Electron beam welding (EBW) is a fusion welding process in which a beam of high-velocity electrons is applied to the materials being joined. The workpieces melt as the kinetic energy of the electrons is transformed into heat upon impact, and the filler metal, if used, also melts to form part of the weld. The welding is often done in conditions of a vacuum to prevent dispersion of the electron beam. German physicist Karl-Heinz Steigerwald, who was at the time working on various electron beam applications, perceived and developed the first practical electron beam welding machine which began operation in 1958.[1]
It is well known that electrons are elementary particles possessing the mass m = 9.1E10-28 g and negative electrical charge e = 1.6E10-19 As. They exist either bound to an atomic nucleus, as conduction electrons in the atomic lattice of metals, or free electrons in vacuum.
The free electrons in vacuum can be accelerated and their orbits controlled by electric and magnetic fields. In this way we can form narrow beams of electrons carrying high kinetic energy, which at collisions with atoms in solids transform their kinetic energy into heat. Thanks to some specific conditions, this way of heating gives us exceptional possibilities. These conditions are:
Resulting effect of the electron beam under such circumstances depends on conditions; -first of all on physical properties of the material. Any material in very short time can be melted, or even evaporated. Depending on conditions, the intensity of evaporation may vary, - from negligible to essential. At lower values of surface power density (in the range of about 103 W/mm2) the loss of material by evaporation for most metals is negligible, which is favorable for welding. In the upper region of the power density the material affected by the beam may be evaporated totally in a very short time, which can be applied for “machining”.
Conduction electrons (that are not bound to the nucleus of atoms) move in crystal lattice of metals with velocities distributed according to Gauss law, depending on temperature. They can not leave the metal unless their kinetic energy (in eV) is higher than the potential barrier at the metal surface. Number of electrons fulfilling this condition increases with increasing temperature of the metal exponentially, according to Richardson's rule.
As a source of electrons for electron beam welders, the material must fulfil more requirements:
The emitter must be mechanically stable, chemically not sensitive to gases present in vacuum atmosphere (like oxygen and water vapour), easily available, etc.
These and some other conditions limit the choice of material for the emitter to metals with high melting points, - practically only two of them, tantalum and tungsten. With tungsten cathodes, emission current densities about 100 mA/mm2 can be achieved, but only a small portion of emitted electrons takes part in beam formation, depending on the electric field produced by anode and control electrode voltages. The type of cathode most frequently used in electron beam welders is made of tungsten strip, about 0.05 mm thick, shaped as shown in Fig. 1a. The appropriate width of the strip depends on the highest required value of emission current. For the lower range of beam power, up to about 2 kW, the width w=0.5 mm is appropriate.
Electrons emitted from the cathode possess very low energy of only a few eV. To give them the required high speed, they are to be accelerated by strong electric field applied between the emitter and another, positively charged, electrode, - the anode. The accelerating field must also navigate the electrons to form a narrow converging “bundle” around the axis. This can be achieved by an electric field in the proximity of the emitting cathode surface, which has, in addition to an axial component also a radial one, forcing the electrons in the direction to the axis. Due to this effect, the electron beam converges to some minimum diameter in a plane close to the anode.
For practical applications the power of the electron beam must, of course, be controllable. This can be accomplished by another electric field produced by another, with respect to the cathode negatively charged
At least this part of electron gun must be evacuated to "high" vacuum, to prevent "burning" the cathode and emergence of electrical discharges.
After leaving the anode the divergent electron beam does not have power density sufficient for welding metals and has to be focused. This can be accomplished by magnetic field produced by electric current in a cylindrical coil.
The focusing effect of a rotationally symmetrical magnetic field on the trajectory of electrons is the result of complicated influence of magnetic field on a moving electron. This effect is a force proportional to the induction B of the field and electron velocity v. The vector product of the radial component of induction Br and axial component of velocity va is a force perpendicular to those vectors, making the electron to move around the axis. Additional effect of this motion in the same magnetic field is another force F oriented radially to the axis, which is responsible for the focusing effect of the magnetic lens. The resulting trajectory of electrons in magnetic lens is a curve similar to a helix. In this context it should be mentioned that variations of focal length (exciting current) shall course a slight rotation of the beam cross-section.
As mentioned above, the beam spot should be very precisely positioned to the joint to be welded. This is commonly accomplished mechanically, by moving the workpiece with respect to the electron gun, but sometimes it is preferable to do this by deflecting the beam. Most often a system of 4 coils positioned symmetrically around the gun axis behind the focusing lens, producing magnetic field perpendicular to the gun axis, are used for this purpose.
There are more practical reasons why the most appropriate deflection system is such that is used in TV CRT or PC monitors. It applies to both the deflecting coils, as well as to the necessary electronics. Such a system enables not only “static” deflection of the beam for positioning purposes mentioned above, but also precise and fast dynamic control of the beam spot position by a computer. This enables e.g.:
Both possibilities do find many useful applications in EB welding practice.
Penetration of electrons
To explain the capability of the electron beam to produce deep and narrow welds, we have to explain the process of "penetration". First of all let us consider the process for a "single" electron.
