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Final Report Summary - COMPACT (A concurrent approach to manufacturing induced part distortion in aerospace components)

The project COMPACT intended to establish a thorough, rigorous and deterministic understanding of RS and management of PD in high strength aluminium alloys. A multi-functional approach in multiple disciplines, i.e. material processing, manufacturing, design, finite element modelling and Knowledge-based engineering (KBE) have been established to address these challenges that lead to cost savings in materials, rework, scrap, concessions, process rationalisation and design optimisation for weight reduction.

In the quest to deliver greener and more environmental-friendly aircraft, the overall aircraft and the detailed parts have to be reengineered to meet these challenges whilst still meeting the mechanical, fatigue and fuel performance requirements. The results are the use of more exotic materials including high strength alloys and composite materials, and design for larger and thinner monolithic parts to reduce the aircraft weight. Unfortunately, manufacturing challenges, in particular machining of thin-walled structures, distortion and chatter / vibration marks have been difficult to address at the design stage.

Distortion or when a part deviate from its original intent, has been a recurring problem in the manufacturing of many high valued parts, i.e. large monolithic aluminium wing-box rib and skin panels, gear disc, bearing ring, castings and etc. In instances where high uncontrolled and unpredictable distortion was manifested, unfavourable recurring costs would add to the part as a distortion correction strategy would need to be deployed in order to minimise the distortion to design tolerance.

Due to the lack of published knowledge in the field of residual stress and part distortion, COMPACT was launched in October 2005 to fill the knowledge gap and to advance the state-of-the-art in predicting and managing distortion in the manufacture of airframe components. COMPACT was a four-year European Commission (EC)-funded project that brought together 12 European academia, industrial end-users and SMEs. The project was aimed at developing knowledge in different streams, i.e. material processing, manufacturing processes (machining, stretch bending, and post machining distortion correction processes) and design to better understand the influence of process parameters and prediction of residual stresses and their influence on distortion. The knowledge was captured and subsequently integrated in a knowledge-based engineering environment.

The key objectives of COMPACT since the project start were the following:
- In the quest to further reduce RS the first objective is to determine the unknown influential parameters of quenching and stress relieving. Furthermore, the largely unknown influences of hot-working conditions and artificial ageing (and their impact on microstructure and texture) must be addressed. Some novel strategies for reducing the impact of RS on distortion were investigated by looking at machining of a pre-form before the quench and subsequent stress-relieving and ageing operations. This technology was further developed within COMPACT, starting with optimisation of the quenching, of the pre-form geometry, of the stress-relieving method and the verification of the RS in the final component. Process and cost knowledge were modelled, formalised and validated for integration in a knowledge based advisory system.
- To develop a reliable and repeatable RS measurement method based on existing methods with low cost and quick to implement for routine industrial use.
- To advance the relatively novel and immature technology of using ultrasound to measure through thickness RS distribution and develop and optimise it to the extent that it became a viable technique for routine industrial application.
- Creation of knowledge on the influence of machining induced mechanical and thermal loads on RS and consequently development of numerical and process models of the influence of machining on RS for high strength 7 000 series aluminium alloys. To develop understanding on the effect of bending operating parameters on RS for high strength 7 000 series aerospace aluminium alloys using experimental data and numerical modelling.
- As distortion needs to be managed after machining, it was therefore important to study the effect of different treatment processes on RS and microstructure that could further alleviate distortion after machining. Analytical and numerical models for these processes were also developed. Three processes were identified: mechanical shot peening, thermo-mechanical and thermal laser beam treatment. The key objective is to introduce a distortion correction process that relieved RS while maintaining mechanical performance.
- Finite element modelling would enable RS and PD to be simulated such that advice could be given on the corrective measures to be taken. The modelling of specific phenomena related to materials and manufacturing processes were performed out. Subsequently the models and data from individual streams of research were concatenated together to form a 'single' modelling environment. This work focused on enabling this functionality to be integrated. A review and decision board were convened that stipulated hypotheses and methodologies used that assisted in the final integration. USFD were cognisant of the potential risks and limitations of the technology and worked with the board to introduce processes that assisted in the integration. Functionality was integrated and validated as effective for simulation of RS and PD from the aspects of design, material processing and manufacturing.
- Development of a sesign against PD methodology, establishing new design principles and validating the design process using case studies. Ultimately, an integration framework was derived for design against PD.
- The principal objective was to design, develop and plan the exploitation of a knowledge-based advisory system that could be applied to the complex problem of PD. The evolving process and system required the integration of the emergent best practices from the four key work packages, materials, machining, finite element analysis and design, which were developed through cutting edge research carried out in COMPACT.
- Development, formalisation and validation of the process mapping, decision making and cost models for integration in a knowledge based advisory system.

The highlights of COMPACT include improved understanding of the influence of rolling, forging, machining, stretch bending, shot peening, thermo-mechanical treatment and laser peening parameters on residual stress. Key process parameters that influence the magnitude and profile of residual stresses have been identified. Residual stress measurement techniques have also been greatly improved through optimisation of state-of- the-art measurement techniques and development of a new measurement technique using ultrasound. A novel hybrid method has been developed using incremental distortion data and numerical method to predict residual stress in a part. The influence of various design options on residual stress distribution has also been investigated.

Two distortion prediction tools have been developed in the project. The simple tools is an analytical method using simplified geometry and residual stress profile to provide rapid distortion analysis to designer. Further in-depth analysis can be performed using advanced numerical method executed via a knowledge-based system, which contain captured and formalised knowledge on the management of residual stress and part distortion.

Case studies have demonstrated the application of the knowledge generated during this research project. The machined part case study was challenging and complicated as it considered the influence of design, machining strategy and shot peening on part distortion.

Although the results were satisfactorily, further work is required to strengthen the robustness of the understanding in influence of residual stress on part distortion. The stretch bent part case study has demonstrated a strong correlation in the measurement and prediction of residual stress and part distortion.

The project was structured into six Work packages (WPs), as follows:
- WP1: Project management
- WP2a: Materials processing on residual stress induction
- WP2b: Materials - RS determination method
- WP3a: Manufacturing - machining induced residual stress
- WP3b: Manufacturing - bending induced residual stress
- WP3c: Post machining process induced residual stress
- WP4: Finite element modelling
- WP5: Design against part distortion
- WP6: Knowledge integration.

The research in knowledge engineering has contributed to the following:
(i) a knowledge integration methodology and environment for the management of a transnational multidisciplinary research projects;
(ii) this best practice has delivered a credible and demonstrable framework; and
(iii) tailored business solution for complex, knowledge-Rich, and expert-based problem domains.

The new PD simulation environment has been developed for potential industrial use. This has demonstrated knowledge integration in multidisciplinary domains together to provide a deep disciplinary capability like FEM with a multidisciplinary focus ensuring simulation capability that meets the needs of the end user and not just the whims of the FEM specialist.

From an industrial perspective, the part distortion management arena could contribute to the reduction of PD across a number of industries, especially aeronautics, when exploited.

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