RU2763844C1 - Method for preparation and supply of a protective gas mixture for melting magnesium alloys - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to the field of metallurgy, specifically to the smelting and casting of magnesium-based alloys, and can be used to produce shaped castings, for example, casings of various units used in the aerospace industry and other industries. The method for preparing and supplying a protective gas mixture of a carrier gas, inert or inactive with respect to the melt, and gaseous perfluoroethylisopropyl ketone when melting magnesium alloys includes obtaining a gas mixture containing perfluoroethylisopropyl ketone in an amount of 0.1-10% of the volume of the supplied gas mixture directly in the feed a tube supplying the gas mixture to the melting crucible by mixing the carrier gas and perfluoroethyl isopropyl ketone gas, obtained by evaporation of its liquid phase, supplied by gravity in a droplet manner through a capillary inlet directly into the supply tube, due to convective heat transfer from the metal in the crucible.

EFFECT: invention is aimed at creating a method for the preparation and supply of a protective gas mixture for melting magnesium alloys, which makes it possible to simplify the design of a gas system for introducing a protective gas mixture and to use a liquid material, in particular perfluoroethylisopropyl ketone, to create this mixture.

9 cl, 2 ex, 3 tbl, 3 dwg

Description

Technical field

The invention relates to the field of metallurgy, specifically to the melting and casting of alloys based on magnesium, and can be used to obtain shaped castings with a combination of good strength properties, as well as increased corrosion resistance, for example, housings of various units used in the aerospace industry and in others industries.

State of the art

A feature of almost all currently used magnesium casting alloys is their tendency to intense oxidation up to ignition on contact with air during melting and casting. As a result, special measures are taken to protect the metal surface from contact with the atmosphere.

The most common method of protection against oxidation is melting with the use of coating and refining fluxes based on chloride and fluoride salts (Altman M.B., Lebedev A.A. and others. “Smelting and casting of light alloys”. M., “Metallurgy” , 1969, pp. 332-335). However, inclusions of fluxes in cast metal significantly reduce the corrosion resistance of cast parts, therefore, at present, melting using fluxes is gradually being replaced by melting using a protective gas atmosphere. The gases used for this are various chemical compounds that form a dense protective film on the surface of the liquid magnesium melt.

The most common shield gases for melting magnesium alloys are the highly toxic sulfur dioxide SO ₂ and SF ₆ , and other shield gases are also known (Table 1), with SF6 (SF ₆ ) at low concentrations in the mixture being the preferred shield gas for over 20 years. with air, argon or carbon dioxide.

Table 1 Shielding gases used for melting magnesium alloys

Compound Time of existence in the atmosphere, years Global Warming Potential (GWP), over 100 years CO ₂ 100-150 one N ₂ O 120 360 CH ₄ 12 24 SO ₂ - - SF ₆ 3200 23900

Due to the extreme values of the Global Warming Potential (GWP), SF ₆ has recently been actively replaced by other compounds (Regulation (EU) No 517/2014 of the European Parliament and of the Council of 16 April 2014 on fluorinated greenhouse gases and repealing Regulation (EC) No 842/2006 Text with EEA relevance P. 36).

There is a known method for processing molten reactive metals and alloys using fluorocarbons as a protective gas (US 6,685,764 B2, 03/22/2003), where it is proposed to use a mixture of perfluoroketones, hydrofluoroketones and their mixtures with dried air or other gases to protect magnesium melts. The advantage of this mixture lies in the very low GWP value of the components (less than 10) and their low toxicity to humans. The disadvantage of this method is the presence of a sufficiently large amount of hydrogen fluoride in the reaction products of the gas mixture with air and liquid magnesium, which leads to the destruction of the metal parts of the smelter.

Also known is the material Novec 612, developed by 3M Innovative Properties Company (USA), which is designed to protect molten reactive metals such as magnesium and its alloys, mixed with CO ₂ or dried air, and the method of its application (https://multimedia. 3m.com/mws/media/713947O/3m-novec-612-magnesium-protection-fluid.pdf). Novec 612 is a fluid that is significantly more reactive at melt temperatures and therefore more efficiently used than SF6. However, since the material is liquid at room temperature (boiling point 49.2°C), creating a gas mixture presents some difficulties. The way to create a protective gas mixture using liquid Novec 612 (prototype) is to enter Novec 612 into the carrier gas flow using a gas bubbler or precision pumping system, resulting in Novec 612 goes into a gaseous state. Novec 612 is an effective protective gas agent for the protection of molten magnesium, however, the implementation of the method of its application is associated with the use of special high-precision equipment, and therefore with an increase in production costs.

