US2003481A - Clad metal and process for producing the same - Google Patents

Description

June 4, 1935. L, F, EHRKE 2,003,481

CLAD METAL AND PRocEss Fon PRoDUcING THE SAME Filed Feb. e; 1932 Aim' INVENTOR L. EHF/(E vprocess for rmly andv permanently Patented `lune 4,v 1935 UNITED STATES PATENT 4OFFICE CLAD METAL AND PROCESS FOR PRODUCING THE SALIE Pennsylvania Application February 6 2 Claims.

This invention relates to clad metals and to a uniting unlike metals. In its more specic aspect the invention is directed to an X-ray anode and to a process for producing 'an X-ray 'anode of high electrical and thermal conductivity, 'having a target rmly embedded therein with the interfaces at the planes of contact of said target, and said anode proper being free from blow holes and pipes and having high thermal and electrical conductivity characteristics.

X-ray tubesin general comprise an enclosing envelope containing a plurality of oppositely disposed electrodes therein. One of the electrodes,

commonly termed the cathode, is ordinarily in the form of a lamentary body adapted to be heated to thermionic temperatures by means of an external source of electrical supply, as for example the secondary Winding of a low voltage transformer. The other electrode of said tube, commonly termed the anode, generally comprises a solid body of copper or other similar high heat and electrical conducting material, having a target, such as tungsten or the like, embedded in the face thereof opposite said cathode.

In order that X-rays of the desired hardness may be produced, there is applied between the incandescent cathode and the cold" anode a voltage of suicient magnitude to cause the electrons to migrate with sufficient velocity from the cathode to said anode. These electrons strike the target to produce X-rays; the hardness of the X-rays so produced depending upon the voltage applied between these electrodes.

In the commercial operation of tubes of this.

character it is frequently necessary that the volt age between the electrode terminals be above 20,000 volts and sometimes as high as 500,000 volts.

It is Well known that when the electrons emanating from the cathode are directed to and strike the target, the target becomes heated. When the heat so developed in said target, due to the electron bombardment thereof, is not readily dissipated in some manner, the target will become overheated and ultimately fused, which is obviously undesirable.

This particular phenomenon has heretofore been appreciated by those in the art, as evidenced by the anode constructions heretofore proposed for the purposes of attempting to provide an anode containing a target, and which has high electrical and heat conducting characteristics. Heretofore a number of processes have been proposed for producing a clad metal and especially 1932, Serial No. 591,353 (Cl. fil-70.2)

a unitary structure comprising a refractory metal embedded in comparatively high thermal' and electrical conductivity metal, such as copper or the like.

One of the methods heretofore proposed for this purpose contemplated uniting a low melting point metal of high electrical and heat conductivities, such as copper, to a relatively high melting point, high resistance metal, such as tungsten. The specific method suggested for uniting these two metals so as to obtain a juncture of good thermal conductivity and `ffirm mechanical char.- acteristics involved the following steps:

. A refractory metal, as for instance tungsten in the form of a button, is placed in a suitable crucible and surrounded with a large mass of copper metal. This crucible containing the refractory metal and copper metal is placed in a reducing furnace containing an atmosphere of hydrogen gas therein. In the presence of this hydrogen, reducing, ambient the temperature of the crucible together with its constituents is elevated to a temperature higher than that necessary to fuse the copper so that it ows over the tungsten and in such a fluid condition that it may wet the same. Thereafter the resultant mass is allowed to cool and the bi-metallic structure is extracted from the crucible.

It has been found that the bi-metallic structure, generally in the form of a slug, produced in this manner, exhibits at least one desirable property, namely that of a strong mechanical union at the joint. This bi-metallic slug, however, although exhibiting this desirable property, is not suitable for all purposes and especially for `X-ray anode purposes, because of the internal ,physical characteristicsof the copper of said slug. Upon shearing the copper of said slug either lengthwise or crosswse it is generally found that its internal structure has a relatively large number of small voids, sometimes simulating a honeycomb and commonly termed blow holes and/or one or more large cavities, commonly referred to as piping.

