METHOD OF CONTROLLING THE RATE OF HEAT EXTRACTION IN MOULD CASTING

Abstract: A process is described for producing castings comprising the steps of introducing a charge of molten metal, e.g. aluminum into a mould, cooling the molten metal in the mould until the outer layer of the metal solidifies and contracts leaving a gap between the surface of the casting and the surface of the mould, injecting into the gap a gas having a different thermal conductivity than air whereby the rate of further cooling of the casting can be controlled, continuing the cooling of the casting and removing the casting from the mould.

Description: Method of Controlling the Rate of Heat Extraction in Mould Casting Background of the Invention This invention relates to a method for forming castings in which a charge of molten metal is introduced into a mould. Traditionally, metal castings are formed either by pouring a charge of molten metal into a reusable permanent mould or into a one-time use mould consisting of non-bonded or bonded moulding media in a casting box. There have been proposals to use a gas other than air in the interstitial spaces of the moulding media for the purpose of modifying the rate of heat transfer. For instance, Doutre, Canadian Patent Application 598,137, filed May 1, 1989, describes a process in which the air normally present in the interstitial spaces of non-bonded moulding media is replaced by a gas, such as helium, which has a higher thermal conductivity than the air. 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 for producing metal strip. The purpose of the helium is 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 AlSi 12.8", Giesserie 6± (9) 227-230 (1977) describes 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. Japanese Patent Publication No. 44-20733 describes the cooling of steel ingots using mass flow of heated air away from the casting to achieve cooling. It describes the use of a very large flow, e.g. 1,500 m 3/h of air and 5 m3/h of water, for a steel casting which can be calculated to 3 have a volume of 1.15 m . The weight of the ingot is approximately 9,000 kg and this converts to a flow rate of air of 25,000 1/min for 9,000 kg of casting, or 2.8 1/min/kg. U.S. Patent 3,344,840, issued October 3, 1967, relates to direct chill casting of ingots in which a shrinkage space forms between the ingot and the sides of the mould. This is the typical wide shrinkage space that forms between the mould and the metal is DC casting and is usually in the order of 3.2 to 1.6 mm. A gas, such as helium, is introduced into this wide gap to improve the conductive heat transfer. It is the object of the present invention to provide an improved casting process that can be used with a variety of moulds. The term moulds is understood to comprise, among others, the following; (a) a "permanent mould" of the type which is reusable to make a large number of castings and may be made of a nonmetallic material such as carbon or graphite, or of metal, such as a ferrous metal, machined to provide a cavity and a plurality of channels to permit molten metal to enter the cavity; (b) a "semi permanent moulding" of the type which includes a non reusable core of bonded moulding medium within the cavity of the permanent mould, in which the core is broken up in order to recover the desired casting; (c) a "non reusable mould" made of any moulding medium, such as a moulding sand, held together by a bonding agent to define the shape of the cavity, in which the mould is broken up to recover the desired casting. Summary of the Invention This invention relates to a process for forming shaped castings comprising the steps of introducing a charge of molten metal into a mould which is preferably coated with 5 a refractory mould coating, cooling and solidifying the metal to form a casting and removing the casting from the mould. According to the novel features, the molten metal in the mould is cooled until the outer layer of the metal 1.7 solidifies and contracts leaving a gap between the surface 10 of the casting and the surface of the mould. At this point, there is injected into the gap between the surface of the casting and the surface- of the mould a gas having a different thermal conductivity than air whereby the cooling rate of the casting can be controlled. The gas in 15 the gap is maintained in a substantially static state whereby the thermal conductivity of the gap is manipulated to control the cooling rate of the casting. This "substantially static state" is a condition in which the gas in the gap is either static or is moving sufficiently 20 slowly that substantially no cooling is taking place because of mass flow of gas. In one aspect of the invention, the cooling of the casting to a temperature below the melting temperature of the metal is accelerated so that the casting can be 25 removed from the mould much sooner than in prior processes. Thus, by injecting gases of different thermal conduc¬ tivity than air into the gap, the thermal conductivity of the gap can be manipulated and relatively large changes in cooling rate of the casting can be produced. For instance, 30 when helium is injected, the cooling time for a casting can be reduced by 30 to 50% depending upon the shrinkage characteristics of the alloy. Since the cooling time of the casting represents a considerable fraction of the cycle time of a die, the use of helium to accelerate cooling is 35 capable of increasing the production rate of a given casting station by 15 to 25% and more. Alternatively, by 86 -4- injecting a gas having lower thermal conductivity than air into localized areas of a casting mould, the cooling rate at that location can be decreased. This can have the effect of creating a riser (a region of metal in the casting that freezes last and feeds solidification shrinkage in other areas of the casting) without the need for providing a larger section thickness and thereby slower solidification rate in the riser section. The simultaneous use of a plurality of gases of different thermal conduc- tivity makes it possible to vary or control the rate of cooling in different parts of the mould, and results in a desired and useful directional solidification. Accordingly, the present invention has the possibility of reducing the total weight of the rough casting and hence increasing the casting yield, i.e. the