Effect of interlayers on softening of aluminum friction stir welds

Effects of the zinc, copper and brass interlayers were studied on the microstructure evolution and mechanical properties of the wrought aluminum friction stir welds. Optical and scanning electron microscopes (SEM) as well as X-ray diffraction (XRD) analysis were used for investigation of the distribution, dissolution and reactions of the interlayers, grain structure in the stir zone and fracture surfaces. Longitudinal and transverse tensile and microhardness tests were also carried out in order to evaluate the mechanical properties of the welds. The results showed that without insertion of the interlayer, the joint efficiency (ratio of the weld strength to the aluminum base metal strength) was ~60%. While zinc and copper interlayers had no significant effect on the joint efficiency, insertion of the brass interlayer increased the joint efficiency up to ~90%. Microstructural observation indicated that brass interlayer was relatively well distributed inside the stir zone and Al₂Cu and Al₄Cu₉ intermetallic compounds were formed due to the solid-state reactions between the aluminum matrix and brass particles during welding leading to the increase in the weld strength. In the case of the zinc interlayer, no intermetallic compound was detected inside the weld zone and zinc was dissolved in the aluminum matrix to form a solid solution. Moreover, the joint welded using copper interlayer suffered from lack of distribution of the detached copper particles. Fracture analysis after tensile test showed that insertion of the interlayers generally changed the fracture location from the advancing side towards the retreating side due to the formation of a composite structure and increase in the strength of the advancing side.