The concept of heat input in welding processes is very important because it greatly affects the cooling rates of welded joints. As the heat input increases, the cooling rate of the welded joint reduces, giving room for increased grain size as well as the Heat Affected Zones (HAZ), causing changes in properties of the material such as reduced toughness.
Melting principles and applications of Electron Beam Welding
The electron beam welding process is carried out in a vacuum chamber at a pressure of about 1.3x10-4Pa to 1.3x10-3Pa. The purpose of this vacuum chamber is to ensure there is no contact between the electrons and air molecules which can lead to loss of heat.
Electrons are emitted from a cathode called the electron beam welding gun, but as a result of the high voltage (up to 150kV) between the cathode and electrode, the electrons accelerate to about 60% of the speed of light, creating enough kinetic energy for the electron beam to produce the required thermal energy to melt the target surface. However, some of the electrons are converted to X-ray irradiation. The convergence coil focuses the electron beam towards a diameter of about 0.2mm and an energy density of about 1,000 times that obtained from the electron gun. The deflection coil channels the accelerated electrons along the surface to be welded.
Electron Beam Welding produces high precision as well as penetration depth. Penetration depth can be as much as 150mm for steel and 500mm for soft metals such as aluminum, and this ensures that thick plates can easily be welded. Also, the surrounding area of the welded joint always experiences low heat, which ensures that there is a thin Heat Affected Zone (HAZ).
Due to the electron beam welding requirements, the process is automated. This implies that specialized fixtures are utilized to move the metals to be welded into the vacuum chamber. However, advancements in the EBW design have made it possible for the electron gun to be enclosed in a vacuum box in the region of the workpiece to be welded, instead of moving the entire workpiece into the vacuum chamber.
Electron Beam Welding Diagram
Image credit: cwbgroup
Construction of Electron Beam Welding Machine
High Voltage Power Supply: The power supply can be classified into two different categories, the low voltage power supply, which is used to weld thin plates, and the high-voltage power supply used to weld thick plates.
Electron Gun: Free electrons are produced in the electron gun through thermo-emission from a hot metal strap. The electron gun then channels the free electrons into an electron beam.
Cathode Grid: This helps in the production of the fee electrons, but the electrons produced at this stage have low eV. These electrons are accelerated as they pass through a high electric field produced by the anode and cathode.
Anode: The anode increases the speed of the electron beam produced by the cathode and electron gun.
Optical Viewing System: The optical viewing system is used to view the point of action of the electron, to properly align the position of the workpiece.
Convergence Coil: The convergence coil absorbs divergent electrons while focusing the convergent electrons into an electron beam with high intensity.
Deflecting Coil: The deflecting coil focuses the electron beam towards the surface of the base material.
Vacuum Chamber: The vacuum chamber is where the welding process takes place in the absence of air. The pressure in this chamber is also either below or at par with the atmospheric pressure.
Workpiece: The workpiece can be similar or dissimilar metals and is attached to the workpiece holding device called fixtures during welding operations.
Workpiece Holding Device: It is called the fixtures and is used to clamp and hold the workpiece in place. It can be made of cast iron, stainless steel, high tensile steel, etc.
Materials that can be welded using Electron Beam Welding
The electron beam welding can be carried out on the following materials:
Most kinds of carbon steels.
Aluminium and its alloys.
Copper and its alloys.
Dissimilar metal combinations such as Cu-Steel, Bronze-Steel, etc.
Electrically conductive ceramics.
Electron Beam Welding Vs Laser Welding
Sometimes people do not easily differentiate between electron beam welding and laser welding. This is because these two welding processes are similar in various ways such as low heat generation during welding as well as achieving high penetration. The major difference between both processes is that laser welding does not require a vacuum chamber, and this also means that there is contact between the laser beam and air molecules leading to the production of lower thermal energy.
Additionally, high welding speeds are achievable in both electron beam welding and laser welding. However, laser welding has a lower penetration depth than electron beam welding, and as such cannot be used to weld thick plates. Also, in cases where the base material possesses high reflectance, utilizing laser beam welding will be inadequate to produce the desired weld.
What are the Advantages of Electron Beam Welding (EBW)?
The electron beam welding process can be used to weld two dissimilar metals, which is difficult to obtain using other welding processes. Refractory metals such as tungsten, niobium, and molybdenum, as well as active metals such as titanium, zirconium, and beryllium, can be welded using electron beam welding.
Electron beam welding uses no filler metal during welding. The surfaces melt and fuse into each other, creating a very strong bond.
Due to its high precision, electron beam welding produces slim weld as well as slim Heat Affected Zone (HAZ).
There are no impurities in the welded joint because the process is carried out in a vacuum.
EBW is easy to control because the process is automated.
Electron beam welding is carried out in a high-pressure vacuum which ensures that the welded joint is free of impurities and produces a weld with a thin heat-affected zone. However, electron beam welding cost has to be considered when choosing this process.