Industrial Applications of Lasers

Industrial Applications

Laser is a source of energy endowed with extra-ordinary properties. These special properties have been detailed in the earlier section on "Properties of lasers". Due to its unique nature, lasers have found applications in almost in every field of human activities, whether it is science, medicine, industry, agriculture, entertainment or informatics, to mention a few. It may be brought to the notice of the reader that all the basic properties of laser are not necessary for its use in every application. In fact, it is the application that decides what all properties are essential in the laser for the successful implementation of an activity. For example, high coherence of laser is an essential requirement in holographic applications, where as the same plays a nominal role, in range finding or for that matter in laser - light shows. In this section Industrial applications of Lasers have been discussed briefly. Other applications are discussed in other sections.

Today, 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. Further, laser material processing (LMP) scores very high in the processing of difficult to machine materials such as, hardened metals, composites and ceramics to name a few, compared to the normal mechanical material processing techniques. In this section our intention is to provide the basic concepts rather than the details.

Pulsed as well as CW lasers are employed for industrial applications. Nd:YAG (~10KW), CO₂ (~25KW) and COIL (~40KW) are some of the most commonly employed high power lasers. High monochromaticity and directionality of the laser gives the laser beam a small wavelength spread and low divergence. Therefore, laser can be focused to a very small spot size, resulting in very high radiance.

Basic laser material processing (LMP) system consists of a high power laser, beam delivery unit with focusing arm and beam parameter-monitoring unit coupled to a CNC machine. Delivery and manipulation of laser beams to extremely complex work shapes and inaccessible places is a critical aspect of an LMP system. Wavelength, energy, power, beam diameter, divergence etc decide the transmission method. Near IR lasers like, Nd:YAG (1064 nm) and COIL (1315 nm) can be transmitted to the work area using optical fiber cables, where one can bend and twist the laser beam as desired. CO₂ laser at 10.6μm wavelength has to use a rather unwieldy mechanical arm, consisting of lenses, prisms, mirrors etc to reach the work area due to the non-availability of a suitable fiber cable for high power laser transmission.

Material properties like, reflectivity, absorption, specific heat, thermal conductivity, heat capacity, diffusivity, melting, latent heat of fusion, vapourisation etc play a major role in LMP. Knowledge of interaction of laser with the target material is of paramount importance in deciding the type of the laser to be used for the particular material-processing environment, since the material should absorb the laser energy. Type of work, speed of operation and the nature of material decide the type of laser to be used. In LMP, the material absorbs the laser energy and distributes the same with in the material and then the material is removed by evaporation or by melt ejection. During LMP, debris, particulate matter, fumes etc can damage the optical components of the focusing system. These can adversely affect the laser beam itself. A gas shield, mostly employing helium gas, is provided as a protection from these. Plasma cutting and oxy-acetylene systems have their own gas jets. As LMP does not have this, an auxiliary jet is provided from a pressurized gas source. Plasma generated during the laser machining, lessens the penetration capability of laser. It is essential to blow away the plasma for the successful performance of the system. High-pressure gas impinging on the work piece from above produces local pressure, which is more than the atmospheric pressure existing below the work piece. Consequently this difference in pressure helps the melt to be blown away from the bottom. Latest LMP systems employ a coaxial nozzle in which the laser and the gas both exit through the same orifice.

Laser is not a bulk heat source like oxy-acetylene flame. The amount of energy absorbed by the material solely decides the rise in temperature leading to melting or boiling. i.e. the laser – material interaction is of paramount importance in LMP. Due to the high power density associated with the laser, rapid heating of the region does not allow the heat to be spread to the bulk of the material; consequently the heat is very much localized. Major laser parameters for LMP system are energy, pulse width and wavelength and monitoring of the same is very important. Along with these, the concept of energy balance and heat transfer also has to be considered. Another plus point regarding laser is that it can be manipulated to process extremely complex shapes or to reach areas where other tools could not have access.

