Investigations Concerning the Removal Rate and Surface Quality for Laser Machining with GHz Bursts in Milling ApplicationsWednesday (24.06.2020) 10:40 - 11:00 Room 1
In literature GHz-bursts were recently reported to show significantly higher removal rates compared to single pulses [1-3]. However, all the presented results are obtained for punching applications (machining of dimples), processes where heat accumulation combined with melt ejection could also lead to an increase in the process efficiency as less material has to be evaporated. It has been shown by experiments and simulations that this high efficiency cannot be transferred from the punching to a milling process due to the different melt flow in the different machining processes .
A PHAROS PH1-20 laser system offering 280 fs pulses at 1028 nm and up to 25 pulses per burst with an intra-burst separation of 185 ps (corresponding to 5.4 GHz) was used to machine squares with a side length of 1 mm and a spot radius of w0 = 16.2 µm into different materials. The square depth and its surface roughness were measured and the specific removal rate was calculated from the average power, the repetition rate, the number of repeats, the square depth as well as the pulse to pulse and the line to line distances. Additionally, in case of copper the residual heat was measured with a calorimetric set-up.
For copper,brass, steel and silicon a tremendous drop in the specific removal for a burst is observed, which is contradictory to the results presented in [1-3]. The surface roughness is significantly reduced for GHz bursts but when its value is concerned as a function of the specific removal rate no advantage is obtained. If ablation cooling would take place the residual heat should decrease in case of bursts, but in contrast, the corresponding experimental results always show the lowest value for single pulses for all investigated situations.
For metals and milling applications neither the reported high removal rates nor the ablation cooling effect could be confirmed for GHz-bursts with up to 25 pulses per burst. All results point to strong melting effects and indicates that the removal rate follows the trend of single pulses of corresponding pulse duration.
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