WO2001026843A1

WO2001026843A1 - Metal casting

Info

Publication number: WO2001026843A1
Application number: PCT/GB2000/003631
Authority: WO
Inventors: Helmut Jaunich; Josef Stemping; Christof Volks
Original assignee: Foseco International Limited
Priority date: 1999-10-11
Filing date: 2000-09-22
Publication date: 2001-04-19
Also published as: GB9923965D0; AU7533600A

Abstract

A method of casting metal in a mould, the mould comprising a mould cavity and a gas expansion chamber in communication with the mould cavity, the gas expansion chamber containing an exothermic composition comprising a metal and an oxidising agent, the method comprising filling the mould cavity with hot molten metal such that a portion of the molten metal overflows into the gas expansion chamber, the heat emitted by the hot molten metal in the gas expansion chamber causing the metal and the oxidising agent of the exothermic composition to react exothermically and thereby to generate gas and/or to cause gas already present in the gas expansion chamber to expand, thereby increasing the gas pressure above the molten metal.

Description

Metal Casting

The present invention relates to the casting of metal in a mould, especially a sand mould The invention particularly relates to counter-gravity low pressure casting, especially of aluminium or aluminium alloys, but it is also applicable to other non-ferrous metals, as well as iron and steel

Japanese patent application 10 -102042 discloses a method of casting in which a gas generating agent is provided in a gas generating chamber connected to the cavity of a mould, the heat of the molten metal causing the gas generating agent to generate gas which pressurizes the molten metal The gas generating agent is preferably polyvinyl chloride, polyvinyl acetate or other resinous materials, calcium carbonate, sodium carbonate or other carbonates, and pulp, cotton or other fibrous materials which decompose or combust as a result of the heat of the molten metal The pressunzation of the molten metal by means of the gas generating agent is stated to avoid the problem of shrinkage cavities being formed in the metal casting as the metal cools and solidifies, without the need for large capacity risers Large risers, which retain excess molten metal to replenish voids produced in the mould cavity due to the contraction of the metal as it solidifies, are stated to be undesirable because they decrease the weight yeild, they increase the labour intensity because of the requirement for cutting them from the casting, and they restrict the freedom of product design

The inventors of the present invention have identified a need for a more efficient and controlled method of pressurizing molten metal in a mould They have also identified a need for a simple and effective way of preventing molten metal flowing out of a counter-gravity mould once the mould cavity has been filled

According to a first aspect, the present invention provides a preformed liner article for insertion in a mould used for the casting of hot molten metal, the iiner article comprising one or more walls formed from an exothermic and/or heat insulating composition, at least one wall of the article having attached thereto a quantity of an exotnermic composition comDπsing a metal and an oxidising selected to react exothermically and thereby to generate gas and/or to cause gas adjacent to the composition to expand.

According to a second aspect, the invention provides a method of casting metal in a mould, the mould comprising a mould cavity and a gas expansion chamber in communication with the mould cavity, the gas expansion chamber containing an exothermic composition comprising a metal and an oxidising agent, the method comprising filling the mould cavity with hot molten metal such that a portion of the molten metal overflows into the gas expansion chamber, the heat emitted by the hot molten metal in the gas expansion chamber causing the metal and the oxidising agent of the exothermic composition to react exothermically and thereby to generate gas and/or to cause gas already present in the gas expansion chamber to expand, thereby increasing the gas pressure above the molten metal.

A third aspect of the invention provides a mould for the casting of molten metal, comprising a mould cavity and a gas expansion chamber in communication with the mould cavity, the gas expansion chamber containing an exothermic composition comprising a metal and an oxidising agent selected to react exothermically due to heat emitted by hot molten metal overflowing from the mould cavity when the mould cavity is filled with the molten metal, the exothermic reaction generating gas and/or causing gas already present in the gas expansion chamber to expand, thereby increasing the gas pressure above the molten metal.

The invention has the advantage that because the pressurization of the molten metal in the mould cavity is achieved by means of an exothermic composition comprising a metal and an oxidising agent, the degree of pressurization can normally be accurately controlled by selecting the appropriate constituents of the composition and their amounts, according to the temperature of the molten metal being cast, the volume of metal being cast, and the shape of the mould cavity. Furthermore, the exothermic reaction between the metal and the oxidising agent is normally efficient and fast-acting. These advantages are in contrast to the generally less efficient and less controllable combustion reaction produced by the gas generating agents disclosed in Japanese patent application 10 -102042, referred to above. The present inventors have also found that the controlled pressurization of the molten metal produced by the exothermic reaction of a metal and an oxidising agent generally reduces the occurrence of unwanted inclusions and gas bubbles (micropores) in the casting.

