effect of resistance spot welding variables on ultimate strength of low carbon steel using design of experiments

In this paper St12 sheets of 0.9 mm thickness were spot welded under different conditions of current, time and electrode pressure to determine the effect of these variables on ultimate shear strength of the spot welds. The capacity of the welding machine was 120KVA and copper-chromium electrodes with 5mm contact diameter were used. Sheets were spot welded using the design of experiments technique and then standard shear-tension tests were carried out on welded specimens to measure the ultimate shear strength of the spot welds. By using the results of the experiments and with the help of surface response method and Minitab software, a second degree mathematical model was developed in order to predict the effect of the above variables on the shear strength. Analysis of variance (ANOVA) at 95% confidence level was performed and validity of the model checked. The modeling results showed that increase in current and time caused an increase and then a decrease in the shear strength whereas increasing the electrode pressure decreased the shear strength.