Innovative treatment of blisk assemblies with a new and patented shot peening process

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Automatic measuring, calibrating and shot peening

Rolls Royce Germany has developed a new, patented shot peening process for the inducement of compressive residual stresses on the surface of blisk assemblies used in aero engines. Rolls Royce Germany together with the shot peening equipment manufacturer Rösler jointly developed a suitable shot peening system that allows fully automatic and precise measurement of the components and calibration of the blast nozzles. Special measuring software guarantees total stability of the shot peening process.

Contrary to conventional compressor elements for turbines where the turbine disk and the respective blades consist of separate components, in the case of blisks the blades and the turbine disk are machined from one single piece („blade integrated disk“) of a titanium or nickel based alloy steel. Besides offering cost savings by eliminating the mounting of the blades into the disk, the use of blisks also results in a significant overall weight reduction. However, the shot peening of these components poses new technological challenges. For this reason, Rolls Royce Germany developed a new, patented shot peening process whose main characteristics are that the suction and pressure sides of each individual blade are simultaneously peened with specially shaped nozzles. The distance between blast nozzles and turbine blades can be as low as a few mm (a small fraction of an inch). This extremely small distance and the high number of turbine blades on the blisk require extremely precise nozzle movements with maximum deviations of a few 1/10 mm (around 0.004 – 0.010 inches). The required precision made manual measuring and calibrating very difficult and time consuming, especially for the processing of blisk assemblies, components which usually consist of multiple blisks.

Absolute precision with a tailor-made measuring software
After careful analysis of all competitive equipment quotes the customer chose the fully automatic shot peening system KU 40 from Rösler. A key factor for the customer’s decision was that the Rösler peening specialists could offer a system that allows practically fully automatic component measuring and calibrating of the blast nozzles. The required software was developed in partnership with an automation company, and, at the same time, the control software for the handling robots was optimized for this sophisticated peening application.

The components to be peened consist of different types of blisk assemblies with varying quantities of turbine blades and different blade positions. After having placed the complete blisk assembly on a special work piece fixture, the operator aligns the assembly and clamps it down. Then the operator mounts the measuring instruments onto the two robots and starts the work cycle. In order to achieve absolute precision and prevent collisions, the robots must not only take readings of the distance to the work piece fixture and the actual component, but must also align themselves. For this purpose, special sensors were mounted on the underside of the turn table. Once the three dimensions of the position of the component have been exactly defined, the operator removes the measuring instruments from the robots.

The various turbine blades are peened with the nozzles making special, individually programmed movements. For this special peening process the blast pressure depends on the blade contours and, therefore, must be individually adjusted. In addition to the online measurement of the media throwing speed, the continuous monitoring and adjustment of all process relevant blast parameters ensures that the specified Almen value tolerance is not exceeded. In addition, for each treated component a process log is automatically generated in which all processing parameters are documented. The frequently required additional shot peening of the blisk base takes place while the component is continuously rotating.

The blast media cleaning and recycling system was designed and built in line with the prevailing aerospace standards and is equipped with a spiral separator and a vibratory screening device for discarding unusable blast media. The continuous supply of media to the blast nozzles is guaranteed by a dual pressure pot blast system.

Offline programming of new shot peening programs
At the moment two shot peening programs are stored in the PLC of the system control panel. The process steps and process parameters for additional components can be easily defined in an offline programming environment with the help of a 3D model of the complete peening system. The new programs can then be directly uploaded into the PLC. The selection of the peening programs and the measuring and calibration operations take place with an easy to operate touch panel.

Higher cost effectiveness and higher parts throughput
The automatic measuring of this type of components and the calibration of the blast nozzles has resulted in a significant reduction of the setup times. At the same time, the new peening system produces a considerably higher parts throughput. Another advantage is that the KU 40 peening system can also be used for overhaul and repair of used components.