Avaliação da soldabilidade a ponto por resistência de AHSS eletrogalvanizados da classe de 1000 MPa de limite de resistência

Abstract

In this study, the effect of resistance spot welding on Dual Phase (DP) and Complex Phase (CP) steels, with a tensile strength class of 1000 MPa, electro-galvanized, using multiple current pulses, was investigated, focusing on improving the mechanical properties of welded components. Welding conditions comprising two current pulses, different in magnitude and/or duration, or three equal pulses were applied. The mechanical strength of the welded joints was evaluated by cross-tension and shear tests, and fatigue life by shear, in comparison to joints welded under the conventional condition (a single current pulse), according to the ISO 18278-2:2016 standard. The DP steel welding with double pulse conditions provided increases in the maximum loads obtained by cross-tension and shear by up to 106% and 45%, respectively. In terms of toughness, increases of up to 496% and 156% were obtained under cross-tension and shear, respectively. These results were attributed to the microstructural refinement of the matrix and a higher density of high-angle grain boundaries in the weld region. Conversely, for CP steel, the multiple pulse conditions resulted in decreases of up to 25% and 17% in the maximum loads obtained by cross-tension and shear, respectively. Regarding toughness, the decreases were 30% and 38% under cross-tension and shear, respectively. These drops were attributed to significant softening of the heat-affected zone (HAZ). The fatigue life of the welded joints from both steels was not significantly influenced by the welding parameters, a situation associated with the joint geometry, which remained unchanged regardless of the welding condition applied. The results showed that welding techniques with more than one current pulse could favor the resistance spot weldability of advanced high-strength steels (AHSS), but this favorability depends on the material to be welded. Among the factors to be considered for the definition or not of using multiple pulses, one can cite the chemical composition and microstructure of the material, besides the undesired increase in welding time. Regarding this, the gains from the increased toughness tend to be a determining factor, as these steels are applied in safety components, many of them with significant responsibility for energy absorption upon impact.