Bjørn Holmedal

Influence of dispersoids on microstructure evolution and work hardening of aluminium alloys during tension and cold rolling

The influence of dispersoids on work hardening of aluminium during tension and cold rolling has been studied by comparing Al–Mn alloys containing similar amounts of solutes but various dispersoid densities. The microstructure evolution with deformation strain was examined in transmission and scanning electron microscopy. It is found that a high density of fine dispersoids strengthens the materials significantly, but their strengthening effect diminishes as the strain increases. From a series of Bauschinger tests, it is found that the internal stress, due to particles, increases rapidly at the initial stage of deformation, but saturates at strains larger than 5%. It is concluded that the internal stress makes a small contribution to the work hardening and contributes to less than 10% of the total flow stress during monotonic loading at strains larger than 5%. The work-hardening behaviour has been correlated to the corresponding microstructure, and the strengthening mechanisms are discussed.

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.

A unified microstructural metal plasticity model applied in testing, processing, and forming of aluminium alloys

Abstract Over the last seven years a collection of models has been developed and put together by Nes, Marthinsen and coworkers in what here will be referred to as the Microstructure-based Metal Plasticity model, or in short as the MMP model. An overview of the most important modelling aspects will be given here. The basic mechanisms are related to the way the dislocations are stored and recovered in the lattice and how they affect the flow stress during deformation. The model at its current state is able to predict the microstructure evolution and the corresponding flow stress for the entire temperature range and for large strain rates as well as creep behaviour. The inherited processing-related quantities, such as grain size, solute content of alloying elements, and the texture, are taken into account, including a model for dynamic strain ageing. Anisotropy of the stress tensor is related mainly to the coupling to a texture model accounting for lattice rotations of the grains. However, a new and novel mo...

Modelling work hardening of aluminium alloys containing dispersoids

The influence of dispersoids on tensile deformation behaviour has been studied by comparison of aluminium alloys containing different dispersoid densities. It was found that a fine dispersion of non-shearable particles led to an increased work hardening at the initial plastic deformation, but the effect was opposite at higher strains. The reason has been attributed to the generation of geometrically necessary dislocations (GNDs). A new model has been proposed for the evolution of GNDs based on a balance of storage and dynamic recovery of GNDs. The model predicts a rapid saturation of GNDs and a reduced work hardening at small strains, consistent with the experimental results.

Precipitation of Non-spherical Particles in Aluminum Alloys Part II: Numerical Simulation and Experimental Characterization During Aging Treatment of an Al-Mg-Si Alloy

This is the second part of the investigation on the precipitation of needle-shaped particles. In part I, two particle aspect ratio-dependent correction factors are introduced to describe the effects of non-spherical precipitate shape on precipitate growth kinetics. In this part II, the two factors are integrated into a CALPHAD-coupled multi-component Kampmann–Wagner numerical model to predict precipitation kinetics of needle-shaped metastable particles during aging treatment of an Al-Mg-Si alloy. The predictions are compared with transmission electron microscopy observations on precipitate number density, volume fraction, and size distribution. Improved agreement is reported, and in particular, the shape of the predicted particle size distribution density function becomes more realistic.

Precipitation of Non-Spherical Particles in Aluminum Alloys Part I: Generalization of the Kampmann–Wagner Numerical Model

Particles precipitated during aging treatments often have non-spherical shapes, e.g., needles or plates, while in the classical Kampmann–Wagner Numerical (KWN) precipitation model, it is assumed that the particles are of spherical shape. This model is here generalized resulting in two correction factors accounting for the effects induced by the particles’ non-spherical shape on their growth kinetics. The first one is for the correction of the growth rate. It is derived from the approximate solution of the diffusion problem on spheroidal coordinate and verified by the three-dimensional numerical solutions for cuboid particles. The second factor is for the energetic correction due to the particle surface curvature. It is derived from chemical potential equality (or Gibbs energy minimization principle) at equilibrium for non-spherical particles and provides a correction factor for the Gibbs–Thomson effect. In the accompanying paper, the two correction factors are implemented into a multi-component KWN modeling framework, and the resulting improvements on the model’s predictive power are demonstrated.

Influence of dispersoids on grain subdivision and texture evolution in aluminium alloys during cold rolling

Abstract Al-Mn alloys containing similar amounts of solutes but various dispersoid densities were cold rolled. The grain subdivision and micro-texture were examined by electron backscatter diffraction and orientation imaging microscopy. Macro-texture was measured by X-ray diffraction. It is found that a high density of fine dispersoids enhances the development of the copper and S textures at large strains (∼3), and also induces a higher fraction of high-angle grain boundaries. At smaller strains, the texture and high-angle grain boundaries are not evidently influenced by the density of dispersoids. It is suggested that the texture evolution, which is enhanced by dispersoid pinning effect, contributes to the grain subdivision and the formation of high-angle grain boundaries.

Crystal Plasticity Calculations of Mechanical Anisotropy of Aluminium Compared to Experiments and to Yield Criterion Fittings

Mechanical anisotropy of a sheet was studied by experiments as well as crystal plasticity calculations. The material is a 99.999% high purity Aluminum with additions of 0.066%Fe and 0.068%Si. Uniaxial tensile tests at every 15° from the rolling to the transverse direction were conducted. Yield stresses were measured and also the r-values for the uniaxial tensile tests. The anisotropic Yld2004-18p yield function for fully three-dimensional stress state was fitted to the experiments. Crystallographic orientation data were measured by EBSD and used as input for the full-constraint Taylor model. The yield locus was calculated by the Taylor model and compared to the Yld2004-18p criterion fitted to the experiments. Since the number of possible mechanical tests is limited and the experimental errors can be challenged, it would be desirable to replace the mechanical tests by one texture measurement and virtual experiments by crystal plasticity calculations. The reliability of this approach is discussed for the case of pure aluminium.

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.

An explicit integration scheme for hypo-elastic viscoplastic crystal plasticity

Abstract An explicit integration scheme for rate-dependent crystal plasticity (CP) was developed. Additive decomposition of the velocity gradient tensor into lattice and plastic parts is adopted for describing the kinematics; the Cauchy stress is calculated by using a hypo-elastic formulation, applying the Jaumann stress rate. This CP scheme has been implemented into a commercial finite element code (CPFEM). Uniaxial compression and rolling processes were simulated. The results show good accuracy and reliability of the integration scheme. The results were compared with simulations using one hyper-elastic CPFEM implementation which involves multiplicative decomposition of the deformation gradient tensor. It is found that the hypo-elastic implementation is only slightly faster and has a similar accuracy as the hyper-elastic formulation.