Solid-state lasers, e.g. Nd:YAG lasers, also operate at a lower wavelength (1.06 µm), which markedly improves the absorption characteristics, i.e. the metal surface absorbs now significantly better energy from the Nd:YAG laser beam.
High power diode lasers (HPDL) were introduced fairly recently. These equipments are available at maximum 6 Kw power level. HPDL equipments represent the newest generation of high power lasers for materials processing; especially for welding (heat conduction welding), coating and surface treatment, polymer welding, brazing and soldering, etc. The still even lower wavelength (typically 0.8 and 0.94 µm) improves further the absorption characteristics of the laser beam. Due to the very high electrical/optical efficiency (30-50%), HPDL equipments are remarkably smaller in size than other lasers of the same kW level.
Lasers generate light energy that can be absorbed into materials and converted to heat energy. By employing a light beam in the visible or infrared portion of the electromagnetic spectrum, we can transmit this energy from its source to the workpiece using delivery optics which can focus and direct the energy to a very small, precise point. Since the laser emits coherent radiation, the beam of energy has minimal divergence and can travel large distances without significant loss of beam quality or energy. Some industrial applications of lasers include
A) Laser Cutting
Today, laser cutting is used extensively for producing profiled flat plate and sheet, for diverse applications in the industrial engineering sectors. For three dimensional components, multi-axis gantry laser beam manipulators have extended laser cutting to the automotive sector, these types of equipments being used for trimming pre-production body panels at all leading car manufacturers. More recently laser cutting has also found its way, very successfully, into other industrial sectors such as shipbuilding, traditionally seen as fairly slow to adopt high technology processes. Metals, ceramics, polymers and natural materials such as wood and rubber can all be cut using CO2 lasers.
Whether laser cutting with CO2 or Nd:YAG lasers, the principles employed are basically the same. The beam from the laser is focused on to the surface of the material being cut by means of a lens. The focused laser beam heats the material surface and a very local melt capillary is quickly established throughout the depth of the material. The diameter of this capillary is usually just slightly greater than the diameter of the focused laser beam. The great majority of CO2 laser cutting is performed using an assist gas. The significant feature of gas assisted laser cutting is that the molten material is ejected from the base of the capillary by a jet of gas coaxial with the laser beam. For some materials this gas can further assist the process by chemical (exothermic) reaction as well as physical work. The cut is generated by either moving the focused laser beam across the surface of the stationary material or by keeping the laser beam stationary and moving the workpiece. In this way, thick sections (up to 20mm) can be cut commercially and the cut quality and speed are generally considered high when compared with other thermal cutting processes. Laser cutting is also generally regarded as a 'low distortion' process, compared with other thermal cutting options. Stainless steel, aluminium and titanium are also cut using lasers, this time using a high pressure (up to 15 bars) inert assist gas to aid the process and blow material from the cut kerf. Less cutting of thermoplastic materials is performed currently because of the nature of the fume generated when some plastics are vaporised. Most laser cutting with CO2 lasers is performed in the power range 1 - 1.5kW.
B) LASER WELDING
In the past, laser welding was in its infancy and used primarily for exotic applications where no other welding process would be suitable. But today, laser welding is a full-fledged part of the metalworking industry, routinely producing welds for common items such as cigarette lighters, watch springs, motor/transformer lamination, hermetic seals, battery and pacemaker cans and hybrid circuit packages. Laser welding could be used in place of many different standard processes, such as resistance (spot or seam), submerged arc, RF induction, high-frequency resistance, ultrasonic and electron-beam. While each of these techniques has established an independent niche in the manufacturing world, the versatile laser welding approach will operate efficiently and economically in many different applications. Its versatility will even permit the welding system to be used for other machining functions, such as cutting, drilling, scribing, sealing and serializing.
To appreciate the potential of employing lasers in welding operations, you must redefine some of the traditional approaches to viewing "efficiency" as it relates to energy conversion. The laser is a relatively inefficient converter of electrical energy into output light, with the best lasers achieving about 2 to 15 percent energy conversion, depending upon the type of laser being used. However, virtually all of this output light energy is delivered to a small spot, as small as a few thousandths of an inch or less. By heating the spot of laser focus above the boiling point, a vaporized hole is formed in the metal. This is filled with ionized metallic gas and becomes an effective absorber, trapping about 95 percent of the laser energy into a cylindrical volume, known as a keyhole. Temperatures within this keyhole can reach as high as 25,000 °C, making the keyholing technique very efficient. Instead of heat being conducted mainly downward from the surface, it is conducted radially outward from the keyhole, forming a molten region surrounding the vapour. As the laser beam moves along the work-piece, the molten metal fills in behind the keyhole and solidifies to form the weld. This technique permits welding speeds of hundreds of centimetres per minute or greater, depending on laser size. The effects of welding on various materials depends upon many of their metallurgical properties such as "hot strength." After the applied energy is removed, the melt pool solidifies and then it slowly cools to the same temperature as the surrounding material. During this cooling, the material contracts, creating tensile stresses in the fusion zone. Materials that have a low tensile strength at temperatures near their melting point are said to exhibit "hot shortness," which often results in cracks appearing in the weld. Similarly, other thermal transformation, such as the martensitic transformation of high carbon steel, also can lead to cracking in or near the weld. To overcome this tendency, special precautions such as pre- and post-welding heating of the material is necessary.
