ELECTRON BEAM WELDING (EBW)
Electron Beam Welding (EBW) is a fusion welding in which coalescence is produced by heating the work piece due to impingement of the concentrated electron beam of high kinetic energy on the work piece. As the electron beam impinges the workpiece, kinetic energy of the electron beams converts into thermal energy resulting in melting and even evaporation of the work material.
Principles of ELECTRON BEAM WELDING (EBW)
In general, electron beam welding process is carried out in vacuum. In this process, electrons are emitted from the heated filament called electrode. These electrons are accelerated by applying high potential difference (30 kV to 175 kV) between cathode and anode. The higher the potential difference, the higher would be the acceleration of the electrons. The electrons get the speed in the range of 50,000 to 200,000 km/s. The electron beam is focused by means of electromagnetic lenses. When this high kinetic energy electron beam strikes on the workpiece, high heat is generated on the work piece resulting in melting of the work material. Molten metal fills into the gap between parts to be joined and subsequently it gets solidified and forms the weld joint.
An EBW set up consists of the following major equipment:
Work pie e handling device.
An electron gun generates, accelerates and aligns the electron beam in required direction and spots onto the work piece. The gun is of two types: Self accelerated and work accelerated. The work accelerated gun accelerates the electrons by providing potential difference
between the workpiece and cathode. In the self accelerate gun,the electrons are accelerated by applying potential difference between cathode and anode. The anode and cathode are enclosed within the gun itself. The control of electron density is better in this type of electron gun. A schematic of an electron beam gun used in EBW is shown in Fig. Major parts of an electron gun are briefly introduced in the following sections.
|Schematic of an electron beam gun used in EBW
It generates t he electrons on direct or indirect heating.
Anode: It is a positively charged element near cathode, across which the high voltage is applied to accelerate the electrons. The potential difference for high voltage equipment ranges from 70-150 kV and for low voltage equipment from 15-30 kV.
Grid cup is a part of triode type electron gun. A negative voltage with respect to cathode is applied to the grid. The grid controls the beam.
It has two parts:
Electron focusing lens and deflection coil. Electron focusing lens focuses the beam into work area. The focusing of the electrons can be carried out by deflection of beams. The electromagnetic lens contains a coil encased in iron. As the electrons enter into the magnetic field, the electron beam path is rotated and refracted into a convergent beam. The extent of spread of the beam can be controlled by controlling the amount of DC voltage applied across the deflection plates.
Electron gun power supply
It consists of mainly the high voltage DC power supply source, emitter power supply source, electromagnetic lens and deflection coil source. In the high voltage DC power supply source the required load varies within 3-100 kW. It provides power supply for acceleration of the electrons. The potential difference for high voltage equipment ranges from 70-150 kV and for low voltage equipment 15-30 kV. The current level ranges from 50-1000 mA. In emitter power supply, AC or DC current is required to heat the filament for emission of electrons. However DC current is preferred as it affects the direction of the beam. The amount of current depends upon the diameter and type of the filament. The current and voltage varies from 25-70 A and 5-30 V respectively. The power to the electromagnetic lens and deflection coil is supplied through a solid state device.
In the vacuum chamber pressure is reduced by the vacuum pump. It consists of a roughing mechanical pump and a diffusion pump. The pressure ranges from 100 kPa for open atmosphere to 0.13-13 Pa for partial vacuum and 0.13-133 mPa for hard vacuum. As the extent of vacuum increases, the scattering of the electrons in the beam increases. It causes the increase in penetration.
Work Piece Handling Device
Quality and precision of the weld profile depends upon the accuracy of the movement of work piece. There is also provision for the movement of the work piece to control the welding speed. The movements of the work piece are easily adaptable to computer numerical control.
Advantages of EBW
High penetration to width can be obtained, which is difficult with other welding processes.
High welding speed is obtained.
Material of high melting temperature can be welded.
Superior weld quality due to welding in vacuum.
High precision of the welding is obtained.
Distortion is less due to less heat affected zone.
Dissimilar materials can be welded.
Low operating cost.
Cleaning cost is negligible.
Reactive materials like beryllium, titanium etc. can be welded.
Materials of high melting point like columbium, tungsten etc. can be welded.
Inaccessible joints can be made.
Very wide range of sheet thickness can be joined (0.025 mm to 100 mm)
Disadvantages of EBW
Very high equipment cost.
High vacuum is required.
High safety measures are required.
Large jobs are difficult to weld.
Skilled man power is required.
Applications of EBW
Electron beam welding process is mostly used in joining of refractive materials like columbium, tungsten, ceramic etc. which are used in missiles.
In space shuttle applications wherein reactive materials like beryllium, zirconium, titanium etc. are used.
In high precession welding for electronic components, nuclear fuel elements, special alloy jet engine components and pressure vessels for rocket plants.
Dissimilar material can be welded like invar with stainless steel.