Influence of Cu powder on strength, failure and metallurgical characterization of single, double pass friction stir welded AA6061-AA7075 joints

The present work investigates the effect of copper powder addition during single pass and double pass friction stir welding (FSW) between AA6061-T6 and AA7075-T651 to appraise the formation and detailed characterization of the intermetallic compounds (IMCs) in addition to study tensile, flexural strength, material flow, and fracture behavior. The copper powder was added in three different ways namely over the plate top surface (along weld path, below the tool shoulder), inside the slots of 1 mm and 2 mm (combined slot width at faying surfaces of adjoining plates) in addition to plate top surface. Analysis was also carried out employing a quick weld stop approach keeping the tool pin inside the weld seam and sectioning through it. Small improvement in joint ultimate tensile strength and flexural load was noticed when copper powder was added on plate top surface. For joints obtained with the addition of Cu powders in slots, formations of large clusters of particulates and/or IMCs were observed. Detailed characterization revealed that the formation of IMCs in small quantity with finer size distributed evenly could be beneficial for improving joint strength. However, large chunks/lumps can promote quick and brittle failure. In double pass FSW (with second pass in the reverse direction of first pass) good material flowability and slightly finer grains in stir zone (SZ) were observed. Comparatively smaller size agglomerates were also noticed distributed on either side of the SZ after the second pass. Fractographs showed the presence of numerous micro-voids and dimples in sound joints indicating the ductile fracture. However, tearing/rupture and brittle failure were witnessed in the joints prepared with addition of Cu powder in slots.