Strain path effects on Timetal 834 under hot working conditions.

Little work has been dedicated to the magnitudes of the strain paths that are imposed on
a workpiece during various industrial thermomechanical processing (TMP) routes.
Furthermore, previous strain path work has concentrated on linear, sequential and full
reversal strain paths due to the ease of performing such tests. No work has been carried
out on the complex concurrent strain paths which are imposed during TMP. By
combining finite element (FE) modelling and the new Arbitrary Strain Path (ASPII -
the second generation of its kind at the University of Sheffield) machine such work is
now possible.
The ASPII machine is a test rig capable of imposing independent or concurrent torsion
and axial components of deformation under fixed or free end conditions. The machine
in equipped with an induction heater capable of testing materials up to 1100ne and a
water quenching system to capture high temperature deformation microstructures. An
induction coil heating system also allows controlled slow air cooling to be carried out to
closely reproduce industrial cooling rates. The machine has been calibrated to
accurately carry out full reversal in the deformation direction (torsion and
tension/compression) over the temperature range of 600-11 oooe up to a strain rate of
IOs' I with negligible delay or overshoot.
Model parameter sensitivity analysis, material flow behaviour and model validation
have been carried out using axisymmetric FE models for a range of temperatures (950,
990, 1030°C) and strain rates (0.2, 2, 20s·l
) combined with actual tests carried out
within IMMPETUS. The simulations demonstrated that over the tested temperature.
strain and strain rate ranges, the models were largely unaffected by most thermal input
parameters (e.g. thermal conduction.), The mesh density and friction coefficient have
been shown to have the largest influence on FE model output.
FE simulation of a two stage closed die forging of an arbitrary aero engine compressor
disc has been carried out. This model has provided 'typical' process parameters to carry
out extreme strain path change tests i.e. full reversal tests. Such tests were undertaken
to evaluate any effects on the similarly orientated primary alpha grain clusters that are
responsible for reductions in fatigue life under dwell loading situations. Such extremes
changes in strain path were unsuccessful in breaking up the clusters.
By tracking nodes within the FE model the deformation history has been extracted and
subsequent strain paths have been calculated for three points of interest within the
forged material. This information has been transferred to the ASPII machine and the
deformation has successfully been replayed. Initial tests were followed by a simulated
industrial slow air cool. Later work quenched the samples after the forging strain path
simulation. It was seen that different strain paths do influence the morphology of the
microstructure however in terms of micro and macro texture no significant difference
can be found.