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Economic Advanced Shaping Processes for Integral Structures (ECOSHAPE)

Final Report Summary - ECOSHAPE (Economic Advanced Shaping Processes for Integral Structures)

The main target of the ECOSHAPE project was the development of such laser forming processes for integral fuselage and wing structures. Thus, relevant laser parameters with respect to minimum material degradation on one side and maximum formable sheet thickness on the other side were evaluated. A simulation tool for the forming process was built up and integrated into a control system. Key to the control of the process was the development of a predictive model to provide scan strategies based on a required geometry. This system included online three-dimensional (3D) shape measurement to enable straight-line laser forming to the required final geometry. The main innovation of the project was the combination of the simulation, the control system and the online 3D shape measurement to a tool offering a self-correcting, reliable, quick, robust and thus economic laser forming process for Al-based structures.

The following objectives were pursued:
1. forming stiffened structures to single curvature of 1 250 to 3 000 mm radii along stiffeners;
2. forming bi-axially curved structures with additional 10 000 mm radius across stiffeners;
3. verification of estimated shell manufacturing cost reduction of 10 % by more processing in a flat condition and a further 10 % by avoidance of heavy and complex tooling;
4. shell weight reduction of 10 % with new alloys, less useful with conventional forming.

The LBF process uses the power provided by a laser beam to inject heat within a sheet. The heat flows into the sheet and modifies the temperature distribution. The temperature has two effects in the hot areas:
- an expansion;
- a decrease of the mechanical properties (and in particular of the yield stress).

Because of the temperature gradient in sheet thickness, mechanical incompatibilities develop themselves and create compressive stresses in the hottest and tensile stresses in the coldest layers. Since the yield stress in the hottest layers is lower, a compressive plastic strain field is created, which modifies the dimensions of the concerned layers. Cooled down to room temperature, the resolution of these incompatibilities leads to a residual stress field and distortions. Considering the motion of the laser, the summation of the distortions creates a line of bending, which can be described with a bending angle.

The project was structured into the following work packages (WPs):

WP1: Processes and materials basics
WP1 consisted of three tasks with the objective to analyse basic laser forming influence for all selected materials on a specimen level.

WP2: Process development and characterisation
WP2 consisted of four tasks with the objective to develop and up-scale the laser forming process for single curvature (two-dimensional (2D)) using laser geometry analysis, simulation and self learning path generation.

WP3: Process development and characterisation
WP3 had the objective to develop further and enhance the forming process to cope with stiffened, biaxial (3D) curved generic shapes.

WP4: Simulation and verification
WP4 consisted originally of four tasks with the objective to develop a thermo-mechanical and a benchmark model to simulate the laser beam forming, using a local-global approach.

WP5: Economical evaluation, exploitation and dissemination
WP5 consisted originally of three tasks with the objective to prepare an economical evaluation of laser-based forming processes developed during the project to create an exploitation and dissemination plan based on all partners inputs.

Before project start, activities on shaping by laser have been carried out only outside the aerospace industry and mainly regarding small electro-optical-mechanical precision components such as fibre couplings for telecommunication industry, complex optical lens systems (photocopiers, wafer steppers), computer peripherals (disk, CD drives, DVD), recording heads (digital audio / video), modern display systems (cathode ray electron gun, opto-electronics), illumination systems (automotive lamps), micro-electro-mechanical systems (MEMS) as well as electrical contacts and switches. Until project start, laser forming has been performed using empirically gained results. Thus, the results were only applicable for the specific cases investigated in these publications. To overcome these shortcomings, several efforts have been made to simulate the process. A number of finite element models have been developed, but most of them were proposed for a single laser beam pass of the work piece only. Only one more generic model has been developed for the simulation of more complex shapes such as the sine shape, but this was dedicated to steel sheet. Analytical models have been developed as well. Again, most of them were case specific, whereas one more common model was done for steel sheets.

The achievements are a major step beyond the state of the art and summarised as following:
- Influence on material properties. Determination of material properties and influences (static and fatigue strength, crack propagation, corrosion properties) as a function of process parameters.
- Interactions of process parameters, different materials and forming strategy. Influence of the intensity of the heat source, exposure time and material on the process results. Timing and locality of material heat exposure for increased process predictability.
- Process development. Development and validation of an online measurement and control system for laser beam forming of Al structures by iterative and analytic development and combination of optimised laser beam path strategy; geometry measurements; computer control based on stored strategies with self-learning correction functions; control algorithms for a robot based laser forming application; simulation.
- Development and validation of an online measurement and control system for laser beam forming of Al structures by iterative and analytic development and combination.
- Spin-off. While laser beam forming is attractive for the manufacturing of thin panels, it is of limited suitability for thicker structures. Indeed, the necessary amount of plastic strain for bending presumably requires a through-thickness temperature gradient that may lead to important micro-structural material alterations. For this type of components, the laser peen forming has been evaluated to be a very promising alternative.

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