REVIEW Critical review of automotive steels spot welding: process, structure and properties M. Pouranvari* 1 and S. P. H. Marashi 2 Spot welding, particularly resistance spot welding (RSW), is a critical joining process in automotive industry. The development of advanced high strength steels for applications in automotive industry is accompanied with a challenge to better understand the physical and mechanical metallurgy of these materials during RSW. The present paper critically reviews the fundamental understanding of structure–properties relationship in automotive steels resistance spot welds. The focus is on the metallurgical characteristics, hardness–microstructure correlation, interfacial to pullout failure mode transition and mechanical performance of steel resistance spot welds under quasi-static, fatigue and impact loading conditions. A brief review of friction stir spot welding, as an alternative to RSW, is also included. Keywords: Resistance spot welding, Automotive steels, AHSS, Failure mode, Microstructure, Mechanical performance Introduction Weldability of a material is one of the key factors governing its application in auto industry. 1 The vehicle components (body in white, cradle, doors, etc.) are made of thin metal sheets that are connected with spot weld- ed joints. Spot welding is a vital joining process for automotive production. 2 Despite the fact that many advanced spot welding technologies are available such as gas tungsten arc spot, laser spot and friction stir spot welding (FSSW) processes, the conventional resistance spot welding (RSW) is the predominant process in sheet metal joining particularly in automotive industry. 3 The most important features of RSW process are high operating speeds and its suitability for automation or robotisation. Resistance welding is a fusion welding process in which heat is generated by the resistance of the parts being welded by the flow of a localised electrical current. Pressure is applied to ensure adequate contact between the parts being welded. The welding current and force are applied to the workpieces via copper alloy electro- des, which are shaped to provide the required current density and pressure at the point of welding. Resistance to current flow through the metal workpieces and their interface generates heat; therefore, temperature rises at the interface of the workpieces. When the melting point of the metal is reached, the metal will begin to fuse and a nugget begins to form and an autogenous fusion weld is made between the workpieces. The current is then switched off and the nugget is cooled down to solidify under pressure via heat conduction through water cool- ed electrodes. 4,5 The physical principal for achieving this heat genera- tion is defined by Joule’s law, which in its simplest form can be expressed by the equation 4 H~RI 2 t (1) where H is the weld heat input in joules, I is the current in amperes, R is the resistance in ohms and t is the time in seconds. The current density and pressure must be such that a weld nugget of the required diameter/size is formed, but not so high that molten metal is expelled from the weld zone (i.e. expulsion). Resistance spot welding involves a complex interaction between the physical and metallurgical properties of the material being welded and the electrical/mechanical phenomena of the process or welding machine. 4 Therefore, to obtain a weld nugget with sufficient size and reliable mechanical properties, a thorough understanding of the process is needed. Typically, there are about 2000–5000 spot welds in a modern vehicle. Automotive structural assemblies use groups of spot welds to transfer load through the struc- ture during a crash. Additionally, spot welds can act as fold initiation sites to manage impact energy. 6 Vehicle crashworthiness, which is defined as the capability of a car structure to provide adequate protection to its passengers against injuries in the event of a crash, largely depends on the integrity and the mechanical performance of the spot welds. 7,8 Moreover, the joint failure, e.g. resistance spot weld joint failure, has been identified as one of the key failure types when vehicle crashes. 9 Failure of spot welds may affect the vehicle’s stiffness and noise, vibration and harshness performance on a global level. 2 Therefore, the quality, performance and the failure characteristics of resistance spot welds are important for determination of durability and safety design of the vehicles. This is why there is a great deal of much attention on design- ing, monitoring and controlling systems for RSW. 10–15 1 Materials and Metallurgical Engineering Department, Dezful Branch, Islamic Azad University, Dezful, Iran 2 Mining and Metallurgical Engineering Department, Amirkabir University of Technology, Tehran, Iran *Corresponding author, email mpouranvari@yahoo.com ß 2013 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 25 January 2013; accepted 16 March 2013 DOI 10.1179/1362171813Y.0000000120 Science and Technology of Welding and Joining 2013 VOL 18 NO 5 361