Metallographic and fracture characteristics of resistance spot welded TWIP steels

Abstract

In this study, the microstructural characterisation, mechanical testing and fractography investigation were performed on twinning induced plasticity (TWIP) steels, fabricated with resistance spot welding. Failure mode during the cross-tensile test was found to follow the sequences of strain localisation of both sheets, crack initiation at notch tip, crack following along the fusion boundary and, finally, ductile shear fracture along the sheet thickness direction. On the other hand, failure in the tensile shear test was always directed along the sheet/sheet (s/s) interface; the interfacial failure and shear deformation were observed at the weld centreline. Solidification occurred as a primary austenitic solidification mode, and no martensitic transformations were detected through electron backscatter diffraction analysis. The fusion zone was mainly composed of austenite with directional solidification towards the centreline; the columnar dendritic and equiaxed structures were identified. Interdendritic C and Mn cosegregation were confirmed by electron probe microanalysis.

Properties of Resistance Spot-Welded TWIP Steels

“…The detailed fracture surface of the sample that failed in PIF mode is shown in Figure 4a-d. As seen in Figure 4a,b, the crack initiated at the steel sheet/steel sheet interface and propagated along the weld nugget circumference in PIF failing mode [11]. Figure 4c,d shows the PIF mode with ductile characteristics in the weld nugget periphery.…”

“…The primary cause of weakening of the weldment may be identified as the second phase or precipitate phases in the HAZ, which may weaken the transition zone from weld nugget and HAZ and grain growth occurred in HAZ adjacent to the weld nugget. Transition from PIF to PF mode depends on the physical characteristics, strength of the crack initiation zone, and especially the weld nugget sizes [11,27,28]. In this study, the transition from PIF Transition from PIF to PF mode depends on the physical characteristics, strength of the crack initiation zone, and especially the weld nugget sizes [11,27,28].…”

“…Their study show that there is an almost linear relationship between the tensile shear strength and the weld spot size that can be defined for the samples that failed with an interfacial fracture [10]. Saha et al [11] conducted a study on resistance spot welded high-Mn (18% Mn) TWIP steels with 1.4 mm thickness. They confirm that due to high chemical composition in high-Mn steels; expulsion occurred earlier and high welding current increases the cracking tendency because of increasing nugget pressure and the tensile stress in heat affected zone (HAZ) during cooling [11].…”

“…Transition from PIF to PF mode depends on the physical characteristics, strength of the crack initiation zone, and especially the weld nugget sizes [11,27,28]. In this study, the transition from PIF Transition from PIF to PF mode depends on the physical characteristics, strength of the crack initiation zone, and especially the weld nugget sizes [11,27,28]. In this study, the transition from PIF Transition from PIF to PF mode depends on the physical characteristics, strength of the crack initiation zone, and especially the weld nugget sizes [11,27,28].…”

“…Saha et al [11] conducted a study on resistance spot welded high-Mn (18% Mn) TWIP steels with 1.4 mm thickness. They confirm that due to high chemical composition in high-Mn steels; expulsion occurred earlier and high welding current increases the cracking tendency because of increasing nugget pressure and the tensile stress in heat affected zone (HAZ) during cooling [11]. Deformation behavior of high-manganese TWIP steels has been widely studied in relation to microstructure and texture evolution by microscopy analysis scanning electron microscope (SEM) and transmission electron microscopy (TEM), X-ray diffraction (XRD) measurements [2,[12][13][14][15][16][17][18][19][20][21].…”