Trond Furu

Work Hardening Behaviour of Heat-Treatable Al-Mg-Si-Alloys

The work hardening of alloys hardened by precipitate heat treatments depends on the distribution of the precipitate sizes and the solute level left in the metal matrix. A mean field theory for precipitation is first applied for the ageing and subsequently it is coupled to a work hardening model to study the stress-strain responses of age hardened conditions of AA6xxx alloys. The predictions are compared to mechanical experiments and to TEM characterisations.

The Influence of Grain Structure and Texture on Formability and Toughness of Extruded Aluminium Alloys

The main focus in this work is to investigate the effect of crystallographic texture, grain structure and dispersoids on formability and toughness in some industrial 6xxx and 7xxx series alloys. Materials of these alloys showing strong cube textures or β-fibre deformation textures in as extruded condition have been compared with the same alloys processed by rolling and heat treatment to obtain a random texture. It is found that the formability depends on the temper and the texture and that the effect of the latter is path dependent. Materials with a random texture have a significant higher formability in terms of uniform elongation than materials with cube texture when deformed in the W-temper condition. Forming in other deformation modes shows less difference between the cube and random texture. However, a fibrous grain structure with a sharp β-fibre texture shows an anomalous behaviour when deformed in the biaxial deformation regime. Toughness, in terms of Charpy energy and local strains in the necking area, is significantly higher for materials with a cube texture as compared to materials with random textures. This difference is explained by variations in the dispersoid levels, grain structures (size and grain boundary misorientation) and the texture.

The Effect of Chemical Composition and Microstructure on the Flow Stress during Hot Deformation of Aluminium Alloys

The effect of magnesium and silicon in solid solution on the flow stress during hot deformation is determined by means of torsion and uniaxial compression. The flow stress increases both with magnesium and silicon in solid solution, but the effect of magnesium is much stronger than the effect of silicon. Homogenising AA6082 at 480°C resulted in the formation of smaller dispersoids than homogenising the same alloy at 600°C. The material with the smallest dispersoids exhibited the highest flow stress. Manganese additions to AA6005 facilitated the transformation of the elongated β-AlFeSi to the more rounded α-AlFeSi, especially at high homogenisation temperatures. The flow stress was lower for the manganese-containing alloy homogenised at 580°C compared with the alloy homogenised at 550°C. For the AA6005 alloys without the manganese addition the flow stress increased with increasing homogenisation temperature.

Texture evolution during extrusion of flat AA6060 and AA6082 aluminium profiles

Extrusion trials have been carried out in a laboratory extrusion press on the two important industrial aluminium alloys AA6060 and AA6082 The cooling devices positioned just below the outlet of the die in this laboratory press give the possibility to freeze the entire deformed structure developed during the extrusion process in such a way that the recrystallization process can be avoided. Textures were measured through the profile thickness for both alloys in the as-deformed state and after recrystallization annealing. The deformation textures in the two investigated alloys follow the same pattern In both cases the textures are dominated by a β-fibre in the centre section where the Bs orientation is the sharpest, while the β-fiber is rotated/degenerated towards the surface. The strengths of the fibres are comparable in both alloys, but alloy AA6082 has a somewhat weaker Cu component. There is, however, a difference for the starting point where the fibres begin to rotate and degenerate. This indicates that the shear deformation penetrates deeper into the material in the softer AA6060 alloy. Both alloys display a significant cube component in the deformation texture with comparable strength and similar scatterings around ED and ND. The recrystallization textures are in both materials dominated by the cube orientation with scatterings as observed in the deformation texture The strength of the cube orientation is very high in the centre, especially for 6060, but decreases towards the surface as the cube transforms into a fibre and is gone for the outer 75μm. The outer surface zone is dominated by a weak texture (close to random) with some goss and 45° ND-rotated cube.

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.

Study of microstructure and texture evolution using in-situ EBSD investigations and SE imaging in SEM

The crystallographic slip activity in several grains deformed by simple tension is determined by use of in-situ deformation in combination with Electron Back Scattering Diffraction (EBSD)-investigations and Secondary Electron (SE) imaging. This technique is also used to determine grain lattice rotation paths of grains with different initial orientation, providing information on basic deformation mechanisms of grains present in texture gradients. Both slip activity and grain lattice rotation paths depend on the initial orientation and are influenced by the neighbouring grain orientations. This indicates that predictions of the forming behaviour of extruded profiles with a strong through thickness texture gradient relate to a very complex nature.

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.

Modelling the Recrystallization Behaviour during Industrial Processing of Aluminium Alloys

The microstructure evolution in commercial AlMgSi alloys during and after extrusion of a simple U-shaped profile has been modelled. The strain, strain rate and temperature along a set of particle paths are taken from FE-HyperXtrude simulations and used as input to the work hardening model ALFLOW, to predict the evolution of the subgrain size and dislocation density during deformation. As soon as the profile leaves the die, the subsequent recovery and recrystallization behaviour is modelled with the softening model ALSOFT. This procedure enables the modelling of recrystallization profiles, i.e. the fraction recrystallized through the wall thickness of the extruded profile. The sensitivity to chemistry (alloy composition), profile deflection and the cooling rate at the die exit has been investigated by means of a set of generic modelling cases.

Influence of Processing Route on the Work-Hardening and Ductile Fracture of an AA6060 Aluminium Alloy

Tensile tests on smooth and notched axisymmetric specimens were carried out to determine the large strain work-hardening curves and the ductile fracture characteristics of an AA6060 aluminium alloy for three different processing routes. The alloy was processed in three subsequent steps: 1) casting and homogenization, 2) extrusion, and 3) cold rolling and heat treatment to obtain a recrystallized grain structure. After each processing step, the material was tested after natural ageing for more than one week. A laser-based extensometer was used to continuously measure the average true strains to failure in the minimum cross-section of the specimens and the true stress-strain curves were calculated. Since these curves are influenced by necking, they do not represent the correct work-hardening of the material. Accordingly, finite element (FE) simulations of the tensile tests on the smooth axisymmetric specimens were conducted to determine the work-hardening curves to failure, using an optimization tool that interfaced with the nonlinear FE code and the experimental stress-strain curves as objectives. The microstructure of the alloy was characterized after the three processing steps by optical and scanning electron microscopy, and fractography was used to investigate the failure mechanisms.