Research On Removing Non-Metallic Inclusions From AZ91 Magnesium Alloy

Due to the active chemical properties of magnesium, burning often occurs when smelting magnesium or magnesium alloy. It brings lots of non-metallic inclusions in molten magnesium alloy which may impair properties of the casting. In order to increase the quality of castings, it has important meaning to study more effective methods to remove the non-metallic inclusion from molten magnesium alloy. This paper studies the purification effectiveness and purification mechanics when adding Ce or LaF₃ to eliminate non-metallic inclusions in AZ91 magnesium alloy.Add different weight percents of Ce in molten magnesium alloy under different temperature. The content of non-metallic inclusions in magnesium alloy has the trend of reduce when adding 0.1wt%⁰.7wt% Ce at 730℃. The size of the inclusion is diminished as well, which has been less than 10μm when adding 0.7wt% Ce;when the addition of Ce greater than 0.9wt%, the content of non-metallic inclusions arises contrarily. Burning occurs when adding Ce at higher temperatures, which disadvantages the purification of magnesium alloy. Ce has the effect to purify molten magnesium alloy because it is able to react with non-metallic inclusions such as MgO and MgCl₂. The reaction products are compounds of Ce which have much more greater density than molten magnesium alloy, so they can deposit to the bottom of crucible quickly.Add the mixture of LaF₃ and 1.5wt% RJ-2 flux in molten magnesium alloy at 750℃. The experimental result indicate that it has obvious purification effectiveness when adding 0.3wt%⁰.5wt% LaF₃, the volume-fraction of the non-metallic inclusion reduces from 0.742% in original magnesium to 0.121%, the mean size of non-metallic inclusions is diminished markedly;But when addition of LaF₃ is greater than 0.5wt%, the content of non-metallic inclusions arises contrarily, which may matter with the ratio of LaF₃ and RJ-2 flux.LaF₃ is able to reacts with MgCl₂ and Al in molten magnesium alloy. The reaction products are LaCl₃ and [La] respectively, which can father react with non-metallic inclusions such as MgO and the final reaction product LaOCl depositfrom molten magnesium alloy.The paper also studies the changing of AZ91 microscopic structure after adding Ce or LaF3. It indicates that besides the effect of purification, they also have the effect of grain refinement and so on.