An important aspect in the industrial scenario is the safety of the personnel, since many of them are unaware of the biological and physical hazards that can be caused by laser. For details, the reader may kindly see the section on "LASER SAFETY".

Though laser is a versatile tool, it has certain advantages and disadvantages. Some of them are listed below.

The advantages are,

• 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.

As already mentioned that Lasers have unique properties for surface modification applications. The electromagnetic radiations are absorbed within the first few atomic layers for opaque materials such as metals thus making it possible to put the applied energy precisely on the area of surface of interest. Common advantages of laser surface treatment include chemical cleanliness, controlled thermal penetration, remote non-contact processing, and localized heating. 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

• 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

Some of the important features related to some important industrial applications of Lasers are listed below.

Salient Features of interest

Physical mechanism of LMP is characterized and controlled by the interaction of laser radiation with matter. LMP is a thermal process and the material removal is by either melting or vaporizing the volume of interest. Optical and thermal properties of material play a major role in LMP and mechanical properties have only a minor role. The material with low thermal conductivity and thermal diffusivity is the best candidate for LMP. Being a non-contact process, the energy transfer occurs through irradiation and as such, mechanically introduced vibration damages are avoided and that also without any tool wear. The same laser machine can be used for welding, drilling, cutting and heat treatment processes with a multi-axis work piece positioning station. This flexible environment gives LMP its unique nature, hitherto unavailable in mechanical systems. The most interesting feature of LMP is that it is a multidimensional process. Drilling can be considered as a single dimensional process, since laser beam is stationary relative to work piece. If we take the cutting process, laser and the work piece are in relative motion perpendicular to each other. This situation can be considered as a two dimensional process. If more than one laser beam is employed, each laser will have a two dimensional nature with one edge being common. This situation makes the LMP as a three dimensional process.

To identify the specific high power laser for a particular industrial application requires an understanding of the interaction of laser beam with the work piece and the concept of heat transfer. The absorption of laser energy by the material decides the temperature rise in the material. Type of operation like drilling, cutting, welding etc along with the speed of operation is also of importance in deciding the types of laser to be used. It is not only the type of laser but also its nature such as wavelength, pulsed or continuous, spot size, divergence etc are also very significant. Further, monitoring of the laser beam parameters like power, intensity distribution and beam diameter is also very important.

Though laser beams can be manipulated to process extremely complex shapes, its delivery to work area is critical. Wavelength, power, energy, beam diameter, divergence etc of the laser beam decide the transmission method. Flexible optical fiber can be used to transmit near infra-red lasers like Nd: YAG (1.06 micron) and COIL (1.13 micron), but the same cannot be used to transmit CO₂ laser (10.6 micron), where mirrors and prisms have to be employed. High power lasers can heat the optics thus altering the shape of the surface quality of the optics adversely affecting the beam quality. It is likely that the space between the optics may get heated up due to the high intensity of the laser beam, which reduces the beam quality due to thermal blooming.

Safety of personnel is of paramount importance as the potential laser hazards include biological, fire, electrical shock, fumes, debris etc. Safety aspect of laser beams will be discussed in detail in another section. It is also to be remembered that the training of the personnel is costly, time consuming and laborious.

To have an idea of radiance, let us look at the radiance of sun and 1 mw laser. The radiance of a 1 milli-watt He-Ne laser with an output diameter of 1mm and of divergence of 1 milli-radian is 160 x 10⁶ Watts/m² – steradian where as the radiance of the sun with emission power of 10²⁶ Watts is only about 10⁶ Watts/m² steradian.

Material processing applications require very high laser intensity at the working region. As can be seen, laser beam can be focused to a very small spot size, thus generating the highest intensity possible. We know that diffraction limits the minimum spot size that can be achieved. When a laser beam with divergence Θ is focused with a lens of focal length f, then the estimated focal spot focus radius r is,

Since the divergence of the beam is determined by the diffraction at the aperture of the laser, the divergence angle Θ can be approximated to,

where λ is the wavelength of the laser and d is the diameter of the limiting aperture.

