EP0470968B1

EP0470968B1 - Shape casting in mouldable media

Info

Publication number: EP0470968B1
Application number: EP90906083A
Authority: EP
Inventors: Don Allen Doutre
Original assignee: Alcan International Ltd Canada
Current assignee: Rio Tinto Alcan International Ltd
Priority date: 1989-05-01
Filing date: 1990-04-12
Publication date: 1993-06-02
Anticipated expiration: 2010-04-12
Also published as: CN1047233A; JPH04507064A; ES2041531T3; KR920700803A; CA1328554C; EP0470968A1; AU5421190A; PL285017A1; CS215590A3; AU633077B2; DD293971A5; WO1990013374A1; BR9007342A

Abstract

A process is described for forming castings using non-bonded (loose) moulding media, e.g. an evaporative pattern casting process. According to the novel feature, a very significant improvement can be achieved in the cooling and solidification rate of the casting if the air normally present in the interstitial spaces of the non-bonded moulding media is replaced by a gas having a higher thermal conductivity than the air. Helium has been found to be particularly useful for this purpose.

Description

This invention relates to a process for forming castings according to the first portion of claim 1
Traditionally, silica sand has been the moulding medium used in shape casting various metals and their alloys. Among the well known casting processes there may be mentioned sand casting in which the metal is poured into a hollow mould made of a sand and a binder, CO₂ casting in which the binder (water glass) is reacted with CO₂ gas to activate it, investment casting in which the mould is produced by surrounding an expendible pattern with a refractory slurry and shell mould casting where the mould is made by bonding sand particles together to make a shell which has taken up the contours of the metal pattern. Another casting system of particular interest is the evaporative foam casting process in which a foam pattern, generally comprising polystyrene foam, of the item to be cast is made. This foam pattern is coated with a suitable refractory wash, placed in a casting box or flask and surrounded with unbonded silica sand as moulding medium. A foam sprue extends from the pattern to the upper surface of the moulding medium, providing a passageway for the entry of molten metal. The casting box is vibrated to achieve maximum compaction and density of the sand. The molten metal is then poured into the casting box via the sprue, whereby the molten metal evaporates the sprue and pattern, thereby displacing it.
The result is a casting which perfectly reproduces the shape of the pattern. Gases formed from the vapourized polystyrene permeate through the wash, the sand and out through vents in the casting box.
Various metallic materials have been tried as moulding media to provide increased thermal conductivity. For instance, there have been studies in U.S.S.R. on the use of pig iron or steel shot, either as cast or fragmented, as a ferromagnetic moulding medium used in conjunction with the evaporative foam process. These studies were reported in the Transactions of a Symposium "Lit'e po Gazifitsiruemym Modelyam" (in English "Full Mould Casting") published in 1979 by the Institute of Foundry Problems Ukranian S.S.R. Academy of Sciences, Kiev, U.S.S.R. While the above materials do provide the desired thermal conductivity, they are very heavy materials which tend to distort the polystyrene patterns used in the evaporative foam process resulting in imprecise-castings. Moreover, such heavy moulding media cannot be handled in the conventional equipment used for silica sand.
U.K. Patent application 2,183,517, published June 10, 1987, describes the use of zircon sand as moulding medium in the evaporative foam process. Since zircon sand has a higher bulk density than silica sand, about equal to that of molten metal being cast, it is believed that the hydrostatic forces acting on the moulding feature are reduced thereby greatly improving mould stability and hence greatly improving the final accuracy of the casting. On the other hand, at temperature of 600°C the thermal conductivity of zircon, 0.83 W/m°K, is only twice that of quartz (silica), 0.54 W/m°K. Since the rate of heat extraction is roughly proportional to the square root of the thermal conductivity of the moulding medium, zircon provides an increase in cooling rate of approximately 24%.
