In-process measurement of a keyhole using a low-coherence interferometer with a high repetition rate

Area of Science:

• Materials Science and Engineering
• Laser Material Processing
• Metrology

Background:

• High-power laser drilling creates keyholes, affecting material properties.
• Accurate characterization of keyhole geometry is crucial for process control.
• Existing measurement techniques may lack precision for micro-scale features.

Purpose of the Study:

• To precisely measure the three-dimensional shape of a laser-induced keyhole in stainless steel.
• To evaluate the effectiveness of low-coherence interferometry for keyhole metrology.
• To provide dimensional data for validating laser drilling models.

Main Methods:

• A 3-mm-thick stainless-steel plate was drilled using an 8-kW peak power laser (1070 nm center wavelength).
• A low-coherence interferometer (10 MHz repetition rate, 1550 nm center wavelength) was employed for measurement.
• The interferometer achieved an absolute spatial resolution of 10 µm over a 5 mm measurement range.

Main Results:

• The cross-sectional area of the keyhole was measured as 0.42 mm × 0.78 mm (width × depth).
• The side dimensions of the keyhole were determined to be 0.46 mm × 0.78 mm (width × depth).
• The measurements provided detailed geometric data of the laser-drilled keyhole.

Conclusions:

• Low-coherence interferometry is a viable technique for precise keyhole metrology.
• The study provides quantitative data on laser-drilled keyhole dimensions in stainless steel.
• Accurate dimensional data aids in understanding laser-material interactions and optimizing drilling processes.