Development of Complex Spot Welding Technologies for Automotive DP Steels with FEM Support

In this paper spot-welding technology of sheets made of Dual-Phase (DP) steel, which are one of the most important materials in the automotive industry, has been analyzed with experimental research and finite element modelling. Besides conventional mild steels that have been dominant for decades in automobile production, the application of advanced high-strength steels (AHSS) comes into focus. Among the first generation AHSS, DP steels are of the utmost importance concerning their automotive use. In Hungary a lot of small and medium sized enterprises function as the suppliers of the big automakers settled in our country. Nowadays these enterprises, besides the welding of conventional mild steel parts, often have the task of joining ferrite-martensitic DP steels with spot welding. During the spot welding of DP steels, when using the technology familiar with mild steels, the risk of hardening, the unfavourable failure of the joint or even cracking during operation must be taken into account, therefore welding technology must be planned on different theoretical bases. The traditional, continuous energy input and the symmetric double pulse as the non-continuous energy input were compared for resistance spot welding, focused on the advantages of pulsed energy input. For numerical analysis an axisymmetric coupled finite element model (FEM) is developed to study the effect of welding time and current intensity on nugget size and thermal history in resistance spot welding process using MSC.Marc software package. The cross-section macrostructures of the welded specimens are examined and compared to the predicted size of the weld nugget and heat affected zone (HAZ). Shear testing, cross tension testing, peel testing and hardness measurements are also performed on specimens for each welding process to examine the effect of the changes in different welding parameters on the load bearing capacity of the joint.