ratio of the finished casting to the weight of the as-cast casting including gates and risers. As a gas having a higher thermal conductivity than air, helium is preferred 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. Of course, it is also possible to use mixtures of helium with other non-reactive gases of lower thermal conductivity to provide advantages in applications where carefully selected rates of cooling are required. As suitable gases having lower thermal conductivity than air, there may be mentioned argon, sulphur hexafluoride, and carbon dioxide. The present invention can be used with moulds in which the internal surface are, or are not, covered by a mould coating based on a refractory, pr graphitic, or other material. The invention can be used with moulds which are subjected to an additional control of their temperature during and after introduction of the molten metal, either by a temperature control fluid circulating through internal channels inside the mould, or by surrounding the external surfaces of the mould by a fluid. In the shape casting of the present invention, the shrinkage space between the ingot and the side of the mould is much smaller than in the case of direct chill casting. Even with the very small gap present in the shape casting mould of the present invention, it has surprisingly been found that simply maintaining a gas different from air in that small gap is highly effective in changing the cooling rate of the casting. It has also surprisingly been found according to the present invention that the use of the modified gas is highly effective in modifying the cooling rate particularly when the mould surface is coated with a refractory coating. In fact, it has been found that the introduction of helium into the narrow gap according to this invention is capable of restoring the heat conductivity of the interface to the same value as that existing in a metal/gas/metal interface. Thus, the benefits of this invention are particularly significant in association with a hot metal/gas/refractory coating/metal system. Description of the Preferred Embodiments Preferred embodiments of this invention are illustrated by the following non-limiting examples. Example 1 In this example, a split steel mould was used dimensioned to cast cylindrical test specimens measuring 38 x 152 mm. A hole 1.6 mm in diameter was drilled through the base of the mould through which various gases were introduced. In order to prevent the hole from becoming clogged with metal during casting, the bottom of the mould was covered with layer, about 13 mm in thickness of a porous insulating refractory felt (Fibre-Frax) . The internal surfaces of the mould were coated with a commercially available refractory base mould coating ("Stahlcoat" insulating mould coating sold by the Stahl Speciality Company, Kingsville Missouri, U.S.A.). The temperature of the mould was monitored by a thermocouple inserted into the mould to a depth of 13 mm. The castings were made with the mould temperature at 340°C, and the molten metal temperature at 700°C. The metal used was A319, an aluminum alloy containing 4.5% Cu, commonly employed in permanent mould casting. Three tests were conducted, one with air in the gap formed between the casting and the mould, one in which helium was introduced into the gap and one in which helium was introduced into the gap and the mould was subjected to convection cooling. The helium flow rate was 50 cc/min. The temperature of the casting was monitored during cooling with a K-type thermocouple inserted through a hole in the mould at a point 76 mm from the bottom of the test specimen to a depth corresponding to the center line of the cylinder being cast. The casting was removed at 400°C. The average results of the tests are shown in Table 1 below: Table 1 ♦Time between extracting the solidified casting from the mould and the time of filling the mould again with molten metal. It will be seen that the dead time for both air and helium are the same. The dead time is the time between extracting the solidified casting from the mould and the time of filling the mould again with molten metal. This is greater when helium alone, without convection cooling, is used, because the heat absorbed per unit time during cooling with helium is greater than with air and consequently the mould temperature after extracting is higher than with air. Therefore, it takes a longer time for the mould to cool to the pouring temperature, because it is hotter. By continuously blowing air across the mould, thereby providing the -convection cooling, this extra heat is dissipated more quickly and the dead time was reduced. The convection cooling was achieved by blowing cool air from a fan over the mould. In other similar tests using a variety of alloys, including A319, A356, 332, and a 4.5% Cu, a 11.5% Si alloy and a 99.7% Al alloy, the increased rate of cooling and solidification obtained by providing helium rather than air in the gap between the solidified layer of metal of the casting and the wall of the mould gave productivity increases of 6 to 13% with helium cooling alone and a 37 to 48% increases with both helium and convection cooling. Example 2 For this test, a flat mould was used having a cavity to form flat test specimens measuring 152 mm x 152 mm x 25 mm with a riser measuring 38 mm x 25 mm x 152 mm. Holes 1/16" diameter were provided through both the top and the bottom of the mould. The mould was coated with a commercial refractory base mould coating (Stahlcoat) and was preheated to 300°C. A molten A319 aluminum alloy was poured into the mould cavity at a temperature of 700°C. Three tests were conducted, one with air in the gaps formed between the casting and' the mould, one in which helium was introduced into the gaps and one in which helium was introduced into the gaps and the mould was -8- subjected to convection cooling. The helium flow rate was 100 cc/min. and the helium flow was started after the riser had solidified (approximately 50 seconds after the metal had been introduced). Castings were removed when they had cooled to 400°C. The results obtained are shown in Table 2 below: Table 2 The above tables shows that the time required for the casting to cool to 400 β C was reduced by 34% by the use of helium. In this example the production rate of castings increased by 13 to 