Laser Transmission Welding of Solid and Foam Thermoplastics: Parametric Analysis and Optimization

Lightweight polymeric structures are increasingly used in industries such as automotive, aerospace and packaging, driving the demand for flexible and sustainable joining techniques. Currently, joining these structures typically relies on traditional adhesives, which inherently involve toxic substances and require long curing times, negatively impacting process sustainability and flexibility. This study analysed the joining of polypropylene (PP) to a low-density polyethene (PE) foam, which is widely used in automotive parts such as bumpers and interior panels, using laser transmission welding (LTW). While LTW, a technique increasingly employed to join solid thermoplastics due to its unparalleled flexibility, weld quality, and suitability for mechanically fragile components, presents challenges when applied to morphologically dissimilar materials, particularly regarding their thermal, chemical, and morphological structural differences. Initially, a baseline study analysed the influence of the two parameters that most directly affect heat input onto the joint interface: laser power and scan speed. This phase yielded promising results, achieving joint strengths comparable to those of the base foam. These joints were characterised by various techniques, including shear lap testing, optical microscopy, and computerised microtomography, to assess mechanical performance, interfacial adhesion, and morphological defects. In a second stage, to optimise the initial results, additional parameters were systematically varied in the study. These included clamping force, laser frequency and wobble, various scanning patterns, and fill laser spacing. This approach proved successful, as high weld strengths were achieved while morphological impacts were minimised. These results prove the capability of LTW as a viable alternative to adhesives, providing significant insights into the parametric interplay and the mechanical and morphological mechanisms governing these joined structures.