Avoiding Back-wall Damage in Trepanning

Certain turbine engine combustor designs involve the use of a louver (see picture at the right) to direct cooling air at a right angle to the flow in order to cover a surface with air and more effectively cool the surface. There is often a challenge to prevent back-wall damage when laser drilling these combustors.

Section of louver-style combustor with laser drilled holes.

Impingement of the laser beam on the surface behind the one being laser drilled (referred to as the ‘back-wall’) can cause unacceptable melting of the back-wall surface. It is a common practice to use materials such as Teflon® or copper sheet to form a protective barrier to the laser energy that passes through the primary surface toward the back-wall. This has been marginally effective sometimes still allowing for back-wall damage as the laser beam penetrates the protection material. This is especially true when the back-wall is relatively close to the drilled surface. In other cases the useful life of the barrier material may be short or the process results inconsistent.

Recent developments based on advanced laser parameter control capability of the LASERDYNE S94P have made it possible to avoid back-wall damage even in parts with small (for example, 1 mm) gaps between the back-wall and drilled surface. Note that protection material continues to be used, though the laser control consistently prevents penetration of the material by the laser beam and extends the useful life of the material.

The pictures below show holes produced using previous techniques with an Nd:YAG laser (left) and results with fiber laser and proprietary LASERDYNE control of the fiber laser (right).

Laser trepanned 1 mm diameter holes using Nd:YAG laser. Note the damage (melting) of the back-wall (blue colored region).	Laser trepanned 1 mm diameter holes using fiber laser and proprietary LASERDYNE® control. Note the absence of damage (melting) of the back-wall (blue colored region).

LASERDYNE S94P control of a fiber laser means that laser parameters can be consistently changed quickly. Results using the recently developed macro provide throughput of 1.0 to 1.5 seconds per hole. Also, Optical Focus Control, or OFC, mapping for which there is no ‘side sensing’ (as can be the case with the capacitance-based AFC) ensures precise location of the holes whether they are perpendicular or at angles to the surface. This is critical if there is another surface near that being drilled.