Journal of Materials Processing Technology 246 (2017) 252–261 Contents lists available at ScienceDirect Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec Resistance spot welding of galvannealed high strength interstitial free steel Shravan Singh Rao a , Rahul Chhibber a,∗ , Kanwer Singh Arora b , Mahadev Shome b a Department of Mechanical Engineering, Indian Institute of Technology, Jodhpur, India b R&D, Tata Steel Limited, Jamshedpur, India a r t i c l e i n f o Article history: Received 11 November 2016 Received in revised form 24 March 2017 Accepted 25 March 2017 Available online 28 March 2017 Keywords: Dynamic contact resistance Nugget formation Resistance spot welding Galvannealed steel Failure mode a b s t r a c t Variation in dynamic contact resistance with the change in welding process parameters such as weld cur- rent, weld time and electrode force were taken into account for establishing the range of adequate nugget formation parameters. Influence of the welding process parameters on the shear – tensile strength, nugget diameter and the observed failure mode was analysed. The adequate resistance spot welding process parameters for galvannealed high strength interstitial free steel sheets of 1.6 mm thickness were esti- mated as 8 kA weld current, 250 ms weld time and 3.5 kN electrode force. Increase in the mean dynamic contact resistance led to a significant reduction in nugget diameter. A critical nugget diameter distin- guishing between the IF and PF mode was experimentally determined by failure mode analysis. Different numerical models for estimation of critical nugget diameter were evaluated. © 2017 Elsevier B.V. All rights reserved. 1. Introduction Resistance spot welding (RSW) is widely used in automotive Industries. It is comparatively a clean process as it does not involve any filler material. The joint is created with the application of pres- sure and heat. In the case of RSW, the flow of electric current causes heating. This heating further leads to an occurrence of localized melting and coalescence of a small volume of the material. This localized heat input is estimated as a product of squared value of weld current times the electrical resistance of material to be welded. The electrical contact resistance of the material plays an important role in the nugget formation during spot welding. It is dif- ficult to monitor the nugget formation as the nugget is not directly exposed and exists between the electrodes. Aktas et al. (2012) used the RSW process to join DP600 steel sheets of different thick- nesses and compared the strength of the joints under shear – tensile and tensile – peel loading and also studied the microstructure and hardness variations in the weld specimens. Tsai et al. (1992) used finite element methods and found the weld nugget to initiate in a ring shape at a certain distance from the electrode centre which expands both inward and outward during the welding process to form the nugget. For analysing the spot welding process, a system ∗ Corresponding author at: Mechanical Engineering Department, IIT Jodhpur, Jodhpur 342001, India. E-mail addresses: rahul chhibber@iitj.ac.in, rahulchh@gmail.com (R. Chhibber). is needed which can evaluate the process dynamically and moni- tor the different process parameters. Dickinson et al. (1980) built a dynamic electrical monitoring system to non – discretely monitor the current, voltage, resistance and power during RSW and related the dynamic resistance curves obtained to the nugget formation phenomenon during the spot welding process. Luo et al. (2016) monitored the change in welding current and electrode voltage in real time in the secondary circuit. Cho and Cho (1985) presented an analytical model to predict the resistance variation during the resistance spot welding and studied the complex phenomenon of thermo – electric interaction at the weld interface to analyse both temperature and voltage distributions in the weldment. Cho and Rhee (2003) analysed the effect of weld thickness, corona bond and resistivity on the dynamic contact resistance and observed the nugget formation phenomenon using high speed camera. Shome (2009) has analysed and compared the dynamic contact resistance (DCR) of DP590 and DP780 steel weld joints. Kianersi et al. (2014) studied the optimization of RSW welding parameters for joining of austenitic stainless steel sheets and defined the transition region between interfacial and failure modes as the optimum welding condition. Gedeon and Eager (1986a,b) studied the material varia- tions and process modifications in order to determine their effects on the adequate range of spot welding parameters for galvanized steel sheet and performed dynamic inspection monitoring of weld- ing parameters on resistance spot welding of uncoated and coated steel sheet. Tumuluru (2007) in his study of Dual Phase (DP) steel found that welding current range for galvannealed steel is wider http://dx.doi.org/10.1016/j.jmatprotec.2017.03.027 0924-0136/© 2017 Elsevier B.V. All rights reserved.