In the domestic industry, freon gas FK 5-1-12 (perfluoroethylisopropyl ketone) is known, which belongs to fluoroketones and is intended primarily for use as a refrigerant and fire extinguishing agent. Its main advantage when used as an active gas to create a gas mixture of perfluoroethylisopropylketone over other active gases is its low GWP, which greatly increases the compliance of the technology of melting magnesium alloys in a gaseous atmosphere with constantly tightening environmental requirements.

Despite its similarity in chemical formula to Novec 612, it has not previously been used as a shielding gas material for magnesium alloy melting because PFEK is a liquid at room temperature and does not actively evaporate in air, thus creating a gas mixture in which perfluoroethylisopropylketone would reach the required concentration, it is difficult.

Disclosure of the essence of the invention

The technical objective of the invention is to create a method for preparing and supplying a protective gas mixture for melting magnesium alloys, which makes it possible to simplify the design of a gas system for introducing a protective gas mixture and use liquid material, in particular perfluoroethylisopropyl ketone, to create this mixture.

The technical result of the invention is to simplify the design of the gas system for introducing a protective gas mixture for melting most industrial magnesium alloys in terms of dosing, mixing and supplying gas mixture components to the melt surface.

The technical result is achieved by the proposed method for preparing and supplying a protective gas mixture of a carrier gas, inert or inactive with respect to the melt, and gaseous perfluoroethylisopropyl ketone when melting magnesium alloys, characterized in that the gas mixture containing perfluoroethylisopropyl ketone in an amount of 0.1-10 % of the volume of the supplied gas mixture, is obtained directly in the supply tube supplying the gas mixture to the melting crucible, by mixing the carrier gas and gaseous perfluoroethylisopropyl ketone, obtained by evaporation of its liquid phase (form), supplied by gravity in a drip way through a capillary inlet directly into the supply tube, due to convective heat transfer from the metal in the crucible.

The content of perfluoroethylisopropylketone in the gas mixture in the amount of 0.1-10% of the volume of the supplied gas mixture is due to the minimum amount of perfluorethylisopropylketone necessary for the formation of a protective film of magnesium oxyfluorides on the surface of the melt, and on the upper side - the content of perfluoroethylisopropylketone in the gas mixture, the excess of which will be lead to active destruction of the walls of the melting crucible.

It is permissible to use argon, carbon dioxide, helium, nitrogen, dried air, as well as a mixture of two or more of these gases as a carrier gas. The choice of these gases as carrier gases is due to their inertness or low chemical activity with respect to the magnesium melt at the melting temperatures used.

It is preferable that the crucible lid is provided with a working opening with a lid, which can be opened during the melting process for working with metal in the furnace, while the flow rate of the gas mixture supplied to the crucible is increased to compensate for its dispersion in the atmosphere of the working area of the shop.

It is important that the capillary inlet used to introduce the liquid perfluoroethylisopropylketone into the inlet tube has a capillary hole diameter of 0.1-1.5 mm. The use of a capillary hole of a different diameter does not contribute to stable drip dosing of perfluoroethylisopropylketone.

The main differences from the prototype are:

the use of an original method for preparing and supplying a gas mixture, which is not created in advance using a gas bubbler or a precision pumping system, but is created directly in the inlet pipe supplying the gas mixture to the melting crucible, due to the mixing of gases when carrier gas and liquid enter the inlet pipe perfluoroethylisopropylketone, which evaporates when heated from the heat of the metal in the crucible, which greatly simplifies the design of the gas system;

the supply of liquid perfluoroethylisopropylketone to form a mixture is carried out by drip method directly into the inlet pipe through a capillary inlet by gravity, the dosage is carried out according to the volume of the outflowing liquid, the flow rate of which is determined by the scale applied to the transparent wall of the vessel with liquid perfluoroethylisopropylketone based on the formation of approximately 120 ml from 1 ml of liquid perfluoroethylisopropylketone gaseous. This method of dosing makes it possible to visually control the consumption of perfluoroethylisopropylketone and quickly change the supplied amount of active gas without using complex and expensive devices for dosing and measuring gases.

