Electron Beam machining

EBM - Unit 5

1. Electron beam machining D.PALANI KUMAR, Assistant Prof. / Mech. Engg ., Kamaraj College of Engg . & Tech. Virudhunagar.

2. ELECTRON BEAM MACHINING (EBM) Invented in Germany in 1952 by Dr. K. H. Steiger wald . EBM is a High-Energy-Beam Machining process Electrical energy is used to generate high-energy electrons The mechanism of material removal is primarily by melting and rapid vaporization due to intense heating by the electrons. An electron beam

3. EBM (LINE DIAGRAM)

4. EBM – PROCESS Electron beam (negatively charged particles) is generated in an electron beam gun. Electron beam gun provides high velocity electrons over a very small spot size. Due to pattern of electrostatic field produced by grid cup, electrons are focused and made to flow in the form of a converging beam through anode.

5. EBM – PROCESS (cont.) The electrons are accelerated while passing through the anode by applying high voltage at anode. A magnetic deflection coil is used to make electron beam circular and to focus electron beam at a point (localized heating). The workpiece to be machined is located under the electron beam and is kept under vacuum. The high-energy focused electron beam is made to impinge on the workpiece with a spot size of 10 – 100 μm The kinetic energy of the electrons, upon striking the workpiece , changes to heat, which melts and vaporizes minute amounts of the material.

6. EBM – PROCESS (cont.) The “melt – vaporization” front gradually progresses Finally the molten material, if any at the top of the front, is expelled from the cutting zone by the high vapour pressure at the lower part. Localized heating by focused electron beam Gradual formation of hole

7. EBM – PROCESS (cont.) The whole process is carried out in a vacuum chamber The gun in EBM is used in pulsed mode. Holes can be drilled in thin sheets using a single pulse. For thicker plates, multiple pulses would be required. Penetration till the auxiliary support Removal due to high vapour pressure

8. WHY VACUUM CHAMBER ? The entire process occurs in a vacuum chamber because a collision between an electron and an air molecule causes the electrons to scatter and thus loose their energy and cutting ability .

9. EBM – EQUIPMENTS Electron Beam Gun

10. Electron beam gun is the heart of EBM. The basic functions of any electron beam gun are to generate free electrons at the cathode, accelerate them to a sufficiently high velocity and to focus them over a small spot size. Cathode is generally made of tungsten or tantalum. Such cathode filaments are heated, often inductively, to a temperature of around 2500 C. Heating leads to thermo-ionic emission of electrons. A combination of repelling forces from the negative cathode and the attracting forces from the positive anode causes the free electrons to be accelerated and directed toward the work piece. One of the major requirements of EBM operation of electron beam gun is maintenance of desired vacuum.

11. vacuum is achieved and maintained using a combination of rotary pump and diffusion pump. Diffusion pump is attached to the diffusion pump port of the electron beam gun Diffusion pump is essentially an oil heater. As the oil is heated the oil vapour rushes upward. Nozzles change the direction of motion of the oil vapour and the oil vapour starts moving downward at a high velocity. Such high velocity jets of oil vapour entrain any air molecule present within the gun. This oil is evacuated by a rotary pump via the backing line.

12. Power Supply The high-voltage power supply used for EBM systems generates voltages of up to 150 kv to accelerate the electrons. The most powerful electron beam machining systems are capable of delivering enough power to operate guns at average power levels of up to 12 kw . Individual pulse energy can reach 120 joules/pulse. To avoid the possibility of arcing and short circuits, the high-voltage sections of the power supply are submerged in an insulating dielectric oil.

13. EBM PROCESS – PARAMETERS Process parameters which directly affect the machining characteristics in EBM are: The accelerating voltage electrons get accelerated at high voltage. The beam current related to the number of electrons emitted by the cathode or available in the beam. Beam current can be as low as 200 μamp to 1 amp. Pulse duration pulse duration can be as low as 50 μs to as long as 15 ms. Energy per pulse

14. Power per pulse Lens current Spot size Spot size is controlled by degree of focusing achieved by the electromagnetic lenses. For a lower spot size, the material removal would be faster though the size of the hole would be smaller. Power density The energy density and power density is governed by energy per pulse duration and spot size .

