Laser Drilling

Some of the main benefits of laser drilling are:

• Non-contact process (no tooling wear or breakage, minimal material distortion)
• Highly accurate and consistent results
• Precise control of heat input
• Ability to produce small diameter holes with high aspect ratios
• Ease of programming and ready adaptability to automation
• Increased production rates with faster setup times and less tooling
• Flexibility in changeovers for prototypes and small-batch manufacturing
• Versatility (the same tool can be used for laser welding and laser cutting)
• Capacity for a high degree of beam manipulation (including the ability to drill at shallow angles and to drill shaped holes)
• Capability of some laser systems to drill multiple features simultaneously
• Ability to process a wide range of materials

The JK Lasers range of pulsed Nd:YAG lasers drill metals, coated metals, some ceramics, and graphite composites.  These lasers create discrete pulses of controllable energy, peak power, and temporal profile (or shape), for drilling.  It is the control of these pulses that make the pulsed YAG such a versatile drilling system.  The key to drilling with a pulsed YAG is the peak power:  Even a lower average power pulsed YAG laser can drill well since the interaction with the material is defined by the peak power and pulse energy parameters with average power having little effect. 

The Lumonics Lasers range of pulsed TEA CO2 lasers and Excimer lasers use an ablative drilling mechanism to produce high quality, very low heat input features of any shape and even multiple features being drilled at the same time.  These lasers can break the bonds of the target material and create almost a “cold” ablation with very fine depth control resolution usually within a few tens of microns since each pulse removes a very controlled thin-thickness layer.  The wavelength of these lasers can be adjusted or chosen to match the absorption bands of the target material.  Features are typically created via mask imaging of the large laser beam.  Because of the ablative nature of this drilling the feature sizes can be held to a few microns in tolerance.
The eCO2 laser range drill non-metals such as plastics, paper, fabrics, ceramics, and polymers.  Their excellent beam quality and high power make it easy to drill most all non-metals and some ceramics.

Percussion drilling with Nd: YAG lasers
Percussion drilling means adjusting the laser’s focus spot size to determine the hole size.  There is no relative motion between the laser beam and the part during the drilling process.  The parameters and number of pulses are chosen to produce a good quality hole.  Holes in the 25um to 1000um diameter range can be drilled using this method but the limits vary according to the material to be drilled and the thickness.  Most percussion drilled holes are in the 300um-600um diameter range.  Percussion drilling allows a special drilling regime termed drilling “on-the-fly” to greatly increase hole drilling speed.  Drilling on the fly is usually performed on round turbine engine parts and uses signals from the motion systems encoders to trigger the laser at specific, consistently-spaced, hole locations around the part.  If 10 pulses are required to percussion drill a hole then a drill on the fly system will rotate the part 10 times sending a laser pulse to each hole location per revolution.  Drilling speed is increased because indexing time is eliminated from the cycle time.  Percussion drilling can take advantage of pulse-shaping to improve the interaction of the laser beam with the material and help control taper and improve drilling speed.  Pulse shaping is programming the laser’s pulse temporal profile.  By breaking up a long drilling pulse into two three or four shorter segments separated by off-time the hole quality can be improved and speed increased.  Debris coming out of the hole can interfere with end of a long duration pulse so breaking it up improves efficiency and reduces drilling time.  Using a pulse shape with a lower peak power segment at the beginning can create a hole that has less initial bell-mouth taper.

Trepan Drilling with Nd:YAG lasers
Trepan drilling requires a motion system to allow piercing with the laser and then motion of the beam relative to the part to “cut out the hole”.  Trepanning allows for a diameter tolerance that about half that of percussion drilling and the ability to make shaped holes with a tailored taper or changing cross section with depth with a multi-axis system.  This requires a system with good accurate motion capability and high-level programming capability.  Trepanning also allows cutting out specific shapes and large holes as well as complex 3-D trimming of large parts.
Holes drilled in metals are judged by the hole diameter tolerance, taper, recast thickness, and micro cracking.  Hole diameter in percussion drilled holes is generally less than +/- 50um and in trepanned holes the tolerance tightens to about +/- 25um.  Recast is molten metal that resolidifies around the hole’s inner diameter and recast thickness varies with the alloy and hole depth but is generally held to less than 100um.  Hole depth can be as high a 50mm but most drilling tasks will have hole depths of less than 15mm.

Ablative drilling with Excimer Lasers
Excimer lasers produce intense pulses of radiation at short, UV, wavelengths with high peak powers in megawatts and short pulses of nanoseconds’ duration. These intense pulses of radiation can be used drill holes in selected materials by non-thermal ablation resulting in extremely clean edges to the drilled holes with a minimal heat affected zone 
Tight tolerances on the diameter, taper and circularity of holes require excellent beam profile homogeneity and good pulse-to-pulse stability. A high repetition rate (of hundreds of pulses per second) and high pulse energfy alloweconic production rates to be achieved