Friction Stir Welding of High Strength and Toughness Cast Al-Si7-Cu4-Mg0.3 Alloys Manipulated by In-Situ Nanocrystals

The cast aluminum alloys with light weight and good castability show great potential in aerospace and automotive applications. Due to their poor weldability and the low strength of welded joints, these alloys are difficult to fuse using conventional fusion welding techniques. Here, through friction stir welding (FSW) for cast Al-Si7Cu4-Mg0.3 alloy manipulated by in-situ nanocrystals, a novel method to realize the automatic solid-state joining of cast aluminum alloys was proposed, along with the guarantee of high strength-plasticity combination of the joints. The microstructure of cast Al-Si7-Cu4-Mg0.3 alloy comprehensively manipulated by in-situ nanocrystals ensured the complete globalization of eutectic silicon and comprehensive mechanical performances of base materials to fulfill FSW. The microstructure evolution across various welded regions of the joints was revealed. The dendritic grains with a certain degree of coarsening and short needle-like eutectic silicon were formed in the heat affected zone (HAZ), and the mechanism of microstructure evolution was mainly dynamic recovery (DRV). The dynamic recrystallization (DRX) of grains in the nugget zone (NZ) resulted in regular fine equiaxed grains, and nearly spherical eutectic silicon distributed uniformly in the matrix. The optimal yield strength (YS) and ultimate tensile strength (UTS) of friction stir welded joints were 280 MPa and 400 MPa, which achieved 84.8% and 83.3% of the base material respectively (330 MPa and 480 MPa). This study contributes to producing friction stir joint of cast aluminum alloys with complex structures, as an alternative to traditional stamping-welding process of deformed aluminum alloys.