US3174026A - Method and means of circumventing cathode maintenance in electron beam devices - Google Patents

Description

"Skim-H March 16, 1965 l.. T. BowERs ETAL 3,174,026

METHOD AND MEANS OF CIRCUMVENTING CATHODE NCE IN ELECTRON BEAM DEVICES MAINTENA Filed June 20, 1962 j l l l l l I ATTORNEY H R .w S S R R 0 O 1W. S M 0 M M m V R D E T T R S E E B L R E H Y B nited States Patent METHOD AND MEANS F CIRCUMVENTING CATHDE MAINTENANCE 1N ELECTRON BEAM DEVICES Lester T. Bowers, Oreland, and Herbert D. Van Sciver II,

Merion, Pa., assignors to The Budd Company, Philadelphia, Pa., a corporation of Pennsylvania Filed June 20, 1962, Ser. No. 203,931 5 Claims. (Cl. 219--117) This invention relates to electron beam devices and more particularly to an anti-oxidation cathode structure.

In recent years electron beam welding, melting, metal deposition or vaporization, and machining have become well known experimental techniques. In some instances such as metal cutting and welding, involving small objects, these laboratory techniques have been used for small specialized production processes; but technical problems have heretofore limited widespread production use.

The principle of an electron beam tool depends upon electrons emitted from a heated lament in a vacuum chamber. Usually a heated tungsten emissive cathode acts as an electron source and is adjustable to very high negative voltages. Surrounding the negative cathode is a beam-forming cup or negative electrode which may be held more negative than the negative cathode. Below the electrode is a target or positive anode which is held near ground potential, as is the workpiece, and acts as an accelerator for the cloud of electrons emitted from the negative cathode and formed by the beam-forming cup negative electrode. The cathode, electrode and anode constitute an electron beam gun. Two common systems are employed in shaping and focusing the electron beam; the iirst system is commonly known as the Rogowski system. Directly below the anode of the gun in the Rogowski system there is usually employed a first set of beam alignment electromagnetic coils or plates to dellect and adjust the stream of electrons being emitted from the gun. ln some electron beam machine tools an auxiliary or secondary pair of coils or plates are placed below the alignment coils to act as a manual adjustment for detlecting the beam of electrons. Below the auxiliary positioning coils or plates the Rogowski system employs a visual light microscope concentricaliy surrounding the beam of electrons. Below the light microscope a magnetic lens refocuses and converges the electron beam for critical focusing. Below the magnetic focusing lens another pair of beam dellection electromagnetic coils similar to the beam alignment electromagnetic coils is employed to permit critical positioning of the electrode beam. The above-mentioned Rogowski system employing a magnetic lens permits critical focusing in a relatively short electron beam distance.

A secondary system known as the Steigerwald system omits the light microscope and the magnetic lens but is capable of welding by extending the length of travel of the electron beam approximately 50% further than the Rogowski system. Both above-mentioned systems have been employed to perform welding.

The great advantage achieved by electron beam welding is predicated upon the cleanliness and lack of contaminating gases in the welding area. It is presently necessary, when employing an electron beam machine tool, to create a high vacuum in the order of l0-4 or 10*5 mm, of mercury (.1 to .01 micron pressure). At these vacua the impurity level in the vacuum chamber is 0.1 to 0.01 p.p.m. It was formerly believed that such a high vacuum and high purity was necessary to insure an obstacle free path for the electron beam and prevent emissive cathode deterioration.

3,174,026 Patented Mar. 16, 1965 lCe It has been discovered that partial vacua of 1 to 150 microns pressure are suflicient to insure that the electron beam can be adequately maintained to perform critical focus welding, melting, metal deposition or removal and machining for oxygen remaining in the partial vacua may be substantially removed to prevent rapid deterioration of the electron emitting cathode without the necessity of creating a high vacuum. It was further discovered that the electron beam caused oxygen and residual gases to be released from the workpiece, which would ordinarily accelerate the deterioration of the electron emitting cathode.

A method of prolonging the life of the emissive cathode in electron beam devices operating at a partial vacuum has been discovered.

Therefore, it is a general object of the present invention to provide an incandescent electrode in the vicinity of the workpiece of an electron beam device to absorb residual gases;

It is a further object of the present invention to provide an incandescent electrode having a high affinity for oxygen, nitrogen, hydrogen, etc. in the vicinity of the workpiece but removed from the effective electromagnetic path of the electron beam;

Another object of the present invention is to provide a method of electron beam welding without the necessity of a high vacuum;

Another object of the present invention is to provide an electron beam welding device for welding metals outside the vacuum chamber;

Another object of the present invention is to provide a method of extending the life of an electron emitting cathode in a contaminated relatively low vacuum atmosphere.

