BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention generally involves the centrifugal casting of molten metal. More specifically, 5 the invention relates to an improvement in the casting of molten metal in a rotating mold wherein the molten metal is derived from an arc -gap between a pair of oppos- sing electrodes, at least one of which is formed of the metal to be cast.

^{10 2} _* Description of the Prior Art

It is known to cast metal, particularly the production of fine grain castings from prealloyed metals, wherein the metal to be cast is configured as at least of a pair of opposing electrodes which, upon the applica-

15 tion of electrical energy thereto, strike an arc there¬ between to provide the heat necessary for melting the consumable electrode. The molten metal droplets developed at the arc gap are thereafter cast within a rotating mold. The electrodes are conventionally each of a

20 corresponding cylindrical configuration and supported for rotation relative to each other about a common longitudi¬ nal axis. The centrifugal force developed, by the rotat¬ ing mold causes the molten metal received therein to be solidified into the desired casting. 5 Casting systems of this type have typically been

utilized for the production of fine grain castings which are intended for subsequent workings so as to impart desired strength and other strucutral properties to the final product. The presence of a fine crystalline structure greatly facilitates subsequent working of the casting, such as through forging and related mechanical procedures. It is therefore the primary aim of these conventional systems to produce castings wherein the rate of solidification of the molten metal from its origina- tion at the arc gap to its final destination in the mold is as rapid as possible in order to produce the desired fine grain structure.

However, these known systems have been charac¬ terized by certain disadvantages. For example, the elec- trodes are extremely heavy and the supporting of same for rotation is difficult because the electrodes have a tendancy to wobble about the end ,of their support stubs. Moreover, it is very difficult to realize a uniform de¬ pletion of the consumable electrode or electrodes as the electrodes are advanced towards one another to maintain a constant arc gap during melting.' Though these systems are generally capable of producing fine grain castings, it is this characteristic which also precludes the pro¬ duction of a sound casting having a uniform structure throughout the casting. This is because rapid solidifica¬ tion is required for the realization of a fine grain struc¬ ture and this causes the metal droplets to accumulate and solidify in the mold in clustered form, thus resulting in an unsound cast product.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved method of centrifugal casting through which sound castings having a uniform structure are produced. It is another object of the invention to pro¬ vide an improved method of centrifugal casting utilizing at least one consumable electrode of the metal being

cast wherein uniform depletion of the consumable electrode is realized.

The foregoing and other objects of the invention are achieved ^* by providing a method of casting wherein at least one of the pair of opposing electrodes serving to define an arc gap therebetween is made of the metal to be cast. The electrodes are carried on the ends of stubs supported for rotation about a common longitudinal. axis within a vacuum chamber. The electrodes are disposed eccentrically on their respective support stubs so that upon application of current to the electrodes and the rotation of same, preferably in the same direction and at the same rate, shall cause a uniform melting and de¬ pletion of the at least one consumable electrode. A circular mold is disposed around the arc gap and is rotated so that centrifugal force causes the molten metal droplets developed at the arc gap to be uniformly deposi¬ ted and accumulated in layers within the mold and seque- tially solidified to form the cast product. The position of the arc gap is longitudinally reciprocated by shifting the electrodes together through the use of hydraulic rams in order to provide a uniform deposition of molten metal in the mold. Sufficient current is applied to the elec¬ trodes in order to maintain the molten droplets in a" molten state until they have been deposited and smoothly layered within the mold, thereby resulting in a sound casting that is structurally homogeneous and built up from layers of molten metal.

Other objects, features and advantages of the invention shall become apparent from the following detailed description of preferred embodiments thereof, with refer¬ ence to the accompanying drawings which form a part of the specification, wherein like reference characters designate corresponding parts of the several views.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a plan view of an apparatus used to practice a preferred embodiment of the present invention.

shown with a car used to support a movable section of the vacuum chamber being displaced to a position wherein the movable section normally supported thereon would be

_» detached from the vacuum chamber for the purpose of gaining access to the interior thereof;

Figure 2 is a side elevational view of the apparatus of Figure 1;

Figure 3 is an enlarged vertical sectional view taken through the -apparatus of Figure 1; Figure 4 is an enlarged transverse sectional view taken on the. line 4-4 of Figure 3; and

Figure 5 is an enlarged fragmentary vertical sectional view, taken on the line 5-5 of Figure 2, show¬ ing details of the coupling between the electrode shaft and hydraulic ram shaft.

