Design-Production And Heat Transfer Mechanism Of Al-Si Alloys With High Thermal Conductivity

The influence of different alloy elements and the second phase on the thermal conductivity of Al alloys was investigated by thermodynamic simulation and first-principle calculation in this study.The Al-Si alloy was trial-produced based on the results of simulation and verification experiments,which has high thermal conductivity and good mechanical properties.The microstructure evolution of the high thermal conductivity Al-Si alloy under different processings,including Sr-modification,rolling deformation and heat treatment,was systematically discussed.The influence mechanism of the second phase shape,size and distribution on the thermal conductivity of the alloy was revealed.The effect of microstructure evolution on the mechanical properties of high thermal conductivity Al-Si alloy and the mechanism of strengthening and toughening was clarified.This research provides a new idea and theoretical guide for the preparation of high thermal conductivity Al-Si alloy by the“ingot-hot deformation-heat treatment”process.The change of thermal conductivity of aluminum alloy with different contents of Cu,Fe,Mg and Si was studied by thermodynamic calculation.The results indicated that the reduction of thermal conductivity of Al alloys affected by the Si element is the least.Further composition design was carried out for Al-12Si alloy and the results showed that the order for the thermal conductivity reduction of Al-12Si alloy is Fe,Cu and Mg,while the adding contents are the same.Meanwhile,the thermal conductivity of Al-Si alloy decreases with the increase of the second phase contents.The analysis and discussion on thermodynamic properties and mechanical properties of second phases in Al-Si alloy,which contains Cu,Fe and Mg elements were implemented by first-principle calculation.The results showed that the stiffness and theoretical hardness of ternary Al_xFe_ySi_z phases are best,and the theoretical calculation value of Debye temperature and thermal conductivity of those are also highest.That indicated ternary Al_xFe_ySi_z phases have excellent heat transfer and strengthening ability.The composition optimization of Al-Si alloy was based on the results of simulation and verification experiments,and the high conductivity Al-Si-Fe-Mg alloy was finally obtained.Its thermal conductivity in the as-cast state was 155.99 W/(m·K)at room temperature.After Sr modification,the eutectic silicon in the Al-Si-Fe-Mg alloy changed from the morphology of coarse plate-like into typical fibrous morphology.Due to the transformation of the size and morphology of the eutectic silicon,the heat conduction of Si phases has been improved by reducing the scattering effect on the free electrons in the heat transfer process.Thus,the thermal conductivity of the Sr-modified alloy was 3.86%higher than that of the unmodified alloy.Meanwhile,the separate effect of?-Al matrix was weakened by the fibrous eutectic silicon,then the mechanical properties of Sr-modified alloy were also increased.After T6 heat treatment,the eutectic Si particles of unmodified alloy and Sr-modified alloy were further refined,the average size and aspect ratio of eutectic silicon particles have changed significantly.According to the statistical results,the fibrous eutectic Si phases with high branch and fine grain had stronger spheroidizing ability than that of coarse lamellar eutectic silicons.The segregation phenomenon was found in the?-Al matrix after fusing,hence,the further improvement on the thermal conductivity of Sr-modified alloy after T6heat treatment was restricted.After T6 heat treatment,the mechanical properties of Sr-modified alloy were better than those of unmodified alloy,but the thermal conductivity of Sr-modified alloy is slightly lower.The size and distribution of the second phase in Al-Si alloy could also be improved by treatment of hot rolling,which was beneficial to enhance the thermal conductivity and mechanical properties of Al-Si alloy.With the increasing of hot rolling reduction,the fracture degree of eutectic Si phase and Fe-rich phase in the alloy increased.,meanwhile,the distribution of Si particles in?-Al matrix is more uniform,and deformation twins were formed in the eutectic silicons.The thermal conductivities and mechanical properties of the hot-rolled alloys also increased with the increasing of hot rolling reduction.The results revealed that the alloys had good thermal conductivity and strength when the eutectic Si particles of them were small and dispersed.After T6 heat treatment,the spheroidization occurred in eutectic silicon phase of the hot-rolled alloy,and the proportion of small-size eutectic silicon particles increased,which reduced the scattering effects on the free electrons in the heat transfer process and increased the average free paths of electrons.At the same time,the separate effect of?-Al matrix was reduced and the dispersion strengthening effect was enhanced due to the morphology change of eutectic silicon particles so that the thermal conductivity and mechanical properties of the hot-rolled alloy after T6heat treatment were both improved.The high thermal conductivity Al-Si-Fe-Mg alloy obtained in this paper can get a higher thermal conductivity and mechanical properties under hot rolling with 71.25%reduction after T6 heat treatment.The eutectic silicon and Fe-rich phase were fine and disperse in?-Al matrix of Al-Si-Fe-Mg alloy.The thermal conductivity at room temperature is 188.22 W/(m·K),the ultimate tensile strength at room temperature is 295.21 MPa,and the elongation reaches to 9.56%.