Electron Beam Welding (EBM)

 Electron Beam Welding (EBW) is a fusion welding process in which a beam of high- velocity electrons is used for producing high temperature and melting the workpiece to be welded. The electrons strike the workpiece and their kinetic energy is converted into thermal energy by releasing heat. This heat is used to heat the metal so that the edges of workpiece are fused and joined together forming a weld.

1. Working Principle

If a filament of tungsten or tantalum is heated to high temperature in a vacuum either directly by means of an electric current or indirectly by means of an adjacent heater, a greater number of electrons are given off from the filament. These electrons carry a negative charge which is passed through the anode hole. The greater is the filament current, the higher will be the temperature and greater will be the electron emission.

If a metal disc with a central hole is placed near the filament and charged to a high positive potential relative to the filament, the emitted electrons are attracted to the disc because of their kinetic energy pass through the hole as a divergent beam. So, the filament is the cathode and the disc is the anode. The electron beam is focused by the focusing lens as shown in Figure 2.28. The focus is done electrostatically or magnetically by means of coils situated adjacent to the beam and through which a current is passed. The beam is now convergent and it can be spot focused. The basic arrangement of an electron 'gun' is done which is similar to television tubes and electron microscopes.

When the focused electron beam strikes the workpiece, the kinetic energy of this electron beam is converted into heat energy. This heat energy is used to weld the metals. The kinetic energy of an electron is mV2/2.

where m is the mass of an electron (9.1 × 10-28 g)

V is its velocity.

The variables which are controlled in the electron beam welding are as follows:

1. Voltage

2. Speed

3. Distance between beam gun and workpiece.

The electron mass m is small but increasing the emission from the filament by raising the filament current increases the number of electrons. Hence, it produces the mass effect because the kinetic energy varies directly as the square of the velocity V, accelerats the selectrons up to velocities comparable with the velocity of light by using anode voltages (up to 200 kV) and greatly increases the beam energy. The smaller the spot into which the beam is focused, greater will be the energy density. So, it is possible to weld holes. The beams are focused about 0.25 mm to 1 mm diameter and the power density is of 10 kW/mm2. Aluminium material has focusing length of about 40 mm and steel has about 30 mm.

Focusing coils can concentrate the beam on a spot of a few micron in diameter. With this concentrated spot, a weld bead of narrow width relative to the plate thickness is formed.

When the beam strikes a metal surface X-rays are generated, adequate precautions must be taken for screening personnel from rays. If the beam emerges into the atmosphere, energy is reduced by collision of electrons with atmospheric molecules and focus is impaired. Hence, the operation is carried out in vacuum. The vacuum may be created either in the gun chamber or in a separate steel component chamber fixed to the gun chamber. Welding in non-vacuum atmospheric conditions requires much greater power than the vacuum method because of the effects of the atmosphere on the beam and the greater distance from gun to work. A shielding gas may be required around the weld area.

Welds made with this process on thicker sections are narrow with deep penetration with minimum thermal disturbance. At present, welds are performed in titanium, niobium, tungsten, tantalum, beryllium, nickel alloys, inconel, aluminium alloys and magnesium, mostly in the aero and space research industries.

2. Advantages, Limitations and Applications of EBW

Advantages:

1. High-quality welds, deep and narrow profiles are produced.

2. Clean and bright weld can be obtained.

3. High speed operation can be achieved.

4. Dimensional accuracy is good.

5. Very small part can be welded.

6. There is no need of using electrodes.

7. Accurate control over welding conditions is possible by the control of electron emission and beam focus.

8. No flux or shielding gases or filler metal is needed.

9. It has limited heat affected zone and low thermal distortion.

10. Because of the vacuum conditions, it is possible to weld more reactive metals successfully.

11. This type of weld is more suitable for welding dissimilar materials.

12. Tight continuous weld can be produced.

Limitations:

1. The welding cost is high.

2. Skilled persons are required.

3. It is limited to small size welding.

4. Welding should be carried out in vacuum seal only.

5. It is a time consuming process.

6: The weld suffers from contamination if it is performed in atmospheric condition.

7. Precise joint preparation and alignment are required.

8. X-ray irradiation occurs.

9. As EBW generates X-rays, there must be protection against radiation hazards.

Applications:

1. Dissimilar metals can be welded.

2. Refractory and reacting metals can be welded.

3. It is used in aircrafts, missile, nuclear component, gears and shafts.

4. It is suitable for large scale.

5. It is used in cams.