Microstructural characterizations and mechanical properties in underwater friction stir welding of aluminum and magnesium dissimilar alloys

• Aluminum and magnesium alloys were joined by underwater friction stir welding.
• Underwater FSW was conducted to improve properties of joint with lower heat input.
• Microstructures and mechanical properties of dissimilar joint were investigated.
• Intermetallic compounds developed in the fracture interface were analyzed.
• Fracture features of the tensile samples were analyzed.

Formation of intermetallic compounds in the stir zone of dissimilar welds affects the mechanical properties of the joints significantly. In order to reduce heat input and control the amount and morphological characteristics of brittle intermetallic compounds underwater friction stir welding of 6013 Al alloy and AZ31 Mg alloy was carried out. Microstructures, mechanical properties, elements distribution, and the fracture surface of the joints were analyzed by optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy, etc. The result shows that sound dissimilar joint with good mechanical properties can be obtained by underwater friction stir welding. Al and Mg alloys were stirred together and undergone the process of recrystallization, forming complex intercalated flow patterns in the stir zone. Tensile strength of the dissimilar joint was up to 152.3 MPa. Maximum hardness (142HV) appeared in the middle of the centerline of the specimen. Intermetallic compounds layer consisting of Al3Mg2 and Mg17Al12 formed in the Al/Mg interface and resulted in the fracture of the joint.