One other method heretofore proposed for the same general purpose contemplates the substitution of the hydrogen reducing ambient with an ordinary atmospheric ambient. Bi-metallic slugs comprising a refractory metal, as for example tungsten, having its bottom and sides coated with a low melting point metal, as for example copper, vproduced according to this method, have not been satisfactory for X-ray anode purposes, although the copper of said slug is a homogeneous mass substantially free from blow holes or piping.

Although the employment of the latter method overcomes the disadvantages inherent in the method utilizing the hydrogen ambient so as to eliminate the voids or cavities, it is found that the resulting product lacks the advantages of firm mechanical union between the rare refractory metal and the copper which is provided by employing the method using a hydrogen ambient.

It is apparent that in an oxidizing medium .the tungsten will become coatedl with an external thin skin or film of oxide which prevents a complete wetting of the said tungsten at the surfaces of contact between said tungsten and said copper. This is my explanation of the lack of a rm, tenacious, mechanical union between said tungsten and said copper. The oxide film also acts as a heat and electrical insulator to prevent the heat and electrical energy from being very quickly conducted between the tungsten and the copper, the latter of which has good heat capacity and electrical conductivity characteristics.

Still another method proposed for the same general purpose comprises plating -a tungsten button with a metal, such as nickel, iron or Monel metal, then uniting the plated button to a copper backing by supplying sufficient heat and pressure to thoroughly unify the intermediate binding metal with the tungsten and the copper. The purpose of this intermediate binding metal is to aid in the prevention of the loosening of the tungsten button from the copper backing due to the differences of the co-efcients of expansion between the tungsten and copper.

It is obvious that electroplating a metal, such as tungsten, with nickel that at least some portions of the tungsten will become oxidized upon being immersed in the nickel bath and that when nickel is deposited thereon that someof the hydrogen that may be deposited at the cathode will become entrapped between said nickel electro-deposited particles in the course of electro-deposition of said nickel.

This process has other disadvantages among which may be noted the following: The heat conductivity of the nickel is not as great as copper and, therefore, an anode constructed in this manner embodies an extraneous element, nickel, whose conductivity both electrically and thermally is materially below that of copper.

A still further method suggested for the same general purposes comprises rolling platinum on a sheet of copper and brazing the sheet to a conducting mass. This process obviously necessitates the utilization of a soft metal as an X-ray target. Besides being so limited the so constructed anode depends for its union upon a brazing material which includes an alloy of copper and zinc.

When an anode so constructed is employed in an X-ray apparatus the low melting point metal, as for example the zinc, becomes vaporized therefrom and is present within the tube either as a vaporized material or condensation product upon the interior walls of the envelope to disturb the electrical characteristics of the tube. Moreover, because of this general breakdown of said uniting material the target becomes detached from the anode proper.

In order that these diculties as Well as others may be obviated, I have developed the following invention which contemplates the combination of the rst two described processes in a novel and unique manner to provide a clad metal, and

especially one adapted to be employed as the anode of an X-ray tube.

Briefly stated one aspect of my invention comprises coating a refractory metal, such as tungsten or the like, with a relatively thin lm or layer of a low melting point metal, such as copper, in a reducing atmosphere of hydrogen or the like, thereafter adding to this bi-metallic structure a relatively large quantity of a low melting point metal, such as copper, the latter step being carried out in air.

Another aspect of the invention is to provide a clad metal comprising a rare refractory such as tungsten or the like, secured to a low melting point metal, such as copper, the juncture of said refractory and low melting point metal being tenacious and providing a firm mechanical union having high thermal and electrical conductivity characteristics, with the low melting point metallic mass being substantially free from blow holes and pipes.

An object of my invention is to provide an element comprising a refractory metal and a relatively low melting point metal, the low melting point metal being substantially free from blow holes or pipes, the joint between the low melting point metal and the refractory metal being tenacious and of high thermal and electrical characteristics.

Another object of my invention is to provide an element comprising tungsten and copper, the copper being substantially free from blow holes or pipes, the union between said tungsten and copper being mechanically strong and of high thermal and electrical conductivity characteristics.

Another object of my invention is to provide an X-ray anode comprising an element composed of a rare refractory metal and a low melting point metal, the low melting point metal being substantially free from voids or pipes, the juncture between said refractory and low melting point metals being tenacious and of high thermal and electrical conductivity characteristics.

