RU2798498C1 - Method for producing magnesium alloys of the magnesium-zinc-calcium system and device for its implementation - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: methods for producing magnesium alloys of the magnesium-zinc-calcium system with increased strength and corrosion-resistant properties. The method for producing magnesium alloys of the magnesium-zinc-calcium system includes sequential melting of magnesium and zinc in a crucible of a melting furnace in the presence of a protective gaseous medium using a stirring agent and, after stirring, introducing calcium into the melt. When melting magnesium and zinc in the crucible of a melting furnace, technical argon is used as a protective gas medium, stirring is carried out using a stirring means, which is driven into rotation at a speed of 400-600 rpm and which is provided with oscillations along the axis with a frequency of 12-14 Hz and an amplitude of 0.5-0.8 mm, and the introduction of calcium into the melt is carried out with a bell, which is brought into rotation at a speed of 700-900 rpm and which is given vibrations in the axial direction with a frequency of 12-14 Hz and an amplitude of 0.7-0.9 mm. The device for producing magnesium alloys contains a crucible of a melting furnace, equipped with a means for dosed supply of a protective gas medium in the form of an annular steel pipe with holes, as well as a stirring means. An annular steel pipe with holes is rigidly fastened to the upper part of the crucible protruding above the heating furnace along the entire perimeter of the pipe, with the exception of the break for pouring the alloy, and the holes in the pipe are made in opposite directions at an angle of 45° relative to the melt surface.

EFFECT: obtaining the mechanical properties of the alloy and corrosion resistance.

2 cl, 5 dwg

Description

The invention relates to the field of metallurgy of light alloys, in particular to methods for producing castings from magnesium alloys of the magnesium-zinc-calcium system with increased strength and corrosion-resistant properties.

Currently, in domestic and foreign sources, much attention is paid to magnesium alloys of the magnesium-zinc-calcium system - one of the most promising alloys for medicine in terms of biocompatibility.

For decades, in the domestic and foreign practice of smelting magnesium alloys, two main methods have been used: flux-free and with the use of flux. Fluxes are chlorine-containing salts, and for flux-free melting, sulfur-containing gas is used. In the process of melting, modifying and refining under a flux layer with all modifications of the method, the formation of magnesium chloride takes place, which, when interacting with moisture, actively destroys the casting metal. Gas protection of the molten metal using sulfur dioxide or sulfur hexafluoride SF ₆ - SF gas displaces air above the surface of the molten metal, however, at the same time, we form a strong film of magnesium sulfide, which can be retained on the surface of the melt for a long time, being an undesirable inclusion.

For medicine, magnesium alloys must be of high purity. The presence of impurities not more than 0.001-0.004% of the mass is allowed. In this regard, there is a need to improve the technology of smelting magnesium alloys.

A method is known for flux-free melting of magnesium alloys (patent RU No. 2623965, IPC B22D 1/00, publ. 06/29/2017), including magnesium melting, the introduction of alloy components in a protective atmosphere and blowing with a modifier at a temperature of 730-750 ° C. This technology refers to alloys of the Mg-Al-Zn-Mn system.

The disadvantage of this method is that a wide range of gases is needed to create a protective atmosphere, which complicates the melting process, and carbon-containing gas mixtures worsen working conditions in the shop.

A known method for manufacturing a medical implant made of magnesium alloy (patent RU No. 2608152, IPC A61L 31/02, A61F 2/02, publ. 01/16/2017), which includes the following manufacturing steps:

a) melting magnesium alloy;

b) spraying the melt in a protective gas atmosphere and cooling it below the solidification point;

c) molding the resulting powder by pressing to obtain a raw material;

d) extruding the raw alloy to obtain a product.

The proposed method is laborious due to the large number of technological operations, at the same time it does not guarantee high mechanical and corrosion-resistant properties that meet the requirements for implants, including biocompatibility.

A known method of smelting magnesium alloy (patent RU No. 2601718, IPC S22S 23/00, publ. 11/10/2016), including flux-free melting with treatment of the melt with carbon fluoride compounds at the rate of 1-4 g per 100 kg of melt at a temperature of 750-770 ° C within 3-10 minutes. When processing the melt, it is degassed, and before pouring into the mold, it is purged with sulfur hexafluoride. This method is used to obtain alloys of the magnesium-zirconium system.