DETAILED DESCRIPTION OF THE{ PREFERRED EMBODIMENTS

An apparatus 1 for the practice of the present invention according to a preferred embodiment thereof * shall now be described with reference to Figs. 1 and 2. Apparatus 1 includes a vacu'um chamber 3 comprised of a stationary section 5 and a movable section 7 which are detachably secured together through a flanged seal assem¬ bly 9. Seal assembly 9 may be of any conventional type well known in the art and deemed suitable for the practice of the invention as described herein.

Movable section 7 is supported on a car 11 which in turn is supported for lateral movement on a pair of rails 13. This arrangement permits movable sec¬ tion 7 to be laterally ^' displaced away from stationary section 5 upon disconnection of seal assembly 9, as schematically depicted in the two positions of car 11 as shown in Fig. 1. It is also preferable that car 11 be provided with a turntable 15 to permit rotation of movable section 7 thereon and thereby facilitate its connection and disconnection with respect to stationary section 5. Chamber 3 is also provided with an outlet 17 for connection to a vacuum pump (not shown) for the

purpose of evacuating the interior of chamber 3. As shown in Fig. 2, stationary section 5 is rigidly secured by a pair of support frames 19 and 21, the structural

_» configurations of which may be of any type that is con- ventionally known in the art for this purpose.

Stationary section 5 is provided with an elec¬ trode shaft 23 that is preferably sectioned and secured together through a disconnect coupling 25. One end of shaft 23 extends longitudinally into stationary section 5 and is supported for rotation through a bearing seal assembly.27. The other end of shaft 23 is supported for rotation through a commutator 29 and a support bearing 31 both of which are in turn carried by a support frame 33. Commutator 29 is provided with an electrical connection line 35 for receiving current from an appropriate source (not shown). The .terminal end of shaft 23 extends out¬ wardly from support bearing 31 and is attached to a ram shaft 37 forming a* part of a hydraulic ram (not shown) for imparting longitudinal movement to shaft 23 with respect to section 5.

Likewise, movable section 7 is" also provided with a bearing seal assembly 39 for rotatably supporting an electrode shaft 41, the latter being preferably sectioned and secured together by a disconnect coupling 43. Shaft 41 extends longitudinally into the interior of section 7 and is further provided with an exterior end that is supported for rotation by a commutator 45 and a support bearing 47, both of which are in turn carried by a support frame 49. Commutator 45 is provided with an electrical connection line 51 for receiving current from an appropriate source (not shown). The terminal ^v end of shaft 41 extending outwardly from support bearing 47 is attached to a ram shaft 53 forming a part of the hydrau¬ lic ram (not shown) for imparting longitudinal movement to shaft 41 with respect to section 7.

The internal details of chamber 3 shall now be described with reference to Fig. 3, As shown therein, bearing seal assembly 27 includes a ball bearing assembly

55, an inner electrical insulating sleeve 57 and an outer support sleeve 59, all of which are disposed within and secured to an external casing 61 by a ring clip 62. Assembly 27 functions to support electrode shaft 23 for 5 both rotary and ^" longitudinal movements with respect to stationary section 5, and also provide a vacuum seal to permit evacuation of chamber 3.

Similarly, bearing seal assembly 39 associated with movable section 7 is also provided with a corres- 0 ponding ball bearing assembly 63, an inner electrical insulating sleeve 65 and an outer support sleeve 67, all of which are secured within an external casing ^* 69 by a ring clip 70. Electrode shaft 41 is also supported for both rotary and longitudinal movements with respect to 5 movable section 7 by assembly 39, the latter further serving to provide an appropriate vacuum seal for chamber 3.

Electrode shafts 23 and 41 are each secured at their internal ends within chamber 3 to a pair of stub 0 shafts 71 and 73, respectively. Shafts 23 and 71 are connected by a disconnect coupling 75 and a similar dis¬ connect coupling 77 is utilized for connecting shafts 41 and 73. Stub shafts 71 and 73 are also preferably provided with a pair of stop clamps 79 and 81, res-

25 pectively, for the purpose of limiting their longitudinal movement within chamber 3.

Stub shaft 71 is carried within a stub cage 83 which is in turn supported for rotation by a plurality of opposed idler rollers 85 and driven rollers 87.