26% for helium, and helium plus convection cooling respectively. Example 3 This test was conducted using the same mould and refractory coating used in Example 2. The mould was at 300°C and molten A319 aluminum alloy was poured into the mould cavity at a temperature of 700°C. -9- Helium flow rates were between 30 and more than 5,000 cc/min. These were started 50 sees after the metal was introduced. The castings were removed when they reached a temperature of 400°C. The results are shown in Table 3 below: Table 3 The results show that flow rates in excess of 50 to 100 cc/min do not significantly increase the rate of cooling. The higher flow rates result only in excess helium spilling from the mould and are not useful in transporting heat away from the casting. Example 4 This test was conducted to determine the time when the helium flow should be started, i.e. the time lapsed since the metal was introduced into the mould. Again for this test a flat mould of Example 2, coated with the refractory coating (Stahlcoat) was used. 6386 -10- The mould was at 300°C and a molten A319 aluminum alloy was poured into the mould cavity at a temperature of 725°C. The helium flow rate was 100 cc/min. The casting was removed from the mould at 400°C. The results are shown in Table 4 below: Table 4 ♦ Elapsed time between introducing metal and starting flow of gas Thus, when the flow of helium was started only 30 second after pouring, the cooling was slower than when the flow started 50 seconds after the metal was poured. It is believed that this is because after only 30 seconds in the mould coated with a refractory base coating the outer layer of the metal lacks mechanical strength, and the pressure of the incoming gas deforms the surface of the casting, enlarging the gap, which in turn reduces the rate of heat transfer. However, after 50 seconds, the surface of the metal is strong enough to resist deformation. Example 5 This experiment demonstrates the different cooling rates obtained by using different gases of varying heat conductivity. A mould similar to that of Example 2 was used for these tests. However, the mould surface was coated with a graphite based coating (Acheson Aerodag G dry graphite spray, supplied by Acheson Company, Brantford Ontario). The mould temperature was 350°C. A molten A319 aluminum alloy was poured into the mould cavity at a temperature of 700 β C. The gas flow rate into the gaps was 100 cc/min. and was started 30 sees after the metal was introduced. The castings were removed when they reached 400°C. The results are shown in Table 5 below: • Table 5 The above results show that helium cools the casting more rapidly than air, while argon cools slightly more slowly than air and SF β cools still more slowly than air. By the proper choise of gas, it is thereby possible to control or vary the rate of cooling of the metal and th casting. Example 6 This test involved casting 500 specimens in an industrial type mould in the shape of an intake manifold for a 5.0 liter V-8 engine. The mould was of the semi¬ permanent type for a gravity-die cast process. For this purpose, a bonded sand core was used and the overall dimensions of the finished casting were approximately 150 x 370 x 370 mm. The weight of the metal in the casting was 15 kg. Holes were provided at different locations through the mould. Molten A319 aluminum alloy at 720-750°C was poured into the mould at an average temperature of 445°C. The resulting castings weighed 15 kg and the finished casting weighed 8.5 kg. Castings were produced using both the normal plant practice with no gas injection and by injecting helium into the gap region between the casting and the die once the temperature of the casting had fallen to 540 β C. The helium was introduced at a pressure of 20 psi and a flow rate of 0.6 1/min. through two slotted vent plugs which were centrally located on both the lower and upper dies. Using the normal casting practice, the casting was removed from the mould 170 seconds after pouring, at an average temperature of 488 β C. When helium was injected as described above, castings had cooled to 488°C, on the average 125 seconds after pouring. The time required to cool at 488°C was reduced by 45 sees. i.e. from 170 to 125 sees.

Claims: 91/06386 -13- Clai s : 1. In a process for forming shaped castings comprising the steps of introducing a charge of molten metal into a mould coated with a refractory mould casting, cooling and solidifying the metal to form a casting and removing the casting from the mould, the improvement comprising the steps of cooling the molten metal in the mould until the outer layer of the metal solidifies and contracts leaving a gap between the surface of the casting the surface of the mould, and injecting into said gap a gas having a different thermal conductivity than air and maintaining said gas in the gap in substantially static state whereby the thermal conductivity of the gap is manipulated to control the cooling rate of the casting. 2. A process according to claim 1 wherein said gas is introduced through small holes in the mould walls adjacent where gaps form. 3. A process according to claim 2 wherein a cooling gas is used having a higher thermal conductivity than air, ' whereby the cooling of the metal is accelerated and the productivity of the casting process is increased. 4. A process according to claim 2 wherein gases of different conductivities are injected into different areas of a single casting. 5. A process according to claim 4 wherein the gases of different conductivities are injected in a manner to provide directional solidification. 6. A process according to claim 1 wherein the gas is helium. 7. A process according to claim 4 wherein the mould is also subjected to additional cooling. 8. A process according to claim 1 wherein the metal being cast is aluminum or an all'oy thereof. 9. A process according to claim 1 wherein additional temperature control is provided by circulating fluid through channels of the mould or by circulating fluid around the mould. 10. A process according to claim 7 wherein the mould is subjected to additional cooling by blowing air on the outer surface of the mould.

Inventors: DOUTRE DON ALLEN, DOUTRE, DON, ALLEN
Applicants: ALCAN INT LTD, ALCAN INTERNATIONAL LIMITED
Application year: 1990
Main classification: B22D27
IPC codes: B22C9, B22D11, B22D27