آلیاژهای al-si-mg

To evaluate the effect of magnesium content on the microstructure and hardness of the Al-Si-Mg composites in the centrifugal casting method, three cylinders with the chemical composition of Al-20Si-XMg (X = 6, 9, 12) (as weight percent) were cast. Then the microstructure and hardness of the different radial sections were studied by optical microscope, SEM equipped with a micro-analysis system (EDS), and standard brinell hardness testing method, respectively. The phase diagram of Al-20Si-XMg system was plotted as a function of Mg% using Thermo-Calc software. Also JMat Pro software was employed to plot the variation of the mass fraction and density of the in situ formed phases during the solidification of the alloys. The results show clearly that while the coarse Mg2Si particles are formed in high Mg content alloys; however, these particles along with the primary Si particles, both, due to the low density, based on Stokes' law in fluid mechanics, are centripetally segregated towards the inner layers of the cylinders. In addition, by increasing the Mg content of the alloys from 6% to 9% then 12% the volume fraction and average size of the Mg2Si particles in inner layer of the cylinders, both, increase respectively from less than 7% to about 28% and from less than 54 microns to about 166 microns. But, since Mg2Si particles are softer than Si particles, by increasing the volume fraction of the Mg2Si particles, the hardness of the inner layers of the cylinders reduces from 86 to 81 and then 78 brinell.

In this study, centrifugal casting method was used to produce a hyper-eutectic composite Al-Mg2Si ‎with hybrid microstructure and graded hardness. For this purpose, two cylinders with a chemical ‎composition of Al-20Si and Al-20Si-9Mg (weight percent) were cast using a vertical centrifugal ‎casting machine. Then the microstructure and hardness of the cast cylinders were studied along the ‎radial direction by optical and scanning electron microscopes and brinell method, respectively. The ‎results show that in Al-20Si cast cylinder; in contrary to the outer layer which shows only a binary ‎eutectic Al-Si microstructure, the inner layer contains a high volume fraction of in situ segregated ‎primary silicon particles inside the eutectic matrix. By addition of 9% Mg to Al-20Si alloy, the inner ‎layer of the cast cylinder form a hybrid microstructure containing of both Si and Mg2Si particles ‎inside the eutectic matrix, while the outer layer shows only a ternary eutectic Al-Si-Mg2Si ‎microstructure. As a result of these microstructure changes along the radial direction of the ‎cylinders, not only the hardness of both cylinders are smoothly increased from the outer towards the ‎inner layers, but also all radial sections of the Al-20Si-9Mg cylinder in compared with the other one, ‎indicate higher hardness (about 18 brinell).