Resistance Spot Welding Behavior of Automotive Steels

In present-day automobiles, new high-strength steel grades are being extensively utilized, which have different alloy concentration, microstructure and properties, and results in different spot welding behavior. In the present work, the effect of weld current was studied on four widely used automotive grades, such as IF 270, TRIP 690, DP 780 and TRIP 980, in a laboratory welding machine. The welding current was varied between 5 and 15 kA. Weld quality was assessed based on tensile shear, mode of failure, weld nugget diameter, metallographic observations and hardness tests. Experimental results showed that weld strength increased with an increase in base metal tensile strength but with a drop at the point of expulsion. Variation in hardness along the cross section was found to be higher in high-strength steels. The initial microstructure and phase transformation during cooling affected the final properties. Critical currents for peak strength, expulsion, sticking and failure mode transition were determined for each grade for industrial applications. This study helped in determining the criterion-based optimized welding parameters for specific applications of these automotive-grade steels.