The above and other objects and novel features of the invention will be apparent from the following description and accompanying drawing in which:

FIG. l is a schematic diagram of an electron beam device illustrating a preferred embodiment of the invention;

FIG. 2 is a schematic diagram of an alternative embodiment of the invention shown in FIG. 1 showing through-metal welding with a portable unit.

In accordance with the present invention there is provided an electron beam device comprising an electron beam gun for emitting a stream of electrons in a partial vacuum, a magnetic lens for focusing the stream of electrons in said partial vacuum, an incandescent electrode physically located between the magnetic lens and a workpiece for collecting residual impurities in said partial vacuum and an orifice in the side of the partial vacuum enclosure cut by and having the approximate size and shape of said stream of electrons through which the electron beam leaves the partial vacuum and enters the atmosphere.

Referring now to FIG. l where a vacuum beam device 10 is shown surrounded by a vacuum box 12. Negative cathode 14 is heated to approximately 4000 F. to provide an emissive cloud of electrons 16 therefrom. The cloud of electrons 16 is shaped by the negative electrode beam-forming cup 18. Cup 18 with a control oriiice 20 therein, is electrically insulated from the negative cathode 14, and maintained at a direct current voltage level adjustable from 0 to 100 volts D.C. negative (shown as VB) with respect to the negative cathode. The beam of electrons 16 is shaped by the beam-forming cup 18 and accelerated by a high potential positive anode 22. Depending upon the velocity of the electron beam desired, the Voltage V0 may be varied from approximately 20,000 volts to 150,000 volts. The electrostatic eld generated by the beam-forming cup 18 focuses the cloud of electrons 16 from the emissive surface of the negative cathode and also limits the number of electrons (i.e. beam current) projected therefrom. A high potential positive anode 22 is provided with an aperture 24 therein through which the partially focused electron beam 16 passes. Below the positive anode 22 there may be provided a pair of beam alignment electromagnetic coils or plates 26 and 28 to position the electron beam 16 and prevent inadvertent defiections which could cause damage to the electron beam device. In some embodiments of electron beam devices a tungsten diaphragm or disk 30 shaped similar to the positive anode 22 is placed adjacent to the coils 26 and 28 to insure that there is no damage incurred before the beam alignment electromagnetic coils can be brought into effect. Directly below the beam alignment coils 26, 28 there is provided a magnetic focusing lens 32. FIG. 1 exaggerates the diffusion of the electron beam 18 to illustrate the principle of operation of the magnetic focusing lens 32. The D.C. potential on the magnetic focusing lens 32 may be varied over a positive potential range to insure critical focus of the electron beam 16. The finer the focal spot is adjusted the greater the ratio of depth to width of penetration. Higher negative cathode voltages produce greater depth to width ratios.

Should the electron beam 16 be initiated prior t0 obtaining a high vacuum of the order of l*4 mm. of mercury, the negative cathode 14 would ordinarily be oxidized by oxygen present in the partial vacuum. Other contaminating gases present in the partial vacuum would combine with the cathode 14 and reduce the potential current density of the beam.

Even after a high vacuum has been obtained and the electron beam 16 initiated in a relatively clean chamber, the electron beam process whether welding, machining, or vaporizing causes some outgassing of the workpiece 34. Outgassing of the workpiece 34 not only reduces the vacuum pressure but introduces contaminating gases into the vacuum chamber 10, and can be so great as to render the process inoperable.

In order to prevent deterioration of the negative emissive cathode 14 and to prolong its life there is provided an incandescent or auxiliary electrode 34 shielded from the magnetic lens 32 by a heat shield insulator 36. In many cases the incandescent electrode 34 may be made of material identical to cathode 14, such as tungsten, molybdenum and tantalum, but better results are achieved when the incandescent electrode 34 has a higher affinity for oxygen than the emissve cathode 14. In the closed vacuum chamber 12 of FIG. 1 the cathode 34 will cornbine with oxygen and absorb residual gases. Absorption of residual gases reduces the pressure in the vacuum chamber. Elimination of free oxygen preserves the emissive cathode 14. It was found that the electron beam device may be operated at partial vacuum pressures hundreds of times lower than the high vacuum pressure heretofore proposed, thus enabling electron beam process to be performed on materials which outgas excessively.

Heretofore, electron beam devices were limited to operation on the workpiece with the electron beam gun inside the vacuum chamber with the workpiece. Such a requirement is no longer necessary, for the novel device permits introduction of gases into the vacuum chamber.

