Electron-beam welding


Electron-beam welding is a fusion welding process in which a beam of high-velocity electrons is applied to two materials to be joined. The workpieces melt and flow together as the kinetic energy of the electrons is transformed into heat upon impact. EBW is often performed under vacuum conditions to prevent dissipation of the electron beam.

History

Electron-beam welding was developed by the German physicist in 1949, who was at the time working on various electron-beam applications. Steigerwald conceived and developed the first practical electron-beam welding machine, which began operation in 1958. American inventor James T. Russell was also credited with designing and building the first electron-beam welder.
In 2023–2025, driven by small modular reactor development, robotic EBW techniques were able to weld pressure vessels in hours/days instead of months. In 2024 a 3 m diameter SMR pressure vessel demonstrator was completed in under 24 hours with a single-pass full penetration welding robot.

Physics

s are elementary particles possessing a mass m = 9.1 · 10−31 kg and a negative electrical charge e = 1.6 · 10−19 C. They exist either bound to an atomic nucleus, as conduction electrons in the atomic lattice of metals, or as free electrons in vacuum.
Free electrons in vacuum can be accelerated, with their paths controlled by electric and magnetic fields. In this way beams of electrons carrying high kinetic energy can be formed. Upon collision with atoms in solids their kinetic energy transforms into heat. EBW provides excellent welding conditions because it involves:
  • Strong electric fields, which can accelerate electrons to high speed that carry high power, equal to the product of beam current and accelerating voltage. By increasing the beam current and the accelerating voltage, the beam power can be increased to practically any desired value.
  • Magnetic lenses can shape the beam into a narrow cone and focus to a small diameter. This allows for a high power density on the surface to be welded. Values of power density in the crossover of the beam can be as high as 104–106 W/mm2.
  • Penetration depths can be on the order of hundredths of a millimeter. This provides a high volumetric power density, which can reach values of the order 105–107 W/mm3. The temperature in this volume can increase rapidly, up to 108–1010 K/s.
Beam effectiveness depends on many factors. The most important are the physical properties of the materials to be welded, especially the ease with which they can be melted or vaporize under low-pressure conditions. EBW can be so intense that material can boil way, which must be taken into account. At lower values of surface power density the loss of material by evaporation is negligible for most metals, which is favorable for welding. At higher power, the material affected by the beam can quickly evaporate; switching from welding to machining.

Beam formation

Cathode

s move in a crystal lattice of metals with velocities distributed according to Gauss's law and depending on temperature. They cannot leave the metal unless their kinetic energy is higher than the potential barrier at the metal surface. The number of electrons fulfilling this condition increases exponentially with increasing metal temperature, following Richardson's rule.
As a source of electrons for electron-beam welders, the material must fulfill certain requirements:
  • To achieve high power density, the emission current density , hence the working temperature, should be as high as possible,
  • To keep evaporation in vacuum low, the material must have a low enough vapour pressure at the working temperature.
  • The emitter must be mechanically stable, not chemically sensitive to gases present in the vacuum atmosphere, easily available, etc.
These and other conditions limit the choice of material for the emitter to metals with high melting points, practically to only tantalum and tungsten. Tungsten cathodes allow emission current densities about 100 mA/mm2, but only a small portion of the emitted electrons takes part in beam formation, depending on the electric field produced by the anode and control electrode voltages. The most frequently used cathode is made of a tungsten strip, about 0.05 mm thick, shaped as shown in Figure 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.

Acceleration

Electrons emitted from the cathode are low energy, only a few eV. To give them the required speed, they are accelerated by an electric field applied between the emitter and the anode. The accelerating field must also direct the electrons to form a narrow converging “bundle” around an axis. This can be achieved by an electric field in the proximity of the cathode which has a radial addition and an axial component, forcing the electrons in the direction of the axis. Due to this effect, the electron beam converges to some diameter in a plane close to the anode.
For practical applications the power of the electron beam must be controllable. This can be accomplished by another electric field produced by another cathode negatively charged with respect to the first.
At least this part of electron gun must be evacuated to high vacuum, to prevent "burning" the cathode and the emergence of electrical discharges.

Focusing

After leaving the anode, the divergent electron beam does not have a power density sufficient for welding metals and has to be focused. This can be accomplished by a 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 the complicated influence of a 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, causing the electron to move around the axis. An 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 the magnetic lens is a curve similar to a helix. In this context variations of focal length cause a slight rotation of the beam cross-section.

Beam deflection system

The beam spot must be precisely positioned with respect 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 deflect the beam instead. A system of four coils positioned symmetrically around the gun axis behind the focusing lens, producing a magnetic field perpendicular to the gun axis, is typically used for this purpose.

Penetration

Electron penetration

When electrons from the beam impact the surface of a solid, some of them are reflected, while others penetrate the surface, where they collide with the solid. In non-elastic collisions they lose their kinetic energy. Electrons can "travel" only a small distance below the surface before they transform their kinetic energy into heat. This distance is proportional to their initial energy and inversely proportional to the density of the solid. Under typical conditions the "travel distance" is on the order of hundredths of a millimeter.

Beam penetration

By increasing the number of electrons the power of the beam can be increased to any desired value. By focusing the beam onto a small diameter, planar power density values as high as 104 up to 107 W/mm2 can be reached. Because electrons transfer their energy into heat in a thin layer of the solid, the power density in this volume can be high. The volume density can reach values of the order 105–107 W/mm3. Consequently, the temperature in this volume increases rapidly, by 108–109 K/s.

Results

The results of the beam application depend on several factors:
  1. Beam powerThe power of the beam is the product of the accelerating voltage and beam current , which are easily measured and must be precisely controlled. The power is controlled by the beam current at constant voltage, usually the highest accessible.
  2. Power density The power density at the spot of incidence depends on factors like the size of the cathode electron source, the optical quality of the accelerating electric lens and the focusing magnetic lens, alignment of the beam, the value of the accelerating voltage, and the focal length. All these factors are a function of the design.
  3. Welding speedThe welding equipment enables adjustment of the relative speed of motion of the workpiece with respect to the beam in wide enough limits, e.g., between 2 and 50 mm/s.
  4. Material propertiesDepending on conditions, the extent of evaporation may vary, from negligible to complete. At values of surface power density of around 103 W/mm2 the loss of material by evaporation is negligible for most metals, which is favorable for welding.
  5. Geometry of the joint
The final effect depends on the particular combination of these parameters.
  • Action of the beam at low power density or over a short interval results in melting a thin surface layer.
  • A defocused beam does not penetrate, and the material at low welding speeds is heated only by conduction of the heat from the surface, producing a hemispherical melted zone.
  • High power density and low speed produces a deeper and slightly conical melt zone.
  • A high power density, focused beam penetrates deeper in proportion to total power.