The operations described in the patent have the following disadvantages:

a) sulfur hexafluoride protects the melt from oxidation (combustion), however, the resulting protective film of magnesium sulfide adversely affects the ductility of the alloy;

b) sulfur hexafluoride worsens the working conditions of foundry workers. In a number of EU countries, the use of SF6-SF gas in the smelting of magnesium alloys is prohibited.

Of the known methods for the flux-free production of magnesium alloys, the closest to the claimed method is the method and device (patent RU No. 2701248, IPC S22S 1/02, S22S 23/02, publ. 09/25/2019).

The method for producing magnesium alloys includes the introduction and melting of the alloy components in the crucible of a melting furnace in a protective gas environment created above the surface of the melt by supplying gas through an annular pipe with holes in the lower part, with simultaneous mixing of the melt by means of a device for mixing it, and the gas supply is carried out over surface of the melt until the mixing device is removed and the melt is poured into molds.

When implementing the known method, a protective gaseous medium above the surface of the melt is created in the form of a mixture of oxygen-free carbon-containing gases and an inert gas in the ratio (1-10) ÷ (1-20), when the alloy reaches a temperature of 730-750 ° C, it is modified and refined for 10-20 minutes by immersion in the melt of the said lower annular part of the pipe, through which the said mixture of gases is supplied with simultaneous mixing of the melt.

This method was developed to obtain alloys of the magnesium-zinc-manganese system.

The disadvantage of this method is that to protect the surface of the melt from ignition and improve the quality of the alloy, a mixture of chlorine-containing and carbon-containing gases is used, as a result of which not only the ecology in the workshop deteriorates, but also the quality of the alloy decreases due to the presence of residual impurities of the gas mixture components in it. . As a result, the alloy obtained by the known technology does not have sufficient mechanical and corrosion-resistant properties for use in implantology. In addition, there is no information about the sufficient effectiveness of this method when used as a modifying calcium additive, since it is able to actively interact with the above gases.

The technical result of the present invention is to improve the quality of alloys of the magnesium-zinc-calcium system, which consists in increasing (compared to materials obtained by known technology) mechanical and corrosion-resistant properties.

The technical result is achieved by the fact that when implementing a method for producing magnesium alloys of the magnesium-zinc-calcium system, including the sequential melting of magnesium and zinc in the crucible of a melting furnace in the presence of a protective gaseous medium using a mixing agent and after mixing, introducing calcium into the melt, the difference is in the fact that when magnesium and zinc are melted in the crucible of a melting furnace, technical argon is used as a protective gas medium, stirring is carried out using a stirring means, which is driven into rotation at a speed of 400-600 rpm and which is provided with vibrations along the axis with a frequency 12-14 Hz and an amplitude of 0.5-0.8 mm, and the introduction of calcium into the melt is carried out with a bell, which is rotated at a speed of 700-900 rpm and which is given vibrations in the axial direction with a frequency of 12-14 Hz and an amplitude 0.7-0.9 mm.

The claimed method for producing magnesium alloys of the magnesium-zinc-calcium system is carried out in a device that contains a melting furnace crucible equipped with a means for dosed supply of a protective gas medium in the form of an annular steel pipe with holes, as well as a means for mixing and is characterized in that the annular steel pipe with holes is rigidly attached to the upper part of the crucible, protruding above the heating furnace, along the entire perimeter of the pipe, except for the gap for pouring the alloy, and the holes in the pipe are made in opposite directions at an angle of 45 degrees relative to the melt surface.

The technical result of the invention is due to the following factors:

1. The components of alloys of the magnesium-zinc-calcium system have different specific gravity (1.73, 7.13 and 1.5 g / cm ³ , respectively), therefore, for their uniform distribution, the sequence of input and mixing into the melt is established, namely: 1st in order magnesium, 2nd in order zinc, 3rd in order calcium. This order makes it possible to first ensure a uniform distribution of zinc in the magnesium molten base, and then introduce calcium into the melt as a modifying additive, making the alloy finer-grained. The amount of calcium is about 0.5% by weight of the melt. Calcium is introduced at the final stage of melting in order to avoid uncontrolled waste due to high oxidative activity.

