1. What are the major Industrial applications of Lasers?
7. Describe salient features of Laser welding?
Laser is probably the most versatile tool available for various material processing applications like welding, drilling, cutting, heat treatment (hardening, annealing, glazing, cladding etc) as well as certain very special applications like clearing of space debris, laser balancing, remote decontamination and decommissioning of components of unused nuclear installations, laser ablation, oil and gas exploration, automotive industry etc.
2. What are the advantages and disadvantages of using lasers in material processing applications?
The advantages are,
Disadvantages are,
3. What are the areas of interest in laser surface processing and how these are related to laser power?
- High maneuverability.
- Highly focused spot, where the heating is very much localized.
- Minimum distortion to target material.
- No force is exerted on the work piece.
- Non-contact process and as such there is no wear and tear of tool.
- Not affected by magnetic field.
- Angular operation is possible.
- High speed of processing (improved productivity).
- Adaptability with existing machines.
- Ability to operate in inaccessible areas.
Disadvantages are,
- Cost of machine and operator is high.
- High cost of operational maintenance.
- Laser safety.
The adjoining figure highlights the areas of interest. There are three distinct regions - surface heating, melting and vaporization. Processes, which come under these categories, include:
Surface Heating
Melting
Vaporization
4. How is surface temperature related to Laser power?
Surface Heating
- Hardness increase
- Strength increase
- Reduced friction
- Wear reduction
- Increase in fatigue life
- Surface carbide creation
- Magnetic domain control
- Stereolithography
- The depth of hardness is proportional to P/(DV)1/2
Surface Heating
Melting
- Moderate to rapid solidification rates thus produce almost homogeneous structures
- Little thermal penetration results in little distortion and thus the process can be used for thermally sensitive materials.
- Surface finish typically of the order of 20-25 micron thus reducing the post laser processing of the work piece
- Cleaning
- Glazing
- Marking
- Welding
- Cladding
- Laser surface alloying
- Ion implantation
- Diffusion
- Reactive gas shrouding to form nitride, hydride etc.
Vaporization
- Shock Hardening
- Drilling
- Cutting
One dimensional heat flow model is the most convenient model to explain most of the experimental results. As per this model, if the heat flows in one direction and there is no convection or heat generation, it is assumed that there is a constant extended surface heat input and constant thermal properties, with no radiant heat loss or melting then
At z = 0, the surface power density is
where T = temperature, z = depth, t is time , k is thermal conductivity, α is thermal diffusivity, F0 is the absorbed power density, rf is the reflectivity of the surface.
The surface temperature T0t can thus be written as:
For a continuous gaussian source, the temperature of a surface central point under stationary condition is given by 
Maximum possible temperature is
where D is the diameter of the laser spot.
The amount of power effective on any other point on the surface within the beam depends on the power distribution. For example, for a Gaussian mode structure, TEM00, the power at any point is given as 
where
and rb is the beam radius at the work piece.
5. Describe salient features of laser drilling?
For a continuous gaussian source, the temperature of a surface central point under stationary condition is given by
The amount of power effective on any other point on the surface within the beam depends on the power distribution. For example, for a Gaussian mode structure, TEM00, the power at any point is given as
Drilling operation requires focusing of the laser beam at the point of interest. When laser is focused on the work piece, say metals, the surface gets heated up first and then conduction heats the subsurface. Drilling of metals by laser is based on surface heating. Laser material interaction depends on material properties like reflectivity, absorption, thermal conductivity and diffusivity, specific heat, melting and vaporization, latent heat of fusion, heat capacity etc.
For aluminum, the processing velocity increases from zero to 25 m/s, as the laser intensity is increased from 300 watts/cm2 to 500W/cm2 and it remains same as the laser intensity increase to 2Kw/cm2. For copper, processing velocity increases from zero to 20 m/sec when the laser intensity is raised from 700 W/cm2 to 1 kW/cm2 and then remaining almost constant for higher laser intensities.
6. Describe salient features of Laser cutting?
For aluminum, the processing velocity increases from zero to 25 m/s, as the laser intensity is increased from 300 watts/cm2 to 500W/cm2 and it remains same as the laser intensity increase to 2Kw/cm2. For copper, processing velocity increases from zero to 20 m/sec when the laser intensity is raised from 700 W/cm2 to 1 kW/cm2 and then remaining almost constant for higher laser intensities.
