Effect of expulsion on peak load and energy absorption of low carbon steel resistance spot welds M. Pouranvari 1 , A. Abedi 2 , P. Marashi* 3 and M. Goodarzi 4 The effects of weld nugget size and expulsion on the performance of low carbon steel resistance spot weld have been investigated in the present paper. Failure mode, peak load and failure energy obtained in tensile–shear test have been used to describe the performance of spot weld. The influence of voids and porosity as well as electrode indentation associated with expulsion on peak load and failure energy is discussed. The results showed that although expulsion does not reduce the load carrying capacity of spot welds, it decreases their energy absorption capability which was attributed to the change of failure location due to excessive electrode indentation associated with expulsion. Keywords: Resistance spot weld, Weld nugget size, Expulsion, Failure Introduction 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 structural mechanical behaviour of spot weld. One of the most important parameters determining the mechanical properties and performance of spot weld is the fusion zone (weld nugget) size, which is governed by welding process parameters. 1,2 Expulsion (molten metal ejection) during welding is a common phenomenon in resistance spot welding pro- cess. To ensure a large weld nugget, it is a common practice in industry to use a high welding current up to and beyond the expulsion limit. Indeed, many spot welds are made under the expulsion condition. Expulsion is often used as a visual indicator of a correct welding process in steel spot welding. 3 The effect of expulsion on the overload performance of steel resistance spot welds has been investigated by some researchers. Kimichi 4 and Karagoulis 5 have reported that expulsion does not significantly affect the performance of spot weld which makes it acceptable to some extent. On the other hand, Zhang 6 reported that the quality of spot weld can be significantly affected by expulsion. However, it seems that an elaborated explanation was not given in these studies. Regarding the importance of expulsion, studies of some researchers 7–10 focused on the detection, monitoring and controlling of expulsion during resis- tance spot welding. The aim of the research is to investigate and analyse the effect of expulsion on the overload performance of resistance spot welds of a typical thin low carbon steel sheet used in car body production. Experimental A0?8 mm thick uncoated low carbon steel of the type used in automotive industry was used in the investiga- tion. The yield strength and ultimate tensile strength of this sheet steel were 180 and 330 MPa respectively. The chemical composition of the steel is Fe–0?045C– 0?189Mn–0?032Si–0?009P. Spot welding was performed using a 120 kVA AC pedestal type resistance spot welding machine, controlled by a PLC. Welding was conducted using a 45u truncated cone RWMA Class 2 electrode with a face diameter of 5 mm. To study the effects of both nugget size and expulsion on the weld quality, several welding schedules were used. The electrode force was in the range of 2?8 to 4?5 kN, the weld current was in the range of 7 to 11 kA and the weld time was between 5 and 9 cycle (1 cycle550 Hz). Critical welding conditions leading to expulsion were recorded. Static tensile–shear test samples were prepared according to ANSI/AWS/SAE/D8?9-97 standard. 11 Figure 1 shows the tensile–shear sample dimensions. Static tensile–shear tests were performed at a crosshead of 2 mm min 21 with an Instron universal testing machine. The peak load and failure energy were extracted from the load displacement curve. The failure energy was calculated as the area under the load displacement curve up to the peak load. It should be noted that when the specimen finally fails, the 1 Mateials Science and Engineering Department, Sharif University of Technology, Tehran, Iran 2 Metallurgy Department, Shahid Rajaee University, Tehran, Iran 3 Mining and Metallurgical Engineering Department, Amirkabir University of Technology, Tehran, Iran 4 Materials Science and Engineering Department, Iran University of Science and Technology, Tehran, Iran *Corresponding author, email pirmarashi@yahoo.co.uk; ß 2008 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 26 July 2007; accepted 14 September 2007 DOI 10.1179/174329307X249342 Science and Technology of Welding and Joining 2008 VOL 13 NO 1 39