RU2632754C1 - Moulding mixture for producing casting mould when casting magnesium alloys - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: moulding mixture contains quartz sand, a binder and a non-stick additive in the form of silicon carbide in an amount of 4% to 36% by weight with respect to the total weight of the moulding mixture. Silicon carbide is a powder or micropowder with an average particle size of less than the average particle size of the sand used in the moulding mixture. The moulding mixture may further comprise oleophilic bentonite in the form of a dry powder or in a swollen state in an amount of 0.5% to 12% by weight with respect to the total weight of the moulding mixture.

EFFECT: increase reliability of magnesium alloys protection against oxidation, cleanliness of castings surface.

3 cl, 2 tbl

Description

The invention relates to foundry, and in particular to non-stick additives for foundry sand-tar and sand-clay molds used in casting magnesium alloys.

Modern non-stick protective agents are formulations (paints) for application to the surface of foundry molds or non-stick additives, including various additives added to the mold material.

Non-stick paints serve as a separation barrier between the molten magnesium alloys and the material of the mold or core coated with these paints. The disadvantages of most modern non-stick coatings are low heat resistance, insufficient mechanical strength of the film. Destruction of the coating as a result of any of the factors (thermal degradation as a result of local overheating, cracking as a result of thermal deformations, mechanical stress, and other causes) entails the risk of direct contact of magnesium with sand and moisture of the material of the molds and cores that interact intensively, which under conditions of oxygen deficiency, part of the binder form and coating burns out, forming burns on the surface of castings of magnesium alloys, and in conditions of a sufficient amount of oxygen, Rania melt and also leads to hydrogen evolution, which is ignited by the explosion. Therefore, the use of paints does not always reliably protect magnesium alloys from oxidation during their casting into sand forms, therefore, special measures are required to prevent their oxidation, which complicates the manufacturing process of castings.

The known composition of the non-stick coating for foundry molds and cores, which uses silicon carbide and water (see, for example, RF patent No. 2170155, IPC B22C 3/00, publ. 10.07.2001).

The composition contains a significant amount of water (from 19 to 28%), which is unacceptable for compositions used for magnesium casting, because when heated, the released water vapor coming into contact with the magnesium melt will cause its ignition with a possible explosion due to hydrogen evolution. Therefore, the proposed composition cannot be applied as a non-stick coating for rods and molds used in casting magnesium alloys. In addition, the organic binder in this composition is not stabilized by the builder, which will increase its heat resistance to temperatures of 800-850 ° C, which will cause its thermal degradation with possible burnout or mechanical destruction, leading to corresponding casting defects. In the above patent, silicon carbide is used in an amount of 37-40% or 83-85 wt.%, Which provides a very intensive heat accumulation at the melt-mold (melt-core) boundary, and without further intensive heat removal into the mold or core volume this will cause destruction of a thin film.

Known non-stick additives in molding mixtures of casting molds and cores for casting magnesium alloys, including non-stick additives and binders (see, for example, RF patent No. 2221668, IPC B22C 1/06, publ. 20.10.2004).

These additives contain components that cause the release of unwanted gaseous products - for example, ammonia (in the case of urea, amines, ammonium salts), toxic boron trifluoride (in the case of tetrafluoroborates), carbon dioxide, which enhances the combustion of magnesium in case of ignition ( in the case of the use of carbonates, organic salts), toxic sulfur dioxide, hydrogen sulfide and other undesirable components.

It is also highly undesirable to use halides of alkali and alkaline earth metals, other halogen-containing additives that decompose with the release of toxic gases. In addition, many inorganic salts serving as such components contain crystallization water, which, when heated, is released and upon contact with the molten magnesium can accelerate its oxidation with the possibility of fire. Substances that can decompose with increasing temperature to 300-400 ° C with the release of water vapor (for example, boric acid, a number of organic compounds) are not the best choice for the same reason. In addition, the use of non-stick compositions based on water or with moisture-containing components is not recommended due to the fact that moisture can react with magnesium to produce hydrogen, which ignites with explosion.

