Method for casting a directionally solidified article

Abstract: Method of casting a directionally solidified article with a casting furnace (1) comprising a heating chamber (2), a liquid cooling bath (3) as a cooling chamber (3) and a shell mold (5), whereby the shell mold (5) is fed with liquid metal (6a), and withdrawn from the heating chamber (2) to the cooling chamber (3), characterised in that the method takes place in an inert atmosphere of Ar and/or He in the pressure range of 0.01 to 1 atmosphere, with a preferred range of 0.05 to 0.25 atmosphere.

Description: FIELD OF INVENTION The invention relates to a method for casting a directionally solidified article. STATE OF THE ART The Liquid Metal Cooling Process (LMC) is a well known variation of the investment casting directional solidification process in which a shell mold which forms a cavity resembling the final part to be manufactured is filled with a liquid alloy, and lowered from a hot zone into a cold zone. The hot zone is usually maintained at above the temperature of solidification of the alloy being used and the cold zone is well below this temperature. As the mold is drawn at a controlled rate into the cold zone, solidification occurs directionally beginning with the first portion of the mold to enter the cold zone. The mold may be supported on a chill plate used for a structural base and also to rapidly remove heat from the bottom-most portions of the casting. The typical Bridgman process (disclosed in the document US 1,793,672) uses a water-cooled copper or steel lined vacuum chamber as a means of cooling the shell mold by radiation. There is a baffle made of an insulating material between the cold zone and hot zone which prevents excessive heat flow between them. The baffle is shaped so as to let the shell mold pass through it with only a minimum amount of clearance space between the mold and the baffle. This further minimizes heat flow from hot zone to cold zone, maintains the solidification front of the material in the shell mold at a level close to that of the baffle, which is desirable, and maximizes the thermal gradient across the solidification front. The LMC process as disclosed in US 3,763,926 uses a liquid medium (usually aluminum or tin) kept at a controlled temperature as the cold zone. While Patent US 3,763,926 discloses a stationary baffle similar to that used in the Bridgman process, a variation of this, using a baffle which floats on the surface of the liquid coolant, is disclosed in US 4,108,236. Those familiar with the art of directional solidification understand that the rate of heat transfer achievable at the surface of the shell mold is much greater when using liquid metal as a coolant than simply relying on radiation cooling in vacuum. It can be shown by calculations that the effect is 70% - 100% greater heat transfer away from the shell mold surface compared to pure radiation heat transfer. The Bridgman and LMC directional solidification processes are used mostly today for the industrial production of columnar grained and single crystal articles destined for high temperature service and composed of Ni-based superalloys. Those familiar with the solidification of Ni based superalloys understand the great advantages of the higher gradients across the solidification front achieved during the LMC process: as outlined by Giamei and Tschinkel in Metallurgical Transactions A Volume 7A Sept 1976, pgs 1427-1434, these lead to finer dendrite arm spacings, smaller pores, the suppression of freckle defects, and smaller diffusion distances required during solution heat treatment. The effect of high gradient solidification on the lifetime of single crystal Ni based superalloys is well known. One example, the commercially available CMSX-2, exhibits approximately a doubling in Low Cycle Fatigue life and an order of magnitude improvement in creep rupture life at 870°C when solidified with a high gradient process compared to a low gradient process (p.1005, "Directionally Solidified and Single Crystal Superalloys" - The Metals handbook, Tenth Edition, Volume 1, ASM International 1990). However, it is also well known in the industry that there are relatively few LMC furnaces in production compared to the very numerous Bridgman furnaces. One reason for this is the very high cost of the LMC furnace: maintaining a bath of liquid aluminum or tin at a controlled temperature is very complicated and costly. The high cost offsets the improvements in material properties achieved by using the process in most cases. One way to improve this situation would be to increase the