[Thesis]. Manchester, UK: The University of Manchester; 2017.
Initial investigations were conducted to determine the feasibility of applying a post
solution heat treatment stress relief stretching operation to near-net-shaped taper
rolled aerospace aluminium plate, in terms of the possible process window and the
impact on key mechanical properties. Finite element modelling (FEM) and simulations,
validated using digital image correlation to measure the strain distribution in flat
tapered tensile specimens, have demonstrated the existence of a suitable process window
for the production of near-net-shape wing skin blanks. It was revealed that the limits
of the available taper geometry are dictated by the work hardening behaviour of the
specific aluminium alloy. In addition, in the case of two typical upper wing skin
alloys AA2195 and AA7055, tensile and toughness testing revealed that the resultant
strain gradient offers the potential for graded mechanical properties suited to this
application, with a higher level of toughness at the wing root and an increase in
the yield strength of the material at the wing tip. Further, investigations explore
the effect of increased tensile pre-stretching, to the maximum achievable before tensile
instability, on the strength, microstructure, and precipitation kinetics seen during
ageing alloys AA2195 and AA7055. Subsequently, in both alloys, microstructure data
have been used to deconvolute and model the effects of increasing pre-strain on the
main strengthening components that contribute to the T8 yield strength of the material.
In the case of the AA2195 alloy, the T8 yield strength was observed to increase continually
as a function of pre-strain to a maximum value of ~670MPa at 15%. It was shown that
refinement of the T1 phase continued to the maximum obtainable pre-strain, which was
predicted to result in a reduction in the precipitate strengthening contribution.
The increase in the T8 yield strength resulting from high levels of pre-strain was
attributed to a large increase in residual strain hardening following artificial ageing,
owing to a low level of recovery.In the AA7055 alloy, increased pre-strain led to
a substantial increase in the T3 yield strength, which was revealed to be due to the
dynamic precipitation of GP-Zones. In the T8 temper the yield strength of the alloy
increased continually as a function of pre-strain to a maximum of ~607MPa at 24%.
Increasing pre-strain was shown to result in a continual and significant increase
in the residual strain hardening contribution to the alloys T8 yield strength, due
to the occurrence of limited recovery. However, it was also shown that increasing
pre-strain led to a progressive reduction in the precipitate strengthening contribution
from the eta prime phase, due to an increase in the volume fraction of precipitate
free zones (PFZs) with increased dislocation density. Therefore, the increase in the
T8 yield strength, as a function of pre-strain, is attributed to an increased residual
strain hardening contribution which outweighs a smaller progressive reduction in the
precipitate strengthening contribution.