Then,

If we use a lens of F number one to focus the laser beam, then r = λ, which means that a laser beam can at best be focused to a spot size equal to the order of its wavelength, due to diffraction.

The above discussion is truly applicable to beams with Gaussian profile only, where the laser beam oscillates in the lowest mode. The lowest Gaussian mode is TEM₀₀ and the divergence is lowest. But practical laser systems operate with much higher divergence and F-numbers. The minimum spot size that can be achieved is about 6 microns for Nd:YAG (λ = 1.06micron) and 60 microns for CO₂ (λ = 10.6 micron) lasers. Another important parameter is depth of focus and the beam waist. When a laser beam is focused with a lens, the minimum size of the beam at the focal plane is referred to as beam waist. The intensity of the beam does not reduce much on either side of the beam waist as it propagates. The depth of focus is the distance over which the intensity on either side of the beam falls to 50% of its peak value.

The diameter of the focal spot (focal diameter) and the depth of focus are very much related to the focal length of the lens system. A short focal length lens can produce a smaller focal diameter compared to a longer focal length lens. But depth of focus in the shorter focal length lens will be shorter than the one that can be obtained with the longer focal length lens. Therefore, one has to be very judicious in designing the focal length of the focusing system, which will be decided by the application, say as in welding operations. Further, one also has to remember that a short focal length lens will have large spherical aberration. To reduce this defect one has to employ aspherical or multi-element lens system.

• 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, r_f is the reflectivity of the surface. The surface temperature T_0t 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 r_b is the beam radius at the work piece.

Some of the important features related to some important industrial applications of Lasers are given below:

Drilling

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.

Metals are basically very good reflectors. Since reflectivity varies with wavelength, this aspect has to be kept in mind while selecting a laser. The reflectivity of polished silver is about 15% at 300 nm and it increases steeply to about 95% in the visible region, reaching 98% for far infrared region. In the case of copper, reflectivity is 30% at 200 to 400 nm and it increases to 90% at 700 nm, reaching to 98% at 3 mm wavelength. Reflectivity of aluminum is 80% at 400 nm, reducing to 75% at 1μm and then increasing to 90% at 2.5 μm, remaining same for longer wavelengths. In case of carbon steel, reflectivity increases from 40% at 400 nm to 85% at 4μm. CO₂ lasers and free running as well as Q-switched Nd:YAG / Nd:Glass lasers are normally employed for drilling of holes in various materials with thickness varying from millimeter to few centimeters. From the above discussion regarding reflectivity aspects, it can be seen that Nd:YAG (1064 nm) is better suited for drilling operation than CO₂ laser (10.6 μm). The average power levels of the lasers employed vary from tens of watts to few KWs. The power levels and the pulse duration of lasers are decided by the nature and the thickness of the work piece. Absorption of laser energy by the target material is another important aspect. Oxidized surfaces absorb laser energy much better than unoxidised surfaces since the reflectivity of the former is much less than the latter.

As the penetration depth of the laser increases, the absorbed energy heats up the work piece and at high irradiance level of 10⁶ W/cm² onwards, the laser focal spot starts melting. The melted material is removed by flushing. As the vaporized material is removed, a new surface is formed for further drilling.

Laser intensity plays a very important part in drilling. The ratio of vapor and liquid material removal is proportional to laser intensity. Consider nickel as example. It takes 1.84 millisecond to reach vaporization for laser intensity of 10⁵ W/cm², where as it is only 1.84 nano second when the laser power is increased to 10⁷ W/cm². Another aspect is the dependence of process velocity on laser intensity, especially when drilling holes with single pulse. The process velocity is defined as the constant velocity with which the vapor pressure drives the interface melt into the material. It is found that above certain threshold intensity, the processing velocity increases from zero to a higher value, which is material specific and then remains constant as the laser intensity increases. Let us take two materials, aluminum and copper as examples to illustrate the above statement. 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.

