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Metals 2017, 7(1), 14; https://doi.org/10.3390/met7010014
2. Materials and Methods
3. Results and Discussions
3.1. Fracture Characteristics of Sample
3.2. Effect of Welding Parameters on Weld Nugget Size, Strength, and Hardness
3.3. Microstructure Evaluation of Weldment
- The tensile shear strength of welded samples increased up to 7 kA welding current for 20 cycles of welding time. The results indicate that, except for 3 kA and 5 kA welding current for 20 cycles of welding time and 7 kA welding current for five cycles of welding time, all of the combinations of the welding process parameters in this study provide acceptable tensile strength for the automotive industry. Over the critical heat input level (greater than 25 cycles of welding time for 7 kA welding current) the strength of the welded sample decreases due to expulsion or decreasing in cross-section thickness of the nugget.
- The failure in the tensile shear test sample occurred in PIF mode for lower welding parameters (3 kA welding current, up to 15 cycles of welding time). PIF mode was present in the ductile characteristics in the weld nugget. Over these welding parameters (except expulsion) PF mode was started by a crack in HAZ and then the crack propagation occurred by tearing from the sheet. It is thought that the primary cause of weakening in HAZ could be the grain growth mechanism.
- The fully austenitic solidification present in the weld nugget was due to a high amount of manganese and a low amount of carbon in chemical composition. Since the fusion zone microstructure has been fully austenitic, the weld thermal cycle has not changed the structure. The ferritic or martensitic transformation has not been detected through metallographic investigation. The formed particles observed in transition zone from the weld nugget to HAZ, at which point EDS analysis was carried out. Results indicate that this formed particle contains high amounts of Al and Mn that can be cause the formation of second-phase particles.
- The hardness of the weld nugget and HAZ were found to be lower than those of the base metal due to the nature of the weld thermal cycles, the chemical composition of TWIP steel, and grain coarsening. The hardness results also indicate that the high strength caused by twinning-induced plasticity is almost lost due to the weld thermal cycle.
- Due to electrode force, high heat input, or any other parameters that produce excessive weld heat, the shrinkage cavities and cracks were observed in the fusion zone of the weldment. It is believed that the cracks at the periphery of a weld nugget where the load stresses are highly concentrated were formed due to the solidification mode, interdendritic aluminum segregation, high electrode pressure, and residual stress in the weld nugget.
- In conclusion, the optimum welding parameters that guarantee acceptable tensile shear strength and fracture mode (PF) for the automotive industry were obtained at 7 kA welding current for 20 and 25 cycles of welding time in the examined range. The results indicate that the acceptable welding parameter area is very narrow for resistance spot-welded TWIP steels, because of cracks and cavities in the weld nugget, surface cracks in the HAZ that causes PIF mode, unacceptable weld nugget geometry in adequate welding parameters because of low heat input, and high metal expulsion reducing the partial thickness due to high heat input.
Conflicts of Interest
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|Welding Current (kA)||Electrode Force (kN)||Weld Time (Cycle)||Holding Time (Cycle)||Squeeze Time (Cycle)||Clamping Time (Cycle)|
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