Papers by Keyword: Primary Silicon

Abstract: Normally, the microstructure of eutectic and hypereutectic Al-Si alloys consists of Al-Si eutectic or Al-Si eutectic + Si_I, that is why they have good wear-resistance properties, but their ductility decreases somewhat, which can be improved by the appearance of non-equilibrium α-aluminum grains. In present work, a recrystallization and partial melting (RAP) technique was applied in the preliminary research to investigate the solidification behavior of the eutectics at high cooling rate. The results show that numerous α-Al dendrites appeared instead of the eutectic structure. In the followed researches, the combination of some technological procedures, based on the competitive growth of dendrites and eutectics, such as heterogeneous nucleation and high cooling rate, were applied: A413.0 and A390.0 alloys were melted in resistant furnace and poured into a mold, made of copper at one side and of steel at another, which is connected to the temperature measuring device via four K-class thermocouples. The various pouring regimes: gravity, via 45⁰ tilt cooling slope and injection with Argon gas for 2 min. before pouring, were applied at different temperatures of 680, 650, 630⁰ C. The results show that the hetero-structure of eutectic and hyper-eutectic Al-Si alloys, consists of non-equilibrium α-Al grains, primary silicon and eutectics was obtained. Grain size varies with cooling rate, with minimum value of 10 μm when the specimen thickness is 5 mm. Non-dendritic grains were achieved with semi-solid treatment (pouring via cooling slope). The elongation of alloy is expected to be enhanced due to the appearance of α-Al grains.

Abstract: s: The semi-solid slurry of Al-25%Si hypereutectic aluminum alloy was prepared through a copper serpentine pouring channel, the effect of pouring temperature and numbers of channel bend on the slurry microstructure was investigated. The results show that the primary silicon grains in Al-25%Si hypereutectic alloy solidified at a traditional condition are very large and the average silicon grain size is about 65.3μm, however, when the liquid Al-25%Si alloy is poured through a copper serpentine pouring channel, the primary silicon grains are fined obviously. If the channel has three bends and the pouring temperature is 747°C,the average silicon grain size in the slurry is about 33.7μm. If the channel has four bends and the pouring temperature is 747°C, the average silicon grain size in the slurry is about 30.9μm. If the channel has seven bends and the pouring temperature is 747°C, the average silicon grain size in the slurry is about 28.6μm. The analysis shows that the chilling effect of the inner channel wall precipitates primary silicon nuclei, and so the primary silicon grains of Al-25%Si alloy are fined greatly. Meanwhile, the subsequent washing of the alloy melt also promotes the separation of primary silicon grains from the inner wall and the primary silicon grains are further fined. The above research work demonstrates eventually that the copper serpentine pouring channel process is a good method for fining the primary silicon grains in hypereutectic Al-25%Si alloy rather than using chemical fining agent phosphorus as in traditional process.

Abstract: In order to study the possibility of refining the primary silicon grains in the microstructure of hypereutectic Al-Si alloy through a serpentine pouring channel, the semi-solid slurry of A390 aluminum alloy was prepared through a water-cooled copper serpentine pouring channel, which is a new method proposed recently for semi-solid forming process, and the effect of pouring temperature on the slurry microstructure was investigated. The results show that the slurry of A390 aluminum alloy with refined primary silicon grains can be prepared under given conditions and especially when the pouring temperatures is 690°C, the primary silicon grains can be refined obviously, the equivalent silicon grain size is 19.7mm and the average shape factor is about 0.7. The analysis shows that the chilling effect of the inner channel wall precipitates a large number of primary silicon nuclei, and so the primary silicon grains are refined greatly. Meanwhile, the subsequent alloy melt washing also promotes the separation of primary silicon grains from the inner wall and the primary silicon grains are further refined. The work undertaken demonstrates eventually that the serpentine pouring channel process is a good method for refining the primary silicon grains in hypereutectic A390 alloy rather than using chemical fining agent phosphorus as in traditional process, and provides an alternative process choice.

Abstract: P-doped γ-Al₂O₃ was found to be a potent substrate to nucleate primary silicon whilst good modification of the eutectic matrix is retained during solidification of hypereutectic Al-Si alloys. On using P-doped γ-Al₂O₃ could be a perfect and clean source of P without additional impurities. The optical micrographs show that the morphologies of primary silicon crystals in solidification of Al-18Si alloy are changed from irregular coarse morphologies to fine regular particles. The average particle size of primary Si decreased from 52 μm to 25 μm and 22 μm in adding P-doped α-Al₂O₃ and P-doped γ-Al₂O₃ respectively. It was clear that P-doped γ-Al₂O₃ led to good refinement of primary Si and the modification effect on eutectic Si was retained in solidification of commercial purity Al-18Si alloy. Adding P-doped γ-Al₂O₃ give a good primary Si refinement to Al-18Si alloy if compared with the addition of P and using finer P doped γ-Al₂O₃ powder give narrower particle size range similar to that of adding P.

