Transmission Electron Microscopy Studies of Plasma Arc-Welded DP600 Dual-Phase Steel in Keyhole Mode
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NTRODUCTION
TO minimize vehicle weight, enhance fuel efficiency, and lower CO2 emissions,[1] the use of thinner gauge advanced high-strength steels (AHSS) is imperative for the automobile industry. Dual-phase (DP) steels with a soft ferrite (a) matrix and dispersed martensite (a¢) islands offer high tensile strength and good ductility, which meet the safety standards of the automobile industry.[1,2] AHSS face issues during resistance spot welding such as weld discontinuity, inhomogeneities, stress concentration factors, etc. These issues lead to liquation cracking, early expulsion, indentation of electrodes, and surface cracks.[3] Ashiri et al.[4,5] studied welding issues during RSW of twinning-induced plasticity (TWIP) steels such as liquid metal embrittlement (LME) cracking. They reported that LME occurs because of the simultaneous presence of the high-temperature, high enough tensile stress, and susceptible alloy with the presence of liquid zinc at the workpiece surface, which is in contact with electrodes. They
AMIT A. KURIL, M. JAGANNATHAM, G.D. JANAKI RAM, and SRINIVASA R. BAKSHI are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai, Tamil Nadu 600036, India. Contact e-mail: [email protected] Manuscript submitted April 12, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
showed that avoiding the effect of one or more components, such as by using impulse welding, gradual heating to bring down the temperature and have less liquid zinc availability, and no tensile stress, helps to suppress the LME phenomenon. Ahsan et al.[6] studied the porosity formation mechanisms in zinc-coated galvannealed steel of 2.3-mm-thick sheets with CMT-GMA welding in a lap joint configuration. The study was carried out for low heat input (< 250 J mm 1), medium heat input (250 to 350 J mm 1), and high heat input (> 350 J mm 1). They observed that low heat input resulted in less zinc evaporation, while high heat input resulted in the formation and escape of zinc vapor bubbles and resulted in low porosity formation in the weld metal. Implementing DP steels in practical automobile applications is a challenge as hardness reduction occurs with respect to the base metal (BM) during welding.[7] Ashrafi et al.[8] observed that after gas tungsten arc welding (GTAW) of 2.0-mm-thick DP700 steel there was an increase in the hardness of the fusion zone (FZ) due to formation of bainite, whereas martensite tempering in the sub-critical heat-affected zone led to a reduction in the hardness value below the BM. Ahiale et al.[9] performed and analyzed joining of 2.0-mm-thick DP590 steel in lap joint configuration using gas metal arc welding (GMAW) and plasma arc welding (PAW). They observed martensite, allotriomorphic, Widmansta¨tten, and acicular ferrite in the FZ, which led to an increase in hardness compared with BM. Softening
in the heat-affected zone (HAZ) had a negative influence on the formability and joint efficiency. Sreenivasan et al.[10] showed the detrimental effect of laser welding using
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