Hot-stamped ultra-high strength steels have been widely used in automobile structural\nparts. Considering the high splash tendency in resistance spot welding due to their extremely high\nhardness, in this work, microstructural characteristics and mechanical performance of the resistance\nspot welded ultra-high strength steels are investigated. The results indicate that the interface between\nthe nugget and heat-affected zone (HAZ) is the weakest zone where fractures initiate. In tensile\nshearing tests, a qualified spot welding joint failed with a button-shaped fracture. Welding defects\nwould significantly decrease the load-carrying capacity and lead to interfacial fracture, except for\na button-shaped fracture. In spot welding, it was found that a specific mid-frequency alternating\ncurrent (AC) input mode, in which a 6 ms cooling cycle was inserted between every two neighboring\ncurrent pulses, can avoid the splash in the spot welding of hot-stamped hardened steels.
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