To satisfy the need of polymer connection in lightweight automobiles, a study on laser transmission spot welding using polymethyl methacrylate (PMMA) is conducted by using an Nd:YAG pulse laser. The influence of three variables, namely peak voltages, defocusing distances and the welding type (type I (pulse frequency and the duration is 25 Hz, 0.6 s) and type II (pulse frequency and the duration is 5 Hz, 3 s)) to the welding quality was investigated. The result showed that, in the case of the same peak voltages and defocusing distances, the number of bubbles for type I was obviously more than type II. The failure mode of type I was the base plate fracture along the solder joint, and the connection strength of type I was greater than type II. The weld pool diameter:depth ratio for type I was significantly greater than type II. It could be seen that there was a certain relationship between the weld pool diameter:depth ratio and the welding strength. By the finite element simulation, the weld pool for type I was more slender than type II, which was approximately the same as the experimental results.

Due to their large compatibility difference, polyethylene (PE) and polyoxymethylene (POM) cannot be welded together by laser transmission welding. In this study, PE and POM are pretreated using plasma that significantly enhances their laser transmission welding strength. To understand the mechanism underlying the laser welding strength enhancement, surface modification is analyzed using contact angle measurements, atomic force microscopy (AFM), optical microscopy, and X-ray photoelectron spectroscopy (XPS). Characterization results show that the plasma surface treatment improves the surface free energy, significantly enhancing the wettability of the materials. The increase in surface roughness and the generation of homogeneous bubbles contribute to the formation of mechanical micro-interlocking. The oxygen-containing groups introduced by the oxygen plasma treatment improve the compatibility of PE and POM, and facilitate the diffusion and entanglement of molecular chains and the formation of van der Waals force.

In this paper a novel laser impact spot welding (LISW) method is described, in which a hump was formed on the flyer plate on the intended welding spot location by local pre-forming. When the flyer and base plates were placed together to perform welding, the two plates kept in contact over their entire surfaces except at the hump, where a local air gap was enough to guarantee the impact velocity and collision angle to achieve spot welding using laser pulse energy. The presented approach was implemented to join thin titanium foils to copper foils under low laser energy system. Joints with regular shapes were obtained. The microstructure in the weld interface was studied with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It was found that the jetting occurred at the central region of the weld spots due to oblique impact. Wave features were observed in the weld interfaces. The impact energy was found to have significant influence on the wave’s characteristics. Moreover, SEM images and EDS analysis did not show apparent element diffusion across the weld interface. Besides, the lap shearing test was used to characterize the mechanical properties of the spot welded joints.