When electrons of the beam impact the surface of a solid, some of them may be reflected (as "backscattered" electrons), and others penetrate under the surface, where they collide with the particles of the solid. In non-elastic collisions they loose their kinetic energy. It has been proved, both theoretically and experimentally, that they can "travel" only a very small distance under the surface before they transfer all their kinetic energy into heat. This distance is proportional to their initial energy and inversely proportional to the density of the solid. Under conditions usual in welding practice the "travel distance" is of the order hundreds of a millimeter. Just this fact enables, under certain conditions, the fast penetration of the beam.
Penetration of the electron beam The contribution of single electrons to heat is very small, but they can be accelerated by very high voltage, and by increasing their number (the beam current), the power of the beam can be increased to any desired value. By focusing the beam to a small diameter on the surface of a solid object, values of planar power density as high as 104 up to 107 W/mm2 can be reached. Due to the fact that electrons transfer their energy into heat in very thin layer of the solid, as explained above, the power density in this volume can be extremely high. The volume density of power in the small volume in which the kinetic energy of electrons is transformed into heat, can reach values of the order 105 – 107 W/mm3. Consecutively, the temperature in this volume increases extremely rapidly, by 108 – 109 K/s.
Resulting effect of electron beam under such circumstances depends on conditions; -first of all on physical properties of the material. Any material in very short time can be melted, or even evaporated. Depending on conditions, the intensity of evaporation may vary, - from negligible to essential. At lower values of surface power density (in the range of about 103 W/mm2) the loss of material by evaporation for most metals is negligible, which is favorable for welding. In the upper region of the power density the material may be evaporated totally in a very short time, which can be applied for “machining.
The results of the beam appliction depend on several factors: Many experiments and innumerable practical applications of electron beam in welding technology prove that the resulting effect of the beam, i.e. the size and shape of the zone influenced by the beam depends on:
(1) power of the beam,
(2) power density (focusing of the beam) but also on:
(3) welding speed,
(4) material properties, and in some cases also on
(5) geometry (shape and dimensions) of the joint.
(ad 1) – The power of the beam [W] is the product of the accelerating voltage [kV] and beam current [mA], parameters easily measurable and precisely controllable. The power is controlled by the beam current at constant accelerating voltage, usually the highest accessible.
(ad 2) – The power density in the spot of incidence of the beam with the “workpiece” depends on more factors, like the size of electron source on the cathode, “optical quality” of the accelerating electric lens and the focusing magnetic lens, alignment of the beam, on the value of the accelerating voltage, and on the focal length. All these factors are dependent (except the focal length) on the design of the machine.
(ad 3) – The construction of the welding equipment should enable to adjust the speed of relative motion of the workpiece with respect to the beam in wide enough limits, e.g. between 2 and 50 mm/s.
The final effect of the beam depends on combination of these parameters.
a) Action of the beam at low power density or in a very short time will result in melting only a thin surface layer.
b) A defocused beam will not penetrate and the material at low welding speed will be heated only by conduction of the heat from the surface, producing a hemispherical melted zone.
c) At higher power density and lower speed a deeper and slightly conical melted zone will be produced. In case of very high power density the beam (well focused) penetrates deeper, proportionally to its total power.
For welding thin walled parts, generally, some appropriate welding aids are needed. Their construction must provide the perfect contact of the parts and prevent their deformation during welding. Usually they have to be designed individually for the given workpieces.
Not all materials could be welded by electron beam in vacuum. This technology can not be applied to materials with high vapour pressure at the melting temperature, like zinc, cadmium, magnesium and practically all non-metals.
Another limitation of weldability may be the change of material properties inflicted by the welding process, as e.g. the high speed of cooling. As detailed discussion of this matter exceeds the scope of this article, the reader is recommended to look for more information to other literature.[1]
Joining two metal components by welding, i.e. by melting part of both in the vicinity of the joint, in case of two materials with very different properties is often not applicable because of unsuitable properties of their alloy, due to creation of brittle intermetallic compounds. This fact cannot be changed even by electron beam heating in vacuum, but nevertheless it makes possible to realize joints meeting high demands on mechanical compactness that are perfectly vacuum-tight. The principal rule of the method is not to melt both parts, but only that one with lower melting point, while the other remains in solid state. Advantage of the electron beam is in the possibility to localize the heating to a proper point and to control exactly the energy needed for the process. High vacuum atmosphere, no doubt, substantially contributes positively to the success. General rule of the construction of joints that should be made in the way mentioned above is that the part with the lower melting point should be directly accessible for the beam.
The material melted by the beam shrinks during cooling after solidification, which may have unwanted consequences, like cracking, deformations and changes of shape, depending on conditions.
The butt weld of two plates will result in bending of the weldment due to the fact that more material has been melted at the head than at the root of the weld. This effect is of course not as substantial as by arc welding.
Another potential danger is the emergence of cracks in the weld. If both parts are rigid, the shrinkage of the weld produces high stress in the weld which may lead to cracks if the material is brittle (even if only after remelting by welding). Consequences of the weld contractions should always be considered by the construction of the parts to be welded.
