GB2275054A - Tungsten articles and method for making them - Google Patents

Abstract

Relatively thin articles of tungsten e.g. caps 22 for copper electrodes 12, 14 of a xenon arc discharge lamb 10 can be made by forging the articles from tungsten sponge. The forging can be carried out at a single face or at a pair of opposed faces at a temperature of 1800 degrees C in a protective atmosphere. The thickness of the article may be not more than 1.0 mm. The article may be incorporated by an investment casting process into a larger body. As an intermediate stage in the investment casting process the shaped article is incorporated into a fired shell mold to be filled with copper or other material. <IMAGE>

Description

TUNGSTEN ARTICLES AND METHOD FOR MAKING THEM The present invention relates to a method for making thin articles of tungsten by a forging process. It also relates to an investment casting process in which the aforesaid forged article can be used.

In one aspect the invention is concerned with a process for forging a part direct from sintered tungsten sponge taking advantage of the fact that in this process a thin layer of the sintered tungsten adjacent to the punch, but a layer of useful depth, will become adequately dense. The slug will in practice be large enough to allow and contain flow of metal around the punch without fracture, but need not contain any amount of material beyond that and whose removal would increase costs. This process enables any preselected first face of the article to be forged, but if desired a second face opposite the first face can be forged to produce a fully compacted article, optionally after machining to remove surplus material and improve the structure.A particular benefit of this process is that the forged articles do not assume the highly directional grain structure more usually associated with tungsten.

In a second aspect the invention is concerned with an investment casting process into which a forged article as aforesaid becomes part of an investment casting shell" so that in a subsequent casting operation the tungsten part becomes integrally bonded with a second metal or other material of the casting e.g. copper.

In the fabrication of tungsten tipped copper electrodes for use in arc lamps or the like this avoids a separate brazing operation and the consequent disadvantage of a low melting point interlayer and much subseqent machining. The casting operation can be carried out by placing the "shell" or mold with copper or other second metal into a hydrogen furnace heated to the melting temperature of the copper or other metal, and can give rise to a very pure product.

Various embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic sectional view of an electrical arc lamp having electrode tips according to the invention; Figure 2 is a fragmentary part sectional view of the tip regions of the electrodes of figure 1; Figures 3A to 3E are diagrams illustrating the successive stages in the formation of an electrode cap or tip; Figures 4A to 4F show successive stages in the process for making an electrode having a cap or tip according to the invention; and Figure 5 is a vertical section of a pair of electrodes of a second kind.

In figure 1 there is shown a xenon arc lamp 10 having a cathode 12 and an anode 14. The arc formed between the electrodes 12, 14 is approximately at the focus of a reflector 16 so that the light emerges as a generally parallel beam through a window 20. The cathode 12 is supported at the required small spacing from and in axial alignment with the anode 14 by means of three or more fins 18. The electrodes 12, 14 generally take the form of copper rods and these are formed with tungsten caps or tips 22 in the region where the arc is formed.

The use of tungsten as a material for arc lamp electrodes is known. Solid tungsten may be used with or without additives, for example The2, CeO2, LaO2, etc. The electrodes may be made from a solid rod of tungsten, but these electrodes can exhibit tip splitting as a result of poor on axis grain structure which can give rise to meltback, erosion and instability of the arc produced. The electrodes can be difficult to cool because tungsten has a relatively low thermal conductivity, and cooling can be particularly troublesome with small high power arc lamps.

Arc lamp electrodes in the form of a rod of copper having a tungsten tip of significant thickness e.g.

about 3 mm which provides a face in the region where the arc is formed are already known. However, where the thickness of tungsten is a multiple of the arc length and the lamp is of significant power, the behaviour of the tip is in all significant respects the same as a rod of solid tungsten and it is difficult to cool because of the poor thermal conductivity of tungsten.

We have conducted experiments on a cap of thin tungsten i.e. less than 1 mm thickness which when in face-to-face contact with a good electrical and thermal conductor such as copper could offer superior lamp performance and in particular life. A preformed tungsten cap of the required thinness can be attached to the end of a copper rod by grinding or spark eroding a hollow into the end of a tungsten rod and then brazing copper into the hollow, and grinding the tungsten back to the required thickness. However, this does not solve the problem that the tungsten rod has an unfavourable grain structure which still leaves the above mentioned problems unsolved.

