Aluminum alloy is the largest single consumable material in die casting production. During aluminum alloy smelting process, significant aluminum loss occurs due to oxidation and operational factors. This not only results in waste of large amounts of aluminum but also in significant energy consumption. Furthermore, generation of large amounts of smelting oxides affects quality of molten aluminum, leading to various adverse consequences for product. Effectively controlling metal loss can significantly increase a company's profitability. Conversely, poor control measures during production will cause excessive material loss, directly increasing production costs.
Aluminum alloy material loss during die casting includes following aspects: Burn-off and volatilization during smelting; Slag carry-over during slag removal; Burrs and flash on die castings; Burn-off and volatilization from remelting of gates, sprues, and scrap castings; Increased weight of die castings due to machine tools, molds, etc.

In aluminum alloy smelting process, loss of unrecoverable metal caused by aluminum oxidation and interaction between aluminum and furnace wall and refining agents is called burn-off. Burn-off and metal contained in slag are collectively referred to as melting loss. Whenever aluminum is smelted under atmospheric conditions, whether it is pure aluminum or an alloy, various elements within it undergo oxidation at high temperatures, forming various metal oxides, resulting in loss of molten aluminum. Naturally, different oxides are generated according to different components in alloy, thus causing aluminum material to be lost. After molten aluminum is melted, it still needs to be refined. Flux added during refining decomposes and generates heat at high temperatures, further promoting oxidation and burn-off of various elements in molten aluminum. Even more noteworthy is that after refining, many intact aluminum droplets are adhered to slag due to interfacial tension, becoming a slag-aluminum mixture, which is then scraped out of furnace as slag. Although this mixture can be separated and recycled using various methods, it still consumes a lot of energy, recycling process involves oxidation and loss of various elements. This "re-oxidation" results in a significant amount of aluminum becoming unrecoverable powder. At most, only 30% to 50% of aluminum content can be recycled. Unrecoverable powdered aluminum is often used as "ash" for road paving and other waste, while the rest becomes waste. In summary, main destinations of losses during aluminum smelting are:
1. Oxidation and loss of various elements under high-temperature conditions during smelting;
2. During refining, aluminum is mixed with slag and removed from furnace. This loss is several times greater than the former.

 

1. Gases: In aluminum alloy smelting process, main gas is hydrogen, followed by nitrogen, oxygen, carbon monoxide, etc. Hydrogen in aluminum alloys mainly comes from water vapor in furnace gas. Water vapor reacts with molten aluminum to produce atomic hydrogen. A significant partial pressure is established at interface, causing hydrogen to dissolve rapidly and increase substantially in molten aluminum. Hydrogen content in molten aluminum increases with hydrogen partial pressure and temperature.
2. Oxide Inclusions: Inclusions refer to any exogenous impurities in solid or liquid phase above liquidus temperature. Common non-metallic inclusions include oxides, carbides, and borides. These inclusions mostly exist as particles or films. Molten aluminum reacts with various gases to form aluminum nitride, aluminum carbide, and alumina. Alumina has high chemical stability and does not decompose in molten aluminum. Besides above inclusions, molten aluminum may also contain refractory bricks from smelting furnace and ladle, solvents, and other forms of intermediate compounds.

 

1. Charge Condition: Experience has shown that whether charge is cleaned before entering furnace has a significant impact on the formation of slag. This is because impurities and oil stains on charge surface burn and generate heat at high temperatures, leading to a rapid increase in temperature and intense oxidation, resulting in a large amount of slag. Therefore, keeping charge clean is beneficial in combating loss.
2. Charge Size: Size of charge reflects area of contact between charge and oxygen in furnace atmosphere during smelting. Smaller charge size means a larger contact area with oxygen, which promotes oxidation, especially in reverberatory furnaces with highly oxidizing atmospheres. Therefore, excessively small charge sizes should be avoided.
3. Heating Conditions: In smelting conditions with high oxygen concentrations in furnace atmosphere on molten pool surface, such as in furnaces with oxygen-assisted combustion nozzles, flame directly contacts a large area of aluminum alloy. Excessive heating will intensify oxidation loss of furnace charge. Melting temperature and heating rate have a significant impact on oxidation loss; excessively high melting temperatures promote alloy oxidation, while faster heating rates shorten melting time, thus reducing oxidation period and minimizing oxidation loss. Therefore, selecting a high-efficiency melting furnace is beneficial for reducing metal oxidation loss.
In summary, selection of alloy melting heating conditions essentially revolves around minimizing interaction time between alloy and oxygen at high temperatures during melting. Factors such as furnace atmosphere, melting temperature, heating rate should be considered. The longer alloy remains in furnace, especially during high-temperature stage, the more likely it is to experience accelerated oxidation loss. This is a crucial aspect of production management; therefore, prolonged retention of molten aluminum in furnace after alloy melting is highly detrimental.