Any electron beam equipment comprises:
1 - : electron gun, generating the electron beam,
2 - : working chamber, mostly evacuated to "low" or "high" vacuum,
3 - : work-piece manipulator (positioning mechanism),
4 - : supply and control/monitoring electronics.
Electron gun
In the electron gun, the free electrons are gained by thermo-emission from a hot metal strap (or wire), which are then accelerated and formed into a narrow convergent beam by electric field produced by three electrodes: the electron emitting strap, the cathode, connected to the -pole of the high (accelerating) voltage power supply (30 - 200 kV) and the +H.V. electrode, the anode. There is a third electrode charged negatively with respect to the cathode, called Wehnelt or control electrode. Its negative potential controls the portion of emitted electrons entering into the accelerating field, i.e. the electron beam current.
After passing the anode opening the electrons move with constant speed in a slightly divergent cone. For the technological applications the divergent beam has to be focused, which is realized by the magnetic field of a coil, the "magnetic focusing lens.
For the proper function of the electron gun, it is necessary that the beam is perfectly adjusted with respect to the optical axes of the accelerating electrical lens and the magnetic focusing lens. This can be done by applying magnetic field of some specific radial direction and strength, perpendicular to the optical axis before the focusing lens. This is usually realized by a simple correction system consisting of two pairs of coils. By adjusting the currents in these coils any required correcting field can be produced.
After passing the focusing lens the beam can be applied for welding directly or after being deflected by the deflection system. This consists of two pairs of coils, each pair for one of the X and Y directions. These can be used for "static" or "dynamic" deflection. The static deflection is useful for exact positioning of the beam by welding. The dynamic deflection is realized by supplying the deflection coils by currents which can be controlled by the computer. This opens new possibilities of electron beam applications, like e.g. surface hardening or annealing, exact beam positioning, etc.
The fast deflection system can also be applied (if provided with appropriate electronics) for imaging and engraving. In this case the equipment is operated similarly as a scanning electron microscope, with the resolution of about 0,1 mm (limited by the beam diameter). In a similar mode the fine computer controlled beam can "write" or "draw" a picture on the metal surface by melting a thin surface layer.
Working chamber
Since the publication of the first electron beam welding machines at the end of 1950s, the application of electron beam welding spread rapidly into industry and research in all highly developed countries. Up to nowadays uncountable number of various types of electron beam equipments have been designed and realized. In most of them the welding takes place in the working vacuum chamber in high or low vacuum environment.
The vacuum working chamber may have any desired volume from a few liters up to hundreds of cubic meters. They can be provided with electron guns supplying electron beam with any required power up to 100 kW, or even more if needed. In micro-electron beam devices the components in the tenths of a millimeter dimension range can be precisely welded. In welders disposing with high enough power electron beams, welds up to 300 mm deep can be realized.
There are also welding machines in which the electron beam is brought out of vacuum into the atmosphere. With such equipments very large objects can be welded without huge working chambers.
Work-piece manipulators
The electron beam welding can never be "hand-manipulated", even if not realized in vacuum, as there is always strong X-radiation. The relative motion of the beam and the work-piece is most often rotation or linear travel of the work-piece. In some cases the welding is realized by the beam being moved by the computer controlled deflection system. The work-piece manipulators are mostly designed individually to meet specific requirements of the welding equipment.
Power supply and control/monitoring electronics
Any electron beam equipment must be provided with appropriate supply of power for the beam generator. The accelerating voltage may be chosen between 30 and 200 kV. Usually it is about 60 or 150 kV, depending on various conditions. With rising voltage the technical problems and the price of the equipment are rising rapidly, hence, whenever it is possible the lower voltage about 60 kV is to be chosen. The maximum power of the H.V. supply depends on the maximum depth of weld required.
The high voltage equipment must also supply the low voltage above 5 V for the cathode heating, and negative voltage up to about 1000 V for the control electrode.
The electron gun also needs low voltage supplies for the correction system, the focusing lens, and the deflection system. The last one may be very complex when it should provide the computer controlled imaging, engraving and similar applications of the beam.
Complex electronics may also be needed for the control of the work-piece manipulator.
Since the publication of the first practical electron beam welding equipment by Steigerwald in 1958 the electron beam (EB) welding spread rapidly in all branches of engineering where welding can be applied. To cover the most various requirements, a countless number of EB welder types has been designed, differing in construction, working space volume, work-piece manipulators and beam power. The electron beam generators (electron guns) designed for welding applications can supply beams with power ranging from a few watts up to about one hundred kilowatts. "Micro-welds" of tiny components can be realized, as well as deep welds up to 300 mm (or even more if needed.
The vacuum working chambers of various design may have the volume of only a few liters, but vacuum chambers with the volume of several hundreds cubic meters have also been built.
http://ebt.isibrno.cz/en/special-technologies). "Standardizing the Art of Electron-Beam Welding". Lawrence Livermore National Laboratory. https://www.llnl.gov/str/MarApr08/elmer.html. Retrieved 2008-10-16.
|
||||||||||||||||||||||