What is Electron Beam Welding?

Electron beam welding (EBW) is a fusion welding process that uses a high energy electron beam to join metals together, with a wide range of applications in many industries.

 

How Does Electron Beam Welding Work?

An electron beam welding machine consists of an electron gun, which is a device for producing a focused beam of high-speed electrons with velocities of between 0.3 and 0.7 times the speed of light.

The electron gun is normally mounted on, or in, a high vacuum chamber that contains tooling used for holding and moving a workpiece.

The electron beam passes through the anode and on towards the workpiece, using a focusing lens to focus the beam and achieve sufficient power density to weld the two metals.

This creates a high weld quality, with deep penetration and a narrow fusion zone.

 

Components of an Electron Gun

An electron beam is usually formed by employing a triode-style electron gun that 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 to the hot cathode emitter
  • An anode, a ground potential electrode through which the electron flow passes in the form of a collimated beam

 

How is the electron beam generated?

The hot cathode, or filament, is made from a high emission material such as tungsten or tantalum, with tungsten being the most common. 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.

 

How is the electron beam controlled?

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 workpiece.

As the electrons travel to the workpiece, they pass through a focus coil or focusing lens (electromagnetic lens) so that the beam can be focused to a fine point 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.

The addition of some hardware enables functions such as backscattered imaging, joint finding, real-time seam tracking, automatic beam alignment, engraving, beam splitting and Surfi-Sculpt™*.

 

Electron Beam Welding Machine Schematic Diagram

 

Electron Beam Welding Schematic Diagram

 

Find out more about electron beam welding on our electron beam welding page, or watch our CGI animation video here.

*Surfi-Sculpt is a trade mark of TWI Ltd.

Electron Beam Welding vs Laser Beam Welding

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

 

The difference between laser beam welding and electron beam welding

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

Electron beam welding is lesser-known than laser welding out of the two techniques. This is not because it is an inferior process to laser, but mostly due to people’s perceptions. Most have heard about or have seen Star Wars, James Bond, and a host of other hi-tech 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, the electron beam process has taken a back seat.

The following article examines some of the differences between electron beam welding in a vacuum vs laser welding with a shielding gas.

 

Vacuum Environment

Electron beam welding in a vacuum aids the weld quality, as it tends to pull contamination away from the weld pool. Welding in a vacuum also results in the operator not becoming exposed to the hazardous welding environment.

Conventional laser welds at atmospheric pressure, with an additional shielding gas. However, you can laser weld in a vacuum, which significantly increases the depth of the weld.

 

Shielding Gas

Shielding gas is not required for electron beam welding as the process is done in either low or high vacuum.

Laser welding in a normal atmosphere requires a shielding gas; it is an expensive, but essential, consumable. Fume extraction may also be an issue.

 

Component Size

Electron beam welding in a vacuum places restrictions on component size, as it must fit inside the vacuum chamber. Chamber volumes are kept to a minimum to reduce evacuation times.

Laser welding with a shielding gas can accommodate any component size, as there is no vacuum chamber. Furthermore, fibre optic delivery systems can be used. This allows the welding head to be remote from the power source.

 

Welding Speed

Electron beam welding can achieve deep penetration welds over a wide range of speeds, whereas laser welding with a shielding gas requires high welding speeds due to the plume of metal vapour that forms.

 

Weld Quality

Electron beam welding produces high-quality welds in a wide variety due to the inert atmosphere, which creates a very stable and repeatable environment. Joint finding and imaging using backscattered electrons are advanced options that can further increase the weld quality.

Laser welding 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, but they can improve weld quality.

 

Single Pass Welding of Thick Sections

Electron beam welding in a vacuum can achieve 20mm penetration in stainless steel when using 6kW beam power at 60kV. Up to 300mm thicknesses can be welded in a single pass.

Laser welding with shielding gas can achieve approximately 1kW per mm depth of weld in steel. However, limited availability and high cost of high-power laser systems is a factor.

 

Automated Process

Electron beam welding can be highly automated with the evacuation time of the chamber in a few seconds. 40 seconds per components is a typical cycle time within the automotive industry, but time is dependent upon the length and complexity of the weld.

Laser welding can also be highly automated with high production rates, in addition to there being no waiting time for chamber evacuation. Beam splitting and beam sharing are also possible.

 

Wearing Components

The main wearing component within the electron beam welding process is the filament. Metal vapours can deposit on the viewing prism, but this has no effect on weld characteristics and the prism can be cleaned.

During the laser welding process, optical devices such as mirrors and lenses can be coated by a metal vapour that is produced during welding, leading to a drop in beam power.

 

Power Efficiency

Electron beam welding is a very efficient process, typically converting 85% of electrical power.

Laser welding typically converts up to 40%, when using modern fibre and disc lasers.

 

Cost Comparison

Electron beam welding is more expensive than tungsten inert gas (TIG) and metal inert gas (MIG) welding.

Laser welding is also more expensive than TIG and MIG, with prices increasing steeply with increasing power.

 

Turnkey Solutions

Electron beam systems include a chamber, fully automatic vacuum system, work handling, and control system.

Laser welding usually requires a systems integrator to provide an integrated solution, as the laser source does not include a control system or work manipulation.

 

 

The best process to use is often dependent on the given application. If you are not sure which system is right for your application, please get in touch! Our machines are built-to-order and manufactured at our Cambridge Headquarters. With 60-years of process know-how in providing turn-key solutions, we can find the right solution for your application.

What is near-net shape?

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 be 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.

 

How does near net shape work?

With near net shape, the 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.

 

What are the advantages?

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.

  • 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 feedstock can be wire fed or powder
  • Electron beam outperforms 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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