Lasers & EDM - working together

Modern Machine Shop, Feb, 1992 by Reza K. Mosavi

In gas turbine engine design and manufacturing, continuous efforts are being made to improve engine efficiency. This is directly related to the engine's operating temperature. Although it is clear that higher efficiency levels can be achieved with higher operating temperatures, the practical limitation is the damage caused by hot gases impinging on the surfaces of turbine components such as blades and vanes.

In the past decade, new and exotic alloys that can withstand a high-temperature environment have been developed, and turbine air foils have been fabricated from them. New and sophisticated thermal barrier coating technologies have allowed higher temperature operation and have increased the life cycle of engine components. However, to attain higher temperature, it is necessary to protect these vanes and blades by effectively air-cooling their surfaces. Holes of various sizes and different patterns are manufactured in vanes and blades for the passage of cooling air. The holes are basically cylindrical and drilled in both the air foils and platforms of vanes and blades. Presently, these holes are produced either by electrical discharge machines (EDM) or by lasers. Figure 1 shows a typical hole pattern drilled in a turbine vane, by a laser.

A new "shaped hole" is now being implemented to more effectively cover the air foil surface with a thin film of cooling air. More effective cooling is accomplished through holes that terminate in diffuser formations at the air foil surface. Figure 2 shows a typical diffuser hole pattern. To produce such holes with EDM, a single comb-type electrode is configured to form multiple cooling holes and their diffuser openings. Although the above method produces reliably uniform products, over all it is expensive due to the slow nature of the EDM process.

Laser-EDM Integrated Operation

A new approach for producing diffuser holes was developed and patented at Chromalloy Research and Technology Division whereby the cooling holes are drilled by laser and the diffuser shapes are generated by EDM. The process is composed of a two-step procedure in which a laser beam drills the holes very quickly and cost-effectively and an EDM step is utilized only to form the diffuser shape. The EDM part of the procedure occupies only a fraction of the time previously required. Substantial overall economy is realized by integrating these two steps, preferably performed first by laser drilling, and then by the EDM operation. If desired, the reverse order may be employed.

Figure 3 shows a comb-type electrode generating diffuser shapes in the laser-drilled holes of a high-pressure first-stage turbine blade. Figure 4 shows the same blade after completion of the laser-EDM integrated operation. Producing the cooling holes in the vane shown in Figure 2 and the blade shown in Figure 4 solely by EDM comb-type electrodes made of copper would take about 120 and 75 minutes, respectively.

The vane is a cobalt-based alloy with holes ranging from 0.100 to 0.250 inch. There are 140 shaped holes on this vane. The blade is a nickel-based single crystal alloy with 50 shaped holes varying from 0.12 to 0.018 inch in diameter and 0.060 to 0.200 inch long. When applying the laser-EDM integrated process, the same diffused cooling hole patterns are generated in 80 and 47 minutes in the vane and blade respectively. Thus, time savings of 40 minutes per vane and 28 minutes per blade are achieved. Considering the number of vanes and blades per engine set and the number of engines produced annually, the savings potential is enormous.

Hole Quality

One of the major reasons users selected EDM hole drilling rather than laser for aerospace components has been the quality of the drilled holes. EDM does produce high-quality uniform holes which, in contrast to laser, have almost no recast layers. However, the technology of high-power industrial Nd:YAG lasers has reached a level of maturity and reliability so that it is indeed possible, in some applications, to drill holes with quality approaching EDM. The laser drilling parameters such as pulse width, pulse per second, energy per pulse, focusing lens, assist gas, and so on, can be optimized to produce holes with minimum recast layers. These laser-drilled holes now have gained acceptance in aerospace industries.

Figure 5 shows an enlaged cross-section, at 100X magnification, of a laser-drilled hole in a high-pressure first-stage gas tubine blade. The laser used was an industrial Nd:YAG (neodymium yttrium aluminum garnet) laser with a 250-watt power capability. The blade was drilled with a beam of 0.6-millisecond pulse width, 3 pulses per second, and a power range of 10 to 20 watts. As the picture shows, the hole is clean and practically free of recast layers.

The critical step in the integration of the laser-EDM process when forming shaped holes in alignment of the laser-drilled holes with their EDM diffuser shapes. NC programming, tooling and setups must be well coordinated in this effort.