Spot Welding Effects in Multi-Material Combinations

As multi-material combinations consisting of high strength (HS) and advanced high strength (AHS) steels are increasingly being considered for automotive structural applications, it is posing challenge to the resistance spot welding process to produce successful joints that meet expected performance levels. The nugget diameter, type of weld nugget failure and joint strength under applied tensile loads are critical aspects that justify the merit of such combinations. While such practices have been adopted in shop floors based on the weldability of individual grades, proper laboratory studies are necessary to substantiate the appropriateness of the prevailing practices. The current investigation involves spot welding dual phase DP590 (1.6mm) steel with DP780 (1mm) steel, and DP590 (1 mm) steel with carbon manganese CMn440 (1.4 mm) steel by electrodes with appropriate geometry. It is evident that for a given sheet combination the β-peak in the dynamic contact resistance (DCR) plot occurs at distinctly different time scales with variation in heat input.  The asymmetric heat distribution because of material and thickness variations causes the material resistance to behave differently with increasing temperature, which in turn influences the melting conditions and location of nugget formation with respect to the sheet interface.  The associated thermal cycles promotes higher volume fraction of hard phases in the weld and HAZ on the thicker side, which is reflected in the hardness variation across the nugget. In both combinations interfacial fracture is prevalent during tensile loading, particularly in small size nuggets. By adjusting the welding parameters it is possible to have proper mixing of the chemistries and thereby minimize hardness variation. This not only ensures plug type fracture, but also more acceptable and consistent mechanical properties.