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Laser drilling: the hole story.

Many components in today's ground and airborne turbine engines require literally tens of thousands of cooling holes. As recently as 10 years ago, they were produced using conventional drilling techniques. Now they are drilled with lasers at substantial savings. In fact, one major US turbine-engine manufacturer has over 20 CNC, five-axis laser systems doing production drilling.

Part of the reason for turning to lasers comes from improvements in equipment, which has led to better drilling performance, and from broader acceptance criteria of laser-hole characteristics. Holes ranging from 0.25-mm to 1.25-mm dia can be made in most alloys at angles as small as 15 degrees from the work surface.

How it works

In laser drilling, focused power densities are greater than 1 million W/sq cm. Absorbed energy is converted to heat, resulting in controlled material removal partially by vaporization, but primarily by expansion of molten metal by localized boiling.

A number of factors determine the range of hole sizes that can be drilled with a pulsed CO.sub.2 or YAG laser. Limitations are optical in the small sizes. For large diameter holes, the practical limit is pulse energy (today, about 50 joules).

Trepanning is important in enhancing laser drilling performance. This technique, which uses a coaxial gas jet and either lens or part movement to cut out the desired hole size, often will dramatically improve hole quality regarding taper, diameter, and recast.

In general, the more conventional form of laser drilling, known as percussion drilling, is preferred for making holes under 0.030" dia, particularly when depths of several tenths of an inch are required.

The greatest potential for advance in laser drilling is improvement of beam quality in high-power units. Divergence (angle at which the beam spreads) of a typical high-performance laser increases with average power. Tests concerning the impact of divergence on hole quality found that with constant pulse energy, as pulse rate increases a hole's entrance becomes larger and penetration decreases. Similarly, by holding pulse rate constant and increasing pulse energy, entrance diameter of laser drilled holes increases linearly with average power.

Hole depth is increased by adjusting the laser's focal point so it is beneath the part's surface. In tests conducted at Raytheon's Laser Center (Waltham, MA), a series of 181 single-shot holes were drilled at 12 focal positions for three or four energy levels per pulse at four different pulse widths.

The firm's technical staff concluded that hole depth for single-shot holes could be improved significantly by focusing into the workpiece. Increased depth could be as great as 15 percent, but entrance diameter (and hence taper) also would increase by about 10 percent. Whether this is acceptable must be judged on an application basis.

Another study, performed by Solar Co, evaluated small hole production in large combustor liners made of 2.36-mm thick Hastelloy-Y. Traditional holemaking methods were expensive or impractical. More than 9000 holes were laser generated by contour cutting. The study indicated that laser drilling was the least expensive method of producing functional holes.

Commercial applications for percussion laser drilling continue to grow as potential users become more familiar with its advantages, i.e., perishable-tool costs are eliminated, hole consistency and accuracy are unaffected by traditional tool wear and breakage, and lasers are compatible with automated production. This last characteristic is the focus of our cover story "CIM blends lasers into automated manufacturing" in this month's Advanced Manufacturing Technology section.

For more information about holemaking with lasers from Raytheon, circle E63.
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Publication:Tooling & Production
Date:Mar 1, 1985
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