Vaporization and material removal depend on the materials. While drilling, these two reach saturation level due to absorption and refraction of laser by the expanding plasma, which shields the work piece from further drilling. Drilling efficiency depends on power density, pulse duration and number of pulses. Higher machining rate can be achieved, when drilling with high repetition rate pulses and lower laser energies rather than vice versa. Laser with longer pulses with lower energies produce deeper holes compared to shorter pulses with higher energies and the former require less number of pulses as well. Nd:YAG laser in the free running mode as well as in the Q-switched mode with power levels varying from hundreds of watts to few kilowatts and pulse width varying from 0.1 millisecond to 3milliseconds are normally employed for drilling holes with diameter in the millimeter range through metals up to several centimeters thick. CO₂ lasers in the CW mode as well as in the pulsed mode with 0.5-millisecond pulse length are used for drilling holes in polymers and ceramics. Nd:YAG lasers are used for drilling hundreds holes, at a rate of 60 holes per second with diameters of 50 micrometers to 100 micrometers in the production of filters for fuel injection purposes. Radio frequency excited sealed CO₂ lasers as well as TEA CO₂ lasers producing high pulse repetition rate of tens of thousands with peak power of few hundred to 1000 watts and average power of few hundreds of watts, with pulse duration ranging from tens of microseconds to milli-second duration are also used for drilling. Basically, Nd:YAG lasers are better suited for drilling operations compared to CO₂ lasers as wavelength of former (1.06 μm) is ten times shorter than the latter (10.6 μm) and consequently the focal spot size is ten times shorter for the same focusing system. Further, absorption of energy at 1.06 μm is much more than at 10.6 μm in metals.

Laser 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.
• Tool wear is zero since the process is non-contact one.
• The noise level is low.
• The process can be made easily automatic.
• Nearly all materials can be cut. They can be brittle, electric conductors or non-conductors, hard or soft. Only high reflective materials such as aluminum or copper can pose a problem but proper beam control these can also be cut.

The major components of the laser cutting process include laser with some shutter control, beam guidance, focusing optics, CNC drive for precisely moving the workpiece. When not in use, the laser beam is directed towards beam dump, which may be a water calorimeter. Gas jet not only assists the cutting process but also works as an air knife that blows sideways across the exit from the optic train thus deflecting any smoke and splatter.

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: These processes require the minimum power. Scribing is a process for making a groove or line of holes in order to make the make the structure weak so that it can be mechanically broken. Particularly silicon chips and alumina substrates use this technique. Low energy, high density pulses are used to remove the material mainly as vapour. In case of brittle material, thermal stress cracking is usually preferred. These materials are neatly severed, by guiding a crack with a fine spot heated by a laser. The laser heats a small volume of surface causing it to expand and hence to cause tensile stresses all around it. If there is a crack in this region, it will act as a stress enhancer and the cracking continues in the direction of hot spot. The speeds of the order of meter / sec can be achieved with this. Material like glass, quartz, alumina, sapphire can be cut with powers as low as 10 W with speed upto half a meter per second.

Burning Stabilized Laser Gas cutting: In this mode, laser is used more of a matchstick to ignite the metal in an oxygen stream. Very thick sections can be cut with relatively less power. The process is essentially oxygen cutting with wide kerf widths of the order of 3 – 4 mm, however the quality of edge and squareness is far better as compared to oxy/plasma cutting. Typical rates for cutting 80 mm thick mild steel are 0.2 mm/min with 1.2 kW and 1 mm/sec with 2 kW of laser power.

