the stepped structure is not limited to a specific region, but rather the entire stepped structure as a whole experiences the temperature increase. The stepped structure supports Al adhesion without altering IMCs distribution. By increasing welding speed from 50 to 80 mm/min, SFSW achieves a 60 % boost in welding efficiency, with ultimate tensile strength reaching 320.4 MPa, an 18.4 % improvement over conventional FSW. In summary, the stepped structure is crucial in promoting metallurgical bonding and optimizing heat and mass transfer.