T. Sheppard

Simulation and control of microstructure evolution during hot extrusion of hard aluminium alloys

Controlling the microstructure distribution in the final product is challenging and demanding work for the aluminium extrusion industry. This can only be achieved when the microstructure evolution during deformation is fundamentally understood. In this paper, some physical models which are based on dislocation density, subgrain size, and misorientation have been integrated into a commercial finite element modelling program simulating extrusion. The volume fraction recrystallised after press quench and the recrystallised grain size after solution treatment are studied. The influences of die configuration, container temperature and die temperature on the recrystallisation behaviour are detailed. Following this successful prediction, a uniform distribution of microstructure along the length of the extrudate has been obtained by adjusting the ram speed according to the calculated subgrain size at die exit. The calculated histories of temperature and subgrain size during deformation are presented. The problems of this new process are illustrated. Further work on simulation and control of microstructure is finally discussed.

Dynamic recrystallization in Al–7Mg alloy

AbstractThe microstructural changes which occur during the extrusion of an Al–7Mg alloy have been investigated using optical and electron microscopy. Light microscopy has shown that recrystallized grains are present both at the original grain boundaries and within the grains, even at the breakthrough pressure. Electron microscopy has shown that a large percentage of recrystallized grains contain a substructure and are thus dynamically recrystallized. The grain size of these dynamically recrystallized grains is usually less than 5 μm. Some hot rolling, and cold rolling followed by annealing, experiments have also been performed to substantiate the evidence for dynamic recrystallization. A large number of recrystallized grains were observed, although the bulk of the material deformed with dynamic recovery as the operative mechanism. It is finally concluded that the high solute content hinders dislocation motion, and nucleation of dynamic recrystallization occurs at dislocation clusters and/or at small parti...

Influence of forming parameters on static recrystallization behaviour during hot rolling aluminium alloy 5083

In this paper, the influence of rolling parameters (i.e. rolling temperature, roll speed, roll temperature, friction and the ratio of the mean thickness to the contact length in the roll gap Hm/L) on static recrystallization (SRX) behaviour is studied by the combination of the finite element method (FEM) with the Taguchi experimental method. The FEM is first applied to simulate a single pass laboratory rolling experiment by the use of both empirical and physical models. A new approach is used to generate the mean value of the Zener-Hollomon parameter, which is necessary for the prediction of the volume fraction recrystallized (XV) when the empirical SRX model is used. A physical model which considers the density of recrystallization nuclei and the total stored energy is also applied based on the prediction of internal dislocation density, subgrain size and misorientation. The predicted XV and the recrystallized grain size at the centre and subsurface fit well with experimental measurements from the literature. Then, the Taguchi method is applied to design an orthogonal experimental table, L9({34}), which indicates that there are four parameters, each parameter has three levels and a total of nine test runs need to be conducted. These nine virtual experiments are analysed by the use of FEM. The predicted results are then analysed by the use of the Taguchi method from which the influence of each rolling parameter on XV is given. The studies show that rolling temperature has the greatest influence on XV for the centre point whilst friction is the most important parameter on the determination of XV within the surface region. The roll temperature and roll speed have little influence on XV for both the centre and subsurface point.

Fabrication, properties and structure of a high temperature light alloy composite

Paniculate alumina reinforced Al-4Cr-1 Fe alloys were fabricated from rapidly solidified aluminium alloy powder and commercially purchased alumina powder by traditional powder metallurgical techniques involving powder mixing and cold compaction followed by hot extrusion. The tensile tests at ambient temperature indicated a considerable improvement in the mechanical strength at the expense of ductility and modulus. Poor values of modulus were explained by the presence of porosity in the composites. The high temperature mechanical properties of the matrix, tested at 350 °C after prolonged exposure to the test temperature under static air conditions, were intrinsically poor. Additions of the filler material, alumina particles, up to a weight fraction of 15% did not improve the high temperature performance of the matrix substantially. Possible causes for this are discussed and alternatives proposed.

Direct and indirect extrusion of a high strength aerospace alloy (AA 7075)

Abstract There is renewed industrial interest in the process of indirect extrusion because lower pressures are required and the greater homogeneity of deformation produces a surface structure which is less heavily worked than in the direct process. The parameters which limit the extrusion process are the available press pressure and surface effects which may be cosmetic (i.e. surface checking) or metallurgical (i.e. surface recrystallisation). These features can be presented on an extrusion limit diagram together with other structural features (i.e. subgrain size), which may then be used to determine the optimum operating parameters. In this paper the specific pressure required for the direct and indirect extrusion of an Alue5f8Znue5f8Cuue5f8Mg alloy is investigated and it is shown that the pressure required may be presented as a function of the constants contained in the constitutive equation and the extrusion ratio. The strain rate obtained during indirect extrusion is evaluated and compared with the predictions of existing equations. Equations are presented describing the onset of surface checking during extrusion and parameters presented predicting surface recrystallisation. This communication illustrates that mathematically derived equations do not describe the pressure limitations sufficiently accurately and that the mathematical description of the loci presented on limit diagrams are best described by equations based on the material constants in the hot working equation. Direct and indirect extrusion are compared and limit diagrams for each mode of deformation are presented.

