Study of al-Mn-Fe intermetallics in Mg-Al Alloys

Field: Materials Science (Physical Sciences)

Al-Mn-Fe intermetallics are very important to magnesium metallurgy. They are responsible for removal of iron from the melt and improved corrosion protection of the solidified alloy. They have also been linked to grain-refinement and die-casting sludge. Despite their importance, there are a number of questions raised in the literature concerning the properties of Al-Mn-Fe intermetallics which remain unanswered. This thesis addresses this knowledge gap with a thorough examination and documentation of the properties of Al-Mn-Fe intermetallics and their relevance to the industrial processing of Mg. The thesis starts by investigating the sludge material from two industrial die-casting operations in North America. Contrary to the literature, it was observed that the majority of sludge was made up of entrapped Mg and Al-Mn-Fe intermetallics and not MgO. The intermetallics in this sludge were found to be rich in Mn with very little Fe present, and this suggests that much of the Mn used for corrosion protection ends up wasted as excess Al-Mn-Fe intermetallics at the bottom of the furnace collectively termed “sludge”. Sludge was observed to be made up of a network of agglomerates which serve as a very good skeleton for Mg metal entrapment and this explains why sludge removal can account for a significant proportion of melt loss. Prior to this current work, there have been no studies reported in the literature which measure settling rates for Al-Mn-Fe intermetallics in molten Mg alloys. An experimental program was therefore designed and implemented to do so. In the course of the investigation, it was determined that there are different morphologies for intermetallic particles and that these have an effect on their settling behaviour. A classification regime for intermetallic morphology has been developed. It has also been observed that a particle transformation occurred during the settling trials with polyhedral-shaped particles growing into structures which have long arms or petals. These floral-shaped particles did not settle readily from the melt and it has been proposed that particle shape along with convection effects are the cause for this. Since analysis of the die-casting sludge showed that settled intermetallics were mostly made up of Mn, a continuous crystallization experiment was designed and carried out to determine the feasibility of using the Gibbs-Thompson relationship between solubility and particle size to reduce the Mn usage required for Fe removal. The experiment showed that crystallization of Al-Mn-Fe intermetallics is not commercially viable. The reasons for this include: . • intermetallic growth is too slow; . • particles agglomerate with a loose packing density giving little reduction in surface area, and . • intermetallics being much denser than the Mg liquid are difficult to maintain in suspension once agglomerates form. The nature and properties of Al-Mn-Fe intermetallics have also been examined during the melt superheating process for grain-refinement. There are a number of Fe-rich intermetallics that are present just after cooling the melt from superheat temperatures. These deplete in number as the holding time increases, corresponding with an increase in grain-size. A number of Al-Mn-Fe particle characteristics match well with the grain refining behaviour exhibited in Mg-Al alloys following superheating. Nevertheless, a definitive mechanism for the grain-refinement effect remains elusive. Overall, this thesis contributes broadly to the growing body of knowledge on industrial magnesium processing and, in particular, it contributes substantially to a new understanding of the role and effects that Al-Mn-Fe intermetallics have in molten and solidified Mg-Al alloys.