production rate at which components are cast using the LMC process. As outlined by Giamei and Tschinkel, as the rate of withdrawal of the shell mold from the hot zone into the liquid coolant increases, so does the curvature of the solidification front. This is highly undesirable (leading to freckle defects and other problems) and so places an upper limit on the rate of withdrawal and hence the production rate of LMC cast components. What is needed is a means of avoiding this solidification front curvature at higher withdrawal rates, and generally increasing the amount of heat transfer in the process, in order to increase production rates using LMC and hence lower the cost of the process and further improve material properties. In Processing a Microstructure of Investment casting turbine blade NITAC In-Situ Composites, Fras et al, Journal of Materials engineering and performance, US, ASM International, vol. 5, no. 1, 1. February 1996, p. 103-110, XP 000554304, ISSN 1059-9495 a LMC process is disclosed in which the directional solidification of turbine blade NITAC alloys was carried out in a Bridgman apparatus (liquid-cooling method). The method was carried out under pure argon. EP-A1-0 749 790 discloses a casting furnace. In the production of a casting in a vacuum chamber, a mould with molten alloy is transferred, through a hole in a separating baffle, from a heating chamber into a cooling chamber in which the alloy solidifies in an oriented manner. The method is characterized by the fact that below the baffle the mould is additionally cooled from outside by means of a gas flow. Also claimed is an apparatus. for implementation of the method. The apparatus is characterized by the presence of means for producing and directing a gas flow in the cooling chamber below the baffle. The gas used for cooling the mould is an inert gas such as, in particular, argon or helium. The gas is introduced into the cooling chamber after entry of the bottom of the mould. The gas is directed in the direction of the mould surface, and is then removed from the vacuum chamber. The gas is cooled, filtered and again introduced into the cooling chamber. SUMMARY OF THE INVENTION It is the object of the present invention to find a method for manufacturing a directionally solidified articles using a LMC process overcoming the disadvantages described above and enhancing the LMC process (higher withdrawal rate, temperature gradient) without solidification front curvature. According to the invention a method of casting a directionally solidified article with a casting furnace comprising a heating chamber, a liquid cooling bath as a cooling chamber and a shell mold, whereby the shell mold is fed with liquid metal, and withdrawn from the heating chamber to the cooling chamber, characterised in that the method takes place in an inert atmosphere of Ar and/or He in the pressure range of 5·1.013·10 3 to 25·1.013·10 5 Pa [0.05 to 0.25 atmosphere] pressure and with a preferred gas mixture of between He and Ar with (vol.-%) 60 - 100 % He. In another embodiment the gas will be provided from a nozzle under or above the baffle between the heating chamber and the cooling chamber. The surprising discovery was made that using a casting furnace with a liquid metal bath as cooling medium a controlled pressure of the inert gas enhances the heat transfer between the shell mold and the liquid coolant greatly and increases the production rate substantially without solidification front curvature. It is also possible to introduce the gas within the liquid cooling bath using porous nozzles enhancing the method of solidification and at the same time inducing convection currents within the liquid cooling bath, which further enhances heat transfer. SHORT DESCRIPTION OF THE DRAWINGS It is shown by Fig. 1 a first embodiment of a LMC casting furnace for carrying out the method according to the invention, with a nozzle under/above the baffle between the heating chamber and cooling chamber and Fig. 2 a second embodiment of a LMC casting furnace for carrying out the method according to the invention, with a nozzle at the bottom of the cooling chamber and immersed in the liquid coolant. The drawings show only the elements important for the invention. Same elements will be numbered in the same ways in different drawings. PREFERRED EMBODIMENT OF THE INVENTION The present invention relates to a method of manufacturing a directionally solidified articles using an Liquid Metal Cooling (LMC)-process. The essential features of the method are defined in claim 1. Preferred embodiments are defined in claims 2 to 7. Figure 