Heat accumulation investigation in ultrashort pulse laser micromachiningFriday (26.06.2020) 10:50 - 11:10 Room 3
Laser micromachining and texturing is a growing field with interesting applications in industry. With increase of average power of ultrashort pulsed lasers, heat accumulation starts to play a role even when using so called “cold ablation”. Up to now, most of the works analyse the accumulation by numerical modelling. The present work focusses on measurement of heat accumulation directly during the process in nanosecond and microsecond time range. Measurement system was setup using liquid nitrogen cooled fast HgCdTe photodetector and paraboloid mirrors. Micromachining of lines was done using 14 W picosecond laser with different pulse energies and scanning speeds. Heat accumulation signal was registered for all parameters. Interesting dependences were obtained. Calibration was done in order to obtain temperatures from the measured signal. The calibration is not straightforward due to very small laser spot and changing of heated area in low scanning speeds (significant accumulation).
The measurement system is based on previous systems [1,2]. Here a liquid nitrogen cooled detector is used for high sensitivity measurements. The signal is low due to small laser spot (25 μm) and thus also small heated area. For small scanning speeds the heated area increased, according to microscope measurements, to more than 100 μm. Different calibration approaches were tested based on reference radiation source and a small aperture placed in front of it. The sizes of apertures were from 25 to 200 μm. In Figure 1b, there is a calibration curve obtained for aperture diameter of 100 μm. Using such a calibration curve gives heat accumulation temperatures from 200°C to 1600°C for 30 μJ laser pulses for scanning speeds from 8 m/s to 0.07 m/s. According to SEM images, the material is already partially melted (small droplets on boarders) for the scanning speed 0.07 m/s.
Measured heat accumulation is higher for higher pulse repetition frequency (and lower pulse energy), than for low frequency (and high energy) for most scanning speeds. This is in accordance with prediction from Weber et al. .
The work has been supported by the Ministry of Education, Youth and Sports of the Czech Republic (OP RDE program, LABIR-PAV project, No. CZ.02.1.01/0.0/0.0/18_069/0010018) and SGS-2019-008 project.
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