Electrolytic Aluminum Direct Casting Process for Producing High-quality Aluminum Alloy Ingots

【China Aluminum Industry Network】 I. Preface At present, most electrolytic aluminum plants in China use electrolytic aluminum liquids for refining, removing slag and gas, and then casting them into remelted aluminum ingots for sale. The aluminum profile factory repurchases the above aluminum ingots and remelts them. Casting, production of extruded aluminum alloy ingots.

If high-temperature electrolytic aluminum alloy is added and the alloy is directly melted and cast into aluminum alloy ingots, the process is shortened, the aluminum ingot remelting process is omitted, and the equipment investment and labor salary are saved. The use of electrolyzed aluminum liquid directly produces round ingots. As long as the process is properly implemented, the quality of the product fully meets the requirements of the extrusion production process. Melting and casting cost is 300 yuan/tA, including about 0.8% of burning loss, 4kg/tA of fuel consumption, 14kw/tA of electricity consumption, and 24 yuan/tA of artificial wages.

The use of a short-flow production process makes full use of the high-temperature thermal energy of electrolytic aluminum liquid, saves valuable energy, reduces the pollution of flue gas and dust to the environment, and reduces the burning loss of aluminum liquid by about 1%. Oil savings per ton of product can be achieved. More than 60kg, save more than 30 degrees, each ton of products to create at least 500-600 yuan of economic benefits.

However, some production units use equipment that is too simple, lack understanding of the properties of electrolytic aluminum, improper process measures, and the quality of round ingots has many non-metallic inclusions, high gas content, large crystals, cracks and other quality defects. As long as appropriate technological measures are taken to strengthen management, high-quality round ingots can be produced to meet the needs of users.

Second, the characteristics of electrolytic aluminum liquid 1, the electrolytic aluminum liquid is high temperature, high gas content is well known, electrolytic aluminum production process is an electrochemical process under high temperature conditions. Nowadays, with the development of science and technology, the capacity of electrolytic cells is getting larger and larger, the current intensity is as high as 350-500 kA, and the current efficiency is 94-95%. The electrolysis cell uses the carbon block as the anode and the semi-graphitizing carbon block as the cathode. The general temperature is between 950-960°C. Electrolyte composition is very complex, in addition to the main ingredient cryolite, but also added Al2O3, CaF2, MgF2, etc., their content were 1.38-2.88%, 4.88-5.88% and 0.47-0.87%; moisture content in addition to AlF3 ≤ 7.5% Others are all less than or equal to 1.0%. The moisture in the above materials is adhered water and crystal water, and the adhered water is easily evaporated under high temperature conditions, while the crystal water is chemically reacted in the electrolytic cell 3H2O+2AL→Al2O3+6[H], [H] dissolved In the electrolytic aluminum liquid, the higher the temperature of the aluminum liquid, the greater the saturation concentration of atomic hydrogen.

In addition to hydrogen, electrolytic aluminum liquids also contain CO2, CO, CH4, and N2. The electrolytic aluminum liquid has a wide gas composition range, H2: 53-96%, CO2: 2.5-30%, CO: 20%, CH4: 2.5%, and N2: 2.5%. For electrolytic aluminum liquids with high gas content, effective degassing measures must be taken to eliminate their hazards.

2. Electrolytic aluminum liquid impurity content The impurities in liquid primary aluminum are mainly non-metallic impurities, but also a small amount of metal impurities. Non-metallic impurities are more aluminum oxide, followed by fluoride salts, aluminum carbide and aluminum nitride. In the electrolytic process, the alumina added to the electrolytic cell is sand-like alumina. Sand-like alumina consists of α-Al2O3 and γ-Al2O3, α-Al2O3 ≥ 25%; α-Al2O3 is spherical, large in specific gravity, compact in texture, and small in surface area. In the electrolyte, the dissolution rate is slow, and there is no time to dissolve α-Al2O3. Precipitate at the bottom of the tank and mix a small amount into the cathode aluminum liquid. However, γ-Al2O3 has a large activity and a fast dissolution rate and is generally not deposited on the bottom of the electrolytic cell. In general, the electrolytic cell draws aluminum liquid once in 24 hours, and α-Al2O3 mixed into the molten aluminum enters the molten aluminum liquid together with the molten aluminum and is poured into the smelting furnace of the fusion casting production system.

