Patent Application
20240153739
Welding is a process by which a work piece including two components are joined together. In general, surfaces of the two components are placed in contact with one another and heated. The heat causes the two components to melt and, when subsequently cooled, fuse together.
Electron-beam welding (EBW) is one technique that has been developed to heat metal components for welding. High velocity electrons are extracted from a source and focused into a beam. The focal region of the electron beam is directed to the weld location where surfaces of the two metal components are in contact. A portion of the kinetic energy of the electrons within the beam is converted into thermal energy, supplying heat for the welding process.
EBW provides a number of advantages. In one aspect, electron beams can exhibit a relatively high penetration depth-to-width ratio, which can eliminate the need to perform multiple passes. In another aspect, electron beams can be focused with high accuracy, providing precise control and reproducibility. The energy of an electron beam can also be tailored to avoid overheating the weld region, reducing shrinkage and distortion.
Existing EBW systems typically employ thermionic cathodes as an electron source. However, thermionic cathodes can present challenges. As an example, thermionic cathodes require relatively high vacuum (e.g., about 10−4 torr or lower) to operate. At higher pressures, the thermionic cathode can react with residual gases in the vacuum and form compounds that readily erode the thermionic cathode due to evaporation. This erosion reduces the lifespan of the thermionic cathode. Additionally, in order to maintain the thermionic cathode at high vacuum, the components to be joined, as well as work piece fixturing and positioning mechanisms, must be installed in a large welding chamber that is also under high vacuum. Enclosing the fixturing and positioning mechanisms in vacuum increases the cost, complexity, and size of EBW systems, as compared to other welding technologies. In addition to these lifespan and cost limitations, there are also limits to the electron currents that can be generated by thermionic cathodes. The electron current can restrict the depth and extent of welding and the types of materials that can be welded.
Embodiments of the present disclosure provide improved systems and methods for electron beam welding. As discussed in greater detail below, the hot thermionic cathode is replaced with a cold plasma cathode. The cold plasma cathode is relatively inert at operating temperatures without substantive erosion. As a result, the lifespan of the plasma cathode can be significantly increased (e.g., more than 30 times). The inert nature of the plasma cathode also enables operation at pressures that are significantly higher than conventional thermionic cathodes. As a result, in one embodiment in which the plasma cathode gun is retrofit into the welding chamber containing the work piece, the welding chamber can also be maintained at higher pressure than e-beam welders with thermionic cathodes. For example, the weld chamber pressure can operate between about 1 millitorr and about 50 millitorr without significant electron beam scatter from residual gases. In another embodiment, in which the plasma cathode gun is installed in a differentially pumped housing (referred to as a snorkel), the welding chamber to be eliminated. The components to be welded and the support table and fixturing are positioned outside, rather than within, the vacuum enclosure. The components, support table, and fixturing are at atmospheric pressure. With the welding chamber outside of the vacuum enclosure, the volume of the vacuum enclosure can be significantly reduced, providing a reduction in the cost and complexity of plasma cathode-based EBW systems, as compared to thermionic cathode-based EBW systems.
In an embodiment, a system is provided that includes an electron gun, a focusing system, and a housing. The electron gun can include a cold cathode electron source and an extraction electrode. The focusing system can be configured to focus a beam of electrons extracted from the electron gun to a focal region. The housing can include the electron gun and extend along a housing axis in the direction of the electron beam. The cold cathode source is configured to emit electrons at a first operating pressure that is higher than a second operating pressure at the focal region of the electron beam.
In another embodiment, the cold cathode electron source is a plasma cathode. The plasma cathode can include a plasma cathode chamber, a first plasma electrode, and a second plasma electrode. The first plasma electrode can be mounted to a first wall of the plasma cathode chamber. The second plasma electrode can be mounted to a second wall of the plasma cathode chamber, opposite the first wall. An axis of the plasma cathode chamber can extend in the direction between the first and second plasma electrodes.
In another embodiment, the plasma cathode can be configured to generate a plasma having an electron temperature less than about 200° C.
