Applications In Material Processing: Dr. N. Venkatanathan

APPLICATIONS IN MATERIAL PROCESSING Dr. N. Venkatanathan

LASER WELDING High-power

lasers have found applications in welding. Pulsed ruby lasers have also been used in welding. Laser welding found important application in the fields of electronics and microelectronics Requires precise welding of very thin wires (as small as 10 µm ) or welding of two thin films together.

INTRODUCTION Laser Beam Welding (LBW) is a modern

welding process. It is a high energy beam process and has many advantages like deep weld penetration and minimizing heat inputs. Use of laser and computers improved the product quality through more accurate control of welding processes.

How it works? The focal spot is targeted on the work piece

surface which will be welded. At the surface the large concentration of light energy is converted into thermal energy. The surface of the work piece starts melting and progresses through it by surface conductance. For welding, the beam energy is maintained below the vaporization temperature of the work piece material.

The penetration of the work piece depends on

conducted heat. Therefore the thickness of the materials to be welded is generally less than 0.80 inches. The role of focusing lenses in this process is it concentrates the beam energy into a focal spot as small as 0.005 inches diameters or even less. Concentrated energy produces melting and joining of two bodies. The fatigue strength of the welded joint will be excellent. So welding is in pure form.

Laser weld can easily performed between two

dissimilar metals. Thus, a thermocouple may easily be welded to a substrate without much damage to adjacent materials. One can indeed simultaneously form the junction and attach the junction to the substrate. This method has been used in attaching measuring probes to transistors, turbine blades, etc.

Laser welds not only achieves welding

between dissimilar metal but also allow precise location of the weld. The Nd-YAG lasers and CO2 lasers are the two important kinds of lasers that find-wide ranging applications in welding.

In welding, materials are added to join the

two components. Thus the laser power must not be too high to evaporate the material. Removal of material leads, in general, to bad weld. Thus the laser used in welding process must have a high average power rather than high peak power.

Examples for LASER Welding A weld of ¼ inch thick stainless steel can be carried out

by CO2 laser having an output power of 3.5 KW. The material was moved at a speed of 2 cm/sec in the focal plane of the lens of focal length of 25 cm. Pulsed ruby beam having energy of 5 J with pulse length of about 5 nanosecond was used in welding 0.18 mm thick stainless steel. The weld was made using overlapping spots and the laser was pulsed at the rate of 20 pulses per minutes. The focused spots was about 1 mm in diameter and the associated power density was ≈6 × 105 W/cm2

Laser offers some unique advantages, requires

extremely short time for the laser welding process. Welding can be done in the region adjacent to the heat-sensitive areas with out affecting these elements. Furthermore, welding in otherwise inaccessible areas can also be done using a laser beam. In welding of two wires, one may have a effective weld even without the removal of insulation.

Most Popular types of LBW Nd:YAG (neodymium-yttrium aluminum garnet)

Laser: Produces light with a 1.06-micron wavelength. Carbon Dioxide Lasers: It produces light with a 10.6-micron wavelength. The Diode Laser: produces several wavelengths. Industries Served: Aerospace, Defense/military, Electronics, Research & development, Medical, Sensors & instrumentation, Petrochemical refining and Communications & energy.

ADVANTAGES Deep and narrow welds can be done. Absence of distortion in welds. Minimal heat affected zones in welds created. Excellent

metallurgical quality will be established in welds. Ability to weld smaller, thinner components. Increased travel speeds.

HOLE DRILLING Drilling of holes in various substances is

another interesting application of the laser. Drilling holes less than about 250 µm in diameter by using metal bits becomes very difficult and is also accompanied by frequent breakage of drill bits. With lasers one can easily drill holes as small as 10 µm through the hardest of substances.

The Swiss watch industry has been using flash-

pumped neodymium-YAG lasers to drill ruby stones used in timepieces. So laser ensures that the absence of problem like drill breakage. Laser hole drilling has the advantage of precise location of the hole. A laser pulse having a pulse width of about 1/1000 of a second and an energy of approximately 0.05J can burn through a 1-mm thick steel plate leaving behind a hole of radius ≈ 0.1 mm.

LASER CUTTING Laser cutting is an advantageous technology choice. The most common laser that is used in cutting

process is the CO2 laser due to its high output power. In the cutting process, one essentially removes the material along the cut. Using pulsed lasers, the repetition frequency of the pulse and the motion of the laser across the material is adjusted so that a series of partially overlapping holes are produced.

The width of the cut should be as small as possible

and it should avoid any re-welding of the cut material. The efficiency of the laser cutting can be increased by making use of gas jet coaxial with the laser. In some cases, one uses a highly reactive gas like oxygen so that when the laser heats up the material, it interacts with the gas and gets burnt. The gas jet also helps in expelling the molten material.

Such a method has been used to cut the

materials like stainless steel, low-carbon steel, titanium etc. For e.g., a 0.13 cm thick stainless steel plate was cut at the rate of 0.8 m per minute using a 190 watt CO2 laser using oxygen jet. In some methods, one uses inert gases in the

place of oxygen. Such a gas jet helps in expelling the molten material.

Such a technique would be very efficient with

materials which absorb most radiation at the laser wavelength. Wood, paper, plastic, etc. have been cut using this method. A gas jet assisted CO2 laser can be used for obtaining parallel cuts of up to 50 mm depth in wood products. At the cut edges carbonization occurs, but it is usually limited to a small depth of the material.