At least part (and in some embodiments of the invention, substantially all) of the gas expansion chamber is preferably located at a higher level than the highest part of the mould cavity. The higher part of the gas expansion chamber preferably contains the exothermic composition. As the mould cavity is filled, some of the molten metal overflows from the mould cavity into the gas expansion chamber, preferably only partially filling the gas expansion chamber. Advantageously, therefore, the exothermic composition may be selected to react exothermically by ignition caused by radiant heat emitted from hot molten metal adjacent to, but spaced a distance from, the composition. This has the advantage of substantially preventing the contamination of the molten metal by the exothermic composition.

[With regard to the temperature of the molten metal, the term "hot" merely refers to the fact that the metal will be at a significantly elevated temperature with respect to the ambient temperature; the actual temperature of the molten metal will of course depend upon the composition of the metal or alloy being cast.]

The metal of the exothermic composition preferably comprises a metal from Group IA, Group IIA and/or Group MIA of the Periodic table. Particularly preferred metals are aluminium and/or magnesium and/or silicon. The oxidising agent of the exothermic composition may, for example, comprise an oxide and/or a nitrate, preferably iron oxide and/or sodium nitrate and/or manganese dioxide. In at least some preferred embodiments, the exothermic composition includes a fluoride-containing compound (or other accelerator), preferably an inorganic fluoride salt, for example sodium fluoride, potassium fluoride, lithium fluoride and/or magnesium fluoride, and/or one or more complex fluorides, such as sodium silicofluoride, potassium silicofluoride, sodium aluminium fluoride and/or potassium aluminium fluoride. Organic fluoride compounds may also be used. The exothermic composition preferably includes a binder to bind the constituents together, for example a resin binder, preferably in combination with an organic solvent. The composition is preferably in the form of a tablet or other shaped solid mass.

The amount of metal in the exothermic composition is preferably at least 40%, more preferably at least 50%, especially at least 55%, based on the weight of metal and oxidising agent. The metal preferably comprises no more than 90%, more preferably no more than 80%, especially no more than 75%, for example approximately 64%, based on the weight of metal and oxidising agent.

The amount of oxidising agent in the exothermic composition is preferably at least 15%, more preferably at least 20%, especially at least 30%, based on the weight of metal and oxidising agent. The oxidising agent preferably comprises no more than 60%, more preferably no more than 50%, especially no more than 40%, for example approximately 36%, based on the weight of metal and oxidising agent.

In particularly preferred exothermic compositions, the metal is aluminium, and the oxidising agent is a mixture of sodium nitrate and iron oxide. In such embodiments, the possible exothermic redox reactions include:

(1) 2 Al + 3 NaNO₃ - AI2O3 + 3 NaNO₂

(2) 2 Al + 2 NaNO₂ - AI₂O₃ + Na₂O +N₂

(3) 10 Al + 6 NaNO₃ - 5 AI₂O₃ + 3 Na₂O + 3 N₂ [i.e. (1 ) + (2)]

(4) 8 Al + 3 Fe₃O₄ -» 4 AI₂O₃ + 9 Fe

Advantageously, the increase in gas pressure generated by the exothermic reaction of the exothermic composition may be equivalent to an increase in gas volume of at least 50 ml, preferably at least 60 ml, more preferably at least 70 ml, especially at least 80 ml, per gram of exothermic material, at a constant pressure of 1 atmosphere. The increase in gas pressure is preferably equivalent to an increase in gas volume of no more than 200 ml, more preferably no more than 150 ml.