In another embodiment, the plasma cathode chamber axis can be approximately aligned with the housing axis.
In another embodiment, the plasma cathode chamber axis can be approximately perpendicular to the housing axis.
In another embodiment, the first operating pressure can be within the range from about 50 millitorr to about 500 millitorr.
In another embodiment, the second operating pressure can be within the range from about 1 millitorr to about 50 millitorr.
In another embodiment, the housing can include the electron gun and a welding chamber enclosing the focal region.
In another embodiment, the housing can further include a differentially pumped snorkel extending between a first end coupled to the electron gun and a second free end. The snorkel can be configured to provide a selected pressure gradient between the first end and the second end, and the focal region can be approximately positioned at the second free end.
In another embodiment, the snorkel can include a plurality of vacuum enclosures, each in fluid communication with a respective vacuum pump.
In an embodiment, a method is provided. The method can include generating, by an electron gun including a cold cathode source and an extraction electrode, electrons at a first pressure. The method can further include extracting, by the extraction electrode, electrons emitted from the cold cathode source. The method can additionally include focusing a beam of the extracted electrons to a focal region along an axis of a housing containing the electron gun. The method can further include receiving, incident upon a surface of the work piece the focal region of the electron beam, wherein a second pressure at the work piece is less than the first pressure.
In another embodiment, generating the electrons can include receiving a flow of gas within a plasma cathode chamber of the electron gun, and generating an electric field between a first plasma electrode mounted to a first wall of the plasma cathode chamber and a second plasma electrode mounted to a second wall of the plasma cathode chamber, opposite the first wall. The electric field can be configured to form a plasma from the gas and an axis of the plasma cathode chamber can extend in the direction between the first and second plasma electrodes.
In another embodiment, the generated plasma can have an electron temperature less than about 200° C.
In another embodiment, the plasma cathode chamber axis can be approximately aligned with the housing axis.
In another embodiment, the plasma cathode chamber axis can be approximately perpendicular to the housing axis.
In another embodiment, the first pressure can be within the range from about 50 millitorr to about 500 millitorr.
In another embodiment, the second pressure can be within the range from about 1 millitorr to about 50 millitorr.
In another embodiment, the method can further include enclosing the work piece within a welding chamber. The welding chamber can be in fluid communication with the electron gun.
In another embodiment, the method can further include forming a vacuum seal between a surface of the work piece and a free end of a snorkel, the snorkel extending from the free end to the electron gun. The method can also include establishing a selected pressure gradient along the length of the snorkel between the electron gun and the free end.
In another embodiment, establishing the selected pressure gradient can include applying vacuum pressure of different levels to respective vacuum enclosures of the snorkel.
These and other features will be more readily understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
It is noted that the drawings are not necessarily to scale. The drawings are intended to depict only typical aspects of the subject matter disclosed herein, and therefore should not be considered as limiting the scope of the disclosure.
To better appreciate the features and advantages of embodiments of the disclosed cold plasma cathode-based EBW systems, as compared to hot thermionic cathode-based EBW systems, an exemplary embodiment of a thermionic cathode-based EBW system 100 is discussed below with reference to
The thermionic cathode 106 can take the form of a filament (e.g., a tungsten “hair-pin” filament). In use, an electric current from the high-voltage power supply is transmitted through the thermionic cathode 106, resulting in resistive heating of the thermionic cathode 106 and emission of electrons therefrom. The thermionic cathode 106 is biased at a large negative voltage, for example, within the range from about 60 kV to about 150 kV. An extractor electrode 132 is aligned in close proximity to the thermionic cathode 106 and is biased at a large positive voltage, within the range from about +400 V to about +1000 V, relative to the thermionic cathode 106. The extractor electrode 132 has an aperture 134 that accelerates the thermionic electrons towards the anode 120 (e.g., in the z-direction), which is at approximately ground potential, forming an electron beam (thermionic e-beam) 136. The thermionic e-beam 136 is transmitted through the anode 120 at high energy towards the focusing and deflection coils 122, 124. The focusing coil 122 generates a first magnetic field that is configured to focus the thermionic e-beam 136 to a focal region 140 of predetermined size (e.g., diameter) at a surface of the work piece 114 (e.g., an incipient weld location at the interface between the first component 116a and the second component 116b). The deflection coil 124 generates a second magnetic field that is configured to control in-plane deflection of the thermionic e-beam 136 (e.g., in the x-y plane). In this manner, the thermionic e-beam 136 forms a weld 142 joining the first component 116a and the second component 116b.
