Study on underwater-assisted friction stir lap welding of magnesium alloy/CFRTP

With the rapid advancement of the transportation and electronic industries, there is an increasing demand for reliable joining technologies for metal/polymer composites, particularly in the context of lightweight design solutions. Friction stir welding (FSW) has garnered considerable attention due to its environmentally friendly characteristics and high efficiency. Underwater environments have been demonstrated to minimize the formation of holes and other welding defects, thereby enhancing the quality and strength of joints. This study investigates the FSW of AZ31 B magnesium alloy and carbon fiber-reinforced thermoplastic polymer (CFRTP), comparing conventional air FSW (AFSW) with an underwater FSW (UFSW). The key finding is that while AFSW consistently resulted in severe defects and failed bonding, UFSW significantly improved joint integrity. The welds were evaluated by optical microscope image and scanning electron microscope of the fracture and tensile analysis. Microscopic analysis revealed that the underwater process effectively minimized defects such as cracks and tunnels, establishing mechanical interlocking as the primary bonding mechanism. As a result, UFSW achieved a maximum average tensile strength of 27.27 MPa (51.45% of the CFRTP base material), which opened up a new way for the connection between magnesium alloys and polymers.