Another object of my invention is to provide an X-ray anode comprising a tungsten plate embedded in a copper backing, said copper backing being substantially free from pipes or blow holes and the union at the interfaces of said metals being mechanically strong and of high thermal and electrical conductivity characteristics.

Another object of my invention is to provide a relatively simple process for producing an element composed of a refractory metal and a low melting point metal, the contact areas of said two metals forming a strong mechanical joint and the low melting point metal being substantially free from blow holes or pipes.

Another object of my invention is to provide a process for producing an element composed of tungsten and copper, the union between the tungsten and copper being mechanically strong and of high thermal and electrical characteristics, the copper being substantially free from blow holes or pipes.

Other objects and advantages of my invention will become readily apparent from the follow description taken in conjunction with the appended drawings wherein Figure 1 represents a vertical cross section, with some of th'e parts in elevation, of a furnace employed in carrying out my invention.

Figure 2 represents a cross sectional view of process; and

- ing material.

supports on the base of said furnace is a crucibleA the product obtained following at one stage'of my DIOCGSSI yFigure 3 represents'a vertical cross section of another furnace employed in carrying out my invention;

Figure 4V represents a cross sectional view o f the productv obtained at a later stage of my Figure 5 represents a vertical cross section with apart thereof in elevation of an X-ray anode embodying my invention.

Although my invention is broadly directed to clad metalsand to the process of producing the same, for the sake of simplicity of description and ease of understanding and because it embodies X-ray anodes and is especially applicable thereto, I will describe my invention as applied to an X-ray anode and to the process of producing thesame.

According to my invention I take a rare refractory metal .I, such as tungsten or the like, of appropriate size, and for X-ray purposes in the form of a button orplate, and cleanse, by any suitable means, all of the surfaces thereof and especially the surfaces to be coated with a high electrical and thermal conductivity low melting point metal, preferably copper.

The surfaces to be coated with copper are preferably subjected to an initial mechanical, chemical, or other treatment in order that they may be substantially freed from any oxide or other coating which might prevent the copper from forming a tenacious bond thereat. Thereafter in order that I may obtain this tenacious bond between the tungsten and the metal, copper, which is to be united therewith and so that the juncture between the` adjacent faces of the tungsten, and the copper may be of high' heat and electrical conductivity placed and coated with va thin layer of molten copper in an atmosphere of hydrogen.

One means suitable for carrying out this step and employed herein only so that a cleark understanding of my invention may be ascertained, comprises a furnace I0 having gas tight walls and composed of any suitable refractory heat insulat- Resting on a plurality of spaced The interior diameter of this ,crucible is slightly and preferably about 1A; to of an inch greater` than the diameter of the button, which in this case happensr to be of a circular peripheral configuration. Snugly fitting along the sides of this crucible and having its lower surface located on the upper surface of the base of this crucible is a removable plate I2, composed of the same mate` rial as the crucible per se.

Embedded in this plate I2, diametrically op posed and spacedfrom the center thereof a distance approximately half that ofthe diameter of the button I, are a bent wires I3, preferably of the same composition as said button. Said crucible with the face to be exposed adjacent the upper surface of the plate I2 and is` preferably so positioned that its center is in approximate contiguity with the center of the plate I2.

The bent tungsten wires I3 extend towards each other and lap on to the lupper face of said button I to hold the same in'position during the perplaced upon said button'and insaidcrucible. Only characteristics'the tungsten button is plurality of relatively thin button I isplaced in the 3 `a sumcient quantity thereof, however, is preferably contained therein as is necessary upon the fusion thereof to coat said exposed surfaces of the button with a copper lm measuring aboutfrom to; of an inch in thickness. Prior to the I heating of said metals and at this stage hydrogen gas is continually admitted into said closed chamber through a conduit I4 and allowed to egress .therefrom through a conduitli. yHeat is now supplied to said chamber by any appropriate means, as for example, high resistance glow bars (not shown). I tained and hydrogen is allowed to pass through the furnace by virtue of the conduit I4 and the outlet I5, so that the copper fragments in the crucible maybeheated to a temperature sufficiently high to cause the copper tobecome sufficiently molten to flow over button and wet said surfaces.

