ã€China Aluminum Network】 Aluminum alloy castings are used more and more, and the requirements for castings are getting higher and higher. In addition to the requirements for ensuring chemical composition, mechanical properties and dimensional accuracy, aluminum alloy castings are not allowed to have casting defects such as shrinkage holes, shrinkage porosity, porosity, and slag holes. Purification of aluminum liquid is one of the measures to ensure high quality aluminum alloy products, and it is also the main means to improve the overall quality of aluminum alloys. The refining effect of aluminum liquid has an important influence on the formation of pores, shrinkage holes, and inclusions, and directly affects the physical and mechanical properties of aluminum alloy castings. Without high-quality aluminum liquid, even if the subsequent processing is advanced, once defects are created, they are always present in the product, and high-quality castings are difficult to obtain. Therefore, it must pay attention to the gas and inclusions in the aluminum liquid, and take measures to remove the gas and inclusions in the aluminum liquid.
1. Gases and Inclusions in Aluminum Liquids The gases in aluminum liquids are mainly hydrogen (about 80% to 90%), followed by nitrogen, oxygen, and carbon monoxide. Hydrogen is almost insoluble in solid aluminum, and its solubility in the liquid is large. The solubility of hydrogen below the solidus line is 0.65 mL and 0.034 mL (hydrogen at 0.1 MPa) per 100 g of aluminum liquid, that is, the difference in the solubility of hydrogen in solid and liquid phases is 19.1 times, and per 100 g of molten aluminum. The normal hydrogen content is 0.1 to 0.4 mL. Because of the different solubility of hydrogen, it tends to escape from the melt. When the hydrogen pressure is greater than the surface tension and the hydrostatic pressure, bubbles are formed and pinholes are produced in the casting. Therefore, in the melt purification of aluminum alloys, the main problem is that the aluminum alloy has a high hydrogen content, but the existing methods cannot meet the production requirements of high quality aluminum alloy castings. Generally, the hydrogen content per 100g of aluminum can meet the production requirements when the hydrogen content is 0.1 to 0.2mL. For a special casting (such as an aviation casting), the hydrogen content per 100g of aluminum should be less than 0.06mL.
Inclusion refers to anything other than liquidus above solids and liquids. Common non-metallic impurities in aluminum liquids are oxides, nitrides, carbides, borides, etc., mostly in the form of particles, the typical particle size in the range of 1 ~ 30μm. In addition to the charge, it is mainly the product of chemical reactions during the melting process. The thickness of the oxide film on the surface of aluminum is 2~10μm, and it increases to 200μm when it is close to the melting point. The oxide film on the liquid surface is not only thicker, but also the structure has changed; the side facing the aluminum liquid is dense and has protective effect on the aluminum liquid. The outer side of the aluminum liquid is loose, with 5~10μm diameter holes and is filled with hydrogen, air, and water vapor. If the liquid film is stirred into the aluminum liquid, it will increase impurities and increase gas. In addition, some undesired primary intermetallic compounds such as aluminum zirconium, aluminum titanium, etc. may also appear in high-alloy melts. Iron-containing aluminum alloys also form iron-rich aluminum-iron phases, aluminum-silicon-ferrosine and aluminum-silicon. The iron phase is an acicular compound that severely destroys the aluminum matrix and affects the mechanical properties.
There is a strong interaction between the inclusions in the aluminum alloy and the gas. The hydrogen content in the aluminum liquid is greatly affected by inclusions. When the inclusion content is 0.002% and 0.02%, the corresponding hydrogen content is 0.2mL/100gAl and 0.35. mL/100gAl. In the case of the same amount of hydrogen, the higher the content of inclusions, the higher the pinhole rate; conversely, when the aluminum solution contains a very low amount of impurities, the amount of hydrogen is also very low; even if hydrogen is artificially introduced, it will be automatically precipitated. Soon recovered to its original content. Even a small amount of inclusions can significantly reduce the critical concentration of pores. On the other hand, in the absence of inclusion (or very low inclusion content), the critical hydrogen concentration to form pores can reach 0.3 mL/100 g Al. Therefore, it is equally important that impurities and degassing should be carried out simultaneously. No matter what kind of refining method is used, the effect of degassing and removing impurities is often a combination of both, but each has its own emphasis.
2. Status quo of aluminum alloy melt purification technology Currently, the comprehensive treatment of aluminum liquid purification and grain refinement is the basic problem of obtaining high quality aluminum alloy. Aluminum liquid purification treatment can be divided into in-furnace treatment and off-furnace treatment according to different production links. Aluminum alloy furnace treatment according to the purification mechanism can be divided into adsorption purification technology and non-adsorption purification technology.
Adsorption purification mainly depends on the adsorption of oxide inclusions generated by the refining agent, at the same time remove the oxide inclusions and hydrogen attached to the surface, to achieve the purpose of purification of aluminum liquid. Non-adsorption purification depends on other physical and chemical effects to achieve the purpose of purification of liquid aluminum. Adsorption purification occurs only at the adsorption interface, and the non-adsorption purification action acts simultaneously on the entire aluminum liquid. The main adsorption methods include inert gas purge, reactive gas purge, mixed gas purge, chlorine salt (hexachloroethane) purification, non-toxic refining agent purification, solvent purification, etc.; non-adsorption methods, mainly vacuum purification methods (Static vacuum treatment, dynamic vacuum treatment), ultrasonic purification treatment method, electromagnetic purification treatment method, pressure crystallization method, rare earth element solid hydrogen method and the like.
