LASER DRILLING makes it possible to machine both very small and precise holes in a variety of shapes and orientations, in a wide variety of materials, including difficult-to-machine aerospace alloys. These holes can be tapered or shaped to enhance the amount and direction of air or liquid flowing through them. The drilling of holes in aerospace/turbine engine parts generally serves to enhance the cooling characteristics of the part. These holes can be drilled at extreme angles to the surface. Hundreds or thousands of cooling holes can be drilled in one part with a single set-up in cylindrical or unusually shaped parts. A high power pulsed Nd:YAG laser is normally used although a CO
2 laser can be used with non-metallic parts. Processing is accomplished through either percussion drilling or trepanning. In the laser drilling process, high power density is accomplished by using a high power laser and a focused spot size of 0.05 mm (0.002") to 0.75 mm (0.030").
Assist gas helps remove the molten material from the hole by a coaxially delivered flow during the laser pulse. Percussion drilling can be accomplished with or without assist gas. If no gas jet is utilized, an alternative lens protection method must be employed. Oxygen can be used to produce an exothermic reaction with many metals. Exothermic percussion drilling provides more efficient metal removal. Air or inert gas can also be used in percussion drilling. Percussion drilling can be described as delivering one or more pulses from the laser to a part while the laser beam and part are stationary. More than one pulse may be required depending on the material thickness. A variation of percussion drilling is "drill-on-the-fly" where pulses are delivered to the part by a stationary laser while the part is rotated. Hole placement is a function of rotational speed and laser pulse frequency. If multiple pulses are required, "Fire-On-The Fly", a software package developed by Laserdyne engineers, is utilized to synchronize the movement of the part to the laser pulses, ensuring that multiple pulses are delivered to the exact location required. By changing the laser pulse energy, pulse count or lens focal length, the characteristics of the drilled hole size and taper can be controlled to meet the design requirements of the part. Fire-On-The-Fly software can also change the pulse shape during the process to improve hole geometry.
Trepanning is another process for drilling holes. The part is held stationary and the laser beam is moved to create a hole or feature by cutting the shape. The term "drilling" is generally used until the diameter of the hole or feature exceeds the material thickness. The advent of extremely accurate and repeatable laser positioning systems allows for very unique and tight tolerance trepanned features. Shaped hole drilling is an emerging variation of trepanning where designers of aerospace and land based turbine components now have increased flexibility to create new designs and cooling concepts.
Laser hole drilling technology is employed on every turbine engine used in aircraft and/or land (power generation) applications. Automotive engineers, filtration designers and medical manufacturers are a few of the other users of laser technology that would not be able to manufacture their parts without the laser drilling process.
Power, or average power, is determined by pulse frequency and pulse energies selected. Power is limited by the duty cycle at which a laser can operate without degradation of performance. Power supply and resonator design also limit maximum usable laser power. Percussion drilling is accomplished using average power from less than 100 watts to 400 watts. Pulse length is selected to optimize the quality of the hole. Shorter pulse lengths may limit the maximum energy achievable in a single pulse. Typical drilling pulse lengths range from 0.5 to 2 microseconds.
Pulse frequency is determined by striking an optimal balance between throughput and quality requirements. Pulse frequency used for percussion drilling ranges from 5 to 20 Hz (with Nd:YAG lasers); up to 1000 Hz with C0
2 lasers.
Pulse energy required is fundamentally determined by material thickness, composition and hole diameter required. Higher pulse energy also provides faster drilling rates, but this can also be detrimental to the hole quality.
Focusing lens will determine the spot size for a given laser setup. The spot size correlates to the desired hole diameter to be drilled. The spot size is equal to the hole size in thinner metals (i.e., less than 0.25"). As metal thickness increases, the range of hole diameters that can be percussion drilled decreases. Thicker metals and larger holes dictate the use of longer focal length lenses. Focal lengths usually range from 4.0 inches to over 10.0 inches.
Focal position can be optimal above, below, or on the surface of the work piece, depending on the desired results. Most often, focus lies 5-15% of the metal thickness below the surface. Best focus for a desired result will most often be determined empirically following evaluation of hole quality. Hole quality is composed by roundness, taper, recast and micro-cracks.
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