Laser Beam Machining (LBM)

• LASER stands for Light Amplification by Stimulated Emission of Radiation.

• LBM uses the light energy (focused coherent beam of monochromatic light) to remove material by vaporization and ablation (evaporation or melting of a surface through heating by friction).

• The light energy is pulsed so that the released energy results in an impulse against the work surface that produces a combination of evaporation and melting with the melted material evacuating the surface at high velocities.

• Energy densities of the order of 100,000 kW/cm2.

• Many types of laser exist – they can be classified as solid-state, gas or liquid. In metal processing, solid state and gas are generally used. Principal lasers used in metal processing are the neodymium -glass (Nd:glass), the carbon dioxide (CO2) and the neodymiumdoped yttrium aluminium garnet (Nd:YAG) lasers.

• Laser beams are being used for a variety of industrial applications, including heat treatment, welding, measurement as well as cutting, drilling, slitting, slotting, marking operations, scribbing.

• LBM does not involve mass material removal but does provide rapid material

Characteristics of LBM

• Highly collimated (light rays in the beam are almost perfectly parallel – a diverging angle of less than 1-2 milli radians)

• Monochromatic (all the photons have the same energy and hence the same frequency and wavelength)

Principle of Laser Beam Machining (LBM)

• Key principle behind the operation of laser was first published in 1917 by Albert Einstein when he hypothesized that under the proper conditions light energy of a particular frequency could be used to stimulate the electrons in an atom to emit additional light exactly with the same characteristics as the original stimulating light source

Conditions required to produce a laser

• Many, if not all, materials can be made to undergo stimulated emission given the right circumstances

• However, to build a working laser two conditions need to be met:

1. Energy source that provides the initial stimulation must be powerful enough to ensure that the majority of the atoms or molecules in the material to be lased are at their higher energy level

2. Feedback mechanism – captures and redirects a portion of the coherent photons back into the active medium to stimulate the emission of still more photons of the same frequency and phase

3. The feedback mechanism is designed to allow a small percentage of the coherent photons to exit the system in the form of laser light; although some of the photon are allowed to escape the system, most will still be available to maintain the amplification process through stimulated emission

Equipment

• Three important elements of any laser device are:

1. Laser medium (collection of atoms, molecules, or ions)

2. Pumping energy source – required to excite these atoms to higher energy level

3. Optical feedback system

Types of lasers

1. Solid state lasers

All solid state lasers share several common characteristics – they all use light derived from a high-intensity light source as the excitation mechanism

Xenon or krypton filled flashlamps are used for pulsed lasers and krypton filled arc lamps are used to produce a continuous beam

Lamp configurations can be straight or helical

Nd:YAG laser, having the highest available power of all the solidstate lasers, is used to perform cutting, drilling and seam welding

2. Gas lasers

1. Construction of the most common type of carbon dioxide gas laser is very similar to that of the solid state laser

2. A glass tube containing a flowing mixture of CO2, helium and nitrogen essentially replaces the crystal that acted as the lasing medium for the solid state laser

3. Instead of a flashlamp, direct electrical energy is used to provide the energy for stimulating the lasing medium

4. The gases flowing through the laser are often recirculated and replenished to reduce operating costs

5. Design types: axial-flow; transverse flow or gas transport

Pumping process

• Generally, the number of atoms or molecule raised to a higher energy level N2 (population of the higher energy level atoms) is less than the population N1 of lower energy level.

• For laser action, the population of atoms in the higher energy state should be increased. The

process of increasing the population of higher energy level i.e. making N2  N1 is called as

population inversion.

• The method of achieving population inversion is called pumping.

Emission Types

1. Spontaneous emission: For example, by the absorption of photon when the electrons are excited to higher state, they will almost immediately decay back to the ground state in about 10 ns and happens spontaneously.

Spontaneous decay often results in spontaneous emission of photons. These have exactly the same frequency as the exciting photons.

2. Stimulated emission: If an atom or molecule is raised to a higher energy level by an outside energy source (e.g heat, light, chemical reactions etc.) and is allowed to decay back to its ground state energy level, a photon is released.

If that photon contacts another atom or molecule that has also been raised to higher energy level, the second atom will be triggered to return back to the ground state. The return to ground

state causes the second atom or molecule to release a second photon along with the trigger photon.

The pair of photons produced by this chain of events are identical in wave length, phase, direction and energy. This sequence of triggering “clone” photons from stimulated atoms or molecules is known as stimulated emission.

Advantages

• Any solid material that can be melted without decomposition can be cut with the laser beam. The other major advantages of the laser beam as a cutting tool are:

1. There is no mechanical contact between the tool and work

2. LBM could be used to drill micro holes with very large depth to diameter ratio (L/D aspect ratio)

3. Large mechanical forces are not exerted upon the work piece

4. Laser operates in any transparent environment, including air, inert gas, vacuum and ever certain liquids

5. The laser head need not be in close proximity for performing cutting and drilling operations in locations of difficult accessibility

6. The beam can be projected through a transparent window

7. No burrs are produced in this process, so the cut is clean

8. Any metal or non-metal can be machined. E.g. tungsten, ceramics,hastealloy, zirconium, wood, paper etc.

9. Work piece need not be electrically conductive – e.g solid CBN cannot be cut using EDM

10.Unlike with other thermal machining devices, the laser can be used with materials sensitive to heat shock such as ceramics. The properties of heat treated material or magnetic material are not

affected by LBM. There is very small heat affected zone (HAZ)

11.A laser beam can be split into several beams, each beam can be used to perform same operations or different operations

Limitations

1. Currently, it cannot be used to cut metals that have high heat conductivity or high reflectivity. For eg. Aluminium, copper and their alloys cannot be cut satisfactorily. However, this can be taken as an advantage, in that the laser beam is a self-selective cutter for different materials – useful in cutting complex profiles without damaging the work table

2. The machined area can be irregular due to off-axis modes that may be generated during laser action

3. The least diameter to which laser beam can be focused depends upon the laser beam divergence; this in turn, is a function of the quality of the laser material and the laser cavity length

4. Output energy from laser is difficult to control precisely

5. The laser system is quite inefficient

6. Pulse repetition rates are low

7. The metal removal rate of 0.0065 cm3/hr in LBM is among the slowest as compared to other machining processes

8. The holes produced by LBM may taper from entry to exit. The taper

on one side of the hole can be as much as 0.05mm/mm

9. Circular holes produced by LBM are restricted to 3.20mm in diameter

in material thickness exceeding 12.5mm

10. Due to metal cooling on the sides of the hole, LBM results in a series

of ridges along the edge of the work piece

11. High capital investment

12. Not suitable for machining blind holes in metal

Applications

• LBM is again a micromachining method which can be used for a wide range of metal processing applications such as metal removal

– drilling, trepanning; metal shaping – cutting, scribing and controlled fracturing; welding and surface treatment (modification of metals to produce structures suitable for the intended component

function – heat treatment, cladding, surfacing, glazing and marking)

• Some specific applications:

1. Most of the LBM drilling applications are in small hole drilling such as fuel filters, carburetor nozzles, surgical and hypodermic needles, hole for lock-nut safety wires, jet engine blade cooling holes,

diamond drawing dies, holes in rubber baby bottle nipples, relief holes in pressure plugs, holes in nylon buttons

2. Round holes with diameter ranging from 0.127 to 1.27mm can be produced with L/D ration of 100:1.

3. Laser beam can be used for making or engraving so as to produce controlled surface pattern on a workpiece – company logos, part number, bar codes or serial number can be made. Metal, glass and paper can also be marked in this way.

4. For vaporizing foreign material clogged in electron microscope apertures.