In the purification of chlorine by adsorption purification technology, chlorine gas and liquid aluminum generate aluminum trichloride, aluminum chloride and hydrogen gas generate hydrogen chloride, both physical and chemical effects, refining effect is obvious, but pollute the environment, harmful to the human body. Later, inert gas (nitrogen and argon) was used instead of 99.999% of inert gas, but the effect was not good and the cost was increased. Various methods are used to improve the refining process but the effect of chlorine refining is always not achieved. In recent years, the use of mixed gas refining in foreign countries, ie, inert gas plus reactive gas, is beneficial to the removal of inclusions in aluminum alloys while strengthening the dehydrogenation of the melt, and the formation of dry slag on the surface of aluminum alloy melts can be achieved well. Refined effect. And can easily control the amount of chlorine gas mixed, the impact on the environment is not great.
In the adsorption purification process technology, many practical aluminum liquid treatment methods have also been produced through process innovations and process transformations, such as inert gas rotating degassing, ceramic foam filtration, dusting treatment, and the combined use of rotary degassing and powder spraying. A variety of methods. In the non-adsorption purification process, the hydrogen fixation of rare earth elements is also one of the future development directions.
3. Application status of aluminum liquid purification technology in China Aluminum alloy casting enterprises are not very large in size. There are only 100 companies with an annual production capacity of more than 50,000 tons. Among these enterprises, die casting products account for a considerable proportion. In addition, at present, the production volume of high-quality castings of enterprises in our country is not very large, so the development of the purification treatment of aluminum liquids is slow. China's melt purification equipment, testing equipment manufacturing companies a lot, but the company's scale is not large. Some large-scale production companies use equipment such as Morgan, Foseco and other foreign brands. Domestic equipment is widely used along with technological progress.
The casting and aluminum alloy castings in the aerospace field are demanding, but in China, they are still in a mode of production with multiple batches and small batch sizes. The scale of the enterprises is not large; the aluminum purification of most enterprises still uses inert gas to rotate the degassing and then hit. The working mode of the slag; for the highly demanding liquid aluminum, the product requirements can be met after the hexachloroethane treatment, which is one of the reasons for the slow development of high-end purification facilities in China.
Small aluminum alloy die-casting enterprises, due to low requirements for castings and backward equipment technology, are basically melted at the edge of the furnace and do simple slag treatment to produce castings. Better still use inert gas or non-toxic refining agent to do some simple refining process or filter the aluminum liquid with filter plate. The treatment effect of aluminum liquid will start to produce without testing; this treatment method has poor quality of aluminum liquid. , Low production quality, relatively high cost, and difficult business development. Some large and medium-sized aluminum alloy die-casting enterprises mostly use centralized melting, furnace slagging, subcontracted degassing and slagging, and transfer to the edge furnace (the aluminum liquid transferred to the edge furnace is tested). Some of the most demanding castings, in this type of enterprise, generally use a continuous melting furnace as an edge furnace, and the aluminum fluid is produced by in-situ degassing of inert gas and foam ceramic filtration. The advanced aluminum liquid purification technology is also used by many companies in China, such as the melting and purification facilities of an aluminum alloy lost foam production line of a casting company in Shaanxi. The alloy materials used in the production line are: ZL101A, Sr deterioration; Melting furnace: Jack 1.5t/h Continuous Melting Furnace of the West Company; Pyrotek's on-line continuous processing equipment is used in the purification treatment; the Ar gas rotation degassing and the 2-channel filter plate filtration are used in the purification treatment; the equivalent liquid density of the treated aluminum liquid is less than 1, and the processing capacity is 1.5 t/h; Advanced processing equipment like this is used more often by newer-scale manufacturing companies. The enterprises that choose metal gravity casting and mass production of automobile and motorcycle castings basically use continuous melting aluminum, inert gas T-type degassing pipe on-line degassing, and ceramic filter plate filtration to treat aluminum liquid. Metal-type low-pressure casting, mass production of automotive, motorcycle castings companies are also mostly used continuous melting furnace aluminum, rotary inert gas inert gas in the turnover package (some use rotary degassing plus powder refining), slag. The quality of molten aluminum processed by these methods can be guaranteed, the efficiency is high, and the production cost is relatively low; it is beneficial to mass production.
4. Summary In the field of casting in China, from technology, equipment, and management, the world's more advanced and more primitive two modes coexist, and in this mode, through continuous learning of advanced technology and management, especially in China's reform and opening up For more than 30 years, great changes have taken place in the casting of aluminum alloys. Both technology and productivity are improving. The detection methods for the quality of molten aluminum have also been continuously improved, from the more primitive experience to the qualitative analysis of equipment and the quantitative analysis up to now. Different enterprises in the quantitative analysis have chosen their own suitable methods (density equivalent method, vacuum method and direct reading of hydrogen content); and these equipment can be produced in our country, and have not much difference with foreign products. More and more high-quality aluminum alloy castings have already begun to be produced in China. The requirements for aluminum liquids are constantly increasing. It is a necessary way to improve the equipment and process of aluminum liquid purification.
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