There is a strong interaction between inclusions in aluminum alloy and gas. The hydrogen content in molten aluminum 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 hydrogen content, the higher the inclusion content, the higher the pinhole rate; on the contrary, when the aluminum liquid contains very low impurities, the hydrogen content is also very low; even if hydrogen is artificially injected, it will automatically precipitate , Quickly restored to the original content. Even if a small amount of inclusions are present, the critical concentration value for the formation of pores can be significantly reduced. On the other hand, when there is no inclusion (or the inclusion content is very low), the critical hydrogen concentration to form pores can reach 0.3mL/100gAl. Therefore, it is equally important to remove impurities and gas at the same time. No matter which refining method is used, the effects of degassing and impurity removal are often both, but each has its own focus.
Current status of aluminum alloy melt purification technology
At present, the comprehensive treatment of the purification and grain refinement of liquid aluminum is the basic problem for obtaining high-quality aluminum alloy. According to the different production links, the purification treatment of molten aluminum can be divided into treatment in the furnace and treatment outside the furnace. The aluminum alloy furnace treatment can be divided into adsorption purification technology and non-adsorption purification technology according to the purification mechanism.
The purification of the adsorption method mainly relies on the effect of adsorbing oxidized inclusions produced by the refining agent, and at the same time, removes the oxidized inclusions and the hydrogen attached to the surface to achieve the purpose of purifying the molten aluminum. Non-adsorption purification relies on other physical and chemical effects to achieve the purpose of purifying molten aluminum. The purification effect of the adsorption method only occurs on the adsorption interface, and the purification effect of the non-adsorption method acts on the entire aluminum liquid at the same time. The adsorption method mainly includes inert gas purging, active gas purging, mixed gas purging, chlorine salt (hexachloroethane) purification, non-toxic refining agent purification, solvent purification, etc.; non-adsorption method, mainly vacuum purification treatment method (Static vacuum treatment, dynamic vacuum treatment), ultrasonic purification treatment method, electromagnetic purification treatment method, pressure crystallization method, rare earth element hydrogen fixation method, etc.
When refining with chlorine in the adsorption purification treatment technology, chlorine and liquid aluminum generate aluminum trichloride, and aluminum trichloride and hydrogen generate hydrogen chloride, which has both physical and chemical effects. The refining effect is obvious, but it pollutes the environment and is harmful to the human body. Later, inert gas (nitrogen and argon) was used for refining, and even 99.999% inert gas was used, 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, foreign countries have adopted mixed gas refining, that is, inert gas plus active gas, which can strengthen the hydrogen removal effect of the melt and help remove the inclusions in the aluminum alloy, and form dry slag on the surface of the aluminum alloy melt, which can achieve good results. Refining effect. Moreover, the mixing amount of chlorine can be conveniently controlled, and the impact on the environment is not large.
In the adsorption purification treatment technology, many practical aluminum liquid treatment methods have been produced through process innovation and process transformation: such as inert gas rotating degassing, foam ceramic filtration, powder spraying, and the combined use of rotating degassing and powder spraying, etc. Multiple methods. Among the non-adsorption purification treatment technologies, the rare earth element hydrogen fixation method is also one of the future development directions.
Application status of aluminum liquid purification technology in my country
The scale of aluminum alloy casting enterprises is not very large, and there are only a hundred enterprises with an annual production capacity of more than 50,000 tons, and die casting products account for a considerable proportion of these enterprises. In addition, the current production volume of high-quality castings of Chinese enterprises is not very large, so the development of purification treatment technology of aluminum liquid is slow. There are many manufacturers of melt purification equipment and testing equipment in my country, but the scale of the enterprises is not large. Some large-scale production companies use equipment from foreign brands such as Morgan and Foseco, and domestic equipment is being widely used as technology advances.
In the aerospace field, the requirements for cast aluminum alloy castings are high, but in our country, it is still in a production mode with multiple varieties and small batches. The scale of the enterprise is not very large; most enterprises still use inert gas to rotate and degas and then degas. The operation mode of slag: For the demanding aluminum liquid, it can meet the product requirements after being processed by hexachloroethane. This is also one of the reasons for the slow development of my country’s high-end purification facilities.
Small aluminum alloy die-casting enterprises, due to low casting requirements and backward equipment technology, are basically machine-side furnace melting, and do some simple slagging treatment to produce castings. It is better to use inert gas or non-toxic refining agent to do some simple refining treatment or filter the aluminum liquid with a filter plate. The processing effect of the aluminum liquid is not tested before the production; this processing method, the quality of the aluminum liquid is poor , The production level is low, the cost is relatively high, and the development of the enterprise is difficult. Some large and medium-sized aluminum alloy die-casting enterprises mostly use centralized melting, slagging in the furnace, subcontracting degassing and slagging treatment, and then transfer to the side furnace (the aluminum liquid transferred to the machine side furnace must be tested) to organize production. For some high-demand castings, continuous melting furnaces are generally used as machine-side furnaces in such enterprises. The molten aluminum is degassed by inert gas on-line and filtered by foam ceramics to produce castings. Advanced aluminum liquid purification treatment technology is also used by many enterprises in my country, such as the melting and purification treatment facility of the aluminum alloy lost foam production line of a casting company in Shaanxi. The alloy materials used on the production line: ZL101A, Sr are modified; melting furnace: Jack West Company’s 1.5t/h continuous melting furnace; Pyrotek’s online continuous processing equipment is used for purification treatment, Ar gas rotating degassing plus 2 filter plates are used for purification treatment, the equivalent density of the molten aluminum after treatment is less than 1, and the processing capacity is 1.5 t/h; advanced cnc machining equipment like this is used more by new large-scale production enterprises. Companies that choose metal gravity casting and mass-produce automobile and motorcycle castings basically use continuous melting furnace aluminum, inert gas T-type degassing pipe on-line degassing, and ceramic filter plate filtration to process molten aluminum. Metal mold low-pressure casting, and most of the enterprises that mass produce automobile and motorcycle castings also use continuous melting furnace aluminum, inert gas rotating degassing in the turnover package (some also use rotating degassing and powder spraying refining), and slag removal. 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 good for mass production.
In the casting field of our country, the two most advanced and primitive modes coexist in the field of technology, equipment, and management. Under this mode, we will continue to learn advanced technology and management, especially since our country has been in reform and opening for more than 30 years. Here, aluminum alloy casting has undergone tremendous changes. Both technology and production capacity are improving. The detection methods for the quality of molten aluminum are constantly improving, from the most primitive experience to the qualitative analysis of the equipment, to the current quantitative analysis. In terms of quantitative analysis, different companies choose their own suitable methods (density equivalent method, vacuum method and hydrogen content direct reading method); and these equipment can be produced in my country, and there is not much difference with foreign products. More and more high-quality aluminum alloy castings have begun to be produced in my country, and the requirements for molten aluminum are constantly improving. It is an inevitable way to improve the purification equipment and technology of molten aluminum.
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