2. The use of argon as a protective gas medium eliminates the formation of impurities and films of undesirable compounds that occur when using gas media with chlorine and carbon. An additional and important advantage is the improvement of the environmental situation in the shop.

3. The uniform distribution of the alloy components is due to the relatively high speed of rotation of the mixing means - about 400-600 rpm, causing turbulence. In addition, a relatively high rotation speed provides metal bubbling, which occurs due to the funnel formed during rotation of the mixing device, into which argon is drawn from the atmosphere above the molten metal and is evenly distributed over the volume of the crucible. This eliminates the usual procedures for refining and modifying the alloy associated with the accumulation of impurities. For the purposes of the invention, a device that provides the fastest mixing of the melt [patent RU No. 2625471, IPC B01F 7/18, B01F 7/26, publ. 07/14/2017]. The working body of this device is represented by perforated disks located on a common axis and equipped with counter-oriented pins installed on their periphery.

4. The use of a bell is a typical technique for the introduction of alloying additives in the production of cast alloys. Information about the use of the bell in the casting industry is available, for example, in the source [Bell [Electronic resource] URL: http://delta-grup.ru/bibliot/31/290.htm (accessed 06.06.2022)]. The bell is a hollow cylinder with holes around the circumference and with a muffled one side. A holder is attached to the plug. Due to the low specific gravity of calcium, which is 1.5 g/cm ³ , a special kneading procedure has been proposed to prevent its floating and prolonged contact with the atmosphere. It consists in the fact that the bell is driven into rotational motion with an angular speed of 700÷900 rpm with simultaneous vibration in the axial direction with a frequency of 12÷44 Hz and an amplitude of 0.7÷0.9 mm. Rotation speed, vibration frequency and amplitude are selected experimentally.

5. The peculiarity of the means for supplying a protective gas medium, namely the hermetic connection of the crucible with an annular tube provided with holes, makes it possible to create a more reliable protective gas dome of argon above the melt surface. The ring has only a single gap to drain the molten metal. The jets of argon emanating from the holes in the specified configuration flow around the surface of the melt, creating a continuous flow that cuts off the air.

6. Making holes in the lower part of the pipe at an angle of 45 degrees is due to the fact that the lower part is hermetically connected to the edge of the crucible. Thus, making holes directly from below is not possible. As a result, the location of the holes at an angle in converging directions was chosen. As a rule, holes are made with a diameter of 3 mm, placing them at a distance of 15 mm from each other. When draining, the jet of molten metal is also protected by argon, because. the casting mold cup enters the overlapping area of the gas dome, which makes it possible to limit the contact of the jet with air.

The invention is illustrated by the illustrations of Fig. 1-5.

In FIG. 1 shows a general view of a device for producing castings from magnesium alloys of the magnesium-zinc-calcium system. The numbers indicate: 1 - crucible cover; 2 - perforated pipe for supplying argon; 3 - nozzles for installing handles for pouring the alloy; 4 - steel crucible; 5 - cover of the heating furnace; 6 - heating furnace; 7 - heaters; 8 - refractory insulation of the furnace; 9 - molten metal; 10 - pipe for supplying argon; 11 - gap in the perforated pipe for pouring the alloy. In order not to clutter the drawing, the stirring means in FIG. 1 is not shown.

In FIG. 2 separately shows a view of the cover 1 with a mixing device installed on it, consisting of a drive 12 and a working body 13.

In FIG. 3 shows a typical view of the bell.

In FIG. 4 shows micrographs of the alloy obtained by the known method (a) and by the proposed method (b).

In FIG. 5 shows SEM images of the alloy obtained by the known method (a) and by the proposed method (b).

The essence of the proposed method and device for obtaining magnesium alloys of the magnesium-zinc-calcium system is as follows.

The removable steel crucible 5, together with the protective gas supply means, is inserted into the furnace 6 heated to 850-900 ° C. Depending on the volume of the crucible, magnesium is preliminarily loaded into it in accordance with the calculation of the charge amount and then the protective gas is supplied through the gas supply means. Means for dosed supply of the protective gas medium is represented by an annular pipe 2, as a rule, steel, rigidly fastened to the upper part of the crucible, protruding above the heating furnace, and provided with holes made in opposite directions at an angle of 45 degrees relative to the melt surface.

