Simple Solutions That Work! Issue 17

JOHN REYNOLDS Wedron Flux Product Manager HA-International LLC ARTICLE TAKEAWAYS: • Develop and document a good furnace cleaning procedure • Furnace Tenders must have ample time and tools • Avoid metal loss • Let the wall cleaners do the bulk of the work Aluminum Fluxes 101 - Basics of Furnace Cleaning, Dross Handling and Wall Cleaning C asting defects, energy usage, metal recovery, ergonomics, and labor are cost variables addressed by properly identifying and applying fluxes for the aluminum casting operation. Proper selection and application are especially important to realize the benefits that flux can provide. Defects stemming from the use of dirty metal are plentiful. Metallic aluminum is very reactive with atmospheric oxygen, and a thin passivation layer of aluminum oxide (4 nm thickness) forms on any exposed aluminum surface in a matter of hundreds of picoseconds to form aluminum oxide (Al2O3). Hydrogen forms whenever molten aluminum meets water vapor, and easily dissolves into the melt. The gas tends to come out of the solution and forms bubbles when the melt solidifies. The detrimental effects arising from the presence of an excess of dissolved hydrogen in aluminum are numerous. Aluminum oxide and hydrogen gas H2 are the two most common defects found in aluminum castings. Magnesium formation is also common in aluminum melting due to reaction with the alloy or with the refractory which typically contains magnesium and oxidizes rapidly forming magnesium aluminate (spinel), which visually forms black speck inclusions in the metal surface. Aluminum fluxes will address all three potential inclusions from the process of melting to final molten metal preparation. This article will cover two of the seven fluxes listed below*, drossing and wall cleaning fluxes, and some of the key savings. WHAT’S IN FLUX? Aluminum fluxes are available in several forms. Powder blends are the most widely used however granular and tablet forms are also available. Potassium Chloride (KCI) and Sodium Chloride (NaCI) inorganic chemical compounds are the base for most flux formulations. Sulfates, fluorides, carbonates, nitrates, and other chemicals are used with the base compounds depending on the intended application. Types of fluxes used by aluminum casters • Drossing Fluxes / Exothermic * • Wall Cleaning Fluxes * • Cleaning and Degassing Fluxes • Cover Fluxes • Charging Fluxes • Grain Refining Fluxes • Silicon Modification Fluxes DROSSING FLUXES / EXOTHERMIC REACTIVITY Drossing is the most common type of flux used by Foundries and Die Casters. Drossing fluxes are used on crucible furnaces, reverberatory furnaces, tower furnaces, holding furnaces, and transfer and pouring ladles. During the melting process, a thick layer of aluminum oxide forms on the surface of the molten bath. The dross layer contains crumpled films of Al2O3, MgO, or MgAl2O4 spinel. Trapped in the Continued on next page 11 BACK TO BASICS