By: Mohammed Naeem

Fiber lasers with its high beam quality (M²~ 1.10 are routinely being used for a welding and cutting for a rage of industrial applications. These fiber lasers are very compact and robust and have an edge over lamp pumped Nd: YAG lasers in terms of beam quality and wall plug efficiency (approx 20%).

To date majority of the laser material processing work with these high beam quality lasers has been carried out either with a continuous wave (CW) or modulated output because currently conventional CW fibre lasers have no peak power over maximum average power capability.  This is due to the peak power limitations of the diode pump sources used.  Significant lifetime degradation occurs if the junction temperature of a laser diode is increased during operation for any significant length of time which would normally be needed for percussion drilling of aerospace alloys (i.e. milliseconds and above). However it may be possible to use these high beam quality lasers to trepan various sizes holes. Unlike percussion drilling where high pulse energies (up to 20 joules) and high peak powers (up 20kW) are needed to drill holes of 0.3-0.75mm diameter. For trepanning applications the main laser requirements are good beam quality with CW/high frequency modulated output to drill holes at reasonable drilling speeds.

First part of the work describes laser trepanning of aerospace alloys with a 400W single mode (SM) fiber. The second part of the work describes laser ablation of aerospace alloys with the same laser. Normally laser ablation of variety of materials is Q-switched lamp pumped or diode pumped solid state lasers. Here we show that it is possible to the CW fiber for ablation work. Turning the SM fiber laser on and off typically produces a relaxation pulse which is 4-5x the CW power as shown in Figure 1, which is very useful for ablating a range of materials.

Figure 1: Typical output waveform with relaxation pulse, when turning the SM fiber laser on and off.

During drilling primary concern to the component designer is achieving adequate airflow through the holes so that the appropriate cooling is provided. Airflow is governed principally by the size and shape of the hole and hence the need for tight control of size, roundness and taper. There are other factors also to consider; holes are often very closely positioned to one another on a component and any deviation in size may adversely encroach on other holes or even weaken the component locally. Excessive bell- mouthing or barreling is therefore undesirable in addition to recast layer and heat-affected zone.

The geometrical features and the metallurgical characteristics of each laser drilled hole generated during the present study were investigated. The cross sections of some of holes drilled with 400W SM fiber laser are highlighted in Figures 2-3.

Figure 2: 1.25mm thick HastalloyX, 0.5mm dia hole very parallel hole with low recast layer (<25µm thick)

Figure 3: 4mm thick Haynes alloy, 3.6mm dia hole, average recast layer < 40µm

The above holes were drilled with SM fiber laser with CW output, however it possible to use the same laser to remove thermal barrier coatings (TBC) prior to laser drilling. The ablation tests were carried out by optimizing the modulation frequency and the peak power in the initial spike of the relaxation pulse. Some of the ablation results are highlighted in Figures 4-5.

Figure 4: HastalloyX alloy with 0.4mm thick TBC coating; material removal rate 13.8mm³/min; 15 kHz, 40MW/cm²

Figure 5: 2mm thick Haynes alloy with 0.5mm thick TBC coating, 15 kHz, 40MW/cm³ material removal rate 14mm³/min; 1mm dia trepanned hole with the same laser after removing the TBC.

The work reported here show that by optimizing laser and processing parameters it is possible to produce good drilling and ablation results (dual processing) with the same SM fiber laser.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here