Defect mode determination of dual phase DP800 steel sheets joint by electrical resistance spot welding

Use of high-strength steels in automotive industry has become pretty widespread in recent years. Among these, dual phase (DP) steels come into prominence in terms of high-strength and forming ability. In automotive industry, decrease of costs and weight have vital importance, therefore ultra high strengtehed and having high toughness steels are required. In 1990s, niobium (Nb) was added to steels as an alloying element to strengten them up to 400 grades. However, the needs of customers have not been provided accuretely, so new steels having easily formable, and strain hardenable were developed. In 2000s, the dual-phase steels have been started to use in many areas. The main phase in this type of steel is ferrite which was specifically heat-treated in order to form easily, and the second phase is martensite to strengthen the whole structure. In this study, Dual-Phase (DP 800 type) steel sheets having 1.0 mm thickness were welded by electrical resistance spot welding method. Welded joints were prepared in 5, 10, 15, 20, 25 and 30 cycles welding times, and 6 kA, 7 kA, 8,2 kA, 9,6 kA, 11,1 kA, 12,5 kA, 14 kA, 15,5 kA, 16,5 kA weld currents. The tensile-peel and tensile-shear tests of these joints were performed and microstructural properties of them were investigated by SEM (Scanning Electron Microscope) and optical microscope and EDS (Energy Dispersive Analysis) were done. Finally, the optimum welding parameters (i.e. Weld Lobe) were advised to users.