30 Rollers 85 and 87 are supported by an upper roller cage 89 and a lower roller cage 91, respectively. Rollers 87 are driven by a motor 93 through a chain drive 95 for the purpose of imparting rotation to stub cage 83. Likewise, stub shaft 73 is also carried within a stub cage

35 97 which is supported for rotation between a plurality of idler rollers 99 and driven rollers 101. Rollers 99 and 101 are supported by an upper roller cage 103 "and a lower roller cage 105, respectively. Rollers 101 are

driven by a motor 107 through a chain drive 109.

The internal end of stub shaft 71 is provided with an elect _.rode 111 and the internal end of stub shaft

73 is also provided with a corresponding opposed elec- trode 113. Electrodes 111 and 113 are preferably of cylindrical configuration and secured to the ends of their corresponding stub shafts 71 and 73 in any manner well known in the art. It is preferable that at least one of electrodes 111 and 113 be made of the metal desired to be cast, and thus consumable during the practice of the invention. The remaining electrode is also prefer- ably of the same metal, but may be of nonconsumable material. It is important to note from Fig. 3 that the longitudinal axes of rotation of stub shafts 71 and 73 are coaxial, but -electrodes 111 and ^' 113 are supported with their longitudinal axes disposed eccentrially tfith respect to the common axis of rotation defined by stub shafts 71 and 73. It is preferred that the amount of offset between the longitudinal axis of each electrode and its corresponding stub shaft be at least four inches. As evident in Fig. 3, an arc gap G is defined by the spacing between the offset and faces of electrodes 111 and 113. The eccentric offset between electrodes 111 and 113 has been found to provide uniform depletion of the at least one consumable electrode or, in the case of both electrodes being consumable, uniform depletion of both electrodes when an arc is struck between electrodes 111 and 113. It has also been found that the eccentric dis¬ positions of electrodes 111 and 113 serve to eliminate end wobble, an undesirable characteristic normally asso¬ ciated with known electrode structures of this type.

When sufficient current is provided electrodes 111 and 113, an arc is struck at gap G, thereby providing intense heat for melting the at least one consumable electrode. As the electrode is consumed during this process, a plurality of molten metal droplets 115 are developed and dropped downwardly to contact a casting surface 117 of a circular mold 119. Casting surface 117

is preferably formed of high temperature resistant refractory material and encased within an outer metal shell 121. Mold 119 is of circular configuration and provided with opposed openings 123 and 125 for receiving electrodes 111 and 113 therein. A plurality of upper -idler rollers 127 carried by an upper roller cage 129 and a plurality of lower driven rollers 131 carried by a lower roller cage 113 support mold 119 for rotation about an axis that is preferably coaxial with the common axis of rotation of stub shafts 71 and 73. Rollers 131 are driven by a motor 135 through an appropriate drive shaft 137.

The details of the connection between ram shaft 37 and electrode shaft 23 shall now be described with reference to Fig. ^" 5. It is understood that this connec¬ tion configuration is exactly the same for ram shaft 53 and its corresponding electrode shaft 41, and therefore applies equally thereto. The terminal end of ram shaft 37 is secured against longitudinal movement to a tapered rolling bearing assembly 139, the latter being in turn secured in position between ram shaft 37 and electrode shaft 23 by an inner ring clip 141 carried by shaft 37 and an outer ring clip 143 carried by shaft 23. An electri¬ cal insulating liner 145 is disposed between bearing assembly 139 and the inner wall of shaft 23. Therefore, movement between shafts 23 and 37 is limited only to the relative rotation of shaft 23 about shaft 37. Because of this configuration, longitudinal movement of shaft 37 through actuation of its associated hydraulic ram causes corresponding longitudinal movement of shaft 23, thereby advancing or retracting electrode 111 relative to elec¬ trode 113.

Referring now to Fig. 4, there is shown the details of stub cage 97 and its associated upper and lower roller cages 103 and 105. It is of course understood that the structural details described for Fig. 4 also equally apply for corresponding stub cage 83 and its associated roller cages 89 and 91. Stub cage 97 supports stub shaft

73 coaxially therein through a plurality of circumfer- entially spaced struts 147 which have their opposite ends rigidly __»secured to the interior wall of a cylindri- cal shell 149 and the exterior wall of a cylindrical inner shell 151. An electrical insulating sleeve 153 is disposed between stub shaft 73 and inner shell 151. It is preferred that plural sets of struts 147 be lon¬ gitudinally spaced along the annular spacing defined by outer and inner shells 149 and 151. As previously described, stub cage 97 is supported for rotation between upper idler rollers 99 and lower driven rollers 101, the driving of the latter rollers being accomplished through electric motor 107 and chain drive 109. The connection between chain drive 109 and rollers 101 is achieved by a pair of sprockets 155, and the connection between chain drive 109 and motor 107 is achieved through a sprocket 157 carried by a drive shaft 159 of motor 107. Idler rollers 99 are carried by roller cage 103 which is in turn rigidly secured to the upper carrier wall of movable section 7. Driven rollers 101 are carried by roller cage 105 which is in turn secured to an opposing lower interior wall portion of movable section 7.

