| Magnesium alloys have low density, high strength, etc. It was used in the automotive and aerospace in recent years and become the focus in the new materials field. Mg-Al alloy is the most important magnesium alloy system. The solidification characteristics, characteristic temperature during solidification, the solid fraction in coherent dendrite point, cast microstructure, grain size and mechanical properties of Mg-Al alloy were studied by the double thermocouples thermal analysis, thermodynamic calculation, microstructure observation and mechanical testingafter changing the content of solute elements, adding a small amount of alloying elements, modification treatment and casting method.Study the cast microstructure, T6state microstructure and mechanical properties of Mg-6Alx%Zn (abbreviated as AZ6x, x=0,2,4,6wt.%) alloy. The cast microstructure of the AZ60alloy composition is:a-Mg and β-Mg17Al12phase. The Φ-Mg21(Al, Zn)17phase appeared after adding2%Zn. After T6treatment, Mg-6Al-x%Zn (x=0,2,4,6) at room temperature yield strength increases by about20MPa; tensile strength of AZ60alloy increases by about40MPa, the tensile strength of about three other alloys improve20MPa. Since the matrix and the second phase softens at high temperatures, the tensile strength of the alloy increases with temperature were significantly reduced, but a significant increase in elongation with increasing temperature, the yield strength, but less affected by the temperature.Study the solidification path of sand casting Mg-6Al-x%Zn (x=0,2,4,6) alloy by using double thermocouples thermal analysis. The β-Mg17Al12phase is generated in the temperature range of439～436℃in AZ60alloy and decreased to372-369℃in AZ66alloy;Φ phase is generated in the temperature range of359-362℃. The dendrite solid fraction in coherent point calculation results by using double thermocouple analysis method show that Zn content increase from0to6%, the fsDCP gradually reduced from36%to23%.The Mg-6Al-x%Zn (x=0,2,4,6) ternary variable temperature equilibrium phase and Scheil non-equilibrium solidification condition were calculated by thermodynamic software Pandat. The microstructure of the Mg-6A1alloy composition is:α-Mg and β-Mg17Al12phase. The Φ-Mg21(Al, Zn)17phase appeared in AZ62-AZ64alloy. Under equilibrium conditions, the β-Mg17Al12phase alloys cannot be directly formed from the liquid phase during solidification in all Mg-6Al-x%Zn (x=0,2,4,6); The Φ-Mg21(Al,Zn)17directly from the liquid phase precipitation in AZ66alloy. The β-Mg17Al12phase generated by the eutectic reaction Lâ†'α-Mg+β-Mg17Al12and the Φ-Mg21(Al, Zn)17phase generated by the peritectic reaction L+P-Mg17Al12â†'α-Mg+Φ-Mg21(Al, Zn)17in Scheil non-equilibrium.Calculate the dendrite growth restrain factor QAl,QZn and Q values by using the parameter of binary and ternary equilibrium phase diagram. Calculate the dendrite growth restrain factor Q values by using Scheil non-equilibrium model. The results showed thatternary equilibrium phase diagram and non-equilibrium Scheil conditions is the same.The results showed that Q value increase with Zn content increases using the different calculation method; The method by using non-equilibrium solidification Scheil model was more convenient and accurate. The grain size decreases with the Q value increases of the sand cast Mg-6Al-x%Zn (x=0,2,4,6) alloys. When the Q value reaches a certain value, the grain size decreases is not obvious. The Q value is quite different between binary phase diagram parameter Q and ternary equilibrium phase diagram.Q value of Mg-6Al-x%Zn (x=0,2,4,6) alloy which obtained by Scheil non-equilibrium model was increased from21.3to41.8. The average grain size reduced from557μm to235μm.The average grain size and1/Q is not linear relationship.The effect of small amounts of Sb elements on microstructure and properties of AZ91alloy has been studied. The results show:Sb phase formed Mg3Sb2during the solidification process of AZ91alloy. The mechanical properties of the AZ91+x%Sb (x=0,0.1,0.5,1) at room temperaturetest results showed that AZ91+1.0%Sb alloy has a high tensile strength. The fracture form of four alloy is mainly intergranular fracture; AZ9I+x%Sb alloy tensile strength and yield strength can be increased80-100MPa after T6treatment, the fracture form is cleavage fracture; AZ91+x%Sb still has higher strength at120℃, the tensile strength of AZ91+0.1%Sb alloy is235.9MPa, the yield strength is153.9MPa; The characteristic temperature decreases and the dendrite coherent point decreases from40%to35%when the Sb content increases from0to1%, while the grain size change from217μm to246μm.Thermodynamic calculations show that Mg3Sb2phase formation temperature is lower than a-Mg nucleation temperature, and the dendrite growth restraint factor Q value does not change significantly with increasing Sb. So Sb elements cannot inhibit a-Mg dendrite growth and the Mg3Sb2phase cannot be used as the heterogeneous nucleation during the solidification in AZ91alloy. The grain size does not change significantly with Sb content increases.Microstructure and properties of AZ91alloy was studied in metal casting, sand casting and lost foam casting process. The results showed that AZ91alloy composed of a-Mg, β-Mg17A12and AlMn phases. A greater impact on grain size of AZ91alloy, the number and distribution of the second phase in different casting process. Because of the fastest cooling rate of the metal casting condition method, the grain size is minimum and about186μm. The cooling rate of lost foam casting conditions is slow. The grain size is about284μm and the second phase is thick and continuous; The study about mechanical properties of AZ91under three different casting methods indicated, AZ91tensile strength increased from120MPa under LFC increase to176MPa under metal casting method, yield strength increased from72MPa to96MPa, elongation rate increased from0.65%to2.4%. The crack of AZ91alloy mainly along the grain boundaries initiate and propagate. The fracture form of cast AZ91alloy is cleavage fracture. |
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