Characterization of the Early Surface Films Dynamically Formed on Molten AZ91D and AM60B Alloys under Different Atmospheres: A Comparative Study

In the present study, the early stages of the surface oxidation and fluoridation of liquid AZ91D and AM60B alloys under ultra-high purity (UHP) argon, dry air, and air mixed with two different protective fluorine-bearing gases were studied. The chemical composition, morphology and thickness of the surface films formed inside the trapped bubbles were characterized by SEM equipped with EDS analysis. It is found that the molten AM60B alloy is more sensitive to impurities under UHP argon gas than AZ91D alloy. Under dry air atmosphere, the entire surface of molten AZ91D alloy is covered with an oxide layer and thinner thickness than the surface film formed on AM60B alloy which has rough surface exhibiting granular growth in later stages of oxidation. The EDS analysis shows that the film chemistry is mainly composed of Mg, Al and O elements. Under fluorine-bearing gas/air mixtures with either SF6 or HFC-R134a at 3.5%vol., a fresh surface film formed with a flat and dense morphology with a uniform thickness consisting of Mg, F, Al and O elements. It is observed that there is a lower O:F intensity ratio in the surface film formed on the molten AZ91D alloy under 1,1,1,2-tetra-fluoroethane (HFC-R134a) mixed with dry air compared to the AM60B alloy under both air/ R134a and air/SF6 mixtures which shows a higher fluorine concentration in the surface film a leading to a better oxidation resistance.