Tanja Pettersen

Effect of Changing Homogenization Treatment on the Particle Structure in Mn-Containing Aluminium Alloys

During casting and homogenisation of aluminium the microstructural fundament for further processing is made. Particle structure (dispersoids and primary particles), grain structure and level of elements in solid solution govern the mechanical and annealing properties of the material. In 3xxx-alloys, Mn in solid solution and Mn-containing dispersoids formed during homogenisation play an important role in controlling the recrystallization behaviour of the material [e.g. 1, 2, 3]. Other elements, such as Si, will have an influence on the formation of dispersoids [4, 5]. Hence, to control the annealing behaviour of the material, it becomes important to control the particle structure. In the present investigation, an AA3103 alloy, and modified versions of this alloy, have been investigated. Various homogenisation treatments have been performed and the resulting material has been studied. Electrical conductivity has been measured and microstructural investigations have been carried out.

Modelling the Phase Transformation from Al6(Mn,Fe) to α-Al(Mn,Fe)Si Phase during Homogenization of AA3xxx Alloys

A simplified numerical model for the solid state phase transformation from Al6(Mn,Fe) to α-Al(Mn,Fe)Si phase in 3xxx alloys has been constructed. In this model, the phase transformation is assumed to be initiated by the heterogeneous nucleation of α-Al(Mn,Fe)Si dispersoids at the interface between Al6(Mn,Fe) particle and matrix and the growth of the α- Al(Mn,Fe)Si phase into the Al6(Mn,Fe) particle is controlled by the diffusion of Si from the matrix. The model has been implemented into a numerical homogenization model. The simulation results show that the implementation of the phase transformation model improves much the prediction results of the homogenization model on the evolution of solid solution level of alloying elements and the volume fraction evolution of dispersoids in 3xxx alloys during homogenization.

Modelling the Metallurgical Reactions during Homogenisation of an AA3103 Alloy

The as cast microstructure of a DC cast AA3103 alloy consists of equiaxed grains with a cellular structure. The periphery of the cells contains high volume fractions of intermetallic phases and there are large variations in the solid solution level across the cells. During a typical homogenisation heat treatment the material is heated at 50 to l00(degrees)C/hour up to a temperature of 500-600(degrees)C and held there for some hours. The material is then cooled to room temperature (extrusion ingot) or fed into the hot-rolling mill (sheet ingot). A model for the metallurgical reactions occurring in this system is constructed based on a cylindrical cell geometry. The as cast microstructure is adopted from a solidification model (Alstruc) that predicts the micro segregation, the volume fraction and the composition of the primary phases. A thermodynamic description of the two phases Al6(Mn,Fe) and Al15(Mn,Fe)3Si is proposed, assuming matrix to be a dilute solution and the phases to be regular solutions. Fe and Mn are allowed to Subscriptstitute each other completely. Precipitation, growth and coarsening of the phases are modelled individually in each position across the cell, each particle is designated to a size class and infinite diffusion is assumed inside particles. Diffusion across the cell is accounted for. Model results are compared with measured number density and size distribution of precipitates and electrical conductivity.

Phase Transformation for Primary Particles in the Surface Regions of an AA1200 Alloy

In the present investigation the particle structure in an AA1200 sheet ingot used for litho applications has been studied. Caustic etching of the as-cast material was seen to result in a zone close to the surface with a different etching response. This zone was identified as what is known as a fir-tree zone or an Altenpohl zone [1,2,3,4]. A variation in particle type over the cross section of the as-cast ingot was seen to follow the differences in etching response. After heat treatment of the material, the fir-tree zones were no longer visible, and the accompanying change in particle structure was studied. Samples from the subsurface regions and from a distance of ~20 cm from the surface has been investigated before and after heat treatment. In the as-cast material, the sample from the surface was dominated by featherlike particles with long strings of particles, identified as AlmFe. While closer to the centre Al3Fe and Al6Fe were seen to be the main phases, however, some AlmFe and probably some α-AlFeSi was also found in this sample. After heat treatment, the particle structure was seen to change, and the surface sample contained mainly Al3Fe in addition to a small amount of AlmFe. The change in particle structure during heat treatment is discussed with reference to the change in etching response.

Characterization and Modelling of the Microstructure and Texture Evolution In AlMgSi-Extrusions

The properties and surface appearance of aluminium extrusion are critically dependent on the microstructure and texture of the extruded profiles, and the requirements with respect to these aspects may vary with applications. Moreover it is often a challenge to produce extrusions with a consistent and homogenous grain structure and texture along as well as through the cross section of the profiles. It is thus vital to understand and be able to predict (model) how different microstructures and textures are formed and how they evolve during and after extrusion. In the present work a model framework has been implemented which includes a FEM model to account for the strain, strain rate and temperature along a set of particle paths during extrusion. From these the deformation texture and grain structure are calculated with an appropriate deformation texture model and a sub-structure evolution model, respectively. The sub-structure model have in the present work been coupled to a crystal plasticity model to provide an orientation dependent subgrain size and dislocation density during deformation which provides the driving force for the post-extrusion recovery and possible recrystallization behaviour. The post-extrusion microstructure and texture evolution is calculated with a recovery and recrystallization model, which is accompanied by a recrystallization texture model. The framework and its constituent models and their interplay are presented, and some preliminary results when applying this modelling framework to Al-Mg-Si extrusions are presented and discussed in view of corresponding experimental results.

Recrystallization behaviour of AA6063 extrusions

Cylindrical profiles of an AA6063 aluminium alloy were produced in a lab-scale direct extrusion set-up. The extrusion was performed at 300 °C, 450 °C and 550 °C, respectively, with the same ram speed. Immediate water quenching was applied to the profiles and the end of billet (butt-end) after extrusion. Microstructure and texture of the material in different states were measured by electron back-scattered diffraction. Only the profile extruded at 300 °C, was found in the deformed state after extrusion, featuring a fibrous grain structure and a strong and weak double fibre texture. Post-extrusion annealing of this profile at 450 °C resulted in an almost fully recrystallized structure (recrystallized fraction of 87%) and with a texture similar to that of the as-deformed state. The profile extruded at 450 °C was almost fully recrystallized (recrystallization fraction 91%) already after quenching, and with a texture characterized by a weak and strong double fibre. The profile extruded at 550 °C showed a partially recrystallized grain structure with recrystallization fraction of 71%, and with a texture dominated by a fibre. The influence of the deformation conditions on the recrystallization behaviour, in terms of recrystallization kinetics and mechanisms, are discussed in view of these results.

Hot Deformation to High Strains; Microstructure, Texture and Flow Stress Behaviour

An investigation of the development of the stress-strain curve during deformation in torsion has been carried out. A fall in the flow stress of about 20% was found from the peak of the stress-strain curve to a second steady state. To investigate the origin of the fall in the flow stress the change in texture and microstructure during deformation was followed. The texture was found to change from a close to random texture at the peak of the stress-strain curve to a texture mainly consisting of the components {001} and {112} at the second steady state strain. A fall in the stress of 5%-7% could hence be ascribed to the change in texture. The subgrain size was found to increase by about 20% corresponding to a fall in the flow stress of 10%.