Instead of placing fragments of solid copper in the crucible together with the tungsten button, I may first heat the button I to a temperature of about 1150 C. in the presence of hydrogen. When this temperature is attained, suflicient molten copper I may be added directly to said crucible.

In the course of my experimentations with metals of t s character I have found that copper heated to a temperature of about 1150 C. is suiiciently molten for the aforesaid purposes. This temperature may be maintained for a short period and then the heating is removed and. the `hydrogen is turned oif at I4. The crucible, together with its contents, is allowed to cool to room temperature. The entir'e contents of the crucible including the removable graphite plate 'IP may then be extracted therefromv en masse.

'I'he graphite plate I2 may be detached from the metallic structure by shipping the tungsten wires at the point of juncture offsaid plate and said button leaving a slug composed of the tungsten button I having its bottom and side walls coated with alayer 2 of copper metal,` the juncture of said tungsten button and said'copper metal being mechanically strong and having a high thermal and electrical 'conductivitycharacteristic.

It is found that when copper is applied to said button in this manner that there may sometimes'appear small pipes or blow holes extending in the direction of the depth of said copper, and rgenerally above the surface skin of copper in contact with the tungsten button.

Because of the thinness .of said metalthese pipes or blow holes are apparent to the eye, or the copper outer surface or skin has indications thereon generally locating, if not definitely establishing, the exact position of said; blow holes or pipes. These indications on the surface of said copper appear as blisters or depressions. In order that the plug may be'prepared for the-next cause the outer surface of said ,copper coating is preferably machinedbefore the next step', Xwhich machining opens substantially all blow holes or pipes that may be therein. Instead of detaching the copper coated button I from the plate I2,l prior to machining, the button I may be machined while attached to said plate.

'I'he next step of the process comprises placing this slug with its lower uncoated face adjacent the upper face of a removable plate of graphite or the like, located in thefbottomof a crucible 20 within a furnace 2| containing air `and free from the hydrogen reducing atmosphere,

the exposed surfaces of said 'I'he heating operation is main- 1 The slug comprising the button I and the coppensed with by increasing the size of the copper per coating 2 is now heated to a temperature just casting 3, thus obviating the mechanical joint beslightly below the melting point of copper and tween the head 5 and the rod 6. y

preferably about 1000 C. When the slug becomes Although I have described my invention with 5 heated to this temperature there is admitted into minute particularity, this is not to be taken by 5 said crucible above said slug a quantity of molten way of limitation, but by way of illustration and copper, at a temperature of about 1l25 C., very the invention is to be limited only by the prior art.

large in comparison to the quantity of copper `What is claimed is: coating said slug and sufficient to form an anode 1. The plOCeSS 0f making Clad metals @OnSiSthead 5 of desired length which may be of any ing of tungsten and copper comprising, in the 10 dimension and about one inch. presence of hydrogen, coating said tungsten with Thereafter the crucible together with the anode a relatively thin layer of molten copper, solidifyhead 5 comprising the button I, the copper coating said molten layer, mechanically treating said ing 2 and the copper casting or body 3 are allowed copper layer and thereafter elevating the temto cool to room temperature. Subsequently any perature of said thin layer and coating said thin 15 appropriate machining and cleansing operations layer with a second molten layer of copper whose may be performed thereon to provide an X-ray mass is great as compared to that of said thin anode head comprising a rare refractory metal, layer and solidifying said second layer. tungsten, button or target I having adjacent 2. The process of making clad metals consistthereon a mass of copper which is substantially ing of tungsten and copper comprising, in the 20 homogeneous and free from blow holes or pipes, presence of hydrogen, coating said tungsten with the juncture between said button and said copa relatively thin layer of molten copper, solidper being tenacious and forming a firm mechaniifying said thin layer, mechanically treating said cal, high heat conductivity, electrical conductivcopper and thereafter elevating the temperature ity union therebetween. of said thin layer to slightly below its melting 25 This anode head 5 may then be machined and point and then adding thereto a molten laye:` mounted ina copper rod 6 to provide a suitable of copper whose mass is great as compared to X-ray anode shown in Figure 5. If desired the, that of said thin layer and solidifying said copper. machiing of the anode head 5 and subsequently mounting the same in a copper rod 6, may be dis- LOUIS F. EHRKE. 30

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