- Laser cutting process is a function of a multiple parameters like laser beam properties, work piece transport properties, gas properties and material properties. Beam parameters include spot size and mode, power, pulsed or CW, polarization and wavelength. Transport properties speed of the stage carrying work piece and focal position of the laser. Gas properties comprise of jet velocity, nozzle position, nozzle shape and alignment and gas composition. Material properties of relevance are mainly optical and thermal.
- There are various processes, which can be utilized for cutting depending on the power available and the material. These include:
- Scribing and Thermal Stress Cracking
- Burning Stabilized Laser Gas cutting
- Fusion Cutting
- Vaporization cutting
- Cold Cutting
- Laser cutting is today the most common industrial application of lasers. In Japan, around 80 % of the industrial lasers are used for this application only. The advantages of using lasers are that these can cut faster and with a higher quality as compared to other competing processes like abrasive fluid jet, sawing, oxy flame, wire EDM, ultrasonic, plasma and NC milling.
- The cut can have a very narrow kerf width (width of the cut opening) resulting in substantial saving of material.
- The cutting edges can be square and not rounded as with most hot jet processes or other thermal cutting techniques.
- The cut edges can be smooth and clean thus do not need any further treatment.
- There is no edge burr as with mechanical cutting techniques
- There is very narrow Heat Affected Zone as a result of resolidification. This results in minimum distortions.
- Cut depth is limited and depends on laser power. 10-20 mm is the current range for high quality cuts.
- Fastest cutting process.
The intensity of focused laser beam is comparable to electron beam and is one of the highest power densities available in industry today, At energy densities in the range of 1010-1012 W/m2, almost all materials are likely to evaporate provided the energy is completely absorbed. In laser welding, a hole is usually formed by evaporation, which traverses through the material with molten walls sealing up behind it. This is known as keyhole weld, which is characterized by its parallel-sided fusion zone with a narrow width. The concept of welding efficiency is known as joining efficiency and is defined as mm2 joined per kJ of energy supplied. In terms of power and thickness and traverse speed it is equal to [Vt/P], where V, t and P are traverse speed in mm/sec, thickness welded in mm and laser power in kW respectively. The higher the value of joining efficiency, lower is the laser power used and thus lower are the distortions and heat affected zone.
High frequency Resistance welding is the best in this respect having joining efficiency of the order of 65-100 mm2/kJ as compared to 15-30 mm2/kJ achievable in Laser and electron beam welding. Nevertheless it is far more efficient than oxy acetylene flame and tungsten inert gas welding. As Lasers offer high quality, high speed welding, the process is capturing fast and is likely to take 25-30% of world market share for neat and reliable welding.
There are two modes of welding. Conduction limited welding occurs when the laser power density is insufficient to cause boiling particularly in the case of broad beams required for welding variable gaps. In this case, it generates the keyhole at a given traverse speed. The weld pool in this case has a strong stirring forces resulting from the variation in surface tension with temperature. The other mode is keyhole welding in which there is sufficient laser energy to cause evaporation and hence the hole is in the melt pool. The pressure from the vapour being generated stabilizes this hole. The keyhole behaves as an optical black body in that the radiations enter the hole and are subjected to multiple reflections and are unable to escape.
Penetration is inversely proportional to the weld speed for a given lode, focal spot size and laser power. Typically, for welding stainless steel (304), the penetration depth increases from 3mm to more than 20 mm for a 5 kW laser power when welding speed is reduced from 150 mm/sec to about 10 mm/sec.
High frequency Resistance welding is the best in this respect having joining efficiency of the order of 65-100 mm2/kJ as compared to 15-30 mm2/kJ achievable in Laser and electron beam welding. Nevertheless it is far more efficient than oxy acetylene flame and tungsten inert gas welding. As Lasers offer high quality, high speed welding, the process is capturing fast and is likely to take 25-30% of world market share for neat and reliable welding.
There are two modes of welding. Conduction limited welding occurs when the laser power density is insufficient to cause boiling particularly in the case of broad beams required for welding variable gaps. In this case, it generates the keyhole at a given traverse speed. The weld pool in this case has a strong stirring forces resulting from the variation in surface tension with temperature. The other mode is keyhole welding in which there is sufficient laser energy to cause evaporation and hence the hole is in the melt pool. The pressure from the vapour being generated stabilizes this hole. The keyhole behaves as an optical black body in that the radiations enter the hole and are subjected to multiple reflections and are unable to escape.
Penetration is inversely proportional to the weld speed for a given lode, focal spot size and laser power. Typically, for welding stainless steel (304), the penetration depth increases from 3mm to more than 20 mm for a 5 kW laser power when welding speed is reduced from 150 mm/sec to about 10 mm/sec.