Research On Co₂ Laser Welding With Filler Wire Of Dissimilar High Strength Aluminum Alloy 2024 And 7075

Aluminum alloy 2024 and 7075 are heat-treatable aluminum alloy with high strength, which are widely used in the fabrication of fuselage skin and wing spar of airplane, automobile and railway transport system demand for high specific strength. Their application result in the joint of these two kinds of alloy, but mechanical joint, such as riveting, increased the weight, while, by using arc welding techniques, 2024 has only limited weld ability and 7075 is not recommended for welding. Compared with an arc welding, laser beam has a much higher energy density, faster heating and cooling rates make laser welding an applicable choice for these alloy. However, the high laser power density may produce an unstable keyhole and bad weld appearance, especially for 7075, with a higher content of Mg, Zn element. In this work, Laser welding with filler wire process was studied to solve the problem of instability; microstructure and mechanical property are tested and analyzed.The effect of welding procedure to the instability of welding process was studied, and the mechanism was discussed. My study on the droplet transfer process led me to the conclusion that laser heating and heat conduction are the primary mechanisms, which results in the change of droplet transfer form in different distance between laser and wire. Experiments showed that the most stable droplet transfer process can be obtained when the distance is 0. Through analyzing the change of droplet transfer form and plasma plume area, we came to the conclusion that the droplets play a role of regulating laser power and stabilizing the keyhole front wall. By increasing the wire feed speed, the droplet transfer frequency was increased, as a result, the welding process becomes more stable. Cleaning method and welding procedure were studied respectively to reduce hydrogen porosity and procedure porosity, and found that physical cleaning is superior to chemical cleaning. The rotation of laser beam and the increasing of wire feed speed in fusion-through welding will decrease the procedure porosity.Microstructure, micro-area element content, fracture surface, tensile strength and micro-hardness were analyzed in two kinds of welds which are welded by filling 2319 and 4043 filler wires. The reason for appearing of overheated heat affected zone, columnar grain zone near the fusion zone, V-shaped grain-coarsening zone and large dendrite were also discussed. And mechanical property of the joint using two kinds of filler wire was tested; result showed that tensile strength reached to 339.5Mpa with 2319 filler wire, a little higher than joint with 4043, but with a lower toughness. Finally, fracture position and fracture surface were analyzed which showed relationship between fracture and microstructure.