Effect of weld nugget size on overload failure mode of resistance spot welds M. Pouranvari 1 , H. R. Asgari 2 , S. M. Mosavizadch 3 , P. H. Marashi* 4 and M. Goodarzi 5 In the present paper, effects of welding current, welding time, electrode pressure and holding time on the weld nugget size were studied. A failure mechanism was proposed to describe both interfacial and pullout failure modes. This mechanism was confirmed by SEM investigations. In the light of this mechanism, the effect of welding parameters on static weld strength and failure mode was studied. Then, an analytical model was proposed to predict failure mode and to estimate minimum nugget diameter (critical diameter) to ensure pullout failure mode in shear tensile test. On the contrary to existing industrial standards, in this model, critical nugget diameter is attributed to metallurgical characterisation of material (weld nugget hardness to failure location hardness ratio), in addition to sheet thickness. For a given sheet thickness, decreasing H WN H FL increases interfacial failure mode tendency. The results of this model were compared with experimental data and also with the literature. Keywords: Resistance spot welding, Weld nugget size, Failure mode, Shear tensile test Introduction Resistance spot welding (RSW) is considered as the dominant process for joining sheet metals in automotive industry. Typically, there are about 2000–5000 spot welds in a modern vehicle. Simplicity, low cost, high speed (low process time) and automation possibility are among the advantages of this process. Owing to the weld thermal cycle a heterogeneous structure will be created in spot weld and the region around it. Spot weld and its surrounding area can be divided into three zones: (i) fusion zone or weld nugget (melted and resoli- dified region) (ii) heat affected zone (HAZ), a region which is not melted but undergoes structural changes owing to welding heat (iii) base metal (BM) (see Fig. 1). Geometrically, spot weld causes an external crack at the joint. 1 Also, electrode forces create an indentation and therefore stress concentration in the sheet (Fig. 1). These two factors (structural and geometrical changes) reduce load capacity of the joint compared with the BM. Vehicle crashworthiness depends on the weld structural integrity. Therefore, understanding spot welds mechan- ical behaviour under different loading conditions is important. Spot weld failure analysis is largely divided in two: the overload failure and the fatigue analysis. Accidents, rough roads or driving conditions which apply excessive load on the vehicle, are some of the cases which can cause overload failure. 2 Spot weld failure mode is a qualitative measure of the weld quality. Generally, spot weld failure occurs in two modes: interfacial and pullout. In interfacial mode, failure occurs through nugget, while in pullout mode, failure occurs by complete (or partial) withdrawal of nugget from one sheet. The shape of load displacement curve under shear tensile test for both interfacial and pullout modes is drawn schematically in Fig. 2. Load carrying capacity and energy absorption capability for those welds fail under interfacial mode, are much less than those which fail under pullout mode. To ensure reliability of spot welds during vehicle lifetime, process parameters should be adjusted so that pullout failure mode is guaranteed. Weld nugget size is the most important parameter determining its mechanical behaviour. 3–5 Various indus- trial standards have recommended a minimum weld size for a given sheet thickness. For example, American Welding Society/American National Standards Institute/Society of Automotive Engineers (AWS/ ANSI/SAE) 6 has recommended equation (1) d ~4t 1=2 (1) where d and t are weld nugget diameter and sheet thickness in mm respectively. However, this criterion 1 Material science and engineering Department, Sharif University of Technology, Tehran, Iran 2 Material Science Department, Shiraz University, Shiraz, Iran 3 Material Science and Metallurgical Engineering Department, Tehran University, Iran 4 Mining and Metallurgical Engineering Department, Amirkabir University of Technology, Tehran, PO Box 15875 4413, Iran 5 Material science and engineering Department, University of Science and Technology of Iran, Tehran, Iran *Corresponding author, email pirmarashi@yahoo.co.uk ß 2007 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 10 August 2006; accepted DOI 10.1179/174329307X164409 Science and Technology of Welding and Joining 2007 VOL 12 NO 3 217