A Phenomenological Study of Weld Discontinuities and Defects in Resistance Spot Welding of Advanced High Strength TRIP S
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INTRODUCTION
SORTING the conventional steels in a curve presenting their fracture elongation vs strength results in a banana shape loop.[1] This means that the conventional steels do not exhibit high strength and high ductility (and thus high toughness), simultaneously.[1] Automakers require strong and tough steels in order to manufacture environmentally friendly and crash-resistant vehicles.[2,3] In reaction to this demand, steelmakers have done significant efforts in the last decades and developed the advanced high strength automotive steels (AHSASs).[1,4,5] Development of the dual phase (DP) steels,[6] twinning-induced plasticity (TWIP) steels,[7–9] transformation-induced plasticity (TRIP) steels,[10,11] low-density steels,[8] and high modulus steels[12,13] is the consequence of those efforts.[1] Most of these steels do not fall into the banana shape loop of the conventional steels; they exhibit strength and toughness ROUHOLAH ASHIRI is with the Department of Materials Science and Engineering, Dezful Branch, Islamic Azad University, P.O. Box 313, Dezful, Iran. Contact e-mail: [email protected] HOSSEIN MOSTAAN is with the Department of Materials and Metallurgical Engineering, Faculty of Engineering, Arak University, Arak, Iran. YEONG-DO PARK is with the Department of Advanced Materials and Industrial Management Engineering, Dong-Eui University, Busan, Republic of Korea. Manuscript submitted April 1, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
synergy.[2,3] TRIP-assisted steels[10,11] benefit from the mechanically induced transformation mechanism to synergy strength, ductility, and thus toughness; as the result, they are good candidates for manufacturing the autobody.[1,14,15] Alloy design and thermomechanical processing of TRIP steel are performed in such ways that enhance the mechanical performance of the alloy; however, a few studies[14,15] considered the welding problems[16] of the alloy. Since the automotive steels are designed to be used in the autobody and resistance spot welding (RSW) process is the most widely used joining process in the automotive industry,[2,3] the assessment of weldability of AHSASs in RSW process is of significant importance. Especially this assessment for TRIP-assisted steels is critically essential due to its great potential of application in the autobody.[1,14,15] On the other hand, most recent studies focused on the relation between microstructure and mechanical performance of TRIP steels[17]; studies on the impact of weld defects and discontinuities on the mechanical performance of the welds are relatively scarce. There are some researches in the literature aiming at studying the susceptibility of the welds of Mg[18] and Al[19] alloys as well as DP steels[20–24] to defect formation; however, the studies devoted to TRIP steels are lacking. As another drawback of the existing background, the works have not presented in-depth discussions on the matter and have not discussed the cause of the limited weldability of AHSASs and their susceptibility to the defect formation.
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