Fusion Cutting: The process is also called Melt and Blow. Once a penetration hole is made or cut is started from the edge, then it is possible with sufficiently strong gas jet to blow the molten material out of the cut kerf thus avoiding the temperature increase of workpiece. The melt is removed before any significant conduction occurs. In this manner one requires almost one tenth of the power otherwise required for vaporization. The laser beam after arriving at the surface, most of it passes through the hole or kerf; while some part is reflected off the unmelted surface. At slow speeds the melt starts at the leading edge of the beam and much of the beam passes clean through the kerf without touching the material particularly when the workpiece is thin. The absorption takes place on the steeply sloped cut via Fresenel absorption – that is direct interaction of the beam with the material and secondly by plasma absorption. The plasma build up is not very significant as it is blown away by the gas. At high speeds, the beam is coupled to the workpiece more efficiently by less being lost in the kerf. The beam tends to ride ahead onto the unmelted surface. When this happens, the power density increases since the surface is not sloped and so the melt proceeds faster and is swept down into the kerf.

If the gas used in cutting is capable of reacting exothermally with the workpiece then another heat source is added to the process thus overall reducing the laser energies. Oxygen or oxygen containing mixtures is usually used for this application.

Typically mild steel, stainless steel and titanium can be cut with speed upto 80 mm/sec using oxygen jet with energies of 5.7J/mm², 5J/mm² and 3J/mm² respectively. Further the energy required for cutting with nitrogen or argon is higher as compared to that required for oxygen. Typically mild steel requires energy of 10J/mm² with nitrogen as compared to 5.7J/mm² with oxygen. Similarly stainless cutting requires energies of the order of 13J/mm², 8J/mm² and 5J/mm² with argon, nitrogen and oxygen respectively.

Vaporization cutting: In this cutting mode, the process relies on vaporization. The laser beam first heats up the surface to boiling point and thus generates the keyhole. The keyhole causes a sudden increase in the absorptivity due to multiple reflections and the hole deepens quickly. As a result, the vapors are generated which escape blowing the material out of hole thereby stabilizing the walls temperature of the hole. This is a common method of cutting the materials, which do not melt like wood, carbon, and plastics mainly employing pulsed lasers. When the metals are cut using this technique, the heat-affected zone is minimum in this case usually of the order of few microns. Typically, if we use a laser of 2 kW focused to a 0.2mm beam, the power density is 6.3 x 10¹⁰ W/m². With this power density, the vaporization temperature of most of the metals like tungsten, titanium, steel etc can be achieved within a microsecond and the speeds of cutting can be high as one meter per second.

Cold Cutting: This is a relatively new technique. Mainly lasers in the ultraviolet range like Excimer, have been used for this mode of application. For example the energy of the ultraviolet photon is 4.9eV, which is similar to the bond energy for many organic materials. Thus if a bond is radiated with such a photon, the bond is broken. When this radiation is impinged onto plastic with sufficient flux of photons such that at least there is one photon for each bond, then the material just disappears without heating leaving a hole without leaving any debris. The process is being widely used for laser ablation of materials for thin film applications. There are potential medical applications also including microsurgery and conventional ablation of tumor cells.

There are numerous applications of laser cutting. These include:

• Profile cutting in metals
• Cutting of quartz tubes
• Kevlar cutting
• Cutting alumina and dielectric boards
• Cutting radioactive materials
• Cutting of materials in prototype car production and shipbuilding
• Hole drilling in electronic industry
• Laser machining

Laser Welding

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 – 10¹²W/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.

The advantages of use of lasers in welding can be summarized as follows:

• High energy density "keyhole" type weld leading to less distortion
• High processing speed
• Rapid start
• Weld at ambient pressure unlike electron beam welding
• No X-ray generated unlike electron welding
• Narrow weld
• Little heat affected zone
• No contamination
• Easy to automate
• Accurate and reliable welding

The welding relies mainly on a tightly focused laser beam and the general set up is shown in the adjoining figure. Shrouding is a feature that is almost used in all the welding techniques. It protects the optics as well from spatter. 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. There are two principle areas of interest in the mechanism of keyhole welding. The first is the flow structure since this directly affects the wave formation on the weld pool and hence the final frozen weld bead geometry, which is a measure of weld quality. The second is the mechanism for absorption within the keyhole, which may affect both the flow and the entrapped porosity and hence decides about the quality of the weld.

