Investigation on Resistance Spot Welding of TWIP Steel Sheets

Abstract

A widespread usage of new advanced high strength TWIP steels in the automotive industry is conditional on proper assembly operations by welding. Therefore, the effects of the main important process parameters on the mechanical and microstructural properties of resistance spot welded TWIP sheets are examined here. The welding parameters used were the welding current, clamping force, and welding time. An L‐9(33) orthogonally array was used to design the experimental tests. The mechanical response of the welded joints was assessed by tensile shear tests and Vickers microhardness measurements. The effects of the welding parameters on the fracture mode of the tensile shear specimens were also investigated. Some combinations of the welding process parameters guarantee acceptable tensile shear strengths for the automotive industry. The tensile shear samples mainly failed by interfacial fracture mode, while partial thickness with pull out fractures were observed in the samples when a high welding current and clamping force were used. Improper clamping force and welding current promoted excessive metal expulsions and, in turn, the formation of welding defects in the weld spots.

Effect of Weld Current on the Microstructure and Mechanical Properties of a Resistance Spot-Welded TWIP Steel Sheet

“…Yu et al [16] identified the RSW characteristics of 1 GPa grade TWIP steel. Spena et al [17] investigated the effects of the process parameters on the mechanical and microstructural properties of resistance spot-welded TWIP sheets and reported that an improper clamping force and weld current promoted excessive metal expulsions and the formation of welding defects in the weld spots. Yu et al [18] considered improvements to the weldability of 1-GPa grade TWIP steel, and found that a larger nugget size (NS) and higher tensile shear strength were obtained in constant power-control welding compared to constant current-control welding.…”

Microstructure and Tensile-Shear Properties of Resistance Spot-Welded Medium Mn Steel

“…Russo Spena et al [24] reported that spot-welded 1.5 mm thickness TWIP steel joint failed in IF or PIF mode, while Saha et al [25] found that the 1.4 mm thickness TWIP steel joint always failed in IF mode.…”

“…For instance, Zeytin et al [21] found a larger nugget diameter than the recommended value 4√h was needed to obtain the PF mode in terms of 1.0 mm thickness TWIP steel. Russo Spena et al [24] reported that spot-welded 1.5 mm thickness TWIP steel joint failed in IF or PIF mode, while Saha et al [25] found that the 1.4 mm thickness TWIP steel joint always failed in IF mode. Typical load-displacement curves of RSW joints under various welding current with constant welding time (20 cyc) are illustrated in Figure 12.…”

“…Thus, the joint that welded at 14 kA, 20 cyc and failed in PIF mode reached higher tensile-shear strength in this study. Russo Spena et al [24] concluded that the high stress concentration around the weld nugget led to the PIF mode. In general, the transition of failure mode from IF to PF mainly depends on the nugget size and the strength of the BM and HAZ for expulsion-free spot-welds.…”

Microstructure Evolution and Failure Modes of a Resistance Spot Welded TWIP Steel

“…They reported a spot weld fusion zone (FZ) harder than the heat affected zone (HAZ), which facilitates the occurrence of ductile pullout failure (PF) mode, differing from the observed by Saha et al [6]. Russo Spena et al [10] and Kazdal Zeytin et al [11] on the other hand, observed softening in both HAZ and FZ when compared to the BM, indicating that softening mechanisms take part in the HAZ. The cited works, however, agree in that there is a distribution of mechanical properties throughout the weldment and such distribution in sensible to the welding parameters.…”