Laser Beam Machining

Laser:

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Laser is defined as the Light Amplification by Stimulated Emission of Radiation.

Laser beam machining

Laser beam machining is a thermal energy based on the non-conventional machining process.  Laser is an optical transducer which converts the electrical energy into the high coherent light beam. When the laser is focused on the conventional optical lens, at that instance high power density is generated.

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Working principle of laser beam machining:

Laser beam is focused on the work piece material, to remove the unwanted material from the work piece by heating often. In the laser beam machining the metal removal rate must depend on the wave length used in the laser machine.

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Laser beam machining process:

In the process the negatively charged electrons are rotates around the positively charged nucleus in the specified orbital path. Based on the variety parameters like atoms, electrons, electron structure, plus the presence of electromagnetic field the radii and the geometry of orbital paths are changes. The energy of the orbital must be related to the single energy levels. At the zero level condition the atom is considered at the ground level, this is because all the electrons occupy the lower potential energy. From the external sources, the ground level electrons absorb some energy to stimulate high stage. At the elevated temperatures the electronic vibrations are increased, and in chemical reactions they absorb energy from the photons which are external sources. The electrons are moving from the lower to higher energy level.

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When the electron reaches the higher energy level, at that instance we observe the unstable band of energy. By releasing the photons within a short time, it reaches the ground level. So the process is known as the spontaneous emission. The existing photon and the emitted photon have the same frequency.

In some conditions the energy state changes the positions of the electrons into the metastable energy band. In a material if the electrons are pumped to the higher metastable state, and are compared to the atoms present in the ground state where the phenomenon is known as population inversion.

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They are returned to the ground level in the form of rush, moved by a photon of appropriate energy or frequency which is called as stimulated emission.

If the photon is stimulated with a suitable energy at such instance the electron moves down to the lower energy state, and comes to the original position. This emits photon by recreation having some longitudinal phase and progressive phase which is available. Coherent laser beam is created in this way.

  • The cylindrical glass vessel consists of gas, the gas is known as lasing medium.
  • The one end of the glass vessel is closed with reflective mirror, and the other end of the vessel is arranged with partial mirror
  • The existing gas molecules or atoms are sent into the flash lamps there population inversion takes place. Simulation emission would be start lasing action.
  • The stimulated emission photons are in all directions and they generate waste heat.
  • The photons present in the longitudinal direction long for procedure coherent, intense laser beam.

LASING MEDIUM

Depending up on the lasing medium the laser is classified in to two types they are

  • Gas laser
  • Solid laser

Gas lasers are classified in to three types they are

  • Helium and neon
  • Argon
  • CO_2

Solid lasers are classified in to three types they are

  • Nd- YAG
  • Nd- glass
  • Ruby- alumina

Lasers operations are operated in a pulsed or continuous mode

Generally the  gas is used for the continuous modes and from solid lasers Nd – YAG laser is used for the pulsed modes.

Construction of a laser

Nd – YAG laser is sent into the flash tube. The flash tube must be helical in shape or in flat shape. The helical flash tubes provide better pumping, but there is difficult of maintenance; through the flash tube is circulated to the crystal. By charging and discharging the capacitor the flash tube must be operated in the pulsed mode. The on time of the pulse is decided with the help of resistance on the flash tube, and the off time is decided by the charging resistance. In the beginning high pulses switching voltage is supplied.

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While coming to the CO_2 laser, the flow of gas must be in axial direction. It maintains a power of 100W per meter out of the tube length. ‘He’ (Helium) and  N_2 continuously circulated in the gas tube. So, the continuous recirculation of gas is done to minimize the consumption of gas. In the process the CO_2 acts as main lasing medium and the nitrogen helps in supporting the gas plasma. Helium is used to cool the gases. High voltage is supplied to the both the ends, that leads to the formation and discharge of the gas plasma. Discharged energy in-turn leads to the population inversion. One of the end consists of reflector which redirects the photons to push inwards of the tube, and the partial reflector send the laser outwards to get rid of the material, where high power is achieved by using the CO_2  laser.

S.No Lasing Material Ruby Nd-YAG Nd- Glass
1 Type gas Solid state Solid state Solid state
2 composition 0.03 to 0.7% Nd in A 1% Nd doped YAG 2 to 6% Nd in glass
3 Spot size 0.075mm 0.015mm 0.015mm 0.025mm
4 Wave length 10.6 0.69 1.064 1.064
5 Pulse repetition rate Clock wise 1 to 10 pps Clock wise or 1 to 300 pps 1 to 3 pps
6 Beam mode Clock wise or pulsed Clock wise or pulsed Clock wise or pulsed pulsed
7 Beam output 0.1 to 10 kW 10 to 100 W 10 to 1000 W 10 to 100 W
8 Peak power 100 kW 200 kW 400 kW 200 kW
9 efficiency 10 to 15 % 1% 2% 2%

Advantages of laser beam machining:

  • Lubricants are not utilized in the process
  • The laser beam machining have an ability to cut most of the material
  • The laser point moves in all directions and paths
  • There is no limit for the cutting path
  • In the process there is no requirement of tool holding device
  • There is an absence of contact between the work piece and the tool material
  • Without any support the fragile materials are stress-free to cut
  • There is no tool cost involved and no tool wear cost involved in the machine
  • By using the laser beam machining we can produce the cuts with high quality and without involving extra finishing.

Disadvantages of laser beam machining:

  • Material which are not suitable for the conventional methods are made on the laser beam machining process.
  • The initial cost and the maintenance cost of the laser equipment is comparatively high.
  • In the laser beam machining the tapered holes are limited to some extent only. They are approximately in the drill depth of 1%.
  • Based upon the thickness of the work piece material the drill holes are made to fixed depth.
  • Heat is released during the process from the laser, at that process the alloy and metallic materials change the mechanical properties.
  • Light rays released from the laser process leads to hazards.
  • Skilled operators are required for machining process
  • Never try the process on the high heat conductivity material and light reflecting materials.
  • Production rate is slow
  • For safety purposes cover gases are required during the process time.

Applications of laser beam machining:

  • Mainly the laser beam machining is used to make a undersized hole with accuracy.
  • The holes are possible at 5 microns in ceramics, composites and metals.
  • Mainly the laser beam machining process is used in the cutting of the non-metallic and metallic materials for getting an accurate drilling.
  • Holes are made up to 1.5 mm diameter
  • On the thin slitting of metals and plastics the laser operation is performed
  • The process can be performed on the ceramics, organics, non-metals, metals, plastic etc.
  • Holes with a diameter of less than .25 mm can also be made

Click Here To Know More About: Electrical Discharge Machining

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