Apparatus For Casting Directionally Solidified Articles

Abstract

Casting apparatus for the production of directionally solidified castings comprises first, second and third chambers and valves between the first and second and second and third chambers. The first chamber holds a charge melting and pouring arrangement. The second comprises a casting chamber and the third is a withdrawal chamber so that the casting may be withdrawn in a controlled fashion from the second chamber to produce the required directional solidification.

Description

This invention relates to apparatus for casting and is particularly concerned with apparatus for carrying out the process known as directional solidification.

The term directional solidification refers to a process of casting in which the heat flow from the casting during solidification is controlled so that the grains within the final casting extend at least substantially unidirectionally. It has been found that this process can produce castings which have improved properties, and particularly in the production of such articles as turbine blades for gas turbine engines, it is possible by the use of directional solidification to produce an article whose properties may be more reliably specified than when using conventional casting.

The present invention relates to apparatus in which directional solidification may be carried out in a simple fashion and which lends itself to automation.

According to the present invention apparatus for casting comprises first second and third adjacent chambers each of which may be sealed and evacuated, the first and second valve means dividing said first from said second chamber and said second from said third chamber respectively, and a charge melting arrangement in said first chamber adapted to melt metal and discharge it through said first valve means into said second chamber, a mould heating furnace in the second chamber adapted to heat a mould which receives said molten metal, and a movable chill support adapted to carry a chill and a mould between said second and said third chambers whereby the mould may be introduced into the furnace and removed when charged from the second to the third chamber so that a directional grain structure may be formed in the casting.

Preferably said first, second and third chambers are mounted on above the other with the first chamber being the topmost.

Said charge melting arrangement may also comprise an automatic bottom pouring arrangement.

There may also be charging means for the bottom pouring arrangement which comprises a flange member which carries a crucible which holds the new charge, the flange sealing a charging hole in the first chamber when the crucible is in position in the bottom pouring arrangement.

Preferably there are separate ducts connected to said first, second and third chambers for evacuation of these chambers; in this case the valves may be so sequenced as to maintain at all times during the operation a vacuum within the second chamber.

We prefer that the charged mould should be introduced and withdrawn from the second chamber at a non-constant rate.

There may be a pair of said charge holding crucibles and flanges and a pair of said third chambers and chill supports which may be alternately moved into position with respect to the first and second chambers, thus enabling loading and unloading of the crucible and mould respectively on the crucible and third chamber which are not adjacent the first and second chambers.

In a combined production facility it would be possible to use a single vacuum pumping unit (which may comprise a number of pumps) and a single power supply for the bottom pouring equipment; with suitable switching and sequencing means it is possible for such a single unit to service a plurality of the chamber combinations. Alternatively a single power supply could be used together with a number of vacuum units each servicing a number of chambers.

The invention will now be particularly described merely by way of example with reference to the accompanying drawings in which:

FIG. 1 is a sectional view of apparatus for casting in accordance with the present invention, and

FIG. 2 is a diagrammatic plan view of the lay-out of a production casting unit using the apparatus of the invention.

In FIG. 1 there is shown casting apparatus comprising a first melting chamber 10, which is mounted on top of a second furnace chamber 11 and which in the condition shown in FIG. 1 is on top of a third withdrawal chamber 12. The melting chamber is separated from the furnace chamber by an upper gate valve 13 while the furnace chamber is separated from the withdrawal chamber by a second lower gate valve 14. It should be noted that it may be preferable to use a different form of valve, e.g. a lift and swing valve, and that in order to prevent damage to the valve seals and to provide a more rapid rate of change of temperature between the furnace chamber and the withdrawal chamber it may be desirable to water cool the valve itself. The upper gate valve is operated by a ram 15 while the lower gate valve is operated by a ram 16. All the chambers are arranged to be evacuated by way of ducts 17, 18 and 19 respectively which are connected to a vacuum pump when evacuation is necessary; ducts 17 and 19 are connected to the pump by way of a single valve 20 while duct 18 has a separate valve 21. A further valve 22 is also provided by which ducts 17 and 19 may be vented to atmosphere.

Within the chamber 10 there is positioned as induction coil 23 which forms part of an automatic bottom pouring arrangement. The coil is supplied with electricity through bus bars 24 and 25 and is cooled with water through pipes 26 and 27. The electricity supply to the coil is controlled by an automatic sequencing unit which is not shown in FIG. 1.

In the condition shown in FIG. 1 a crucible 28 is mounted in position within the induction coil 23. The crucible 28 carries a charge of metal 29 and is suspended from a flange 30. The bottom of the crucible may be supported on a location plate (not shown) above the valve 13. The flange 30 is moved vertically by a ram 31, and seals the charging hole 32 in the top surface of the chamber 10.