According to the principle of action, the components of protective agents (non-stick paints or additives in the molding mixture) in order to avoid oxidation of magnesium in the magnesium alloy provide a protective gaseous atmosphere at the melt-mold interface, or a protective strong film (glaze), or are able to provide a rapid decrease in temperature on the interface molten form due to the high thermal conductivity of the components or their ability to decompose with heat absorption. Often, protective non-stick protective agents involve several of the listed methods to ensure the conditions for eliminating or minimizing the possibility of magnesium oxidation, leading to corresponding surface oxide defects or fires [1].

The basis of the invention is the task of creating a composition of non-stick additives to improve the reliability of protection of liquid magnesium alloys from oxidation, the absence of surface defects on castings from these alloys, reduce the cost of molding mixtures with non-stick additives, improve sanitary and hygienic working conditions, increase the fire and explosion safety of the casting process .

The problem is solved in that the non-stick additive in the molding mixes of casting molds and cores for casting magnesium alloys, including non-stick additives and binders, contains silicon carbide in the amount from 4% to 36% of the mass as the main component. in relation to the total mass of the molding mixture, which may be a powder or micropowder with an average particle size less than the average particle size of the sand used in the molding mixture, and as an additional component can be used oleophilic bentonite in the form of a dry powder or in a swollen state in an amount of 0.5% to 12% of the mass. in relation to the total weight of the moldable mixture.

Since as a main component the non-stick additive contains silicon carbide in an amount of from 4% to 36% of the mass. in relation to the total mass of the molding mixture, which may be a powder or micropowder with an average particle size less than the average particle size of the sand used in the molding mixture, and as an additional component can be used oleophilic bentonite in the form of a dry powder or in a swollen state in an amount of 0.5% to 12% of the mass. in relation to the total mass of the molding mixture, it is possible to increase the reliability of protection of liquid magnesium alloys from oxidation, the absence of surface defects on castings from these alloys, reduce the cost of molding compounds with non-stick additives, improve sanitary and hygienic working conditions, and increase the fire and explosion safety of the casting process.

The molding mixture of casting molds and rods for casting magnesium alloys are made in the following sequence.

Take the necessary fraction of sand and clay and add green silicon carbide, which contains less impurities, in particular iron, to the prepared sand-clay mixture, add a binder and, if necessary, introduce bentonite in a dry state. As a binder, a 4GU TU 6-10-1317-76 fastener or bard is used, and the choice can be made based on the presence of one or another.

In the private manufacture of sand, resin, resin is added to it. The choice of resins is determined by the temperature regime of casting, in which the hardened casting core or mold must have the required heat resistance, the required strength characteristics of hardened casting molds and cores, as well as cost characteristics.

The composition of non-stick additives proposed in the present invention eliminates the release of water, since its components do not contain crystallization water, or substances that can decompose with the release of water. Also, these non-stick additives do not contain substances that emit any toxic fumes, which significantly improves the environmental component as a result of their use.

As the main component of the composition of the present invention, the non-stick additive is silicon carbide, which has a high melting point, chemical inertness with respect to many materials, including well-known components of molding compounds, the introduction of which significantly increases the heat resistance of the compositions with their presence. Its presence is capable of providing a very intensive heat removal and eliminating the accumulation of heat in the zone of contact with the melt, which accelerates the solidification of the melt with the improvement of its crystallization parameters and reduces the risk of fire due to the acceleration of the temperature gradient equalization in the surface layer. This is due to the fact that the thermal conductivity of silicon carbide is one of the highest among non-metallic materials. In addition, powders and micropowders of silicon carbide are widely used in the abrasive industry, which ensures their availability and their cost is low.

The use of silicon carbide in order to increase the thermal conductivity of various materials has been carried out for a long time, however, it has not been previously used for molding mixtures because of fears of its possible reaction with moisture at high temperatures, as well as the formation of magnesium carbide and magnesium silicide, which, upon contact with moisture, can produce unwanted fire hazard products. Laboratory experiments showed that even in the presence of moisture in the sand of the sand mixture, these fears are groundless, since in mixtures of sand with silicon carbide in contact with magnesium alloy melt (including melts burning in air), the effect of magnesium quenching and inhibition of its reaction with the sand. This is explained by the formation of magnesium silicates (talc, forsterite, clino-enstantine enstantite, cristobalite), which bind moisture and are able to hold it to temperatures of 800-900 ° C, which are refractories, as well as the burning of a large amount of oxygen due to the direct reaction of silicon carbide with oxygen. The data were then confirmed in the foundry production of products from magnesium alloys.

