The Interface Microstructure Of The Connected Mg/Al Bimetal

In the face of increasingly severe environmental pollution and energy crisis, the lightweight of automobile is becoming more and more important. So the lightweight magnesium and aluminum alloys become the preferred materials for automotive lightweighting. And the solid-liquid composite casting is a very simple and effective method to produce automobile cylinder that has very complex shapes.In this study, the basic materials are AZ91D magnesium alloy and ZL105 aluminum alloy. On the surface of aluminum alloy substrate the basic pretreatment, zinc chemical plating, zinc electroplating and hot dip tin were accomplished and casting parameters such as pouring temperature, heat preservation temperature, holding time, were designed before the casting experiment. Finally, AZ91D magnesium alloy and ZL105 aluminum alloy were fabricated by the solid-liquid composite casting. Metallographic microscope, scanning electron microscopy (SEM) analysis, energy dispersive spectrum analysis, micro-hardness analysis and X-ray diffraction (XRD) phase analysis were used after the casting experiment to research the interface microstructure and its distribution of magnesium-aluminum bimetal and explore the interface behavior and the mechanism of magnesium-aluminum bimetal connection. The results prove that the magnesium and aluminum alloy can be connected by four methods above with heat preservation at a certain temperature. Two alloys are connected under the influence of elements diffusion and phase transition.The datas of SEM, EDS and XRD prove that the interface microstructure of the basic pretreatment sample with 30min heat preservation can be divided into three small diffusion transition zones from AZ91D to ZL105, which are 8-Mg solid solution and Mg17Al12 phase, Mg17Al12 phase Mg2Al3 phase and Mg2Si phase,Mg2Al3 phase Mg2Si phase and ?-Al solid solution, respectively. However, compared with the basic pretreatment sample with 30min heat preservation, the interface microstructure has a very big difference in the zinc chemical plating and zinc electroplating samples with 30min heat preservation.The zinc chemical plating and zinc electroplating samples with 30min heat preservation have the ?-Zn solid solution that is not found in the basic pretreatment sample with 30min heat preservation, however the basic pretreatment sample with 30min heat preservation has the Mg2Al3 phase and Mg2Si phase that are not found in the zinc chemical plating and zinc electroplating samples with 30min heat preservation. It can be proved that the zinc layer from zinc chemical plating and zinc electroplating can limit the diffusion of Al and Si elements. In addition, the diffusion layer thickness of the basic pretreatment sample is 2 mm which is wider than the zinc chemical plating sample with a 1.5 mm diffusion layer. And the zinc electroplating sample has the most narrow diffusion layer of 400?m. For the basic pretreatment, zinc chemical plating and zinc electroplating samples with 60min heat preservation, the interface microstructure is similar with the basic pretreatment sample with 30min heat preservation. The transition zone has the same interface microstructure and the same distribution. In addition,the three kinds of samples with 60min heat preservation have the similar difusion layer thickness which is about 4mm. It can be proved that the zinc layer disappeared under the influence of the long time difusion. The zinc layer can not limit the diffusion of Al and Si elements. The hot dip tin samples with 30min and 60min heat preservation have the similar interface microstructure and distribution. There are a large number of Mg2Sn phase,some ?-Mg solid solution and Mg17Al12 phase among the Mg2Sn phase, and some ?-Sn solid solution near the aluminum alloy substrate.The measurement of micro-hardness proves that the hardness on the sample interface is obvious higher than the hardness on the substrate. This is because of the formation of Mg17Al12? Mg2Al3 and Mg2Sn intermetallic compound on the interface.In this study, the metal layers formed by zinc chemical plating, zinc electroplating and hot dip tin were used to control the thickness of the intermetallic compound and obtain the Mg/Al bimetal composite material which is reliable performance and easy formation.