Microstructural Characterization and Mechanical Properties of 120-mm Ultra-thick SiCp/Al Composite Plates Joined by Double-Sided Friction Stir Welding

Applications of aluminum matrix composite (AMC) engineering components are being quickly developed and their numbers are increasing, but difficulties still exist in the direct fabrication of large AMC components. Joining thick composite plates without drawbacks has become an effective and alternative route, and friction stir welding (FSW) is the most promising and efficient technique, which should be selected first. In this work, 120-mm ultra-thick plates of 16 vol pct SiCp/2014Al composites were successfully joined by double-sided FSW. Microstructural characterization and mechanical properties of the composite joint along the transverse and thickness directions are investigated, respectively. Along the transverse direction changing from the base material (BM) to the heat-affected zone (HAZ) to the nugget zone (NZ), SiC particles are broken, blunted and uniformly distributed, and the aluminum grains are refined. The coarse precipitates are dissolved and then re-precipitated. Vickers microhardness of the composite joint gradually increases, and the peak values appear in the NZ. For the composite joint, the tensile strength is close to that of the BM showing a joint efficiency of 97 pct. Along the thickness direction varying from the top surface to middle section of the composite joint, the SiC particle size has no significant differences in the NZ, the aluminum grain size gradually decreases, and the precipitate size has some differences. Vickers microhardness of the composite joint decreases with increasing thickness in the NZ and HAZ, and no clear differences exist in the BM; tensile properties of the composite joint present few differences with varying thickness, and tensile failures usually occur in the BM. A good understanding and guide can be provided for joining ultra-thick plates of metal matrix composites by FSW in industrial applications.