For use in molding sand mixtures, it is advisable to use powders and micropowders of silicon carbide with an average particle size equal to or smaller than the average particle size of the sand, since in the latter case, compaction of the molding sand is observed, and the high thermal conductivity of silicon carbide provides a significant increase in their thermal conductivity due to the possibility of closer the contact of its grains with each other and the possibility of placing sand between grains even with their extremely tight packing.

Silicon carbide as a non-stick additive of the present invention serves both in sand-clay and sand-resin molding sand mixtures of any composition, since it is inert to their components in the entire temperature range of their use.

The minimum amount introduced into the molding mixture of silicon carbide in an amount of 4% of the mass. with respect to the total mass of the molding sand, it is due to the fact that, at lower concentrations, a sufficiently effective heat removal and inhibition of the magnesium oxidation reaction are not provided. The maximum amount of silicon carbide in the amount of 36% of the mass. with respect to the total mass of the molding sand, it is necessary to minimize the rise in the cost of the molding sand, while large values of thermal conductivity are not required to ensure reliable protection of magnesium alloys from oxidation. The value of the silicon carbide content in the molding sand depends on its composition (to a large extent on the type of binder used, the particle size distribution of sand and their content in the molding sand), as well as the type of magnesium alloy that is cast using molds and rods made from such mixtures .

An additional component of non-stick additives is oleophilic bentonite (organobentonite or its analogue Benton-34), which is a product of the interaction of natural montmorillonite clays (bentonites) with oleophilizers - quaternary ammonium salts, which serves as a universal structure-forming agent for the binder. It increases the thermal stability of the binder, increasing the threshold for its thermal degradation to temperatures above 850 ° C, which exceeds the range of working temperatures for casting magnesium alloys. The amount of oleophilic bentonite depends on the type of binder used and its content in the molding sand. This additional component is suitable for organic binders - synthetic resins (phenol-formaldehyde, urea-formaldehyde-furan, urea-formaldehyde, urea-phenol-formaldehyde-furan, phenol-urea-formaldehyde, furyl-phenol-formaldehyde). The method of administration depends on the degree of swelling of oleophilic bentonite in the binder, therefore, in some cases, preliminary soaking (swelling) of oleophilic bentonite in the binder is required. In other cases, it can be used in the form of a dry powder.

In the case of sand-clay mixtures, the use of oleophilic bentonite is selective and depends on the presence of other hydrophobic components in them.

The physicochemical nature of the non-stick additives in the invention provides an increase in the thermal conductivity and heat resistance of the molding compounds, chemically inhibits magnesium to prevent its oxidation, which ensures the absence of surface oxide defects on the castings and will be able to suppress combustion in case of magnesium ignition, and can also ensure high surface cleanliness castings.

All components of the proposed non-stick additives are available and inexpensive, which gives a significant advantage over analogues. The use of non-stick additives in the molding sand according to the invention significantly improves the environmental performance of production associated with the casting of magnesium alloys and provides a high level of fire and explosion safety.

Examples of the use of non-stick additives in the compositions of molding mixtures of foundry molds and cores for casting magnesium alloys are given in tables No. 1 and No. 2.

Literature

[1] Duyunova V.A. Methods for the protection of magnesium alloys in domestic foundry since the 1930s. until now // Foundry worker of Russia. 2010. No. 10. S. 35-37.

Claims (3)

1. The molding mixture for the manufacture of molds for casting magnesium alloys, containing quartz sand, a binder and non-stick additive, characterized in that as a non-stick additive it contains silicon carbide in an amount of from 4 to 36 wt. % relative to the total weight of the molding sand. 2. The molding mixture according to claim 1, characterized in that the silicon carbide is a powder or micropowder with an average particle size of not less than the average particle size of the sand. 3. The molding mixture according to claim 1 or 2, characterized in that it further comprises oleophilic bentonite in the form of a dry powder or in a swollen state in an amount of from 0.5 to 12 wt. % relative to the total weight of the molding sand.