Modelling of age hardening for non-steady-state heat treatment of aluminium alloys

Abstract

Most aluminium alloys gain their strength by heat treatment through age-hardening (ageing or precipitation hardening). During the process of ageing, if the temperature does not change with time, it is called isothermal age hardening; if the temperature changes with time, it is called non-isothermal ( or non-steady state ) age-hardening, which occurs in most commercial heat treatments. Computer modelling, due to it saving time and money, has been widely used in industrial simulation. In this thesis, various computer models of precipitation kinetics and strengthening in age hardening have been developed and applied to both wrought and cast aluminium alloys under isothermal or non-isothermal heat treatment conditions. The models are validated by experimental data as well as data obtained from the open literature. All the modeling works are programmed by the author using Matlab software. Basically there are two modelling methods developed and compared in this project: one is based on Schercliff-Ashby methodology in which only one average particle size is assumed; the other is which based on the Kampman and Wagner Numerical (KWN) model in which the particle size is assumed as a continuous parameter. In the Schercliff-Ashby method, most constants are determined by a calibration process. Some researchers thought the calibration process was too complicated, however, by choosing suitable software and functions, the calibration work can be greatly reduced. This method is applied to different Al-Mg-Si alloys (A356, A357 and 6061) for different artificial ageing temperatures. By combining the concept of internal state variables approach, the Schercliff-Ashby method was extended and applied to non-isothermal age hardening. In the other approach, the numerical Kampmann and Wagner model (KWN model) is a powerful method capable of describing the particle size distribution (PSD) in the time domain, while dealing with nucleation-growth-coarsening phenomena within the same formulation. This method was developed and applied to different aluminium alloys under isothermal or non-isothermal heat treatment conditions. And this is the first time that the KWN model had been applied to the casting aluminium alloys. However, since there are some limitations in the original KWN model, the KWN model has been improved in the two ways. One big improvement is that since there is lack of relationship between each group of particles in the small size intervals in the original KWN model, in order to solve the problem in a more rigorous way, the population balance equation (PBE) which is an important tool in analyzing any particulate phase systems is introduced into the KWN model in the present modelling work. The modeling results show that by combination of PBE with the KWN model, the particle size distribution (PSD) function can be solved more rigorously. The other big improvement is that the mixed-mode controlled model for the growth rate of the second-phase precipitated particles is developed and replaces the diffusion controlled model in the original KWN model. The mixed-mode control model is the intermediate case to the extreme of "diffusion control" and "interface control". In fact, every real case is undoubtedly "mixed control" with the two extremes being just a matter of degree. This is the first time that the mixed control model has been developed and introduced into the KWN model. However, because of a time limit, only preliminary results are shown for the present modelling work using this method and more work still can be done with the mixed mode control model to improve it further in the future.