Title of Invention

"APPARATUS FOR UNIDIRECTIONAL SOLIDIFICATION OF COMPONENTS SUCH AS TURBINE BLADES & VANES"

Abstract

This invention relates to an apparatus for unidirectional solidification of components. The apparatus has a single vacuum chamber having an upper and lower portion. At least a first charging cup provided with said chamber for loading of remelt alloy bar into said chamber. At least a first mould charging cup is provided for loading a ceramic mould within said chamber. A crucible actuator is disposed within said charging cup for raising and lowering of a crucible containing said remelt alloy bar. A chill block is disposed in said mould charging cup. A chill block actuator is provided to lower or raise the chill block. The ceramic mould is clamped to said chill block.

Full Text

BACKGROUND OF THE INVENTION 1.Technical Field This invention relates to apparatus for unidirectional solidification of components such as turbine blades and vanes. 2.Description of Prior Art Apparatus for directional solidification of turbine parts have come are known in the art and comprise an open bottom hot ceramic mould containing molten alloy mounted on a water cooled copper chill block is pulled out of a mould heater in a controlled manner. The casting solidifies from the top of the chill—block (i.e.mould bottom) unidirectionally upward by ~ heat conduction into the chill-block via the solidified part of the casting initially and later by radiative heat transfer from the mould-wall to the water cooled furnace-wall as the resistance against heat conduction to the chill-block through the increasing length of solidified part goes above that of heat conduction from solidifying casting to mould wall and radiative of heat transfer from mould wall to the water-'tooled furnace wall. A variety of equipment designs have come about since then, ranging from large capacity furnaces employing as much as 50 Kg of superalloy melting crucible and large moulds of about 150 mm diameter and 60O mm height to furnace design for casting in moulds as small as 14O mm diameter and height about 200 mm. Large sized furnaces offer higher production rate but poor casting quality owing to lower temperature gradient across solid-liquid interface compared to those with relatively smaller mould heaters. Higher processing temperature in large sized furnaces provide improved temperature gradient, but results in poor casting quality due to increased rnelt-mould reaction or else would require costly moulding systems to avoid or reduce melt-mould reaction. Smaller furnace design on the other hand leads to better quality castings within small to medium sizes only. Because, adequate mould rigidity against metallostatic pressure in the case of large and tall components require increasingly thick mould wall (in proportion to mould height) which in turn deteriorates the temperature gradient across the solid-liquid interface of the solidifying casting. Moreover, such furnaces of prior art often do not have precise control over the crystallographic orientation of single crystal components since the crystal generally emerges here' through random nucleation and grain growth competition in helical grain selectors or other geometrical constrictions. In the furnace of US patent no.4469161 seed crystal of desired crystal lographic. orientation is placed in the mould bottom so that the same seed texture will extend into the component as solidication would proceed from the bottom to the top of the mould cavity. The casting yield however, in such case,, has been only marginal since the seed crystal surface is subjected to an aggressive atmosphere during mould heating due to vaporisation of volatile compounds and also to oxidation in the dynamic vacuum at high temperature (-1500'C). All these increase the risk of polycrystalline growth in stead„ Better control of crystal orientation through seed implanatation can be achieved if the seed crystal enters the mould after heating the mould to desired temperature, just at the event of melt pouring into the mould. European Patent no.0496978 Al and US patent no.5261480 disclose seeding method of single crystal component casting has been described to have been successfully adopted by a complex arrangement of splitting the furnace into four or more number of separate chambers employing several vacuum interlocks and transporting devices for loading, unloading and heating of remelt bar, mould and seed crystal in isolation of one another and then simultaneously bringing melt (by tilt pouring) and seed crystal (by quick movement of seed carriage from lower to upper chamber and actuating a clamping device to hold the mould tightly against any melt leakage) and simultaneously inserting the seed crystals into the mould. All these call for precision movement of various parts with faithful interlocking, sequencing and quick start-stop movements with positional and durational accuracy and repeatability in vacuum at high temperature avoiding any impact between various parts or melt leakage where occurrence of refractory debris with time and melt droplets due to tilt pouring is unavoidable. SUMMARY OF THE INVENTION An object of this invention is to propose an apparatus for