Strain path effects on Timetal 834 under hot working conditions

• Main Author: Blackmore, Michael
• Published: University of Sheffield 2010
• Subjects: 669
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.515428

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.