As noted above, the thermionic cathode 106 is heated to generate electrons. Therefore, the thermionic cathode 106 is classified as a “hot cathode.” However, at typical operating temperatures (e.g., about 1200° C.), the thermionic cathode 106 is highly reactive and easily forms compounds with gases present in the cathode chamber 104. Formation of these compounds is undesirable, as they reduce the level of electron emission and readily evaporate, as compared to the pure cathode material. Thus, the lifespan of the thermionic cathode 106 is limited by its evaporation rate. As an example, assuming a typical rate of evaporation, when the thermionic cathode 106 is maintained at a pressure of approximately 10−4 torr, its lifespan is limited to roughly 30 hours.
The lifespan of the thermionic cathode 106 can be increased by reducing the pressure in the cathode chamber 104. However, maintaining high vacuum pressure within the electron gun 130 requires that the welding chamber 110 is also enclosed and maintained under high vacuum. Thus, reducing the pressure in the cathode chamber 104 also requires a concurrent reduction of the pressure in the welding chamber 110. As an example, in order to maintain a pressure of the thermionic cathode 106 at about 10−5 torr, the welding chamber 110 is maintained at a pressure of about 10−4 torr. The need to maintain high vacuum in both the cathode chamber 104 and the welding chamber 110 can make electron beam welding relatively expensive compared to other joining and fabrication techniques.
Additionally, eliminating the requirement of high vacuum within the plasma cathode chamber also reduces the time required to pump down the welding chamber, simplifying the system's vacuum pump requirements, and increasing the rate of the system's processing time (e.g., throughput). For example, in normal welding operations, the welding chamber can operate at pressures less than or equal to about 10 millitorr (e.g., from about 1 millitorr up to about 10 millitorr. The operating parameters of the disclosed embodiments (e.g., plasma pressure, electrode bias, pulse frequency, pulse width, etc.) can also be changed to achieve high beam current and attendant improvements in weld speed, component throughput, and cost.
In certain embodiments, the pressure of the cathode chamber can be higher than that at the focal region of the plasma electron beam (e.g., within the welding chamber). That is, the pressure gradient between the cathode chamber and the welding chamber in EBW systems employing the plasma cathode can be reversed as compared to EBW systems employing a thermionic cathode. In certain embodiments, discussed with regards to
As shown, in
The plasma power supply can be a pulsed, high-voltage alternating current AC power supply. As an example, the power voltage can be ±ENSP=about ±15 kV to about ±24 kV, with a 20 kHz frequency, and a few nanosecond pulse length.
In use, a neutral gas (also referred to as an inert gas) is supplied to the plasma cathode chamber 204. As an example, the neutral gas can be argon. Other examples of the neutral gas can include helium, nitrogen, and hydrogen. Combinations of two or more neutral gases can also be employed. The pressure within the plasma cathode chamber 204 can be within the range from about 50 millitorr to about 500 millitorr for normal welding operation.
Concurrently, the plasma power supply applies a potential difference between the plasma electrodes 216a, 216b. The potential difference generates an electric field between the plasma electrodes 216a and 216b. The pulsed voltage creates a discharge in the neutral gas to generate a plasma 242 (e.g., a cloud of electrons and ions. A high, negatively biased electrode (referred herein as −Egun) is positioned at an extraction opening or aperture 234 in the plasma cathode chamber 204. Electrons are extracted from the plasma 242 with an additional electrode that is positively biased relative to the negatively biased extractor electrode. A portion of the plasma electrons formed in this manner can be extracted and collimated by the extractor electrode 208.