Another process to increase the rate of solidification is described in Ryntz et al, U.S. Patent 4,520,858, issued June 4, 1985. In that patent, a chill member of metal, acting as a potential heat sink, is attached to an evaporative foam pattern. When metal is cast into the mould, the chill member accelerates cooling and solidification. However, the attaching of a chill member to each pattern is an expensive procedure and it provides a very limited increase in solidification rate.
It has also been proposed to improve the moulding media procedure is described in Rikker, U.S. Patent 4,651,798 issued March 24, 1987 wherein silica sand, alumina, zirconia or glass particles are coated with such a refractory layer. This layer also modifies the shapes of the particles to make them more spherical, so that they flow more evenly around the pattern thereby improving precision. However, these materials again do not have the high thermal conductivity required to increase the solidification rate.
Another moulding media that inproves the evaporative pattern casting process is aluminum granules. This medium has been found to be highly effective in its ability to increase the rate of heat extraction while avoiding the problems of the heavier metals as moulding medium. However, all of the above investigations have focused upon the properties of the solid phase of the moulding medium and have not considered the influence of the gas phase occupying the interstices between the particles in controlling the thermal transport properties of the moulding medium.
There have been prior proposals to use helium gas for the purpose of modifying the rate of heat transfer. For instance, U.S.S.R. Patent 369,972, published November 15, 1973 discloses a method of freezing sand moulds, presumably to bind the particles of media prior to casting, in which, in order to increase the freezing rate, the moulds are filled with a gas having a higher thermal conductivity coefficient than that of air. However, the patent was concerned only with the cooling of moulds to temperatures below 0°C and not with casting molten metal.
Russian Patent 1,161,224 published June 15, 1985 relates to a mould with a porous core whose porosity varies from fine pores at the surface to coarse penetrating cavities at the middle. These coarse cavities of the core can be filled with different cooling media, including helium to change both the heat storage capacity of the core and the cooling rate of the casting in contact with the core.
U.S. Patent 4,749,027 issued June 7, 1987 describes the use of a film of helium between molten metal and the front face of a moving casting belt, in a continuous casting machine to produce metal strip. However the purpose of the helium is solely to produce a gaseous film between the metal and the belt.
S. Engler and R. Ellerbrok, "Influence of Various Gas Atmospheres and Gas Pressures in Some Casting Characteristics in Example Alloy Al Si 12.8", Giesserie 64 (9) 227-230 (1977) describe the effect of argon and other gases present in the atmosphere surrounding molten metal in a melting furnace, in a transfer ladle and when it is being poured from the ladle into a mould. The object of this gas was to reduce the rate of cooling of the metal during melting and transfer. The article teaches that reducing the pressure of any gas and replacing air by argon will achieve the objective of reducing the rate of cooling, i.e. increasing the time of solidification.
It is the object of the present invention to provide an improved moulding system with greater heat transfer through the moulding media.

