| With the rapid development of automobile industry,people put forward higher requirements on passenger protection,comfort and environmental protection,and development of manufacturing technology and application of new materials also poses a severe challenge.The development of lightweight technology opens up a new way to energy saving,emission reduction and sustainability.The dual phase steel has become one of the first choice of lightweight steels due to its low yield strength ratio,high initial work hardening rate,good strength and plasticity.The ultra-low carbon IF steel DC54 D plays an important role in car covering parts owing to its super deep drawing performance.The combination of dual phase steel and ultra-low carbon IF steel is beneficial to lightweight,safety,energy saving,environmental protection and corrosion resistance.What’s more,resistance spot welding is the most widely used method in the production of white body.Therefore,the research on the resistance spot welding process of DP600 and ultra-low carbon sheet DC54 D has an important theoretical and guiding significance.The welded spots of 1.6 mm DP600 dual phase steel and 1.0 mm ultra-low carbon steel DC54 D were as the research object,and the macro/micro structure,mechanical properties,joining mechanisms and failure behavior were studied.Effects of welding current,welding time and electrode force on macro properties,microstructure evolution,microstructure in different zones and mechanical properties of DP600(1.6)and DC54D(1.0)welded spot were characterized.The formation and joining mechanisms of nugget during DP600 and DC54 D spot welding were studied.Failure behavior and fracture mechanisms of DP600 and DC54 D welded spot under tensile shear loading were analyzed in depth.By orthogonal test,the welded spot had a high mechanical properties when the welding current is I=9 kA,the welding time is T=14 cycles and the electrode pressure is F=2.6 kN.At the same time,the effects of different welding parameters on nugget diameter,the depth of indentation and the microstructure of the welded spot were studied by the using of control variables.The results show that the nugget diameter increased first and then decreased with the raise of welding current,welding time and electrode force.But the expulsion occurred,the nugget diameter and indentation decreased,which led to the decrease of the tensile shear strength.According to the effect of welding thermal cycle,the welded spot can be divided into: nugget zone(FZ),heat affected zone(HAZ)and base metal zone(BM).The microstructure of nugget zone was thick lath martensite.Because of the difference of microstructure,the DP600 heat affected zone can be divided into three sub regions: coarse grain zone,fine grain zone and two-phase region.The microstructure of coarse grained HAZ was coarse martensite grain,the fine grain zone is fine martensite and the two-phase region corresponds to the inter-critical zone including martensite and ferrite.The microstructure of base metal was ferrite substrate with the distribution of martensite particles.The heat affected zone of DC54 D was coarse ferrite grain,while the DC54 D base material was equiaxed ferrite grain.In the process of spot welding of DP600 and DC54 D,the formation and joining mechanism of nugget were studied.The results show that the fusion zone(FZ)was not formed while the base metal was not melted due to its low heat input.But the heat-affected zone(HAZ)occurred with a “butterfly” shape with the microstructure of fine martensite when the welding time was short.In 4 cycles,the FZ in dissimilar sheets occurred with an “abnormal butterfly” shape because of nugget shift.When the welding time increased to 14 cycles,the FZ exhibited a “bread loaf” shape and the weld shifted to “dog bones”.The FZ consisted of lath martensite,and the zinc element in the weld is pushed to the periphery under the pressure of the electrode force.Through the comparative analysis of the failure mechanism of DC54D(1.0)/DC54D(1.0)and DP600(1.6)/DP600(1.6)welded spots,the tensile shear failure mode of DP600(1.6)/DC54D(1.0)was a typical interface fracture.During the tensile shear test,the microstructure of the welded spot was not changed,and the fracture occurred at the needle gap of the two plate,and extend between the edge of the nugget and the DC54 D heat affected zone.The failure mode of DP600(1.6)/DC54D(1.0)of base metal showed that the DC54 D heat affected zone and the base material area of the welded spot had a bending deformation with the increase of the tensile displacement.The DC54 D BM became thinner because the strength of DP600 was higher.When the deformation reached a certain critical value in DP600 side due to work hardening,the fracture occurred at the DC54 D side in the necking place.The grain orientation changed from <111> direction to <101> direction after deformation,and the grains were elongated to about 70-80 μm,and the deformation was as high as 200-300%. |
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