The microstructure and mechanical properties of single-pass and double-pass lap joint of Al 5754H-11 and Mg AZ31-O alloys by friction stir welding

Abstract

Friction stir welding (FSW) is an efficient method for joining dissimilar alloys as it increases joining efficiency and quality by creating sound welds. This research investigates the effects of rotation & travel speed of the welding tool, single-pass and double-pass welding on defects, microstructure, hardness and tensile-shear force of lap joint between AZ31-O alloy and Al 5754H-111 alloy created by FSW. FSW was performed at two rotation speeds 500 RPM and 600 RPM and two travel speeds of 30 and 45 mm/min. The highest tensile-shear force was about 3.2 KN. It was related to the single-pass welding specimen created at the rotation speed of 500 rpm and travel speed of 45 mm/min. The intermetallic compounds Al12Mg17 and Al3Mg2 observed at the stir zone. Due to the presence of intermetallics and recrystallization induced by welding, the stir zone has higher hardness than the base metal. The results Showed that mechanical properties and formation of intermetallic compounds in the stir zone were influenced by FSW parameters. Investigation the cross-sections of welds showed that the hook defect had formed at different sizes around the stir zone. Moreover in the tensile-shear test, the crack initiated from this defects and propagated along the contact area between the intermetallic compounds and base metal. The fracture in all specimens occurred as a combination of soft and brittle failure.

Keywords

• Al 5754H-111 alloy
• AZ31-O alloy
• Single-pass welding
• Double-pass welding
• Lap joint
• Friction stir welding