Laser transmission joining of plasma treated polypropylene to stainless steel: Interfacial microstructure and mechanical properties

Hybrid joints between polypropylene (PP) and 316L stainless steel (316L) have been widely used in various industrial fields, such as automotive, medical equipment, and electronic devices. The difficulty in the joining PP to 316L is that the former is a non-polar polymer, which makes it difficult to initiate a chemical reaction at the interface between the two materials. This ultimately results in a joint with low strength. Additionally, the melting point and thermal conductivity of the two materials differ significantly, so it is necessary to properly control the laser heat input. To tackle this challenge, the PP surface was pre-treated by plasma and then joined to 316L by laser transmission welding technology. The polar groups introduced by the plasma-treated PP form new chemical bonds with the metal and metal oxides of 316L, resulting in high quality dissimilar joints. The macromorphology and microstructure of the interface were investigated comparatively with different scanning speeds. The results showed that the optimal scanning speed was 10 mm/s at a laser power of 60 W and a defocusing distance of 0 mm, resulting in a maximum lap shear force of 149.18 N and an optimal macroscopic morphology. Furthermore, the correlation between the change in weld morphology and the mechanical properties was investigated, and the morphological and chemical bonding of the fracture were analyzed to elucidate the joint connection mechanisms.