in Vol. 1 - September Issue - Year 2000
ARIANE Tanks - shaped with KSA-Controlled Shot Peen Forming
peen forming of an ARIANE 4 tank segment
peen forming of an spherically curved ARIANE 5 tank bulkhead
Small balls with a great impact: 30,000 tools per minute forming the tanks of the Ariane.
KSA Kugelstrahlzentrum Aachen GmbH is a spin-off of the RWTH Aachen (Technical University of Rhineland-Westphalia) and belongs to the RAG Group in Essen through Beteiligungsgesellschaft Aachener Region mbH. Dr. Wüstefeld and his team form large integral metallic structures for the aerospace industry quasi without limitation of power and size in an exceptionally flexible method due to controlled peen forming. The specialists in Aachen are leading in this discipline in Germany. Already more than 500 side shells for the Airbus A310 and more than 600 tank segments for the European rockets Ariane 4 and Ariane 5 as well as 300 front claddings for the Dutch diesel trains DM90 were peen formed in Aachen. The main KSA customers in the European space rocket program ARIANE are MAN Technologie AG and Fokker Space B. V..
With the small steel balls as tools it is possible to achieve astounding results. Shot peening knows two different methods: The common shot peening is a hardening blasting process for saturation peening, influencing only the material itself but without shaping. Hardening blasting takes place using very small balls with a diameter of 0.1 to 0.5 mm at speeds of up to 100m/s. The balls induce local hardening and favourable internal stress in the work piece, thus increasing load bearing and extending service life. That is why the critical components of an aircraft are often shot-peen treated.
KSA Kugelstrahlzentrum Aachen has specialised in the peen forming method. The main aim is the new shape of the work piece. The increased blasting intensity required for this method is achieved mainly by considerably increasing the weight of the balls used that are between 2 to 10 mm in diameter. In the course of forming the impact of the balls on the work piece also induces local hardening and thus favourable internal stress, which consequently increases the long-term stability of the component.
In the case of concave forming or 'sheet beating' the balls contact at such a high speed that the whole cross section of the sheet is effected. On the other hand, in the case of convex forming lower speed causes only slight effect on the surface treated. Just a small layer directly on the surface of the component is increased in size, whereas the deeper layers of the sheet and the back are not effected at all. The impact of the balls displaces the material aside and the surface material is enlarged, in principle similar to forging. Each individual ball has only a minor impact, but the large number of impacts add up over the total surface area. The material is, however, only enlarged on the one side of the component, so that the sheet bends towards the balls in the process.
Peen forming shows its strong points when forming work pieces with large surface areas. For example, within an Airbus side shell there are different strengths from 3 mm to 8 mm corresponding to the strain on each component. For this reason it is not possible to achieve a controlled forming using common bending processes, not the least because bending often effects an undesired diffusion of stress. However, with peen forming it is possible to vary the speed of the balls dependent upon the strength of the sheet and thus effect evenly distributed bending of the material.
Another example: The base segments of the Ariane 5 tanks are flat prior to processing and through forming are to take on a spherical curvature. This forming could also be effected by stretch-form processing, but due to the size of the component excessive power and correspondingly sophisticated and expensive tools would be required. Every alteration to the design would also require new tools. Peen forming only requires a new programme. Thus apart from cost the aspect "time to market" also speaks in favour of peen forming.
The size of the component is of no importance for peen forming as partially applied forming process. The work piece is only formed directly where the impact is aimed. Thus the diffusion of stress in the material can be controlled more precisely. Complex structures can also be produced flexibly and at a reasonable price. Depending upon the kind of nozzle the diameter of the shot is between 20 and 200 mm and the overlay of staggered tracks permits production of almost any desired profile.
Online process visualisation permits documentation of work piece impact for each component and is a world-wide innovation in the field of peen forming. Complex test series in KSA Kugelstrahlzentrum Aachen have verified empirically the correlation between the speed of the balls, degree of coverage and other peen parameters of peen treatment and the resulting material properties in the various metals treated. The results of these tests can now be documented with a process control, that apart from the tasks of control and regulation, can be transmitted with an unusually high quality of reproduction: Tolerances in the per mil area are the result, as for example the Ariane 4 - in tank segments treated with a radius of 1900 mm there are deviation of only +/-0.15mm. Only KSA-Controlled Shot Peening provides the basis for full automation of the peening process.