Study On Resistance Spot Welding Of Aluminum Alloy A6061and Low-carbon Steel Q235

In the course of vehicle lightweighting, the resistance spot welding betweenaluminum alloy and steel has a good prospect of application. Because of the lowresistivity and high heat conductivities of aluminum alloy, there are some defectsduring the process of aluminum alloy and steel welding by normal resistance spotwelding such as generating little heat and radiating heat fast on the side of aluminumalloy, which easily limit the application of aluminum steel composite in vehiclemanufacture. To overcome these defects, this project puts forward the thermalcompensation resistance spot welding to weld aluminum alloy with steel. In this study,the materials were aluminum alloy A6061and low-carbon steel Q235, in which thealuminum alloy A6061was air-cooling and natural aging while the Q235steel washot-rolled. Adopting the orthogonal test, the effects of welding current, welding timeand electrode pressure on the properties of joints, the microstructure and the fractureappearance were researched, and obtained the optimum welding parameter of thethermal compensation resistance spot welding of aluminum alloy A6061with low-carbon steel Q235, providing a new method for the resistance spot welding ofaluminum alloy and steel.According to the experiment results, welding current is the main factor of thejoint properties, its influence on the joint diameter and tensile shear load is larger thanthat of welding time and electrode pressure. Both the tensile shear load and nuggetdiameter increased with the increasing of welding current within the range of10~20kA.There was splash when the welding current increased to20kA. The joint with themaximum tensile shear load of4.25kN without splash was obtained under thecondition of17.5kA welding current,3kN electrode pressure and10cyc welding time.After welding, there were two kinds of time-load curves: unimodal curve andmultiple peak curves. The fracture with multiple peak curves was pullout failure andhad superior tensile shear load, however the other fracture was interfacial failure. Compared with the hardness of the base metal, the hardness of the heat affected zoneincreased, and the ductility of heat affected zone decreased. The heat affected zone isthe weak point of the spot welding joint.During the thermal compensation resistance spot welding process the aluminumalloy melted while the cover plate and steel did not melt. The reaction layer wasdetected in the boundary of aluminum/steel. The reaction layer showed the tongue-likemorphology, and it was thick at the central region, its thickness decreased with thedistance from the joint center increasing, and at last the reaction layer becamediscontinuous. As the welding current increasing, the thickness of reaction layer in thejoint center boundary increased. The bases intermetallic compounds of the reactionlayer were FeAl3and Fe2Al5, and near the steel side the intermetallic compound wasFe2Al5while near the aluminum side the intermetallic compound was FeAl3. Theshear strength decreased with the reaction layer thickness increasing. There were somecommon flaws during the thermal compensation resistance spot welding process, suchas blowholes, splash, burned to black and so on. The discontinuous blowholes innuggets had little influence on the static load strength when the percent ofdiscontinuous blowholes was less than24.2%.The thermal compensation resistance spot welding acquires larger nugget andhigher tensile shear load with lower welding current, which reveals that this weldingmethod is effective for the welding of aluminum alloy and steel.