A review of various improvement strategies for joint quality of AA 6061-T6 friction stir weldments

Aluminium alloys are one of the choice materials with ever-increasing demands in manufacturing industries. The aluminium alloy 6 xx series such as AA6061-T6, has emerged as one of the promising materials utilized owing to its combination of favourable properties which include high strength to weight ratio, good ductility, excellent corrosion resistance and relatively low cost. These superior properties are responsible for its emergence and usage in the fabrication of aircraft wings and fuselages, yacht/ship construction, automotive rims and wheel spacers. However, joining of AA6061-T6 including the use of friction stir welding (FSW) has serious concerns because the mechanical and tribological properties of the AA6061-T6 welded joints deteriorate significantly compared with the base metal. This phenomenon has been attributed to the severe softening encountered at the stir zone (SZ) of the aluminium matrix during FSW. Other inherent challenges of FSW such as weld thinning, kissing bond and keyhole formation also contribute to the reduction in the weld joint quality. The softening phenomenon has been linked to the dissolution of the strengthening precipitates (B″-Mg5Si6) as a result of high heat input during the welding process. Hence, this paper attempts to review the various improvement strategies adopted in the existing studies to improve the quality of AA 6061-T6 welded joint. These include parametric optimization, selection of appropriate tool design, pre and post heat treatments, adoption of different groove/hole designs for particle addition as well as the addition of reinforcement particles to the weld joint. The variants of FSW recently developed will also be considered. The findings from the review will generally be useful for future work on FSW of heat treated aluminium alloys. The evolution of FSW and its associated challenges are briefly discussed while the research areas yet to be harnessed are suggested for future works.