Comparative effects of Al-Si and galvannealed coatings on the properties of resistance spot welded hot stamping steel joints

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

Section: Introductionmentioning

confidence: 99%

Microstructure and Properties of Spot Welded Joints of Hot-Stamped Ultra-High Strength Steel Used for Automotive Body Structures

Qiao

Lian-jin

et al. 2019

“…Subsequently, the attempt to use lubricant oils was abandoned due to complex cleaning procedures and a high risk for hydrogen-induced embrittlement [15]. Currently, various types of coatings, including aluminized coating [16][17][18][19][20], galvanized and galvannealed coatings [21][22][23], Zn-Ni alloy coating [24,25], sol-gel hybrid coating [26] and Al-Zn alloy coating [27], were developed industrially. Among which, Type 1 aluminized coating, Al-Si with a near eutectic composition, has been widely applied on high-strength boron steel sheets [28].…”

Section: Introductionmentioning

confidence: 99%

Enhancement of Fatigue Endurance by Al-Si Coating in Hot-Stamping Boron Steel Sheet

Tan

et al. 2019

Most structural components undertake cyclic loads in engineering and failures always cause catastrophic economic losses and casualties. In the present work, the phase evolution of Al-Si coating of high-strength boron steel during hot stamping was investigated. Two types of 1500 MPa grade boron steel sheets, one with Al-Si coating and the other without, were studied to reveal the effect on the high-cycle fatigue behavior. The as-received continuously hot-dip Al-Si coating was composed of α(Al), eutectic Al-Si and τ5. After hot stamping at 1193 K, three phases formed in this coating: β2, Fe(Al,Si)2 and α(Fe). The experimental results showed that the endurance limit of the coated steel sheet was 370 MPa under 107 fully reversed tension-compression loading cycles as opposed to 305 MPa in the uncoated sheet. Both the coated and the uncoated specimens showed surface-induced transgranular fatigue fractures. In the uncoated sheet, the fatigue cracks were generated from the decarburization surface, but the Al-Si coating effectively prevented the occurrence of near-surface decarburization during high-temperature hot stamping, and the only cracks in the coated steel sheet were initiated at wire-cutting surfaces.

“…Resistance spot welding (RSW) is the most widely used method to join lapped steel because of its stable quality and low cost. The lap-shear test is usually used to evaluate the strength of the spot welded joint [2][3][4]. As fracture mode can significantly affect maximum lap-shear force and energy absorption capability of the resistance spot welds [5][6][7], it is vital to determine the fracture mode of the spot welded joint and identify the key influencing factors.…”

Section: Introductionmentioning

confidence: 99%

Analysis of Fracture Modes of Resistance Spot Welded Hot-Stamped Boron Steel

Zhao

Zhang

Lai

2018

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.