On the Failure Behavior of Highly Cold Worked Low Carbon Steel Resistance Spot Welds
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RODUCTION
CONSTRAINED groove pressing (CGP) is wellknown severe plastic deformation (SPD) process for production of nano-structured (NS) or ultra-fine grained (UFG) sheet metals.[1–5] Microstructure evolution and mechanical properties improvement of low carbon steel sheets through CGP process were investigated by present authors.[6,7] They had shown that, CGP at higher passes (three or four) can effectively refine the microstructure of sheets into 200 to 300 nm and enhance the tensile strength of sheets about two times. These CGPed low carbon steel sheets in comparison with the as-received one not only exhibit superior strength to weight ration, but also suffer from a great loss in formability. Properties of these SPDed sheets are very susceptible to heating during additional operations. Their annealing responses were also investigated by present authors[8] and it was shown that with a short time annealing at low temperatures about 673 K (400 °C), structure of CGPed low carbon steel sheets is thermally stabilized, their strength is not reduced so much, but their elongation is increased considerably. Due to limitation in size and shape of processed sheets F. KHODABAKHSHI, Ph.D. Student, M. KAZEMINEZHAD, Associate Professor, and A.H. KOKABI, Professor, are with the Department of Materials Science and Engineering, Sharif University of Technology, Azadi Avenue, P.O. Box 11155-9466, Tehran, Iran. Contact e-mail: [email protected] Manuscript submitted March 14, 2013. Article published online October 30, 2013 1376—VOLUME 45A, MARCH 2014
through CGP, for practical applications, joining of these sheets is essential. Resistance spot welding (RSW) is the dominant process for joining of sheet metals in automotive industry. Typically, there are about 2000 to 5000 spot welds in the modern vehicles. Simplicity, low cost, high speed (low process time), and automation possibility are advantages of this process.[9–13] The variations in mechanical properties and microstructure of CGPed steel sheets as a result of joining via RSW process were examined in previous works by present researchers.[14,15] The objective of this research is focused on the failure behavior of the highly cold worked (HCW) low carbon steel after RSW using the tensile-shear (TS) test in comparison to the as-received low carbon steel samples. Failure behavior includes some concepts such as; failure peak load (FPL), failure absorbed energy (FAE), failure mode, fracture surface, etc. Failure mode is the manner in which spot weld fails. Failure mode of spot welds is a qualitative measure of the weld reliability and very important in indicating the performance of spot welds.[9–11,16–24] Generally, resistance spot welds failed in three modes; interfacial failure (IF) mode, pullout failure (PF) mode, and tearing failure (TF) mode. In the IF mode, failure occurs via crack propagation through the fusion zone from joint interface. In the PF mode, crack propagates through base metal (BM) or heat affected zone (HAZ) and failure occurs via complete (or partial) nugget withdrawal
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