Aluminum and aluminum alloys can be smelted in a variety of different ways. Commonly used are coreless induction furnaces and trough induction furnaces, crucible furnaces and reflective open hearth furnaces (using natural gas or fuel oil combustion) as well as electric resistance furnaces and electric radiation furnaces. Furnace materials range from high-quality pre-alloyed ingots to furnaces made up of low-grade waste. However, even under the most suitable conditions for smelting and casting, molten aluminum is susceptible to three types of adverse effects:
• Under high temperature conditions, the adsorption of hydrogen causes an increase in hydrogen dissolved in the melt over time.
• At high temperatures, the melt oxidizes over time.
· Loss of alloying elements.
Hydrogen is easily adsorbed by molten aluminum. Unfortunately, in molten aluminum alloys, the solubility of hydrogen is substantially greater than its solubility in solid aluminum. When the aluminum alloy solidifies, hydrogen gas is discharged from the melt, and the shrinkage porosity is enlarged and amplified, accompanied by loss of mechanical properties. Hydrogen is generally derived from wet charge and wet melting tools, but the primary source of hydrogen is moisture in the environment. Since it is almost impossible to prevent the adsorption of hydrogen gas during smelting, hydrogen must be removed from the melt before pouring. The most commonly used method is to blow dry nitrogen or argon bubbles into the melt. The use of chlorine to remove hydrogen is particularly effective. However, it is often excluded from production for environmental and safety reasons.
In the past, the amount of hydrogen dissolved in the melt has been measured by a reduced pressure test by injecting a sample of molten aluminum into a steel cup and allowing it to solidify in a vacuum chamber. Observing the solidification process, it was found that the degree of bubble change during solidification indicates the amount of hydrogen present. At the same time, the size of the bubble formed can be checked by using the sliced sample after solidification. Unfortunately, these methods are not precise and are greatly affected by the presence of oxide particles in the melt as nucleation of hydrogen bubbles. A better way to test dissolved hydrogen is to use a specially designed instrument that uses liquid extraction techniques to display hydrogen.
Aluminum instantaneously forms a very stable oxide on the surface of the melt. The rate of oxidation increases with increasing temperature and the presence of certain alloying elements such as magnesium and barium. However, if the surface of the aluminum melt is not disturbed, the oxide film formed on the surface is self-limiting, and any turbulent flow will stir the oxide film into most of the melt and produce a fresh surface to facilitate More oxide formation. The resulting oxide film and oxide impurities are very detrimental to the properties of the cast aluminum part, however, turbulence is caused during alloy smelting, transfer of molten metal or casting and casting.
The oxide particles in the melt become cores that form craters and pores. In the absence of oxide impurities, the pores and micropores are substantially eliminated. For the production of cast aluminum parts, reducing oxide impurities is a particularly important condition. Since they usually have a very large difference between the liquidus and the solidus, and condense in a porous state, it is difficult to supply the pores.
The oxide film of the casting forms a fragile surface that is extremely vulnerable to failure. The non-uniformity of the mechanical properties of the cast aluminum alloy is precisely caused by the existence of these oxide films. Without these oxide films, the unevenness will be reduced and the casting properties will be reduced. The repeatability is better than that of forgings. When X-rays are used, these oxide films are usually invisible, but they must be prevented beforehand and not repaired afterwards.
In the molten state, the coverage of the flux can be utilized to control the oxide. These fluxes are typically magnesium chloride salts. They float on the surface of the melt. However, it is still necessary to periodically remove oxides from the surface of the melt, and the molten oxide impurities can be removed from the melting furnace by passing the melt through the filter bed. For smaller scale production, filters can be placed in the gating system to remove oxides.
In order to prevent the formation of an oxide film in the casting, it is necessary to let the metal enter the cavity of the mold in a state of turbulent flow. For most castings, this is not possible with gravity casting because the head height of the sprue speeds up the flow and causes turbulence, so anti-gravity or liquid level casting techniques must be used. This filter slows the flow of metal, making it slow enough to prevent oxide formation. In addition, the cavity of the mold must be injected from the bottom, and the order of the injection of each liquid level of the casting must be carefully designed to avoid the occurrence of a “waterfall”—the liquid metal in the mold drops from a higher liquid level to a lower liquid level. An oxide is formed on the surface of the nascent metal. By injecting the mold from the bottom, the oxide layer on top of the liquid metal will rise to the top of the upper flask level and into the top of the riser so that the casting is not damaged.
Many cast aluminum alloys contain elements such as magnesium that slowly react with oxygen. The molten metal is stored for too long, and these elements are gradually oxidized, resulting in chemical composition of the casting that is not up to standard, while other alloying elements, For example, zinc with a low gasification pressure will also evaporate from the surface of the bath.