MODE OF OPERATION A preferred mode of practicing the present -invention shall now be described with general reference to the drawings, and with particular reference to Fig. 3. In the production of a uniform and structurally homogen¬ eous casting in mold 119, it is preferred that both elec¬ trodes 111 and 113 be consumable and formed of the metal to be cast. Since chamber 13 may be evacuated to produce a vacuum therein, the casting of highly reactive metals, such as titanium, niobium and zirconium, can be advanta¬ geously conducted in apparatus 1.

Electrodes 111 and 113, formed of the metal to be cast, are attached to their respective stub shafts 71 and 73 to provide an eccentric offset of about four inches between the corresponding longitudinal axes of each

electrode and its corresponding stub shaft. Actuation of the hydraulic rams serve to dispose electrodes 111 and 113 a- desired distance from each other to form an arc gap G

_» defined by the offset eccentric faces of same. Activation of motors 93 and 107 causes stub cages 83 and 97 to rotate, thereby rotating their asso¬ ciated electrodes 111 and 113, preferably in the same direction and, if desired, at different speeds of rotation. Mold 119 is also caused to rotate by electrodes 111 and 113 by the activation of motor 135.

Electrical current is then supplied to commutators

_.

29 and 45 through their respective electrical connection lines 35 and 51, with such current being of sufficient intensity in order to strike an electric arc at arc gap G and cause the depletion of electrodes 111 and 113 into molten droplets 115. It is important that sufficient current- applied to electrodes 111 and 113 to -assure - that molten droplets 115 remain molten until they are in contact with mold surface 117 and form a smooth molten layer thereagainst under centrifugal force prior to soli¬ dification. Once the initial layer of molten metal has solidified against mold surface 117, additional molten droplets 115 are crushed against the initial solidified layer of metal due to the centrifugal force and are in- corporated immediately into the body of the accumulating casting. Solidification of molten droplets 115 should not occur until they have been smoothly layered against the casting to become an integral and structurally homogeneous portion thereof. Through the actuation of the hydraulic rams, arc gap G may be positioned back and forth across the mold surface in order to deposit alternate smooth layers of molten metal in mold 119. The rotation of mold 119 should be maintained at sufficient speed.so that the resulting centrifugal force causes an immediate and uniform layering of molten droplets 115 across the circumferential interior of mold 119 so that, upon solidification, a ring-shaped casting is produced.

Because of the eccentric mounting of electrodes 111 and 113, the adjacent faces thereof realize full over¬ lapping disposition with respect to each other during rotation. This produces an extremely uniform and even depletion of electrodes 111 and 113 during their consump¬ tion. As electrodes 111 and 113 are continuously con¬ sumed, their respective hydraulic rams are actuated to move them longitudinally towards each other so as to always maintain an optimum spacing for arc gap G. An important aspect of the present invention comprises the production of a sound casting which is

_> extremely uniform and homogeneous in structure. This is in contrast to castings produced by heretofore known systems wherein rapid solidification of the molten metal must be effected in order to achieve a fine grain struc¬ ture. Such rapid solidification causes the molten metal droplets developing from the arc gap -to essentially under¬ go solidification prior to their contact with the mold surface. This necessarily produces a nonuniform casting which is characterized by an uneven structure defined by bunches of metal nodules agglomerated together. The primary characteristic desired of a casting produced through the practice of the present invention is that it be structurally uniform without the undesirable charac- teristics associated with rapidly solidified castings. It has been discovered that extremely uniform castings may be produced by applying sufficient current to main¬ tain the molten droplets in a molten form until they have been properly accumulated and layered within the mold prior to the initiation of solidification.

While the invention has been described and illustrated with reference to certain preferred embodi¬ ments and operating parameters, it shall be appreciated that various modifications, changes, additions, omissions and substitutions may be resorted to by those skilled in the art and considered to be within the spirit and scope of the invention and appended claims.