The process of electron beam machining is divided into two following thermal and non- thermal. In non-thermal electron beam machining process, the electron stream is applied to cause a chemical reaction. Thermal electron beam machining process employs a beam of high – intensity electrons which are focused onto the work piece with an aim of melting and vaporizing the metal work piece.
Electron beam is generated in an electron beam gun. The free stream of electrons is made in order to move towards the anode at very high velocity in the form of a very small diameter electron beam. The work piece is intensely heated in a localized point by the bombarding high speed electron stream, to a temperature that melts as well as vaporizing sometimes. The high energy focused stream of electrons is made to hit the work piece with a spot size of 9-101µm. The kinetic energy of the high velocity beam of electrons is then very fast converted to heat energy.
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Due to very high power density, instant melting and vaporization of the work piece commences and then gradually progresses. If there is any molten material at the top front, it is expelled from the cutting zone by the help of a high vapor pressure at the bottom part. This process is illustrated in the diagrams below.
The electron beam machining process is carried out in a vacuum so that the electrons do not interact with the air molecules. The interaction of the electrons with the air molecules would make the electrons lose energy and their cutting ability.
Note that the material properties of the work piece surface such as strength and hardness do not contribute any effect to the electron beam machining process performance. The electron stream after absolute complete penetration into the work piece partly penetrates into the auxiliary material at the bottom, which vaporizes then flows out of drilled hole at very high pressure. This assists in the expulsion of the molten material from the cavity together with the vaporized backing material. The electron stream can be deflected by the assistance of the computer control round the perimeter of the hole to be made. Otherwise, the electron beam is maintained at a stationary position while the working – table is moved in relevant path. This process implements computer numerical control (CNC)
Tungsten is heated to temperatures above 2000 C. During this process a beam of electrons, approximately one billion per second is emitted from the tungsten. The produced electrons are let to deflect and then be focused by means of magnetic field. Then electrostatic optics is applied in order to focus the stream of electrons. The latest technology employed provides an option of programming the desired pattern in the work piece.
The diameter of the stream of electrons is in the range of 0.012 – 0.025mm. Therefore, narrow slits with depth-width ratios of 100: 1 can be obtained with very high precision. Note that the interaction of the stream of electron with the work piece surface produces very dangerous X-rays. Thus, electromagnetic shielding of the process becomes quite significant for safety purposes of the operators.
Electron beam machines use very high voltages that lie between 50 to 200 kV to accelerate the beam of electrons at speeds of 0.5 – 08 times the speed of light (3.0 x 108 m/s); 1.5 x 108m/s to 2.4 x 108m/s. It is always recommended that the electron beam machine is operating by well trained personnel.
Advantages of Electron Beam Machining
There is no mechanical contact between the tool and the work piece; thus there is no tool wear and residual stresses;
There is a possibility of machining very small holes in all types of material and at the same time attaining high accuracy;
Electron beam machines provide high efficiency and better surface finish;
Closer finishing can be obtained;
It is possible to machine brittle work pieces such as ceramics.
Disadvantages of Electron Beam Machines
The rate at which materials are removed is relatively low;
The cost obtaining the machine and machining the work piece is also relatively high;
It’s expensive to high a skilled operator even though it is a mandatory requirement;
Size of the work piece is restricted, thus speed improvement is therefore limited to the size of the electron beam machine available for use;
Electron beam machining is significantly associated with non-productive pump down period necessary towards attaining desired vacuum conditions. However, this is always taken care of by the vacuum load locks.
Current Limitations of the Manufacturing Process
It has proved uneconomical when handling high volumes of material as compared to stamping. Electron beam machining process has limitations relating to thickness of the work piece. The user is forced to incur high acquisition cost of the equipment. Moreover, high maintenance cost is incurred in order keep the machine adequately serviceable. Some other primary limitations are very high capital cost of the electron beam equipment and significant timely regular maintenance applied to all equipments implementing the vacuum systems. In the determination of temperature distribution, a direct spatially and period resolved temperature measurement by the help of the finite element method. In addition, the influence of different base plates and their associated heat dissipation is properly examined.
Moreover, the experimentally determined temperature course; furthermore, the temperature course that had been calculated by the finite element method reveal a drastic temperature drop with respect to distance to the bonding surface area.
Examination of the bond welds by use of scanning electron microscopy shows – with adequate or sufficient bonding temperature such as a clear-to-define and smooth boundary layer in between glass and silicon. At high temperature, beside silicon, the glass melts as well. Melting silicon and glass results in interlocking of both materials.
State of the Art (Applications) of the Process
Electron beam machining as a manufacturing process has made it possible to design and make accurate complex shaped work pieces. During the process, the complex products are easily obtained since all the machine parameters such as focus, power and the various mechanical motions are numerically controlled and computerized.
As a result, electron beam machining process is mostly used during manufacturing of fine gas orifices in the space nuclear reactors, metering holes in injector nozzles and very fine holes in dies.
Moreover, this process is as well used for pattern generation during integrated circuit fabrication. Manufacturing of holes less than 1mm in very thin plates that are used for turbine engine combustor dome, considerably require this process. Filters used in food and textile processing industries greatly require electron beam machining. Finally, the process is applied in industries like, aerospace, insulation and chemical.