SMEs should increase the use of advanced technologies for the purification of liquid aluminum

According to the bulk non-ferrous network reproduced: With the development of the automotive industry, 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 liquids, 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 Liquid

The gas in the aluminum liquid is 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, hydrogen has a 19.1 fold difference in solubility between solid and liquid phases, and every 100 g of molten aluminum. The normal hydrogen content is 0.1 to 0.4 mL. Due to 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 special castings (such as aviation castings), 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 has a change in structure; the side facing the aluminum liquid is dense and has a 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-ferro-silicon, 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 impurity content in the aluminum solution is very low, the amount of hydrogen is also very low; even if artificially introduced with hydrogen, 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, the status of aluminum alloy melt purification technology

At present, the comprehensive treatment of the purification and grain refinement of aluminum liquid is the fundamental problem of obtaining high-quality aluminum alloys. 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 produce 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 removing the inclusions inside the aluminum alloy while strengthening the dehydrogenation of the melt, and forms dry slag on the surface of the aluminum alloy melt, which 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, the application of aluminum purification technology in China

The scale of aluminum alloy casting companies is not very large, but only 100 companies have 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 enterprises 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 many varieties and small batch sizes. The scale of the enterprises is not large; most of the company's liquid aluminum purification still uses inert gas to rotate the degassing and then hit. The working mode of the slag; for the highly demanding aluminum liquid, 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 casting requirements and equipment technology, are basically melted by an edge furnace and do simple slag treatment to produce castings. Better still use inert gas or non-toxic refining agent to do some simple refining treatment or filter the aluminum liquid with filter plate. The treatment effect of aluminum liquid will start to be produced 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, slagging in the furnace, subcontracting and degassing and slagging, and transferring to the side furnace (the aluminum liquid transferred to the side-furnace is subject to detection). Some high-demand castings are generally used in these enterprises as a continuous melting furnace as an edge furnace, and the aluminum liquid 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 foundry 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 process; Ar gas rotation degassing and two-channel filter plate filtration are used in the purification process; the equivalent liquid density of the treated 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 plates to treat aluminum liquid. Metal-type low-pressure casting, mass production of automotive, motorcycle castings, most companies also use 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, we have co-existed from the world's most advanced and primitive models in terms of technology, equipment, and management. Through continuous learning of advanced technologies and management in this model, especially in China's more than 30 years of reform and opening up. In the aluminum alloy casting, great changes have taken place. Both technology and productivity are improving. The detection method of the quality of liquid aluminum is also constantly improving, and it is judged from the most primitive experience to rely on qualitative analysis of equipment and 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 China and have little difference with foreign products. More and more high-quality aluminum alloy castings have already begun to be produced in China, and the requirements for aluminum liquids are constantly increasing. It is a necessary way to improve the aluminum liquid purification equipment and processes.