In preferred embodiments of the second and third aspects of the invention, the gas expansion chamber contains or comprises a preformed liner article according to the first aspect of the invention The preformed liner article preferably comprises a sleeve, e g a feeder sleeve, containing the exothermic composition

The sleeve or other preformed liner article may advantageously have a closed end wall to which the exothermic composition is attached The exothermic composition is preferably attached to the preformed liner article by means of an adhesive or polymer composition

It is particularly preferred that the method and mould of the invention further comprise a shut-off valve to prevent outflow of the molten metal from the mould once it has been filled The shut-off valve preferably comprises a sleeve formed from refractory material, the sleeve including first and second spaced apart transverse walls defining a valve compartment in the sleeve, at least one aperture extending through each wall, the valve compartment containing a valve member which is movable between a first position in which the valve member closes the or each aperture in the first transverse wall, thereby closing the valve, and a second position in which apertures in both walls are at least partially open, and the valve is therefore open

According to a fourth aspect, therefore, the present invention provides a valve for controlling the flow of molten metal, comprising a sleeve formed from refractory material, the sleeve including first and second spaced apart transverse walls defining a valve compartment in the sleeve, at least one aperture extending through each wall, the valve compartment containing a valve member which is movable between a first position in which the valve member closes the or each aperture in the first transverse wall, thereby closing the valve, and a second position in which apertures in both walls are at least partially open, and the valve is therefore open

The sleeve of the valve, and/or the preformed liner article mentioned above (which may also be a sleeve) is/are generally formed from a material which is sufficiently refractory to withstand the temperature of the hot molten metal Suitable materials include metals, ceramic materials, bonded particulate refractory materials (e g silica sand) and/or bonded refractory materials containing refractory fibres In some embodiments, the material of the sleeve or article may include exothermic materials.

The sleeve and/or preformed liner article is preferably made from bonded refractory heat insulating material; it is preferably made by dewatering onto a suitable former an aqueous slurry containing particulate and/or fibrous material and a binder, removing the sleeve (or other article) from the former and then heating the sleeve to remove the water and to harden or cure the binder.

For ease of manufacture, the sleeve (whether the valve sleeve or the preformed liner article) will usually be of substantially circular horizontal cross-section, but other cross-sectional shapes are possible, e.g. oval, oblong or square.

Each transverse wall of the valve preferably contains a single aperture, the apertures and the valve member being sized and shaped such that the valve member closes the aperture in the first transverse wall when it is in the first position, but the valve member is incapable of fully closing the aperture in the second transverse wall. The valve member preferably comprises a ball or the like. The aperture in the first transverse wall is preferably substantially circular (and therefore, for example, fully closable by a suitably sized ball), whereas the aperture in the second transverse wall is preferably non-circular, for example oval or otherwise elongated in cross-section (and therefore, for example, incapable of being fully closed by a spherical ball).

In preferred embodiments, the valve member has a higher density than that of the molten metal. The valve member may, for example, be formed from chromium or zirconium sand, e.g. bound with an aluminium phosphate binder. In such embodiments, the first transverse wall is located below the second transverse wall, the valve is closed by the valve member being maintained in the first position by gravity, and the valve is opened by a generally upwardly-directed flow of the molten metal through the sleeve which maintains the valve member spaced apart from the or each aperture in the first transverse wall. The closure of the valve preferably occurs automatically once the mould cavity has been filled. For example, the closure of the valve, i.e. movement of the valve member to close one or more apertures in the valve, may occur because the inward and upward flow of molten metal ceases or at least significantly decreases, and therefore the valve member falls under gravity since it is no longer held in a position spaced apart from the lower aperture by the upward flow of the molten metal. The inward flow of molten metal may be terminated (or at least significantly reduced) once a predetermined quantity of the metal has entered the mould (i.e. sufficient metal to fill the mould cavity, and optionally also partially to fill the gas expansion chamber, where present). The quantity of metal entering the mould may be measured by electromagnetic induction, for example.

The sleeve of the valve may advantageously include a ceramic filter through which the molten metal flows. The ceramic filter may, for example, be a conventional ceramic filter, e.g. a ceramic foam filter or an extruded ceramic filter, both of which are well known in the art. Ceramic foam filters are normally formed by impregnating an open cell polymeric foam (e.g. a polyurethane foam) article with an aqueous slurry of ceramic material containing a binder, followed by drying the impregnated foam article to remove the water and then firing it to burn off the polymeric material. The ceramic filter is preferably located spaced apart from the valve compartment of the sleeve. The filter is preferably retained in its location in the sleeve by means of one or more projections formed on the internal wall of the sleeve. The filter is preferably located downstream (with respect to the inward flow of molten metal into the mould) of the valve compartment.