Description of illustrations

The invention is illustrated by illustrations, where in Fig. 1. shows a device for preparing and supplying a protective gas mixture, where:

1 - balloon; 2 - gas reducer; 3 - manometer; 4 - rotameter; 5 - crucible; 6 - crucible cavity; 7 - crucible cover; 8 - melt surface; 9 - graduated transparent wall; 10 - inlet tube; 11 - system; 12 - dropper; 13 - capillary input; 14 - flow regulator; 15 - outlet tube.

In FIG. 2 shows a protective film on the surface of an ML5 magnesium alloy (image from a scanning electron microscope), melted in a protective gas atmosphere consisting of 99 vol.% argon and 0.5 vol.% perfluoroethylisopropyl ketone, prepared using a device for preparing and supplying a protective gas mixture, consisting of fluoride and magnesium oxide with an admixture of carbon.

In FIG. 3 shows a protective film on the surface of an ML10 magnesium alloy (image from a scanning electron microscope), melted in a protective gas atmosphere consisting of 99 vol.% argon and 0.5 vol.% perfluoroethylisopropylketone, prepared using a device for preparing and supplying a protective gas mixture, consisting of fluoride and magnesium oxide with an admixture of carbon.

Implementation of the invention

From the cylinder (1), the carrier gas through the gas reducer (2) equipped with a pressure gauge (3) and rotameter (4) is fed into the inlet pipe, which has an inclination towards the crucible (5), from which it enters the cavity of the crucible (6) through tight cover of the crucible (7), along the way mixing with evaporating perfluoroethylisopropylketone, where it is distributed over the surface of the melt (8), at the same time, perfluoroethylisopropylketone, which is in a liquid state in a container with a graduated transparent wall (9), located above the inlet pipe (10), with the possibility of replenishing the loss of perfluoroethylisopropyl ketone consumed during the operation of the system (11), flows in a drip way through a dropper with transparent walls (12) into a tube that is connected to the supply tube through a tee with a capillary inlet with a diameter of 0.1-1, 5 mm (13), and the freon outflow rate is regulated by the flow regulator (14), while the perfluoroethylisopropyl ketone passes into a gaseous state and mixes with gas-n oscillator only in the inlet pipe due to the heat coming from the crucible, when it flows down the inclined inlet pipe towards the metal, and the excess gas mixture is removed from the crucible through the outlet pipe (15) into the exhaust ventilation system.

Example 1

The melting of aluminum-containing magnesium alloy ML5 was carried out in a steel crucible under a steel lid equipped with holes for the inlet and outlet of the gas mixture. An inlet tube was installed in one hole, through which the gas mixture of the carrier gas and gaseous perfluoroethylisopropyl ketone was fed into the melting crucible, an outlet tube was installed in the other hole, connected to the exhaust ventilation system to evacuate the reaction products from the room, the places where the tubes were inserted and the steel lid adjoining the crucible were sealed . In the inlet tube, through which the carrier gas was supplied, a tube was installed through a tee from above, coming from a glass vessel with liquid perfluoroethylisopropyl ketone, on the wall of which a graduation scale was applied, which made it possible to determine the flow rate. The flow rate of liquid PFK was regulated using a valve installed on the tube coming from the glass vessel, and was set at the level of 0.5% gaseous PFK based on the volume of the supplied gas mixture. The required amount to obtain a gas mixture was determined from the calculation of obtaining 120 ml of gas from 1 ml of liquid perfluoroethylisopropyl ketone. The supply of perfluoroethylisopropyl ketone into the supply tube with a channel diameter of 15 mm was carried out by the drop method through a capillary inlet with a hole diameter of 0.4 mm. The inclination of the inlet tube towards the melting crucible allowed liquid perfluoroethylisopropyl ketone to flow unhindered along its wall until complete evaporation and mixing with the carrier gas, which was argon. The amount of perfluoroethylisopropyl ketone in the gas mixture varied during the melting process from 1 to 10%. The products of the reaction with magnesium exited through the exhaust pipe in the form of clearly distinguishable smoke, which was heavier than air, and fell into the exhaust ventilation. Molten alloy ML5 was in the crucible in a protective atmosphere for 1 hour at a temperature of 800±10°C, after which the heating was turned off and the alloy solidified in the crucible. There were no traces of ignition on the metal surface. After solidification of the alloy, the surface film was studied, which consisted mainly of fluoride and magnesium oxide with an admixture of carbon.