4.1 Although wetting ability of oxides generated under high-temperature conditions to molten metal is very poor, molten aluminum and oxides are still mechanically mixed together. This mixing phenomenon is more likely to occur with non-dried flux. Drying additives can not only relatively reduce wetting effect but also promote exothermic reaction of additives, helping to separate two and allowing molten aluminum to flow back into molten pool. Therefore, non-dried flux easily introduces gas, causing more serious oxidation losses.
4.2 The Influence of Refining Agents: Adding refining agents will decompose and generate heat, further accelerating oxidation of molten aluminum. To reduce wettability of aluminum slag and promote separation, it is necessary to add them. Therefore, it is essential not only to select right type of refining agent but also to control its appropriate amount. For example, NaCl has a lower decomposition temperature and generates less heat than NH4Cl, resulting in a lower oxidation slag production rate. However, adding too much will also increase amount of oxide slag and increase loss of molten aluminum.
4.3 Improving slag-molten metal separation performance: Slag that floats to surface after refining has good wettability with molten metal, and two are tightly bound together in a mixed state. At this time, it is necessary to add an appropriate amount of "heat-generating slag-forming agent". After being added to surface, it is forced to burn and generate heat, which raises temperature of slag at the surface of liquid. This makes wettability of oxides and metal droplets in slag worse, so that large metal droplets in slag will remelt back into molten metal. Such forced combustion and heating will inevitably produce a small amount of metal burn-off, but compared with loss of large aluminum droplets being scraped out of furnace, benefits outweigh the disadvantages, that is, the total loss can be greatly reduced.
In summary, basic starting point for reducing smelting losses can be summarized as:
(1) Reducing all solid elements that promote intense oxidation during smelting process, including:
a. Strictly use treated, clean-surfaced furnace charge. Metal ingots and die-casting burrs should be stored in a dry place and kept away from other materials. Outdoor storage of metal materials should be avoided as much as possible to prevent increased oxidation.
b. Minimize use of small-sized furnace charge pieces.
c. Select furnaces and aluminum melting conditions with good furnace atmosphere (low oxygen concentration) and implement measures to reduce oxygen concentration in furnace atmosphere.
d. Maximize heating rate and shorten smelting time.
(2) Strictly control types and amounts of additives and fluxes added during refining process.
a. Using inert gas refining methods can reduce oxidation loss during refining process.
b. Using a good slag remover to promote slag-liquid separation is a crucial key to significantly reducing burn-off.
(3) When melting metal ingots and rough edges, some oxidation is unavoidable. However, when adding metal to molten metal pool, pool should be kept as turbulent as possible. A thin layer of slag should be left on the surface of molten metal pool to reduce chance of fresh molten metal being exposed to air. Slag should only be removed when it reaches a thickness of 1-2 cm. Fine burrs, rough edges, and scrap from ground have a high surface area/volume ratio. For wet/oily thin rework parts or machining scrap, a targeted approach can be taken to determine whether it is worthwhile to clean, dry, and compress fine scrap into small pieces before remelting.

 

To properly manage burn-off losses, maintaining accurate metal balance records is essential. This includes purchase quantity, weight of castings produced, and amount of slag generated. Accurate records allow for timely detection of any changes in slag quantity by comparing data across different time periods (perhaps monthly). In addition to losses during alloy smelting process, losses from production process, product processing, weight errors during receipt and shipment, improper inventory records, and other unforeseen factors should be deducted when calculating company's overall burn losses.
Metals experience losses during smelting due to oxidation, refining, and slag removal. In subsequent casting, castings containing process waste such as risers and gating gates, as well as scrap castings, need to be returned for remelting. Besides head and tail sections and casting waste requiring remelting, billets also contain process waste and scrap that need to be returned to foundry for remelting. A similar metal cycle exists in processing of any metal, during which some metal is lost. Obviously, amount of metal loss in this cycle is related to smelting conditions and properties of metal, as well as casting yield and finished product rate of processing workshop. Under certain production equipment and process conditions, a certain amount of metal raw materials are processed into finished products through repeated cycles. How much metal raw material is lost, and how many products can be produced? These are questions that every producer and operator is concerned about. According to material consumption process quota basis in JB/T9169.6-1998 process quota compilation standard of Ministry of Machinery Industry, loss in smelting process can be calculated with reference to following formula:
(1) Smelting and casting loss rate a a＝Ws/W
Where: WS-Mass of metal lost in smelting and casting (including metal carried away by oxidation and slag);
Note: Since slag can be recycled and ultimately disposed of, value of recycling should be considered in calculation; W-The total mass of metal put into furnace.
(2) Casting yield b = Wc/W Where: Wc - Product quality of foundry workshop
(3) Recyclable rate = Weight of recycled material/W
(4) Non-recyclable rate = (W - Wc - Weight of recycled material)/W
(5) Furnace consumption rate = (W - Weight of molten metal)/W
(6) Molten metal recovery rate = Weight of molten metal/W
(7) Metal charge to coke ratio = W/Coke weight
According to above formulas, when simply calculating burn-off during smelting process, the total weight of materials input within a certain period should be subtracted from the total weight of alloy liquid taken each time, then recovered slag (final slag value of smelting process) should be subtracted to accurately calculate burn-off rate.
Formula 7, coke ratio, should be converted when using natural gas or other energy sources. Above company's calculation is based on a standard weight setting and does not consider loss due to weight increase. To calculate material loss of the entire die-casting process, all five related factors mentioned above should be considered to more accurately calculate material cost of production process.