Summary of the Invention

According to this invention it has been discovered that by replacing air normally present in the interstitial space of a non-bonded (loose) moulding media by a gas of higher thermal conductivity, such as helium, a much greater rate of cooling and solidification can be achieved.
Thus, the present invention in its broadest aspect relates to a process for forming castings comprising the steps of producing a pattern of the product to be cast in a casting box by means of a non-bonded moulding media, e.g. non-bonded particles of a heat resistant material, and pouring a charge of molten metal into the casting box to produce a casting in the shape of the pattern within the moulding media. According to the novel feature, the air normally present in the interstitial spaces of the non-bonded (loose) moulding media is replaced either before casting is commenced or immediately after pouring the molten metal into the casting box by a gas having a higher thermal conductivity than the air.
A preferred feature of the invention relates to a process for forming castings comprising the steps of producing a pattern of the product to be cast from a material which is gasifiable substantially without residue upon subjection to a molten casting charge and having a shape conforming to the product to be cast, surrounding the pattern in a casting box with moulding media comprising unbound particulate material and pouring a charge of molten metal into the casting box to evaporate the pattern and produce a casting in the shape of the pattern. The novel feature comprises the step of replacing air normally present in the interstitial spaces of the particulate moulding media by a gas having a higher thermal conductivity than the air.
Helium is the preferred gas because it is inert, non-toxic, non-corrosive and relatively inexpensive. Other gases with high thermal conductivities exist, notably hydrogen and neon, but the practical limitations of their use in terms of the safety of hydrogen and the cost of neon are readily evident. Mixtures of helium with other non-reactive gases of lower thermal conductivity provide advantages in certain applications where carefully selected rates of cooling, more rapid than those obtained by air, but slower than those provided by helium, are required. For these applications the required rate of cooling is obtained by using mixtures of helium with air, or helium with nitrogen, or helium with argon, or helium with any gas which does not react with either the molten or solidified metal or the moulding media. Use of such selected mixtures present "tailor making" of the cooling and solidification rate.
In one embodiment of the invention, the interstitial areas of the particulate moulding media are simply filled with the high conductivity gas before casting is commenced. Alternatively, the mould may be filled with molten metal before introducing the high thermal conductivity gas in order to fill the mould completely under conditions of low rates of heat extraction and subsequently increasing the cooling rate by introducing the gas, such as helium, or using helium/air mixtures as described above in order to obtain intermediate rates of heat extraction.
A wide variety of particulate materials can be used as the moulding media, including silica sand, zircon sand, chrome-magnesite sand, steel shot, silicon carbide, alumina, aluminum granules, etc. A wide variety of metals may also be moulded by the process of this invention, including such materials as aluminum, magnesium, zinc and their alloys.
Preferred embodiments of this invention are illustrated by the following non-limiting examples.

Example 1

Evaporative Foam Casting

Patterns made of expanded polystyrene were prepared (38.1 mm x 50.8 mm x 152.4 mm) and coated with a mould coating consisting of Styro-Kote 250.1 (trade mark of The Thiem Corporation). These were packed in various moulding media (Aluminum granules -20/+80 mesh, SiC #24 grit and foundry sand) and castings were produced by pouring an Al-4.5% Cu alloy onto the pattern at 750°C. A thermocouple was positioned at the mid-section of the casting and the cooling times were recorded under the conditions indicated in Table 1.
It can be seen that, under these conditions cooling rates in sand and helium were equivalent to those obtained in aluminum granules and helium and superior to those obtained in either aluminum granules or silicon carbide in air. The use of helium roughly doubled the rate of cooling of the parts cast in sand in an air atmosphere.

Example 2

A separate series of experiments were conducted in order to evaluate the effect of helium on the rate of heat extraction under conditions more closely approximating a conventional sand casting operation.
During these experiments the moulding media were packed around unused, empty can bodies. Metal (Al-4.5% Cu) was poured directly into the cans at 700°C and an insulating cover was placed over the mould. Temperature-time recordings were obtained in order to compare the relative cooling rates under various casting conditions.
The results obtained are shown in Tables 2 and 3. Inspection of those Tables reveals that the presence of helium had a major (enhancing) effect under all conditions examined an that introducing helium into silica sand is a very effective means of increasing the cooling rate of castings.

Example 3

A series of tests were carried out using the evaporative foam pattern casting technique with various different moulding media.
Cylindrical expanded polystyrene patterns (Density = 22.5 Kg/m³) measuring 38.1 mm in diameter by 152 mm in length were obtained from Lost Foam Technologies, Sheboygan Falls, Wisconsin. The weight of metal required to fill these patterns was 0.5 Kg.
Coated patterns were prepared by dipping into a coating slurry of Styro-Kote 250.1 whose specific gravity had been adjusted to 1.56 in order to provide a coating thickness of 0.2 mm. Following dipping, the patterns were either air dried overnight or dried in a microwave oven.
Prior to packing the pattern in the moulding media, a thermocouple was inserted at the midpoint along the length to the depth of the center line of the cylinder. The pattern was then inserted into a flask and the latter filled with the moulding media while vibrating the whole assembly. In order to prevent heat losses through the bottom of the flask, an insulating layer (either 2.7 mm of fiber-board or 2 layers of Fiber-Frax* paper) was placed at the bottom of the flask. For tests in which the gas phase was varied, a perforated stainless steel gas distributor was placed in the bottom of the flask,and connected to the gas supply and used to purge the particle bed prior to casting. For the purge 2.7 SLPM of helium was injected for 2-3 minutes. Immediately before casting, the gas flow was reduced to about 0.3 SLPM in order to maintain the gas atmosphere during cooling.
*Trade Mark of Carborundum Corporation
Samples were cast of a binary Al-4.5% Cu alloy at a pouring temperature of 700°C and the temperature was monitored using a strip chart recorder. This alloy was chosen because it exhibits a well defined eutectic arrest at 548°C which allows easy recognition of the solidification time. When casting at 700°C, the metal reaching the thermocouple was already at the liquidus temperature and the cooling rates were calculated by dividing the liquidus to eutectic arrest temperature range (100°C) by the time elapsed between pouring and the time of the end of eutectic arrest.

