Research On Microstructure And Properties Of CMT Welding-brazing Of Aluminum And Steel

Now, with the application of aluminum and aluminum alloy in automobile manufacturing, shipbuilding, aerospace and other fields is increasing year by year, the welding of "Al + steel" has become a hot issue in the field. CMT welding has low heat input, no splash and being easy to control, can welding thin plate, so connect aluminum/steel by using CMT welding brazing method is meaningful and valuable.The 1.5mm thick 6061 aluminum alloy and 1.85 mm thick DP780 galvanized steel plate and thickness of 1.5mm 304 austenitic stainless steel plate as the research object, with ER4043(Al Si5 wire) and ER4047(Al Si12 flux cored wire) as filler metal,CMT welding brazing process was carried out, studying the influence of welding technological parameters and alloying elements on weld macro morphology,microstructure and mechanical properties.First of all, the effects of different welding parameters on the surface forming,mechanical properties and microstructure of welded was studied through single variable experiment. The welding process parameters of 6061 aluminum alloy and DP780 steel are determined as follows: the wire feeding speed is 4.7m/min, the welding speed is 600mm/min, the welding current is 80 A, and the arc voltage is 12.4V.At this time, we can get a lap joint with uniform and good appearance, the tensile strength of the joint reaches 48.5% of the aluminum base material.Aluminum/steel joints using lap, after welding using SEM and XRD observing and analyzing microstructure and phase composition of joint, welded joint as part of the melting zone, zinc rich zone and interface zone. The melting zone is composition of liquidus Al and Al-Si eutectic phase in the intergranular that vertical to the direction of the interface and the fusion line. And in this region, in order to avoid the formation of pores, with the help of the change of welding process parameters and variables in the process to achieve. The interface layer near the steel side is smooth,close to the aluminum side is uneven, and the interfacial area is composed of dense Fe-Al intermetallic compounds.Secondly, with Al Si5 wire and Al Si12 flux cored wire as filler metal; using CMTwelding 6061 aluminum alloy and DP780 galvanized steel. Test results show that two kinds of welding wire can be used to obtain a good weld brazing joint. Si elements can refine the microstructure of the welded joint, and can effectively prevent the formation of the intermediate compound layer. However, the tensile strength of the welded joint is greatly reduced, and the tensile property of the joint is severely deteriorated when the Si element in the solder is too much.Finally, welding 6061 aluminum alloy and 304 austenitic stainless steel by CMT,the stainless steel surface is coated with a layer of special brazing agent(KAl F4 and K3 Al F6 plus 10% of Zn powder) to obtain forming good aluminum/stainless steel lap joints. Under the special flux, weld metal wetting and spreading well, and the microstructure of Al-stainless steel joint were observed and analyzed by OM, SEM and XRD. In the aluminum/steel interface the obvious IMC layer generates, while the layer is the main component of Fe Al2, and due to the effect of flux and Si elements also participated in the reaction, formation of the Al0.5Fe3Si0.5. Vivtorinox hardness testing of the joints found that the hardness of the interface layer reaches 108.8HV,which is due to the formation of a brittle intermetallic compound at the aluminum/steel interface. The tensile test results found that joint fracture occurs in the binding site of aluminum/steel, and the strength of brazed joints reaches 84.1MPa, it is also deeply verified that this position is a weak region of Al/steel dissimilar material welded joint.