The Laser Expert
By Simon L. Engel, President, HDE Technologies Inc.
Set up to standard
Calibrating the process when laser drilling small holes
I

n the December 2019 edition of FFJournal, this column covered the surface effect and basic characteristics of percussion laser drilled, small-diameter holes. Two metal removal technologies were discussed: photo ablative drilling (PAD) and thermal laser drilling (TLD). The distinction between the two methods is the significant difference in power density of the focused laser beam as it is focused on the material.

In everyday practice, we need to calibrate the performance of the laser drilling system before we perform actual drilling. The following steps are applicable to both PAD and TLD:

Equipment qualification record (EQR). In published technical standards, calibration of the laser beam characteristics is a requirement. The spatial and temporal characteristics are measured and recorded to make sure the laser and the beam delivery optics are within the limits specified by the manufacturer and meet the settings developed for the specific drilling process.

Figure 1. When adjusting the aperture, we change the cavity configuration to generate different spatial profiles. An aperture that is as large as the cross section of the cavity generates a large M2 spatial profile and a large-diameter hole. Smaller apertures generate progressively smaller M2 spatial profiles and progressively smaller diameter holes (see Figure 2.) The apertures are designed not to heat up.
adjusting the aperture and changing the cavity configuration
Procedure qualification record (PQR). This information is typically created when the laser drilling process is developed. The procedure and the process-related data is contained in the PQR.

Setting the diameter of the hole—control of spatial profile of the beam. One method is to adjust of the spatial profile of the laser beam in a system where the laser cavity is accessible (by design) and the laser beam is delivered by “free-air” optics. Examples are Nd:YAG, CO2 and some solid-state lasers. (See Figure 1.) Another method is to select the correct diameter process fiber and beam delivery optics (See Figure 3.)

Relationship between the diameter of the intra-cavity aperture
Figure 2. Relationship between the diameter of the intra-cavity aperture and the diameter of the hole drilled.
laser beam being delivered with a process fiber
Figure 3. In systems where the laser beam is delivered with a process fiber, the diameter of the process fiber and the correct collimating and focusing lenses are selected to achieve the correct diameter of the focused beam and the diameter of the laser drilled hole. When the focal length of the collimating lens equals the focal length of the focusing lens, the diameter of the fiber is “imaged” on the target. Given that fiber diameters are available as small as 50 micrometers (0.002 in.), it is easy to get very high power densities to drill even Class I metals.
first hole as a function of the energy per pulse
Figure 4. Depth of the first hole as a function of the energy per pulse. It is assumed that correct power density is applied to the surface. Subsequent pulses will remove less and less material. This is critical when partial penetration holes are to be drilled or in the case of through holes when the material under the hole must be protected. Material: Class II. Hole diameter = 0.20 inch. In all cases, the I = 2.0E+07 w/in.^2. This graph may be expanded with a family of curves that show the number of pulses required to reach a desired depth. Compute the aspect ratio of the first holes at 1.0 j, 3.0 j and 10.0 j.
Setting the temporal parameters of the laser beam. There are several purposes for these settings:

  • To improve the coupling efficiency of the laser beam into the material. Perform pulse shaping as covered in FFJ’s September 2018 issue and as shown in Figure 5.
  • To optimize the ejection of the molten/vaporized metal. Instead of one 450-microsecond pulse, use a train of shorter pulses, 50 microseconds each, at 50 percent or lower duty cycle (Figure 5).
sectors having higher power density values
Figure 5. Rather than one 450-microsecond pulse, use a train of shorter pulses, 50 microseconds each, at 50 percent or lower duty cycle. The peak power density of the 450 microsecond pulse is = 1.3E+08 watts/in.^2. For Sector 1 = 1.1E+09 watts/in.^2; for Sector 2 = 8.6E+08 watts/in.^2. Both sectors have higher power density values. Information is based on U.S. patent 6,252,195.
  • To prevent the buildup of re-solidified metal on the walls and the entrance of the hole, by using an optimal-duty cycle at about 50 percent or higher.
  • Minimize the HAZ by selecting a duty cycle of lowest value to allow the metal to cool off completely between pulses.
  • Facilitate the drilling of holes at very shallow angle to the surface (~12 degrees). This is accomplished by firing a train of high-peak power pulses to establish a new surface that is effectively perpendicular to the axis of the laser beam. Then continue with lower peak power pulses to complete the drilling cycle.

After all this preparation, you will have a solid set of data to proceed with percussion laser drilling.

SIMON L. ENGEL, president of HDE Technologies Inc., has taught laser cutting and drilling applications for over 48 years. He was vice chairman of the AWS C7C (laser welding) Subcommittee and launched laser welding programs at community colleges. To learn laser cutting and drilling techniques, the Laser Cutting and Drilling Technology Engineering Manual is available at www.hdetechnologies.com. Engel can be reached at simonlaser@hdetechnologies.com.
Back to issue