This causes a discoloration only and can be

decreased by increasing the cutting speed. Laser cutting of stainless steel, nickel alloys and other materials finds widespread application in the aircraft and automobile industry. It has been recently tested and shown that aluminum sheet metal can be efficiently cut with high-powered laser beam.

In fact, it is believed that it could be as

much as 60% to 70% less expensive than the conventional techniques. Laser cutting has also been used in the textile industry for cutting cloth. It is even claimed that this is the greatest advance in apparel manufacturing since the sewing machine.

Advantages Reduction of total work times Increase in production quality. It is precise, clean and silent. The beam can be focalized on an

extremely small area (from 0.1 to 1mm in diameter). The area in proximity to the cut edge has a very low heat alteration.

Moreover, the laser cut has the capacity of operating

on complex profiles and with very small rays of curvature. Unlike water and traditional cutting systems, light exerts no mechanical pressure on the piece. Absence of wear in the instrument. Cutting capability independent of hardness of the material. Has the capability to cut coated or surface treated materials.

Ease of integration with other automated

systems Very high trimming capability Capability of adapting immediately to changes in production requirements. In many cases, laser cutting can produce finished pieces that do not require further processing (polishing, de-burring, finishing etc.).

Laser Ablation Laser

ablation is the process of removing material from a solid (or occasionally liquid) surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimates. At high laser flux, the material is typically converted to a plasma.

Usually, laser ablation refers to removing

material with a pulsed laser. But it is possible to ablate material with a continuous wave laser beam if the laser intensity is high enough. The amount of material removed by a single pulse is depending on the depth over which the laser energy is absorbed. The absorption of laser energy depends on the material's optical properties and the laser wavelength.

Laser pulses can vary over a very wide range of

duration from milliseconds to femtoseconds. The laser fluxes can be precisely controlled. This makes laser ablation very valuable for both research and industrial applications. The simplest application of laser ablation is to remove material from a solid surface in a controlled fashion. Laser drilling is good example.

Pulsed

lasers can drill extremely small,

deep holes through very hard materials. Very short laser pulses remove material so quickly that the surrounding material absorbs very little heat. Laser drilling can be done on delicate or heat-sensitive materials, including tooth enamel which called as laser dentistry.

Laser energy can be selectively absorbed by

coatings, particularly on metal. So CO2 or Nd:YAG pulsed lasers can be used to clean surfaces. Removal of paint or coating, or prepare surfaces for painting without damaging the underlying surface. High power lasers clean a large spot with a single pulse. Lower power lasers use many small pulses which may be scanned across an area.

ADVANTAGES No solvents are used, so it is environmentally friendly

and operators are not exposed to chemicals. It is relatively easy to automate, e.g., by using robots. The running costs are lower than dry media or CO2 ice blasting, although the capital investment costs are much higher. The process is gentler than abrasive techniques, e.g. carbon fibres within a composite material are not damaged. Heating of the target is minimal.

High level Applications To process the material removed into new forms either

not possible or difficult to produce by other means. A recent example is the production of carbon nano tubes. To create coatings by ablating the coating material from a source and letting it deposit on the surface to be coated, called special type of physical vapor deposition. Can create coatings from materials that cannot readily be evaporated any other way. This process is used to manufacture some types of high temperature superconductor.

Remote laser spectroscopy uses laser ablation to

create a plasma from the surface material. The composition of the surface can be determined by analyzing the wavelengths of light emitted by the plasma. Finally, laser ablation can be used to transfer momentum to a surface. The ablated material applies a pulse of high pressure to the surface underneath it as it expands. The effect is similar to hitting the surface with a hammer.

This process is used in industry to harden

metal surfaces. Used as a damage mechanism for a laser weapon. It is also the basis of pulsed laser propulsion for spacecraft. Laser ablation has biological applications and can be used to destroy nerves and other tissues.

Medical Applications Used to remove part of biological tissue. Surface ablation in the skin (also called

resurfacing, because it induces regeneration) can be carried out by lasers. Its purpose is to remove skin spots, aged skin, wrinkles, thus rejuvenating it. Surface ablation is also employed in the ENT treatment. For several kinds of surgery, such as for snoring.

Ablation therapy using radiofrequency waves

on the heart is used to cure a various of cardiac. It is a process by which the molecular bonds of a material are dissolved by a laser. For a laser to be able to ablate tissues, the power density has to be very high. Otherwise thermo-coagulation will happen, which is just a thermal vaporization of the tissues.

Roto-ablation is a type of arterial cleansing

that consists of inserting a tiny, diamondtipped, drill-like device into the affected artery to remove fatty deposits . The procedure is used in the treatment of coronary heart disease to restore blood flow. Bone marrow ablation is a process whereby the human bone marrow cells are eliminated in preparation for a bone marrow transplant.

Ablation of brain tissue is used for treating

certain neurological disorders like Parkinson’s disease and some time psychiatric disorders. Recently, some researchers reported successful results with genetic ablation. In particular, genetic ablation is potentially a much more efficient method of removing unwanted cells, such as tumor cells.

Laser ablation is greatly affected by the nature of the

material and its ability to absorb energy. Therefore the wavelength of the ablation laser should have a minimum absorption depth. Surface ablation of the cornea for several types of eye refractive surgery is now common, using an excimer laser system (LASIK and LASEK). Since the cornea does not grow back, laser is used to remodel the cornea refractive properties, in order to correct refraction errors, such as astigmatism, myopia and hyperopia.