Microstructure And Mechanical Properties Of Castex Mg-cu-zn Alloy

The capacity of plastic deformation of magnesium alloys is the main constraint on the cost of magnesium alloy products which is much higher than aluminum products. The CASTEX (continuous casting and extrusion) as an efficient, energy saving, simple equipment technology, will significantly reduce the cost of rod, wire, sectional material and tubes and pipes of magnesium alloys if it could be successfully applied in commercial. Zn is an important alloying element of magnesium alloys, it not only provide the solid solution strengthening and aging hardening effect but also improve the melt fluidity of magnesium alloys. That makes the casting of magnesium becomes easier. The adding of Cu on Mg-Zn alloys can reduce the thermal cracking tendency, increase the low limit of the thermal cracking temperature to460℃, control the morphology of eutectic MgZnCu and improve Mg alloy plasticity. Therefore, in this work, actualize the CASTEX of Mg-Zn-Cu by control the parameter in the process. And then the microstructure, mechanical properties and electrochemical properties of Mg-6%Zn-3%Cu (wt%)(Referred to as ZC63) alloy in state of casting and CASTEX was obtained by using optical microscopy, XRD, SEM and other analysis. The influence of solution and aging treatment on the microstructure and mechanical properties of CASTEX ZC63was be studied. The influence of extrusion rate on microstructure of the alloy in the mold and the product was also be studied. Some valuable results were got as follow.There are only two phases in the casting and CASTEX ZC63:a-Mg phase and MgZnCu phase. The MgZnCu phase in the ZC63has a lamellar shape in the casting state and grainy in CASTEX (about300nm).When the CASTEX extrusion rate is2, a large deformation along the extrusion direction occurred on the edge section of the ZC63. The eutectic structure is also be elongated along the extrusion direction. The microstructure of the center section has the characteristics of casting alloy which shows a network of eutectic structure is the surrounding of the α-Mg matrix. The fluctuation of tensile strength shows that the control in the microstructure need to be further optimized. The polarization curve showed that the corrosion resistance of CASTEX ZC63with extrusion ratio of2is better than casting alloy in the pH=8.3,5%(wt%) NaCl solution.Second-phase MgZnCu in the CASTEX ZC63after solid solution treatment is grainy and it distributes at the grain boundaries discontinuously. The hardness of the alloy reach the peak when the alloy aging15h at200℃, and the precipitates are mainly distributed within the grain.When the extrusion ratio increases from γ=2to2.5or3, the uniformity of microstructure is improved significantly, the deformation along the extrusion direction occurred on the center section and edge section, the grain size of matrix drives to the same. Eutectic structure has a zonal distribution along the extrusion direction.The most intense shear deformation occurred in the location where the CASTEX wheel groove accesses to the block fend off the metal. The deformation becomes more serious with the increase of extrusion ratio and stronger shear deformed structure would be generated. When the extrusion ratio be larger (greater than2.5), the metal flow like "water wave" is found in the cavity of mold. And the flow is formed by the dim organization and bright organization by alternately.