The invention will now be described, by way of example, with reference to the accompanying drawing.

Figure 1 shows, in cross-section and schematically, a sand mould 1 formed from sand 2, for low pressure counter-gravity casting, e.g of an aluminium part. The mould comprises a mould cavity 3, a gas expansion chamber 5 in communication with the mould cavity (the gas expansion chamber being located in a cope box 4), and an inlet for the mould cavity comprising a sleeve 7. The gas expansion chamber 5 is provided by a preformed liner article in the form of a sleeve 9. The sleeve 9 contains an exothermic composition 11 attached to an upper end wall 13 of the sleeve interior by means of an adhesive composition (not shown). The sleeve itself is in the form of a conventional feeder sleeve, formed from bonded refractory material, and including a breaker core 15 at the junction with the mould cavity 3, the breaker core having a circular aperture 17 through which molten metal overflowing from the mould cavity may flow into the gas expansion chamber 5.

In use, when the mould cavity 3 is filled with hot molten metal some of the metal overflows into the gas expansion chamber, partially filling the chamber. Ignition of the exothermic composition is then initiated by the radiant heat emitted by the hot molten metal, and the metal and oxidising agent of the composition react exothermically, generating heat. This causes the gas (air etc) present in the gas expansion chamber to expand, and also generates new gases produced by the exothermic reaction, resulting in pressurization of the molten metal from above. The pressurization of the molten metal as it cools and solidifies has been found substantially to prevent the formation of shrinkage cavities and micropores in the casting.

The inlet sleeve 7 below the mould cavity 3, is also formed from bonded refractory material. The sleeve includes a valve compartment 19 formed by first and second spaced apart transverse walls 21 and 23 respectively. The first (lower) wall 21 contains a circular aperture 25, and the second (upper) wall 23 contains an oval aperture 27. The valve compartment 19 contains a spherical valve member 29 which is free to move within the valve compartment, and which has a density greater than that of the molten metal to be cast. An upper region of the sleeve contains a ceramic filter 28 through which, in use, molten metal entering the mould cavity flows. Below, and separate from the mould 1 , is a connecting part 30 which in use connects a conduit for conducting molten metal from a dosing furnace (not shown) into the mould cavity. In use, while molten metal is supplied to the inlet sleeve from a dosing furnace under a head of pressure, the spherical valve member 29 is kept spaced above the lower (first) wall, i.e. spaced from the circular aperture 25, by the inward flow of the molten metal, thereby permitting the molten metal to flow into the valve compartment. Because the aperture 27 in the upper (second) transverse wall 23 is non-circular (i.e. oval), even though the spherical valve member 29 will generally mate with this aperture, it cannot fully close it, and therefore the molten metal is permitted to flow into the mould cavity. When a pre-determined quantity of molten metal has flown into the mould cavity (filling the mould cavity and partially filling the gas expansion chamber), the head of pressure from the dosing furnace is reduced and the inward flow of metal into the mould cavity is terminated. Once this occurs, the spherical valve member (which, as mentioned above, is more dense than the molten metal) sinks under gravity within the valve compartment until it mates with, and fully closes the circular aperture 25 in the lower first transverse wall 21 , thereby preventing outflow of the metal from the mould cavity. The valve therefore functions as a shut-off valve once the mould cavity has been filled.

Claims

1. A preformed liner article for insertion in a mould used for the casting of hot molten metal, the liner article comprising one or more walls formed from an exothermic and/or heat insulating composition, at least one wall of the article having attached thereto a quantity of an exothermic composition comprising a metal and an oxidising agent selected to react exothermically and thereby to generate gas and/or to cause gas adjacent to the composition to expand.

2. A method of casting metal in a mould, the mould comprising a mould cavity and a gas expansion chamber in communication with the mould cavity, the gas expansion chamber containing an exothermic composition comprising a metal and an oxidising agent, the method comprising filling the mould cavity with hot molten metal such that a portion of the molten metal overflows into the gas expansion chamber, the heat emitted by the hot molten metal in the gas expansion chamber causing the metal and the oxidising agent of the exothermic composition to react exothermically and thereby to generate gas and/or to cause gas already present in the gas expansion chamber to expand, thereby increasing the gas pressure above the molten metal.

