What are the requirements for aluminum alloys in the manufacturing and synthesis process? Aluminum and aluminum alloys can be melted using various methods. Commonly used are coreless induction furnaces and slot induction furnaces, crucible furnaces and reflective open hearth furnaces (using natural gas or fuel oil combustion), as well as resistance furnaces and electric radiation furnaces. There are a wide variety of furnace materials, ranging from high-quality pre alloyed ingots to furnace materials specifically composed of low-level waste. However, even under the most suitable conditions for melting and pouring, molten aluminum is susceptible to three types of adverse effects:
Under high temperature conditions, the adsorption of hydrogen gas leads to an increase in hydrogen gas dissolved in the molten liquid over time.
·Under high temperature conditions, the molten liquid undergoes oxidation over time.
·Loss of alloying elements.
Hydrogen is easily adsorbed by molten aluminum. Unfortunately, in molten aluminum alloys, the solubility of hydrogen gas is basically greater than its solubility in solid aluminum. When aluminum alloy solidifies, hydrogen gas is discharged from the melt, causing shrinkage porosity to expand and amplify, accompanied by a loss of mechanical properties. Hydrogen gas generally originates from wet furnace materials and wet melting tools, but the main source of hydrogen gas is humidity in the environment.
Because it is almost impossible to prevent the adsorption of hydrogen during melting, it is necessary to remove hydrogen from the molten liquid before pouring. The most commonly used method is to blow dry nitrogen or argon bubbles into the molten liquid. The use of chlorine gas to remove hydrogen gas is particularly effective. However, due to environmental and safety reasons, its use in production is often ruled out.
In the past, a pressure reducing test method has been used to measure the amount of hydrogen dissolved in the molten liquid. The process involves 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 changes during the solidification process indicates the amount of hydrogen present. Simultaneously using solidified sample slices can check the size of bubbles formed.
Unfortunately, these methods are not precise and are greatly influenced by the presence of oxide particles as hydrogen bubble nuclei in the melt. A better way to test dissolved hydrogen gas is to use specially designed instruments that utilize liquid extraction technology to display hydrogen gas.
Aluminum forms a very stable oxide instantly on the surface of the molten liquid. The rate of oxidation increases with the increase of temperature and the presence of certain alloying elements such as magnesium and beryllium. If the surface of the aluminum melt is not disturbed, the oxide film formed on its surface is self limiting, and any turbulence will stir the oxide film into most of the melt and produce a fresh surface to facilitate the formation of more oxides. The generated oxide film and oxide impurities are very harmful to the performance of aluminum castings. However, turbulence can be caused during alloy smelting, transportation or pouring of molten metal, and mold filling.
The oxide particles in the molten liquid become the nuclei that form shrinkage and porosity. When there is a lack of oxide impurities, the pores and micro pores also disappear. Reducing oxide impurities is a particularly important condition for the production of aluminum castings. Because there is usually a very large amplitude difference between their liquidus and solidus, and condensation in a porous state makes it difficult to provide supply to the pores.
The oxide film on the casting forms a fragile surface that is prone to failure. The non-uniformity of the mechanical properties of cast aluminum alloy is precisely caused by the presence of these oxide films. Without these oxide films, the non-uniformity will be reduced, and the repeatability of the casting performance will be better than that of the forging. When inspected by X-ray, these oxide films are usually invisible, but they must be prevented in advance rather than repaired after discovery.
In the molten state, the oxide can be controlled by covering it with flux. These fluxes are generally magnesium chloride salts. They float on the surface of the molten liquid. However, it is still necessary to regularly remove oxides from the surface of the molten liquid. These suspended oxide impurities can be removed from the large melting furnace by passing the molten liquid through a filter bed. In small-scale production, filters can be installed in the pouring system to remove oxides.
In order to prevent the formation of oxide film in the casting, it is necessary to allow the metal to enter the mold cavity in a highly turbulent state. For most castings, gravity casting cannot achieve this because the height of the water head of the sprue will accelerate the flow rate and generate turbulence. Therefore, it is necessary to use counter gravity or liquid level mold casting technology. This way, the filter slows down the speed of metal flow enough to prevent the production of oxides.
In addition, it is necessary to inject the mold cavity from the bottom, and the order of injection into each liquid level of the casting should be carefully designed to avoid a "waterfall" - the liquid metal in the mold falls from a higher liquid level to a lower liquid level, thereby forming oxides on the surface of the new metal. By injecting the mold from the bottom, the oxide layer on the top of the liquid metal will rise to the top of the upper sand box layer and flow into the top of the riser, so as not to damage the casting.
Many cast aluminum alloys contain elements such as magnesium that slowly react with oxygen. If the molten metal is stored for too long, these elements will gradually oxidize, leading to substandard chemical composition of the casting. Other alloy elements, such as zinc with low gasification pressure, will also evaporate from the surface of the bath.