A further possibility is to form a tungsten cap from sheet tungsten by spinning and then attaching the cap to the copper rod, but this method still has a number of problems. The variety of tungsten sheets with different additives which are available is restricted, and the spinning operation still leaves the sheet with an undesirable grain structure that is prone to splitting. The cap could be forged from fully dense tungsten bar stock. This would be a costly and difficult method of making a cap, and residual grain structure in the bar which is carried over into the cap at the time when the cap is made may also give rise to problems. In the present invention it is proposed to make a cap, or other relatively thin relatively small component from sintered porous tungsten sponge by: (a) Forging a male cone into a piece of sponge.This creates a very dense layer over the surface of the cone, and allows the shape or configuration of a reference surface to be incorporated if required.

(b) Removing excess metal; and (c) Forging the cap between male/female cone tools to improve the density and final form an outer face for the cap.

In figure 3 there is shown diagrammatically a method for forming a cap of tungsten from sintered porous tungsten. The tungsten is forged in a die 30 having a cylindrical region 31 and a gently tapering region 32 at the base of which there is an ejector 33.

The gently tapering region 32, which is preferred to be present in the die, provides an external compression to the slug of tungsten sponge during forging to help reduce splitting of the slug. A tungsten sponge 35 is forged to define the normally concealed surface of the cap by means of a punch 37.

The tungsten sponge 35 is a sintered material having a density typically 60 to 85% of its final value, and the forging is carried out at an elevated temperature, typically about 18000C and in a protective atmosphere (Hz). The result of the forging operation is to produce a compacted zone of depth about 0.5 mm which is equal to or more than the final thickness of the cap, which in one example is only 0.2 mm and is preferably in the range 50 micrometres to 0.7 mm. The result of the forging operation is to produce a microstructure in which the grains of tungsten lie flat in a plane defined by the surface of the punch and optionally close to right angle to the axis of the slug 35.

The first stage forging 39 is removed from the die 30 and excess tungsten is machined away e.g. by grinding to provide a machined cap of near net shape 41. In a second forging step (which may be omitted as described below) the preformed cap 41 is subjected to a second forging step, again at a temperature of about 18000C in a protective atmosphere between a punch 43 and a female die 45. The result of this second forging operation is to produce a finished cap 47 with both of its surfaces fully dense. The finished cap can then be bonded to a electrode support of copper or other suitable material, and any residual excess tungsten can be machined away. The cap may, of course, be configured to incorporate features which enable it to be controlled within a jig during the time when the cap is bonded to the copper or other electrode.

In the foregoing method, it will be appreciated that the roughly formed cap may be attached to the electrode support at the steps shown by figures 3B or 3C provided that the fully dense region is sufficiently deep. It may then be finish machined without a second forging operation.

The shape of the cap and the form of electrodes which are used in figure 1 is shown in figure 2. The cathode 12 has a cap e.g. of diameter about 5 mm and of cone angle of 450 and with an end flat of diameter 0.42 mm and a thickness 0.20 mm bonded to a copper rod wiiose diameter is no less than to the radius of the cap e.g. about 10 mm. Because the tungsten cap is relatively thin and is in intimate heat-transmitting contact with the copper rod to which it is attached, heat is rapidly and effectively removed from the region where the arc is struck. The side surface of the copper rod may be formed with a relatively deep grooves or threads 45 which assist convective heat transfer from the copper rod to the gas within the lamp. This heat transfer increases the convective flow within the lamp and reduces the temperature of the gas which is involved in that convective flow.As a result the tendency for development of regions within the lamp in which the gas has different refractive index is reduced, and the tendency to instability in the light output of the lamp is correspondingly reduced. Such considerations are important where a stable well-defined arc is vital, for example where the lamp is being used in a projector. The structure of the anode 14 and its cap is the same as that of the cathode 12 except that at the anode 14 the cap has a cone angle of 200.