3. A mould for the casting of molten metal, comprising a mould cavity and a gas expansion chamber in communication with the mould cavity, the gas expansion chamber containing an exothermic composition comprising a metal and an oxidising agent selected to react exothermically due to heat emitted by hot molten metal overflowing from the mould cavity when the mould cavity is filled with the molten metal, the exothermic reaction generating gas and/or causing gas already present in the gas expansion chamber to expand, thereby increasing the gas pressure above the molten metal.

4. A method according to Claim 2, or a mould according to Claim 3, in which at least part of the gas expansion chamber is located at a higher level than the highest part of the mould cavity.

5. A method or mould according to Claim 4, in which said part of the gas expansion chamber contains the exothermic composition.

6. An article, method or mould according to any of the preceding claims, in which the exothermic composition is selected to react exothermically by ignition caused by radiant heat emitted from hot molten metal adjacent to, but spaced a distance from, the composition.

7. A method or mould according to any one of claims 2 to 6, in which the gas expansion chamber contains or comprises a preformed liner article according to Claim!

8. An article according to Claim 1 , or a method or mould according to Claim 7, in which the exothermic composition is attached to the preformed liner article by means of an adhesive or polymer composition.

9. An article according to Claim 1 , or a method or mould according to Claim 7 or Claim 8, in which the preformed liner article comprises a sleeve containing the exothermic composition.

10. An article, method or mould according to Claim 8 or Claim 9, in which the preformed liner article has a closed end wall to which the exothermic composition is attached.

11. An article, method or mould according to any preceding claim, in which the metal of the exothermic composition comprises aluminium and/or magnesium and/or silicon.

12. An article, method or mould according to any preceding claim, in which the oxidising agent of the exothermic composition comprises an oxide and/or a nitrate.

13. An article, method or mould according to Claim 12, in which the oxidising agent comprises iron oxide and/or sodium nitrate.

14. An article, method or mould according to any preceding claim, in which the exothermic composition includes a fluoride-containing compound, preferably sodium aluminium fluoride.

15. An article, method or mould according to any preceding claim, in which the exothermic composition includes a binder.

16. An article, method or mould according to any preceding claim, in which the increase in gas pressure generated by the exothermic reaction of the exothermic composition is equivalent to an increase in gas volume of at least 50 ml, preferably at least 60 ml, more preferably at least 70 ml, especially at least 80 ml, per gram of exothermic material, at a constant pressure of 1 atmosphere.

17. A method according to Claim 2 or a mould according to Claim 3, or any claim dependent thereon, further comprising a shut-off valve to prevent outflow of the molten metal from the mould once it has been filled.

18. A method or mould according to Claim 17, in which the shut-off valve comprises a sleeve formed from refractory material, the sleeve including first and second spaced apart transverse walls defining a valve compartment in the sleeve, at least one aperture extending through each wall, the valve compartment containing a valve member which is movable between a first position in which the valve member closes the or each aperture in the first transverse wall, thereby closing the valve, and a second position in which apertures in both walls are at least partially open, and the valve is therefore open.

19. A valve for controlling the flow of molten metal, comprising a sleeve formed from refractory material, the sleeve including first and second spaced apart transverse walls defining a valve compartment in the sleeve, at least one aperture extending through each wall, the valve compartment containing a valve member which is movable between a first position in which the valve member closes the or each aperture in the first transverse wall, thereby closing the valve, and a second position in which apertures in both walls are at least partially open, and the valve is therefore open.

20. A method or mould according to Claim 18, or a valve according to Claim 19, in which each transverse wall contains a single aperture, the apertures and the valve member being sized and shaped such that the valve member closes the aperture in the first transverse wall when it is in the first position, but the valve member is incapable of fully closing the aperture in the second transverse wall.

21. A method, mould or valve according to any one of claims 18 to 20, in which the valve member comprises a ball.

22. A method, mould or valve according to any one of claims 18 to 21 , in which the valve member has a higher density than that of the molten metal, the first transverse wall is located below the second transverse wall, the valve is closed by the valve member being maintained in the first position by gravity, and the valve is opened by a generally upwardly-directed flow of the molten metal through the sleeve which maintains the valve member spaced apart from the or each aperture in the first transverse wall.

23. A method, mould or valve according to any one of claims 18 to 21 , in which the sleeve includes a ceramic filter through which the molten metal flows.