FIG. 2 is a modified embodiment of the electron beam device shown in FIG. 1. This embodiment illustrates a portable electron beam Welder for through-metal welding wherein the workpiece 38 is placed outside the vacuum chamber 12. In the modified embodiment the beam passes through a very small orifice 4t) in a replaceable diaphragm or cap 42 at the target end of the vacuum chamber. The orifice is maintained usually of the order of several microns, and the chamber 12 is easily maintained at partial vacuum stability by continuous pumping of pump P. A two inch diffusion pump is capable of pumping seventy liters per second at *4 mm. of

CTI

mercury, thus a two inch pump is large enough to maintain vacuum stability at 10-4 mm. with an Orifice having a diameter of ten microns.

The operation of the modified electron beam device is practically the same as before. Cathode 14 initiates a cloud of electrons 16' shaped by the cup 18 and accelerated by the anode 22. The electron beam 16 is focused by the magnetic lens 32' and passes through the orifice 40 in the target 42 to impinge on the workpiece 38'. The electron beam 16 inside the chamber 12 is operated as before and at similar partial pressures. Well known pressure sensitive devices should be incorporated inside the chamber to shut down the beam in case of loss' of partial vacuum, and provision made for insulating the coils to avoid damage due to spark over if the beam is initiated in a concentrated ionizable gas.

A preferred method of operating the modified electron beam device is to initiate the beam 16 after a partial vacuum is obtained and after the electrode 34 is initiated. Initial focus of the beam is made at a low voltage level on the target 42. When it is desired to start the electron beam process the energy level of beam 16 is increased to cut an orifice 40 through the target 42. The hot center portion of the beam 16 cuts through the target 42 creating a very small orifice 40 approximately the size and shape of the beam. Not only does this insure that the orifice is very small, but the orifice 40 serves as a re-focusing device to insure that the beam 16', as it leaves the vacuum chamber 10', will be critically focused while traveling to workpiece 38. The beam 16 as it passes from the vacuum chamber 12 must pass through a high density gas before reaching the workpiece 38. Part of the electron beam is ionized, but being of high velocity and high energy content remains critically focused causing a beam of electrons and high velocity ionized particles to reach the workpiece.

It is intended that the target 42 of the electron beam device be placed close to the workpiece in operation so that the electron beam, even though ionized, is not diffused, but will continue in a concentrated stream to the workpiece maintaining sufficient energy and critical `focus to perform electron beam welding and other processes.

A replaceable thin metal diaphragm 42 may be employed at the target end of the vacuum chamber 12' as a seal between the vacuum chamber 12' and the workpiece 38 located outside the vacuum chamber. The diaphragm or target 42 may take the form of a ribbon valve which may be changed by sliding a new piece of ribbon over the target end for replacing or sealing off the existing orifice. A baffle 44 having an orifice to permit the beam to pass therethrough may be employed at a critical location to deflect gases entering through the orifice 40 to the vacuum pump P.

The incandescent electrode 34 is shown as a wound wire helix in a cylindrical portion of the vacuum charnber 12'. A larger surface area of cathode 34 is more effective, but cannot be made too large without providing external cooling means such as cooling fins as shown.

An inert gas shroud may be introduced at the workpiece to insure that there is no contamination of the workpiece as is well known in the welding art, and the incandescent cathode can be modified to absorb the shlielding gas.

It is apparent that electron beam devices may be successfully operated at partial vacuum and further that an electron beam may operate on a workpiece outside the vacuum chamber. While a preferred embodiment has been illustrated showing an incandescent electrode with a high afiinity for oxygen it is apparent that plates of active metals at reaction temperatures (below their fusion temperatures) may also be employed inside the vacuum chamber along with incandescent cathode 34 or a continuous stream of active gas may be released into the vacuum chamber to absorb gases without any effect on the vacuum pressure in the chamber to further reduce contamination of the emmissive cathode.

Modifications and variation of the specific embodiments described herein may be made without departing from the spirit of the invention which is limited only by the terms of the appended claims.

What is claimed is:

1. A method of electron beam welding at atmospheric pressures comprismg e step''" evacuating a singlestage vacuum chamber of an electron beam Welder to a partial vacuum pressure of approximately -4 mm. of Hg with a vacuum pump, heating an auxiliary electrode in said vacuum chamber to absorb residual gases and oxygen in said vacuum chamber, initiating a low energy stream of electrons upon a target located at the side of said vacum chamber, said target being located directly in the path of a workpiece located outside of said vacuum chamber, increasing the intensity of said electron beam to a higher energy level to cut an orifice in said target and permit said electron beam to impinge upon the workpiece located outside of said vacuum chamber, maintaining said auxiliary electrode in said vacuum chamber active to absorb gases entering said vacuum chamber through said orice, and maintaining said vacuum pump active to create a partial vacuum pressure in said vacuum chamber.

