Project CORE - Computerized Refurbishment

22/03/2022

• Challenge
• Objectives
• Methods
• Partners

Highly stressed components in aircrafts are typically replaced when local defects are present. A potential alternative is repair using gas dynamic cold spraying. In this process, powder particles are applied to a surface at supersonic speeds to selectively rebuild damaged material. This can save considerable costs and resources.

To be able to guarantee the required material properties in a reproducible manner, CORE is pursuing the approach of computer-aided automation of the repair process. This is intended to enable the remanufacturing of defective components in a way that is practical for industrial applications. The properties of the built-up material must not lead to premature failure of the components. Critical factors are, for example, introduced defects and residual stresses.

The adhesion of the coating material to the base material is crucial for the mechanical integrity of the repair. To optimize it, investigations are carried out at BAM on the structuring of adhesive surfaces using pulsed laser radiation. In addition, the mechanical properties of the repair material are determined. The focus is on material fatigue and corrosion. The data obtained will serve as a basis for automating the repair process.

Partners
Helmut-Schmidt University | University of the Federal Armed Forces Hamburg
Bundesanstalt für Materialforschung und -prüfung
Lufthansa Technik AG
Airbus Defence and Space GmbH
GTT-Technologies
H. Loitz Robotik
Impact Innovations GmbH
KIT - Karlsruhe Institute of Technoloty
Funding
dtec.bw

CORE - Computerized Refurbishment

Cold spraying as a repair process in the aerospace industry

The production of highly stressed components in the aerospace industry is usually cost and resource intensive. Due to high safety requirements, components that are defective because of faulty manufacturing or mechanical stress are often replaced. As an alternative, local repair of defects can lead to large cost savings and is in line with sustainable, resource-saving business practices. In view of that, a promising process is cold spraying, in which metal particles are accelerated to supersonic speed in a gas stream and applied to a surface. High, local plastic deformation removes oxide layers and creates a strong bond between the particle and the surface. This allows new material to be applied layer by layer. For component repair, a defect is first worked out (e.g. by milling). The removed material is then reapplied near-net-shape by means of cold spraying. Subsequently, the the original component geometry is mechanically worked out. The advantage of the cold spraying lies in the low heat input. In other additive manufacturing processes, this can lead to a degradation of the material properties in the component.

The CORE project: Automation of the cold spraying process

Ideally, the goal of a repair process is to restore the component properties of the undamaged (new) component. If the process parameters are not optimized, this cannot be guaranteed, as defects, microcracks and residual stresses may be introduced. In particular, the cyclic strength can be greatly reduced compared to a new part. The challenge in cold spraying is the large number of parameters that must be set. Different component and defect geometries imply a great variation of the optimum parameters. Until now, these have had to be determined individually for each repair.

The CORE project is pursuing the approach of digitally recording all critical framework conditions of cold spraying and implementing a process chain for the repair procedure in an automated and computer-controlled manner. This is intended to ensure the reproducibility of the required properties. High-strength aluminum alloys from the aerospace industry are used as the base material in the project. Typical component and defect geometries will be defined together with the industrial partners. In cooperation with the industrial partners, a test cell for cold spraying is being set up at Helmut Schmidt University for the repair of components. In the further course of the project, this will be used to determine algorithms for defining optimum process parameters regarding mechanical properties. Accompanying the investigation of the mechanical properties, material tests will be carried out to determine residual stresses and defect distributions.

BAM's contribution to the CORE project

An important aspect affecting the mechanical properties of cold sprayed repair-material is the adhesion to the base material. To optimize it, the adhesion of the coating is investigated after prior laser structuring of the surface to be coated. The parameters of the surface preparation are chosen to achieve the best possible adhesion. The work on laser structuring is being carried out at BAM's Additive Manufacturing of Metallic Components division.

Component fatigue plays an important role, especially in the aerospace sector. In order to determine the cyclic mechanical properties of repaired components, fatigue tests are carried out in the Weld Mechanics division using repair materials produced with different process parameters. The tests include fatigue life testings, fatigue crack growth determination, and cyclic plastic deformation behaviour testing. Subsequently, fractographic and microstructural investigations of the cause of failure will be performed. Besides purely mechanical damage, corrosion plays an important role for component integrity. Stress corrosion cracking is considered particularly critical in this respect, as it can lead to sudden, brittle failure of components without prior external indication. The sensitivity of repaired material to stress corrosion cracking is determined in the Component Safety department.

The results of the fatigue tests are used in the development of algorithms for automated reprocessing of component defects at Helmut Schmidt University. Fatigue crack growth is important regarding the evaluation of the material's defect tolerance and can thus serve to identify possible permissible defects in the component. The experimentally determined cyclic deformation behaviour is used to create a material model for simulating specimen and component deformation.

Project coordination

Helmut-Schmidt University | University of the Federal Armed Forces Hamburg
Institute of Materials Engineering, Prof. Dr.-Ing. T. Klassen, Dr. rer. nat. F. Gärtner

Project partners

Bundesanstalt für Materialforschung und -prüfung (BAM)
Division Weld Mechanics division
Additive Manufacturing of Metallic Components division
Helmut-Schmidt University | University of the Federal Armed Forces Hamburg
Institute of Materials Engineering,
Professorship for Automation Technology

Other project partners

Lufthansa Technik AG
Airbus Defence and Space
GTT-Technologies
H. Loitz Robotik
Impact Innovations GmbH
KIT - Karlsruhe Institute for Technology

Funding

The CORE project is funded by the Center for Digitization and Technology Research of the German Armed Forces dtec.bw