The temperature of the plasma 242 is less than about 200° C. under operating conditions and can be classified as a “cold cathode.” In contrast, a typical operating temperature of the hot thermionic cathode 106 can be about 1200° C. Due to its lower operating temperature, the plasma cathode exhibits significantly smaller thermal emittance than the thermionic cathode 106. As a result, the plasma electrons extracted from the plasma cathode, and collimated by the aperture 214 of the extractor electrode 208, can exhibit a lower beam emittance (also referred to as beam divergence) than the thermionic cathode 106. Thus, a focal region 240 of a plasma electron beam (plasma e-beam) generated using the plasma cathode can have a smaller diameter at the weld, allowing achievement of more precise welds.
Embodiments of the plasma cathode chamber can adopt a variety of configurations.
An advantage of this first configuration of the plasma cathode chamber 204 is that it requires three high-voltage feedthroughs, one each for the −NSP electrode 216a, the +NSP electrode 216b, and the extractor electrode 208. As the thermionic cathode-based EBW system 100 of
However, this first configuration of the plasma cathode chamber 204 can also face challenges with regards to the plasma power supply. Notably, the plasma power supply has voltage isolation of the output voltages but the pulsing can be asymmetric. The pulse can be unbalanced relative to the plasma power supply common because the positive plasma electrode +NSP 216b is connected to −Egun. This can cause the isolation transformer to shift the average bias voltage, creating a virtual common of unknown voltage and necessitate modification of the plasma power supply. One way to affect this modification can be by the use of a fast switching transformer to reduce the transformer's inductance and allow the stored energy to be dissipated sufficiently quickly to bring the virtual common to about zero volts.
Another plasma cathode-based EBW system in the form of plasma cathode-based EBW system 300 having a second plasma cathode configuration is illustrated in
An advantage of the second configuration of the plasma cathode chamber 304 is that the plasma power supply output is symmetric relative to the electron gun voltage −Egun. That is, −Egun is connected to neutral voltage (common ground) output of the plasma power supply. Therefore, the plasma power supply does not require modification to implement the second configuration of the plasma cathode chamber 304. However, the plasma cathode chamber 304 and electron gun vacuum flange 344 can require modification versus their conventional design. In one aspect, modification can be required to include a fourth high voltage feedthrough, in addition to the three feedthroughs employed by the thermionic electron gun portion 230.
In another aspect, extraction of electrons from the plasma 242 and acceleration of electrons to the (grounded) anode 220 can require a new extractor electrode 308 and configuration. Generally, thermionic electron guns for electron beam welders use a Wehnelt electron optics design, where the thermionic cathode is biased at a high negative potential relative to the anode, in order to accelerate the electrons. In the second configuration of
Table 1, below, illustrates operating parameters for the plasma cathode chamber 204 in the first configuration of
Table 2, below, illustrates a maximum range of plasma operating conditions and output current and power for a 60 kV electron beam welding gun employing the plasma cathode. Plasma parameters for normal welding operations have a smaller range. For example, Table 1 gives exemplary parameters for 100 mA operation, which can produce an approximately 6 kW plasma electron beam. This is nominally the lowest power that is useful for welding. These operating parameters are applicable to all configurations of the plasma cathode discussed herein (e.g.,
Additional operating arrangements are envisioned, as an example, for output power greater than about 60 kW. However, it can be understood that the plasma power supply would require modification to operate at higher output power of the plasma e-beam. Specifically, the power output would need to be increased to accelerate the plasma's higher electron flux.
As noted above, the plasma cathode-based EBW systems 200, 300 of
C
ring*(PTC−PWC)=SWC*PWC (1)
where Cring is the gas conductance, PTC is the pressure of the plasma cathode chamber, PWC is the pressure of the welding chamber, and SWC is the pumping speed of the welding chamber. For a pressure at 10 millitorr in the plasma cathode chamber, a pump generating less than about 1 liters per second can maintain an approximately one order of magnitude pressure differential between the plasma cathode chamber and the welding chamber, assuming no leaks, due to the time required for pressure transmission therebetween (e.g., between about 10 millitorr within the plasma cathode chamber and about 1 millitorr within the welding chamber). For a plasma pressure of about 100 millitorr, the pump speed is about 10 liters per second.
