Study On Microstructures And Mechanical Properties Of Dissimilar Thickness Dual-phase Steel DP780/DP600RSW Joints

Based on the narrow weldability lobe, heat affected zone softening, expulsionand facial failure of the dual phase steel resistance spot welding (RSW) joints due todifferent thicknesses, microstructures, compositions, physical and mechanicalproperties and processes, in this paper the microstructures, mechanical properties andprocess parameters of dual phase steel DP780/DP600RSW joints were researched,and the failure modes and mechanism of joints were also discussed.The results show that the dissimilar thickness dual phase steel DP780/DP600RSW joint consists of fusion zone (FZ), heat affected zone (HAZ) and base metal(BM). The FZ consists of coarse lath martensite (volume fraction78.6%) and ferrite,and the phase of FZ contains α-Fe, Fe19Mn and MnP4. Based on the microstructures,HAZ can be divided into three subregions: the intercritical, fne grained and coarsegrained heat affected zones. The intercritical HAZ contains tempered martensite andferrite; the fine gained HAZ contains acicular martensite and ferrite; the coarse gainedHAZ contains lath martensite and ferrite.The FZ hardness of dissimilar thickness DP780/DP600RSW joint issignificantly higher than HAZ and BM, and the highest hardness value in FZ is446HV. The peak load and failure energy of the RSW joint at welding current8.0kA,welding time18cycles and electrode force4.0kN are17.44kN and28.55J,respectively. The DP780/DP600RSW joints in the tensile shear test occur in threefailure modes: interfacial failure (IF); partial-interfacial failure (P-IF) and pulloutfailure (PF). The first mode is interfacial failure (IF). Cracks propagate through theweld nugget and IF is mainly brittle fracture. The second mode is partial-interfacialfailure (P-IF). The cracks initiate from the faying surface of base metals andpropagate towards DP780side. P-IF is mixed fracture of ductile and brittle fracture.The third mode is pullout failure (PF). The fracture of PF is initiated from DP780steelside and occurs near the HAZ with failure propagating through the BM. The fractureis initiated from the DP780sheet due to the more serious stress concentration and HAZ softening of DP780side. The surface of the fracture exhibits a lot of dimplesand PF is mainly ductile fracture.The excellent nugget critical size of dissimilar thickness DP780/DP600RSWjoint is5.5t0.5, and the amount of martensite is formed in FZ and the weld nuggetmoves towards the thicker base metal. The both side melting and indentation rates ofjoints are also unequal. With the increasing of welding current and time, the coarselath martensite and ferrite are formed in FZ, and the peak load and failure energygradually increase and hen decrease. The peak load and failure energy reach themaximum at electrode force4.0kN, and the joints occurs in the softening region in theHAZ of DP780side at electrode force5.0kN. The orthogonal experiment results showthat the optimum process parameter are welding current8.0kA, welding time18cyclesand electrode force4.0kN.