Analysis of the Weld Seam Morphology of Polypropylene in Laser Transmission Welding

Laser transmission welding is a well-known joining technology for welding thermoplastics. Although the process is already used industrially, fundamental process-structure-property relationships are not fully understood and are therefore the subject of current research. One aspect of these mentioned process-structure-property relationships is the interaction between the temperature field during the welding process, the weld seam morphology of semi-crystalline thermoplastics, and the weld seam strength. In this study, the influence of the line energy on the weld seam morphology of polypropylenes is analyzed. For this purpose, the size of spherulites in the weld seam is investigated, as well as different occurring phases of polypropylene (α- and β-phase). It is shown that both the spherulite size of the α-phase and the amount of β-phase increase with increasing line energy. For the explanation and discussion of the results, a temperature-dependent thermal simulation model is used to derive characteristic attributes of the temperature field (maximum temperatures, cooling rates, temperature gradients).