producing columnar grained as well as single crystal castings over wide range of shape and size (from about 50 to about 5OO mm length) with very high casting quality and productivity. Another object of this invention is to propose an apparatus with minimum vacuum sealing joints in order to minimise leak rate and to rely upon very few mechanisms involving very few moving or sliding parts and achieve thereby greater reliability, operational ease and less maintenance relative to the apparatus of prior art. Yet another object of the invention is to propose an apparatus for casting large and tall components in much thinner moulds employing much less induction melting capacity than what the furnaces of prior art require. According to this invention there is provided an apparatus for unidirectional solidification of components characterized in that a. a single vacuum charrber (1) having an upper and lower portion; b. at least cup alloy charging cup (7) provided for loading of the remelt alloy bar into said chamber; c. at least a mould charging cup (8) for loading of a ceran.ic mould within the said chamber; d. a crucible actuator (18) disposed within said charging cup (7) for raising and lowering of a crucible containing said remelt alloy bar (19); e. a chill block (23) disposed in said mould charging cup (8); f. a chill block actuator adapted to lower or raise said chill block; g. said ceramic mould (24) adapted to be clamped to said chill block. The present invention provides a single chamber design where remelt alloy bar and ceramic mould are loaded from their respective charging cups at the chamber top and bottom via small isolation valves. For efficient production of single crystal components with desired crystal orientation, a heat conductive chill block that clamps and carries the ceramic mould is provided with an ejector to insert seed crystals into the preheated mould up to casting position at the instant of melt pouring. A hot sane for mould heating is designed for precise control of solid-liquid position and desired level of temperature gradient at the interface employing optimum aperture of radiation baffle to accomplish defect free unidirectional freezing at minimum hot zone temperature. The apparatus of this invention is capable of casting either one component at a time or several components together as clustered in the ceramic, mould depending upon the component cross-section. Unidirectional solidification at tall components having large cross-sections in relatively thin walled moulds as compared to the prior art takes place in this invention because of its multiple tundish arrangement •for melt pouring in instalments. Added advantage of this arrangement is the scope to make larger castings than the induction melting capacity with the concept of quick melting in open—bottom—smal1-crucibles and quiet pouring of clean melt therefrom. In a preferred construction, several such single chamber units are coupled to single vacuum pumping system comprising a centralised diffusion pump along with its roughing, fore-line and hold pump then a roots-blower-mechanical-pump combination for the remelt alloy bar and mould charging cups for increased production and efficient maching utilisation. In another preferred construction for unidirectional solidification of components upto very large size, both in height, cross-section, and casting thickness, a low melting liquid metal bath is provided for efficient heat transfer and high temperature gradient across the solid-liquid interface of the solidifying casting. BRIEF DESCRIPTION OF THE FIGURES OF THE Accompanying DRAWINGS Fig.l is an elevation view of the apparatus far-unidirectional solidification of components. Fig. 2 is a plan view of several single chamber casting modules coupled to a single vacuum pumping system. Fig. 3 is an elevation view of the apparatus for unidirectional solidification of very thick and large casting employing liquid metal coolant. Fig. 4 is an isometric view of casting configuration employing orthogonally helical crystal. selector for longitudinal as well as transverse control of crystal lographic orientation in the casting- DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Design Feature Fig.l shows the overall arrangement of the apparatus in a. vertical sectional view. It comprises essentially a single chamber 1 made out of two cylindrical vessels 2 and 3 which are water cooled and sealed against each other at the vacuum tight flange 4. This split design of the chamber is for the ease of apparatus fabrication, installation and maintenance. The dome shaped ends of each vessel are fitted with vacuum valves 5 and 6 for connecting the alloy bar charging cup 7 at the top and the mould charging cup 8 at the bottom after evacuation. The top half of the chamber has an induction coil 9 for melting alloy bar. It also has an intermediate tundish 10 and an indexing shaft 11. The bottom half of the chamber is equipped with two mould heaters 12,13 and the actuating means 14 for the seed crystal ejecting mechanism 15. The alloy bar charging cup 7 has a transport mechanism 16 to move the crucible holder 17 upward along with the crucible 18 containing remelt bar 19 and metal plug 20 or down