In this article we will discuss about the meaning and characteristics of Electron Beam Machining (EBM) in industries.
Meaning of Electron Beam Machining (EBM):
In this process, the material is removed with the help of a high velocity (travelling at half the speed of light, i.e. 160,000 km/sec.) focused stream of electrons which are focused magnetically upon a very small area.
These heat and raise the temperature locally above the boiling point and thus melt and vaporise the work material at the point of bombardment. This process is best suited for micro-cutting of material (in milligram/sec) because the evaporated area is a function of the beam power and the method of focusing which can be easily controlled.
The electrons are obtained in free state by heating the cathode metal in vacuum to the temperature at which they attain sufficient speed for escaping to the space around the cathode. These can then be made to move under the effect of electric or magnetic field and can be accelerated greatly. The acceleration is carried out by electric field, and focusing and concentration is done by controllable magnetic fields.
From Fig. 10.64, it would be seen that electrons first penetrate through a layer undisturbed (transparent layer) and then they start colliding with lattice atoms and its kinetic energy (K.E.) is converted into heat and finally all of K.E. is lost. The heat is produced below the transparent layer and in very small zone around impinging point of electrons.
Total penetration range can be estimated as:
It is found that if the applied voltage be in the range of 106 to 108 volts then the electron velocity would be in the range of the velocity of light and under that condition electrons are having a high amount of kinetic energy. When these electrons reach the anode surface, a rapid change in momentum takes place through collisions with the atoms of metal and thus a lot of concentrated thermal energy is obtained, which can be easily controlled.
Thus in this process there is no tool pressure or tool wear. By this process fine gas orifices, wire drawing dies, turbine blades for supersonic aero engines, metering holes in injector nozzles can be fabricated.
The total power for a beam current of I amperes is given by relation P = EI watts and; Force of the beam from the electron beam on the molten metal is given by the relation F = 0.34 I √E dynes.
Due to electron bombardment, the electrons of the work material require some velocity which is given by the relation:
This process of moving the electron beam over the workpiece for either welding or cutting process depends on several parameters.
The circular area of diameter D where the beam is to be concentrated is given by the relation:
Power requirement (P) in EBM process is approximately proportional to the rate of material removal (MRR), i.e. P ∝ MRR or P = C x MRR where C is specific power consumption = 12 W/mm3/min for tungsten and 7, 6, and 4 W/mm3/min for iron, titanium and aluminium respectively.
In this process, production of high vacuum is, essential to ensure free movement of electrons, to protect the cathode from chemical contamination and heat losses, and to prevent the possibility of an arc discharge between the electrodes.
This requirement poses a limit to the size of the specimens that can be machined by this process. The initial cost is also high, and high operator skill is required.
An optical viewing system is required in order to allow the operator to see the work, in order to position the beam. The deflecting coils allow the beam to have a range of movement of 12 mm. The electron beam can be controlled very accurately, and machining tolerances of 0.005 mm are possible.
Power consumption is exceptionally high, but applications at the moment are very specialized, and the normal machining parameters have little significance. The beam is transmitted at 20,000 Hz and the thermal effects can be controlled in such a way that only surface layers of the work material are affected.
The process is not suitable for sinking deep holes where side walls must be parallel. It can be used for drilling or cutting, and holes or slots of less than 0.002 mm can be produced in thin materials. It is particularly suitable when used upon materials having a high melting point combined with low thermal conductivity.
Automatic production machines are available having more than one beam with production rates of several 100’s/hour. These are used, for example, in drilling synthetic jewels in the watch industry.
Characteristics of EBM Process:
i. Material Removal Technique – High speed electrons impinge on surface and kinetic energy of electrons produces intense heating to melt or vaporise the metal.
ii. Voltage – 150 kV
iii. Electron velocity – 228 x 1000 km/sec.
iv. Power density – 6500 billion W/mm2
v. Operations performed – Annealing, welding, or metal removal by cutting narrow slots, drilling holes of 25—125 µm in 1.25 mm thick shells.
Complex contours possible by deflection by coils
vi. Medium – Vacuum (10-5 mm Hg)
vii. Materials of workpiece – All materials
viii. Material removal rate – 10 mm3/min (max)
ix. Specific power consumption – 500 W/mm3 min
x. Limitations – Not suitable for large workpieces. Small crates produced on beam incident side of work. A little taper produced on holes.
xi. Very high specific energy consumption, necessity of vacuum, high cost of machine.
xii. Advantage – There is no effect of local heat on workpiece as the temperature of surrounding material (25—50 µm away from the machining spot) is not raised.
Estimate the thermal velocity acquired by an electron of the work material due to electron bombardment, if the vaporisation temperature of the work material concerned is 3327°C.