Composition for refining aluminum and its alloys

Abstract

FIELD: metallurgy.SUBSTANCE: metallurgy of aluminum and its alloys and foundry production. Refining composition contains, wt.%: sodium hexafluoroaluminate 15-30, potassium hexafluorozirconate 10-40, a component releasing free nitrogen upon contact with liquid metal 10-15, sulfur - the rest.EFFECT: reducing the content of gas and non-metallic inclusions in the metal and reducing the content of aluminum in the slag without deteriorating the mechanical and technological properties of the alloy.3 cl, 3 tbl

Description

[0001] The invention relates to the foundry industry and metallurgy of aluminum and its alloys.

[0002] A characteristic feature of the melting of aluminum and its alloys is the need to clean the melt from dissolved hydrogen and suspended non-metallic inclusions, in particular, aluminum oxide, which inevitably forms when the metal comes into contact with atmospheric oxygen.

[0003] A known modifier for aluminum alloys contains sulfur and potassium chloride taken in a certain ratio [1].

[0004] Its disadvantage is the formation of volatile toxic aluminum chloride upon contact with molten aluminum, which worsens the working conditions of the personnel.

[0005] A flux is known for processing hypereutectic aluminum alloys, containing sulfur, aluminum oxide, hexachloroethane and sodium phosphate, taken in a certain ratio [2].

[0006] Its disadvantage is a decrease in environmental safety due to the intensive gas evolution of chlorine-containing compounds when the flux comes into contact with the molten metal.

[0007] A carbonate mixture for refining aluminum alloys with a modifying effect is known, consisting of calcium carbonate and strontium carbonate taken in a certain ratio [3].

[0008] However, it is not free from disadvantages, the main ones being the technological complexity of preparation and application, the high price and the fact that the calcium present in the mixture contributes to the formation of a large number of coarse inclusions of crystallization origin, leading to a decrease in the mechanical properties of the alloy.

[0009] A degasser for aluminum and aluminum alloys is known, containing sulfur and sodium carbonate taken in a certain ratio [4].

[0010] Its main disadvantages include the low refining capacity of sodium carbonate, as well as the high risks associated with saturating the molten metal with sodium, such as a decrease in the density of the material, the formation of shrinkage porosity in castings, and, in some cases, undesirable modification of the eutectic due to the grinding of silicon, which is unacceptable, for example, for heat-resistant piston alloys.

[0011] The closest to the invention is a degasser for aluminum or its alloys, containing sulfur, sodium hexafluoroaluminate and potassium hexafluorozirconate in the following ratio of components, wt. %:

[0012] Sodium hexafluoroaluminate - 15-30

[0013] Potassium hexafluorozirconate - 10-40

[0014] sulfur - rest [5].

[0015] The prototype's shortcomings include an excessively high sulfur content, which quickly turns into a gas at temperatures as low as 445°C, causing intense bubbling of the melt. The chemical conversion of hydrogen dissolved in the metal into hydrogen sulfide is incomplete due to the formation of large gas bubbles, while oxide films and slag from the metal surface are actively mixed into the melt, ultimately reducing refining efficiency. Additionally, a strong sulfur odor is observed and the risk of melt ejection from the crucible increases, potentially resulting in industrial injuries.

[0016] These shortcomings are addressed by the proposed solution.

[0017] The objective of the proposed solution is to increase the efficiency of refining and improve the working conditions of production personnel.

[0018] Technical result: a composition for refining aluminum and its alloys, ensuring a reduction in the content of gas and non-metallic inclusions in the metal and a reduction in the aluminum content in the slag without deteriorating the mechanical and technological properties of the alloy relative to the prototype.

[0019] The technical result is achieved in that the composition for refining aluminum and its alloys, which contains sodium hexafluoroaluminate, potassium hexafluorozirconate and sulfur, additionally contains a component that releases free nitrogen upon contact with liquid metal, in the following ratio, %, mass.:

[0020] Sodium hexafluoroaluminate - 15-30

[0021] Potassium hexafluorozirconate - 10-40

[0022] a component that releases free nitrogen upon contact with liquid metal - 10-15

[0023] sulfur - the rest.

