Mark W. Rainforth

Combined Discrete/Finite Element Multiscale Approach for Modelling of the Tool/Workpiece Interface during High Shear Processing: Hot Rolling and Friction Stir Welding Applications

The concept of combining the latest finite element (FE) and discrete element (DE) multiscale numerical technologies for modelling of the tool/workpiece interface during high shear processing is described. The potential of FE tools and techniques merged with DE based transient dynamics is highlighted. Linking of the modelling scales is based on transferring the corresponding boundary conditions from the macro model to the representative cell, considered as the meso- level model. This meso- model consists of a large number of bodies that interact with each other. The transfer processes are described by the system of diffusion and motion equations including contact detection and interaction solutions for particles integrated in time. Modelling of the tool/workpiece interface including both mixing of the oxide particles into the subsurface layer during hot rolling of aluminum and heat generation during friction stir welding (FSW) are considered.

Characteristics of the Deformed and Recrystallised Grains Obtained after Hot Plane Strain Compression of a Model Fe-30wt%Ni Alloy

The development of physically-based models of microstructural evolution during hot deformation of metallic materials requires knowledge of the grain/subgrain structure and crystallographic texture characteristics over a range of processing conditions. A Fe-30wt%Ni based alloy, retaining a stable austenitic structure at room temperature, was used for modelling the development of austenite microstructure during hot deformation of conventional carbon-manganese steels. A series of plane strain compression tests was carried out at a temperature of 950 °C and strain rates of 10 s-1 and 0.1 s-1 to several strain levels. Evolution of the grain/subgrain structure and crystallographic texture was characterised in detail using quantitative light microscopy and highresolution electron backscatter diffraction. Crystallographic texture characteristics were determined separately for the observed deformed and recrystallised grains. The subgrain geometry and dimensions together with the misorientation vectors across sub-boundaries were quantified in detail across large sample areas and the orientation dependence of these characteristics was determined. Formation mechanisms of the recrystallised grains were established in relation to the deformation microstructure.

An Analysis of Deformation Microstructure and Subsequent Recrystallisation in Hot Deformed Aluminium Alloy AA5052 Using Forward and Reverse Torsion

The effects of strain path reversal on the microstructure in AA5052 have been studied using high resolution EBSD. Deformation was carried out using two equal steps of forward/forward (F/F) or forward/reverse (F/R) torsion at a temperature of 300°C and strain rate of 1s-1 to a total strain of 0.5. In both cases the deformation microstructure in the majority of grains analysed consisted of microband arrays clustering at specific angles to the macroscopic deformation axes. For the F/F condition microbands clustered around -20° and +45° to the maximum principle stress direction, whilst for the F/R condition significantly more spread in microband angle was observed. This suggests that the microbands formed in the forward deformation have or are dissolving and any new microbands formed are related to the deformation conditions of the final strain path. This leads to the conclusion that instantaneous deformation mode determines the orientation of new microbands formed whilst a non-linear strain path history influences the range of misorientation angle in the material through the dissociation of previously formed microbands and the formation of new microbands at the new straining condition, leading to a lower level of misorientation angle. Analysis of material subjected to static annealing at 400°C for 1 hour appears to correspond with these observations as the F/F material was completely recrystallised with a fine grain structure whilst the F/R material had no major signs of recrystallisation.

Observations of Strain Induced Precipitation during the Thermomechanical Processing of AA6111 Alloy

The effect of interpass time during thermomechanical processing of AA61111 on flow behaviour and microstructure evolution has been investigated. This was achieved using plane strain compression testing undertaken on the Sheffield thermomechanical compression (TMC) facility, using the hit-hold-hit-quench approach. Following solution treatment at 560°C for 1200s, samples were water mist quenched to 320°C and deformed at a constant strain rate of 85s-1 to an initial strain of 0.5, unloaded and held for delay times of 0.019, 6, 60, 600 and 6000s and then given a second deformation for a further strain of 0.5, followed by a water quench to room temperature. Hardening of the alloy was observed, the extent of which was dependent on the hold time. The microstructure of the samples was quantified by TEM in order to determine the extent of strain induced precipitation. TEM identified precipitation, predominantly β and Q phases, on dislocation lines, the size and volume fraction of which were a function of the hold time. The coarsening rate during the hold period of the precipitates was considerably faster than for coarsening following a conventional precipitation treatment. The size of the microband structure at the end of the double deformation was a function of the hold time, suggesting that coarsening of the precipitates during the hold had altered the Zener pinning potential. The implication of these observations on the thermomechanical processing of 6xxx alloys is discussed.

