Welding Strategies for Improving Joint Performance in FSW of Dissimilar Al/Mg Alloys

Al- and Mg-alloys are continually attracting significant interest for use in lightweight structures in aerospace and automotive applications owing to their low density and excellent combination of mechanical and corrosion properties. The demand for joining these alloys in dissimilar configurations to produce both the butt- and lap-welded joints is also increasing in various transport industries. This growing demand for lightweight materials in transport industries has triggered extensive research on joining dissimilar alloys, particularly Al and Mg alloys (Al/Mg), in recent years. Although both alloys have some difficulties in welding such as porosity formation, cracking, and mechanical property deterioration, various fusion (e.g. arc, laser, and electron beam welding) and solid-state joining techniques (e.g. friction stir welding) are used to join these alloys in similar combinations. However, joining these dissimilar alloys is very challenging. The main obstacle that hinders progress is the formation of brittle intermetallic compounds (IMCs). This problem is more exaggerated in fusion welding due to the much higher heat inputs involved. Friction stir welding (FSW) is a promising technique for joining Al/Mg alloys, as it avoids the melting of the base metals, leading to refined microstructures, higher joint quality, and reduced porosity and intermetallic formation. However, undesirable IMCs also occur in this solid-state technique. Several novel weld strategies have recently been developed for mitigating and thinning IMCs forming in the stir zone, thus improving joint performance. This paper will discuss these novel approaches, which can employed in FSW to achieve sound joints with acceptable mechanical properties.