Research On Residual Stress And Its Relief Methods In Aluminum Alloys For Aircraft Structures

Several aluminum alloy types from Al-Mg-Cu-Zn （7×××series） and Al-Cu-Mg（2×××series）not only have lots of characters such as small density, high strength,extraordinary anti-corrupting and anti-fatigue, but also be easy to machine. So they becomethe main materials in aerospace, weapon equipments and many other fields. In which,7075aluminum alloy thick plates are more and more used as the key structures in transport rockets,war aircrafts, passenger transport aircrafts, high speed trains and so on. Usually, theseaeronautical monolithic components have large and intricate structure shapes by NC millingor incremental pressing. However, the needed shapes are always not available because ofresidual stresses, but only several flaws such as distortions, deformations, damages and evenfractures. So investigation on residual stresses is of great importance and can makecontributions to the relief, effective use or control of them. In this paper, several aspects areresearched as follow:First, not only the developments and applications of aluminum and its alloys, aerospacealuminum alloys, but also the origins, the classes and endangers of residual stresses weresystematically described.Second, several residual stress measurement methods were systematically described suchas X-ray diffraction method, neutron diffraction method, hole drilling method and crackcompliance method (CCM).Third, a kind of lately residual stess measurement methods, named as Indentation StrainMethod, was brought into the measurement of the quench-induced residual stresses in seriesthickness of7075aluminum alloy plates. An equipment for Indentation Strain Method wasdesigned and made. Then the measurement principle, experiment demarcation and a lot ofother experiment details were described and analyzed in the measurement processing. Thesurface residual stresses from a50mm thick quenched7075aluminum alloy plate were thenmeasured using Indentation Strain Method and simulated using Ansys software. The result shows that there is a good agreement between theoretical predictions and experimental results.For further research, a series thicknesses of quenched7075aluminum alloy plates weresimulated by Ansys software and several important regulations were found.Fourth, a relief method to plate residual stresses which named as pre-stretching methodwas verified using finite element analysis software，Ansys. Besides, the Stretch-bendingmethod was firstly used to relieve the plate residual stresses and simulated by Ansys. Theresults showed that this method can effectively reduce the amplitudes of residual stresses fromquenched7075aluminum alloy thick plates.