Study On Homogeneity Of Microstructure And Properties Of AZ61and AZ31Alloy Of Semi-Continuous Casted Ingot With Large-Diameter

Magnesium alloys are increasingly used in automotive, aerospace, communications,electronics and other fields. In these applications, because of the needing of large–sizedmagnesium alloy sectional bars, high-quality magnesium alloy billet with large size wasrequired. Various defects in the billets, such as: uneven microstructure, segregation,shrinkage, etc., have seriously effect on the application of large magnesium alloyprofiles. Semi-continuous casting process of magnesium alloy mainly included castingtemperature, mold size, casting speed and cooling water intensity. When the castingtemperature and mold size is fixed, the casting speed is very important processparameters, which combines the primary cooling water and secondary cooling waterintensity, and has an influence on the uniformity of the microstructure and properties oflarge-sized billets.The paper focus on the AZ61and AZ31magnesium alloys, by means of OM(Optical Microscope), SEM, EDS, the X-ray diffraction, hardness and mechanicalproperties testing, mainly study the uniformity of the microstructure and mechanicalproperties of AZ61magnesium alloy ingot with the size of φ500mm diameter. Theeffects of the homogenization annealing on the microstructures, second phases andmechanical properties of AZ61alloy were also researched. In addition, the effects of thecasting speed on the microstructures and mechanical properties of AZ31alloy were alsoresearched.The results show that the grain size reduced at the semi-continuous casting AZ61alloy, form the center to the edge of ingot. The as-cast structure consists of the region ofequiaxed grain, a region of columnar grain and a fine grained region. The α-Mg,Mg17Al12and Al8Mn5were found in the as-cast microstructure. At the edge of ingot, thegrain, the secondary dendrite arm and the second phase were refined, and the solidsolubility of alloy elements increased. On the contrary, the grain, the secondary dendritearm and second phases were massive, and the content of solute elements reduced at thecenter of ingot. The microhardness value increased from the center to the edge of ingot.In the equiaxed grain area, the tensile properties were uniform. In the columnar grainzone, the elongation significantly reduced. After homogenization annealing treatment of450℃×15h, the segregation have been basically eliminated. But the Al8Mn5still existin AZ61magnesium alloy matrix. The inhomogeneity of microhardness and mechanical properties reduced, significantly.The as-cast structure consists of the region of equiaxed grain, a region of columnargrain and a fine grained region in semi-continuous casting AZ31alloy ingot with thesize of φ500mm diameter. The second phases of Mg17Al12and Al8Mn5were found in theas-cast microstructure. At the edge of ingot, the grain and the second phase were refined,and the solid solubility of alloy elements increased. With the increasing of casting speed,the content of second phase increased and the microhardness value reduced. Themechanical properties were nonuniform, and elongation at the ingot edge issignificantly lower than the at ingot center. The effect casting speed on the homogeneityof mechanical properties of AZ31alloy was distinct. Increasing the casting speed, thehomogeneity of mechanical properties reduced. The segregation have been basicallyeliminated by homogenization annealing treatment. But the Al8Mn5still exist in AZ31magnesium alloy matrix. With the increasing of casting speed, the inhomogeneity ofmechanical properties of AZ31alloy ingot with large size diameter reduced.