The gate valve 13 is shown in the open position and hence allows free passage of molten metal from the crucible 28 into the furnace chamber 11. Within this chamber there is a mould heating furnace (32) electrically heated either by a resistance heater in the form of either a single element or by a plurality of elements, which is the preferred method, or alternatively an induction heated susceptor may be used. The heater surrounds a central area within which a mould 33 is shown carried on a water cooled chill 34 which is in turn supported on a spindle 35 which is carried in bushes 36 and 37 which are formed in support arms 38 and 39. The arms 38 and 39 are mounted on an indexing spindle 40, the arm 38 being the major supporting member for the third chamber 12.

Water cooling for the chill 34 is effected by ducts which extend within the spindle 35 from inlet and outlet ports 41 and 42 at its extremity.

The arm 38 carries the major portion of the withdrawal chamber 12 which comprises a cylindrical chamber mounted co-axially on the spindle 35 and sealed to it just above the bush 36. The walls of the cylindrical chamber are cooled by water which flows through ducts 100 in the walls, the cooling water being supplied from flexible pipes (not shown). The open end of the chamber 12 seals against the stationary plate 43 which is carried from the lower portion of the chamber 11 and which encloses that part of the chamber 12 which provides space for the gate valve 16. This valve is shown as being open and consequently allows the chill 34 and hence the mould 33 to be withdrawn vertically from chamber 11, vertical movement of the spindle 35 being permitted by its sliding within the bushes 36 and 37.

To effect vertical movement of the spindle 35, a ram 44 is mounted to move a plate 45 in a vertical direction. The plate 45 engages in the slot of a flange bush 46 which is fixed to the spindle 35. Hence, vertical movement of the plate 45 will cause vertical movement of the spindle 35. To control the rate at which this vertical movement occurs, a cam 47 is also mounted from the plate 45, and the cam engages with a feedback device 48 which controls the pressure of fluid supplied to the ram 44, thus providing a predetermined rate of rise and fall of the spindle 35. Microswitches 49 and 50 co-operate with end faces of the cam 47 to limit the vertical travel of the spindle 35 in the upward and downward direction respectively.

The hydraulic ram 44 may be replaced by other devices, thus a leadscrew could be used to raise and lower the spindle 35, the leadscrew being rotated by a hydraulic or electric variable speed motor. The leadscrew would enable easier control of the vertical position of the spindle.

To provide a simple charging and mould removing arrangement the apparatus is provided with two charge crucible support arrangements and two mould support arrangements. Thus the ram 31 is supported by an arm 51 from the upper end of the indexing spindle 40, the arm 51 extending the same distance the other side of the spindle 40 to support a ram 52 exactly similar to the ram 31 and which carries a flange 53 similar to the flange 30. The flange 53 is shown carrying a fresh crucible 54 which contains a fresh charge of metal 55. In a similar fashion the arms 38 and 39 extend on the other side of the spindle 40 and carry a further supporting spindle 56 which is similar to the spindle 35 and which carries a further withdrawal chamber 57 within which are a chill 58 and mould 59.

It would be appreciated that the crucible 54 and the mould 59 may easily be replaced while the spindle 56 is in the position shown and clear of the first and second chambers 10 and 11.

To enable either of the chambers 12 and 57 to be engaged with the lower surface of the plate 43, the indexing spindle 40 is mounted on a ram unit 60 which comprises an indexing ram 61 which rotates the spindle so as to bring either the flange 30 and chamber 12 or the flange 53 of the chamber 57 into position with respect to the first and second chambers 10 and 11. A second cylinder 62 raises or lowers the spindle 40 so as to engage the upper surface of the chamber 12 or 57 with the remainder of the apparatus.

FIG. 2 shows how a number of these units may be layed out to take advantage of a single vacuum unit and induction coil supply unit, although depending on the pipework etc. required it may be more economic to use a vacuum unit for a smaller plurality of casting units. It will be understood that each vacuum will comprise a plurality of pumps each operating over a different pressure range. It will be seen that a plurality of separate units 70, 71, 72 etc. are mounted in a ring about in this case a single central vacuum unit 80 and induction coil supply unit 81. A sequencing unit 83 is is provided which operates the various valves of each unit in turn and which actuates the vacuum unit and the induction coil supply.

Operation of the system is as follows:

In the condition shown in FIG. 1 a charge crucible is mounted in position in the induction coil 23, and the sequencing unit operates the induction coil supply to initiate its melting cycle. The charge of metal in the crucible is melted in a predetermined fashion so that the entire charge is melted before a fusible plug at the lower extremity of the crucible melts to allow the molten metal to flow out. When this occurs the metal flows under gravity through the open gate valve 13 and into mould 33 which is pre-heated to the melting temperature of the metal by the heating arrangement 32. The amount of metal within the crucible is carefully arranged to be just sufficient to fill the mould 33. Once the mould 33 has been filled, the gate valve 13 is closed by the ram 15, consequently isolating the chamber 10.

