By Martien van Dijk
Venray, The Netherlands - Magazines Industrial Laser Solutions (ILS) and Laser Focus World (LFW) have just published feature articles on fiber laser drilling aerospace components.
In general, it can be stated that the main advantage of fiber lasers over lamp pumped Nd:YAG lasers are the top hat profile at focus and the square pulse form. Both properties will contribute to a better hole quality.
In both articles, the authors state they performed drilling experiments using pulse lengths in the range of 0.2 to 1 msec — similar pulse lengths that are normally used in Nd:YAG laser drilling. They don’t seem to realize that these pulse length values are used since they are the shortest pulse lengths that can be achieved due to limitations in the discharge process of the flash lamps used in pulsed Nd:YAG lasers. From model studies and from measurement of the light coming out of the hole being drilled, it is clear that the flow of the material coming out of the hole is pulsating strongly during the pulses used in Nd:YAG percussion drilling. This is in line with several publications where it is demonstrated that the quality of holes is improving when shorter pulses are used.
The remark in the LFW paper (4th paragraph) that the recast layer tends to increase with longer pulse durations also confirms the need for short pulse durations in order to improve hole quality.
The long pulse duration (10 msec) used in drilling the holes of Figure 1 in the LFW article seems to contradict the above statement. However, the pulse profile in this experiment is a top hat. At the same pulse duration, using a top hat profile will result in a smaller recast layer than when using a Gaussian beam profile.
The LFW article shows pictures of excellent quality holes. However, no information is given on laser parameters like pulse energy, pulse frequency, number of pulses, focus position, and gas pressure at the surface. So it is not clear whether or not these high quality holes can be achieved at economical costs.
In the LFW article, it is stated that the fiber laser operates in the trepanning mode 10 times faster than the Nd:YAG laser. One would expect that a 2 kW average power laser will trepan 10 times faster than a 200 laser. So this is not an advantage of a fiber laser over a Nd:YAG laser. It is just a matter of average power.
To be able to compare test results in both articles the number of Joules per mm3 to remove material should be given. This figure of merit for the efficiency of the process may be used in comparing all material removal processes. In percussion drilling with Nd:YAG lasers of Ni based supper alloys this figure is in the range of 45 to 50 J/mm3
The ILS article mentions that at high power there is degradation of beam quality of the Nd:YAG laser (presumably a P20 or P50 laser). This may be related to the tuning procedure used. Tests performed with a beam analyzer camera behind the rear mirror demonstrated that this degradation is due to the method of tuning and can be avoided by using the data from a beam analyzer camera as indication of beam quality.
In the LFW article, there is no information about the nozzle and the gas condition used. The ILS article is also lacking this information; however, from the left photo in Figure 1, it is obvious that high pressure is used to blow molten material away from the nozzle. It seems that the ILS authors don’t realize that high pressures counteract the pressure build up in the hole to blow the material out of the hole. This high gas pressure will reduce the drilling efficiency. In percussion drilling, the only function of nozzle and gas flow is protection of the optics. Even by drilling a blind hole, using oxygen as assist gas, one will see a shiny surface at the bottom. This confirms that no oxygen is entering the blind hole. Using sonic gas flow the stand off distance of the nozzle should result in a minimum in the gas pressure at the surface.
Drilling with QCW fiber lasers opens a new field of research. Model studies of the percussion drilling process will help to get a better understanding of the process and to optimize fiber lasers for this application.
The ILS and LFW articles are the first steps on the road to an improved laser drilling process.
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Editor’s note: Martien van Dijk is an international consultant specializing in industrial laser material processing. He has more than 20 years experience in laser material processing. As R&D manager, he has been involved in Nd:YAG trepanning as well as percussion drilling of aero engine and industrial gas turbine components. He also acts as an Editorial Advisor to Industrial Laser Solutions. (Contact: [email protected])