Study On Contact Reactive Brazing Of Magnesium AZ31/Aluminum 6061 Dissimilar Alloys

As the lightest structural material, magnesium alloys have a series of advantages including low density, high specific strength, electromagnetic shielding and easy recoverability etc., and they are now entitled as the'green'engineering material for the 21st century. In the extensive application of magnesium alloys, the sound bonding will show great importance, especially for the case of magnesium and aluminum alloys composite structure, which makes it possible to reduce the weight further and to fulfill the different requirements coming from the sophisticate domains like aviation, aerospace and automobile manufacturing. But the brittle intermetallic phases can be easily formed between magnesium and aluminum elements, which will deteriorate the joint performance. Comparing with the traditional welding methods, the contact reactive brazing technique shows substantial potentials in the connection of dissimilar metals, where the composition and structure of interlayer can be designed carefully to inhibit the emergence of harmful phases and then improve the joint performance.In this thesis, the bonding of AZ31 magnesium and 6061 aluminum alloys was investigated systematically, where the direct bonding and methods with different kinds of interlayer had been essayed. According to the results obtained in measurements of microstructure, phase composition and mechanical performance, the formation mechanism of an optimal joint with high shear strength were explored.For the AZ31/6061 joints bonded directly, the best value of shear strength was 41.3 MPa only, which was obtained from the joint bonded with a moderate heating rate of 110℃/min. The presence of brittle phases like Mg17Al12 intermetallic weakened the joint and it is difficult to stop the formation of a large amount of brittle phases in the directly-bonded joint, and this always leads to a poor performance.For the case of AZ31/6061 joints bonded with an interlayer design, different kinds of metal interlayers were prepared onto the 6061 aluminum alloy samples by a hot dipping method. The interlayers covered pure Zn, Zn-5A1, Zn-8Al and some composition adjusted by adding a small amount of alloy elements, like Ce, Si and Mn. According to testing results, the joint bonded with Zn-5A1 interlayer, heating temperature of 365℃and holding time of 3 seconds gave the highest shear strength, up to 86.1 MPa, the fracture morphology showed plastic characters. Using the Zn-Al-Ce interlayer, the best joint was obtained when heating at 365℃for 3 seconds. The dispersed distribution of Ce element in the initial Zn-Al-Ce interlayer will influence the formation of intermetallic compounds and their thickness during solid-state diffusion. Comparing to the joints bonded directly, the shear strength of joints bonded with interlayer was much more favorable.A physical model corresponding to the contact reactive brazing of AZ31/Zn-5Al/6061 with the highest shear strength was established on considering the element distribution, microstructure feature and bonding conditions. The evolution of structure and phase states formed at different stages was interpreted in detail, i.e. the eutectic liquid generated firstly in Zn-5Al at the place closed to the AZ31 base, and then the AZ31 base began to melt with the increasing solution of Zn element. At this time, there exited two kinds of liquid with different composition, they would continue to expand towards to both sides and interdiffuse with each other. The eutectic component in Zn-5Al interlayer melt fast as compared to the dispersed aluminum solid solution phases, and the particle size became smaller and smaller. It was testified that the best shear strength result was obtained from the joint with a structure phase state composed of large amount and dispersed aluminum solid solution phases distributed on the base of MgZn2 intermetallic. According to our study, this kind of microstructure corresponded to the stage when the liquid phases just formed and began to interdiffuse. To remain this microstructure, the short bonding time and fast cooling method was applied intentionally. The reasons of good shear strength were attributed to the gravitational type interaction between the moving dislocations in MgZn2 and Al solid solution particles. Besides, the numerous Al-rich particles in liquid increased the nucleation number during solidification and refined the grains to improve joint plasticity. If keeping for a overlong time, aluminum solid solution particles dissolved drastically and magnesium element diffused into the 6061 base continuously, the shear strength of joints would decrease.