In the smelting process of aluminum and aluminum alloys, hydrogen and oxide inclusions are the main substances that contaminate the aluminum melt. Al easily reacts with oxygen to form A1202 or secondary alumina (Al2O and A10). At the same time, it is also very easy to absorb gas (H) which accounts for 70-90% of the total gas content in the aluminum melt. The main defects in casted aluminum alloys are voids and inclusions due to the gas and oxidation remaining in the alloy. Caused by solid particles. Therefore, in order to obtain a high-quality melt, it is important not only to select a proper and reasonable melting process, but also to refine and purify the melt. There are many refining and purifying methods for aluminum and aluminum alloy melts, including floatation method, flux refining method, melt filtration method, vacuum method, and joint method. This article describes the application of flux refining method in the smelting of aluminum alloys. 1 The role of flux Salt fluxes are widely used in the production of primary aluminum and recycled aluminum to improve melt quality and metal aluminum recovery [1. 2]. There are four functions of the flux: one is to change the wettability of the aluminum melt to the oxide (aluminum oxide), so that the aluminum melt is easily separated from the oxide (alumina), so that the oxide (alumina) is mostly Into the flux to reduce the content of oxides in the melt. Second, the flux can change the state of the oxide film on the surface of the melt. This is because it can break down the dense and dense oxide film on the surface of the melt into fine particles, which is conducive to the hydrogen in the melt escaping from the particle voids of the oxide film and entering the atmosphere. Thirdly, the presence of the flux layer can block the contact between the water vapor and the aluminum melt in the atmosphere, making it difficult for hydrogen to enter the aluminum melt, and at the same time, preventing oxidation loss of the melt. Fourth, the flux can adsorb the oxides in the aluminum melt and allow the melt to be purified. In short, the effect of removing impurities by flux refining is mainly achieved through adsorption, dissolution and chemical action with the oxide film and non-metallic inclusions in the melt. 2 flux classification and selection 2.1 Flux classification and requirements There are many types of fluxes used in aluminum alloy melting, which can be divided into two categories, covering agents (melt fluxes to prevent oxidation loss and inspiration of the melt) and refining agents (degassing and flux removal of inclusions). Different aluminum alloys are used. The covering agent and refining agent are different. However, any flux used in the smelting of aluminum alloys must meet the following conditions [3.8]. 1 The melting point should be lower than the melting temperature of the aluminum alloy. 2 The specific gravity should be less than that of aluminum alloy. 8 Absorbs and dissolves inclusions in the melt and removes gases from the melt. 4 It should not be chemically reacted with metal and lining. If it works with metal, it should only produce inert gas that is insoluble in metal, and the flux should not dissolve in the melt metal. 5 Hygroscopicity is small and the evaporation pressure is low. 6 should not contain or produce harmful impurities and gases. 7 Have proper viscosity and fluidity. 8 easy to manufacture: cheaper prices. 2.2 flux composition and molten salt discretion Aluminum alloy flux is generally composed of alkali metal and alkaline earth metal chloride and fluoride, its main component is KCl, NaCl, NaF. CaF,. Na3A1F6, Na2SiF6, etc. The physical and chemical properties of the flux (melting point, density, viscosity, volatility, hygroscopicity, and interfacial interaction with the oxide, etc.) play a decisive role in the refining effect. 2.2.1. Chlorine: Chlorine salt is the most common basic component in aluminium alloy fluxes, while the mixed salt of 45% NaCl+55% KCl is widely used. Due to their strong ability to infiltrate solid Al2O3, inclusions, and oxide films (weaker than Al2O3, the wetting angle is more than 20 degrees), the specific gravity of NaCl and KCl is only 1.55 g/cm3 and 1 at the melting temperature. 50g/cm3 is significantly smaller than the specific gravity of the aluminum melt, so it can be spread well on the surface of the aluminum melt and crush and adsorb the oxide film on the melt surface. However, in the case of fluxes containing only chlorine salts, the crushing and adsorption processes proceed slowly, and manual agitation must be performed to accelerate the above process. Chloride has a small surface tension and good wettability and is suitable as a covering agent, among which a chlorine salt of a molecular crystal form such as CCl4 , SiCl4, A1C13, etc. can be used as a scavenger alone, and chloride salts with ionic crystal forms such as LiCl, NaCl hair KCl, MgC12: are suitable as mixed salt fluxes. 2.2.2. Fluoride salt: NaF is added to the chlorine salt mixture. Na3A1F6, CaF2. A small amount of fluoride salt, mainly from refining, such as adsorption, dissolved Al2O3,. Fluoride can also effectively remove the oxide film on the surface of the melt and improve the degassing effect. This is because: a) The fluorine salt can react chemically with the aluminum melt to form gaseous A1F, SiF4, and BF3, etc., which mechanically promote the separation of the oxide film from the aluminum melt and crush the oxide film, pushing Into the flux; b) The current generated at the interface where the above reaction occurs also causes the oxide film to be "washed" and broken. Therefore, the presence of fluoride salt significantly accelerates the destruction of the oxide film on the surface of the aluminum melt, and the hydrogen in the melt can be easily released; c) The fluoride salt (especially CaF2:) can increase the mixing of molten salt The surface tension makes the molten salt with adsorbed oxides spheroidized, which is easy to separate from the melt and reduces the loss caused by enveloping the aluminum in the solid slag. Moreover, due to the increase of the surface tension of the flux, the melt accelerates the adsorption of impurities on the flux. process. 