Determination of Weldability Fields for Dual-Phase 600 Steel in AC and MFDC RSW

The automotive industry faces nowadays a challenge of choosing between two types of current for resistance spot welding equipments: AC (alternating current) and MFDC (medium-frequency direct current). The first technology (AC equipment) is the traditional one and has been successfully applied to accomplish weldments in low-carbon steel sheets, with relatively low cost. On the other hand; the new technology (MFDC equipment) has been propagated by the manufactures as an evolution of the resistance spot welding, where higher productivity, lower energy consumption, better parameters controlling and lower thermal-mechanical stress. Besides these claimed qualities, from the construction point of view, the MFDC equipment demands smaller transformers, since its efficiency is proportional to the input frequency, i.e., the AC equipment uses the 60-Hz (in the Brazilian case) from the power lines, whereas the MFDC ones goes for 1000 Hz, increasing its efficiency and reducing its size. Despite these commercial claims of the MFDC-equipment manufacturers, few scientific results have been published with a thorough methodology of comparison, especially for the recently introduced dual-phase steels. Therefore, this work is aimed to provide the comparison between MFDC and AC resistance spot welding equipment by using an appropriated methodology based on the equivalent size of the obtained spot. It is expected the results lead to further discussion between the use of these two equipments.