Influence of tool offset on microstructure and properties of Mg/Al dissimilar alloys by friction stir welding joints at low heat input

The composition of intermetallic compounds and mechanical properties during friction stir welding of 2024 aluminum and AZ31B magnesium alloys was investigated by using offset tool at low heat input. The macrostructure of the Mg/Al joints was investigated by optical microscope, and microstructure characteristics were analyzed by scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction. The fracture section was also observed. It is found that the mechanical strength of the joint slowly decreases as the value of tool offset increases first and then decreases at the Al-side. The result showed that the joints have the best mechanical properties when offset 0.5 mm and the amount of IMCs is low. And it is found that only IMCs Al12Mg17 is found in the joints. Since Al₃Mg₂ and Al₁₂Mg₁₇ have eutectic temperatures of 450 °C and 437 °C, respectively, the offset caused the temperature to decrease during the FSW process, allowing Al₁₂Mg₁₇, with a lower eutectic temperature, to form easily.