Effect of Mn Addition on Melt Purification and Fe Tolerance in Mg Alloys

Two methods for impeding the detrimental impact of iron (Fe) impurity in magnesium (Mg) alloys on the corrosion properties are studied: the addition of manganese (Mn) and the selection of melting temperature. Experimental work was carried out on Mg and AZ91 alloys with selected additions of Mn and selected settling temperatures. The settling velocity was evaluated using the Stokes equation. Extensive thermodynamic calculations using different databases in comparison were performed and supported by experimental observations. The conditions for the formation of the different types of precipitate phases and their detailed composition are revealed. Practical guidelines for settling temperature and time are proposed. The tolerance limit of Fe, expressed by the critical Fe/Mn ratio, could be related to key phase boundaries in the calculated Mg-Aluminum (Al)-(Zn)-Mn-Fe phase diagrams for AM and AZ types of alloys.