Design For Manufacturing and Producibility in Fabricated Aerospace Structures

Licentiate thesis, 2018

Aircraft component suppliers must adopt new design strategies in order to absorb market growth and become more competitive at the same time as they satisfy environmental demands. To deal with this situation, weight reduction has been the key to success and have made jet engines more fuel efficient. A strategy already adopted by some engine suppliers to reduce weight has been fabrication in which small cast or forged parts are welded together into a final shape. Besides increasing the number of forming suppliers which reduces cost, another main advantage of fabrication is the design freedom due to the possibility of configuring several materials and geometries, which broadens out the design space and allows multioptimization in product weight, performance quality and cost. However, with fabrication, the number of assembly steps and the complexity of the manufacturing process have increased. The use of welding has brought to the forefront important producibility problems related to geometrical variation and weld quality.

The goal of this research is to analyze the current situation in industry and investigate and propose methods and tools within Design and Quality Engineering to solve producibility problems involving welded high performance structures. The research group “Robust Design and Geometry Assurance” at Chalmers University of Technology, in which this thesis has been produced, has the objective to simulate and foresee geometrical quality problems during the early phases of the product realization process to allow the development of robust concepts and the optimization of tolerances, thus solving producibility problems. Virtual manufacturing is a key within the multidisciplinary design process of aerospace components, in which automated processes analyze broad sets of design variants with regard to various disciplines. However, as studied in this thesis, existing methods and tools do not cover all aspects that define the quality of welded structures. Furthermore, to this day, not all phenomena related to welding can be virtually modelled. Understanding causes and effects still relies on expert judgements and physical experimentation to a great deal. However, when it comes to assessing the capability of many geometrical variants, such an effort might be costly. This deficiency indicates the need for virtual assessment methods and systematic experimentation to produce process capability data that can be reused in future projects.

To fulfill that need, this thesis presents a producibility model to represent the fabrication process in order to understand how variation is originated and propagated. With this representation at hand, this thesis builds on the Welding Capability Assessment Method (WCAM). The WCAM is tool with which to support systematic identification and assessment of design issues related to product geometry critical to the welding process. Within this method, a list of potential failure modes during welding is connected to specific design parameters. Once the critical design parameters have been identified, quantitative methods are proposed to calculate tolerances to reduce the likelihood of welding failures.

Combinations of specialized information about welding problems, know-how, inspection and simulation data have been used to evaluate the welding capabilities of a number of product geometries. Patterns and engineering rules can be extracted by combining sources of data, both qualitative and quantitative. With WCAM, evaluations are no longer limited to a single geometry and the study of the process parameter window. Instead, the welding capability space, meaning all geometrical variants that fulfill manufacturing quality, is assessed. This information can be used to perform optimization and evaluate trade-off alternatives in terms of producibility during design space exploration and analysis, thus supporting the multidisciplinary design process.