Development of a new approach to resistance spot weld AW-7075 aluminum alloys for structural applications: a material characterization study – Part 2

The aluminum (Al) alloy AW-7075 is recognized for its high yield strength, making it suitable to replace high strength steels, thereby contributing with lightweight strategies worldwide. However, due to its metallurgical properties it is considered not suitable for joining via fusion methods, exhibiting a wide melting range, proneness to cracking and low solubility of the alloying elements in the solidified Al matrix. To address this problem, a new resistance spot welding (RSW) procedure to properly join AW-7075 was developed in previous work. This procedure involved continuous upslope current profiles, larger electrode force, and copper-silver electrode caps, which were implemented to validate the application of RSW to join AW-7075. In this work, a parametric study was carried out to assess the impacts of these welding parameters on the weld quality. Optical microscopy was used to assess the weld quality, electron back-scattered diffraction (EBSD) for microstructural texture characterization, and scanning-electron microscopy (SEM) along with electron-diffraction spectroscopy (EDS) to identify the resulting phases in the grains and at the grain boundaries. Defect-free weld joints were repeatedly produced, process boundaries were validated, and improvement of the microstructure was verified, whereby finer and equiaxial grains (EG) were detected directly in the vicinities of the heat-affected zone (HAZ). Results show that slower upslope grades favor the formation of EG by changing the solidification mechanism from columnar (directional) to equiaxial (non-directional).