Through-Transmission Laser Welding of Composites

Thermoplastic composite materials are attracting growing interest in a wide range of industrial sectors, especially in the transport sector (aeronautics and automotive in particular). They form components and parts that often need to be assembled to form more complex modules and structures. This has led to growing interest in thermoplastic composite assembly technologies, including laser welding.

Laser welding technology offers specific advantages for industrial applications over other conventional technologies: the process is precise, flexible, easy to control and automate and non-contaminating, an absence of vibration during the welding process, very fast welding speed enabling long parts to be welded with an acceptable welding time (a few seconds).

The laser welding process involves two parts. One part is semi-transparent at the laser wavelength, and the other is absorbent at the same wavelength. The process is based on the principle of heating the interface (forming a weld bead), and consolidating it by applying pressure to the area to be welded. The power of the laser beam is transmitted through the "semi-transparent" material and absorbed at the interface by the "absorbent" material. Contact between the two parts causes the temperature of the weld zone to rise, as heat is transferred by conduction between the two parts. As a result, the two materials fuse together. When welding heterogeneous materials such as composites, a number of difficulties arise. These materials are heterogeneous because they are fiber-reinforced. The passage of the laser beam through such media results in divergence of the laser beam's optical path (refraction phenomenon) caused by the multiplication of fibres-matrix interfaces in the material. The laser power arriving at the welding interface is thus reduced by refraction and absorption in the semi-transparent, scattering medium.

A good quality weld joint depends on a good understanding of the behavior of the materials to be joined under infrared laser irradiation, and on the correct identification of the temperature at the weld interface using modeling and simulation tools.

The aim of this article is to review the principles of laser transmission welding, to highlight the particularities resulting from the heterogeneous structure of the thermoplastic composites to be joined, and to propose virtual engineering tools for optimizing the welding process to produce quality welds.