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Laser-drillingHigh Tech Micro Machining.SOLUTIONS with Effective, Practical Advice
Laser-drilling was briefly addressed in a short note on High Energy Drilling published (7) in Issue 49 of Practical Welding Letter for September 2007. (Find the link further down this page) While the essential information presented there is still valid, unfortunately many of the links provided there are now inactive. 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.
Laser drilling has become an inexpensive alternative to other hole drilling methods, especially for holes of high ratio of depth to diameter. Also for economic mass production 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 systems used to be made of mirrors moving along perpendicular lines to cover all the working space. Recently they are being supplanted by optical fibers with considerable simplification and economy of the mechanical equipment. Consistency Parameters, like accuracy, tight tolerances, repeatability and reproducibility are most sought for. This is equally important for different industries like aerospace, medical devices, semiconductor and nanotechnology. Special applied techniques permit quick modification of laser characteristics. Changing prototype requirements involve dynamic conversion of hole size, depth, profile and edge quality. Laser-drilling is based on the absorption of laser energy by the work piece. Its instant conversion into thermal energy vaporizes the affected volume. The necessary energy density is provided generally by pulsed solid state lasers. As soon as the vaporization temperature of the base material is reached and exceeded, a hole geometry is formed. With proper parameters, sufficient material is removed to deliver the required hole shape, size and depth. 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. Percussion drilling with a stationary beam is similar to piercing as for starting a hole in the middle of a sheet. For trepanning holes of whatever shape, a relative motion is established between beam and workpiece. 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. It should be compared with the other high energy beam drilling process, namely electron beam drilling presented briefly in our page on High Energy Welding Processes.
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. Increasing speed 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 efficiency. 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 20° 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). See Thermal Spray. Such coatings consist 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 re-solidifies 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 micro-structural 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 consider outsourcing 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. An Article on Laser Surface Patterning Pre-Treatment was published (11) in Issue 151 of Practical Welding Letter for March 2016. An Article on A new way to measure Laser Power was published (7) in Issue 152 of Practical Welding Letter for April 2016.
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