Case Study 2.3. Distortions in aeronautical structural parts

Main technical issue
At present, aircraft manufacturing companies are heavily driven by weight  and cost reduction initiatives. To that end, and specifically related to structural  components, the use of highly slender ribbed parts made on newly developed  aluminum alloys has been extensive developed over the last decade. However,  due to the nature of the materials used and the asymmetrical geometries related  to these slender ribbed components, the appearance of geometrical distortions  after the manufacturing of these components is a major issue. Furthermore,  non-repetitive residual stress fields on the stocks make the appearance of nonacceptable  distortions on some parts when following the same manufacturing  process. In order to overcome these distortion problems, the classic solutions  have been based on the incremental removal of material from opposite sides  of the machining stock, so the distortion could be balanced. While this solution  could lead to a distortion-free part, it generates different inconveniences due  to the difficulty to clamp the part on the already machined ribbed zone without  damaging the fins, or the long machine and set-up times required to carry out the  whole machining of the part.  

Proposed technical solution
The present paper presents the work carried out for the development of an easy  to implement solution for the manufacturing of distortion-free parts. On the one  hand, an analytical model has been developed for the calculation of both the  residual stress state of the aluminum stocks and the final distortion of the parts  after the machining process. On the other hand, two fixturing solutions have been  developed. The first one allows modification of the initial state of the stock in case  it is required for the minimization of the final distortion of the part. The control of  this fixture is coupled with the analytical model for the part distortion on a simpleto-  use graphic user interface, so it can be used on the shop-floor without the  need of a high power computer. Based on the calculation model developed,  this interface indicates the best way to machine the component, bringing down  the whole manufacturing process to only two machining steps and reducing the  whole machine time significantly in comparison to solutions based on sequential  passes. Finally, the second fixture helps on clamping the ribbed parts without  damaging the fins. It is based on hydraulic clamps which apply the clamping  force in a balanced way, so the fins are not deformed nor damaged during the  clamping process.

Main advantages of the solution
The application of the present developments in comparison to the conventional  machining process has:

  • Improved the part accuracy from 2 mm distortion to 0.3 mm distortion thanks  to the application of the distortion minimization procedure.
  • Reduced machine time from 4 days to 2 days due to the avoidance of the  sequential clamping procedures.

Thanks to the application of the present solution, it is possible to ensure the quality  on ribbed slender structural components, avoiding the generation of defective  parts and the material and energy waste associated to it. Furthermore, the  machine time required for their manufacturing could be further reduced since  the sequential machining is no longer needed, improving the competitiveness of  the manufacturers on the market.