Influence of forming parameters on the final subgrain size during hot rolling of aluminium alloys

In this paper, the influence of rolling parameters (i.e. the slab temperature, roll speed, roll temperature and the ratio of the mean thickness to the contact length in the roll gap Hm/L) on the subgrain size is studied by the combination of finite element method (FEM) with the Taguchi experimental method. The FEM is first applied to simulate two existing single pass laboratory rolling schedules. The predicted distribution of subgrain size through the thickness agrees well with measurements. Then, the Taguchi method is applied to design an orthogonal experimental table, L9(34). A total of 9 virtual experiments are analysed by the use of FEM. The predicted results are then analysed by the use of the Taguchi method from which the influence of each rolling parameter on the deformed subgrain size is given and expressed in percentage. The study shows that rolling temperature has the greatest influence on the final subgrain size, followed by the parameter Hm/L. The roll speed and roll temperature have little effect on the deformed subgrain size.

Microstructural aspects of extrusion of rapidly solidified Al–10Mg alloy powder

AbstractThe microstructures of rapidly solidified Al–10Mg alloy powders have been observed by optical microscopy, scanning electron microscopy, and transmission electron microscopy. A technique is described which enabled individual powder particles to be made electron transparent. The alloy powders were then consolidated by cold compaction followed by hot extrusion using a wide range of temperature compensated strain rates. The microstructures evolved are reported and it is shown that there is a correlation between the microstructures and the process parameters. It is also shown that the initial powder structure is an important variable. Magnesium is retained in solid solution in the extrudate but is metastable at room temperature leading to decomposition over long periods of time. However, the very fine grain size observed (1–3 μm) is thermally stable up to 300°C and aging processes may be utilized to obtain acceptable structures.

Material flow during the extrusion of simple and complex cross-sections using FEM

Abstract This paper deals with the extrusion of rod and shape sections and uses a 3D finite element model analysis (FEM) to predict the effect of die geometry on maximum extrusion load. A description of material flow in the container is considered in more detail for rod and shape sections in order to fully comprehend the transient conditions occurring during the process cycle. A comparison with experiments is made to assess the relative importance of some extrusion parameters in the extrusion process and to ensure that the numerical discretisation yields a realistic simulation of the process. The usefulness and the limitation of FEM are discussed when modelling complex shapes. Results are presented for velocity contours and shear stress distribution during the extrusion process. It is shown that for most of the shapes investigated, the material making up the extrudate cross-sections originates from differing regions of virgin material within the billet. The outside surface of the extrudate originates from the material moving along the dead metal zone (DMZ) and the core of the extrudate from the central deformation zone. The FE program appears to predict all the major characteristics of the flow observed macroscopically.

Prediction of temperature evolution by FEM during multi-pass hot flat rolling of aluminium alloys

In rolling, deformation is applied by a series of passes, which are necessarily separated by holding periods between passes. The formation of the temperature profile therefore has a complex history. Accurate information on the time variation of temperature is essential for predicting microstructural development during hot working conditions and for the design of an acceptable rolling pass schedule. In the present paper, the finite-element method is employed to compute temperature changes under both laboratory and industrial breakdown rolling conditions. The calculated results are then compared with temperature records extracted from the literature. The difference between laboratory and industrial rolling is discussed. By using inverse analysis methods, appropriate heat transfer coefficients can be ascertained.

Parameters affecting lateral deformation in slabbing mills

AbstractThe nature of deformation in the roll gap during passes in which plane-strain conditions do not obtain is investigated, and it is shown that plastic flow is complex, involving material transfer from the slab edges to the roll-contact face. The factors affecting spread in the lateral directions are investigated, and it is concluded that the homologous temperature and the dimensionless group (deformed roll radius /initial slab thickness) are important factors. A new spread formula, defining spread as the natural lateral strain, is presented, and is shown to agree well with the experimental work reported. The results of over 400 rolling tests, including some tests on plant mills, are incorporated into the analysis. It is shown that a change from sticking to slipping friction modifies the deformation mode, and that the structure of the cast ingot or slab may also be an important factor influencing deformation in the roll gap.