1 shows a casting furnace 1 comprising heating chamber 2 and cooling chamber 3. The heating chamber 2 includes heating elements 2a. Both the heating chamber 2 and the cooling chamber 3 are separated by a baffle 4. The cooling chamber 3 uses as a coolant a liquid metal bath such as for example aluminium at a temperature range between 700° and 900°C or a liquid tin bath at a range of 300°-500°C. In order to keep the temperature in the mentioned range the cooling elements 3a may be arranged around the cooling chamber 3. In the first step of the method for solidifying the directionally solidified article, a shell mold 5 placed within the heating chamber 2 will be filled with a liquid alloy 6a through a feed opening 10 at the top of the heating chamber 2. The shell mold 5 is in the heating chamber 2 in a position which is shown with a dotted line. After filling the shell mold 5 it is withdrawn continuously from the heating chamber 2 to the cooling chamber 3 through an aperture 12 in the baffle 4. The direction of withdrawal 11 is indicated with an arrow. As soon as the shell mold 5 reaches the cooling chamber 2 solidification of the liquid alloy 6a begins. The solidified alloy 6b is found at the lower portion of the shell mold 5, which already reached the cold zone. Approximately at the border between the heating chamber 2 and the cooling chamber 3 is the solidification front 14. The solification front 14 grows from the bottom of the shell mold 5 to the top in opposite direction of the withdrawal direction 11. The surprising discovery that was made, was that when using a casting furnace 1 with a liquid metal bath 3 as a cooling medium, a controlled pressure of inert gas greatly enhances the heat transfer across the shell mold 5 and increases the production rate substantially without solidification front 14 curvature. Experimentation has shown that using inert gas such as Ar or He in the pressure range of 1.013·10 3 to 1.013·10 5 Pa [0.01 to 1 atmosphere], in a more preferred range of 5·1.013·10 3 to 25·1.013·10 3 Pa [0.05 - 0.25 atmosphere], substantially enhances the heat transfer during the LMC process. Tab.1 shows the different results for manufacturing two identical turbine blade-like castings, one cast with conventionally LMC in a vacuum atmosphere, the other using a He atmosphere in the more preferred range of 1.013·10 3 to 25·1.013·10 3 Pa [0.1 to 0.25 atmosphere]. In both cases as coolant an aluminium bath was used, and the alloy was a commercially available Ni based single crystal superalloy. Experimental results Typ Average Primary dendrite arm spacing Average Microporosity Average micropore diameter Largest micropore diameter Conventional LMC 390 µm 0.25 % 9.77 µm 76.5 µm LMC according to invention 321 µm 0.17% 8.63 µm 60 µm The average primary dendrite arm spacing was reduced from 390 µm (conventional) to 321 µm (invention). It is understood that this indicates a substantial increase in thermal gradient across the solidification front 14 during casting. This result will allow a higher withdrawal rate, suppress freckle defects, and lead to longer low cycle fatigue (LCF) and high cycle fatigue (HCF) lives of the cast article. The average microporosity was reduced from 0.25% (conventional) to 0.17% (invention). Less microporosity is well known to lead to significant improvements in HCF, LCF and creep rupture lives. The average diameter of micropore observed was decreased from 9.77 µm (conventional) to 8.63 µm (invention) with the largest micropore decreased from 76.5 µm to 60.0 m. Again, it is understood that not only volume fraction porosity matters for HCF, LCF and creep lives, but also the size of the pores and the size of the single largest pores. From these facts it is clear that the LMC process according the invention offers real lifetime advantages for the cast article over the conventional LMC using vacuum atmosphere. The presence of gas - in particulary the pure He or a mixture of He and Ar, preferable with (vol.