Fibre laser drilling for aerospace applications

Fibre lasers have caught the attention of industrial laser users in a wide range of industries. For a number of welding, marking, and cutting applications, the use of fibre lasers has become the standard. Remarkably, this has happened in a very short time. One area that has remained elusive is the drilling of effusion cooling holes for turbine engine components. This article will explore the reasons for this and more importantly present some of the latest findings that are changing the minds of users of Nd:YAG lasers for this complicated process.

The Nd: YAG drilling process

Nd:YAG laser drilling has made it possible to machine both very small and precise holes, at normal to extreme angles to the surface, in a variety of shapes and orientations and in a wide variety of materials, including difficult-to-machine aerospace alloys. Prima Power Laserdyne, part of Prima Power North America Inc., for example, has installed over 750 systems dedicated to aerospace manufacturing worldwide. A high power (200 to 400 W average power) pulsed Nd:YAG laser is normally used. Processing is accomplished through either percussion drilling or trepanning. In the laser drilling process, high power density is accomplished with a focused spot size of 0.05 mm to 0.75 mm.

Percussion drilling is described as delivering one or more pulses from the laser while the laser beam and part are stationary. More than one pulse may be required depending on the hole depth. A variation of percussion drilling is drill-on-the-fly where pulses are delivered to the part by a stationary laser while the part is rotated. Hole placement is a function of rotational speed and laser pulse frequency. If multiple pulses are required, software is available, such as CylPerf software developed by Laserdyne, which is utilised to synchronize the movement of the part to the laser pulses, ensuring that the multiple pulses are delivered to the exact location required. By changing the laser pulse energy, pulse frequency or lens focal length, the characteristics of the drilled hole size and taper can be adjusted to meet the design requirements of the hole.

Trepanning is another process for drilling holes. The part is held stationary and the laser beam is moved to create a hole or feature by cutting the shape. The term “drilling” is generally used for both percussion and trepanned features. The advent of extremely accurate and repeatable laser positioning systems allows for unique and tight tolerance trepanned features.

Average power is determined by the pulse frequency and pulse energies selected. Power is limited by the duty cycle at which a laser can operate without degradation of performance. Percussion drilling is accomplished using average power from <100 W to 400 W. Pulse length is selected to optimize the quality of the hole. Shorter pulse lengths may limit the maximum energy achievable in a single pulse. Typical drilling pulse lengths range from 0.5 to 2ms.

Pulse energy is the characteristic that separates drilling lasers from lasers capable of other processes. Higher pulse energy may provide faster drilling rates, but this can also be detrimental to the hole quality. The pulse energy required is most often determined by experimentation and is a result of material thickness, composition, and hole diameter required.

Focusing lenses will determine the spot size for a given laser setup. In percussion drilling the spot size correlates to the desired hole diameter to be drilled. The focused spot size is nearly equal to the hole size in thinner materials (i.e., <0.5 mm). As metal thiBlack Friday 19