When smelting aluminum alloys in a flame-reverberatory furnace, slag volume varies from 2% to 5% of charge, while aluminum content in slag is approximately 40-60%. Therefore, proper treatment of aluminum slag and recovery of aluminum from slag to reduce melting loss are crucial. Even with all necessary measures to reduce slag production, some alloy slag still needs to be periodically removed from furnace surface. Die-casting workshops typically require a dedicated smelting furnace, a stirrer or other device for mixing molten metal, and a flux (slag remover) to recover usable die-casting alloys. Die-casting furnaces and accumulators used for casting production are generally unsuitable for slag treatment. Several different patented fluxes are available on the market to separate slag into usable alloys, oxides, and metallic intermediate phases. When used properly, these fluxes will produce little or no smoke. Using smokeless or low-smoke fluxes will reduce emissions. It is recommended to contact flux manufacturers directly to obtain specific process suitable for your particular product.
1.1 Treatment before slag removal: Slag that floats to surface of melt after refining has good wettability with melt, with a wetting angle of less than 90°. Slag contains a considerable amount of melt, which is dispersed in granular form and adheres to slag. Wettability is even better at lower melt temperatures. If slag is removed at this temperature, weight of melt carried away with slag is approximately 60% of slag weight. A slag-forming agent at 1‰–2‰ of slag weight is evenly sprinkled onto surface of melt to reduce aluminum content in slag. Reactant AlF3 undergoes an exothermic reaction with aluminum and oxygen, releasing heat that transforms sticky slag into a loose, powdery, dry slag. This reduces wettability of aluminum melt with oxides in slag, causing granular aluminum droplets mixed in slag to detach and return to melt.
1.2 Treatment of aluminum slag after tapping: It should be noted that slag removed after above treatment still contains aluminum droplets. During slag removal, slag is first shoveled into a perforated iron box to allow some of molten aluminum adhering to slag to seep out. After slag removal, slag is poured into a specially treated pit, a slag-forming agent is sprinkled into it while stirring to mix slag and agent evenly. After 5-10 minutes, slag is removed from pit. Rapid heating effect of slag-forming agent raises slag temperature to approximately 950℃, causing oxide film around aluminum droplets in slag to rupture, aluminum droplets gradually collect and fall to bottom of pit under their own gravity. Slag after this secondary treatment contains only a small amount of aluminum particles. Cooled slag is stored in a designated area, and aluminum particles are then picked out. This method of treating aluminum slag for recycling is simple and feasible. Currently, melting loss during aluminum alloy smelting has been reduced to 1.6%, sometimes reaching 1.4%. For an annual production of 5000 tons of aluminum alloy, economic losses previously caused by melting loss can be reduced by 400,000 yuan, resulting in considerable economic benefits.
Slag formation during melting and casting is unavoidable. However, proper control and management of melting and casting processes can reduce slag formation, thereby increasing profits for die-casting manufacturers. Following steps can minimize metal loss due to slag in die-casting production:
(1) Purchase clean, environmentally friendly, and uncorroded alloy ingots. Store raw materials in clean, dry places, and treat remelted materials in same way.
(2) Reduce amount of remelted material generated by properly designing and assembling die-casting molds. This reduces or eliminates generation of burrs and scrap.
(3) Process damp or oily burrs or cleaning waste into clean, dry blocks before remelting.
(4) Melting temperature should be as low and uniform as possible; temperature of melting furnace and accumulator should not exceed 750 degrees Celsius.
(5) Temperature of molten metal should be checked at least once a month with a portable thermometer to ensure accuracy of furnace thermometer.
(6) When adding new alloy ingots or recycled materials, agitation of molten pool should be minimized.
(7) Carefully remove slag using a porous slag remover, and gently shake or tap remover at the edge of furnace pool to allow useful alloy liquid mixed with slag to flow back into pool.
(8) Remove slag only when necessary. Retaining a layer of slag on the surface of molten pool will reduce oxidation of molten metal by air.
(9) Perform slag treatment according to flux manufacturer's recommended process to ensure maximum metal recovery.
(10) Keep detailed records of slag generation and regularly compare amount of slag generated over a period of time to promptly identify any abnormalities in production.

