Investigation Of Interfacial Structure And Performance Of Dissimilar Bonding Of Mg/Al Bimetallic Castings By Compound Casting Process

Bimetallic castings made by solid-liquid compounding casting have excellent advantage, which can take maximum advantage of their preponderances. These materials can make best use of their performance, and widely used in aviation, aerospace, shipbuilding and other industrial fields due to their higher economy applicable value. Magnesium alloy has the good performaces such as specific strength of magnesium alloy, high damping shock resistance, high thermal conductivity, high electrostatic shielding, high machinability and aluminum alloy has the very low density and good mechanical strength of aluminum, good corrosion resistance performaces, and Mg-Al bimetallic castings can combine these excellent properties. Therefore aluminum magnesium dissimilar metal materials research and development became the various countries’ development and utilization of new materialHowever, the difficulty of producing Mg-Al bimetallic castings is how to get to meet the performance requirements of metallurgical bonding interface. Fist of all, the density oxidation film on the surface of the Al substrates prevents the metallurgical combination of the aluminum and magnesium, due to magnesium and aluminum alloy are extremely easy oxidation in the air. In addition, mutual solubility of two materials is very small, and form much more intermatic phases in the interface which is deleterious to the mechanical properties.According to the difficulty and problem, this paper has carried out the following work: Mg/Al liquid-solid composite interface wettability research and the control of the interface brittle phaseduring the compounding casting processing. In order to get good wettability of Mg/Al liquid-solid composite, the oxide film on aluminum alloy substrate must remove. This paper adopts two ways to remove or destroy the oxide film on the surface of the aluminum alloy:1) Zincate surface treatment can remove the oxide layer and coat with a metallic zinc film by two parallel chemical reactions. The first is an etching procedure which removes the layer; the second is redox reaction, where metallic Al oxidizes and dissolves and Zn anions are reduced and deposed as a dense metallic layer.2) A very small amount of low-melting point metal, Ga and Sn, was introduced to Al substrate and an "electropolishing+anodizing" pre-treatment was applied to the Al substrate. And the surfaces of the Al substrates have much more microviods, which can breakup and separation of oxide film.The welding of the Mg/Al dissimilar metal inevitably form much more brittle phase in the interface, this paper adopts two ways to control or reduce the formation of brittle phase.1) The Al substrate was covered with 5-6μm Ni layer by electroplating technology, which just like a barrier impede the interdiffusion of Al and Mg. Therefore, the brittle phase in the interface reduced.2) The Al substrate with higher Si element was used in this paper, and the results of the interface show that continuous Mg2Si phases strip formed in the interface inhibited the formation of Mg-Al intermetallic compounds during the bimetallic experiment.XPS analysis was carried out on the zinc layer and the results showed that, zincate process can remove the oxide film of aluminum alloy surface. The results show that a characteristic ruptured oxide film is formed on the alloy surface and the oxide film detaches locally from the Al substrate after the anodizing treatment processing. The detachment coincides with formation of the fine voids alloy/film interface, possibly associated with localized oxidation of tin in the enriched alloy layer. The Mg-Al intermetallic compounds are impeded due to the fact that Ni layer prevents the interdiffusion of the aluminum and magnesium elements, under the condition of furnace temperature 700 ℃, substrate preheating temperature 560℃, and instead Al-Ni, Mg-Ni intermetallic compounds. The microstructure of the interface consists of Mg17Al12, 8-Mg eutectic structure and AlNi3 intermetallic compounds.In the region zone of AM60/A380 bimetallic interface, the Mg2Si formed due to the Si prior Al diffused to the Mg alloy. However, the precipitate Mg2Si and the Al alloy is crystal coherent relationship, and continuous Mg2Si phase formed in the interface and was just like a barrier, which effectively inhibited the formation of Al-Mg intermetallic compounds. While the continuous Mg2Si phase is very high hardness and brittleness, which is deleterious to the mechanical properties of bimetallic castings. Thus, the solution treatment was carried out to break up the continuous Mg2Si phase and the optimal shear strength of bi-metallic samples achieve at 74 MPa.