Environmental savings can be made by increasing the use of aluminium alloys in the automotive industry as the vehicles can be made lighter. Increasing the knowledge about the heat treatment process is one task in the direction towards this goal. The aim of this work is to investigate and model the heat treatment process for Al-Si casting alloys. Three alloys containing Mg and/or Cu were cast using the gradient solidification technique to achieve three different coarsenesses of the microstructure and a low amount of defects.Solution treatment was studied by measuring the concentration of Mg, Cu and Si in the α-Al matrix using wavelength dispersive spectroscopy (WDS) after various times at a solution treatment temperature. A diffusion based model was developed which estimates the time needed to obtain a high and homogenous concentration of alloying elements for different alloys, temperatures and coarsenesses of the microstructure. It was shown that the yield strength after artificial ageing is weakly dependent on the coarseness of the microstructure when the solution treatment time is adjusted to achieve complete dissolution and homogenisation.The shape and position of ageing curves (yield strength versus ageing time) was investigated empirically in this work and by studying the literature in order to differentiate the mechanisms involved. A diffusion based model for prediction of the yield strength after different ageing times was developed for Al-Si-Mg alloys. The model was validated using data available in the literature. For Al-Si-Cu-Mg alloys further studies regarding the mechanisms involved need to be performed.Changes in the microstructure during a heat treatment process influence the plastic deformation behaviour. The Hollomon equation describes the plastic deformation of alloys containing shearable precipitates well, while the Ludwigson equation is needed when a supersaturated solid solution is present. When non-coherent precipitates are present, none of the equations describe the plastic deformation well. The evolution of the storage rate and recovery rate of dislocations was studied and coupled to the evolution of the microstructure using the Kocks-Mecking strain hardening theory.
of commercial and high-purity non-heat-treatablealuminum alloys are investigated in this work. It is found that both magnesium and manganesei ns olid solution give an early linear concentration dependence of the strengtha tagiven strain for commercial alloys. This deviates from highpurity AlMg binary alloys, where aparabolic concentration dependence is found. Mn in solid solution is found to give ac onsiderably higher strengthening effect per atom than Mg, both in terms of yield stress and initial work hardening rate. This strengthening effect is stronger comparing commercial grades to high-purity alloys. This enhanced strengthening is believed to be as ynergy or clustering effect caused by interaction betweenMnatoms and trace elements, probably silicon, in solid solution.
This paper aims to assess the role of Cu on Al-Si-Mg alloys, in a range of 0 -5 wt%, qualitatively on microstructure, defect formation, in terms of porosity, and strength in the as-cast conditions. The ternary system of Al-Si-Mg, using the A356 alloy as a base material, were cast using the gradient solidification technique; applying three different solidification rates to produce directional solidified samples with a variety of microstructure coarsenesses. Microstructural observations reveal that as the Cu levels in the alloys are increased, the amounts of intermetallic compounds as well as the Cu concentration in the α-Al matrix are increased. Furthermore, the level of porosity is unaffected and the tensile strength is improved at the expense of ductility.
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.
This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination.
Citation for the original published paper (version of record):Payandeh, M., Sjölander, E., Jarfors, A., Wessén, M. (2016) Influence of microstructure and heat treatment on thermal conductivity of rheocast and liquid die cast Al-6Si-2Cu-Zn alloy.
International Journal of Cast Metals
AbstractThermal conductivity of a rheocast telecom component made from Stenal Rheo1 (Al-6Si-2Cu-Zn) alloy was investigated in as-cast, T5 and T6 conditions. Conventionally liquid die cast samples were used as reference material. In the as-rheocast condition, a thermal conductivity of 153 W/mK at room temperature was measured. A T5 treatment at 250 or 300°C increased thermal conductivity to 174 W/mK. A T6 treatment resulted in further increase in thermal conductivity to 182 W/mK. The liquid die cast material exhibited lower thermal conductivity and higher hardness for all conditions compared to the as-rheocast material.The microstructural investigation revealed that the rheocast material consisted of coarse α1-Al particles formed during slurry preparation and fine α2-Al particles formed during solidification in the die cavity. Macrosegregation was observed as different the ratio of α1-Al particles to α2-Al particles in different locations in the rheocast component. The relation between microstructural characteristics and thermal diffusivity was investigated by determination of local thermal conductivity in the rheocast component and ratio of α1-Al particles to α2-Al particles. The results revealed that regions of rheocast component with a high amount of α1-Al particles showed higher thermal conductivity. WDS measurement showed that α1-Al particles contains lower concentrations of both Si and Cu inside compare to α2-Al particles. The reduced amount of solutes in the α1-Al particles was therefore determined as the root cause to higher thermal conductivity.Silicon precipitation was confirmed using calorimetry and dilatometry to take place between 200 and 250°C. A linear relation between the fraction of Si precipitates formed and the increase in thermal diffusivity was obtained. Silicon in solid solution is shown to have a strong influence (negative) on thermal conductivity. 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 for Si precipitation.
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