Impact of Friction stir lap welding pin positioning on Microstructure and Fracture behavior of Al-Steel

ABSTRACT Al-steel friction stir lap welding (FSLW) experiments have been conducted to study the effects of welding parameters including tool pin positioning on microstructures formed in the Al-steel interface region, and on joint strength.The joint strength is defined as maximum applied force over the width (Fm/ws) of the test sample, of the welds. Different tool pin positioning and speed conditions were used in the Al-steel FSLW experiments. Macrostructure of the joints and the fracture surfaces of the tensile shear tests, including examination on crack propagation in mixed stir zone were observed with optical microscope and scanning electron microscope (SEM). It was found that when the tool pin was close to the top surface of the bottom steel piece, intermetallic compounds formed along theinterface due to the Al-steel reaction, representing incomplete metallurgical joint. When the tool pin position was lowered to just touch the steel piece, a thin and continuous interface intermetallic compounds layer formed. Signs on growth kinetics have suggested that the thin and continuous intermetallic compounds layer formed could only remain thin (≤ 3μm), which could bear a high load during tensile shear testing bydeforming and fracturing aluminum alternatively, and the resulting Fm/wswas high. Similarly when the penetrated to bottom piece steel, Fm/ws significantly reduced due the brittle fracture being dominant inside the mixed stir zone. It was evident that the tool pin penetration has the strong effect on Fm/wsduring FSLW of Al-steel.