Experimental modelling of resistance spot welding of advanced high strength steels

The use of advanced high strength steels (AHSS) is becoming more dominant in the automotive industry due to the excellent balance of strength and lightness of these steels. Although the use of AHSS metals is increasing, there are not enough studies in the scientific literature on resistance spot welding, which is the basic joining method used in automobile production, of AHSS metals each other or to other metals. In this study, MS1500, an AHSS metal, dual-phase steel (DP800) and unalloyed mild steel (DD11) are used. The welding joints created with using single and double pulse resistance spot welding with variable electric currents, which are the results of joining the MS1500-DD11 and MS1500-DP800 metal pairs, respectively, have been investigated from many different aspects by experimental modeling. In resistance spot welding applications where AHSS metals are used, due to the high alloying contents of these metals, it has been observed that interfacial failure occurs, which is absolutely undesirable in the automotive industry, as a failure mode up to the critical current value that will cause expulsion as a result of single pulse. While the first pulse increases the contact area between metals, the second pulse ensures that the weld nugget area of the joint, which is formed after the first pulse, is re-melted. By this way, it is observed that, metals in the nugget zone becomes annealed and nugget size increases, it enables to change the failure modes to the desired mode, pullout failure. When the experimental results such as tensile-shear strength, failure energy, and failure mode analysis obtained after the tensile-shear tests are examined, both the strength of the joints increased and the deviations in the results decreased with the second current, which was gradually increased, following a first current applied just below the expulsion limit in the first pulse. This trend continues until appearance of expulsion because of the second pulse electric current. The optimum welding parameters are determined by evaluating the strength values corresponding to different welding parameters obtained as a result of the experiments, the change of failure modes, the geometric measurements of the weld nugget area and the hardness measurements along the nugget.