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1 Purpose of melting The basic task of melting metal alloys is to put metal feedstock in a certain proportion into a furnace, where it is heated and melted to form a melt; thereafter, the composition of this melted melt is adjusted to obtain an alloy liquid that meets the required specifications. Appropriate measures are also taken during the melting process to control the levels of gases and oxidative inclusions, so as to ensure that the composition meets the specified requirements (including the levels of major components or impurity elements), and to guarantee that the castings obtain an appropriate microstructure (grain refinement) as well as a high-quality alloy melt. Due to the properties of aluminum, aluminum alloys have a strong tendency to develop pores, and they are also highly prone to forming oxidation inclusions. Therefore, preventing and removing gas and oxidation inclusions has become the most prominent issue in the melting process of aluminum alloys. To obtain high-quality aluminum alloy melt, strict control must be exercised over its melting process, with measures taken to regulate it from all aspects. 2 Smelting Process The aluminum alloy smelting process is as follows: charging the furnace → melting (adding copper, zinc, silicon, etc.) → skimming off slag → adding magnesium, beryllium, etc. → stirring → sampling → adjusting the composition → stirring → refining → skimming off slag → transferring to a converter → further refining and aging → casting. Charging the furnace: The correct method of charging the furnace is important for reducing metal loss and shortening the melting time. For a reverberatory furnace, a layer of aluminum ingots is placed at the bottom of the furnace, materials that are prone to burning are added on top, and then more aluminum ingots are placed on top of those. The scrap with a lower melting point is placed on the upper layer, so that it melts first and flows down to cover the materials below that are prone to damage, thereby reducing such damage. Various furnace charges should be evenly and evenly distributed. Melting: The melting process and the melting rate have a significant impact on the quality of aluminum ingots. When the charge is heated to the point where it softens and settles, the flux should be applied appropriately. During the melting process, care must be taken to avoid overheating. Once the liquid level of the melted charge becomes level, the melt should be stirred properly to ensure uniform temperature, which also helps to accelerate the melting process. An excessively long melting time not only reduces the furnace’s production efficiency but also increases the gas content in the melt; therefore, secondary refining of the melt is necessary when the melting time is too long. Scraping the slag can be done once all the charge has melted to the melting temperature. Before skimming off the slag, a powdered flux should be added first (for high-magnesium alloys, a sodium-free flux should be used). Scraping should be done as thoroughly as possible, as the presence of slag can easily contaminate the metal and increase the gas content in the melt. After skimming off the slag, magnesium ingots can be added to the melt; at the same time, a flux should be added for coverage. For high-magnesium alloys, to prevent magnesium burnout, 0.002% to 0.02% beryllium should be added. Beryllium can be obtained by metal reduction from sodium beryllium fluoride, which is added together with a flux. Stirring: There should be sufficient time for stirring before sampling and after adjusting the composition. Stirring should be gentle to avoid damaging the oxide film on the surface of the melt. Sampling: After the melt has been thoroughly stirred, sampling should be carried out immediately for in-furnace analysis. Adjusting the composition: When the composition does not meet the standard requirements, supplementation or dilution should be carried out. After adjusting the composition of the melt in the converter, and once the melt temperature meets the requirements, the surface scum is removed, after which the conversion can proceed. Refining of the melt: Depending on the impurities present, different methods are used for purification. Adjustment of recycled aluminum composition During the melting process, the amounts of various elements in the metal decrease due to their oxidation. The degree of oxidation depends not only on the element’s affinity for oxygen but also on factors such as its concentration (activity) in the liquid alloy, the properties of the oxides formed, and the temperature at which the process takes place. Generally, elements with a higher oxygen affinity lose more; aluminum, magnesium, boron, titanium, and zirconium have a very strong oxygen affinity ； Carbon, silicon, manganese, etc., come next ； Iron, cobalt, nickel, copper, and lead are weaker. Therefore, in the melting of alloys, elements with a higher affinity for oxygen will be \"preferentially oxidized\", resulting in excessive loss ； Conversely, those elements with weaker oxygen affinity are relatively \"protected\" and suffer less degradation. After melting, the content of a certain element in the alloy’s chemical composition may increase or decrease due to oxidation losses, depending on the relative loss of that element compared to the elements of the base metal. The content of elements with higher relative loss will decrease, a phenomenon known as \"burnout\"” ； For elements with relatively low loss, their content will increase, which can be referred to as \"burn-up increase\"” ； To properly control the chemical composition of the melt, when selecting metal feedstocks, the changes that occur after melting must be taken into account, and