Thermal non-linear stresses in an adhesively bonded and laser-spot welded single-lap joint during laser–metal interaction

In this study the transient thermal analysis of an adhesively bonded and laser-spot welded joint was carried out based on a thermal model developed for the laser-spot welding of multi-layered sheets using a pulsed Nd:YAG laser. In the thermal stress analysis the material non-linear properties of adhesive and sheets were considered using the non-linear finite element method. The keyhole formation and temperature distributions around the keyhole were analysed for the different sheet materials, i.e. aluminium, steel and titanium, used widely in the automotive and aerospace applications. The laser beam causes small heat-affected zones in the sheets and adhesive layer. As a result non-uniform temperature and thermal strain distributions arise in the vicinity of the keyhole. However, thermal strains are incompatible along the adhesive–sheet interfaces since the adhesive and sheets have different thermal and mechanical properties. Consequently, non-uniform stress distributions were observed around the keyhole causing permanent deformations in the adhesive and sheets. Especially, the free ends of the adhesive–upper sheet and the adhesive–lower sheet interfaces were subjected to considerable plastic strains. The free ends of the adhesive–plate interfaces corresponding to the keyhole boundaries arise as most critical zones; therefore, the first crack initiation in the adhesive layer can be expected from these zones. The residual stresses around the keyhole due to plastic strains would contribute the fast crack growth along the adhesive–plate interfaces when the laser beam–metal interaction ceases.