Effect Of Ca,Si And B,RE Joint Additions On Microstructures And Mechanical Properties Of AM60 Magnesium Alloy

Magnesium alloys have the advantages of light-weight,high strength/weight ratio,high stiffness/weight ratio, high damping property and electromagnetism shielding, so it will act an important role in automobile and electric industry as a kind of weight reducing material. Following with the lightening of the automobiles,more and more automobiles' components are made with magnesium alloys. The hub for vehicle wheel should have both high strength and high ductility depending on its working environment. AM60 are the most broad used magnesium alloys in industry. Its' strength /toughness ratio and ductility is more meet the needs of hub. But its strength and toughness needs to be improved.In this study AM60 based alloy has been fabricated with microalloying elements Ca/Si and B/RE, and the mechanical properties at room temperature and microhardness were studied. By using modern analysis and test facilities such as optical microscopy(OM), X-ray diffractometer(XRD), electron probe microanalyzer (EPMA),scanning electron microscopy(SEM)with electron dispersion X-ray analysis(EPMA),etc, effects and their mechanism of Ca/Si and B/RE joint addition and heat treatment on the microstructures and mechanical properties have been investigated.The results indicate that new phase didn't assumed ,after a small quantity of Ca addition.Ca dissolves in the MgnAln. After Ca, Si joint additions, Ca and Si formed compound CaSi2 which act as nucleus of Mg2Si. So the morphology of Mg2Si particles can be modified by Ca addition. Morphology of the MgaSi phases changed form coarse Chinese script shape to the fine polygonal shape. The microstructure of AM60 alloy was refined due to Ca, Si joint additions. In this result, Mg2Si particles strengthened the Mg-Al alloy and didn't reduce the toughness.Rare earth combined with aluminum to rod-shaped AluREs preferentially after it has been added to Mg-Al based alloys. Rare earth, aluminum and manganese would combine to an AljoREaMny ternary phase. The addition of rare earth had the effect of second-phase strengthening. Because there are not so much P phase(Mgi7Ali2) in AM60 alloy, so such strengthening was well exhibited.There are many Al-B compounds (A1B2, a - AlB12, P - AlBi2 et.al.)whose melting points are high in the Al-B alloys. Al-B alloys added to Mg-Al based alloys, Al-B compounds didn't melt in fluid magnesium.After fluid magnesium solidified ,a lot of compound AlBi assumed ,most of them on the grain boundary that destoried the integrated of alloys. So the alloys mechanical properties were decreased, despite the refined grains.B element firstly existed around rare earth phases, if B and rare earth were jointly added to Mg-Al based alloys. So rare earth phases became more short and more separated. In a certain extent, B reformed that rod-shaped AluREs phases dissevered the matrix. As a result, the tensile strength was improved after B and rare earth were jointly added to Mg-Al based alloys.Added different metal element to alloys, there were different effects presented. AM60+B alloys' microhardness was decreased by solid solution heat treatment(410"C, 8~12hr), about 4.8%.But AM60+0.3Ca+XSi, AM60+1.6RE alloys' microhardness were higher than as-cast alloys .All the studied alloys' microhardness were improved by T6 heat treatment(410 C solid solution ,205 C aging),because of separated out minute dispersed 3 -phase. AM60+0.3Ca+XSi alloys' tensile strength were improved after T6 or T5 treatment(205 C aging, 18hr), and the raised extent is biger after T6 treatment. AM60+0.3Ca+XSi alloys' elongations also were improved after T6 or T5 treatment, about 10%.