Electron Beam Welding (basic to advanced).pptx

Learn about electron beam welding

1. ELECTRON BEAM WELDING

2. INTRODUCTION Electron-beam welding is fusion welding process in which a beam of high velocity electrons is applied to two materials to be joined. Free electrons in vacuum can be accelerated, with their paths controlled by electric and magnetic fields. Narrow beams of electrons carrying high kinetic energy can be formed, which upon collision with atoms in solids transform their kinetic energy into heat.

3. How does the Process Work? In an electron beam welder electrons are “boiled off” as current passes through filament which is in a vacuum enclosure. Electrons are emitted from the heated filament called electrode. An electrostatic field, generated by a negatively charged filament and bias cup and a positively charged anode, accelerates the electrons to about 50% to 80% of the speed of light and shapes them into a Beam, having high kinetic energies.

4. Electron beams are composed of electrons that are charged particles having a rest mass of 9.1x10-31 kg . The electrons are accelerated from the source with high voltage potential difference (30 kV to 175 kV) between a cathode and anode. The stream of electrons then pass through a hole in the anode. The beam is directed by magnetic forces of focusing and deflecting coils.

5. This beam is directed out of the gun column and strikes the work piece. The kinetic energy of the electrons is transferred to heat upon impact of the work piece and cuts a perfect hole at the weld joint. Molten metal fills in behind the beam, creating a deep finished weld.

6. Electron-beam welding machine

7. Electron gun The electron beam is most often formed by a triode style electron gun under high vacuum conditions. A grid cup, a specially shaped electrode that can be negatively biased with respect to the hot cathode emitter (filament). And an anode, a ground potential electrode through which the electron flow passes in the form of a collimated beam.

8. There are two type of electron guns Self accelerated – Electrons are accelerated by applying potential difference between the cathode and anode. Work accelerated - potential difference is applied between work piece and anode (Diode type).

9. Focusing of electron beam It has two parts: Electron focusing lens and deflection coil. Electron focusing lens focuses the beam into work area. deflection coil (positioned below the magnetic lens) can be employed to “bend” the beam, thus providing the flexibility to move the focused beam spot.

10. Vacuum chamber Generally EBW performed in vacuum. Vacuum in the gun region is needed to maintain gun component cleanliness, prevent filament oxidation, and impede high-pressure short circuiting between the cathode and the anode or the filament and the grid cup.

11. Steps Used in EBW process Joint preparation. Cleaning of work piece. Fixturing of work piece. De-magnetization of work piece. Setting up work piece in chamber. Pump down air form chamber. Carry welding process.

12. Process parameters Major Process Parameters are Accelerating voltage: The potential difference between two electrode which is usually expressed in kilovolts, being utilized to accelerate and increase the energy of the electrons. Increase in the voltage results into increase in the speed of electrons. This equation can use to calculate the acceleration of an 𝑑 𝑉 electron using this equation ∫ − 𝑑𝑥 = 𝑚 𝑒 𝑎

13. Beam current: Beam current is the measure of the quantity of charge (ie: number of electrons),usually expressed in units of milliamperes (mA), that flow per unit time in an electron beam. There is a close relation between electron beam current and depth of penetration as in the graph (Fig 9).

14. Welding speed : Higher welding speeds results into lower depth of penetration Beam Focusing

15. Principle of Deep Penetration Welding

16. Impact of high energy electron beam on w/p surface. The penetration depth into the workpiece is very low, just a few μ m. Most of the kinetic energy is released in the form of heat. The high energy density at the impact point causes the metal to evaporate thus allowing the following electrons a deeper penetration. This finally leads to a metal vapour cavity which is surrounded by a shell of fluid metal, covering the entire weld depth. Capillary action results into formation of weld

17. Joint types favorable to be welded using electron beam we l d i ng

18. In Vacuum Thin and thick plate welding (0,1 mm bis 300 mm). Extremely narrow seams (t:b = 50:1). Low overall heat input => low distortion =>Welding of completely processed components. High welding speed possible. No shielding gas required. High process and plant efficiency. Material dependence, often the only welding method. Advantage of EBW

19. At atmosphere Very high welding velocity. Good gap bridging. No problems with reflection during energy entry into work piece.

20. In Vacuum Electrical conductivity of materials is required. High cooling rates => hardening => cracks. High precision of seam preparation. Beam may be deflected by magnetism. X-ray formation. Size of work piece limited by chamber size. High investment. Disadvantage of EBW

21. At Atmosphere X-ray formation. Limited sheet thickness (max. 10 mm). High investment. Small working distance.

22. Almost all steels. Aluminium and its alloys. Magnesium alloys. Copper and its alloys. Titanium. Tungsten. Gold. Material combinations (e.g. Cu-steel, bronze-steel). Ceramics (electrically conductive). Material favorable for electron beam welding

23. Applications of EBW Mostly used in joining of refectory materials like columbium, tungsten, ceramics. High Precision Welding of electronics components. High precision welding of nuclear fuel elements. Special alloy components of jet engines. Pressure vessels for rocket. Joining of Dis similar metals. Welding of Titanium medical implants.