[0024] Sulfur actively binds hydrogen dissolved in the melt into volatile hydrogen sulfide. Furthermore, when it evaporates upon contact with the liquid metal, it, along with the hydrogen sulfide, forms rising gas bubbles that collect gaseous and non-metallic inclusions on their surface (adsorption refining mechanism) and draw them toward the metal surface (flotation refining mechanism). As a result, the density increases, the mechanical and technological properties of the alloy improve, its gas content decreases, and, consequently, the porosity of castings decreases. Furthermore, sulfur promotes the refinement of the acicular iron-containing structural components of the aluminum alloy.

[0025] Sodium hexafluoroaluminate, or cryolite, promotes the removal of gases and non-metallic inclusions, primarily aluminum oxide, from the melt via an adsorption refining mechanism. However, its main function in the composition is to disperse the gas phase and reduce the rate of sulfur evaporation, thereby increasing the area and duration of its contact with the hydrogen dissolved in the melt. It improves bubbling control, reducing aluminum oxidation, secondary gas saturation, and the mixing of oxide films and slag inclusions into the metal.

[0026] Potassium hexafluorozirconate provides additional chemical binding of hydrogen into stable hydrides, as the zirconium it contains acts as a getter (gas absorber) for hydrogen, retaining it through chemisorption. Furthermore, it promotes the production of a friable slag with minimal aluminum content.

[0027] A component that releases free nitrogen upon contact with molten metal, with a reduced sulfur content compared to the prototype, partially absorbs the functional load of sulfur and cryolite in terms of adsorption and flotation mechanisms for refining molten aluminum, dispersing the gas phase, improving bubbling control, and reducing the risk of melt ejection from the crucible. The released free nitrogen (N2) is the main component of air; it is chemically inert, environmentally safe, and odorless, improving working conditions for production personnel involved in refining aluminum or its alloys.

[0028] The proposed solution uses sodium azide or ammonium dichromate as a component that releases free nitrogen upon contact with liquid metal.

[0029] Sodium azide NaN3 is a sodium salt of the azide ion, produced by the Russian industry and supplied as a white crystalline powder. The density of sodium azide is 1.85 g / cm3. 3 It decomposes when heated above 275°C with the release of free nitrogen N2 according to the reaction

[0030] 2NaN3=2Na+3N2↑,

[0031] and also melts at a temperature of about 300°C and has the ability to form complex compounds with metals that are easily converted into slag.

[0032] Ammonium dichromate (NH4)2Cr2O7 is a salt of ammonium and dichromic acid, produced by the Russian industry and supplied in the form of a crystalline substance of orange-red color with a density of 2.115 g / cm 3 When heated above 180°C, ammonium dichromate decomposes, releasing nitrogen, water vapor, and the emission of hot solid particles of chromium oxide (the "Böttger volcano" or "chemical volcano" reaction):

[0033] (NH4)2Cr2O7=Cr2O3+3N2↑+4H2O↑.

[0034] In order to test the effectiveness of the proposed solution, a series of experimental works were carried out in the laboratory and in the conditions of an existing production facility.

[0035] The percentages of sodium hexafluoroaluminate and potassium hexafluorozirconate in the proposed refining composition were unchanged from the prior art, remaining at 15-30% by weight and 10-40% by weight, respectively. The content of the component releasing free nitrogen upon contact with molten metal was varied from 5 to 20% by weight in 5% by weight increments, with sulfur making up the remainder.

[0036] The starting components were taken in percentages according to Tables 1 and 2 (see the graphic section). A total of nine compositions were tested, including the prototype (composition No. 1). Compositions Nos. 2-5 contained sodium azide as the component releasing free nitrogen upon contact with liquid metal, while compositions Nos. 6-9 contained ammonium dichromate, respectively.

[0037] The materials, with a total weight of 150 g, were thoroughly mixed, dried at 100°C for 4 hours, and then tablets weighing 50 g (three pieces for each composition) were obtained on a hydraulic laboratory press with a force of up to 4 tons.

