Fundmental Research On Solidification Structure Refinement Of Mg-Al Alloys

Magnesium alloys are widely used in the fields of automobile, electronic and aviation. Magnesium alloys possess many advantages, such as high specific strength, excellent damping capacity, better thermal conductivity, and good machinability. But low strength and plasticity restrict its applications to more fields. Structure refinement is an effective method to improve their mechanical properties. The aim of this work is to study structure refinement of Mg-Al alloys by addition of alloy elements and the external physical field.In this work, effects of Al, Zn and Mn on the mechanical properties and the structures of magnesium alloys were investigated by orthogonal experiment design in negative suction casting. For negative suction casting, Al content is the key factor to mechanical properties of Mg-Al alloys. At last, the determined optimal alloy compositions are Mg-6Al-0.6Zn-0.3Mn for negative suction casting, respectively.The influences of Ca and Ce elements on the structures of Mg-6Al-0.6Zn-0.3Mn alloys were systematically studied by negative suction casting. The results show that Ca and Ce elements can obviously refine the solidification structures. Increase of Ca content makes the grain size gradually decrease when it is below 0.9wt.%. Increasing Ce content (below 1.0wt.%) makes the number of the secondary dendrite increase, the link between secondary dendrite and first dendrite become thinner, even more secondary become new small crystals. Moreover, increase of Ce content makes content ofβ-Mg17Al12 phase decrease and a new phase (Al4Ce) appear.The effect mechanisms of Ca and Ce elements on the solidification structures of Mg-6Al-0.6Zn-0.3Mn alloys were systematically studied by liquid quenching. The results show that Ca element can promote nucleation in the melt ahead of dendrite tips and restrain crystal growth. But Ce element restrains the nucleation ofαphase and promotes crystal growth. Ca and Ce elements both can restrain the coarsing of the secondary dendrite and decrease the secondary dendrite spacing.The solidification structures of Mg-9Al-0.8Zn-0.3Mn alloy were examined when it solidified under physical fields including ultrasonic, magneto oscillation and pulsed electric current. The results show that the ultrasonic treatment can significantly refine its solidification structure. In the range of 300~700W, increasing the ultrasonic power makes the grain size gradually decrease. When ultrasonic power is over 500W, the grain size is smaller than that of the samples treated by MgCO3 modifier. The treatment of magneto oscillation also can clearly refine its solidification structures. In the range of 50~200jV, increasing the voltage makes the grain size gradually decrease. When the voltage is over 100jV, the grain size is smaller than that of the samples treated by MgCO3 modifier. The pulsed electric current also markedly refined its solidification structures. In the range of 100~250jV, increasing the voltage makes the grain size gradually decrease. When the voltage is 250jV, the grain size is the same as that of the samples treated by MgCO3 modifier.During solidification course of Mg-9Al-0.8Zn-0.3Mn alloy with different treatment method, the temperature was measured by thermal couple. The measured cooling lines show that treatment of three physical fields makes the solidification time obviously shorter, but have little effects on the initial solidification temperature. However, MgCO3 modification treatment makes its initial solidification temperature clearly increase, and makes its solidification time slightly shorter. Analysis suggests that refinement reason of the physical fields mainly is promoting heat elimination of melt and crystal multiplication, while MgCO3 modification treatment mainly increase the number of nucleation site.