Mechanical Simulation of the milling process for the Aeronautical Parts – SIMP-Aero

The project is based on a numerical simulation tool, which calculates stress relaxation and updates the geometry of the part mesh, from the chronological sequence of the volumes of material to be removed. This gives us the history of the deformations of the part from the rough to the finished part. To validate the consistency of this tool with reality, the initial residual stress fields must be characterized by two different experimental campaigns, depending on the morphology of the initial raw part. Furthermore, the simulation model is recalibrated, via the implementation of an image correlation method (DIC), to measure the deformation of the parts in situ, at each stage of machining. Thus, this simulation tool makes it possible to estimate the deformation of the part, depending on its geometry and the range used. As part of the optimal design of the geometry of a part, this indicator is added to the usual indicators such as the minimization of weight, machining time and machining cost. The optimization then uses a process using two nested optimizations by genetic algorithms which produce and improve a family of part geometries or machining ranges according to a macro-criterion calculated by an AHP method.

The SIMP-Aero project has produced several major results. Experimentally, the results concern the development of methods for measuring residual stress and in-situ deformation during machining. The applicability and accuracy of these methods have been validated. Numerically, the results concern the development of numerical tools for simulating the mechanical behavior of parts and optimizing their geometry and machining range. These tools will be used in the design of new aircraft. The coupling between deformation modeling and in-situ measurement has been a major factor in the progress made in the field of machining deformation prediction. It was also an important scientific step in formalizing a research methodology for manufacturing process performance and multi-criteria optimization. This work has been pursued by several doctoral theses. Nowadays, optimizing a manufacturing line according to cost or productivity criteria alone is insufficient. New indicators need to be taken into account, particularly those related to sustainable development. This project continued with the ANR IMaDe project (ANR-19-CE10-0002).

Modeling the mechanical behavior of parts in machining is now a promising area of ??research, opened up by this project. Indeed, machining suffers from not using effective mechanical models during the processes, unlike forging, die-stamping or foundry. The industrial world has become aware of the impact of machining on the mechanical state of parts. Thanks to these models, we move from a micro point of view linked to the mechanical state of surfaces to a macro point of view in the volume of the part. This work therefore opens new avenues of research, whether in the more precise consideration of certain mechanical states, the consideration of finer geometry, or the application to new materials or new processes.

Rebergue, G.; Blaysat, B.; Chanal, H.; Duc, E. Advanced DIC for accurate part deflection measurement in machining environment. Journal of Manufacturing Processes. 2018, 33, 10-23.

Rebergue, G.; Blaysat, B.; Chanal, H.; Duc, E. In-situ measurement of machining part deflection with Digital Image Correlation. Measurement. 2022, 187, 110301.

Rambaud, P.; Mocellin, K. New numerical approach for the modelling of machining applied to aeronautical structural parts. Esaform 2018, Palermo, AIP Conference Proceedings 1960, 070022 (2018).

Rambaud, P.; Perez, I.; Mocellin, K.; Madariaga, A.; Arrazola, P.J. 1st Esaform Mobility Grant: Modeling of Post Machining Distortions in Thin Walls Applied to Aluminum Parts compared to experimental results. Esaform 2019, Vitoria-Gatseiz, AIP Conference Proceedings 2113, 080015 (2019).

This project, which concerns the production of aircraft structural parts, aims at developing simulation software to predict the deformation of laminated, forged or pre-formed parts during milling operations, in order to optimise the machining process and to validate or modify the geometrical definition. The project follows on from a FUI project carried out by Aubert&Duval, Constellium, the Cemef and Pascal Institute / IFMA, which ended in February 2015 and addressed the post-machining deformation of a new class of aluminium-lithium alloy. The project resulted in the identification of more fundamental scientific issues.
The design of aircraft structural parts must satisfy the requirement of mechanical strength associated with a reduction in weight and a minimisation of the manufacturing cost. Starting from a pre-formed part, between 80 and 90% of the material is removed by milling, which induces a significant re-distribution of the internal stresses generated by the initial forming process. This deforms the workpiece and increases the cost and manufacturing time. Managing this deformation is thus an important factor in reducing costs and optimising the process, by simplifying operations and controlling the volume of metal involved.
During machining, the deformation of the part depends on the milling process and milling parameters, and in particular the type of clamping used and the design of the milling tool path, which produces a continuous re-distribution of internal constraints. The problem of identifying which tool path will give rise to the minimum distortion of the workpiece has not yet been resolved.
The project comprises five parts:
- Characterisation of residual stresses in large pre-formed parts, to identify the stress fields required for the simulation for various different materials and processes. Residual stress measurement and the evaluation of the associated accuracy are complex problems in the case of massive industrial parts.
- Numerical development, using Forge© software to model material removal and estimate machining distortion, with the aim of reducing computation time and improving the accuracy of the estimation.
- Measurement of distortion, and correlation between measurement and simulation, in order to propose a non-intrusive method to measure parts during machining, taking into account the visual interference engendered by the industrial production environment.
- Optimisation of milling process planning in order to check the influence of deformations on the conformance to geometrical specifications and to adapt the design of parts.
- Validation of the simulation by comparison with experimental results on industrial parts.
This project will contribute to a better understanding of the mechanical behaviour of parts during machining, as is already the case for other processes such as casting or forging. Indeed, there are currently no software tools to assess the mechanical behaviour of a workpiece during milling.
At the industrial level, the control of workpiece deformation and the enhancement of the expertise of Aubert & Duval and Constellium is a major competitive advantage in a rapidly changing aeronautical market which must adapt to a significant reduction in production times and the emergence of low-cost competitors in the highly technical pre-formed parts market. Customers now expect their suppliers to be able to advise on the downstream processes with a cost-cutting approach.