[0038] The tablets were used for liquid-phase refining of AK7 alloy from recycled material (sprues, rejected castings) weighing 100 kg in a CAT electric resistance furnace at a flow rate of 0.05% of the melt mass being processed (one tablet). The refining composition was placed in a bell jar and lowered under the metal surface at a temperature of 740-760°C. After a 10-minute hold for the refined alloy, slag was removed from the surface, and samples and test castings were poured into the mold.

[0039] The experiments were carried out three times for each composition of the refining composition.

[0040] During refining, the duration of bubbling, visually observed on the metal surface, was recorded with a stopwatch. The condition of the removed slag was visually monitored, and its residual metal content was estimated by gravimetric analysis. The fluidity of the processed melt was assessed using a Carey spiral test at 750°C. The processed melt was poured into a chill mold to obtain standard samples and conduct mechanical testing (tensile strength and elongation). Gas content was determined on templates cut from test castings using the VIAM porosity scale (GOST 1583-93). On thin sections with an area of ​​4.0 cm 2 , cut from test castings, the content of non-metallic inclusions in the alloy was assessed using metallographic method K (option K1) in accordance with GOST 1778-2022.

[0041] The test results are shown in Table 3 (see the graphic part).

[0042] For compositions No. 5 and 9, the bubbling duration exceeded the technologically necessary time to ensure complete melt processing and was equal to 120 s.

[0043] The slag for compositions No. 2 and 6, unlike all the others, turned out to be lumpy, while the content of residual aluminum in it exceeded the 15% by weight limit, as in the prototype.

[0044] The fluidity for compositions No. 3, 4, 5, 7 and 8 turned out to be higher, for composition No. 2 - lower than that of the prototype, and for compositions No. 6 and 9 - respectively, the same.

[0045] In terms of the tensile strength of the alloy, only composition No. 2 was inferior to the prototype; in terms of the relative elongation of the alloy, no composition was inferior to the prototype.

[0046] The gas content and content of non-metallic inclusions for all compositions were less than that of the prototype.

[0047] During pilot testing, no melt emissions from the furnace or strong, unpleasant sulfur odor were detected for any of the compositions. Monitoring of the melting and mold-pouring work areas revealed no exceedance of maximum permissible gas emission concentrations.

[0048] As a result, we settled on compositions No. 3, 4, 7 and 8 as optimal, allowing us to increase the efficiency of refining and improve the working conditions of production personnel by reducing the content of gas and non-metallic inclusions in the metal and reducing the aluminum content in the slag, as well as improving the mechanical and technological properties of the alloy relative to the prototype.

[0049] Additionally conducted tests have shown that the selected compositions of the refining composition make it possible to achieve results even with a reduction in consumption from 0.05 to 0.03-0.04% of the mass of the metal being processed.

[0050] Sources of information:

[0051] 1. USSR Author's Certificate No. 449957, C22C 1 / 02, C22C 21 / 04, 1974.

[0052] 2. USSR Author's Certificate No. 1214773, C22B 9 / 10, 1986.

[0053] 3. Patent for invention of the Russian Federation No. 2562015, cl. C22C 1 / 06, C22B 9 / 10, C22C 21 / 00, 2015.

[0054] 4. Patent for invention of the Republic of Belarus No. 4022, C22B 21 / 06, C22B 9 / 10, 2001.

[0055] 5. Patent for invention of the Republic of Belarus No. 16132, C22B 9 / 00, C22B 21 / 06, 2011 - prototype.

Claims

1. A composition for refining aluminum and its alloys, comprising sodium hexafluoroaluminate, potassium hexafluorozirconate and sulfur, characterized in that it additionally contains a component that releases free nitrogen upon contact with liquid metal, in the following ratio, % by weight: sodium hexafluoroaluminate 15-30 potassium hexafluorozirconate 10-40 component that releases free nitrogen upon contact with liquid metal 10-15 sulfur is the rest.

2. The composition according to claim 1, characterized in that sodium azide is used as the component that releases free nitrogen upon contact with liquid metal.

3. The composition according to claim 1, characterized in that ammonium dichromate is used as the component that releases free nitrogen upon contact with liquid metal.

Images