Study On Microstructure And Properties With Ca Alloying And Nd Modification Of Mg-8Al Magnesium Alloy

Magnesium alloy has the extremely broad application prospect in the non-ferrousmaterials. Since magnesium alloy enjoys such advantages as hypobarism,good cushioningeffect, light quality,high specific strength, high specific stiffness, etc., it is widely used in theaerospace, electronics, communication, automobile, biological medicine and other fields.However, its further development is limited as its low corrosion resistance, poor shock andheat resistance. In Mg-Al alloy, aluminum element can improve the properties of magnesiumalloys. In the process of alloy liquid-solid transformation, the eutectic structural β-Mg17Al12phase with continuous mesh structure separated out at the grain boundary of α-Mg, which canlead to the decrease of toughness and the increase of brittleness for the alloys, untimately thereduction of comprehensive mechanical properties. Hence, it restrict the development ofMg-Al alloys. If we can reduce the number of eutectic β-Mg17Al12phase, improve distributionand form, the eutecticstructure of alloys will play a positive role to enhance strength andweaken the substrate and reduce toughness of metals. So, the research’s emphasis is how toimprove the eutectic alloy’s eutecticstructure.In this paper, the Mg-Al alloys are selected as the materials. The methods of metalalloying and modification and both conventional gravity casting and liquid die forgingforming are used to study the influences of the alloy elements and forming techniques onmicrostructure and the mechanical properties of alloys. The main adding elements are Al, Ca,Nd and so on. The eutectic β-Mg17Al12phase number, morphology and distribution areinvestigated in different content of adding elements, and their effects on microstructure andmechanical properties are analyzed. Experimental results are shown as followsThe microstructures of alloys adding Ca element are composed of α-Mg, β-Mg17Al12and Mg2Ca phases during gravity casting molding. Within a certain range, Mg–8AL alloy’smicrostructure morphology will be refined with the increase of the content of Ca, and themorphology and distribution of eutecticβ-Mg17Al12phasechange from dendritic to fine acicular or/and short strip. The mechanical properties of refined alloys are also improved.However, the fracture morphology shows cleavage fractureduring tension test, whichindicates the alloysare the brittle materials.After the alloys are treated by the Ca alloyingandthe modification of rare earth elements Nd, their microstructures are composed of thesephases of α-Mg, β-Mg17Al12,Mg2Ca and Al11Nd3inthe process of gravity casting. Rare earthelements added in the alloys can alter the growth form of dendrites α-Mg to refine the grainsize, and they lead to local constitutionalsupercoolingand increase the dendrites of α-Mg. Themorphology of β-Mg17Al12phase is refined by adding Nd elements. When rare earth Nd’saddition amount reaches a certain degree,new hard brittle phase Al11Nd3is produced, which isthe same asβ-Mg17Al12. Distribution of the phase is plate strip, and it is againsttheenhancement of the comprehensive performance of the alloys. Therefore, the addition amountof rare earth Nd should not be more than0.6%.During the liquid die forging forming, the element Ca is added firstly in the Mg alloysfor alloying processing.It can be clearly seen that alloys’ microstructures are refined and themechanical properties are markedly enhanced in comparison with gravity casting alloys.Withthe increase of Ca element, the grains sizeincrease firstly and then decrease, which can beattributed to an excess of the Ca. When the content of Ca element is1.0%,eutecticMg17Al12phases are refined and their distribution are homogeneous after liquid dieforging. Hence, the mechanical properties are best.In the process of liquid die forging forming,the extremely fine microstructure and uniformly dispersed Mg17Al12phase are obtained byadding the rare earth Nd and metal Ca, which have a positive impact on the improvement ofmechanical properties of alloys. When Nd content reaches about0.4%, a small amount of rareearth phases(Al11Nd3)are precipitated to improve the high temperature creep resistance ofalloys; When the rare earth content more than0.6%, the rare earth phases precipitatedcansplitthe substrate and reduce mechanical properties the alloys.