Friction stir welding of thermoplastics with a new heat-assisted tool design: mechanical properties and microstructure

Traditional tools in friction stir welding (FSW) of thermoplastics fail to achieve high quality and strong welds. In this paper, a new heat-assisted stationary shoulder FSW tool design, which provides constant tool temperature during the process using a controllable hot air gun, is introduced. Effect of the process parameters, including tool temperature, rotational speed, and traverse speed, on the microstructure and mechanical properties of polypropylene sheets was investigated using Taguchi method and analysis of variance. Welded joints showed tensile strength up to 96% and elongation at break up to 99% of the base material. Samples with high mechanical performance showed a wider stir zone within thin flow layers, which arranged in a regular pattern, under polarized microscopy. Differential scanning calorimetry demonstrated that higher tool temperature results in lower degree of crystallinity. Scanning electron microscopy revealed that the sample with high mechanical properties has a craze-free fracture surface. However, the sample with the lowest mechanical properties produced a corrugated fracture surface full of crazes.