Dissimilar DP۷۸۰/DP۹۸۰ Resistance Spot Welded joints: Microstructure, Mechanical Properties and Critical Diameter

In this research, microstructure and mechanical performance of dissimilar resistance spot welded DP۷۸۰/DP۹۸۰ dual-phase steels were studied utilizing optical microscope, microhardness, and tensile shear tests. Resistance spot welding (RSW) was performed in the current range of ۷ to ۱۲ kA, with ۰.۵ kA steps. At welding currents lower than ۷ kA low amount of melting led to the very low strength of the joints due to small weld nugget diameter. The results showed that an increase in welding current from ۷ kA up to ۱۱ kA, result in an increase in weld nugget diameter. Further increase of welding current (higher than ۱۱ kA), however decreased the weld nugget diameter due to severe melt expulsion. Microstructural studies showed that weld nugget was primarily comprised of martensite, and the heat-affected zone (HAZ) of both sides of the joint was comprised of three different microstructural zones; upper-critical HAZ (UCHAZ), inter-critical HAZ (ICHAZ), and sub-critical HAZ(SCHAZ). Microhardness test showed that at both sides, softening occurred at SCHAZ. The results of the tensile shear test showed that both peak load and fracture energy of the joints followed approximately the same trend as weld diameter with welding current. Two different fracture modes of interfacial failure (IF) and pullout failure (PF) were observed in the tensile-shear test. At welding currents lower than ۱۰ kA, the failure occurred in IF mode, while at higher welding currents, PF was dominant. Weld nugget diameter at welding current of ۱۰ kA; i.e., critical weld nugget diameter, was ~۸.۵ mm.

Keywords:

Dissimilar resistance spot welding , DP۷۸۰ Dual Phase steel , DP۹۸۰ Dual Phase steel , critical weld nugget diameter , Mechanical Behavior