Cutting costs in aircraft turbine production

Matching aircraft turbine production capacity to increased demand is not easy. Turbine blades are mounted on compressor disks, and to make these disks as light as possible, they are milled from a single piece of material rather than constructed blade-by-blade. Compressor disks made this way are known as blade-integrated disks or blisks, and their job is to compress air in the turbine. But because turbine blades are by nature long and thin, manufacturing them all at once causes problems. They begin to vibrate like a tuning fork during production, which complicates further work. To avoid this, manufacturers don’t mill each blade completely, but instead process the outer edge of the blade to its finished state before moving on. In this manufacturing process, tension on the turbine blades causes their geometry to warp slightly.

A new clamping system from the Fraunhofer Institute for Production Technology IPT in Aachen, Germany, may eliminate blade vibration.

“With the clamping system, absorption in titanium blisks is 12.5%. Without it, absorption falls to just 0.027%,” says Roman Kalocsay, an engineer at IPT. Using the clamping system, manufacturers can roughly mill the blades first, and then perform the precise finishing work since the blades no longer vibrate.

Eliminating vibration with the clamping system slashes manufacturing costs because vibrating blades put a strain on the tools’ cutting edge, forming fissures that quickly increase in size with mechanical and thermal stress. Tools often have to be replaced after cutting only 4m of material. Initial experiments with the clamping system suggest that tools could be used 2x to 3x longer.

“Spring-loaded clamps automatically grasp the blade with equal amounts of tension on both sides,” Kalocsay explains. “As soon as the elements are in position, they are hydraulically clamped in place and hold the workpiece as if it had grown there.”

The clamping system prototype was created by the Fraunhofer Innovation Cluster Adaptive Production for Resource Efficiency in Energy Generation and Mobility AdaM.

During repair, the blades cannot be milled piece by piece out of the material since all of the blades are already in place. Instead, if their edges appear to be frayed, the manufacturer reapplies material using laser metal deposition and then mills it to the desired form. Workers can attempt to hold the blade in place using clamps or rubber, but it is nearly impossible to align them perfectly. Consequently, work­pieces must be re-measured afterward in a time-consuming process – the clamping system helps here.

“It doesn’t change the blisk’s geometry by even a micrometer. The blade is fixed in place in just a few seconds and can be worked on immediately,” Kalocsay says. The process is slightly different than with a new blisk because the clamping elements are arranged in a circle and hold all blades at the same time.

Fraunhofer Institute for Production Technology IPT
www.fraunhofer.de/en.html