Commercial argon is used as a shielding gas. Additional reactive gases are not required.

After the melting of magnesium, zinc is introduced into the crucible. After the melting of zinc, the melt is stirred for 2-5 minutes (depending on the volume of the melt) using a mixing agent known from the patent [patent RU No. 2625471, IPC B01F 7/18, B01F 7/26, publ. 07/14/2017] or any similar, driven into rotation with a relatively high (that is, creating turbulence in the mass of liquid metal) speed. In addition to the rotational movement, the mixing device is provided with vibrations provided by the design along the axis with an amplitude of 0.5÷0.8 mm and a frequency of 12÷14 Hz. After melting and mixing of zinc, calcium in the form of granules is introduced into the melt. Due to the relatively small input volume, as well as the low specific gravity of calcium, a separate device in the form of a bell is provided for this, which makes it possible to quickly and evenly introduce calcium into the melt volume due to a combination of rotation and axial vibrations. The amount of calcium is about 0.5% by weight of the melt. Calcium is introduced at the final stage of melting in order to avoid uncontrolled waste due to high oxidative activity.

The melt is stirred again. Refining and modification are not carried out, because when mixing the components of the mixture with a mixing agent, a funnel is formed into which argon is drawn in and evenly distributed over the volume of the crucible, producing bubbling of the melt.

The resulting melt is poured into a mold. To do this, it is preferable to use the method [patent RU No. 2720331, IPC B22D 15/00, publ. 04/28/2020], providing a more uniform structure of the resulting material. This method is characterized by the use of low-frequency vibration of the mold during the pouring process with a frequency of 50-80 Hz and an amplitude of 0.5 mm and makes it possible to obtain high-quality castings of magnesium alloys for deformation (rolling, drawing, extrusion) and the manufacture of medical products.

Metallographic analysis of alloy samples obtained by the known method and the proposed method showed a uniform grain size distribution and a decrease in the average size from 270 (Fig. 4, a) to 180 μm (Fig. 4, b). SEM images of the microstructure showed that calcium and zinc are more evenly distributed in the magnesium matrix along the grain boundaries (Fig. 5b) without pronounced accumulations observed in the alloy obtained by the known method (Fig. 5a). Due to the finer-grained structure and more uniform distribution of components, the resulting alloy is less subject to local stresses during deformations, which positively affects its mechanical strength and corrosion resistance. The Brinell hardness of the resulting alloy increases from 34 to 45 units, the compressive yield strength from 52 to 66 MPa, the compressive strength from 204 to 294 MPa, and the maximum strain to failure from 16 to 22%. These characteristics make it possible to consider the alloy obtained by the proposed method more suitable for use in medical implantology.

Claims (2)

1. A method for producing magnesium alloys of the magnesium-zinc-calcium system, including the sequential melting of magnesium and zinc in the crucible of a melting furnace in the presence of a protective gaseous medium using a mixing agent and, after mixing, the introduction of calcium into the melt, characterized in that during the melting of magnesium and of zinc in the crucible of a melting furnace, commercial argon is used as a protective gas medium, stirring is carried out using a stirring agent, which is rotated at a speed of 400-600 rpm and which is provided with vibrations along the axis with a frequency of 12-14 Hz and an amplitude of 0, 5-0.8 mm, and the introduction of calcium into the melt is carried out with a bell, which is rotated at a speed of 700-900 rpm and which is given vibrations in the axial direction with a frequency of 12-14 Hz and an amplitude of 0.7-0.9 mm . 2. A device for producing magnesium alloys of the magnesium-zinc-calcium system by the method according to claim 1, containing a crucible of a melting furnace, equipped with a means for dosed supply of a protective gas medium in the form of an annular steel pipe with holes, as well as a means for mixing, characterized in that an annular steel pipe with holes is rigidly fastened to the upper part of the crucible protruding above the heating furnace, along the entire perimeter of the pipe, except for the break for pouring the alloy, and the holes in the pipe are made in opposite directions at an angle of 45° relative to the melt surface.

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