Friction Stir Spot Welding for Structural Aluminum Sheets

The Friction Stir Spot Welding (FSSW) process is a derivative of the friction stir welding (FSW) process, without lateral movement of the tool during the welding process. It has been applied in the production of aluminum joining for various Mazda and Toyota vehicles. Most of the applications and published studies were concentrated in aluminum sheet in the range of 1.0 to 1.5 mm, suitable for non-structural automotive closure applications. The objective of this study is to study the feasibility of FSSW process for automotive structural aluminum joining, up to 3 mm in thickness, for potentially replacement of self-piercing rivets (SPR) process. Joining thicker aluminum with FSSW tooling with a typical smooth concave shoulder and threaded probing pin, requires long process time, which would not be appropriate in mass-production automotive body construction. In this paper, an innovative FSSW tool with grooved shoulder was developed. Results were demonstrated on FSSW of 2 mm 5754 alloy which reached 4 kN of lap-shear strength in 1.4 sec. Critical process parameters, including rotational speed (up to 4,500 rpm) and applied load (up to 15 kN), were studied to achieve higher joint strength and reduced process time. Cross-section micrographs were analyzed to investigate the joining mechanism. It was found that grooved-shoulder tool helped in agitating materials at interface and forming bonded joint larger and quicker than smooth-shoulder tool. Higher rotational speed and applied load were beneficial in reducing joining time.