The advantages of the above described cap structure and manufacturing method are: (i) A wide range of tungsten "alloys" can be used since there are many grades of tungsten incorporating different additives which are available in the form of sintered sponge.

(ii) The grain orientation will be along the surface of the cones, so that the grains are not directed along any plane where splitting is liable to occur.

This reduces deterioration of the electrode tips and can give rise to better and more stable lamp characteristics.

(iii) The second forging provides a cap having a very smooth fully dense outer surface for the cap with the grains lying along the surface.

(iv) The caps are economical to manufacture and have reproducable properties.

(v) The caps do not need to have circular symmetry, which permits a range of different designs to be used, especially when the copper backing is applied by investment casting as described below.

The process shown in figures 4A to 4F is in this instance carried out on a first stage forging as shown in figure 3B though it could, of course, also be carried out using later stage forgings or on other samples made by grinding or by spark erosion. That forging 39 is placed at the base of a mold cavity for which the forging 39 forms a plug. The the interior surface of the mold cavity 50, presents a shape corresponding to that of the intended electrode 12, 14.

In a first molding operation wax is poured into the cavity 50 and the resulting molded body is removed from the mold to provide a composite body in which a wax pattern 52 is joined to the forging 39. The composite body is coated with a layer of refractory material 54 (fig 4C) and dried, after which the wax is melted and poured out from the cavity and the refractory body is fired to produce composite mold comprising a hardened shell 56 (fig 4D) derived from the refractory material and the forging 39 which is to be incorporated into the final product. Typical firing conditions may be at a temperature of 8000 C in a hydrogen atmosphere. The resulting mold is then filled with a metal which will bond to the tungsten.

Filling is either by pouring from a crucible or by introducing pieces of the metal into the mold and heating to the melting temperature of the metal. The shell 56 is then removed (fig 4F) to leave an electrode body 12 of copper or other suitable conductive metal with the forging 39 to form the cap bonded to it. The excess of tungsten is then machined away to give a finished electrode.

Figure 5 shows a further version of the electrodes in which the tungsten tips 51, 53 on the anode and cathode 12, 14 progressively increase in thickness with distance from the tip. It is considered that this shape, where the tungsten cap covers the whole end face of the electrode, will be easier to mold and the increased rim thickness will facilitate forging the subsequent handling of the cap without deteriorating heat conductivity across the cap in the region of the arc.

It will be appreciated that the ideas underlying the invention can also be applied to other types of arc lamp, for example those with a spherical quartz envelope given appropriate design value and choices for factors such as electrode temperature and cooling pattern.

Claims (13)

1. A method for making an article of sintered porous tungsten which comprises forging the article from a body of tungsten sponge.

2. A method according to claim 1, wherein the article has first and second mutually opposed faces which are forged in successive forging steps.

3. A method according to claim 1 or 2, wherein the finished forged article has a thickness of not more than 1.0 mm.

4. A method according to claim 3, wherein the finish forged article is of thickness 50 micrometres to 0.7 mm.

5. A method according to any preceding claim, when used to make a cap for an arc lamp electrode.

6. A method according to any preceding claim, wherein in further steps the article is formed into and becomes part of a mold for use in an investment casting process, and the mold is filled with a second material which is molded to shape and caused to become bonded to the first material.

7. A preform which is an intermediate stage in an investment casting process, wherein a shaped article of refractory material is incorporated into a fired shell mold to be filled with a second material to be united with the tungsten article during the molding process.

8. A preform according to claim 7, wherein the refractory material is tungsten or tungsten carbide.

9. A preform according to claim 7, wherein the refractory material is a forging of sintered tungsten sponge.

10. A preform according to claim 7, wherein the refractory material is tungsten rod which may have been shaped by grinding or spark erosion.

11. Arc lamp electrodes substantially as hereinbefore described with reference to and as illustrated in Figures 2 and 5 of the accompanying drawings.

12. A forging process substantially as described with reference to and as illustrated in Figures 3A-3E of the accompanyig drawings.

13. An investment casting process substantially as described with reference to and as shown in Figure 4A - 4F.