CA1080426A - Apparatus and process for the centrifugal casting of metals - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention provides a process for melting and centrifugal casting of metals in apparatus which comprises rotatable shell having an axial feed tube communicating with the interior of an airtight chamber containing a furnace for melting the metal, said process comprising melting metal in said furnace, maintaining said chamber and said shell constantly under vacuum throughout said melting process moving said shell and said chamber relatively to each other to bring said feed tube from a position outside the molten metal in said furnace to a position immersed in said molten metal, admitting a gas neutral to the metal into said chamber to create a pressure difference between said chamber and said shell, and rotating said shell.

Description

8~2~i The present invention relates to a process for the melting and casting of metals or alloys under vacuum, and it principally applicable to very oxidisable metals. A new integrat-ed device is proposed for carrying out the process.
French Patent No: 1,587,187 describes a process for- -bottom feeding a centrifugal casting mould intended for the casting of solid metal parts.
French Patent No: 1,587,403 describes a centrifugal casting process which allows the mould to be filled without the appearance of eddies for components having a circular bore.
The French Patent application deposited under the number 69~28,972 on the 25th August 1969 describes a process which allows the heating of a central cavity of a mould during centrifugal casting. The metal is thus maintained at a suitable temperature to ensure progressive feeding into the shrinkage hole which forms during solidification. The quality of the components manufactured from oxidisable metals is improved by subjecting the mould to a constant degasification during the entire casting process.
There are therefore known methods of bottom "pouring"
centrifugal casting, without turbulence, by supplying the molten metal under vacuum into a centrifuging shell. However, in all the previousiy proposed methods the metal is brought to its melting point outside the mould and then transferred into a feed device of the centrifuging machine.

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lhe present in~en~io~ is irLte~ded to improve the ~nowrl procedures for centrifugal casting with a view to pre~enting the formation of oxide or other unwanted chemical compounds duri~g the meltin~ a~d casting o~ the metal. This is particularl~ useful in the case o~ metalswhich are ver~
oxidisable or which combine easily with elements other than oxygen.
~ccording to this inve~tion in a process for meltin~
and ce~trifugal casting o~ metals or alloys~ in apparatu~ . :
which comprises a rotatable shell having an axial feed channel in communication with the interior of an airtight chamber containing a furnace for melting the metal or allo~
in which the chamber and the shell are kept constantly under vacuum throughout the melting of the metal or alloy, the furnace and the shell are moved relatively to each other to bring the feed channel from a position outside the molteh metal to a position immersed in the molten metal~ and a gas neutral to the metal is admitted into the chamber to create a pressure differe~ce between the chamber and the shell. ~hus ?0 melting takes place entirel~ under vacuum, which a~oids the formation of oxides or other chemical compounds. I~ addition~
since the feed channel is in its raised position duri~g the melting phase~ the shell and the bath of metal are not in communication, so th~t the bottom pouring does not take place

2~ during the raising of the temperature of the metal~
According to ~nother pre~erred feature of the invention, regulation is carried out o~ the speed at which the liquid metal rises into the shell~ which is at first motionless and then set in rotation. In order to do this, there is a ~; ~ 3 , .
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controlled amoun~ of ~as introduced into the chamber containin~
the furnace. Since the centrifuging shell is maintained under vacuum, and fed by a channel leading into a bath of metal it will be understood that the speed at which the metal rises in this channel will vary with the height between the upper shell and the level of metal in the lower f~rnace, with the density of the metal or the alloy being cast and with the difference in pressure between the upper shell and the lower chamber. By regulating the flow of gas into the fixed airtight chamber, it is possible to control the speed at which the liquid metal flows from the furnace into the centrifuging shell. In particular, this control may be used to feed, progressively the centrifugal component and particularly to feed eddies which have a tendency to form on the component during solidification.
Preferably therefore when the molten metal reaches a pouring temperature, the furnace is raised so that the lower end of the feed channel is immersed in the bath of metal contained in the furnace which is then maintained at the temperature required for centrifugal casting. It is also preferred to ;20 regulate the gas introduced into the chamber containing the furnace to control the fIow of the liquid metal into the then revoIving shell. It is preferred to exercise this control so that the speed of flow of the liquid metal from the furnace into the shell is varied in such a way as to progressively feed molten metal into shrinkage holes which tend to form in the cast component during solidificat~ion.
Preferably the interior of the shell is submit~ed to constant degasification during the entire time of melting and then pouring, and centrifuging the metal, so that saturating vapour of the metal at the pouring -temperatur~

