In automotive industry, new families of coated high strength steel have been introduced in the design of body in white to simultaneously impact safety and light weighting for reducing energy consumption. Some combinations of dissimilar steel sheets, with different thicknesses, coatings, and grades, cause weldability problems and difficulties to the optimization of the process parameters setting. Thanks to the progress achieved in numerical and computer engineering fields, modelling and numerical simulation is a relevant approach, to understand the difficulties encountered during resistance spot welding of these assemblies, and to search solutions to improve the weldability. This work aims at the improvement of the weldability of a dissymmetric combination of three dissimilar sheets: a very thin (0.57mm) zinc coated low carbon steel sheet, a thick (1.47mm) zinc coated advanced high strength steel sheet, and a thick (1.2mm) aluminium-silicium coated press hardened sheet (PHS).

A numerical axisymmetric 2D Electro-Thermo-Mechanical model developed with the software FORGE® is used to improve the knowledge about the mechanisms, which influence the nugget formation and growth, and its penetration inside the cover thin sheet. Experimental evolutions of thermal and electrical contact resistances evolutions, at electrode/sheet and sheet/sheet interfaces, strongly dependent of coatings properties, are embedded in the model and considered dependent on contact temperature and normal stress. The contact radius evolutions, involving the normal contact stresses and the current density distributions in the assembly, are calculated during squeezing, welding, and forging stages.

The model is consistent with several experimental observations (nugget size, contact radii, dynamic resistance) issued from welding tests. The numerical results show that the initial heating appears at the interfaces with the aluminized sheet, due to huge contact resistances. Consequently, the nugget initiates inside the PHS at the opposite of the thin sheet. Firstly the nugget grows quickly in thickness, and then in diameter with the contact radii evolutions when the electrodes indent into the sheets. The influence of some major parameters on the penetration of the nugget inside the thin sheet, such as the welding current, the clamping force, and the curvature radius of the electrodes tips, is also investigated.