In automotive industry, to simultaneously impact safety and light weighting for reducing energy consumption, new families of high strength steel have been introduced in the design of body in white. Some combinations of dissimilar sheets, including a very thin sheet, 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 (0.57 mm) zinc coated low carbon steel sheet, a thick (1.47 mm) zinc coated advanced high strength steel sheet, and a thick (1.2 mm) aluminumsilicon coated press hardened sheet. 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. Thanks to this model, the important effect of the interfacial mechanisms on the formation and the growth of the molten pool have been highlighted. Furthermore, the influence of the process parameters (current, force) and of the curvature radius of the rounded tip electrodes on the penetration of the welding pool into the thin sheet have been investigated.

In automotive industry, to simultaneously impact safety and light weighting for reducing energy consumption, new families of high strength steel have been introduced in the design of body in white. Some combinations of dissimilar sheets, including a very thin sheet, cause weldability problems and difficulties to the optimization of the process parameters setting. 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. 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. 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 is done at the interfaces with the aluminized sheet, opposite the very thin sheet. The lack of penetration of the nugget into the thin sheet is a consequence of the overheating at the interfaces with the aluminized sheet and the uncontrolled growth of the nugget.

During a resistance spot weld, a current of several kiloamperes passes through a stack of sheets clamped between two electrodes. We are interested here in the very first moments of the welding, when the combined effects of electrical and thermal contact resistances at sheet/sheet (S/S) and electrode/sheet (E/S) interfaces are at the origin of intense overheating. In order to estimate the contact temperatures, non-contact infrared thermography measurements coupled with an instrumentation by micro-thermocouples welded on the sheets are performed. The evolution of the emissivity is measured and the temperatures at the interfaces are evaluated.

Welded Blanks applications for AlSi coated Hbt Stamped parts (HS-LWB) are significantly increasing in modem car designs. Automakers use them to achieve lightweight car body architectures, while making the structures stronger to meet the latest crash and safety requirements.Mass savings are achieved due to the higher strength level of the hot-stamped material and due to part integration with the reduction of overlap spot welds. In addition cost savings can be realized related to the overall weight reduction and to a better material utilization by nesting the sub-blanks efficiently out of coil material.In order to assess the mechanical behavior of the HS-LWB, quasi-static, dynamic loading, and three-point bending tests have been carried out. These tests have been performed at room temperature and extra-cold temperatures. The mechanical response of the HS-LWB has been correlated with its weld properties. The main target through the use of these various mechanical tests is to generate different solicitations in the weld area including bending and traction. So, based on these tests, we have demonstrated that the ArcelorMittal welding process is very robust and leading to high crash performances.Furthermore, in order to maximize weight reduction, ArcelorMittal R&D has developed a new simulation tool able to predict crash ductility of the AlSi coated Hot-Stamped Laser Welded Blanks. This weld behavior can be used in a full-vehicle FE-Model. The fracture risk of the stamped HS-LWB part can be assessed for any crash load case. To showcase the potential application of this new tool, several Euro NCAP crash scenarios were applied on a generic car model using a HS-LWB part.

A study looks At improving the welding of automobile body parts involving joints of mild and high-strength steels of varying thicknesses

A laser-welded blank (LWB) offers several notable benefits including decreased part weight, reduced manufacturing costs, increased environmental friendliness and improved dimensional consistency. However, in order to take advantage of these benefits, designers need to overcome the formability of LWBs and be able to accurately predict the LWB forming characteristics early in the design process. We present a new approach to predict the maximum elongation of the laser seam; for example, for Advanced High-Strength Steel (AHSS)-based LWBs. This approach is based on a coupling model taking into account the thermal, metallurgical and mechanical phenomena. A comparison of numerical and experimental results shows very good agreement. These approaches offer a considerable advantage in obtaining the formability limits for all configurations.