Journal

 Critical review of automotive steels spot

welding: process, structure and properties.

Abstract

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 atthe interface of the workpieces. When the melting pointof the metal is reached, the metal will begin to fuse and anugget 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 formcan be expressed by the equation 4H~RI 2t.

Acknowledgements

The authors would like to thank Professor Harry Bhadeshia for providing the opportunity to prepare a critical review on the published data on the metallurgical and mechanical characteristics of automotive steels spot welds. We would also like to thank all the undergradu-ate and postgraduate students who have patiently and carefully conducted a number of projects at Amirkabir Unviersity of Technology, over the years including Hamidreza Asgari, Dr Seyed Mostafa Mouavizadeh Noughabi, Amir Afrouz Daghoughi, Tara Mansouri,Sasan Amirabdollahian, Pooria Movahed, Sina Kolah-gar, Mohammad Sanaee, Dorsasadat Safanama, Farzaneh Alimohammadi, Masoud Alizadeh Shamsabadi, Faryal Hakami, Nina Asadi Moghadas, Samaneh Bayrampour, Maryam Masoumi, Farnoush Falsafi, Ayda Shahryari,Nooshin Mortazavi, Maryam Bazyaran, Mina Salehi, AliTavasolizadeh and Sara Abouzari. Special thanks also go to Novinsazan Setareh Sanat for providing the RSW equipment and Iran University of Science and Technology and Dr Masoud Goodarzi for accommodating the resistance spot welding equipment and their kind coopera-tion for using it.

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