Mechanical properties and failure mechanisms of friction stir spot welds of AA 6061-T4 sheets

► Hook's formation can be attributed to insufficient pressure vertical to the tool. ► Stir zone size and hook geometry are closed related to processing parameters. ► Different variation laws of hardness can be attributed to several factors. ► Rotational speed plays a crucial role in determining the mechanical properties. ► Hook geometry plays a predominant role on the crack propagation.