 

There are many methods for refining, purifying aluminum and aluminum alloy molten metal, mainly including flotation, flux refining, melt filtration, vacuum, and combined methods. This article introduces application of flux refining in aluminum alloy smelting.
1. Role of Flux
Fluids are widely used in production of primary and recycled aluminum to improve melt quality and aluminum recovery rate.
Fluids have four main roles: First, they alter wettability of molten aluminum to oxides (alumina), making it easier for molten aluminum to separate from oxides (alumina). This allows most of oxides (alumina) to enter flux, reducing oxide content in melt. Second, fluxes change state of oxide film on melt surface. This is because they break down dense, solid oxide film into fine particles, facilitating escape of hydrogen from oxide film particles into atmosphere. Third, presence of flux layer isolates atmospheric water vapor from molten aluminum, making it difficult for hydrogen to enter, preventing oxidation and burn-off. Fourth, fluxes adsorb oxides from molten aluminum, purifying melt. In short, removal of inclusions by flux refining is mainly achieved through adsorption, dissolution, chemical reactions with oxide film and non-metallic inclusions in melt.
2. Classification and Selection of Fluxes
2.1 Classification and Requirements of Fluxes
Many types of fluxes are used in aluminum alloy smelting. They can be broadly classified into two categories: covering agents (fluxes that prevent oxidation, burn-off of melt and gas absorption) and refining agents (fluxes that remove gas and inclusions). Different aluminum alloys require different covering agents and refining agents. However, any flux used in aluminum alloy smelting process must meet following conditions:
① Melting point should be lower than melting temperature of aluminum alloy.
② Specific gravity should be lower than specific gravity of aluminum alloy.
③ It should be able to adsorb and dissolve inclusions in melt and remove gases from melt.
④ It should not chemically react with metal or furnace lining. If it reacts with metal, it should only produce inert gases insoluble in metal, and flux should be insoluble in molten metal.
⑤ It should have low hygroscopicity and low vapor pressure.
⑥ It should not contain or produce harmful impurities or gases.
⑦ It should have appropriate viscosity and fluidity.
⑧ Easy to manufacture: inexpensive.
3. Commonly Used Fluxes in Aluminum Alloy Smelting
Fluoride refining is very effective in removing non-metallic inclusions. However, degree of purification of non-metallic inclusions in melt depends not only on physical and chemical properties of flux, but also to a large extent on refining process conditions, such as amount of flux, contact time and contact area between flux and melt, stirring conditions, and temperature. Hundreds of fluxes have been developed for refining aluminum alloy melts, with sodium- and potassium-based chloride fluxes being the most widely used. Sodium- and potassium-based chloride refining agents are widely used for aluminum alloys with low magnesium content, while sodium-free, carnallite-based refining fluxes are used for aluminum alloys with high magnesium content to avoid sodium embrittlement.
3.3 Flux Usage Methods
Following methods are commonly used in flux refining process for aluminum alloy production:
① Refining melt in a ladle. First, a packet of flux is placed in ladle, then molten metal is poured in and thoroughly stirred to increase contact area between the two.
② Molten metal is refined in an induction furnace. Flux is added to induction furnace, flux and molten metal are thoroughly mixed by stirring effect of induction magnetic field, achieving purpose of refining.
③ Refining is performed in ladle or furnace using a stirrer, causing flux to mechanically disperse into molten metal.
④ Molten metal is refined in a magnetic field stirring device. This method relies on electromagnetic force to continuously deliver molten metal to flux-metal interface, achieving active contact between aluminum molten metal and flux. The higher rotation speed of molten metal, the better refining effect.
⑤ Electro-flux refining. This method involves continuously refining molten metal by passing it through a flux layer with an applied electric field (at metal-flux interface).
Summary: Material losses in die-casting process mainly include following aspects
I. Losses during smelting process
Slag removal methods, process still needs improvement; Are various fluxes used as required? Is temperature during smelting process controlled? Are proportions of various materials proportioned according to process? Are slag removal and gas removal processes carried out according to process?
II. Management of Recycled Materials at All Levels
Management of waste and surplus materials during die-casting process is not strict, resulting in significant waste; Recycled materials are not properly graded and managed according to categories; Recycling of recycled materials is not well managed.
III. Losses Due to Increased Weight and Scrap Products in Production Process
Regarding losses due to increased weight, company is actively controlling situation internally and communicating with customers to reconfirm standard weight; Reduce scrap losses during production process.
In conclusion, only by starting from source, strictly controlling each link, and carefully and accurately collecting process data can we truly reflect company's material consumption, calculate production costs, better utilize data to guide operations, reduce consumption, and improve efficiency.