OPTIMIZATION OF ALUMINIUM/POLYMER JOINTS PROPERTIES USING FRICTION STIR SPOT WELDING (FSSW) AND FRICTION STIR CLINCHING (FSC)

Abstract

The best materials choice for product development may call for joining two dissimilar materials to meet the needs of the end users, but their weldability might be challenging and the product not being able to stand the test of time. Thus, this research considers optimizing aluminium and polymer joint using friction stir spot welding (FSSW) and friction stir clinching (FSC). In this research, dissimilar lightweight materials of aluminium and polymer were joined together in a lap configuration using FSC and FSSW methods by varying parameters such as tool rotational speed (900-1400 rpm), plunge depth (5.5-6.5 mm), and dwell time (3-5 s). A 12 mm diameter conical shoulder-cylindrical pin tool was employed for the FSC and FSSW process. This is preferred to flat tool because the conical-cylindrical tool allows lesser frictionally induced heat input into the weldment. The experiment was designed via the use of L9-orthogonal array based on Taguchi method. The mechanical and microstructural properties of the joint were investigated and compared, and the processes were optimized using Taguchi method of optimization. The temperature of the aluminium plate was measured during the joining process with the use of thermocouple. The joining temperature increased with an increase in the speed of rotation of the tool. At tool rotational speeds of 900 rpm, 1120 rpm, and 1400 rpm, the joining temperatures were 271oC, 332oC and peak temperature 3700 C respectively. The high temperature (above melting point of the polymer) facilitates adhesion bonding and this was supported by mechanical interlocked aluminium particles. Observations showed that material interaction and interlocking between aluminium and polymer was higher at zones closer to the center of weldment. Based on the experimental run, the maximum failure loads of 448.91 N and 854.47 N were obtained with the FSC and FSSW processes respectively. Tensile test result showed that FSSW has comparatively higher failure load than FSC. Under tensile load, the joints failed by unbuttoning, partial shear, and complete shear modes. The optimized failure loads of FSC and FSSW joints are 536.84 N (1120 rpm tool rotational speed, 6.0 mm plunge depth and 5 s dwell time) and 863.37 N (900 rpm tool rotational speed, 6.0 mm plunge depth, and 5 s dwell time) respectively. The plunge depth had the most significant influence on the failure load of the joint, followed by the tool speed, and dwell time. The joint formed from FSSW showed better mechanical property in terms of failure load than FSC joint and therefore recommended for use.