Study On Mechanism And Technology Of Carbon-addition Refinement Of Magnesium Alloys

Magnesium alloys have many unique and excellent properties, such as light weight, high specific strength and stiffness, so they have great potential for applications in aerospace, automotive, home appliances and so on. However, the mechanical properties of magnesium alloys can not meet the demand in industry. So the mechanical properties during the process need to be improved. Carbon inoculation is used widely in the casting to refine the Mg-Al alloy. It is supposed to be an effective method. But carbon inoculation still has some shortages. It is necessary to study and discuss the mechanism of carbon inoculation, especially the relationship between trace elements, such as Mn and the heterogeneous nuclei.In this paper, the micro structure and mechanical properties of AZ91 with carbon-containing agents were studied. A further study was carried on the mechanism by which carbon addition causes grain refinement in magnesium alloys containing aluminum. The research also provides the basic research data and theory basis to how to manufacture high-strength magnesium alloys.The effects of pouring process on the microstructure and mechanical properties of AZ91 were studied. It is found that excellent refinement can be got by pouring 1wt% MgC03 powders into the melt at 740℃. Secondly, very good refinement can be got when 5 min gas of CO2 into the melt with the flow of 0.3L/min at 740℃before pouring. By comparing the refinement effects of different carbon inoculation (MgCO3,C2Cl6 and CO2), MgCO3 powders is supposed to be the best one.Experimental results show that, the addition of carbon inoculation can not refine the Mn-free magnesium alloys. On the contrary, the carbon inoculation can refine the magnesium alloys containing Mn element. SEM, EDS, DTA analysis result and a hypothesis proposed can explain the phenomenon. Carbon addition firstly causes the formation of heterogeneous nuclei in the melt, such as aluminum carbide. Then the heterogeneous nuclei causes the change in the morphology. And the Al8Mn5 phase was formed, which was supposed to be the nucleant of a-Mg in the magnesium.