into the induction melting coil. The crucible holder carries at thermo-metric device 21 to measure melt temperature. Two such alloy bar charging cups are mounted on a pivot so that when one is engaged at the alloy melting position, the used crucible from the other one is replaced by new crucible containing fresh alloy bar and metal plug. The mould charging cup is equipped with a water cooled ram-shaft 22 carrying a chill block 23. The chill block has provision to clamp the ceramic mould 24 against melt leakage between mould bottom and itself as well as to keep the seed crystal (/s) 25 water cooled and to insert them into the mould cavity just before the melt would enter into the mould. The ram-shaft may move upward as well as downward along with the ceramic mould over wide range of speed-time schedule in a smooth and vibration free manner. Two such mould charging cups are mounted on a pivot so that the seed crystals and ceramic mould can be placed on one of them while the other is engaged with directional solidification inside the chamber. The chamber containing mould heaters is connected to the diffusion pump of a vacuum pumping system. The alloy bar charging cup and the mould charging cup are connected to another vacuum pump set independently through their respective isolation valves 26 & 27. In a preferred construction, a number of such single chamber units are coupled to the same vacuum pumping system. Fig.2 shows a plan view of such an arrangement where five casting chambers 28,29,30,31 and 32 are connected to a centralised diffusion pump, 33, through their respective isolation valves 34,35,36,37 and 38 so that running or shut down condition of any of the casting chambers can not prevent the functioning of remaining casting chambers. In another preferred construction that is especially suitable for unidirectional solidification of very thick components, a low melting liquid bath 39 is provided as shown in fig.3. The mould charging cup in this case is placed at the top near the alloy charging cup with appropriate modifications (4O) in the chill-block supporting structure that is insulated against hot zone temperature and the low melting liquid in the bath 39. The melting coil is mounted onto a retractable coaxial power feed-through 41. A pivoted pot 42 is provided adjacent to the low melting liquid bath 39 to collect the displaced liquid upon progressive lowering of mould. Operation The chamber 1 is evacuated by valves 5 & 6 being actuated into a closed position. The mould heaters 12 and 13 are then switched on to attain desired temperature. The bottom open melting crucible 18 with its remelt bar 19 and metal plug 20 in position is placed on the crucible holder 17. The crucible transport mechanism 16 is actuated to take the crucible inside the alloy charging cup 7 and the latter being placed on the vacuum sealing flange over the isolation valve 5. After evacuation of the alloy charging cup, 7, valve 5 is opened and crucible 18 is lowered by its transport mechanism 16 into the induction melting coil 9. Single crystal seeds are loaded into the ejecting mechanism inside the chill block 2.3. The ceramic mould 24 is then placed on chill block 23 and clamped after proper indexing with the ejecting mechanism. The chill block 23 is lowered by its transport mechanism 22 to bring the mould below the sealing flange of mould charging cup 8 and the later is then aligned with the chamber bottom and clamped. After evacuation of the mould charging cup,8, the valve 5 is opened and the chill block 23 along with ceramic mould 24 is raised to the initial casting position. After desired duration of mould soaking in the hot zone, power to the induction melting coil 9 is turned on. As soon as the thermo-metric device 21 reads the desired pouring temperature the seed crystals are pushed into the mould cavity by actuating the ejecting mechanism 15. The melt is bottom poured into the ceramic mould 24 automatically within a couple of seconds as the metal plug 2O melts at the set pouring temperature. Solidification of single crystal components proceeds from the seed crystals as the mould is withdrawn from the hot zone in a controlled manner with the help of the transport mechanism of the ram-shaft 22. The used crucible is taken back into the alloy charging cup 7 after melt pouring. The isolation valves 6 & 27 are closed and air is allowed into the alloy charging cup 7 up to atmospheric pressure. This alloy charging cup is replaced with the other one having fresh crucible containing alloy bar and metal plug and the alloy melting-pouring cycle is repeated well before the melt above the solid-liquid interface inside the mould is solidified. Smooth pouring is assured by placing the intermediate tundish 1C in the gap created above the withdrawn mould by actuating the intermediate tundish indexing shaft 11. After complete mould withdrawal, the valve 5 is closed and air is allowed into the mould charging cup 8 up to atmospheric pressure. This mould charging cup containing the casting now is replaced with the other mould charging cup having fresh mould for the next casting cycle. The cast components are taken out and the chill block is prepared for loading fresh seed crystals