The ability of the cold plasma cathode to operate at relatively high pressure, compared to thermionic cathodes, also allows for the use of new configurations as well.
The plasma electron gun 404 of
The snorkel 406 includes a plurality of vacuum enclosures or stages. As shown, a first vacuum enclosure VE1 is positioned at one end of the snorkel 406, adjacent to and in fluid communication with the focusing and steering portion 404b of the plasma electron gun 404. A second vacuum enclosure VE2 is positioned at the opposing end of the snorkel 406. Optionally, one or more third vacuum enclosures VE3 can be interposed between the first vacuum enclosure VE1 and the second vacuum enclosure VE2. Each of the vacuum enclosures VE1, VE2, VE3 is further in fluid communication with a dedicated vacuum pump (not shown) via respective vacuum lines 410.
The walls that separate respective ones of the vacuum enclosures VE1, VE2, VE3 extend from the plasma electron gun 404 (e.g., a distal end of the plasma electron gun 404d) to the surface of the components to be welded (the work piece 114). So configured, each vacuum enclosure VE1, VE2, VE3 of the snorkel 406 can maintain an approximately constant pressure that is different than an adjacent (e.g., nearest neighbor) vacuum enclosure. Thus, a predetermined pressure gradient can be established from the work piece 114 to the plasma electron gun 404. That is, between the second vacuum enclosure VE2 and the first vacuum enclosure VE1. As an example, the pressure within the second enclosure VE2 can be less than the pressure within the first enclosure VE1. When present, the pressure within the third enclosure(s) VE3 is intermediate to the pressure of the first enclosure VE1 and the second enclosure VE2. The plasma cathode chamber can be isolated by use of a small aperture of the extractor electrode (e.g., about 1 mm diameter).
In use, the snorkel 406 (e.g., a distal end of the second vacuum enclosure VE2) can be placed in contact with an outer surface of the work piece, 114 adjacent to the location of the incipient weld 408 and forms a vacuum seal 412. Thus, the portion of work piece 114 adjacent to the weld 408 is held at a pressure approximately equal to VE2. An electron beam 414 extracted from the plasma cathode is transmitted through the focusing and steering portion 404b in a moderate vacuum (e.g., up to several torr) and is incident upon the weld 408. The snorkel 406 is mounted on an alignment stage (not shown) and can be moved along the weld seam using an alignment structure (e.g., a hexapod support). The pressure within the snorkel 406 is low enough to allow the electron beam 414 to be transmitted from the plasma electron gun 404 to the work piece 114 with minimal beam divergence and absorption. Because the portion of the work piece 114 adjacent the weld 408 is maintained at about the pressure of the second vacuum enclosure VE2, the stage and other work piece positioning mechanisms can be at about atmospheric pressure or in a controlled environment, such as a dry argon glove box.
Embodiments of the snorkel 406 can adopt a variety of configurations. In one aspect, the number of vacuum enclosure segments can range from at least 2 (e.g., the third vacuum enclosure(s) VE3 are omitted) to 6 or more (e.g., the first vacuum enclosure VE1, the second vacuum enclosure VE2 and four or more third vacuum enclosures VE3), depending upon the desired pressure gradient between the work piece 114 and the plasma electron gun 404.
