An industrial workflow to minimize part distortion for machining of large monolithic components in aerospace industry

Abstract

Part distortion caused by inevitable residual stresses results in recurring concession, rework and scrap, which worth millions of Euro in the aircraft development and manufacturing life cycle. This thesis is the result of a collaborative work between UNIVERSITY OF GUANAJUATO (Mechanical Engineering Department), AIRBUS OPERATIONS S.A.S. and KAISER ALUMINUM. An overall approach to aerospace industry is explored into the first chapter; materials used in aerospace industry and main manufacturing processes involved in structural aircraft components are described, introducing the reader to the work developed into this thesis. An accurate process to predict distortion after machining of large monolithic components from rolled plates is developed going from characterization and measurement of residual stress (L and LT directions in rolled plates) by layer removal method; data processing to fit mathematical functions (sum-of-cosine function) one for each direction, obtaining 6 coefficients by function; numerical simulations and post-processing results management to find optimal position of the part into the rolled plate. This process is designed to empower manufacturing engineers at the shop floor level. Open source software is used for pre-processing, during the computational numerical analysis and post-processing data analysis; a module of FreeCAD is developed for comfort visual and ease use for user; CalculiX subroutine is developed to residual stress mapping and FEM simulation; R code is generated to visually present the results through plots with position variables and distortion result; sensitive analysis of positioning and residual stress factors is shown by these plots. A representative case study is extensively explored following the proposed workflow, validation of the predicted distortion of the proposed geometry is executed, machining the part in high distortion position and optimal position into the plate; machining process and distortion measurements were performed at the University of Guanajuato workshop. Due several factors that determine the properties of the aluminum plates (micro-structure, manufacturing processes variables, position into the main plate and non-identified variables), the plates are produced in batches, each one with specific properties. Uncertainty propagation of material properties is imitated with ±10% variation of each coefficient from the fitted functions. The variation of the 12 coefficients which defines residual stresses profiles (L and LT directions) is analyzed for the optimal solution of the case study in a 1/16, two levels, fractional factorial design; this experiment has resolution VI with 256 runs and not confounding for interactions. Results from both experimental parts show high agreement with predicted behavior in distortion of the case study in the simulations, having congruence in distortion profiles complementing with good accuracy in distortion values. Results from the fractional factorial design show the forth coefficient of the residual stress function for both directions as the coefficients with more influence, concentrating 75% of the results into low deformation values (maximal distortion in Z direction: 0.125 mm ±0.125 mm).