Laser welding process is a function of a multiple parameters like laser beam properties, work piece transport properties, shroud 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, joint geometries, gap tolerance and focal position of the laser. Shroud Gas properties comprise of composition, shroud design, pressure and velocity. Material properties of relevance are mainly composition, surface condition, optical and thermal.

Good quality weld can be obtained with the right choice of power and weld speed. The welding speed for a given thickness increases with the increase in laser power. Typically for welding a 2 mm titanium alloy, the weld speed can be increased from 5 mm/sec to more than 50 mm/sec if the laser power is increased from a kilowatt to two kilowatts.

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.

For pulsed lasers such as Nd:YAG, pulse width is an important consideration. For example, pulse width less than a millisecond with energies upto 10 J are best suited for cutting and drilling, whereas larger pulse width in the range of 2 – 5 milliseconds with almost similar energies are suitable for welding. Higher energies (> 10 J) and larger pulse widths (> 4 milisecond) are being employed for deep welding.

In welding of butt joints, the gap must be small enough that the beam cannot pass straight through the joint. In other words, the gap should be smaller than half the beam diameter. In case where is a larger gap, either the beam is defocused a bit or a filler material like wire or powder is added in the joint. The gap 'g' that can be tolerated in butt joints is given by

where β is the coefficient of thermal expansion, ΔT is the temperature change usually the melting point , w is the weld width and A is a constant.

However, the gap between the plates 'g' which can be tolerated in case of lap welding is given by

where t_p is the sheet thickness and B is a constant.

The shroud gas can affect the formation of plasma, which may block or distort the beam and thus may affect the absorption of the laser energy. The formation of the plasma is a result of reaction of the hot metal vapors from the keyhole with the shroud gas. The plasma blocking effect is usually less for those gases having a high value of ionization potential. This is the reason why helium is preferred over other gases. However, if the shroud gas is reactive with the weld material, it may form a thin layer such as oxide that results in enhancing the optical coupling.

In order to have an idea about the power requirements for welding, one can assume Laser welding based on keyhole model: model using the moving line source that assumes that the energy is absorbed uniformly along a line in the depth direction. Analytical equations can be used to estimate power or speed of the job

and

where v is the welding speed, w is the weld width, α is the thermal diffusivity, Q is input power per unit time and is given as Q = P(1-rf), g is the job thickness to be welded, k is thermal conductivity and T is the temperature of the plate.

For example, if we wish to weld a 10mm thick stainless steel at a speed of 10mm/s. assuming a usual weld width of 1.5 mm laser power required can be estimated using above relations.

For 304 stainless steel the value of α, k and melting point Tm are 0.49 x 10-5 m2/s, 100 W/m/K, and 1527 oC respectively. The values of Y and X are

If one assumes transfer efficiency of 90%, total power required is 11.8 kW

Some of the important applications of laser welding in industrial applications include:

• Welding of transmission systems and other subsystems for car industry.
• Hermetically sealing of electronic capsules
• Welding of thick pipes
• Repair of nuclear boiler tubes
• Welding of sheet metal products such as washing machines and heat exchangers
• It is now increasingly being used for 3D welding of aircraft and car components because of well-controlled manipulation.
• Welding of polymers and plastics for which typically 20 – 40 W diode lasers operating at 800 – 900 nm are being used.

Surface Treatment

Surface treatment employs lasers of varying energies. For example low power density processes of transformation rely on surface heating without melting and include hardening, bending, laser chemical vapor deposition. Moderately higher power densities, which rely on melting, include surface homogenization, laser glazing, surface alloying and cladding. Much higher power densities rely not on melting but also on evaporation and these processes include instant ablation, shock hardening.

