Behavior of oxide during friction stir welding of aluminum alloy and its influence on mechanical properties

Abstract

In the stirred zone of friction-stir welds of an aluminum alloy 6N01 plate (formed through extrusion), a zigzag bond line was observed after etching by an aqueous solution of 10% NaOH, when the revolution pitch (tool traveling speed/rotation rate) was greater than a critical value. SEM and TEM observations revealed that the bond line consisted mainly of particular inclusions 0.01-0.2μm in size. The inclusion was identified as complex oxide involving Al, Mg, and Si based on EDX analyses. The inclusion can be considered to originate from the oxide film that was formed on the plate surface during the extrusion process, since its amount was reduced significantly by removing the as-extruded surface layer through mechanical cutting or grinding. The presence of the bond line had no significant influence on the tensile strength or fatigue strength of the joint, since it was fractured in the heat-affected zone in the tensile and fatigue tests.