Study On The Mechanism Of Joining Aluminium To Zinc-Coated Steel By CMT Welding-Brazing Process

Joining aluminium to steel is becoming the hot focus in automotive industry with the extensive use of the aluminium alloy in car body. The heat input was decreased by the special droplet transfer process in CMT welding process and the CMT process could be used to make a good lap joint between aluminium and zinc-coated steel by welding-brazing process. The aim of this paper is to study the microstructure characteristics and the interface reaction mechanism of the dissimilar metal joint. The heating behavior and the coating behavior were also studied. This study did not only enrich the formation of weld theory, but also establish a basis in joining dissimilar metals by welding-brazing process.The experiment systems to photograph the arc shape and the molten droplet images by using high-speed charge coupled device camera and to record welding current and arc voltage in synchronism were established. It found the enery input of the welding arc could be reduced by the special wave control and the low short circuiting current also reduced the corresponding energy input during short circuit phase. The heat input to the workpiece can be controlled accurately. During the metal transfer process, the size of the droplet was controlled within some range in order to avoid globular metal transfer. The spatter was eliminated by the adscititious force comparing with the conventional short circuiting metal transfer and the post welding cleaning became needless.The lap joint between aluminum and zinc-coated steel made by CMT welding-brazing process mainly consisted of two kinds of compounds. The homogeneous compact compound near the steel side was Fe2Al5 phase and the needle-like compound near the alumium side was FeAl3 phase. The thickness of the compound layer was sensitive to the welding heat input. When the welding current increased from 66A to 85A, the compound layer thickness could increase from about 10μm to more than 40μm. The interfacial composition was not changed by the enhancement of heat input.The paper also studied the effect of the zinc coating on the arc heating behavior by the arc shape visual sensing system. The zinc coating could stabilize the arc when welding with small current. The evaporization of the zinc coating became obvious with the raise of the welding current and this evaporization changed the arc shape. As a result, the welding heat input was reduced because the changed arc shape decreased the contact area between the arc and the workpiece. The heat flux distribution was fitted by the arc shape and was used to simulate the temperature field during welding. The highest interface temperature could be reduced 200℃owning to the zinc coating evaporization and the thickness of the interfacial compound layer also could be decreased.When joining aluminium to zinc-coated steel, the steel base metal was not melted. The interfacial reaction essentially occurred between liquid aluminum and solid iron. According to the thermodynamics calculation results, only FeAl, Fe2Al5 and FeAl3 phases could be generated at the aluminium-iron interface. The growth kinetics model was established and the growth rate of every phase was calculated. The results showed that the Fe2Al5 phase grew rapidly when the liquid aluminium contacted with the solid steel. The FeAl3 was formed during the cooling step.The clear image of the steel surface was obtained by the passive visual sensing method and the composite filtering technique. After comparing the steel surface features when overlay welding on Q235 steel with that overlay welding on zinc-coated steel, a instantaneous liquid thin film formed by the unevaporated zinc coating was found. During welding process, the surface instantaneous thin film promoted the filller metal spreading extent and guaranteed the quality of the lap joint.Then the"surface instantaneous liquid thin film"method was presented to change the wetting condition of the filler meta. The Q235 steel coated by the zinc powder was joined to aluminium successfully. The appearance of the lap joint obtained by this technology was very good and the tensile stength could reach 80Mpa.