Laser-drillingHigh Tech Micro Machining.SOLUTIONS with Effective, Practical Advice
This is a non-contact process that can be used to form small holes in a wide variety of materials with a high degree of precision and reproducibility. (Sponsored Links) Laser-drilling has become an inexpensive alternative to other hole drilling methods, especially for holes of high ratio of depth to diameter, for economic mass producing of large numbers of tiny holes and for a variety of materials like rubber, plastics, composite materials, glasses, ceramics and various metals. Laser beam is transmitted in air. The pointing system which used to be made of mirrors moving along perpendicular lines to cover all the working space are recently being supplanted by optical fibers with considerable simplification and economy of the mechanical equipment. Accuracy, tight tolerances, repeatability and reproducibility are the consistency parameters most sought for in different industries like aerospace, medical devices, semiconductor and nanotechnology. Special applied techniques permit quick modification of laser characteristics, to accommodate changing prototype requirements involving dynamic conversion of hole size, depth, profile and edge quality. Laser-drilling is based on the absorption of laser energy by the work piece and its conversion into thermal energy forming a large temperature gradient near the affected surface of the work piece. The necessary energy density is provided by pulsed solid state lasers. When the vaporization temperature of the base material is reached and exceeded, the work piece begins to ablate and a hole geometry is formed. With sufficient power and time, more material is removed until the desired hole shape, size and depth are achieved. Sometimes additional gas is delivered coaxially with the laser to assist in removing molten material. Depending on the application there are two common methods used for hole Laser-drilling; percussion and trepanning, similar to conventional laser cutting. The suitability of each method depends on depth requirement, hole diameter, number of holes, edge quality and production quantity. Laser-drilling, using a wide variety of laser wavelengths with integrated laser, motion and vision systems, can provide cost effective solutions but it should be compared with the other high energy beam drilling process, namely electron beam drilling that, although presented in our page on High Energy Welding Processes, will not be further discussed here. Percussion Laser-drilling uses a micro machining method consisting in rapid bursts of pulses. This method, depending on materials, is used for hole diameters from 20 to 1200 µm (micron). The aspect ratio (diameter/hole depth) can be as high as one to 200. It can produce high-quality holes with minimal residue and consistent edge quality. Trepanned Laser-drilling is the method used to remove a cylindrical core, of whatever section, from a substrate. The relative positions of beam and substrate are moved to achieve the desired shape, edge quality and production throughput. Increasing throughput produces a more jagged edge quality. Requirements of finer hole resolution and edge quality decrease throughput. In the gas turbine engines industry, components are exposed to ever-increasing combustion and exhaust gas temperatures to maximize engine thrust. Large combustor components admit compressed air through thousands of small holes of various sizes and shapes, that extend service life by cooling critical components. Holes are made by precision Laser-drilling in carefully designed patterns over contoured surfaces. Holes may be angled from 90 to 200 to the part surface to improve turbulence and combustion, limit fuel consumption and decrease engine noise. Laser-drilling of difficult to machine materials, specifically superalloys and ceramics, permits producing cooling holes in Plasma sprayed thermal barrier coatings (TBCs), consisting of a partially stabilized zirconia top coat and a NiCrAlY bond coat, deposited on a Ni-superalloy substrate. The negative effects of percussion Laser-drilling on material interfaces, on bond strength, and on the individual microstructures such as remelt layers and microcracking, must be assessed for each application. The recast or remelt layer is comprised of molten metal (not ejected from the hole by the vapor pressure of the laser pulse) which resolidifies on the side wall of the hole. For certain applications this layer may be objectionable, and may need to be limited in depth or eliminated by additional operations.The heat affected zone (HAZ) consists of unmelted metal surrounding the hole, that was subjected to microstructural changes due to its thermal history. Operators of laser beam must be aware of the dangers to their eyes and skin if hit by the powerful beam. Industries interested in exploring the capabilities of Laser-drilling, may outsource their business using the services of job-shops before looking for equipment to purchase. Research is being conducted to test Laser-drilling for rock trepanning and fragmenting in view of using this technique for drilling natural gas and oil wells. As mentioned above, a short note with a few links was published (7) in Issue 49 of our Practical Welding Letter for September 2007. Any questions or comments or feedback? Write them down and send them to us by e-mail. Click on the Contact Us button in the NavBar at top left of every page.
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