How Does Electron Beam Welding Work?

An electron beam welding machine consists of a device for producing a focused beam of high-speed electrons, velocities of between 0.3 and 0.7 times the speed of light. This is known as an electron gun, which is normally mounted on or in an evacuated chamber which contains tooling/fixtures used for holding and moving a work piece.

An electron beam is usually formed by employing a triode- style electron gun which consists of a cathode (also known as a filament), a heated source (emitter) of electrons that is maintained at a high negative potential, a grid cup (also known as a bias cup), a specially shaped electrode that can be negatively biased with respect to the hot cathode emitter, and an anode, a ground potential electrode through which the electron flow passes in the form of a collimated beam. The hot cathode or filament is made from a high emission material such as tungsten or tantalum. The most commonly tungsten.

A heating current is passed through to a filament that causes it to emit electrons. These are accelerated from the filament to the anode by applying a high negative voltage to the filament. A control electrode, or grid, is situated close to the filament and is held at a negative potential in respect to it. As the voltage potential between the filament and grid is reduced, electrons are allowed to escape. These are then attracted to the anode, which has more positive potential than the electrons, or in the case of electron beam welding the anode has a hole in its centre which allows the electrons to pass through the anode as a stream, and on towards the work piece. As the electrons travel to the work piece they pass through a focus coil, or focusing lens (electromagnetic lens) so that the beam can be focused to a fine point in order to achieve sufficient power density to weld metals.

The electron beam will also pass through a deflection coil (electromagnetic) at the bottom of the column which can manipulate the beam at very high speeds. With the addition of some hardware this enables functions such as backscattered imaging, joint finding, real-time seam tracking, auto beam alignment, engraving, beam splitting and Surfi-Sculpt.

 

When comparing electron beam welding (EBW) to laser welding (LW) it is important to note that both processes fall under a general heading of power beam welding.

Electron beam welding uses a finely focused stream or beam of electrons, and lasers use monochromatic coherent light, photons. In both cases, the energy of the electrons or photons are turned into heat energy when they impinge upon the surface of metal.

Of the two techniques, electron beam welding is probably less well known than lasers. This is not because it is an inferior process to laser, but probably due to people’s perceptions. Most have heard about or have seen Star Wars, James Bond, and a host of other hi-tec Sci-fi films that have bombarded our screens over many years and coupled with the high profile many respected institutions have been putting forward, unfortunately, electron beam has taken a back seat.

The following table examines some of the differences:

 

Electron Beam Welding in Vacuum Laser Welding with Shielding Gas
Single Pass Welding of Thick Sections
  • 6 kW beam power at 60 kV achieves over 20 mm penetration in stainless steel
  • Up to 300 mm thickness can be welded in a single pass
  • For steels approximately 1 kW per mm depth of weld is required
  • Limited availability and high cost of high power laser systems is a factor
Welding Speed
  • Deep penetration welds possible over a wide range of speeds
  • High welding speeds are required due to the plume of metal vapour formed
Automated Process
  • Can be highly automated with evacuation time of the chamber in a few seconds
  • Typical cycle times found within the automotive industry 40 seconds per component
  • Time is dependent upon length and complexity of weld
  • Can be highly automated with high production rates
  • No waiting time for chamber evacuation
  • Beam splitting and beam sharing are also possible
Component Size
  • Component size is restricted by the size of vacuum chamber
  • Chamber volumes are kept to a minimum to reduce evacuation times
  • Not restricted by component size
  • Fibre optic delivery systems can be used allowing the welding head to be remote from the power source
Vacuum Environment
  • Vacuum aids in the weld quality, as it tends to pull contamination away from the weld pool
  • Operator not exposed to hazardous environment
  • Laser conventionally uses atmosphere with additional shielding gas
  • Laser welding in vacuum significantly increases the depth of weld
Weld Quality
  • High quality weld due to inert atmosphere, very stable and repeatable
  • Deep penetration welds on a wide variety of materials
  • Joint finding and imaging using back scattered electrons is an option
  • Needs a shielding gas, typically nitrogen or argon, to prevent oxidisation of the weld area and stability of the weld pool
  • Real time monitoring of weld depth and quality are expensive options
Shielding Gas
  • Not required as the process is done in either high or low vacuum
  • Essential consumable and expensive
  • Fume extraction may be an issue
Wearing Components
  • Filaments
  • Metal vapours can deposit on viewing prism with no effect on weld characteristics
  • Prism can be cleaned
  • Optical devices such as mirrors and lenses can be coated by metal vapour produced during the welding process leading to drop in beam power
Power Efficiency
  • Typically 85% conversion of electrical power
  • Up to 40% for modern fibre and disc lasers
Cost Comparison
  • More expensive than tungsten inert gas (TIG) and metal inert gas (MIG) welding
  • More expensive than TIG & MIG
  • Price increases steeply with increasing power
Turnkey solutions
  • EB systems includes chamber, fully automatic vacuum system, work handling and control system
  • Usually requires a systems integrator to provide an integrated solution as the laser source does not include work manipulation and control system

 

The best process to use is often dependent on the given application.

Near net shape is a layer-additive process, which is used to build parts using a controlled high energy electron beam to melt a pool on the substrate whilst the wire feed system adds material to build up the required part.

The electron beam provides the energy source used for melting metallic feedstock, which is typically wire (powder can also used). The electron beam is a highly efficient power source that can be both precisely focused and deflected using electromagnetic coils at rates well into thousands of hertz.

A major advantage of using metallic components with electron beams is that the process is conducted within a high vacuum environment of 1×10-4 mBar or greater, providing a contamination-free work zone that does not require the use of additional inert gasses commonly used with laser and arc based processes.

With near net shape, feedstock material is fed into a molten pool created by the electron beam. Through the use of CNC, the molten pool is moved about on a substrate plate, adding material just where it is needed to produce the near net shape. This process is repeated in a layer-by-layer fashion, until the desired 3D shape is produced.

  • Parts are 100% dense and structurally sound without moulds or tooling
  • Intricate, complex geometries can be made in a single operation whilst minimising scrap and manufacturing time
  • Metal feed stock can be wire fed or powder
  • Electron beam out performs lasers with higher integrity coupling and better deposition rates
  • Reduces scrap material wastage
  • Final machining time reduced

Figure 1. Near Net Shape

 

Figure 2. Near Net Shape

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