is removedO

According to .~nother preferred fea-ture, which is aimed at even further improving the quality of the component produced, the shell is submitted to a constant desgasification throughout the casting process, principally in order to remove saturating vapour from the metal or the alloy at the casting temperatureO
~he carrying out o~ the procedure in accordance with the invention is principally intended for the manufacture of a~nular or tubular parts from very oxidisable metal which it is difficult to cast in the presence of air, such as titanium, magnesium a~d their alloys~
According to a~other preferred feature of the invention, apparatus for carrying out the process of melting under vacuum and centrifugal casting comprises a centrifuging shell with a depending central feed tube and an airtight chamber containing a furnace for the ~elting of metal, the airtight chamber being arranged co-axially w~th and below the shell and the central feed tube communicating with the i~-terior o~ the chamber, an air-tight seal between the stationary components of the apparatus a~d the moving components being ensured b~ revolvin~ seals including a first revolving seal effective between the feed tube and the airtight chamber, and a second revolving seal effective between the centrifuging shell and a valve connected to a ~acuum pump~ Further the apparatus preferably comprises a safety system ~or the airtightness of the chamber, this system being constituted by a ca~ enclosing the first revolving seal a~d part of the fced tube, there being a third re~olving seal between the casing and the tube, a~d the casing being filled under ~ 5 --;
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pres~ure with a gas inert in relation to the liquid metal~
so that a leak in the firs-t revol~ing seal would allow the introduction into the chamber only of the inert gas from inside this ca~ingO
~he characteristics of the inver;tion ~ill be better understood from the following descriptio~ of apparatus and its method of use in accordance with the invention, which description is given by way~of ex~mple only, with reference to the accompanying drawings in which:-~igure 1 is a vertical section throu~h a device for achieving the melting and c^ntrifugal casting of metal under vacuum~
~igure 2 is a view similar to Figure 1 showin~ the position of the parts during the supply of molten metal to an upper shell or mould, ~igure 3 is a veiw similar to Figure 2 illustrati~g the pouring of a centrifuged tubular component, ~igure 4 shows in vertical cross-section a safe-ty device : for ensuring air-tightness between a lower chamber and a revolving shelll and ~igure 5 is a graph illustrating how the process according to the invention allows the regulation of the speed at which the metàl flows into the mould~
~here is shown in the drawings a vertical shell 1 which may be rotated by known means (notlshown) about a vertical axis 2~ ~he internal cylindrical wall 3 of this shell defines a ~olume 7. A hollow portion 4 integral with the shell 1 ex*ends vertically below and coaxial with the shell 1~ A hollow shaft 8 havin~ a reduced diameter lower portion 8a defines a channel 9 which is an axial extension of the portion 4 a~d puts the ~ centrifugi~g vol.ume 7 of the shell 1 into communicatio~ with a : - 6 ' ;, . . - ~ . :

