Effect Of Alloying Elements And Solidification Rate On Microstructure And Mechanical Properties Of AM60 Alloy

The mechanical properties of various commercial casting magnesium alloys are generally low, which limits the application of magnesium alloys. To avoid this limitation, grain refinement is one of the methods applied to improve the strength and toughness of magnesium alloys by adding alloying elements or increasing the cooling rate. The alloy elements, especially the rare earth elements, have different effects on the magnesium alloys at different solidification rates; however, no systematic study has been done. AM60 magnesium alloy is one of the most widely used low-cost commercial magnesium alloy. It is necessary to study the influences of alloying elements and solidification rate on AM60 magnesium alloy. Understanding the optimal composition and process of this study, which can lay the foundation of optimizing the performance of casting magnesium alloys.This thesis studied AM60 magnesium alloy with methods, such as the X-ray diffraction, optical microscopy, scanning electron microscope with energy dispersive spectroscope analysis, differential thermal analysis and mechanical properties test. Two rare earth elements, Ce, and Nd, and an alkaline earth element, Sr, were added to AM60 magnesium alloys, to investigate the microstructures and mechanical properties under three different cooling rates(air-cooling, mould-cooling, and water-cooling). The results of this study are listed as following:(1) In the AM60 magnesium alloy with an addition of 0.5wt.% Ce, the number and the size of ?-Mg17Al12 phase decreased; Al11Ce3 precipitates were formed; Al-Mn phase were transformed into Al10Ce2Mn7 at the three different cooling rates. With the increase of the cooling rate, the second dendrite arm spacing was significantly reduced; the secondary phases were fine and distributed dispersively; the number of ? phase decreased; needle-like phase Al11Ce3 increased. Compared with AM60 alloy, the mechanical properties of AM60-0.5Ce alloy are higher under mould-cooling condition, but lower under air-cooling and water-cooling conditions respectively.(2) In the AM60 magnesium alloy with an addition of 0.9wt.% Nd, the number and the size of ?-Mg17Al12 phase decreased, rod-like Al3 Nd phase and needle-like Al11Nd3 phase were formed at the three different cooling rates. With the increase of the cooling rate, the second dendrite arm spacing was reduced; the secondary phases were fine and distributed dispersively; the number of ? phase and Al3 Nd phase decreased; needle-like phase Al11Nd3 increased. Compared with AM60 alloy, the mechanical properties of AM60-0.9Nd alloy are higher under mould-cooling condition, but lower under air-cooling and water-cooling conditions respectively.(3) In the AM60 magnesium alloy with an addition of 0.3wt.% Sr, the number and the size of ?-Mg17Al12 phase decreased, Al4 Sr and Mg58Al38Sr4 phase were formed at the three different cooling rates. With the increase of the cooling rate, the second dendrite arm spacing was reduced; the secondary phases were fine and distributed dispersively; the number of ? phase and Mg58Al38Sr4 phase decreased; Al4 Sr phase increased. and the mechanical properties have been improved. Compared with AM60 alloy, the mechanical properties of AM60-0.3Sr alloy are higher under air-cooling and mould-cooling conditions, but lower under water-cooling condition respectively.(4) In the AM60 magnesium alloy with addition of 0.5wt.% Ce and 0.9wt.% Nd, the number and the size of ?-Mg17Al12 phase decreased; Al3 RE and Al11RE3 precipitates were formed; Al-Mn phase were transformed into Al10RE2Mn7 at the three different cooling rates. With the increase of the cooling rate, the second dendrite arm spacing was reduced; the secondary phases were fine and distributed dispersively; the number of ? phase decreased; needle-like phase Al11Ce3 increased. Compared with AM60 alloy, the mechanical properties of AM60-0.5Ce-0.9Nd alloy are higher under mould-cooling condition, but lower under air-cooling and water-cooling conditions respectively.(5) In the AM60 magnesium alloy with addition of 0.5wt.% Ce and 0.3wt.% Sr, the number and the size of ?-Mg17Al12 phase decreased, Al4 Sr and Al11Ce3 precipitates were formed at the three different cooling rates. With the increase of the cooling rate, the second dendrite arm spacing was reduced; the secondary phases were fine and distributed dispersively; the number of ? phase decreased; Al4 Sr phase increased; the number of needle-like phase Al11Ce3 had not changed obviously. Compared with AM60 alloy, the mechanical properties of AM60-0.5Ce-0.3Sr alloy are higher under mould-cooling condition, but lower under air-cooling and water-cooling conditions respectively.