Transmission Laser Welding of Polyamides: Effect of Process Parameter and Material Properties on the Weld Strength

Abstract

Transmission laser welding (TLW) is an environmentally friendly and versatile process for assembling polymeric parts. Welding by transmission requires the upper element to be transparent to the laser wavelength whereas the inferior part is absorbent to the same wavelength. Thus, the energy of the laser beam is concentrated at the interface, allowing both polymeric parts to be melted. Upon temperature, the interdiffusion of polymeric chains takes place at the interface, creating bridges between both parts, resulting in a strong assembly. Laser welding is in principle applicable to any thermoplastic, provided that it is transparent to the laser wavelength.In the present work, carbon black filled polyamide has been assembled with two types of 2 mm and 4 mm thick partially transparent polyamide specimens. The effect of the process parameters such as energy density, speed of the laser beam and the part thickness have been correlated with the mechanical strength of welds, measured by wedge tests. The thermomechanical and optical properties of the samples have been assessed and correlated to the dimensions of the heat affected zone (HAZ). For a deeper understanding of the effect of optical and thermal properties of the materials on the quality of the welded joint, a numerical modeling is proposed. The ray tracing method is able to determine the light scattering and the absorption at the micro-scale in the polyamide semi-transparent part. The results of the ray tracing are the volume term source and the heat flux that generates temperature elevation.