Effect of annealing treatment on joint strength of aluminum/steel friction stir lap weld

In this work, the effects of the annealing temperature and duration on the joint strength of the steel/aluminum lap joints made by friction stir welding were investigated. The strength of the welded joints was evaluated by shear-tensile test. Formation of intermetallic layer at the joint interface and the fracture locations were studied using scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). The results indicated that longer time of annealing treatment resulted in higher joint strength at 300 and 350 °C and that the rate of the increase in joint strength with time was about 72% higher at the latter temperature than at the former one. In comparison with the atomic diffusion through the joint interface, the formation and development of a thin intermetallic layer in the joint after annealing at 300 and 350 °C (with the thickness of less than 0.49 μm) may have a higher impact on the improvement of the joint strength. However, the joint strength decreased with the increase in annealing duration at 400 °C. In the samples annealed at 400 °C for more than 90 min, the joint strengths were degraded drastically. From the joint strength point of view, intermetallic layer thickness of ∼2.6 μm was a critical thickness.

► Effects of annealing conditions on the joint strength were investigated. ► Longer time of annealing resulted in higher joint strength at 300 and 350 °C. ► Rate of increase in joint strength with time was higher at 350 °C than at 300 °C. ► Fracture loads decreased with increase in annealing duration at 400 °C. ► Intermetallic layer thickness of 2.6 μm was a critical thickness.