Strain Induced Precipitation in Model and Conventional Microalloyed Steels during Thermomechanical Processing

The finishing rolling of microalloyed steels was simulated by double-deformation plane strain compression testing of both model and conventional steels microalloyed with Nb. The flow behavior following interpass delay times of 1-100s was related to the deformed microstructure, the deformation substructure and the strain-induced precipitation. Fe-30wt%Ni is clearly a good model alloy for conventional microalloyed steels, as similar results are observed for both materials. In addition, the location of fine strain-induced precipitates in relation to the deformation substructure can be determined directly using transmission electron microscopy.

Texture Evaluation of Titanium Aerospace Alloy Ti 834 Using Hot Axisymmetric Compression Tests

The effect of hot working strain on texture evolution in the near-α titanium alloy Ti 834 has been investigated using hot axisymmetric compression testing and EBSD. Testing was undertaken sub β transus with 22% α, which upon cooling produced a bimodal microstructure of primary α in a matrix of transformed β. Two distinct deformation induced texture components were identified: i) transverse [(0001)||compression direction] and ii) off-basal [(0001) »^ compression direction]. Orientation image mapping was then used to identify the microstructure associated with each texture component. The transverse texture is associated with the transformed β and the offbasal component is the deformation texture of the primary α.

Advances on Modelling of the Tool/Workpiece Interface during High Shear Processing

Understanding and prediction of physical phenomena in different scales at the same time that are taking place at the tool/workpiece interface during high shear processing is done in different ways combining the latest finite element (FE) and discrete element (DE) analysis technology. The high shear processing is observed during hot rolling of aluminium, when it produces a highly deformed subsurface layer; and also during friction stir welding (FSW), when it results in significant heat generation and flow. The FE analysis is used for macro-scale simulation while the DE method is applied to simulate meso-scale phenomena taking place in the thin, sometimes a few micron thicknesses, surface layer. Different FE models and numerical techniques combined with DE based transient dynamics approaches are discussed in this work.

Hydrothermal Degradation of Zirconia Bioceramics: Effect of Ternary Oxide Additions

Phase transformation of 3Y-TZP with different ternary oxide additions (Al2O3, La2O3, Sc2O3 and CeO2) was evaluated by hydrothermal degradation tests in water vapor at 180°C. Monoclinic phase quantification from XRD patterns showed an improved degradation resistance of the specimens with oxide additions in comparison to the 3Y-TZP base material. Cross sectional sample analysis showed that, even though for all the compositions the exposed surface became saturated with monoclinic phase, the penetration of the transformation varied. Alumina-lanthana doped materials exhibited the thinnest degraded layer.

The use of model systems based on Fe-30%Ni for studying the microstructural evolution during the hot deformation of austenite

The development of physically-based models of microstructural evolution during thermomechanical processing of metallic materials requires knowledge of the internal state variable data, such as microstructure, texture and dislocation substructure characteristics, over a range of processing conditions. This is a particular problem for steels, where transformation of the austenite to a variety of transformation products eradicates the hot deformed microstructure. This paper reports on a model Fe-30wt%Ni based alloy, which retains a stable austenitic structure at room temperature, and has therefore been used to model the development of austenite microstructure during hot deformation of conventional low carbon-manganese steels. It also provides an excellent model alloy system for microalloy additions. Evolution of the microstructure and crystallographic texture was characterised in detail using optical microscopy, XRD, SEM, EBSD, and TEM. The dislocation substructure has been quantified as a function of crystallographic texture component for a variety of deformation conditions for the Fe-30%Ni base alloy. An extension to this study, as the use of a microalloyed Fe-30% Ni-Nb alloy in which the strain-induced precipitation mechanism was studied directly. The work has shown that precipitation can occur at a much finer scale and higher number density than hitherto considered, but that pipe diffusion leads to rapid coarsening. The implications of this for model development are discussed.

Influence of Strain Path on Microstructure Evolution of Low Carbon Steels

Changes in strain path represent one of the most important processing parameters that characterise hot metal forming processes. In the present study, the effect of strain path change on dynamic recrystallisation, strain-induced precipitation processes and phase transformation behaviour in plain carbon and Nb-microalloyed steels was investigated. To assess the effect of strain-path change, forward/forward and forward/reverse torsion tests were conducted. It has been shown that the strain reversal delays the dynamic recrystallisation kinetics whereas its effect on strain-induced precipitation process of Nb(C,N) is rather negligible. Also the onset of austenite-ferrite transformation is delayed; its products however doesn’t change significantly. This can be due to the fact that ferrite nucleation density plays the second order role compared to the geometry of deformation.