Oxide films and particles are intro- duced and/or generated during the charging and melting practices, as well as during the molten metal treatment and handling operations. Different alloys under similar charging practices can have significantly different oxide skins, and identical alloys from differ- ent “heats” will enter the molten metal with different oxide contents. Even primary ingot can introduce oxides. Entrained oxides particles and other inclusions can be floated out with the assistance of inert and/or chlorine gas purging. Te non- metallic particles attach to the surface of the rising gas bubbles, collect on the surface and therefore can be skimmed off. After a meltdown, any molten aluminum alloy will have a large variety of finely divided small quanti- ties of particles suspended in the body of the melt and a layer of wet dross on the surface of the aluminum alloy. Te initial thin oxide film that develops on the surface of the melt offers protec- tion from further oxidation. However, the constant movement of the surface of the molten bath due to the different melting practices (charging, skimming, cleaning, degassing, transferring, and ladling) causes the thin alumina films to break, to crumble, to thicken and to encapsulate unoxidized molten alumi- num, generating what is known as wet dross. Te presence of alkali and alkali earth elements, even in small amounts, could increase the permeability of the film which in turn increases both melt oxidation and dross formation. Te aluminum content of wet

drosses is typically reported to be in the order of 60-80% while the remain- ing 20-40% is aluminum oxide. Te amount of trapped liquid metal in the dross varies according to the melting practice. Te aluminum oxide is a sta- ble compound that cannot be reduced to aluminum under ordinary melt- ing conditions. However, the amount of suspended liquid metal could be reduced from the 60-80% range to 30% by proper fluxing and drossing techniques. Te dross is considered to be the main contributor in influencing the total metal loss during melting. Depending upon the efficiency of the melting furnace and melting practices, the amount of dross generated may be

August 2017 MODERN CASTING | 35

from 5-10% of the total metal melted. However, the total melt loss through- out the operation can also be influ- enced by other process steps that are insensitive to the charged material. Knowing the level of molten

cleanliness at a metalcasting facility is just part of a complete solution to eliminating inclusion-related scrap in castings. Te other two key factors are

establishing a correlation between the defect in the casting and the inclusions in the molten metal, and implement- ing proper corrective action. Tese two factors will be discussed in the second part of this series in the September issue of Modern Casting.

Tis article is based on a paper (17-106) originally presented at the 2017 Metalcasting Congress.

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