The metal impurities in the electrolyzed aluminum bath are silicon, iron, zinc, titanium, sodium, etc., but the main metal impurities are silicon and iron. There are two main sources of these impurities: First, raw materials and materials. For example, alumina raw materials produced by the alkaline method contain impurities such as SiO2, Fe2O3, TiO2, ZnO, and Na2O, and Si, Fe, and Ti are produced through electrolysis. Zn, Na and other metals. The second is from the electrolytic tank lining, tools, equipment, dust and other.

Incidentally, in high-temperature liquid aluminum, carbon reacts with titanium to form TiC. TiC is a very good non-spontaneous nucleus in the solidification process of liquid aluminum alloy. The nucleation of TiC is better than that of TiB2 particles, which is beneficial to the crystallization of aluminum alloy liquid. When the grain is refined, it is a pity that the content is too low.

China is a country with a large amount of primary aluminum production in the world. It uses a short process to use the electrolytic aluminum liquid solar cell to directly produce the aluminum ingots and round ingots required for aluminum processing. This is the only way for the development of the aluminum industry and has also caused the relevant departments to highly valued. Some large-scale electrolytic aluminum companies, such as Guizhou Aluminum Plant, Qinghai Aluminum Plant, Baotou Aluminum Plant, Qingtongxia Aluminum Plant, etc., have introduced advanced technologies and advanced equipment from Europe and the United States in recent years, and have successively established a high-tech first-class fusion casting production line The main equipments include large smelting furnaces, large-scale hydraulic tilting furnaces, high-power electromagnetic stirrers, on-line argon gas degassing systems, hydraulic casting machines and ingot homogenization equipment. Through trial production, the product quality is good, and good economic and environmental benefits have been achieved.

There are also some companies that invest millions of dollars to build three 15-ton smelting furnaces, two wire rope casting machines, a monorail crane, and a set of vomiting, low-grade, low-temperature, low-temperature, 5,000 tons/year. . However, the equipment is too simple, technology and equipment are poor, and disciplines are not respected, management is neglected, production is tedious, product quality is poor, and problems are piled up. For this type of enterprise, how to carry out equipment transformation, strengthen technical management, improve process technology measures, improve the process system, and produce qualified products. Here, talk about your own views for reference by your peers.

Third, the use of electrolytic aluminum liquid production process technology ingots 1, smelting process technology to improve the use of electrolytic aluminum liquid production of aluminum alloy ingots, ordinary 6063 aluminum alloy accounted for more than 90%, and occasionally also produce a little 3003,6061,6082 , 6005, 5052 and the like aluminum alloy. The production of these aluminum alloys has a small range of solid-liquid phase temperatures, small tendency of hot and cold cracks, and low technical difficulty. Unlike the 2XXX series hard aluminum and the 7XXX series super hard aluminum, the alloy contains many elements and has a wide range. Due to the wide temperature range of crystallization, the plasticity in the solid-liquid region is low, and it has a great tendency to form hot cracks and looseness. Taking into account the influence of the proportion of impurity elements Si and Fe on the properties of the alloy, consider adding copper, manganese, and chromium to improve the corrosion resistance of the alloy, and adding titanium and zirconium to improve the ingot and recrystallized structure.

The smelting furnace shall add the dry head and tail material without moisture to the bottom of the smelting furnace before pouring the aluminum water. Aluminum bath liquid temperature is very high, under normal circumstances as high as 950-965 °C, the bottom of the liquid aluminum temperature is not lower than 940 °C. After the molten aluminum is poured into the smelting furnace, there is still 820-860°C. In order to prevent oxidation of the aluminum liquid at high temperatures, a layer of coating agent is immediately sprinkled to cover the melt surface. Because the melting point of the covering agent is lower than that of the aluminum liquid, the density is smaller than that of the melt, and the wettability is good. A continuous liquid protective film is formed on the surface of the melt, and the melt in the furnace is separated from the air.

Under normal circumstances, oxygen or water vapor cannot or can only react with the aluminum liquid through the cover layer, and the hydrogen atoms dissolved in the electrolyte can escape through the cover layer due to its small radius.