(a) Results were recorded for solidification rates when air was replaced by helium on evaporative foam patterns without a coating. These results are shown in Table 5 and the rate of solidification was higher when helium was present.
(b) Another experiment was carried out in which air was replaced by helium on evaporative foam patterns with a coating. These results are shown in Table 6 and these again show that the rate of solidification was higher when helium was present.
(c) Another test was run to show the solidification rate with static air and flowing argon, both of which have lower thermal conductivities than helium. The results of this test are shown in Table 7 and it will be seen that the solidification rates observed in both air and argon were sunstantially lower than those observed when using helium.
(d) Another test was conducted where a larger evaporative foam pattern casting was formed using 8 kg of metal. The results are shown in Table 8 and the same improvement in solidification rate and subsequent cooling to 445°C and 395°C was obtained when helium was substituted for air.

From the above Examples, it will be seen that the rate of solidification and cooling during the evaporative foam process can be significantly increased by the use of high conductivity/heat capacity moulding media. The performance of these media is ultimately limited by the thermal resistance present at particle contact points. The use of a highly conductive gas such as helium increases the solidification and cooling rates substantially. For instance, the use of helium with silica sand was even more effective at increasing the solification rate than the best aluminum granules tested in air. The refractory pattern coatings conventionally used in the evaporative foam process presents a barrier to heat flow that is significant when helium on high conductivity moulding media are used. Optimum results, in terms of solidification rates, were obtained by combining helium with highly conductive media.
The most effective approach was found to be in the use of helium in combination with the conventional evaporative foam process. Although the use of alternative media could, in principle, lead to further increases in solidification rates, it is evident that in order to achieve rates superior to those attainable with helium and sand, highly conductive pattern coatings or coating-free process are required.
While the above detailed description relates primarily to the evaporative foam process, it will be appreciated by those skilled in the art that the invention has much broader application to other moulding processes, such as green sand moulding, shell moulding, investment moulding, sand cores, etc.

Claims

A process for forming castings comprising the steps of producing a pattern of the product to be cast in a casting box by means of non-bonded moulding media and pouring a charge of molten metal into the casting box to produce a casting in the shape of the pattern within the moulding media,
characterized in that air normally present in the interstitial spaces of the moulding media is replaced either before casting is commenced or immediately after pouring the molten metal into the casting box by a gas having a higher thermal conductivity than air.
A process according to claim 1, characterized in that the non-bonded moulding media comprises loose particles of a heat resistant material.
A process according to claim 2, characterized in that the gas is helium.
A process according to claim 2, characterized in that the gas is a mixture of helium with air, nitrogen or a non-reactive gas.
A process according to claim 3, characterized in that the metal being cast is aluminum or an alloy thereof.
A process according to claims 1-5, characterized in that said pattern of the product to be cast is produced from a material which is gasifiable substantially without residue upon subjection to a molten casting charge and having a shape conforming to the product to be cast, this pattern is surrounded in the casting box by the non-bonding moulding media and the molten metal evaporates the pattern to produce the cast shape.