Among metallic materials, the application scope and dosage of aluminum alloy are second only to iron, accounting for about 1/3 of the consumption of non-ferrous metals. With the wide application of aluminum and aluminum alloy, the requirements for its performance are getting higher and higher, and its good performance is inseparable from its melting and casting. Melting and casting is the first process of aluminum processing, which provides ingots for subsequent rolling, aluminum alloy forging, extrusion and other production. The quality of ingots is directly related to the final quality of various aluminum materials, so to obtain good components, we must start with melt treatment. Purification of aluminum alloy melt is one of the basic measures to produce high-quality aluminum castings, and it is also one of the main means to improve the comprehensive properties of aluminum alloys, which has an important impact on the formation of porosity, porosity and inclusions, and directly affects the physical properties, mechanical properties and service performance of aluminum castings.

2 melt purification method

The so-called purification treatment is to adopt various measures to reduce the undesirable gases and solid substances in the aluminum melt to the allowable range to ensure that the properties of the material meet the standards or some special requirements. Purification methods of aluminum alloy can be divided into two basic types according to their mechanism: adsorption purification and non-adsorption purification.

2.1 adsorption purification

Adsorption purification mainly uses the surface function of refining agent. When refining agent (such as various gases, liquids, solid refining agents and filter media) comes into contact with oxide inclusions or gases in aluminum melt, impurities or gases are adsorbed on its surface by refining agent, thus changing the physical properties of impurities and being removed together with refining agent to achieve the purpose of degassing and impurity removal. The main methods of adsorption purification are: floating method, flux method, filtration method and so on.

(1) Floating method

Flotation method, also known as gas purging method, introduces gas into aluminum melt to form bubbles, and hydrogen in the melt diffuses into these bubbles under the action of partial pressure difference, and is eliminated with the floating of bubbles to achieve the purpose of degassing. Flotation method mainly includes inert gas purging, active gas purging, mixed gas purging and chlorine salt purification. Non-toxic refining agent is mainly composed of nitrate and other oxidants and carbon, which can be used to refine nitrogen and carbon dioxide at high temperature. Because it does not produce irritating gas and has good refining effect, it is widely used.

(2) Solvent method

Flux method is to add flux into the melt during the melting of aluminum alloy, and achieve the purpose of degassing and impurity removal through a series of physical and chemical actions. The refining effect of flux mainly depends on its ability to absorb and dissolve oxide inclusions, and the solvent used for aluminum alloy purification is mainly a mixture of alkali metal chloride and fluoride.

(3) filtration method

Filtration method refers to the purification method of letting aluminum melt pass through a filter made of neutral or active materials to separate solid inclusions suspended in the melt. From the principle of slag removal by filtration, it can be roughly divided into mechanical slag removal and physical and chemical slag removal. Mechanical slag removal mainly relies on the blocking effect of filter media, friction or fluid pressure to make impurities settle and malfunctioned, so as to achieve the purpose of purifying the melt. Physical and chemical slag removal mainly depends on the adsorption of medium surface and the action of van der Waals force. Generally speaking, the smaller the gap of the filter medium, the greater the thickness and the lower the velocity of the metal melt, the better the filtering effect. Filter materials generally use glass cloth, corundum balls and foam ceramics. Filtration is mainly to remove the inclusions in the melt, but it has little effect on hydrogen removal, so in practical application, filtration is often combined with blowing.

2.2 Non-adsorption purification

Non-adsorption purification refers to the method of changing the equilibrium state of metal-gas system or metal-inclusion system through some physical action (such as vacuum, ultrasonic wave, density difference, etc.) without adding adsorbent to the melt, so that gas and solid inclusions can be separated from the aluminum melt. Non-adsorption purification methods mainly include: static treatment, vacuum treatment, ultrasonic treatment and so on.

Static treatment means that the aluminum melt is allowed to stand for a period of time before pouring. Because the density of inclusions is higher than that of aluminum melt, inclusions will sink, thus separating inclusions from the melt. However, it is difficult to remove small particles of inclusions by this method. Vacuum treatment refers to the method that the melt is placed in a closed holding furnace with a certain degree of vacuum, and the partial pressure difference of hydrogen in the melt and gas is used to make the hydrogen in the melt continuously generate bubbles, which float up and escape and be removed. Vacuum treatment is an extremely effective method to reduce the hydrogen content in aluminum melt, which can reduce the hydrogen content in the melt to 0.1-0.2 ml/(100g)Al. However, this treatment is not widely used in industrial production because it requires vacuum sealing equipment, has poor economy, and causes great loss of melt temperature and poor impurity removal ability. Ultrasonic treatment is a new method for purifying aluminum alloy melt developed in the 1990s. The principle is to use the "cavity" phenomenon caused by elastic waves in aluminum melt to produce bubble cores, so as to achieve the purpose of degassing. Ultrasonic treatment is also a good method to remove hydrogen. Due to the limitations of ultrasonic generator and poor economy, it is difficult to treat large quantities of aluminum melt, which limits its application in industry.

2.3 On-line purification technology outside the furnace

Because the purification effect of in-furnace melt purification treatment on aluminum alloy melt is very limited, in order to further improve the quality of aluminum alloy melt and effectively remove gas and non-metallic inclusions in aluminum alloy liquid, it is necessary to apply on-line purification treatment outside the furnace. On-line purification treatment outside the furnace can be divided into on-line degassing based on degassing, melt filtration based on slag removal and on-line treatment of both according to treatment methods and purposes. The main development direction of on-line melt treatment outside the furnace is to improve the purity of melt and seek efficient and cheap purification technology to meet the diverse needs of melt purification technology in aluminum processing.

On-line degassing is the key research direction and development object of melting and casting in major aluminum processing enterprises at present. Typical on-line degassing methods include Air-Liquicle, MINT with fixed nozzle and rotary nozzle degassing. At present, the rotary nozzle degassing method is the most widely used. In foreign countries, the rotary nozzle degassing device SNIF was first developed by United Carbonization Company (now Pyrotek Company), and the Alpur degassing device in France, etc. In China, the rotary nozzle degassing devices DFU and DDF were mainly developed by Southwest Aluminum Company. These degassing devices all use nitrogen or argon as refining gas, or their mixed gas with a small amount of active gas such as chlorine as refining gas, which can effectively remove hydrogen, alkali metals or alkaline earth metals from aluminum melt and improve the slag-liquid separation effect.

On-line melt filtration is the most effective and reliable method to remove non-metallic inclusions in melt. According to its principle, it can be divided into cake filtration and deep filtration, and its filtration methods mainly include glass fiber cloth filtration, bed filter corundum tube filtration and foam ceramic filtration. The simplest is glass fiber cloth filtration, and the best effect is filter tube and foam ceramic filter plate.

3 grain refinement

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