corresponding adjustments should be made to the amounts of each element added. In actual melting, the degree of loss of elements in the alloy is also influenced by the quality of the raw materials, fluxes and slag, operating techniques, and particularly the properties of the oxides formed. Prevention of gases and oxides during the melting of recycled aluminum As mentioned earlier, the main source of gases and oxide inclusions in aluminum melt is H2O, which enters the melt from the surface oxide film that comes into contact with the aluminum melt, from the surface of the feed materials (especially those damaged by moisture), from the melting and pouring tools, as well as from the refining and modifying agents used. The oxide film mixed into the aluminum melt, along with low-quality feed materials containing many inclusions (such as slag splashes and remelted ingots), will form oxide inclusions within the aluminum melt. To this end, the following points should be taken into account during the melting and pouring process: ① Crucibles and melting/pouring tools – Before use, any rust, oxide residues, old coating layers, or other contaminants adhering to their surfaces must be carefully removed; thereafter, a new coating should be applied, and the tools should be preheated and dried before use. Both the melting and pouring tools as well as the crucibles used for transferring aluminum melt must be thoroughly preheated before use. ②Furnace charge: The furnace charge should be stored in a dry place before use. If it has been damaged by moisture, sandblasting should be carried out prior to mixing to remove the surface layer of corrosion. The surface of the scrap material often has sand (SiO2) adhered to it, and part of this SiO2 reacts with the molten aluminum as follows: 4 Al + 3 SiO2 → 2 Al2O3 + 3 Si. The Al2O3 that is formed, along with any remaining SiO2, creates oxide inclusions in the molten aluminum; therefore, such material should also be cleaned by sandblasting before being used. The tertiary recycled material, which is remelted from chips, slag splashes, etc. into ingots, often contains a high amount of oxidized inclusions and gases; therefore, its usage should be strictly limited, generally not exceeding 15% of the total amount of feedstock, and it should be completely avoided for important castings. The surface of the furnace charge should also be free from oil, cutting coolants, and similar substances, as various oils are hydrocarbons with complex structures, and when heated, they release hydrogen. The charge must be preheated to above 150–180°C before aluminum melt is added. The purpose of preheating is, on one hand, to ensure safety by preventing an explosion that could occur when the aluminum melt comes into contact with moisture accumulated on the surface of the cold charge ； On the other hand, it is to prevent gases and inclusions from entering the aluminum melt. ③Refining agents, deteriorating agents: Some of these components absorb moisture from the atmosphere easily and thus deliquesce, while others contain crystalline water within them. Therefore, it should be thoroughly dried before use, and certain substances such as ZnCl2 need to be remelted to remove moisture before they can be used. ④Operations during melting and pouring The stirring of the molten aluminum should be done smoothly, in order to prevent the surface oxide layer and air from being introduced into the aluminum melt. The number of transfers of the molten aluminum should be minimized; when transferring, the drop height of the liquid flow should be reduced as well as splashing. During pouring, the nozzle of the pouring ladle should be as close as possible to the gate cup in order to reduce the height the liquid has to fall, and pouring should be done at a steady pace to minimize splashing and turbulence of the aluminum melt. After pouring the casting, the remaining aluminum melt in the spoon should not be poured back into the crucible but rather into the ingot mold; otherwise, the amount of oxidized impurities in the aluminum melt will continue to increase. A large amount of inclusions such as Al2O3 are deposited in the molten aluminum at a depth of about 50–100 mm at the bottom of the crucible; therefore, it cannot be used for casting castings. ⑤Melting temperature, and the duration of the melting and pouring processes. Raising the temperature accelerates the reactions between the aluminum melt and H2O, O2; the solubility of hydrogen in the aluminum melt also increases sharply as the melting temperature rises. When the temperature exceeds 900°C, the oxide layer on the surface of the aluminum melt becomes less dense, which further intensifies the aforementioned reactions. Therefore, the melting temperature for most aluminum alloys is generally kept below 760°C. As for aluminum-magnesium alloys with a loose oxide protective film on the surface of the molten aluminum, the reactions between the molten aluminum and H2O, O2 are more sensitive to temperature increases; therefore, stricter limits are imposed on the melting temperature of such alloys (usually not exceeding 700°C). The longer the duration of the melting and pouring process (especially the time between refining and completion of pouring), the higher the content of gases and oxide inclusions in the aluminum melt. Therefore, the duration of melting and pouring should be shortened as much as possible, especially the time from refining to the completion of pouring. Factories generally require that pouring be completed within 2 hours after refining; if this cannot be achieved, refining must be repeated. In areas with humid weather, or for alloys that require a high level of porelessness or are prone to gas bubbles and inclusions, the pouring time should be even more restricted.