3 commonly used aluminum alloy melting flux The flux refining method has a good effect on the discharge of non-metallic inclusions, but the removal of non-metallic inclusions in the melt, in addition to the physical and chemical properties of the flux, depends to a large extent on the refining process conditions. , such as the amount of flux, the contact time of the flux with the melt, contact area, stirring conditions, temperature, etc. 3.1 Common flux In order to refine the aluminum alloy melt, hundreds of fluxes have been developed. The sodium and potassium based chloride fluxes are widely used. A sodium chloride-based chloride refining agent is widely used for aluminum alloys with low magnesium content, and a sodium-potassium-based refining flux is used for aluminum alloys with high magnesium content in order to avoid sodium embrittlement. The composition and function of the commonly used fluxes in the aluminum alloy smelting process are shown in Table 1 (4-7). Table 1 Composition and Application of Common Fluxes Solvent type component content,% NaCl KCl MgCl2 Na3AlF6 Alloys suitable for other components Covering agent 39 50 6 6 CaF 2 4 4 Al-Cu system, Al-Cu-Mg Series, Al-Cu-Si series Al-Cu-Mg-Zn series Na2CO385. CaF15 General aluminum alloy 50 50 General aluminum alloy KCl, MgCl280 CaF220 Al-Mg Al-Mg-Si Alloys 31 14 CaF210 CaCL244 Al-Mg Alloy 8 67 CaF210, MgF215 Al-Mg Alloys Refining agent 25-35 40-50 18-26 Alloys other than Al-Mg and Al-Mg-Si alloys 8 67 MgF215, CaF210 Al-Mg Alloy KCl, MgCl260, CaF240 Al-Mg Al-Mg--Si Alloys 42 46 Bacl26 (No. 2 flux) Al-Mg alloy 22 56 22 General aluminum alloy 50 35 15 General aluminum alloy 40 50 NaF10 General Aluminum Alloy 50 35 5 CaF210 General aluminum alloy 60 CaF220,NaF20 General Aluminum Alloy 36-45 50-55 3-7 CaF 21. 5-4 General aluminum alloy Na2SiF630-50, C2Cl650-70 General aluminum alloy 40. 5 49. 5 KF10 Alloy Cans From the above table, it can be seen that the content of some flux components varies widely, and can be determined according to actual conditions. First of all, according to the content of alloying elements to determine [8], because the majority of aluminum alloys in the main element content can be changed within a certain range, and secondly according to the impurity components and content to determine. Therefore, in addition to using the flux produced by the flux plant, the manufacturer preferably adjusts the proportion of the flux component according to the composition of the smelted aluminum alloy to find out the preferred flux composition. It is not difficult to see from the above various fluxes that when the composition of the aluminum alloy to be melted is determined, the design of the flux component is firstly the choice of the ratio of the main components (such as chloride), followed by the addition of components (such as fluoride). select. After the flux is properly prepared, it is preferably used after smelting, condensing, and crushing, because the effect of mechanical mixing is not good. 3. 2 flux dosage. When smelting aluminum alloy scrap, the quality of the scrap is different, and the amount of the covering agent and the refining agent are also different. 3. 2. 1. Main cover dosage a) Smelting of good quality wastes, such as bulk materials, tubes, and tablets, when used as covering agents (see Table 2). Table 2 Types and amounts of covering agents Charges for furnace materials and products (% of charged amount) Types of covering agents Electric furnace smelting: General products Special products 0.4-0.5% 5% 5-. 6% Common powdery solvent Ordinary powder solvent gas furnace smelting: primary aluminum ingot scrap 1-2% 2-4% KC1: NaC1 1:1 mixing KC1: NaC1 1:1 mixing Note: For high-magnesium aluminum alloys, all fluxes containing no sodium salt should be covered to avoid contact with sodium-containing fluxes. b) The amount of coating agent used to smelt wastes with poor quality, such as sawdust, car, milling, etc., and melting slag, etc. (see Table 3). Table 3: Coverage dosage Category usage (% of charge) Small debris debris number outside slag 6-810-1515-20 3.2.2 The amount of refining agent Different aluminum alloys, different products, and the amount of refining agent are also different (see Table 4). Table 4 Refining agent dosage Alloy and product melting furnace static furnace High magnesium alloy No. 2 flux 5-6kg/t No. 2 flux 5-6kg/t Special products in addition to high magnesium alloy ordinary flux 5-6kg/t ordinary flux 6-7kg/t LT66, LT62, LG1, LG2, LG3, LG4 with ordinary flux, stacking flux dam Other alloy ordinary flux 5-6kg/t Note: 1 The amount of flux should be increased in humid areas and humid seasons. 2 For large size round ingots, the amount of flux should also be increased. 3. 3 flux use method Flux refining method commonly used in the production of aluminum alloy production 1 The melt is refined in the ladle. First, put a flux in the ladle, then inject the melt and stir it enough to increase the contact area between the two. 2 The melt is refined in an induction furnace. The flux is charged into the induction furnace, and the flux and the melt are fully mixed by the stirring action of the induction magnetic field to achieve the purpose of refining. 3 in the ladle or in the furnace with a mixer refining, so that flux mechanical dispersion in the melt. 4 The melt is refined in a magnetic stirrer. The method relies on the role of electromagnetic force to continuously convey the melt to the flux-metal interface to achieve active contact between the aluminum melt and the flux. The higher the melt rotation speed, the better the refining effect. 5 Flux refining. This method is to continuously refine the melt through a layer of flux that is applied with an electric field (at the metal-flux interface). Among these five methods, the flux refining effect is better. 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Flux Refining of Aluminum Alloy Melts
This article describes the role of flux refining in the melt purification process of aluminum alloys, the classification and requirements of fluxes, the composition of commonly used fluxes, the scope of application, and methods of use.