Papers by Keyword: Hypereutectic Al-Si Alloy

Abstract: The hypereutectic Al-17Si-2Cu-1Ni-0.8Mn alloys with 2% or 3%Fe were fabricated out assisted with ultrasonic vibration (USV) treatment. The coarse plate-like δ-Al4(Fe,Mn)Si2 phase was transformed into fine particles with average diameter of about 17μm~20μm after USV treatment, and the volume fraction of particle-like Fe-bearing compounds is about 3.6%~5%. The polygonal primary Si crystals were also refined into particles with average diameter of about 15μm~23μm, and the volume fraction of primary Si declined to about 5.4%~6.5% after USV treatment. The matrixes of the castings were reinforced with fine Fe-bearing compounds and Si particles. The ultimate tensile strength of the alloys, which were produced by gravity casting process assisted with USV, increases with the increase of Fe content at 350 °C. It is considered the increase of the elevated temperature strength of the samples are mainly attributed to the refinement of δ-Al4(Fe,Mn)Si2 phase by USV and the increase of the volume fraction of the Fe-bearing compounds. The die casting process assisted with USV can further improve the mechanical properties of Al-17Si-2Fe-2Cu-1Ni-0.8Mn alloy.

Abstract: The effects of the thermal cycling on the coefficient of thermal expansion (CTE) and microstructure of the hypereutectic Al-Si alloys were investigated. The hypereutectic Al-Si alloy was produced by spray-forming and extruding process. Experiment results showed that the distribution of Si in Al matrix is uniform for the Al-Si alloy. The primary Si phase grew gradually during the thermal cycle. There is no remarkable change of CTE during thermal cycling for both materials at the same temperature range. The CTE of Al-Si alloy decrease with increasing on thermal cycling temperature up to 300°C due to the compressive thermal stress in the alloys.

Abstract: The combined effects of the Co addition and Ultrasonic Vibration (USV) on microstructure and properties of Al–20Si–2Cu–1Ni–0.6Mg alloy were studied in this paper. The volume fraction of Co-bearing intermetallic compounds was increased with the increasing of Co content when various levels of Co (from 0% to 1.5%) added respectively into the alloys whether they formed in traditional casting or with USV treatment. The Co-bearing compounds presented as Chinese script or granular when Co was equal or less than 0.7% and turned into dendritic or fish-bone shape when Co was more than 0.7%. USV treatment applied around liquidus temperature of the melt could lead to a refinement of Co-containing compounds by promoting nucleus formation, hindering preferential growth on certain direction, and enlarging solute diffusion coefficient. The addition of equal or less than 0.7% Co, as well as application of USV is very effective in promotion of tensile strength of the hypereutectic Al–Si alloys.

Abstract: The paper presents the results of studies on the effect of the A390.0 (AlSi17Cu5) alloy overheating to a temperature of 920°C and modification with phosphorus (CuP10) on the resulting mechanical (HB, R_m, R_0.2) and plastic (A₅ and Z) properties. It has been shown that, so-called, "time-thermal treatment" (TTT) of an alloy in the liquid state, consisting in overheating the metal to about 250÷300°C above T_liq, holding at this temperature and rapid cooling, refines the structure and improves the mechanical properties. It has also been found that strong overheating of alloy above T_liq "enhances" the process of modification, resulting in the formation of fine-grain structure. The primary silicon crystals uniformly distributed in the eutectic and characteristics of the α (Al) solution supersaturated with alloying elements present in the starting alloy composition (Cu, Fe) provide not only an increase of strength at ambient temperature but also at elevated temperature (250°C), which is of particular importance for the automotive applications, especially as regards cast pistons operating in IC.

Abstract: The semi-solid microstructure evolution of hypereutectic Al-20Si-3Fe-1Mn-4Cu-1Mg alloy was studied by unidirectional compression deformation experiment, at a range of deformation temperature of 833K~873K and a range of strain rate of 0.1s^-1~0.001s^-1. The results showed that the microstructure of the reheating alloy was more spherical and fine than the microstructure of as-cast, the alloy was a positive strain rate sensitive material that the flow stress was decreased with increasing in deformation temperature and it was increased with increasing in strain rate. The mechanical properties of the alloy were hardly improved when the deformation temperature was too high to fracture the thick phase of the alloy.The lower strain rate was not only reducing the productivity but also reducing the plastic deformation. The microstructure of the alloy which the thick phase was broken fundamentally and the grain became further refinement can be obtained at 833K~853K, and at 0.1s^-1~0.01s^-1.It can be done that reducing the plastic deformation resistance and strengthening the fabrication procedure of the alloy.

