Electron-beam welding

Introduction

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.

Thermal effects of electron beams (formerly known as “cathode rays”) and their application for melting metals have been reported as early as in 1879, but systematic development of technologies based on this effect were started only in 1950`s by the need to weld new materials used in nuclear research and industry. 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. Since that time innumerable number of electron beam welders has been designed and is being used in numerous branches of industry and research.

1.1Physics of electron beam heating

It is well known that electrons are elementary particles possessing the mass m = 9,1 . 10^-28 g, and negative electrical charge e - 1,6 . 10^19 As. They exist either bound to atomic nucleus, or as conduction electrons in 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:

- Strong electric field can accelerate electrons to a very high speed, i.e. the electron beam can carry high power, equal to the product of beam current and accelerating voltage. Increasing the beam current and the accelerating voltage, the beam power can be increased to any practically desirable value. - Using electromagnetic lenses the beam can be shaped into a narrow cone and focused to a very small diameter with a very high power density in the plane of impingement of the beam on the surface of some solid. Values of power density in crossover (focus) of the beam as high as 10^4 – 10^6 W/mm^2 can be achieved. - The depth of penetration of electrons into solids, as will be shown later, may be in the order of hundredths of mm. 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 10^5 – 10^7 W/mm^3. Consecutively, the temperature in this volume increases extremely rapidly, 10^8 – 10^10 K/s. 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 10^3 W/mm^2) 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 in a very short time, which can be applied for “machining”.

2 Beam formation

2.1 Cathode - the source of free electrons

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 rule. As a source of electrons for electron beam welders, the material must fulfil more requirements:

- to achieve high power density in the beam, the emission current density [A/mm2], hence the working temperature, should be as high as possible, - to keep evaporation in vacuum low, the material must have low enough vapour pressure at working temperature. 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/mm^2 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, as will be discussed later. The type of cathode most frequently used in electron beam welders is made of tungsten strip, about 0,05mm thick. 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. Due to the high working temperature, the ”life” of the cathode is rather limited by evaporation and some other effects, usually to a few hours, and it must therefore be easily replaceable

2.2 Acceleration of electrons, current control

Electrons emitted from the cathode posses 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 axes. Due to this effect, the electron beam converges to some minimum diameter in a plane close to the anode. The word “crossover” is used to indicate this part of the beam in English. 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, control electrode (Wehnelt cylinder).

By application of high enough control voltage all emitted electrons may be prevented to enter the electric field produced by accelerating voltage applied between the anode and the cathode. Decreasing the negative control voltage until the positive gradient area of the electric field reaches the cathode surface, the beam current can be increased continually up to some upper limit dependent on the temperature of the cathode and the “space charge” of electrons surrounding the cathode. After passing the opening in the anode electrode the electrons move with constant speed in a narrow, slightly divergent cone unless they enter the field of focussing lens.

2.3 Focussing effect of magnetic field

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) courses a slight rotation of the beam cross-section. This effect can be observed and utilized by alignment of the beam.

3 Applications of electron beam for welding

3.1 Generally

Possibility to use electron beam for welding metals is based, as in any other welding technology, on its capability to heat any material to its melting point. The fact that the heating by an electron beam can be extraordinary fast provides us with exceptional possibilities. The essential difference, when compared with other methods of heating, is in the fact that the transport of energy into the solid metal is other than by conduction, and may be much faster. As explained above, at high power concentrations in the beam spot, any material is in very short time transmuted into vapor, which, evidently, enables the beam to penetrate, fast and deep, into the metal, albeit the electrons provably penetrate only a few hundredths of mm into the solid. The process of penetration is the most important, but also most difficult to be described in detail. Many attempts had been made to explain it exactly, but the conditions are so complicated and extraordinary that it can not be theoretically analyzed or directly studied experimentally.

3.2Process of penetration

It can be proved, both theoretically and experimentally, that fast electrons striking the solid matter penetrate only a small distance under its surface before they transmit all their kinetic energy to other particles.

That results in high energy concentration in a small volume of the mater, causing the change of its state from solid to vapor in a very short time. The vapor is then overheated (and ionized), expands and presses the melt downwards and to the sides, making so the way for the beam to impinge fresh solid material. In this way the proccess progresses with high speed. With enough power and power density, the beam can penetrate hundreds of mm deep in about one second, which has been proved experimentally many times.84.42.225.153 (talk) 11:12, 2 June 2011 (UTC)

If the above described phenomena should be applied for joining metal parts the workpiece must be moved relative to the beam in some appropriate speed. To describe exactly what takes place around the moving beam is impossible, but we may be sure that the keyhole enabling the deep penetration is traveling with the beam surrounded by envelop of vapor and melted material. The material continuously melts in front of the beam and solidifies in the rear. Pressure of the vapor, the hydrostatic pressure and surface tension of the melted material play important role in the process. The boiling of the melted material and its rapid solidification results in rough surface of the seam It also may be the cause of cavities formed sometimes at the bottom of the keyhole.