Laser Heat Treatment: The main goal of laser heat treatment is selective hardening for wear reduction. However it is also being used to change metallurgical and mechanical properties. Practical uses of laser heat treatment include hardness increase, strength increase, friction reduction, wear reduction, increase in fatigue life, surface carbide creation and for changing metallurgical and mechanical properties. Laser heat treatment is usually carried out on titanium, some aluminum alloys, steels with sufficient carbon contents and cast iron with pearlite structure. An absorbing coating is usually applied to the metal surface to avoid laser power loss. As the laser beam impinges on the metal surface, the temperature starts rising and the thermal energy is conducted into the metal component. The laser energy should be sufficient to result in temperature rise corresponding to transformation temperatures, which are required for a particular process. However, it should not lead to melting. Typical laser power densities required for these applications are in the range of 10³ – 10⁴ W/mm² and the workpiece speed lies in between 5 – 50 mm/sec. The affected depth in laser heat treatment effects depend on the laser power P and the heating time i.e. D/V, where D is the laser spot diameter and V is the traverse speed. Mathematically, the depth of penetration 'd' can be given as

where A and B are constants.

Laser Surface Melting: The main characteristics of laser surface melting process are:

• Rapid solidification rates leading to almost homogeneous structures
• Very little thermal penetration, resulting in little distortion and even thermally sensitive materials can be processed
• Good surface finish
• Process flexibility, because of automation and software control

Powers of the order of 10³ – 10⁶ W/cm² are usually employed for these processes. The surface to be melted is shrouded by an inert gas. There are mainly three metallurgical areas of interest: cast irons, tool steels and certain deep eutectics that can form metallic glasses at high quench rates. All these are essentially non-homogeneous materials, which can be homogenized by laser surface melting. Surface alloying with a laser is similar to laser surface melting except that another material is injected into the melt pool. The alloyed region shows a fine microstructure with nearly homogeneous mixing throughout the melt region. Further most materials can be alloyed into most of the substrates. The high quench rates ensure that the segregation is minimal. The thickness of the treated zone can vary from few microns to a couple of millimeters. Very thin and fast quenched alloy regions can be fabricated using Q-switched Nd: YAG lasers. The metal to be alloyed can be placed on the base material by electroplating or vacuum evaporation or powder coating or ion implantation or diffusion such as boron or reactive gas shroud. Surface alloying of copper, silicon or carbon in mild steel can result in cheap superficially exotic materials. Similarly laser surface hardening of aluminum by alloying with silicon, carbon, nitrogen and nickel has shown excellent properties in car and aircraft industries.

Laser cladding is slightly different than laser alloying. In cladding, the purpose is to overlay one metal over another metal to form a sound interfacial bond or weld but without mixing with one another. Claddings are usually thick greater than 200 micron. For laser cladding one can have powder pre-placed on top of the other metal, or can have layers grown by laser physical vapor deposition of layers grown by laser chemical vapor deposition. Cladding with pre-placed powder is one of the simplest method in which area is covered with powder with some binder and the workpiece is shrouded with inert gas. The powder is scanned with a defocused laser beam resulting the powder to melt and weld with the underlying substrate. Usually laser power of the range of 2 kW can be used to have a clad thickness of few millimeters.

Other Industrial Applications of Lasers

• Enhanced electroplating
• Surface texturing
• Laser ablation
• Laser chemical vapor deposition
• Laser physical vapor deposition
• Non-contact bending
• Magnetic domain control
• Laser cleaning and paint stripping
• Surface roughening
• Micro-machining
• Laser marking
• Shock hardening
• Stereolithography
• Laser direct casting
• Process control using lasers

References

• Industrial applications of Lasers by John F. Reddy: Academic Press, 1997.
• http://lasermatter.umd.edu/
• https://www.princeton.edu/~spikelab/papers/book02.pdf
• http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=926269
• http://en.wikipedia.org/wiki/List_of_laser_applications
• http://www.industrial-lasers.com/index.html
• http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/lasapp.html
• https://www.coherent.com/applications/index.cfm?fuseaction=Forms.page&PageID=250