15. Total penetration range = 2.6 * 10 -17 (V 2 / ρ ) mm where, V = accelerating voltage in volts and ρ = density of material in kg/mm 3 Speed of electron under control of electric field V = E √(2e/m) m/s where, m = mass of electron in kg = 9.1 * 10 -31 kg e = charge of electron in coulomb = 1.6 * 10 -19 E = voltage of electric field in volts Thus, V = 600 E Total power for beam current of I amperes P = E I watts

16. Force of beam on molten metal F = 0.34 I √E dynes Power Requirement (P) for EBM process is approx. proportional to MRR i.e. P α MRR P = C * MRR where C is specific power consumption = 12 W / mm 3 / min for tungsten and 7, 6, and 4 W / mm 3 / min for iron, titanium and aluminium respectively.

17. ELECTRON BEAM PROCESS CAPABILITY EBM can provide holes of diameter in the range of 100 μm to 2 mm with a depth upto 15 mm, i.e., with a l/d ratio of around 10. There would be an edge rounding at the entry point. Materials such as steel, stainless steel, Ti and Ni super-alloys, Al as well as plastics, ceramics, leathers can be machined successfully using EBM. The heat-affected zone is rather narrow due to shorter pulse duration in EBM. Typically the heat-affected zone is around 20 to 30 μm . Typical kerf shape of electron beam drilled hole

18. materials like Al and Ti alloys are more readily machined compared to steel. EBM does not apply any cutting force on the workpieces . Thus very simple work holding is required. Holes can also be drilled at a very shallow angle of as less as 20 to 30 . Variation in drilling speed with volume of material removal

19. DESIGN CONSIDERATIONS Non-reflective workpiece surfaces are preferable Sharp corners are difficult to produce; deep cuts produce tapers Consider the effects of high temperature on the workpiece material Parts should match the size of the vacuum chamber Consider manufacturing the part as a number of smaller components

20. COMPARATIVE PERFORMANCE

21. EBM - ADVANTAGES Extremely close tolerances can be maintained Heat affected zone are minimum It can machine almost any material irrespective of their mechanical properties The beam can be concentrated on a very small area It produces better surface finish and narrow kerf Thermal distortion is least The process is fast because it is entirely non-mechanical.

22. EBM - LIMITATIONS The equipment cost is very high. The interaction of the electron beam with work piece surface produces hazardous X-ray. Hence shielding is necessary Vacuum is essentially required. Because of very low material removal rate, the process is economical only for small volume cuts. Skilled labour is required to accelerate the electrons. Very high voltage is required to accelerate the electrons. The process can machine only thinner parts.

23. EBM - APPLICATIONS EBM is particularly suitable for producing very small diameter holes – down to 0.002 in. It is especially adapted for micromatching . Major applications of EBM include matching in thin materials, cutting of slots and drilling of holes with very high depth to diameter ratios, usually more than 100:1. Machining of wire drawing dies having small cross sectional area. EBM is also used as an alternative to light optics manufacturing methods in the semiconductor industry.

24. Because electrons have a shorter wavelength than light and can be easily focused, electron-beam methods are particularly useful for high-resolution lithography and for the manufacture of complex integrated circuits Welding can also be done with an electron beam, notably in the manufacture of aircraft engine parts

25. EBM CHARACTERISTICS Mechanics of material removal – melting, vaporization Medium – vacuum Tool – beam of electrons moving at very high velocity Maximum MRR = 10 mm 3 /min Specific power consumption = 450 W/mm 3 /min Critical parameters – accelerating voltage, beam diameter , work speed, melting temperature Materials application – all materials Shape application – drilling fine holes, cutting contours in sheets , cutting narrow slots Limitations – very high specific energy consumption, necessity of vacuum, expensive machine

26. Thank you