2. A method of prolonging the life of an emissive cathode of an electron beam welding device having an electron beam gun in a partially evacuated enclosure subject to the influx of contaminating gases comprising, evacuating a single-Stage sealed vacuum chamber to a partial vacuum pressure, initiating an electron beam in said vacuum chamber, creating an opening in said vacuum chamber with said electron beam the approximate size of the electron beam connected to the atmosphere through which contaminating gases enter into said vacuum chamber, exhausting the single-stage vacuum chamber with a vacuum pump at a rate equal to the entrance of said contaminating gases through said opening, passing said electron beam through said opening and into the atmosphere as a concentrated beam, and heating an electrode having a high ainity for oxygen and said contaminating gases inside said vacuum chamber to absorb said oxygen and said contaminating gases, said electrode being placed intermediate said opening and the source of said electron beam.

3. A method of providing an electron stream of energy levels capable of performing operations on a work-piece in the open atmosphere comprising, evacuating a sealed vacuum chamber to a partial vacuum pressure, initiating a concentrated high energy electron beam in said sealed vacuum chamber, cutting an opening to the atmosphere in said sealed vacuum chamber with said electron beam, said opening having the approximate size and shape of said electron beam, exhausting the vacuum chamber with a vacuum pump at a rate equal to or greater than the entrance of atmospheric gases through said opening, and passing said electron beam through said opening and into the atmosphere as a concentrated electron stream having an energy level capable of performing operations on a workpiece in the atmosphere.

4. A method of conducting an electron beam into the open `atmosphere for performing welding operations comprising, evacuating a vacuum chamber containing an electron beam gun to a partial vacuum level of the order of 10-4 mm. of Hg, initiating an electron beam in said partial vacuum and concentrating said beam on the side of said vacuum chamber, cutting an opening in said vacuum chamber by increasing the energy level of said electron beam, said opening forming a passageway between said vacuum chamber and said atmosphere, the diameter of said opening being of the order of 10 microns and having the approximate size and shape of said electron beam, said electron stream passing through said opening and leaving said vacuum chamber, and exhausting said vacuum chamber to maintain said partial vacuum at approximately 10-4 mm. of Hg.

5. An electron beam device for operation at partial vacuum pressures comprising, a vacuum chamber enclosure, a vacuum pump connected to said vacuum chamber enclosure for maintaining a partial vacuum pressure inside said enclosure, an electron beam device for creating a high velocity directed stream of electrons, a target in the path of said high velocity stream of electrons, and an orifice in said target cut by said high velocity stream of electrons having the approximate size and shape of the electron stream, said high velocity stream of electrons passing through said orice leaving said vacuum chamber enclosure providing an electron stream of energy levels capable of performing operations on workpieces in the atmosphere.

References Cited by the Examiner UNITED STATES PATENTS 2,208,987 7/40 Kuhne et al. 313-174 2,792,517 5/57 Holland 313--174 2,899,556 8/ 59 Schopper et al.

2,989,614 6/ 61 Steigerwald.

3,082,316 3/63 Greene 219-117 FOREIGN PATENTS 723,987 2/55 Great Britain.

RICHARD M. WOOD, Primary Examiner.

JOSEPH V. TRUHE, Examiner.

Claims (1)

1. A METHOD OF ELECTRON BEAM WELDING AT ATMOSPHERIC PRESSURES COMPRISING THE STEPS OF EVACUATING A SINGLESTAGE VACUUM CHAMBER OF AN ELECTRON BEAM WELDER TO A PARTIAL VACUUM PRESSURE OF APPROXIMATELY 10-4 MM. OF HG WITH A VACUUM PUMP, HEATING AN AUXILIARY ELECTRODE IN SAID VACUUM CHAMBER TO ABSORB RESIDUAL GASES AND OXYGEN IN SAID VACUUM CHAMBER, INITIATING A LOW ENERGY STREAM OF ELECTRONS UPON A TARGET LOCATED AT THE SIDE OF SAID VACUUM CHAMBER, SAID TARGET BEING LOCATED DIRECTLY IN THE PATH OF A WORKPIECE LOCATED OUTSIDE OF SAID VACUUM CHAMBER, INCREASING THE INTENSITY OF SAID ELECTRON BEAM TO A HIGHER ENERGY LEVEL TO CUT AN ORIFICE IN SAID TARGET AND PERMIT SAID ELECTRON BEAM TO IMPINGE UPON THE WORKPIECE LOCATED OUTSIDE OF SAID VACUUM CHAMBER, MAINTAINING SAID AUXILIARY ELECTRODE IN SAID VACUUM CHAMBER ACTIVE TO ABSORB GASES ENTERING SAID VACUUM CHAMBER THROUGH SAID ORIFICE, AND MAINTAINING SAID VACUUM PUMP ACTIVE TO CREATE A PARTIAL VACUUM PRESSURE IN SAID VACUUM CHAMBER.

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