| Rapid development of the automobile industry have brought up the automobilecivilization and its inherent meaning, influencing the social progresses deeply andproducing three social problems: fuel consumption, emission and safety. In order to solvethese problems, it’s essential to research and develop effective lightweight automobilematerials. Advanced High-Strength Steel (AHSS) meets the demands for the automobileindustry. AHSS posses good combination of high strength and ductility, low yield ratio andhigh initial work-hardening rate. Using AHSS can reduce the thickness of the steel sheetsand the structural weight of the body in white obviously, thus decreasing fuel consumptionand CO2. Passive safety in the event of a car crash can be guaranteed. Resistance spotwelding is an efficient welding technology and widely used in industry fields, such as inaviation, aerospace and automobile. A modern vehicle typically contains2000to5000spotwelds. The strength of the spot weld under quasi-static, impact and fatigue loadingconditions is therefore extremely important to the durability and safety design ofautomobiles. Because of physical chemistry properties and special forming processes ofAHSS, during resistance spot welding there are expulsions, shrinkage cavities and cracks.Furthermore, columnar crystals in weld nugget microstructures tend to form the weaknessin the spot welded joint, which reduce the mechanical properties of the joints. It hasbecome one of main technology problems needed to be solved for AHSS widely used in theautomobile industry.This present research work has systematically studied on the microstructures andmechanical properties of the spot welded joint of DP780GA. Besides, this article hasinvestigated the effects of spot welding process parameters (welding current, welding timeand electrode force) on the microstructures and mechanical properties of the spot weldedjoint of DP780GA. Moreover, the fracture mode of the spot welded joint of DP780GAhave been discussed and analyzed.Tempering process parameters (cooling time, temperingcurrent and tempering time) of resistance spot welded DP780GA have been optimized by the method of orthogonal experiment to study the changes of the microstructures andmechanical properties of the spot welded joint of DP780GA before and after the temperingpulse. Based on the experiment results and careful analysises, the main conclusions of thispaper are summarized as follows:(1) The spot welded joint of DP780GA was mainly composed of the nugget and heataffected zone (HAZ). The nugget formed on the surfaces of semifused grains from theborder, having a characteristically epitaxial growth, grew preferredly towards the centreof the nugget. And the direction of grain growth was nearly perpendicular to the jointline. The nugget was all the columnar transgranular grains. The microstructures of thenugget for spot welded DP780GA were coarse lath martensite, ferrite, bainite and alittle retained austenite, while the microstructures of HAZ were lath martensite andferrite.(2) The spot welding parameters(welding current, welding time and electrode force) hasaffected the microstructures of HAZ and the nugget obviously. With the increasing ofwelding current and welding time, or with the decreasing of electrode force, themicrostructures of the nuggets tend to get coarse. It relates to the heat input during thespot welding and grains growth.(3) There were several problems in the spot welded joint of DP780GA, such as largeindentation, expulsion, shrinkage cavity and the junction line entering. The defectswould affect the appearance of the spot welded joint and reduce the mechanicalproperties. In order to obtain the good spot welded joint, it is necessary to usereasonable process parameters.(4) The influence of the spot welding process parameters (the welding current, weldingtime and electrode force) on the microstructures and nugget diameter of the spot weldedjoint for DP780GA was significant. With the increasing of welding current, thetensile-shear load and nugget diameter increased until expulsion occurred. With theincreasing of welding time, the tensile-shear load and nugget diameter increased andthen maintained stable. With the increasing of electrode force, the tensile-shear loadand nugget diameter changed dramatically, which increased firstly and then decreased.The experiment results showed, the best process parameters of the resistance spot welding of DP780GA were determined, i.e. the welding current was8.5kA, thewelding time was16cycles and the electrode force was4.5kN.Under these conditions,the nugget diameter was6.32mm, the tensile-shear load was17.12kN, themicrohardness of the nugget was378.5kgf/mm2and there was the button pulloutfracture in the joint during the tensile-shear test.(5) The main failure modes for the spot welded joint of DP780GA were the interfacialfracture and the button pullout fracture. The peak load and energy absorption of thespot welded joint for the button pullout fracture were larger than the interfacial fracture.Therefore the button pullout fracture of the spot welds must be guaranteed on theautomotive factory floors. The interfacial fracture was caused by shear stress at theinterface between the two sheets. Cracks initiated at the boundary of HAZ andpropagated along the junction line. It is the typical brittle fracture. The opening betweentwo sheets in the nugget-surrounding area was seen as an initial crack due to the stressconcentration when the button pullout fracture happened to the spot welded joint. Thecracks propagated along the boundaries of HAZ-BM or BM perpendicularly. It is thetypical ductile fracture.(6) The results of the method of orthogonal experiment showed that the factor thatinfluences the nugget diameter of the spot welded joint most was cooling time,secondly was tempering current and tempering time was the least. Apart from that, thefactor that influences the tensile-shear load most was tempering current, secondly wastempering time and cooling time was the least. The best tempering process parameterswere determined by comprehensive balance analyzing, i.e. the cooling time was40cycles, the tempering current was4kA and the tempering time was80cycles.Themicrostructures of the boundaries of FZ-HAZ for the spot welded DP780GA werenearly same before and after tempering. And the microstructures of the centre of nuggetwere mainly lath martensite and tempered martensite. The dislocation density in themartensites reduced significantly after recovery and recrystallization and the internalstress were eliminated. After tempering, the microhardness of the nugget decreased alittle, while the microhardness of HAZ nearly maintained stable. Therefore the ductilityof the spot welded joint was improved. During the tensile-shear test with tempering pulse, the the button pullout fracture happened to the spot welded joints and thestrength and ductility of the joints increased. Therefore the comprehensive properties ofthe spot welded DP780GA have been improved. |
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