Defect formation during dissimilar aluminium friction stir welded T-joints

Abstract

AA8011 remains one of the largest used aluminium alloys due to excellent ductility, corrosion resistance and sufficient strength in H14 conditions. The 5754, due to high strength and corrosion resistance finds key applications in marine, process industry, automobile and nuclear applications. A T-joint combination of the two alloys (when stronger forming is stiffeners) results in significant weight savings. The T-joint between 8011 and 5754 (with the latter forming the stringer) is highly prone to defect formation due to the stronger material being placed away from the heat source. In this paper, friction stir welding of AA5754 and AA8011 aluminium alloys in T-lap configuration is performed. Effects of processing parameters namely tool transverse speed, rotational speed and shoulder diameter on the weld properties are studied with the help of micro-hardness, macrostructure and microstructure analysis. Analysis of defects such as tunnel and kissing bond, and mechanism of defect formation is also discussed. Tunnel defect is observed on advancing side in all the samples, which is attributed to high heat flux on advancing side, insufficient material flow in the vertical direction and asymmetric flow of plasticized material from advancing side to retreating side. The size of the defect is found to increase with increase in shoulder diameter keeping the rotational speed constant. Kissing bond defect was also observed in some samples owing to the presence of oxide layer between the joining surfaces.