Effect of Friction Stir Welding Process on Crystallinity and Degradation of Polypropylene

The aim of this study was to investigate the crystallinity changes and degradation of polypropylene due to heat generated by friction stir welding, i.e., heat generated by friction between the rotation tool and the welded materials. The tool pin was rotated at 620 rpm in the welding process. The travelling speed was varied between 7.3 mm/minute and 13 mm/minute. A cylindrical tool pin, 4.5 mm in diameter and 5.7 mm in length, was used in this experiment. The shoulder dimension was 18 mm in diameter and 90 mm in length. A conventional milling machine was used in the friction stir welding process. The crystallinity test was carried out with X-ray diffraction, hardness was observed using a Shore Type-D durometer, and polymer degradation data was obtained by thermogravimetry analysis. The areas compared were base material, weld nugget area, and thermomechanical affected zone. The results showed that there was a change in the percentage of crystallinity in areas that had undergone friction stir welding, and that the change was inversely proportional to the traveling rate of the friction stir welding process. The friction stir welding process was affected by the initial degradation temperature and hardness property of the polypropylene. This result shows that it is possible to choose specific parameters of friction stir welding in order to obtain good weld joint properties.