Emma Sjölander

Modelling yield strength of heat treated Al–Si–Mg casting alloys

Abstract A model for the yield strength of artificially aged Al–Si–Mg casting alloys has been developed. The model includes Mg concentrations between 0·2 and 0·6 wt-% and aging temperatures between 150 and 210°C. Spherical precipitates with the composition Mg5Si6, which grow by diffusion of Mg from the surrounding α-Al matrix, are assumed in the model. Nucleation is assumed to be instantaneous and growth of the precipitates is modelled using Fick’s second law and mass balance. When supersaturation is lost the continued precipitate growth is modelled using the Lifshitz–Slyozov–Wagner coarsening law. An average precipitate radius is calculated and a precipitate size distribution is introduced by using a relation between the average radius and its standard deviation. The strength contribution from precipitates is calculated using coherency strengthening and Orowan strengthening. The agreement between the model and experimental data is generally good; however, modelling the underaged condition needs further refinement.

Influence of alloy composition, solidification rate and artificial aging on plastic deformation behaviour of Al–Si–Cu–Mg casting alloys

Abstract The plastic deformation behaviour of three Al–Si casting alloys was investigated using the Kocks–Mecking strain hardening theory. Three coarsenesses of the microstructure, two aging temperatures and a number of aging times were used. For Al–Si–Mg and Al–Si–Cu–Mg alloys, the dislocation storage rate decreases while the dislocation recovery rate increases with aging time during underaging, whereas the concentration of alloying elements in solid solution decreases. The storage rate reaches a minimum at the peak aged condition and increases at overaging. The storage and recovery rates of the Al–Si–Cu alloy increase with aging time in the underaged condition and start to decrease during overaging, which indicates that a mixture of shearable and non-shearable precipitates are present during underaging, whereas all precipitates become non-shearable on overaging.

Influence of microstructure and heat treatment on thermal conductivity of rheocast and liquid die cast Al-6Si-2Cu-Zn alloy

Thermal conductivity of a rheocast component made from Stenal Rheo1 (Al-6Si-2Cu-Zn) alloy was investigated in as-cast, T5 and T6 conditions. The thermal conductivity measurement in different locations showed variation of this property throughout the rheocast component. The results of microstructural investigation revealed that the ratio of solute-lean α1-Al particles formed during slurry preparation to fine solute-rich α2-Al particles formed during secondary solidification had significant influence on thermal conductivity. The reduced amount of solutes in the α1-Al particles was determined as the root cause of higher thermal conductivity. A linear relation between the fraction of precipitates and the increase in thermal conductivity was obtained and silicon in solid solution is shown to have a dominant influence. As silicon was precipitated during the heat treatment, thermal conductivity increased. For an optimal combination of thermal and mechanical properties, it is therefore important to use an ageing temperature above the temperature of Si precipitation.

Influence of Quench Rate on the Artificial Ageing Response of an Al-8Si-0.4Mg Cast Alloy

The aim of the study is to present the influence of quench rate on the artificial ageing response of Al-8%Si-0.4%Mg cast alloy in terms of Brinell hardness and yield strength. The investigated material was produced by a gradient solidification technique and exhibited a microstructure that corresponds to the one of gravity die castings, with a dendrite arm spacing of approximately 25 µm. The study comprises two solution treatment temperatures, five quench rates and artificial ageing times exceeding 100 hours at 170 and 220 °C. The microstructure and concentration profiles of Mg and Si were evaluated using energy and wavelength dispersive spectroscopy. Microstructural examination reveals an increment of solutes in the Al-matrix when higher solution treatment temperatures accompanied with high quench rates are applied and shows how both Si and Mg atoms have diffused towards the eutectic during quenching. Consequently, i.e. by increasing the levels of solutes and vacancies, the highest strength levels were realized. The study confirmed that quench rates above 2 °C /s do not offer substantial strength improvement while quenching at lower rates resulted in a lower peak hardness and longer times to peak

Simulation of microstructure and mechanical properties of aluminum components during casting and heat treatment

The combination of alloy composition, metallurgy and local solidification conditions all lead to the development of locally varying microstructure and defects during the production of aluminum alloy castings. In turn, the local microstructure and defects play a critical role in determining the local mechanical properties. In this paper, the integrated modeling and simulation of the development of microstructure in aluminum alloy components from casting through heat treatment will be described. The predicted microstructure at the end of this process chain is used to predict the distribution of mechanical properties in the casting. This information can be used as input for e.g. durability calculations during the design phase of a new component. It can also be used to optimize the casting or heat treatment process to obtain the required mechanical properties using the most cost-effect manufacturing process. The use of the integrated process simulation as well as a comparison with measurements is illustrated using a prototypical cylinder head casting as an example.