| Hypereutectic Al-Si alloys are widely used for making light-weight components such as engine blocks, pistons and cylinder liners, because of their good wear resistance, low thermal expansion coefficient and high heat resistance. Iron is the most common and detrimental impurity element in aluminum casting alloys. The coarse needle-like or plate-like Fe-containing intermetallic compounds are detrimental to the mechanical properties at room temperature. However, it has been proved the Fe-containing intermetallic compounds with high melting point formed in hypereutectic Al-Si alloys can improve the mechanical properties at elevated temperature. The morphology of the Fe-containing intermetallic compounds plays a most important role in determining the mechanical properties of the alloys. Investigations show that the Fe-containing intermetallic compounds can be modified by rapid solidification process and melt superheating. However, applications of these processing are limited because of costly equipment and complicated process. The treatment of molten aluminum and its alloys using ultrasonic vibration (USV) is a relatively new environmentally safe technology with uncomplicated process and low cost. The present study aims to investigate the effects of USV on the microstructure and mechanical properties of high Fe containing hypereutectic Al-Si alloys. The rheological characteristics of the molten hypereutectic Al-Si alloys with high Fe contents were studied. In addition, the mechanism of the effect of USV on the morphology of Fe-containing intermetallic compounds was also discussed.The influence of USV treatment on the Fe-containing intermetallic compounds in hypereutectic Al-Si alloys with different Fe contents (Al-17Si-xFe, x=2,3,4,5mass%) has been systematically investigated. The results show that the size and morphology of Fe-containing intermetallic compounds in the alloys are refined with the application of USV at the liquidus temperature. And they mainly exist as particle-like δ-Al4FeSi2phase with average grain size ranging from26μm to37μm. Acoustic streaming induced by USV increases the start-freezing temperature of δ-Al4FeSi2phase thereby promoting the formation of fine δ-Al4FeSi2particles, and cavitation of USV homogenizes the solute field and temperature field. This leads to the formation and refinement of δ-Al4FeSi2phase.The rheological characteristics of the high Fe content hypereutectic Al-Si alloys were measured with using a high temperature rotational viscometer for the first time. The results show that the viscosities or apparent viscosities of the Al-17Si-(2,3)Fe alloy at the constant temperatures increase with the decrease of temperature. When the molten Al-17Si-(2,3)Fe alloys were continuously cooled at a constant cooling rate, the viscosity increases as the temperature decreases. However, the viscosity increases slowly at the onset of the cooling, and starts to increase abruptly after reaching a critical temperature for all three shear rates. The higher the shear rate, the lower is the critical temperature. The viscosities of the Al-17Si-2Fe alloys with0,0.4%and0.8%Mn content at a constant temperature in the range of600～690℃were also examined. The results show that the viscosities of the molten Al-17Si-2Fe-(0,0.4,0.8)Mn alloys at a constant temperature increase with the increasing Mn content.The combined effects of ultrasonic vibration and the addition of a small amount of Mn on Fe-containing intermetallic compounds and mechanical properties of hypereutectic Al-Si alloys with different Fe contents were studied. The results show that the plate-like δ-Al4(Fe,Mn)Si2phase in Al-17Si-(2,3)Fe-2Cu-1Ni alloys with0.4%or0.8%Mn are refined into small block-like particles after USV treatment. Compared with the counterparts produced by conventional casting process, the room temperature and elevated temperature (350℃) ultimate tensile strengths (UTS) of the USV samples are improved obviously. At the same Fe content, the increase from0.4%to0.8%of the Mn content lead to a small increase of the UTS of the alloys. When Fe content increases from2%to3%, the room temperature UTS of the alloys decrease from289MPa to262MPa. Nevertheless, the elevated temperature UTS increases slightly. The room temperature and elevated temperature mechanical properties can satisfy the engineering application requirements. The dry sliding wear tests of the Al-17Si-(2,3)Fe-2Cu-1Ni-(0.4,0.8)Mn alloys produced by rheocasting process assisted with USV were carried out over a pin-on-disc wear tester. The results show that the wear rate of the USV treated alloy is lower than that of the alloy produced by conventional casting process under the same applied load. The wear resistance is improved significantly. The fine particle-like δ-Al4(Fe,Mn)Si2and polygonal α-Al15(Fe,Mn)3Si2phases help to improve the wear resistance of rheocast Al-17Si-2Cu-1Ni alloys. Thus, at the same Mn content, the wear rate of the USV treated alloy decreases as the Fe content of the alloy increases from2%to3%. For the rheocast alloy with3%Fe, oxidative wear is the main mechanism at low applied load (50N). At higher applied loads, a combination of delamination and oxidation wear is the dominant wear mechanism. This resultes in the wear rate of rheocast alloy increases with, the increase of applied load.The microstructure and mechanical properties of Al-17Si-2Fe-2Cu-1Ni-0.8Mn alloy produced by rheo-diecasting (RDC) assisted with USV treatment were studied. The effect of USV treatment temperature range on the preparation of the semisolid slurry of the alloy was also investigated. The results show that the coarse plate-like δ-Al4(Fe,Mn)Si2phase is refined into fine block-like particles with average grain size of about17μm in the alloy produced by RDC. The porosities in the final RDC sample are reduced or even eliminated. In T6condition, the UTS, elongation and hardness of the RDC sample at room temperature are309MPa,0.8%and141HB, respectively. Compared with the conventional die casting (DC) sample, the UTS, elongation and hardness of the RDC sample at room temperature are increased by34.9%,40.4%and17.5%, respectively. The mechanical properties are improved significantly. In addition, the wear rate of the RDC sample is lower than that of the DC sample under the same applied load. |
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