Investigation of the effect of light scattering on transmitted laser intensity at the weld interface during laser transmission welding of 3D printed thermoplastic parts

3D printing has offered cost-effective, lightweight, and complex parts. To extend their applications, 3D printed parts need to be welded in order to form the larger functional assemblies. For this purpose, Laser Transmission Welding (LTW) is a promising joining technology. This paper aims to investigate the light scattering effect on the intensity profile of the laser heat source during the transmission through the 3D printed laser-transparent part. Indeed, the inherent design of the 3D printing technology results in a complex heterogeneous microstructure with a significant amount of porosity inside the printed parts. Such structure induces the optical diffusion (i.e. light scattering) of the laser beam within the 3D printed parts. This phenomenon leads to the reduction of the transmitted energy arriving at the weld interface, which directly influences the quality of the joint and its mechanical performance. The approach adopted in this paper is to propose a ray-tracing model to simulate the optical paths of the laser beam through the 3D printed laser-transparent part, which is able to evaluate changes in the laser heat source at the weld interface directly linked with the light scattering effect within the microstructure of the parts. Experimental measurements are performed to assess the transmitted intensity flux distribution using an image processing technique, instrumented with a digital camera and macro lens. The numerical results show good accordance with the experimental one, which proves the confidence of the proposed ray-tracing model. Finally, 3D transient thermal model of the LTW process is performed using the FEM software COMSOL Multiphysic (R) to confirm the influence of the scattering effect on the temperature field and thus on the quality of the weld.