The distortion in machining aeronautical aluminum alloy parts (AAAPs) is one of the serious challenges in the aviation industry, and the residual stresses produced in multimanufacturing steps are the main cause. In order to get a comprehensive understanding of the problems about residual stresses and distortion in machining AAAPs, the state-of-the-art in several aspects including the generation reasons of residual stresses, the factors influencing distortion during machining, the measurement methods of residual stresses, the prediction and controlling methods of distortion are summarized in this paper. The generation mechanism of the bulk residual stress inner materials and the machining-induced residual stresses, as well as the factors affecting two kinds of residual stresses are stated. Also, the influences of residual stresses and machining process conditions on distortion are analyzed. Furthermore, the common residual stress measurement methods and its application scope are summarized. Significantly, the differences, advantages, and disadvantages of various prediction methods are analyzed. The methods of controlling distortion before and after machining are summarized. Finally, the paper gives out further research on the distortion in machining AAAPs in aeronautical manufacturing.

Distortion caused by residual stresses in machining aeronautical aluminum alloy parts: recent advances

The manufacturing of aluminium alloy structural aerospace parts involves multiple steps, the principal ones being the forming (rolling, forging etc), the heat treatments and the machining During this last step, the final geometry of the part is obtained Before machining, the workpiece has therefore undergone several manufacturing steps resulting in unequal plastic deformation and metallurgical changes which are both sources of residual stresses On large and complex aluminium alloy aeronautical parts, up to 90 % of the initial workpiece volume can be removed by machining During machining, the mechanical equilibrium of the part is in constant evolution due to the redistribution of the initial residual stressesThe residual stress redistribution is the main cause of workpiece deflections during machining as well as of post-machining distortion (after unclamping) Both can lead to the non-conformity of the part with the geometrical and dimensional tolerance specifications and therefore to a rejection of the part or to additional conforming steps In order to improve the machining accuracy and the robustness of the process, the effect of the residual stresses has to be considered for the definition of the machining process plan In this paper, a specific numerical tool [2] allowing to predict workpiece deflections during machining and post-machining distortion is used to study the influence of the machining sequence on the machining quality in taking into consideration the initial residual stresses A first machining process plan defined as the reference case is simulated Simulation results are then compared with experimental ones showing the feasibility to use the developed tool to predict the machining quality depending on the initial residual stresses, the fixture layout and the machining sequence Using the computational tool, a method to optimise the machining quality depending on the initial workpiece and on the machining sequence is presented A machining process plan allowing to respect the tolerance specifications is then defined This demonstrates the feasibility to adapt and to optimise the machining process plan to ensure conformity of the part with the tolerance specifications

Influence of the machining sequence on the residual stress redistribution and machining quality: analysis and improvement using numerical simulations

The distortion of machined parts is a major concern in the manufacture of aeronautical monolithic structures. We investigated the influence of material removal partition on residual stress in high-strength aluminum alloy parts to minimize machining distortion. In the present study, a methodology of minimizing machining distortion based on an accurate cross-sectional residual stress determination is presented, which can be applied to avoid or minimize part distortions in advance by adapting machining strategies or process conditions. A powerful contour method was used first to measure bulk residual stress within the blank. Next, a finite element model was applied to predict machining distortion based on measured residual stress for analyzing part distortion. Finally, experimental verification was provided by comparing measured distortion and predicted distortion by the finite element analysis. This simulation showed that part distortion is mainly affected by the partition of material removal in T-shaped components. Our results also indicate that distortion can be minimized by optimizing the partition of material removal to ensure a symmetrical distribution of residual stress in the part so that the residual stress-induced bending moment could reach self equilibrium.

Machining distortion minimization for the manufacturing of aeronautical structure

An Industrial Workflow to Minimise Part Distortion for Machining of Large Monolithic Components in Aerospace Industry

The residual stress distributions in two 7449 aluminium alloy rectilinear blocks have been determined using neutron diffraction. Heat treatment included cold water immersion quenching and a period of precipitation hardening. Quenching induced very high magnitude residual stresses into the two blocks. One block was measured in this condition while the other was incrementally machined by milling to half thickness. Neutron diffraction measurements were made on the milled half thickness block at equivalent locations to the unmachined block. This permitted through thickness measurements from both blocks to be compared, revealing the redistribution of residual stresses induced by machining. A square cross section post in the centre of the machined face was left to act as a stress free reference sample. The distortions arising on the face opposite to that being milled were measured using a co-ordinate measuring machine. The residual stresses and distortion arising in the blocks have been compared to finite element analysis prediction and found to generally agree. Material removal only caused distortion and the residual stresses to redistribute; there was no stress relaxation evident.

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Measurement and Prediction of Machining Induced Redistribution of Residual Stress in the Aluminium Alloy 7449

This paper aims to highlight challenges of residual stress and part distortion in civil airframe manufacture as this is a recurring expensive problem as significant costs have been spent to correct part distortion at various stages of the manufacturing life cycle. It is our long term commitment and aspirations to understand and if possible, eliminate part distortion in order to remain competitive. The paper briefly outlines current trends in civil aerospace industry and discussed the fundamental of residual stress and part distortion in high strength aluminium alloys. An insight into industrial state-of-the-art in managing residual stress and part distortion, learning obtained and long-term industrial aspirations are also discussed. Our recent participation in a European-funded collaborative project – COMPACT is also detailed here.

Wei Ming Sim

Papers

Challenges of residual stress and part distortion in the civil airframe industry