C) Laser drilling
The combination of a high-energy laser pulse for melting with a properly tailored high-intensity laser pulse for liquid expulsion results in the efficient drilling of metal targets using lasers. It is being argued that the improvement in drilling is due to the recoil pressure generated by rapid evaporation of the molten material.
It has been found that material is removed in both the vapour and the liquid states. The intense laser energy used for laser drilling is sufficient to melt and subsequently vaporize the material. This vaporization process creates a recoil pressure, which is responsible for expelling the liquid. The amount of material ejected in the liquid state has a direct effect on the laser drilling/cutting efficiency due to the fact that the material is removed without the loss of additional energy required for vaporization.
Many theoretical models have been developed in an attempt to characterize the dynamics of the laser drilling process. In most instances, a gas jet is used to assist the drilling/ cutting of the material. In the case of stand-off drilling/cutting at a distance without gas-assist, the efficiency is rather low. The problem is due to resolidification of the molten pool. Increasing the laser power does not work well. Several novel methods for improving the efficiency of material removal in laser drilling have been developed.
D) Laser Engraving
The CO2 laser and the Neodymium:Yttrium/ Aluminum/Garnet (Nd: YAG) laser are both used for engravings. When engraving, the laser is focused on a polished ceramic surface, and a shutter-type mechanism chops or segments the constant-wave beam. This creates a sharp, segmented burst of the laser onto the surface of the roll while maintaining an undisturbed, consistent and stable laser condition. The beam vaporizes only part of the ceramic and puts the rest into a molten state. This molten ceramic, or recast, is what shapes the cell walls, and in turn creates some of the controversies between the merits of CO2 versus the YAG lasers. In engraving the roll, the laser beam actually moves ceramic, creating a round hole from the recast in the image of a moon crater. Of course, the finished cells are hexagonal shaped, (assuming the accepted 60-degree angle is being used). A hexagonal shape is achieved when the surrounding laser burns move and shape the ceramic recast to form the final cell shape. These burns interact, so to speak, to finish cells and/or create the beginnings of new ones. It actually takes seven hits, or burns for the laser to finish one cell, which includes the burn for the cell itself and the surrounding six cells. During this process, the pushed-up recast creates walls, and therefore, land area. Negative results, such as rough, wide, broken or excessively thin walls, can occur on improperly controlled CO2 anilox surfaces. But if the CO2 is properly applied, these problems will not arise in the first place.
E) Laser surface hardening
Laser transformation hardening (LTH), or laser hardening, is a method of producing hard, wear resistant patterns on discrete surface regions of components. During hardening, a shaped laser beam is scanned across the component, which causes surface regions to heat rapidly. The surrounding material acts as an efficient heat sink, leading to rapid quenching and hardening phase transformations. A hard surface region is thus produced, whilst desirable bulk properties, such as toughness and ductility, remain unaffected. Components made from hardenable ferrous alloys are particularly suitable for laser hardening. Depending on the material, hardness values up to about 1,000 HV can be achieved to a depth of around 1.5 mm, before surface melting occurs.
The benefits of laser surface hardening over conventional hardening include:
- Improved surface hardness, strength, lubrication, wear and fatigue properties
- Greater precision and lower energy input, leading to reduced post-treatment work
- Greater flexibility through the use of software to control the beam heating pattern
- Greater product design flexibility with respect to material selection and geometry
- Minimal environmental impact.
F) Laser coating (Cladding)
Laser coating is an advanced coating technology for improving surface properties of various components and equipments. Laser coatings are surface coatings with an extremely dense, crack-free and non-porous structure. Laser coatings show excellent metallurgical bonding to the base material, have uniform composition and coating thickness. Laser coating produces also very low dilution and low heat input to the component. Laser coating of new components give them surfaces with high resistance against wear, corrosion and high temperatures.
Besides new manufacturing, the process has shown its importance also in maintenance and repair of worn components, often resulting in component performances superior to those of uncoated ones.
In a laser coating process, a fine powder, e.g. 50-150 µm in size, is injected with a carrier gas to the laser beam traversing on the surface of the material or component to be coated. The powder absorbs energy from the laser beam, starts heating and melting in-flight, and deposits on the surface of the base material. Part of the energy is also absorbed by the surface causing controlled melting of a thin layer of the base material. This is then followed by self-quenching to form a martensitic case of high hardness,
ensuring the formation of real metallurgical bonding between the coating and the base material. In laser coating, a melt pool of the coating material is formed, which in turn results in coatings without porosity. The mixing between the two materials (coating and base material), i.e. dilution, must be as small as possible to utilise the properties of the coating material most effectively.
