Study Of Resistance Spot Welding Microstructure And Properties Of Al-mg-si-cu Based Auto Body Sheets

At present, it is a trend to increase the rate of aluminum alloy application in vehicle fields. Among all aluminum alloys of automobile body sheets, Al-Mg-Si-Cu based aluminum alloys have been used wildly because of its good combinational properties of high strength, excellent formability, and good corrosion resistance. Resistance spot welding (RSW) is a major technique means in automobile manufacture because of the characteristics of easy automatization and arrangement and more flexible degree. But Al-Mg-Si-Cu based aluminum alloys has the poor welding ability, resulting in higher application cost and weaker welding joints and so on, so its extended application is limited in the fields of automobile manufacture. Then it is of great significance for automobile industry to improve joints quality as well as to make research on the process mechanism of aluminum alloys RSW.In this paper, the basic materials were the new and higher capability 1mm thickness Al-Mg-Si-Cu based aluminum alloy automobile body sheets including 0.23 wt% Cu element, at first, it used condenser discharge spot welding means with big criterion technics method which was widely used in manufacture fields, The results showed that the joints met the requests of the mechanics capability, but there were some flaws such as splash, crack and so on, just because of the following characters such as the instantaneous big voltage and higher electrode pressure in the method, so the joints qualities were very difficult to guarantee.Then, on the base of above results, and according to the principle, the changes of contact resistances in nugget growth process are important, and a physical model about contact resistances for aluminum alloy RSW is developed. In this model, by adding interlayer medium to two workpieces and using medium resistances, contact resistances distribution could be changed between two electrodes, so it could depress the effect of contact resistances between workpiece and electrode and also reduce the requests for the workpieces surface status. Thus, it was possible to weld workpieces by using mains frequency resistance welding machine with little criterion technics methods. The research indicated that it was better to add double 0.02mm thickness aluminum foils(1235^#) to two workpieces. After a series of experiments with the method of orthogonal experiment, the results showed that the optimal combined factors were welding current 17 kA, welding time 1.0s and welding pressure 2.4 kN. The tensile shear strength was 1943 N and plastic property rate was 53 percents. The joints had perfect shape, microstructures of nugget were mainly composed of equality and exiguous equiaxed grains, and the characteristic of ductile fracture was observed by fracture analysis, and no splash and crack in the process of RSW, so this method was very effective for avoiding those flaws and improving the welding qualities. Then this experiment was done under the condition of simulating painting roasting(180℃×30min) in the optimal RSW qualification, and the results indicated that along with extending aging time, and the strengthen phases of nuggets were separated out gradually, so that joints mechanic properties and elongation rate had increased evidently.The results after RSW for different Cu contents of basic materials indicated that with the increasing of Cu contents in alloys, the quantity of Q phases and other the second phases increased obviously, microstructure characteristics were columniation-state grains and equiaxed grains, and center zone grains were exiguous, joints mechanics properties increased evidently and shearing strength with 0.84wt% Cu could improve about 42.9 percents than that with 0.23wt%, but the plastic properties decreased, elongation rate decreased about 28.4 percents.Then, Corrosion resistance of nugget and basic materials was researched by immersion corrosion and electrochemistry study. The tests indicated that basic materials corrosion characters were pit corrosion, intergranular corrosion in corrosion medium, nugget intergranular corrosion depth was less than basic materials; but with the increasing of Cu contents, nugget corrosion resistance became decreased. Electrochemistry experiments showed that nugget and basic materials stability corrosion potential were all negative, and nugget E_corr and I_corr were lower than that of basic materials, it showed that nugget corrosion resistance was better than basic materials. In addition, nugget E_corr with 0.84wt% Cu was higher about 31.5 percents to positive direction than that with 0.23wt% Cu, I_corr was higher about 85.6%, it indicated that with Cu content increasing, nugget corrosion resistance changed weakness.