Due to the high content of impurities in the electrolytic aluminum liquid gas and impurities, it is necessary to strengthen the melt refining process for the above-mentioned compositional characteristics. Refining usually uses powdered refining agents and is blown with nitrogen; first, the quality of the refining agent and nitrogen must be ensured. The production of 6063 aluminum alloy ingots generally uses 1# refining agent; at present, there are many kinds of domestic commercial refining agents, and the vast majority of enterprises are based on confidentiality, do not explain the composition of components to the users, and the price competition between suppliers is fierce. , Individual bad refining agent manufacturers from the use of chemical raw materials, formulas and processing methods on the cutting corners, refining agents must ensure quality, carefully selected. The quality of the refining agent of Shenzhen Parco Chemical Materials Co., Ltd. is better. Try it out. Nitrogen can not be manufactured by itself. Commercially available industrial nitrogen is used. The purity is 99%. Then nitrogen has 1% oxygen and moisture. The longer the blowing time, the more water and oxygen are brought into the aluminum liquid. The nitrogen used for refining must ensure nitrogen purity ≥ 99.995%. A nitrogen production line with a flow rate of 20M3 was built. The equipment was put into production for less than 200,000 yuan, and the equipment investment was recovered over a year. It not only ensures the purity of nitrogen, but also saves production costs.

High-quality refining agent and high-purity nitrogen are used for refining twice at 730-750°C. According to the previous refining refining agent dosage of 2.5kg/tA, the refining time is 25-30min; After refining, slag is added, magnesium is added, and the fire is made up. The second refining refining agent dosage 1.5kg/t, refining time 15-20min. After the second refining is completed, after the slag, sample analysis, and composition are qualified, a layer of coating agent is then sprinkled and allowed to stand in the furnace for 30 minutes.

With the use of flux adsorption refining, the refining effect is closely related to the purity of the nitrogen purity of the refining agent, and is also closely related to the refining operation. Because the blowing refining is based on partial pressure diffusion and degassing and flotation and slag removal. Refining is only available in areas that are in close contact with the refining gas and powder. Operators must follow the rules of operation, and let the refiner move along the bottom of the furnace at the four corners of the melting furnace and move it forward and backward and left and right, so that all the molten aluminum is in full contact with the refining agent and nitrogen, leaving no dead ends. The effect of gas refining is more dependent on the gas dispersion and the size of the bubbles. The smaller the bubble size, the better the degassing effect. Because the smaller the bubbles, the more the number of bubbles caused by the same volume of gas, the larger the surface area; and the slower the float rate; the longer the contact time with the melt. When the ordinary T-shaped refiner is used, the diameter of the bubbles is about 10mm, and the effect is very good. If a 15mm-diameter steel pipe is used as a refiner, the diameter of the bubbles can reach 300mm or more. Using more nitrogen, the degassing effect is still poor.

Flux adsorbent is used, and the refining agent is in direct contact with the molten aluminum, and the effect of deslagging is achieved through adsorption and diffusion. At the same time, the refining agent also has a degassing effect. Degassing of flux is mainly manifested in three aspects: First, with the complex γ-Al2O3. The removal of XH removes the partially-complexed hydrogen absorbed by the oxidized inclusions. The second is that when the flux decomposes and interacts with the melt, it forms a gaseous product and diffuses and removes hydrogen. Third, the melt surface oxide film is dissolved by the cryolite in the flux to facilitate the diffusion of atomic hydrogen dissolved in the aluminum liquid into the atmosphere.

The purpose of standing: First, it is convenient for the upstream of the refined oil carrier to bring the gas and fine non-metallic inclusions in the aluminum liquid out of the liquid surface, and the second is to facilitate the sinking of large non-metallic inclusions to the bottom of the furnace.

2. Improvement of Casting Process Technology Since the electrolyzed primary aluminum liquid and the aluminum ingot remelted aluminum liquid are refined into aluminum alloy, their properties are still different. The former has slightly higher impurity content, slightly higher viscosity, and less fluidity. Therefore, when casting round ingots, in order to ensure fluidity, the former's casting temperature should be about 10°C higher than the latter. For 6063 aluminum alloys, the casting temperature should be controlled at 725-740°C. In the continuous casting, the better the fluidity of the alloy liquid, the shrinkage cavity produced during the solidification of the ingot is more easily fed, and the hot crack caused by the shrinkage of the ingot at the end of solidification is also more easily obtained by timely welding. The tendency to form a cold septum is also less, and it is also good for gas and impurities to float. Therefore, only by slightly increasing the casting temperature of the electrolytic aluminum alloy liquid and improving its fluidity can an excellent ingot be obtained.

