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Laser transmission welding (LTW) plays an important role in plastic connection for fabricating large and complex products. The laser profile as a key parameter determines the spatial energy distribution, affecting the temperature gradient of the welding path and the quality of the weld seam. In this study, the effect of two laser profiles, the Gaussian and Flat-top beams on the temperature field and weld quality was studied through experiments and simulation. First, the transmission behavior of the laser beams is studied through the spectrometer and photosensitive paper, revealing the laser spot energy distribution and spatial changes after passing through the laser-transparent layer. Furthermore, the tensile force test demonstrated that the weld seam prepared by the Gaussian beam present higher tensile force, with a tensile force of 634.18 N at the line density of 11.2 J/mm, 24.23 % higher than that of the Flat-top beam. Moreover, the simulation result from the finite element model found that the maximum welding temperature of the Flat-top beam is 39.42 % higher than that of the Gaussian beam, consistent with the results of morphological and mechanical results. This study might present a potential way for selecting the laser profile in LTW, further improving weld quality through optimizing process windows. [ABSTRACT FROM AUTHOR]

• Analyzed the enhancement mechanism of MZAP on the fracture process. • Demonstrated positive effect of MZAP on mechanical riveting structure. • Effects of MZAP feature on the joint is investigated firstly. Effectively joining dissimilar transparent thermoplastics plays an important role in lightweight design in vehicles and automotive industry. However, their different physical–chemical properties hamper the welding of thermoplastics directly with diode laser. In this study, magnesium zinc alloy particles (MZAP) are chosen as laser-absorbent to join polycarbonate (PC) and polyarylsulfone (PASF) specimens. First, the comparison of PC/PASF weldments with different laser-absorbents was performed, including adding carbon black (CB) as laser-absorbents. The morphology and mechani-cal properties show the weldments prepared with MZAP have advantages of cleaner appearance and stronger shear strength. Furthermore, the fracture surface and joint microstructure reveal that the mechanical riveting structure, composed of defused MZAP and filled thermoplastics in the gaps between adjacent MZAP, forms when the MZAP was used as laser-absorbent. Comparing with CB aggregates, the fused MZAP has a stronger ability to resist external force and leads to a higher shear strength. Finally, the analysis of factors of MZAP feature reveals that the mechanical riveting structure contributes to improving the shear strength, the defects, such as creaks, holes and bubbles should be eliminated in the welding. The results of analysis of variance illustrated the particle size and the standoff distance are statistically significant for the shear strength. This study provides a potential avenue to effective weld dissimilar thermoplastics. [ABSTRACT FROM AUTHOR]

• The effect of process parameters on the thermal degradation behavior is investigated. • The thermal history model during LTW is simulated by FEM. • The thermal degradation kinetics model is solved from TGA with multi-heating rates. • The prediction model is proposed and matched well with LTW results. Reducing thermal degradation of plastics during laser transmission welding (LTW) process is the key to obtain high-quality weld seam. In this study, a thermal degradation prediction model is proposed. First, using polymethyl methacrylate (PMMA) as an exemplary material, the weld seams are prepared at different process parameters, including the laser power, welding velocity and carbon black (CB) content. Full characterization of the prepared weld seams was performed through shear tests, scanning electronic images, and ultra-depth-of-field microscope. The results reveal that thermal degradation occurs with bubbles at the interface and decreases the shear strength of the weld seam. Second, the thermal history model during LTW is simulated through a 3-D transient heat transfer finite element method (FEM), where the highest temperature is extracted to aid in comprehension of thermal degradation mechanism. Third, a thermal degradation prediction model is proposed based on the thermal history model and thermal degradation kinetic model which is solved by the thermogravimetric analysis (TGA). The results from the proposed prediction model is consistent with the occurrence of the initial thermal degradation and the trend of shear strength of the weld seam. Therefore, this study might present a potential way to optimize the processing window by effectively suppressing the appearance of thermal degradation. [ABSTRACT FROM AUTHOR]

The metal powder absorbents have the advantage of fabricating clean and high-strength joints in the process of laser transmission welding dissimilar transparent thermoplastics. However, the joining behavior between metal powders and thermoplastics is vague. Herein, polyarylsulfone (PASF) and polycarbonate (PC) are selected as model materials to investigate the joining behavior using magnesium zinc alloy (MZA) powders at different laser line energies. The shear force test was performed to reveal the effect of laser line energy on the welding. The µ-CT images reveal the evaluation of welding strength is attributed to the specific joint morphology. The multi-scale morphology characterization shows that the fractured surface height increases with increasing the laser line energy. The voids between MZA powders and resins increase the risks of interface failure, and the bubbles in the resins increase the risks of cohesion failure. The effects of laser line energy on the joint characteristics are elaborated based on the thermo-physical property test and transient temperature field analysis. The above results indicate the optimized welding temperature locates the melting temperature of PASF, because of the good fusion of PASF and less decomposition of PC. The study can provide a guideline for choosing proper process parameters when choosing metal powders as laser absorbents in welding dissimilar transparent thermoplastics. [ABSTRACT FROM AUTHOR]