~L~B0426 lower chamber 6. ~his chamber contains a furnace which in this particular instr~nce i~ shown as GOmprising a crucible 10 made of refractory m~terial carried by a support 11 which is mobile in a vertical directionO ~his crucible is provided with a known type of heating system~ such for example a~ the coils 12 of an induction heaterD Its upper part is closed by a cover 1~ in which a central opening 14 is provided to allow the introductio~ into the chamber 6 of the lower end 15 of the vertical shaft 80 A gas inlet pipe 16a which includes a valve 16, co~nects the i~terior of the chamber 6 with a gas source (not shown~ containing a gas neutral to the metal or alloy to be melted, and a~ exhaust pipe 17a which include~ a valve 17, connects the interior o~ the chamber 6 with a vacuum pump ~not shown)0 '~his inlet and exhau~t s~stem permits the creation o~ a vacuum inside the chr~mber 6 or the introductio~ into it of a gas under con~rolled pressure.
me shell 1, hollow portion ~ and the tube 8 are adapted to revolve, whils-t the chamber 6 ls fixed. hir-tightness between the rotating parts and the stationary part of the a~paratus is ensured by a first revolving seal 18 arranged ; between ~n ~nd wall o~ the chnmber 6 and the wall of the tube 8.
h ~cond revolving seal 19 mounted on a cover 20 o~ the ~hell 1 is connected to a vacuum pump (not shown) via a control valve 24. ~inally one or several airtight doors in the wall ; 25 21 of the chamber 6 allow metal or alloy to be introduced into the crucible 10 of the furnace~ ~he crucible assembly is fitted in the chamber 6 at the time of construction, and it is vertically movable by means of a remote control over the support 11 (Figures 1, 2 and 3).
~or carrying out the process according to the invention the apparatus is used as follow~:-,, , ., . ` .
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~ he shell 1 and the hol.low shaft 8 are stationary~ but they communicate -through the seal 18 with the interior of the open chamber 60 'rhe metal or alloy to be melted is introduced through one of the access doors into the chamber 6 and placed in the crucible 10, after which the access door is closed, so that the interior of the chamber 6 is then ~:
sealed from the surrounding atmosphere~ With the gas inlet valve 16 closed~ a vacuum is created in the chamber 6 and in the communicating centrifuging shell 1, by opening the valves 17 and~or 24 and operating the vacuum pumpO '~he remotely controlled support 11 of the ~urnace is kept in its lower pos1tion so ~that the end 15 of the tube 8 is ou-tside the furnace.
When the required degree o~ vacuum is obtained, the ~-means of heating the furnace are energised to raise the temperature of the crucible 10 and to bring the metal contained in it up to its melting point. During the entire operation of melting under vacuum, the vacuum pump or pumps connccted to - the valves 17 and/or 2'~ continue working, whilst the remotely controlled support 11 is kept in its lower position (~igure 1)O During this first phase, onl~ the melting under vacuum of the metal 25 is carried out, the metal not being ir communication with the feed channel 9 to the shell 1~
Second phase. When the me-tal 25 has reached its pouring temperature, the crucible 10 containing the bath of metal is raised vertically by raising the remotely co~trolled suppor.t 11 along a vertical guide, not shown~ 'rhe lower end 15 of the feed tube 8 is then immersed in the liquid metal 25 (see Figure 2).

3~ 3~ he inlet valve 16 is then opened~ allowing the neutral gas to flow into the chamber 60 Since the interior of the shell 7 is under vacuum9 that is at a minim~m pressure . PO~ the introduction of the neutral gas through the valve 16 ,!~

