| Aluminum(Al) alloy is a kind of light metals, which has high thermal conductivity, absorption, strong impact force, easy recovery, and strong corrosion resistance. So on the lightweight research of cars, Al alloy becomes the preferred material instead of steel. Magnesium(Mg) alloy has the smaller density ratio than Al alloy, high specific strength, high specific stiffness, dimensional stability, good electrical conductivity, thermal conductivity of strong shock absorption and noise reduction effect. It also has been increasingly used in automotive. But the disadvantages of Mg alloy such as bad abrasion resistant perfermance, bad thermal creep resistance make it can’t be used alone on many parts of cars. So the study of aluminum(solid)/magnesium(liqu id) bonding bi-metallic material which can reduce the weight of the car, and also can meet the requirements of the use of strength, is very important. The car engine act as the most important and heaviest part of a car, the reduction of its weight is particularly important. But as it said before, Mg alloy perfermance can not meet the requirement of engine working condition.Therefore, BMW first use hypereutectic aluminum/silicon alloy as the engine inner part, and then use Mg alloy casting on the Al/Si alloy to produce the crankcase. This "process of liquid/solid composite" will be effective in Mg and Al solid/liquid bonding, which both have the great strength and light engineweight.One of the difficulties of aluminum(solid)/magnesium(liquid) bonding is that Al alloy is so easy to be oxidized, and forming a natural oxidation layer on its surface; the heat conduction coefficient of magnesium and aluminum is also a hard point which easily to produce hot crack and air hole, and forming brittle intermetallic compounds phase, which result in the bad weld joints performance. NOW the most Mg/Al welding methods can roughly divided into melting welding (TIG welding, laser welding,) and solid phase welding (FSW, solid-state diffusion welding), but they still can not completely eliminate the effect of alumina layer, and diffusion welding area will still exist in a large number of intermetallic compounds. In this paper, to solve the above problems. The results of the study show that,"aluminum(solid)/magnesium(liquid) bonding" can effectively eliminate the influence of oxidation layer on the Al alloy surface, and forming a good metallurgical combination, intermetallic compound layer is also under conctrol. This paper mainly discussed as follows:(1) The surface of Al alloy was treated, and the the zincate treatment is the point. Using the self-made "high vacuum liquid/solid bonding device" to make Mg alloy and treated Al alloy liquid/solid bonding. Studying on the different effect of different temperature and time of Al preheating. Through the adjustment of preheating temperature and the preheating time of the Al alloy we finally obtain good metallurgical combination. XPS results show that zincate process can remove the oxide layer on Al alloy surface effectively, and before the liquid-solid bonding, zinc film can always protect the aluminum alloy away from two oxidation, thus ensuring the Mg and Al to form a continuous metallurgical bonding interface. Shear strength test results show that the furnace temperature at 700 ℃, the aluminum alloy matrix with increasing the preheating time, shear strength is improved with the increase of Mg/Al composite interface, when the preheating time is 120s, the shear strength reached a maximum Value 60.6MPa.The experimental results indicate the continuous Mg2Si layer was broken by solutionizing at 420℃ for holding time of 3h. The optimal shear strength of diffusion bonding is achieved 74.5MPa, increased by 23% than the samples without solution treatment.(2) We add La2O3 in the coating to improve the structure and the bonding strength of the interface. The experimental results indicate the surface treatment effectively removes the nature oxide layer on solid Al substrate’s surface, meanwhile, a layer containing La2O3 formed on the substrate’s surface. On the one hand, the coating can make aluminum alloy avoid re-oxidation. On the other hand, the La2O3 in the coating can improve the structure and the bonding strength of the interface of AM60/A390 bimetals. The optimal shear strength of diffusion bonding is achieved 60.6MPa. After solution treatment, the shear strength can be increased to 84MPa.(3) Generated by the elements to grain refinement and inhibition of intermetallic compounds. Because the diffusion of the Mg and Al in the bonding process can’t be avoided, so it will form a uniform layer of intermetallic compound. The intermetallic compound belongs to brittle phase, and after testing, we found that fracture mechanics, the basic all occur in the brittle phase position. Rare earth elements lanthanum(La) is used to improve the brittle phase distribution in Mg/ Al liquid/solid bonding interface, the results show that with the increase of La content in the interface, continuous intermetallic compound gradually interrupted separation. La can give priority to with aluminum to form La-Al phase, thus reducing the formation of Al-Mg phase. Shear strength test results show that, with the increase of Lacontent in the interface, the mechanical properties of Mg/Al interface is constantly increasing, when the percent of La in Mg alloy get more than 1%, the shear strength will begin to decline from maximum 88.5MPa. |
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