-%) 60-100% He, increases heat transfer very significantly compared to the case of LMC casting in a vacuum which is state of the art. Preliminary experiments showed that no significant benefits in casting results occurred at pressures over 25·1.013·10 3 Pa [0.25 atmosphere], however significant problems with furnace control equipment may occur as gas pressure rises due to the effect of gas convection on the furnace control thermocouples. In principle it is possible to let the inert gas in at any point of the casting furncace 1 through any inlet. In addition to these general rules Fig. 1 shows a preferred embodiment using a nozzle 7 which is connected with a feeding line 9 and which is in the left side of the figure 1 arranged under the baffle 4. The nozzle 7 directs a flow 8 of the inert gas against the shell mold 5 increasing the temperature gradient at the solidification front 14. For regulation of the pressure within the heating chamber 2 a gas outlet 13 is provided. As shown in the right side of figure 1 it is also possible to arrange a floating baffle 4a on top of the liquid metal bath. The nozzle 4 with the feeding line 9 is arranged above the floating baffle 4a. The number of the nozzles 4 shown in figure 1 is only for example reasons and will not limit the extent of the invention. Figure 2 shows a second embodiment of a casting furnace 1 for carrying out the method according the invention. In general the feeding of the shell mold 5 and the withdrawal from the heating chamber 2 to the cooling chamber 3 is the same as specified in figure 1. But as a difference during the solidification of the liquid alloy 6a the inert gas will be fed through a nozzle 15 located at the bottom of the liquid cooling bath 3. The nozzle 15, as an example a porous refractory nozzle, may be centered in the middle of the cooling chamber 3 under the shell mold 5. With this embodiment not only the advantages during the solidification are achieved as described above but a flow 16 will induce a convection current 18 with no moving parts. This is a further advantage over the patent US 3,763,926 in which a mechanical stirrer is used for generation a current to enhance the heat transfer by ensuring a homogenous temperature within the liquid metal bath. As seen in figure 2 the inert gas will separate into small bubbles 17. This nozzle 15 may be located at the bottom of the liquid coolant bath either centered or at the side, or on the vertical walls of the bath in order to induce convection currents as desired. There may also be more than one immersed nozzle. It is understood that the location of the nozzles 15 and the number of nozzles 15 that will optimise fluid flow and gas introduction will vary as a function of the shape and size of the shell mold being used. NUMBERING 1 Casting furnace 2 Heating chamber 2a Heating elements 3 Cooling chamber, liquid metal bath 3a Cooling Elements 4 Baffle 4a Floating baffle 5 Shell mold 6a Liquid alloy 6b solidified alloy 7 Nozzle 8 flow 9 Feed line 10 Feed opening 11 Direction of withdrawal 12 Aperture 13 Outlet 14 Solidification front 15 nozzle 16 flow 17 bubble 18 current

Claims: Method of casting a directionally solidified article with a casting furnace (1) comprising a heating chamber (2), a liquid cooling bath (3) as a cooling chamber (3) and a shell mold (5), whereby the shell mold (5) is fed with liquid metal (6a), and withdrawn from the heating chamber (2) to the cooling chamber (3) characterised in that the method takes place in an inert atmosphere of Ar and/or He in a pressure range from 5 x 1.013 x 10 3 Pa to 25 x 1.013 x 10 3 Pa (0.05 to 0.25 atmospheres) to enhance the heat transfer between the shell mold (5) and the liquid metal (6a). Method according to claim 1 characterised in that the method takes place in an inert atmosphere of Ar and/or He with an atmosphere containing (vol.-%) 60% to 100% He. Method according to one of the claims 1 and 2 characterised in that the gas Ar and/or He is fed through an inlet at any point of the casting furnace (1) during the withdrawal of the shell mold (5). Method according to one of the claims 1 and 2 characterised in that the gas Ar and/or He is fed through a nozzle (7,15) towards the shell mold (5) during the withdrawal of the shell mold (5). Method according to claim 4 characterised in that the gas Ar and/or He is fed through a nozzle (7) towards the shell mold (5) under a baffle (4), which separates the heating chamber (2) and the cooling chamber (3) during the withdrawal of the shell mold (5). Method according to claim 4 characterised in that the gas Ar and/or He is fed through a nozzle (7) towards the shell mold (5) above a floating baffle (4a), which separates the heating chamber (2) and the cooling chamber (3) during the withdrawal of the shell mold (5). Method according to claim 4 characterised in that the gas Ar and/or He is fed through a nozzle (15) immersed into the liquid cooling bath (3) during the withdrawal of the shell mold (5).

Inventors: FERNIHOUGH JOHN DR, KONTER MAXIM DR, FERNIHOUGH, JOHN DR., KONTER, MAXIM DR., Fernihough, John Dr.,, Konter, Maxim Dr.,
Applicants: ALSTOM TECHNOLOGY LTD
Application year: 2000
Main classification: C30B11
IPC codes: B22D27, C30B11