~8~ 6 causes the pressure in the chamber 6 to rise to a value P~, greater than P0. ~he di~ference (P1 - Po) in pressure causes the liquid metal 25 to rise into the interior volume 7 of the shell 10 ~hus an operation of vacuum i'pouring" from below is carried outO At the beginning of the "pouring" the shell 1 remains stationaryO '~he metal 25 is kept liquid both in the crucible 10 and in the feed channel 9 which is heated by known methodsO
When this pouring has begun, the shell is revolved around the vertical c~xis 20 ~he rotation of this shell causes the molten metal flowing into the shell -to flow -to the inside of the wall 3 of the shell (as shown in Figure 3) by centrifu~al action, and in this way a tube 30 is moulded agai~st the wall of the revolving shell 1.
It has been shown -that when a certain quantity of gas is introduced into the chamber 6, a pressure difference P1-Po is established~ between the airtight interior volume 6 and the upper airtight volume 70 It is obvious that this difference in pressure varies with the flow of gas through the valves 16,17 and 240 I~ it is supposed that the out-flows through the valves 17 and 24 remain constant, the speed at which the metal rises through the channel 9 into the shell 1 will be a function of the flow of gas through the inlet valve 16. ~his flow may be ~egulated so as to control the supply -to the centrifuged . 25 component 30. In particular, at the end of the "pouring", it is possible to supply molten metal progressively into the shrinkage holes which form during the solidification of the centrifuged tube 309 particularly for non-eutectic alloys~
or those with a long solidification period.
~; 3o me pressure difference necessary between the chamber 6 and the interior of the shell for a given rising speed of the : - 9 ;. ' `
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metal~ or ~or machi~es with diffcrent heights~ increases with the density of the metal or alloy to be pourcd~ and with the distance separating the bath of metal in the crucible 10 from the centrifuging shell 1.
In Figure 5, the velocity V of the molten metal flowing upwards into the shell 1 is plotted against three possible bases~ namely the pressure differential P = (P1 Po)i the densit~ d of the metal being cast7 and the level di~erence H ;~
between the ~hell 1 c~d the crucible 10~ ~hc curve A shows ~`
the variation of the rising speed V with the pressure differential P; the curves ~ and C indicate respectively thc variation o~ this speed V with the densi-t~ d of the metal or with the difference in lcvel Ho Since -the closed centrifuging shell 1 is airtight, it is sufficient in theory to create a vacuum through the exhaust pipe 17a and the valve 170 Thus in a simplified construction, the revolving seal 19 and the valve 24 are no-t required~ and ~ :
~ the operation remains the same as previously described.
; Centrifugal casting under vacuum is carried ou-t by ensuring that the lower end 15 of the tube 8 remains constantl~ immersed - in the liquid metal contained in the crucible of the furnace, which is not completely emp-tied during the casting processO
However, the revolving seal 19 and the extraction valve 24 are advantageous~ and in particular allow -the sa-turating ; 25 vapour from the metal at the pouring temperature to be removed from the shell 1~
~or some processes particularly ~hen casting metals or alloys such as titanium allo~s which cannot be melted in the presence of air i-t is desirable to use a safety device 3 against leaks through the revolving seal 18 ~ shown in Figure 40 This device comprises c~n airtight chamber 31 attached to the top of the chamber 6 and effective between _ 10 -., '.
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the seal 18 ~d a supplemen-tary ~seal 32 surrounding -the tube 80 ~his chc~mber 31 is fitted with c~n entry pipe 33 controlled by a valve 34 and with c~n ou-tlet pipe ~5 controlled by a valve 360 The chc~mber 31 is filled, at a pressure P~ with a gas inert in relation to the metal or alloy being meltedO The melting and the centrifugal casting are processes exactly the same as those previously describedO

~he safety device opercates in the following manner:-If the seal 18 de~elopes a leak~ only -the neutral gas con-tained in the chamber 31 penetrates into the chamber 6, from which it is evacuated through the valve 170 If the seals 18 and 32 leak simultaneously, the mixture of gas penetrating into the chamber would be composed of a neutral gas carrying only a very small percentage of airn ~he opening of the ~alve 16 may be programmed in : relation to -the bath of metal to be "poured" in such a w~y as to control the speed of rising of this bath by regulation of the pressure in the chamber. It will be appreciated that the process m~y be used for centrifugal casting of ~:~
all kinds of metals and alloys, including light alloys or . very oxidisable metals~
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Claims (6)

1. A process for melting and centrifugal casting of metals in apparatus which comprises a rotatable shell having an axial feed tube communicating with the interior of an airtight chamber containing a furnace for melting the metal, said process comprising melting metal in said furnace, maintaining said chamber and said shell constantly under vacuum throughout said melting process moving said shell and said chamber relatively to each other to bring said feed tube from a position outside the molten metal in said furnace to a position immersed in said molten metal, admitting a gas neutral to the metal into said chamber to create a pressure difference between said chamber and said shell, and rotating said shell.

2. A process according to Claim 1, wherein when said molten metal attains a pouring temperature, said furnace is raised so that a lower end of said feed tube is immersed in said molten metal, said molten metal being thereafter maintained at a temperature required for centrifugal casting.

3. A process according to Claim 1, wherein the admission of said gas is regulated to control the rate of flow of molten metal into said shell.

4. A process according to Claim 1, wherein the interior of said shell is subjected to degasification during the melting and centrifuging steps whereby saturating vapour is removed from said molten metal.

5. Apparatus for vacuum melting and centrifugal casting of metal comprising a centrifuging shell; an exhaust passage leading from said shell; a feed tube depending from and coaxial with said shell; an airtight chamber below said shell, said feed tube extending down into said airtight chamber, whereby the interior of said chamber is in communication with the interior of said shell through said feed tube; a furnace situated within said chamber; a first revolving seal effective between said feed tube and said chamber and a second revolving seal effective between said shell and said exhaust passage.

6. Apparatus according to Claim 5, comprising a safety casing enclosing said first revolving seal and part of said feed channel, and a third revolving seal effective between said casing and said feed tube.