In general, a snorkel 406 having a greater number of vacuum enclosure segments allows more zones of constant pressure to be established between the second vacuum enclosure VE2 and the first vacuum enclosure VE1 and a smaller rate of change of pressure along the beam axis within the snorkel 406. In the case of two vacuum enclosure segments, the rate of pressure change between the first vacuum enclosure VE1 and the second vacuum enclosure VE2 can be relatively large, and result in significant turbulence between the second enclosure VE2 in fluid communication with a high speed pump and the first vacuum chamber VE1 in which the electron beam 414 is transmitted to the welding joint. This turbulence can cause the plasma electron beam 414 to diverge, which creates a wider weld 408. By adding more vacuum enclosures to the snorkel 406 (e.g., increasing the number of third vacuum enclosures VE3) the rate of change of pressure along the beam axis within the snorkel 406 can be decreased, reducing the turbulence at the electron beam aperture.
One exemplary embodiment of the snorkel of
In a further aspect, under circumstances where the first and second components 116a, 116b to be welded have large, smooth surfaces, the diameter of an aperture of the anode can be relatively large (e.g., about 100 cm in diameter). In this case, the diameter of the anode aperture can be designed to span a large weld joint. The deflection coil can be configured to deflect the plasma electron beam 414 to track the weld.
In another aspect, under circumstances where the first and second components 116a, 116b to be welded have irregular surfaces, it can be difficult to form a reliable vacuum seal 412 between a large outer pumping zone (e.g., vacuum enclosure VE2) and the work piece 114. In this circumstance, the outer pumping zone seal can be too small to steer the electron beam 414 along the weld joint geometry. The VE1 aperture to transmit the electron beam 414 to the weld 408 can be relatively small (e.g., about 2 cm to about 3 cm). In this case, the snorkel 406 can be moved to track the weld 408, rather than steering the plasma electron beam 414.
Irregular work piece surfaces can also create difficulty in controlling the welding process. In general, it is desirable for welding parameters, such as the size of the weld 408 and the temperature of the welding process, to be approximately constant in order to achieve uniformity of the weld 408 along its length. However, the welding parameters are dependent, at least in part, upon the size of the focal region. When the snorkel 406 moves over an irregular surface (e.g., a hill or a valley), a working distance between the work piece 114 and the electron gun 404 changes, which in turn changes the size of the focal region.
Recognizing that irregular surfaces can change the size of the focal region, and result in non-uniform of welds, additional embodiments of the plasma cathode-based EBW system 400 of
The telescoping snorkel 500 is configured to change length in response to changes in the height of the work piece surface. As an example, the vacuum enclosures of the telescoping snorkel 500 can included sliding seals configured to slide in the direction of the housing axis AH, retracting when a distal end of the second vacuum enclosure VE2 contacts a raised portion of the work piece surface and extending when the distal end of the second vacuum enclosure VE2 contacts a recessed portion of the work piece surface. In this manner, the vacuum enclosures adjust to the change of working distance WD between the plasma electron gun 404 and the work piece 114.
In use, the telescoping snorkel 500 can be mounted to a support structure 502 (e.g., a robotic arm, gantry, hexapod structure, track rail, etc.) that is installed above the surface of the work piece 114. The support structure 502 can be configured to move the cathode-based EBW system with respect to the work piece 114. As shown in
Beneficially, embodiments of the differentially-pumped plasma-cathode based EBW systems of
Certain exemplary embodiments have been described to provide an overall understanding of the principles of the structure, function, manufacture, and use of the systems, devices, and methods disclosed herein. One or more examples of these embodiments have been illustrated in the accompanying drawings. Those skilled in the art will understand that the systems, devices, and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments and that the scope of the present invention is defined solely by the claims. The features illustrated or described in connection with one exemplary embodiment may be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present invention. Further, in the present disclosure, like-named components of the embodiments generally have similar features, and thus within a particular embodiment each feature of each like-named component is not necessarily fully elaborated upon.
Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about,” “approximately,” and “substantially,” are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise.
One skilled in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the present application is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated by reference in their entirety.
This application claims the benefit of U.S. Provisional Patent Application No. 62/916,214, filed on Oct. 16, 2019 and entitled “Electron Beam Welding Systems Employing A Plasma Cathode,” the entirety of which is incorporated by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US20/56043 | 10/16/2020 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 62916214 | Oct 2019 | US |
