| Magnesium and aluminum light alloys have been widely utilized in lightweight automotive applications which are an important future research direction of energy conservation and environmental protection for automobile. Magnesium alloys are the lightest metal structural materials, with specific strength and stiffness, good electromagnetic shielding and shock resistance, etc. Application of magnesium alloys for engine block that will undoubtedly reduce the automobile weight and improve integrated performance of cylinder block has a great significance, but the fact that vast majority of magnesium alloys are poor corrosion resistance, wear resistance and general high temperature strength limits the use of magnesium alloys in lightweight automotive, however aluminum alloys can form a natural dense oxide film, with excellent corrosion resistance and self-healing capabilities,in addition aluminum alloys with good wear resistance and high temperature properties that can make up for inadequate performance of magnesium alloys to some extent.In this paper, Mg/Al liquid-solid bi-metal composite materials are prepared successfully with liquid-solid compound casting method, self-designed casting molds,ZL105 aluminum alloys preheated to 300℃ as a solid substrate, molten metal AZ91D magnesium alloys with pouring temperature 600℃～700℃ pouring on the solid substrate and different holding temperature and holding time. Three-point bending strength tests found that bending strength of Mg/Al bimetal composite is different form liquid-solid process parameters, when process parameters are set good pouring temperature for 660℃～680℃, holding temperature for 460℃, holding time as 60min, three-point bending strength of Mg/Al bimetal composite sample have a maximum bonding strength 38.26MPa.The microstructures in bonding zone of Mg/Al bi-metal composite is observed and analyzed by metalloscope and scanning electron microscope(SEM),the results show that bonding zone of Mg/Al bimetal composite is consisted of aluminum alloys matrix, interfacial transition zone and magnesium alloys matrix, while interfacial transition zone can be divided into Al-side transition zone, interfacial intermediate zone and Mg-side transition zone. Line-scanning illustrates that interfacial transition zone and two matrix connections, a obvious gradual process of magnesium and aluminum element is found between interfacial transition zone and two matrices magnesium and aluminum element content in interfacial transition zone is different with matrices for discrepancy of these elsments in diffusion process.XRD indicated Mg/Al bimetal composite bonding zone compounds α-Al 、β(Mg2Al3)、γ(Al12Mg17)、δ-Mg、Mg2Si are formed the locations of themi are Al-side transition zone (a-Alsolid solution+Al3Mg2)、interfacial intermediate zone (Al3Mg2+Mg2Si)and Al-side transition zone (δ-Mg solid solution+Mg17Al12)。Microhardness tests showed that microhardness of the matrix on both sides are about 100HV. Microhardness in different regions of interfacial transition zone is not the same,and microhardness of Al-side transition zone is about 300HV, interfacial intermediate zone and magnesium transition zone are also significantly higher than the two matrix materials. Bending fracture morphology of Mg/Al bimetal composite is gray dark, overall morphology has very obvious brittle fracture characteristics. Fracture micrograph from Al-side showed tear ridge and few dimples, fracture micrograph form Mg-side showed rivers pattern and along intergranular fracture of brittle fracture characteristics. The study also showed that cracks and holes defects were found in the interface fracture. According to structue features of liquid-solid compound casting bonding zone of Mg/Al bimetallic composite, this paper reveals the connection mechanism Mg/Al bimetal composite liquid-solid that is fusion and diffusion,and analyses the formation mechanism of the interfacial phase transition zone and its influence on the interface. |
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