A Comparative Study on Fatigue Performance of DP590 and DP780 Spot Welded and Weld Bonded Steel Sheets

The automotive companies are quickly changing their material bill from low strength formable grades to advanced high strength steels (AHSS) in response to the global call for preserving a green environment and enhanced passenger safety. As compared to low carbon formable steels, the AHSS steels are relatively difficult to weld by the traditional processes and the joint performance is below expected levels. Thereby conventional manufacturing processes are being challenged, and new processes with improved service performance are being increasingly explored. Fatigue performance of material joints is a key design input and indicator of the performance of automobiles during service, particularly for AHSS. In this paper, primary joining process such as spot welding and emerging hybrid joining process such as weld-bonding were carried out on two commercially available AHS dual phase (DP590, DP780) automotive steels. Two dimensional spot welding lobes were developed for each of the steels following nugget diameter criteria of 4 √t to 5.5 √t, where t is the thickness of the sheets. By selecting two sets of parameters from the lobes, spot welding was carried out on shear-tension sheet samples with an AC type resistance spot welding machine. Weld-bonding was done by applying a thin layer of high strength epoxy based structural adhesive followed by spot welding on two overlapping sheet samples. Static shear tensile and fatigue properties under tension-tension mode for all the spot welded and weld-bonded joints of DP590 and DP780 steels were evaluated and compared.