Laser coatings can be prepared on several types of base materials. Most commonly the base materials used are unalloyed steels, alloy steels, hardenable steels, stainless steels, nickel or cobalt based alloys.
Laser cladding
Laser heat treatment
- Laser Surface Alloying
Surface alloying with a laser is similar to laser surface melting except that another material is injected into the melt pool. The melt pool rotates up to five times from melting to solidification in laser surface alloying. This ensures that, the external material taken in the melt pool and alloyed region is nearly homogeneous throughout the melt region. Due to high power and rapid solidification associated with laser surface alloying, most materials can be alloyed into most substrates.
The process of alloying is thus: As the laser pulse hits the substrate surface to be alloyed, significant portion of the laser beam is scattered. The remaining energy is absorbed and instantaneously raises the temperature of the near-surface region above melting point. The liquid-solid interface then propagates inward. Simultaneously the alloying elements get transported by convective flow and diffusion irrespective of their source(i.e. pre-placing, feeding or gaseous environment). As the liquid-solid interface moves inside, the laser beam moved away from it. Hence, the melt pool loses heat and resolidifies outwards(conduction mode of heat transfer is inward). Meanwhile, the laser beam has advanced to the adjoining region and the same phenomenon repeats again. The solidification rate is so rapid that rarely diffusion-controlled phase transformation is observed. Alloying and localized variation in composition leads to amplified variations in constitutional super-cooling leading to significant variation in microstructure (planar, dendritic or cellular structure).
- Laser marking
Laser marking offers many advantages and each user will find specific benefits as they integrate it into their production processes. The generation of the mark is under computer control therefore the process is very flexible, complex marks can be generated with ease to include logo’s, part numbers, symbology, dates, codes. For the generation of characters and shapes, the vector method and various mask marking techniques are used.
With the mask technology, the negative of a character or character set is illuminated by a diverging beam and projected onto a component. This allows very high marking speeds since a complete character or even an entire character set can be produced by a single pulse. However, as this kind of beam shaping is not very flexible (it is difficult to produce bar codes or serial numbers), therefore the industrial significance of this application has decreased.
In the vector process, which has a greater market significance, the radiation is guided via two movable mirrors. The mirror axes are arranged perpendicular to each other so that one mirror can deflect the beam in the direction of the X-axis and the other one in the direction of the Y-axis. With combined control of mirror positioning, virtually any spot can be reached in a field below these two mirrors. If movement of the mirrors is combined with activation/deactivation of the beam by means of a Q switch, any contour can be created. Both beam deflection and activation/deactivation are normally controlled by a standard computer. After the beam has been deflected by the mirrors, it is focused onto the workpiece, which lies under the deflection equipment, through a flat field lens. The laser then marks the required image line by line into the material. The process is very fast with marking speeds up to 1000 characters per second, and requires no chemicals or subsequent operations such as baking in ovens for long periods to cure. Laser marking for that reason, is environmentally friendly. With very few moving parts the process is both repeatable and reliable and produces a clear mark legible to both the human eye and importantly machine vision systems, which can certainly be used to identify the device at some later stage.
Conclusion
Although industrial applications of lasers have enormous advantages over convectional processes, the processes are still not exploited to their full potentials, for a number of reasons. These include:
- The limited availability of suitable types of lasers.
- The widespread availability of more familiar conventional processes
- Limited knowledge of the principles and practices of lasers.
- Limited knowledge of the technical and economic benefits of lasers.
- Perceived difficulties in incorporating lasers into production lines
- Implications of lasers for product design and codes of performance.
- However, some lasers operate at significantly lower electrical/optical efficiency, which makes the equipments bulky and costly to run.
REFERENCES:
1. J.F. Ready: Industrial Applications of Lasers (Academic Press, New
York 1978)
2. M. Von Allmen, A. Blatter: Laser Beam Interaction with Materials,
Physical Principles and Applications, 2nd edn. (Springer, Berlin 1995).
3.James M. Darchuk and Leonard R. Migliore, "The Basics of Laser Welding," Lasers & Applications, March 1985.
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R. F. Duhamel and C. M. Banas, "Laser Welding of Steel and Nickel Alloys," Lasers in Material Processing, American Society of Metals, 1983.
- H. S. Carslaw and J. C. Jaeger, "Conduction of Heat in Solids," Oxford University Press, 1959.
6. Gordon Simpson, "Laser Welding the Large MIC - A New Approach to Hermetic Sealing," Microwave Journal, November 1984.
7. Welding/Brazing/Soldering Spotlight, "It*s a Dirty Job, But Nobody Has To Do It," Modern Applications News, February 1985.
8. Dennis Werth, "Laser Welding of Thermally Sensitive Alloys," Lasers & Applications, March 1985.
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