The results of the beam activity depend on several factors. Many experiments and innumerable many 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.

(1) – The power of the beam [kW] 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.

(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.

(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 semicircular 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 (if well focused) penetrates deeper, proportionally to its total power.

3.2.4 Choice of welding parameters

The result of the welding process depends, as mentioned above, on the beam power, power density (focusing), welding speed and properties of the material. As guidance for welding operator, results of a “test welding” in the given material should be done with most often used materials by the operator with the particular welding equipment individually.

4 EB welding equipment

The electron beam welders are nowadays used and manufactured in countless number of types, differing in working chamber dimensions, beam power, and many other features. The working vacuum chamber volume covers the range from a few lites up to hundreds of cubic meters. The beam power may have any value from a few watts up to 100 kW. 84.42.225.153 (talk) 08:10, 3 June 2011 (UTC)

The EBW can be applied in different welding environments:

4.1: Vacuum welding

The method first developed requires that the welding chamber be at a hard vacuum. Material as thick as 15 cm (6 in) can be welded, and the distance between the welding gun and workpiece (the stand-off distance) can be as great as 0.7 m (30 in). While the most efficient of the three modes, disadvantages include the amount of time required to properly evacuate the chamber and the cost of the entire machine.

4.2 Low pressure welding

As electron beam gun technology advanced, it became possible to perform EBW in a soft vacuum, under pressure of 0.1 torr. This allows for larger welding chambers and reduces the time and equipment required to attain evacuate the chamber, but reduces the maximum stand-off distance by half and decreases the maximum material thickness to 5 cm (2 in).

4.3 In-air welding

The third EBW mode is called nonvacuum or out-of-vacuum EBW, since it is performed at atmospheric pressure. The stand-off distance must be diminished to 4 cm (1.5 in), and the maximum material thickness is about 5 cm (2 in). However, it allows for workpieces of any size to be welded, since the size of the welding chamber is no longer a factor. A schematic drawing may be helpful. [1]

4.4 Equipment

The electron beam gun used in EBW both produces the electrons and accelerates them, using a hot cathode emitter made of tungsten that emits electrons when heated. (Steigerwald also experimented with tantalum filaments because of the lower work function). The electrons are then accelerated to a hollow anode inside the gun column by means of a high voltage differential. They pass through the anode at high speed (approx 1/2 the speed of light) and are then directed to the workpiece with magnetic forces resulting from focusing and deflection coils. These components are all housed in an electron beam gun column, in which a hard vacuum (about 0.00001 torr) is maintained.

The EBW power supply pulls a low current (usually less than 1 A), but provides a voltage as high as 60 kV in low-voltage machines, or 200 kV in high-voltage machines. High-voltage machines supply a current as low as 40 mA, and can provide a weld depth-to-width ratio of 25:1, whereas the ratio with a low-voltage machine is around 12:1. The beam power of a power supply is an indicator of its ability to do work, and determines the power density (generally 40-4000 kW/cm² or 100-10,000 kW/in²).

The welding chamber walls and doors must be thick enough to act as X-ray shielding.

For the hard vacuum and soft vacuum EBW methods, the welding chamber used must be airtight and strong enough to prevent it from being crushed by atmospheric pressure. It must have openings so that the workpieces can be inserted and removed, and its size must be sufficient to hold the workpieces but not significantly larger, as larger chambers require more time to evacuate. The chamber must also be equipped with pumps capable of evacuating it to the desired pressure. For a hard vacuum, a diffusion pump is necessary, while soft vacuums can often be obtained by less costly equipment.

Electron beams can also be sent from their vacuum column through membrane or plasma window for a short distance into the air and this is used for production welding, for example welding the hard teeth of hacksaw blades onto a tougher backing steel. The plasma window is a relatively recent advance which has turned this kind of EBW into a far more practical tool. Previously the vacuum containment membranes were expensive and degraded quickly by the constant stream of high energy electrons.

See also

• Electron beam technology

External links

• Elmer, John (2008-03-03). "Standardizing the Art of Electron-Beam Welding". Lawrence Livermore National Laboratory. Retrieved 2008-10-16.