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Oxides are the most prevalent type of


inclusions, from either direct melt oxida- tion or the oxidation of certain elements during alloying. Because of the nature of molten aluminum to readily oxidize, different oxides can form during differ- ent stages of the melting and liquid metal handling processes. Typical examples are: alumina (Al2


Sedimentation, flotation, filtration, and fluxing are common techniques for removing and separating inclusions from aluminum alloy melts. Any of these techniques will have an impact on metal cleanliness. However, fluxing is the first step for ensuring molten cleanliness by preventing excessive oxide formation, removing non-metallic inclusion from the melt, and preventing and/or remov- ing oxide build up from furnace walls.


(Al2MgO4


Impurities, Oxides, and Dross Te cleanliness of the molten bath


is also greatly affected by the degassing


(CaSiO), magnesia (MgO), magne- tite (Fe3


O3 O4


), silica (SiO2 ).


), calcium silicate ), and spinel


Knowing the level of molten cleanliness at a metalcasting facility is just part of a complete solution to eliminating inclusion-related scrap in castings.


operation, which in turn significantly impacts removal of inclusions and method of flux addition. Te steps taken to prevent hydrogen pickup and dross formation will minimize inclu- sions in molten aluminum alloys. Chemical impurity means an


unwanted chemical element has been dissolved in the molten bath. Inclusion refers to a foreign particle present in the molten metal prior to casting. Dross denotes crumpled aluminum oxide films that encapsulate a significant amount of un-oxidized aluminum, floating on the surface of the molten bath. With the exception of sludging associated with high levels of Cr, Mn, and Fe that could be considered chem- ical impurities, other typical chemi- cal impurities do not cause inclusion


related scrap defects in castings. Scrap related defects due to chemical impu- rities are associated with: • The adverse effect that excessive levels of Fe has on tensile properties.


• The poisoning effects that several ppm of P or Sb have on Na or Sr, during modification of hypoeutec- tic al-Si alloys.


• The high levels of Ca, and Na that cause edge cracking during hot rolling.


• The high levels of Li that produce the “blue” corrosion in aluminum foil.


• The high levels of Na that cause embrittlement in 5XX alloys. Potential methods for remov-


ing unwanted elements from molten aluminum alloys include selective oxi- dation, chlorination, fluorination, and intermetallic compound formation.


GRAPHALLOY® Bearings Can Take the Heat


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34 | MODERN CASTING August 2017


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