Heat-affected zone liquation crack on resistance spot welded TWIP steels

Abstract:In this study the effect of the weld current on the microstructure and mechanical properties of a resistance spot-welded twinning-induced plasticity (TWIP) steel sheet was investigated using optical microscopy, scanning electron microscopy-electron back-scattered diffraction (SEM-EBSD), microhardness measurements, a tensile shear test and fractography. Higher weld currents promoted the formation of a macro expulsion cavity in the fusion zone. Additionally, higher weld currents led to a higher indentation depth, a wider heat-affected zone (HAZ), coarser grain structure and thicker annealing twins in the HAZ, and a relatively equiaxed dendritic structure in the centre of the fusion zone. The hardness values in the weld zone were lower than that of the base metal. The lowest hardness values were observed in the HAZ. No strong relationship was observed between the hardness values in the weld zone and the weld current. A higher joint strength, tensile deformation and failure energy absorption capacity were obtained with a weld current of 12 kA, a welding time of 300 ms and an electrode force of 3 kN. A complex fracture surface with both brittle and limited ductile manner was observed in the joints, while the base metal exhibited a ductile fracture. Joints with a higher tensile shear load (TSL) commonly exhibited more brittle fracture characteristics.

Section: Methodsmentioning

confidence: 99%

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

Tutar

Aydın

Bayram

2017

“…Therefore, it is necessary to minimize the evaporation of manganese or to use filler metals of higher manganese content for compensating the evaporated amount [13,21]. Also, the M 3 C carbide in the weld zone may cause deterioration of the mechanical properties [16,22,23], so solid-solutionizing may be necessary before deformation processes such as forming, drawing and bending.…”

Section: Introductionmentioning

confidence: 99%

Microstructure and Mechanical Properties of Gas Tungsten Arc Welded High Manganese Steel Sheet

Park

et al. 2019

This study investigated microstructure and mechanical properties of high manganese steel sheet fabricated by gas tungsten arc welding (GTAW). The weld zone showed longitudinal coarse grains due to the coalescence of columnar dendrites grown into the direction of heat source, and the HAZ showed equiaxed coarser grains than the base metal due to the thermal effect of GTAW process. Mn segregation occurred in the inter-dendritic regions of the weld zone and Mn depletion thus occurred in the weld matrix. Although the stacking fault energy is expected to be lowered due to the Mn depletion, no noticeable change in the initial phase and deformation mechanism was found in the weld matrix. Lower hardness and strength were shown in the weld zone than the base metal, which was caused by the coarser grain size. The negative strain rate sensitivity observed in the weld zone and the base metal is considered to have originated from the negative strain rate dependency of twinning nucleation stress.

“…The spot-weld quality is critical to the reliability of the overall automobile as cars typically contain thousands of spot-welds [12][13][14]. Saha et al [15] reported that the TWIP steel was highly susceptible to the liquation crack, and some elements such as C, Mn, and Ti showed strong segregation behavior of TWIP steel after RSW. However, the characteristic of resistance spot-welded AHSSs containing medium levels of Mn has still not been reported.…”

Section: Introductionmentioning

confidence: 99%

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

Jia

Liu

Guo

et al. 2018

Abstract:The medium Mn steels are gaining increasing attention due to their excellent combination of mechanical properties and material cost. A cold-rolled 0.1C5Mn medium Mn steel with a ferrite matrix plus metastable austenite duplex microstructure was resistance spot-welded with various welding currents and times. The nugget size rose with the increase of heat input, but when the welding current exceeded the critical value, the tensile-shear load increased slowly and became unstable due to metal expulsion. The fusion zone exhibited a lath martensite microstructure, and the heat-affected zone was composed of a ferrite/martensite matrix with retained austenite. The volume fraction of retained austenite decreased gradually from the base metal to the fusion zone, while the microhardness presented a reverse varying trend. Interfacial failure occurred along the interface of the steel sheets with lower loading capacity. Sufficient heat input along with serious expulsion brought about high stress concentration around the weld nugget, and the joint failed in partial interfacial mode. Pull-out failure was absent in this study.