Investigation of laser transmission welding for joining solid polypropylene to expanded polypropylene: An adhesive-free joining approach for dissimilar morphologies

Foamed and solid thermoplastics are widely combined in lightweight structures for automotive and other industrial applications, typically through adhesive bonding. However, adhesives present limitations, including increased automation complexity, limited process flexibility, curing time constraints, and sustainability concerns associated with thermoset materials. Laser transmission welding (LTW) offers a precise and efficient alternative, enabling localized heating, reduced distortion, and high production rates. This work investigates the LTW of non-foamed polypropylene (PP) to expanded polypropylene (EPP), in which the foam structure strongly influences interfacial heat transfer and contact evolution. To overcome the challenges associated with joining morphologically dissimilar thermoplastics, a systematic parametric study was conducted, varying laser power, scanning speed, clamping pressure, frequency, scanning pattern density and orientation, and beam wobble. The processing window was established based on preliminary differential scanning calorimetry and thermogravimetric analysis. Joint performance and interfacial morphology were evaluated through lap-shear testing, Fourier transform infrared spectroscopy, X-ray micro-computed tomography, and scanning electron microscopy. The results demonstrate that optimized process conditions enable continuous, structurally cohesive joints with strength approaching that of the base foam material. The results further reveal that joint formation is governed by the coupled effect of energy input and pressure, which control interfacial melting, polymer chain interdiffusion, and foam collapse. A transition between distinct joining regimes, ranging from insufficient interfacial fusion to cohesive joining and foam degradation, is identified. The study provides insight into the interplay between energy input, thermal diffusion, and mechanical constraint in polymer–foam laser welding, extending LTW to recyclable, adhesive-free polymer–foam assemblies.