Published by Maney Publishing (c) IOM Communications Ltd ORIGINAL RESEARCH PAPER Resistance welding of thin stainless steel sandwich sheets with fibrous metallic cores: experimental and numerical studies J. C. Tan* 1 , S. A. Westgate 2 and T. W. Clyne 1 This paper concerns resistance spot welding (RSW) of two types of thin stainless steel sandwich sheet. The cores of these materials, made of stainless steel fibres, are highly porous (. around 85 vol.-%) and have low thermal and electrical conductivities. However, these conductivities change during the compression and heating associated with RSW. A sequentially coupled finite element model has been developed, in which the crushed core is treated as a continuum, with properties which vary throughout the process. It is shown that a constitutive relationship of the type commonly used for crushable foams can be successfully employed to simulate the deformation of the sandwich sheets. The thermoelectrical part of the model incorporates the effects of the associated phase transformations and changes in interfacial conductance. It is shown that the predictions are broadly consistent with data obtained during welding experiments. The model is used to explore the effects of welding parameters on weld characteristics (weld pool formation and weld nugget shape). Keywords: Stainless steel, Steel fibres, Sandwich sheets, Lightweight materials, Resistance spot welding, Finite element model Introduction While sandwich panel technology has been commer- cially available since the late 1950s, it is rarely used in high volume applications, such as those in the auto- motive industry. 1 In order for this to be done, sandwich sheets are required that can be handled and processed in a similar manner to conventional monolithic metallic sheet. This requires that the material be relatively thin (around 1–2 mm), formable into complex shapes and suitable for joining by resistance spot welding (RSW). Resistance spot welding is widely used for sheet metal fabrication, particularly for automotive body in white (BIW) assemblies. Typically, there are 3000 to 4000 spot welds in a typical passenger vehicle. 2,3 The process is fast, cost effective, well suited to automation 4 and capable of producing high quality welds. 5 Previous work on RSW has covered numerical modelling of heat and current flow, 6–8 phase change and nugget growth predictions 9–12 and thermoelastic– plastic analyses. 13,14 However, these studies were all focused on monolithic sheets, most commonly steel or aluminium, and there has been very little previous work on resistance welding of sandwich materials incorporat- ing a fibrous metallic core. 15 There have been studies on the resistance welding of vibration-damped steel sheet (VDSS), which consists of two mild steel faceplates (0?3– 1 mm), separated by a fully dense resin core (20– 500 mm). The VDSS core can be made electrically conductive by using a resin containing iron, nickel or graphite particles. 16–18 Unlike monolithic sheets and VDSS, the lightweight sandwich sheets of interest here have collapsible cores with relatively low thermal and electrical conductivities. A preliminary investigation of the resistance weldabil- ity of these sandwich sheets was previously reported by Markaki et al. 15 It was found that the through thickness electrical resistivity is highly significant. The brazed sandwich sheet material was reported to be readily weldable. However, the (adhesively bonded) flocked sheet could not be welded directly, due to its high core resistivity. A weld could only be made by using a shunt – creating a bypass circuit, allowing current to flow through the faceplates without passing through the core, leading to heating and softening of the adhesive at the faceplate to core interface. This reduced the core resistance, subsequently allowing a direct current to flow through the core, generating sufficient heat to initiate melting. Although welding was thus possible, the weld nuggets were of poor quality. The faceplates were susceptible to local burn-through due to the high shunt current, resulting in cracking and melt expulsion. The blow holes formed within the weld nugget were probably caused by vaporisation of the adhesive. Similar problems have also been reported in resistance welding of VDSS. 17 This paper investigates the resistance spot welding characteristics of a novel thin sandwich material 1 Department of Materials Science and Metallurgy, Cambridge University, Pembroke Street, Cambridge CB2 3QZ, UK 2 The Welding Institute (TWI), Great Abington Cambridge CB1 6AL, UK *Corresponding author, email jct33@cam.ac.uk ß 2007 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 6 November 2006; accepted 5 March 2007 DOI 10.1179/174329307X213666 Science and Technology of Welding and Joining 2007 VOL 12 NO 6 490