Abstract: The morphology of eutectic and primary silicon phases was analyzed by OM and SEM. OM and SEM results show that pure Nd can significantly refine both eutectic and primary silicon of hypereutectic Al-20%Si alloy. Morphology of primary silicon is transformed from star-shaped and irregular morphology to fine polyhedral and grain size of primary silicon is refined from 80~120 μm to 20~50 μm. Friction and wear resistance tests show that friction coefficient of Al-20%Si alloy reduces after Nd modification. Wear resistance of Al-20%Si alloy after modification is significantly improved as compared to the initial sample. The dominant wear mechanism for 0.3% Nd modified alloy is abrasive wear, adhesive wear and oxidative wear mechanism, but wear mechanism for unmodified alloy is abrasive wear and adhesive wear mechanism.

Abstract: The modification mechanism was studied by OM, SEM, XRD and Trace Element Analysis. The results of OM and SEM analysis show that pure Nd can effectively refine primary and eutectic silicon in hypereutectic Al-20%Si alloy. Morphology of primary silicon is transformed from pentalpha to block and the average grain size of primary silicon is reduced from 80~120μm to 20~50μm after modification. Trace Element Analysis results show that Nd can purity base metal and reduce impurity contents. XRD patterns show that no new phase formed after Nd modification. The results of mechanical properties test show that whole properties of modified sample are significantly improved. Tensile strength increases about 35.8% from 117 MPa to 148 MPa after modification, Elongation change increases about 193% from 0.56% to 1.64%. The improvement of mechanical properties should be attributed to fine primary Si phase and eutectic silicon phase and purification of base metal after modification.

Abstract: The modification mechanism of hypereutectic Al-17.5%Si alloy with Nd was studied by OM and XRD. The results of OM analysis show that pure Nd could effectively refine primary in hypereutectic Al-17.5%Si alloy. When Nd addition is 0.3%, the average grain size of primary silicon reaches its minimum. The average grain size of primary silicon is reduced from 40~60μm to 10~30μm after modification. XRD patterns show that no new phase forms after Nd modification. The results of mechanical properties tests show that whole properties of modified samples are significantly improved. When Nd addition is 0.3%, tensile strength of the alloy reaches its maximum. Tensile strength is increased about 35.8% from 120 MPa to 163 MPa. Elongation was increased about 175% from 0.8% to 2.2%. The improvement of mechanical properties should attribute to fine primary after modification.

Abstract: Alternating traveling magnetic field (TMF) was introduced to agglomerate the inclusions with a density smaller than surrounding melt. Primary silicon particles precipitating from the solidification process of hypereutectic Al-Si alloy was regarded as inclusions need removing. Results indicated that alternating TMF was more effective to promote the inclusions to agglomerate into clusters than downward TMF. The effect of alternating TMF to agglomerate the inclusions increases with the increase of current and frequency. There exists the best alternating time to get the best agglomeration effect. In this study, 10s is the best alternating time.

Abstract: Hypereutectic Al-Si-Cu alloys which are typical light-weight wear-resistant materials, are required to improve the ductility as well as the strength and wear-resistance for the wider applications. Increase in amounts of primary silicon particles causes the modified wear-resistance of hypereutectic Al-Si-Cu alloys, however, it leads to the poor strength and ductility. It is known that dual phase steels composed of hetero-structure have succeeded to bring contradictory mechanical properties of high strength and ductility concurrently. In order to apply the idea of hetero-structure to hypereutectic Al-Si-Cu alloys for the achievement of high strength and ductility along with wear resistance, ultrasonic irradiation to molten metal during the solidification, which is named sono-solidification, was carried out from its molten state to just above the eutectic temperature. The sono-solidified Al-17Si-4Cu alloy is composed of hetero-structure, that is, hard primary silicon particles, soft non-equilibrium α-Al phase and eutectic region. Rheocasting was performed at just above the eutectic temperature with sono-solidified slurry to shape a disk specimen. After the rheocasting with modified sono-solidified slurry held for 45s at 570^oC, the quantitative optical microscope observation exhibits that the microstructure is composed of 18area% of hard primary silicon particles and 57area% of soft α-Al phase, in contrast there exist only 5area% of primary silicon particles and no α-Al phase rheocast with normally solidified slurry. Hence the tensile tests of T6 treated rheocast specimens with modified sono-solidified slurry exhibit the improved strength and 5% of elongation, regardless of more than 3 times higher amounts of primary silicon particles compared to that rheocast with normally solidified slurry.

Abstract: The present work introduces a novel ultrasonic DC casting process which allows producing billets of hypereutectic Al-Si alloys with greatly refined and uniformly distributed particles of primary Si. In the process, ultrasonic vibrations are introduced into an Al-17Si-0.01~0.03P melt in a hot top positioned on the mold by using a high-amplitude ceramic sonotrode. The hot top design and sonotrode arrangement provided a highly effective cavitation treatment of the melt in the hot top and well-controlled flow in the sump. A simplified model is proposed to explain the obtained results.