Abstract
Fracture modes of resistance spot welded ultra-high strength hot-stamped boron steel via lap-shear test are different from that of the traditional advanced high strength steel due to the difference in geometrical size and material property of the spot welds. In this paper, lap-shear fracture modes of resistance spot welding joints were analyzed and joint characteristics that affecting the fracture behavior were discussed. Three fracture modes were found to change from interfacial fracture (IF) to pull-out fracture (PF) with the increase of nugget diameter. For PF I mode, the fracture initiated at the transition zone between the fusion zone and upper-critical heat affected zone (HAZ) and propagated along the thickness of the nugget. For PF II mode, during which the failure initiated at the sub-critical HAZ where the softest zone occurred, and it propagated to the base material. Obvious hardness decrease was observed in the transition zone with the formation of the delta ferrite at the fusion boundary due to the relatively high amount of alloying element in the hot-stamped boron steel, which could provide the reason for route of PF I extending along this zone. Fluctuation in the hardness in the transition zone led to the existence of both PF I and PF II at the same welding current.
Highlights
With great potential to reduce the weight, ultra-high strength hot-stamped boron steel has been increasingly used in body-in-white [1]
When the nugget diameter was small, the nugget was the weakest position in the joint, and the fracture happened at the interface of the nugget, and this fracture mode is called interfacial fracture (IF)
According to the hardness distribution, the joint could be divided into base metal zone (BM), Heat (HAZ), and fusion zone and (FZ).Fusion
Summary
With great potential to reduce the weight, ultra-high strength hot-stamped boron steel has been increasingly used in body-in-white [1]. Pouranvari et al [8] investigated the failure mode transition from interfacial to pullout mode during tensile-shear, and found that fusion zone size and hardness characteristics were key factors controlling the failure mode of AISI 304 resistance spot welds. Emre et al [11] evaluated the tensile shear strength and failure mode of TRIP800 associated with nugget geometry and electrode indentations, and three distinct failure modes were observed with the difference in fracture route. These researches provided a guide for analyzing the fracture mode of B1500 and revealing the mechanism for various fracture modes. This paper provides a guide to choose the proper welding parameters for guaranteeing the weld quality of hot-stamped boron steel
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