In this study we demonstrate the complementarity of transmission electron microscopy (TEM) and atom probe tomography (APT) in studying the early stages of phase decomposition in the age-hardening ...Al–Mg–Si alloy AA 6016. Samples are subjected to natural ageing at ambient temperature or artificial pre-ageing at elevated temperature in order to produce different types of atomic clusters and early stages of precipitation before age hardening commences. APT is utilized to detect clusters and identify their number density, size and compositions, whereas TEM is applied to analyse and quantify number density, sizes and types of the hardening precipitates during artificial ageing. Finally, the particle statistics derived by APT and/or TEM were utilized to predict the mechanical properties of the various samples and conditions analysed.
In the present investigation a special control volume formulation of the classical precipitation model for coupled nucleation, growth and coarsening has been adopted to describe the evolution of the ...particle size distribution with time during thermal processing of Al–Mg–Si alloys. The analysis includes both isothermal and non-isothermal transformation behaviour. Well established dislocation theory is then used to evaluate the resulting change in hardness or yield strength at room temperature, based on a consideration of the intrinsic resistance to dislocation motion due to solute atoms and particles, respectively following heat treatment. The model is validated by comparison with experimental microstructure data obtained from transmission electron microscope examinations and hardness measurements, covering a broad range in the experimental conditions. It is concluded that the model is sufficiently relevant and comprehensive to be used as a tool for predicting the response of Al–Mg–Si alloys to thermal processing, and some examples are given towards the end.
A numerical solution is presented, capable of handling nucleation, growth and coarsening, and likewise dissolution, of hardening precipitates in Al–Mg–Si alloys during ageing, welding and post weld ...heat treatment (PWHT). By coupling this to a separate strength model for shearing and bypassing of particles by dislocations, the evolution of the macroscopic yield stress at room temperature can be calculated at each time step. Following testing of the model against reliable hardness and transmission electron microscope (TEM) data obtained from dedicated Gleeble simulation experiments, the connectivity on a micro/macro level throughout a multistage manufacturing route is illustrated by means of a numerical example. These simulations show how a past process step influences the microstructure and strength evolution in the subsequent process steps due to interactions between different groups or classes of particles that form at various temperatures.
A framework for modelling coupled nucleation, growth and coarsening in diluted alloys is presented, based on the method of finite differences. According to the assumptions, the particle distribution ...is divided into a series of discrete size classes, each represented by a control volume. By applying a standard procedure for discretization of the governing evolution equation, the flux of particles in and out of the control volumes can be calculated at each time step during thermal cycling. The essential features of the model are illustrated in various numerical examples and case studies, where the memory of a past process step is likely to have an effect on the overall transformation behaviour. These include simulation of up-quenching and linear heating of age hardening Al–Mg–Si alloys under idealized conditions, assuming spherical particles with uniform thermodynamic properties.
•The ballistic properties of AA6070 in four different tempers have been studied.•A nano-scale material model is used to predict the flow–stress curves of the materials.•Finite element simulations of ...the perforation process are then carried out.•The predictions are carried out without any use of data from mechanical tests.•Good agreement with available experimental data is found for all tempers.
Finite element simulations of AA6070 aluminium plates struck by ogival-nose projectiles are performed. The aluminium plates are 20mm thick and heat treated to temper O, T4, T6 and T7. A nano-scale material model, consisting of three parts: a precipitation model, a yield-strength model and a work-hardening model, is used to predict the flow–stress curves of the materials at ambient temperature based on the chemical composition of the alloy and the thermal history defined by the heat treatment. Finite element simulations of the perforation process are then carried out using both 3D solid and 2D axisymmetric elements. The numerically-obtained ballistic limit velocities, predicted without any use of data from mechanical tests, are compared with available experimental data and found to be in good agreement with the experimental ones for all tempers. The same holds for the predicted residual velocities at striking velocities higher than the ballistic limits.
•Tensile tests in different material directions are performed for different tempers of an extruded AA6063 alloy.•A variation in stress ratio and strain ratio are observed in 0° and 45° ...directions.•The microstructure evolution during the heat treatment of the alloy is modelled.•The microstructural parameters are correlated to the variations in the stress and strain ratio.•The possible explanations for the stress ratio variations are analysed.
The plastic anisotropy of an extruded AA6063 alloy after different heat treatments was investigated in a previous study by Khadyko et al. (2017). The material was available in temper T1 (naturally aged to a substantially stable condition) and directly heat treated to the peak aged, overaged and soft annealed conditions without solution heat treatment. Tensile tests in different material directions were performed for these materials and it was found that the plastic anisotropy depended in a complex way on the heat treatment. In the present study, the experimental programme is extended to investigate further the influence of heat treatment on the plastic anisotropy. The AA6063 extruded alloy is first solution heat treated, and then either naturally aged to temper T4 or artificially aged to tempers T6 (peak aged) and T7 (over-aged). Tensile tests in three material directions are performed for these tempers and in the solution heat treated condition (temper W) to reveal the plastic anisotropy. The microstructure evolution of all these materials is then modelled using a nanostructure model (NaMo), which predicts the size distribution of hardening particles, the solute content, the yield strength and the work hardening. The predictions of NaMo are validated using the available data from the tensile tests and TEM observations, and used to analyse the correlation between the stress-strain behaviour and the heat treatment and microstructure of the different materials.
The plastic properties of an aluminium alloy are defined by its microstructure. The most important factors are the presence of alloying elements in the form of solid solution and precipitates of ...various sizes, and the crystallographic texture. A nanoscale model that predicts the work-hardening curves of 6xxx aluminium alloys was proposed by Myhr et al. The model predicts the solid solution concentration and the particle size distributions of different types of metastable precipitates from the chemical composition and thermal history of the alloy. The yield stress and the work hardening of the alloy are then determined from dislocation mechanics. The model was largely used for non-textured materials in previous studies. In this work, a crystal plasticity-based approach is proposed for the work hardening part of the nanoscale model, which allows including the influence of the crystallographic texture. The model is evaluated by comparison with experimental data from uniaxial tensile tests on two textured 6xxx alloys in five temper conditions.
Additivity and isokinetic behaviour in relation to transformations that involve coupled nucleation and growth have been examined. As a starting point, a numerical solution is presented which takes ...into account the independent variations of the nucleation and growth rate with temperature and matrix solute content. This solution is later used to check the validity of two analytical models, based on the internal state variable approach and the Avrami equation, respectively. It is concluded that the analytical solutions provide an approximate but adequate description of the overall transformation behaviour during continuous cooling. The latter is particularly true if the evolution equations are calibrated against experimental microstructure data, as frequently done in high-hierarchy models for process simulation.
Welded components made of age-hardening aluminum alloys are to an increasing extent used within the transport and automotive industries because of their high strength, good formability, low density, ...and good resistance to general corrosion. However, in certain cases, the application of such alloys is restricted by a low heat-affected zone (HAZ) strength level due to softening reactions occurring during welding, which tend to reduce the overall load-bearing capacity of the component. This represents a major challenge in engineering design.
PDF