and mould to continue with the process. Columnar grained castings are produced simply by not using any single crystal seed and leaving the ejecting mechanism in ejected position or on a dedicated chill block with no ejecting mechanism. The operation in case of cooling arrangement through low melting liquid bath of fig.3 is almost similar except the following: i) mould is charged from the chamber top. ii) The alloy charging mechanism leaves the crucible containing alloy bar and metal plug in the induction melting coil that is pushed forward with the help of the retractable power feed through 41 to come just above the pouring cup of the mould. After alloy melting and pouring it goes back to receive next batch of alloy charge just below the alloy charging cup. iii) After complete solidification of casting, the hot zone temperature is lowered well below the incipient melting temperature of the casting. The chill-block along with the mould and casting is lifted up into the mould charging cup and its isolation valve is closed in order to take out the casting and charge a fresh mould for next casting cycle. iv) The low melting liquid bath 39 is lowered and tlv? displaced liquid collected in the pivoted pot "12 is tilt poured into the bath 39. Thc» liquid bath 39 is then pushed up to the cashing position for next casting cycle. Superior texture in the columnar grained casting is obtc/Jned here due to high rate of heat extraction through water cooled chill-block relative to prior art of cooling through low melting bath. Similarly, superior control over longitudinal and transverse texture in single crystal casting is obtained more conveniently by employing orthogonal helix 43 and aligning the transverse axes (X',V) of component with those (X',V) of the orthogonal helix as shown in the fig.4 as compared to prior art of cooling through low melting liquid bath. Although the invention has been described with reference to a specific embodiment thereof, it will become apparent to those skilled in the art that numberous modifications and variations can be made within the scope and spirit of the invention if defined by the following claims. WE CLAIM 1. An apparatus for unidirectional solidification of components characterized in that a. a single vacuum chamber (1) having an upper and lower portion; b. at least cup alloy charging cup (7) provided for loading of the remelt alloy bar into said chamber; c. at least a mould charging cup (8) for loading of a ceramic mould within the said chamber; d. a crucible actuator (18) disposed within said charging cup (7) for raising and lowering of a crucible containing said remelt alloy bar (19); e. a chill block (23) disposed in said mould charging cup (8); f. a chill block actuator adapted to lower or raise said chill block; g. said ceramic mould (24) adapted to be clamped to said chill block. 2. An apparatus as claimed in claim 1 wherein said single chamber is made of two cylindrical vessels (2 & 3) and sealed to each other with a vacuum tight flange (4). 3. An apparatus as claimed in claim 2 wherein each vessel has a vacuum valve (5, 6). 4. An apparatus as claimed in claim 1 wherein said alloy bar charging cup (7) is provided at the upper end of said chamber, and mould charging cup (8) provided at the lower end of said chamber. 5. An apparatus as claimed as claimed in claim 1 comprising a heater or induction coil (9) in the upper portion of said chamber, mould heaters (12, 13) at the lower portion of said chamber. 6. An apparatus as claimed in claim 5 comprising an intermediate tundish (10) and an indexing shaft (11) for said crucible. 7. An apparatus as claimed in claim 1 to 6 comprising an actuator (14) for a seed crystal ejector (15). 8. An apparatus as claimed in any of the claims 1 to 7 wherein a first and second alloy bar charging cup provided in a pivotal relationship to said chamber, a first and second mould charging cup. 9. An apparatus as claimed in any of the claims 1 to 8 wherein said mould heaters are connected to a diffusion pump of a first vacuum pumpset. 10. An apparatus as claimed in any of the claims 1 to 9 wherein said remelt alloy bar charging cup and mould charging cup are connected to a second vacuum pumpset. 11. An apparatus as claimed in any of the claims 1 to 10 comprising a plurality of said single chamber coupled to said first and second vacuum pumpsets. 12. An apparatus as claimed in claim 1 wherein said mould charging cup and rernelt alloy charging cup are provided at the upper end of said chamber. 13. An apparatus as claimed in claim 12 wherein the melting coil is mounted on a retractable power feed. 14. An apparatus as claimed in any of the claims 12 and 1.3 comprising a pivoted pot adjacent to a :o,v melting liquid bath in the lower portion of said chamber. 15. An apparatus for unidirecdenal solidification of components substantially as herein described and illustrated.

Documents:

588-del-1997-abstract.pdf

588-del-1997-claims.pdf

588-del-1997-correspondence-others.pdf

588-del-1997-correspondence-po.pdf

588-del-1997-description (complete).pdf

588-del-1997-drawings.pdf

588-del-1997-form-1.pdf

588-del-1997-form-19.pdf

588-del-1997-form-2.pdf

588-del-1997-form-3.pdf

588-del-1997-gpa.pdf