The ram 44 is now actuated to retract the plate 45 and consequently to withdraw the mould 33 and chill 34 from the chamber 11. The cam 47 regulates the rate of withdrawal such that the mould is quickly withdrawn from the chamber until a stabilised condition is achieved where the heat flow to and from the mould is balanced, the mould is then slowly withdrawn from the chamber until solidification of the main portion of the casting is achieved, and finally the mould is quickly withdrawn completely from the chamber 11 until the cam 47 comes in contact with the micro-switch 50 which halts retraction of the ram 44. In this way optimum properties may be established for different parts of the casting.

In this position the mould 33 is fully within the chamber 12, and consequently the gate valve 14 may be closed by the ram 16, isolating chamber 11 from chamber 12. The valve 22 is now opened while valve 20 is closed, thus venting the chambers 10 and 12 to atmosphere while leaving the chamber 11 evacuated. When the chamber 12 has reached atmospheric pressure the ram 62 operated to lower the spindle 40, while simultaneously the ram 31 withdraws the flange 30 and the spent crucible 28. When these rams have fully operated the ram 61 rotates the spindle 40 so as to bring a new crucible 54 into position above the chamber 10 and the new chamber 57 and mould 59 in position below the plate 43.

Once again the ram 62 raises the spindle 40 and brings the chamber 57 into sealing engagement with the plate 43, and simultaneously the ram 52 lowers the flange 53 into sealing engagement with the top surface of the chamber 10, positioning the crucible 54 within the induction coil 23.

The valve 22 is now closed and the valve 20 opened, evacuating chambers 10 and 57. When this evacuation is complete a pressure sensitive switch causes rams 15 and 16 to open gate valves 13 and 14, putting chambers 10, 11 and 57 into communication with one another. The ram 44 now operates to raise the spindle 56 and consequently the chill 58 and mould 59 into position.

The raising of the mould is again arranged to be at a non-constant rate calculated to avoid thermal shock damage to the mould while giving optimum speed of operation.

It should be noted that various modifications to the apparatus described may be desirable. Thus for instance any or all of the chambers 10, 11 and 12 may be water cooled either by built-in cooling ducts or by soldered-on pipes through which cooling water may flow. Again the charge melting arrangement may differ from that described; for instance a tipping crucible may be used to melt and pour the charge.

Furthermore, it may be desirable to alter the mechanisms used to raise and lower the charging and withdrawal arrangements, for instance electrical motors and chain drives rather than hydraulic drives may be used and it may be possible to use a single ram in place of the rams 61 and 62.

Claims

We claim:

1. Apparatus for casting comprising first, second and third adjacent chambers each of which may be sealed and evacuated, the chambers being mounted one above another with the first chamber uppermost, first and second valve means dividing said first chamber from said second chamber and said second chamber from said third chamber respectively, a charge melting arrangement in said first chamber adapted to melt metal and discharge the molten metal through said first valve means into a mould supported within saidsecond chamber during said discharge, a mould heating furnace in said second chamber adapted to heat said mould, and a movable chill support adapted to carry a chill and said mould between said second and said third chambers wherein said mould is introduced into said heating furnace and removed when charged from said second to said third chamber so that a directional grain structure may be formed in the casting.

2. Apparatus as claimed in claim 1 further comprising charging means for the charge melting arrangement which comprises a flange member adapted to carry a crucible which holds the new charge, the first chamber having an aperture which is sealed by the flange when the crucible is in position in the charge melting arrangement.

3. Apparatus as claimed in claim 1 further comprising separate duct mean connected to said first, second and third chambers for evacuation of these chambers.

4. Apparatus as claimed in claim 3 further comprising control means which operate said valves in sequence so as to maintain at all times during the operation a vacuum within the second chamber.

5. Apparatus as claimed in claim 1 further comprising withdrawal means adapted to withdraw the charged mould from the second chamber at a non-constant predetermined rate.

6. Apparatus as claimed in claim 1 further comprising a single vacuum pumping unit and a single power supply for the bottom pouring equipment which service a plurality of the chamber combinations.

7. Apparatus as claimed in claim 1 further comprising a pair of charge holding crucibles and flanges and a pair of said third chambers and chill supports, said crucibles and third chambers being movable together in a reciprocatory manner between a position in which one of the charge holding crucibles is located above the first chamber and one of the third chambers is located below the second chamber and a position in which the said one of the charge holding crucibles is remote from the first chamber and the said one of the third chambers is remote from the second chamber while the other one of the charge holding crucibles and the other one of the third chambers are positioned adjacent the first and second chambers respectively.

8. Apparatus as claimed in claim 7 in which the pair of charge holding crucibles and the pair of third chambers are mounted on a spindle which can be rotated through an arc between the said two positions.

9. Apparatus as claimed in claim 7 in which the said third chambers are alternately sealed against the lower surface of the second chamber by upward movement of the third chamber.