Research On Process And Mechanism Of Laser Welding-brazing For Al/brass Dissimilar Metals

Facing the domestic shortage of copper resources, the hybrid structure of Al/Cu with Al replacing Cu partly can give full play to the advantages of the two materials and achieve the balance of cost and performance, which has broad application prospects in industries such as electrical, electronic, petrochemical, aeronautics and astronautics. However, joining for Al/Cu dissimilar metals has great difficulties due to the significant differences in physical/chemical properties and the formation of brittle intermetallic compounds(IMCs). In recent years, laser welding-brazing is widely used in the joining of difficult-to-be welded dissimilar metals with the characteristics of high energy-density, short welding thermal cycle time and precision acting position. It is expected to break through the drawbacks of traditional methods in the adaptability of structural, joint quality and other aspects. In this paper, 5052 aluminum alloy and H62 brass dissimilar metals with thickness of 2 mm were laser welding-brazing butt welded using Zn-15%Al flux-cored wire, and microstructure, interface layer and mechanical properties of the joint were investigated. The mechanism of laser welding-brazing for Al/brass were clarified.Microstructure and interface structure of Al/brass laser welding-brazing joint was investigated. The joints could be divided into the fusion joint at the aluminum side, weld seam, and the brazed joint at the brass side, and the brazed joint at the brass side could be further divided into the transition zone and interfacial layer. The phase of Cu Zn5 and Al4.2Cu3.2Zn0.7 were formed in the transition zone, and metastable Cu Zn5 phase nucleated and grown on Al4.2Cu3.2Zn0.7 phase. A straight continuous layer of Cu Zn and serrated Al4.2Cu3.2Zn0.7 were formed at the interface while the melt and dissolution of brass was small, while Al4Cu9 phase was also formed at the interface with more melt and dissolution of brass. The thickness of Cu Zn phase increased with increasing heat input.The micro-hardness distribution, tensile strength and fracture mechanism of Al/brass laser welding-brazing welded joint were studied. The micro-hardness of weld seam was higher than both of the Al/brass base metal and the highest hardness was observed near the interface at brass side. The primary factor to the tensile strength was welding defects, and the tensile strength of the joint increased firstly and then decreased with increasing interfacial IMC layer thickness within a certain range. The tensile strength of the original joint and the one removing the reinforcement could up to 128 MPa and 104 MPa, respectively, at laser power of 2700 w, welding speed of 0.5 m/min and laser beam offset of 0.3mm at the aluminum side. Fracture behavior analysis indicated that there were two typical failure modes of failure in the interface completely and interface partly for the original joint, while failure in the weld seam completely was also found for the joint removing reinforcement. The fractography of joint failed in the weld seam was characterized by quasi-cleavage fracture, while cleavage fracture was observed for that failed at the interfacial layer.The temperature field during Al/brass laser welding-brazing process was simulated by the finite element analysis software MARC, and the energy control for the interfacial layer growth and the mechanism for joint formation were clarified. The results showed that the distribution of temperature in the joint was asymmetrical. The different of peak temperature in the joint along the workpiece thickness direction was within 100 oC and there was little difference in reaction time at high temperature. The interface peak temperature ranged from 930 to 1145 oC and the reaction time was within 8 s under different weld heat input conditions. The interface peak temperature and reaction time could be controlled in an appropriate range at laser power of 2700~3000 w, welding speed of 0.5 m/min and laser beam offset of 0.3~0.5 mm at the aluminum side. Finally, the formation mechanism of Al/brass welding-brazing joint was proposed.