Abstract: The hypereutectic Al-20%Si alloy was fabricated by liquid-liquid mixing of Controlled Diffusion Solidification (CDS), and the mixing interface of two precursor alloys as well as the effects of pouring temperature during liquid-liquid mixing process on microstructure of size, morphology and distribution of primary silicon were studied. The results show that the size of primary phase decreases as the pouring temperature decreases, and the distribution of primary phase becomes uniform, but the morphology changes unobvious. Meanwhile, the liquid-liquid mixing interface is divided into four areas: low temperature alloy area, interface front area, fine grain area and high temperature alloy area.

Abstract: The effect of T6 heat treatment on dry sliding friction and wear characteristics of Al-20Si-5Cu under a fixed sliding speed of 200r/min and sliding time of 30mins was illustrated in this paper. The result reveals that T6 heat treatment can improve morphology of primary Si and Al₂Cu particles, enhance the hardness of the matrix. Wear rate increased with increasing of applied load. T6 heat treated samples have high wear resistance which is related to the improvement of microstructure, particularly with their increased hardness of matrix. But friction coefficient varies with heat treatment processes and has no sensitivity to applied loads, which fluctuates in a small range, so the friction coefficient can be considered to be stable. The wear mechanisms of T6 heat treated samples are also changed with increasing of applied load, from oxidative wear to fatigue wear.

Abstract: By using some types of means such as Optical Microscope (OM), Scanning Electron Microscopy(SEM), the testing of tensile mechanical properties and the testing of friction and wear, the impacts of the Al-5Ti-1B and Al-20Sr modification on the Al-16Si-4Cu-0.5Mg-0.2Mn alloy microstructure, mechanical properties and friction properties are researched. The results turn out that the modification can significantly refine the primary silicon and the eutectic silicon in the alloy, the Al-5Ti-1B mainly refined the primary silicon in the alloy, the Al-20Sr refined the eutectic silicon; the alloy’s effect of refinement after compound modification is better than that in separate metamorphism. Modification can improve the tensile strength and elongation of the alloy: the tensile strength of the alloy has been increased by 65MPa after its compound modification; also the elongation by 0.4%. Modification can improve wear-resisting property of the alloy and also its effect of compound modification is better than that of separate metamorphism. The modification mechanism of Al-5Ti-1B is that Al3Ti and TiB2 belongs to heterogeneous nucleation; while the modification mechanism of Al-20Sr is that the strontium changes the growth pattern of Si phase.

Abstract: In this paper the effect of Ca on the formation of Si phases during solidification of commercial purity Al-15Si alloy was studied. The Ca impurity level of the commercial purity alloy was 200 ppm which was sufficient to lead to a modified Al-Si eutectic. After the addition of K₂SiF₆ flux the Ca impurity level was 20 ppm which was insufficient to modify the eutectic Si, but primary Si was refined from 48 µm to 20 µm. The refinement of primary Si in Al-15Si alloy fluxed with K₂SiF₆, suggests that when Ca level is reduced to less than 20 ppm the impurity level of P (≈20 ppm) in commercial purity hypereutectic Al-Si alloys is sufficient to refine the primary Si without any deliberate addition of P. In the case of adding 0.5 wt.% Ca to the Al-15Si alloy, the eutectic Si was highly modified and the primary Si was coarse and irregular in morphology (unrefined). The addition of such a high level of Ca led to enhanced quantities of entrained oxide inclusions/bifilms. The first phase to solidify was Al₂CaSi­₂ which nucleated on the oxide bifilms. Primary Si then formed by nucleation and growth on the Al_2­CaSi₂ particles.

Abstract: The effect of superheating and cooling rate on primary precipitation of Si was studied in hypereutectic Al-Si alloys. Alloys with compositions of 15, 18 and 20 wt% Si were solidified in unidirectional solidification equipment from different temperatures and drawing speeds. The fraction of primary silicon was measured over a certain distance of the sample in the steady state region. Results show a large variation in primary silicon fraction along the sample length and with varying cooling rate. The fraction of primary silicon and primary aluminium around silicon increases with increasing superheat. These fractions decrease with increasing cooling rate and the structure changes to a more refined dendritic-like primary silicon. Different morphologies of Si and their transformation during solidification can be seen over the sample length. An analysis of the quenched solidification front shows the possibility of strong convection ahead of the solidification front. The convection can be caused by density variation in the liquid due to the cluster structure of the melt which changes the microstructure.