The example is illustrated by the results presented in Table 2.

Table 2. Results of X-ray spectral microanalysis of the surface film formed on the surface of the ML5 alloy during melting

Content of elements % at. C N O F mg Al Zn Other elements Dimension 1 18.7 1.6 12.7 38.8 27.0 0.3 0.03 0.9 Dimension 2 22.3 1.4 12.4 36.6 25.8 0.4 0.02 1.0 Dimension 3 23.4 1.6 11.8 34.9 26.3 1.1 0.03 0.9 Dimension 4 20.0 2.1 14.4 35.1 27.3 0.2 0.02 1.0 Dimension 5 20.7 1.1 7.5 46.3 23.9 0.1 0.02 0.5 Dimension 6 23.6 1.0 9.4 40.6 24.4 0.4 0.02 0.7 Average composition 21.5 1.5 11.4 38.7 25.8 0.4 0.02 0.8

Example 2

The melting of the aluminum-free magnesium alloy ML10 was carried out under the same conditions as in Example 1. There were no traces of ignition on the metal surface. After solidification of the alloy, the surface film was examined, which consisted mainly of magnesium fluoride and carbon. The example is illustrated by the results presented in Table 3.

Table 3. Results of X-ray spectral microanalysis of the surface film formed on the surface of the ML10 alloy during melting

Content of elements % at. C N O F mg Zn Zr Nd Other elements Dimension 1 42.7 0.0 1.7 41.3 13.3 0.0 0.0 0.0 1.0 Dimension 2 26.6 0.0 1.2 51.3 20.2 0.1 0.2 0.1 0.4 Dimension 3 27.7 0.0 2.0 49.3 19.6 0.0 0.1 0.0 1.3 Dimension 4 53.5 0.0 1.3 33.3 11.1 0.0 0.0 0.0 0.8 Dimension 5 47.3 0.0 1.8 37.4 12.8 0.0 0.0 0.0 0.7 Dimension 6 47.9 0.0 1.1 38.3 12.3 0.0 0.0 0.0 0.4 Average composition 40.9 0.0 1.5 41.8 14.9 0.0 0.0 0.0 0.8

Claims (9)

1. A method for preparing and supplying a protective gas mixture of a carrier gas, inert or inactive with respect to the melt, and gaseous perfluoroethylisopropyl ketone during the melting of magnesium alloys, characterized in that the gas mixture containing perfluoroethylisopropyl ketone in an amount of 0.1-10% of the volume of supplied gas mixtures are obtained directly in the inlet pipe supplying the gas mixture to the melting crucible by mixing the carrier gas and gaseous perfluoroethylisopropyl ketone obtained by evaporation of its liquid phase, which is supplied by gravity by drop method through a capillary inlet directly into the inlet pipe due to convective heat transfer from the metal in the crucible . 2. The method according to claim 1, characterized in that argon is used as the carrier gas. 3. The method according to claim 1, characterized in that carbon dioxide is used as the carrier gas. 4. The method according to claim 1, characterized in that helium is used as the carrier gas. 5. The method according to claim 1, characterized in that nitrogen is used as the carrier gas. 6. The method according to claim 1, characterized in that dried air is used as the carrier gas. 7. The method according to claim 1, characterized in that a mixture is used as a carrier gas in any ratio of two or more gases from the series: argon, carbon dioxide, helium, nitrogen, dried air. 8. The method according to claim 1, characterized in that the lid of the crucible is provided with a working opening with a lid. 9. The method according to claim 1, characterized in that the capillary inlet has a capillary hole diameter of 0.1-1.5 mm.

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