Study On Grain Refinement Process And Mechanism Of Mg-Al Alloy

Mg-Al alloy is currently the most widely used magnesium alloy which has been widely used in electronics aerospace automotive and other fields also have better application prospects in the future.As a typical representative of Mg-Al alloy ZM5 magnesium alloy is paid more and more attention with its excellent performance.The crystallization temperature of magnesium alloy is wide the volume shrinkage is large and the grain coarsening tendency is obvious which is easy to produce loose and hot crack and so on and which seriously restricts the development of magnesium alloy.Grain refinement can significantly improve the comprehensive mechanical properties and casting properties of magnesium alloy materials greatly improve the processing performance of magnesium alloy.In this paperZM5 magnesium alloy is used as the Object of study and the grain refinement process and mechanism of Mg-Al alloy are studied by means of optical microscope(OM)scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS).The effects of different cooling rates and different carbonaceous modifiers and different additions on the microstructure and mechanical properties of ZM5 magnesium alloy were investigated.The mechanism of the refinement was also discussed.The effects of different cooling rates(including metal type copper mold metal steel mold stepped resin sand mold with 10 mm thick and 20 mm thick cold iron)on the microstructure and mechanical properties of ZM5 magnesium alloy were studied.The quantitative relationship between the cooling rate and the grain size and the comprehensive mechanical properties was analyzed and the refinement rule was explored.The results showed that the grain size of ZM5 magnesium alloy was obviously refined and the performance became more excellent as the cooling rate increases.In the case of metal-based copper casting the cooling rate was the fastest which is 7.88 ℃ / s the grain size was the most small to 70.31μm;In the sand type ladder mode the area from the thin to the thickness is Ⅰ Ⅱand Ⅲ area;in thesand zone Ⅲ(with 10 mm thick cold iron)conditions the cooling rate was the slowest to2.94 ℃ / s the grain size was also the most coarse to166.43μm.Its grain size from 166.43μm reduced to 70.31μm reduced by about 2.5 times.The shape of the second phase of the ZM5 magnesium alloy was gradually changed from a continuous network a long chain and a block to a discontinuous snowflake needle and granular form.The mechanical properties of ZM5 magnesium alloy have been improved remarkably.Hardness increased from 54.6HB to66.8HB an increase of 22%;tensile strength increased from 165 MPa to 225 MPa an increase of 36%;elongation increased from 2.2% to 3.5% an increase of 59%;The quantitative relationship between grain size(D)tensile strength(Rm)hardness(HB)and elongation(A)was D=221.33v-0.333,Rm=exp(5.21932-0.07165v+0.01246v2),HB=exp(4.05024-0.03v+0.00655v2),A=exp(0.84396-0.002579v+0.00948v2)。The effects of different carbonaceous modifiers(including C2Cl6 Al-5Ti-0.8C master alloy MnCO3)and different additions on the microstructure and properties of ZM5 magnesium alloy were studied.The experimental results showed that the addition of the modifier the magnesium alloy played a different degree of refinement.The best additions of C2Cl6 Al-5Ti-0.8C and MnCO3 were 1.5% 2.4% 0.5%.The average grain size was reduced from unmodified 188.86 μ m to 75.87 μ m 90.34 μ m 103.75 μ m.The tensile strength increased by 16.2%(as-cast)and 39.0%(T6 heat treatment)respectively.5.9%(as-cast)36.6%(T6 heat treatment);5.4% 19.5%.The elongation increased by 25.0%(as-cast)66.7%(T6 heat treatment)respectively;42.8% 70.0%;17.9% 26.7%.The most obvious effect was C2Cl6 followed by Al-5Ti-0.8C and the last one is MnCO3.The refinement mechanism of three carbonaceous Modifier was discussed by SEM and EDS.The results show that the addition of C2Cl6 produces Al4C3 in the magnesium alloy melt which can be used as an effective nucleation of α-Mg which can improve the nucleation rate and refine the grain.Addition of Al-5Ti-0.8C master alloy in the magnesium alloy melt generated TiC and Al4C3 as the primary α-Mg effective nucleation core and then played the role of grain refinement.The addition of MnCO3 on the one hand is due to the formation of Al4C3 in the magnesium alloy melt which can be used as the core of the heterogeneousnucleation.On the other hand the introduction of C and Mn elements is enriched in the grain boundary causing the components to be subcooled and Cooling so that the grain can be refined.