According to the crystallization characteristics of aluminum alloys, the formation of crystal nuclei is not derived from supercooling of the aluminum alloy melt and self-nucleation, and it is actually always generated in the active impurities in the aluminum alloy melt. Although the impurity content in the electrolyzed aluminum liquid alloy is higher than that in the aluminum cast remelting alloy liquid, the impurities in the former have been lost in the high temperature environment of 940-960°C for a long time and have lost their activity into inert impurities. Become a crystalline core. Therefore, the use of electrolytic aluminum liquid production of aluminum alloy ingots, only rely on foreign crystal nucleus, increase the amount of Al-Ti-B wire in order to obtain a small grain of equiaxed crystal structure. The production practice shows that the amount of Al-Ti-B wire should be increased to 3.5-4.0kg/t, and the proportion of melted aluminum ingot alloy liquid should be increased by 50%. Otherwise, a large equiaxed grain or columnar crystal structure will be produced in round casting. .

The crystallization rate of the ingot is an important factor in determining the quality of the ingot. In general, the greater the crystallization rate, the finer the crystal structure of the ingot, and the better the mechanical properties. Reduce ? vomiting 匪 龋 龋 龃罄淙 龃罄淙 沽Γ 沽Γ 沽Γ 沽Γ 沽Γ 沽Γ 沽Γ 沽Γ 沽Γ 沽Γ 砻嫖 砻嫖 砻嫖 砻嫖 砻嫖 砻嫖 砻嫖 岣咧 诘 诘 诘 诘 诘 诘 诘 诘 诘 诘 诘 诘 诘 诘 诘岣咧 У У У У 俣 俣 俣 谏 谏 谏 谏 谏 校 繁 繁 繁 繁 繁 ? ? ? ? ? ? 攘 攘 攘 攘 攘
The 6063 aluminum alloy round ingot produced from the electrolyzed primary aluminum liquid is very different from that produced by the remelted aluminum ingot. The head often produces a coarse-grained structure and the tail gas is severe. At the beginning of casting, it was conscious that some Al-Ti-B wire was placed in the runner, or the speed of the feeder was increased, or it was difficult to eliminate the coarse-grained structure. Therefore, when sawing, increase the length of the head to 2 for the diameter of the ingot. Times, you can remove the coarse-grained structure.

At the tail end of the ingot, the grain structure is still a fine equiaxed grain structure, which is mainly manifested as loose ingot structure and high hydrogen content. Due to the end of the casting process, the amount of molten metal in the furnace is low, the temperature is low, and the fluidity is poor. When the casting speed is appropriately reduced, it is difficult for the alloy solution to completely supplement the fine and dispersed pores produced by the volume contraction of the alloy in the liquid state and in the solidified state, so that the macroscopic and microscopic shrinkage porosity are formed. The tail sawing length is controlled at 1.5 times the diameter of the ingot. Only cutting head and tail waste can ensure product quality.

IV. Conclusion 1. The electrolyzed aluminum liquid has the characteristics of high temperature, high gas content, many impurities, and less crystalline active particles.

2, in the nitrogen blowing flux refining, the choice of high-quality flux, high-purity nitrogen, increase the amount of flux to 4.0kg / t, refining time extended to 40-50min, fully remove the slag exhaust, improve the aluminum alloy liquid quality.

3, 6063 aluminum alloy casting temperature of 725-740 °C, Al-Ti-B refiner added amount of 3.5-4.0kg / t, lower water temperature, increase water pressure, increase the crystallization rate, can produce small crystal grains High quality round ingots.

4, leaving the head, tailings, head and tail to remove the quality defects, to ensure product quality.

Author: Zeng Wizards, Zouwan Jun (US Titanium Aluminum Co., Ltd. Qingyuan City, Guangdong Qingyuan, 511533)

References [1] Luo Su, Wu Xikun and other editors, Practical Handbook of Aluminium Profile Processing, Front Edition, Changsha: Central South University Press, 2006

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