Primary Si Crystals Research Articles

Optimizing microstructures and mechanical properties of hypereutectic Al-18%Si alloy via manipulating its parent liquid state

Liquid state manipulations on solidified Al-18%Si structures and properties were investigated based on its liquid abnormal resistivity-temperature behaviors (LARTB), which suggested a temperature-induced liquid-liquid structural transition (TI-LLST). The primary Si crystals are dramatically refined if solidified from the melt after LARTB range, compared with those before and during it. And then, solidified structures are further ameliorated by suitably mixing the high-temperature melt with low-temperature melt in semisolid state, generating increased tensile strength and ductility synchronously. This work elicits that liquid manipulations for optimizing materials structures and properties can be more effective if information suggesting TI-LLST is detected beforehand.

Influence of melt processing with electromagnetic acoustic fields on the structure and properties of Al–Si system alloys

The research on the influence of melt processing with electromagnetic acoustic fields on the structure and properties of binary alloys Al–12%Si and Al–20%Si was conducted. The frequency of electromagnetic field induced in the loop antenna was changed between 500, 1000 and 2000 kHz during the experiments. Melts were processed after their degassing and refining. It was determined that this method of melt processing reduces the average total time of alloy production by 12 %. Short-term treatment of melts with electromagnetic acoustic fields helps to grind main phase alloy components and to improve their mechanical properties. Grinding of α-Al dendrites (from 30 to 22 μm) and eutectic Si crystals (from 13 to 10 μm) was observed while processing Al–12%Si eutectic alloy with a frequency of 500 kHz. At the same time, Al–20%Si hypereutectic alloy treatment with a frequency of 1000 kHz led to reduction of eutectic Si crystals from 8 to 5 μm, and of primary Si crystals – from 90 to 62 μm. Under specified processing conditions the tensile strength of Al–12%Si eutectic alloy increased by 13 %, and elongation – by 17 %, the same mechanical properties of Al–20%Si hypereutectic alloy increased by 9 and 65 % respectively. Based on the studies performed it was concluded that the choice of parameters for Al–Si melt processing with electromagnetic acoustic fields depended on the silicon content in the alloy. The increase in silica concentration needs treatment with the waves of higher vibration frequency. This processing technique allows modifying the fine crystalline structure of alloys and as a result causes an improvement of their mechanical properties. It can be successfully used for the production of fine-grained ligatures and Al–Si system alloys. However, further research is required to determine optimal processing parameters depending on the structure of the original charge and the nature of the alloys.

The Microstructure And Mechanical Properties Of The AlSi17Cu5 Alloy After Heat Treatment

Abstract In the paper results of the microstructure and mechanical properties (HB, Rm and R0,2) of AlSi17Cu5 alloy, subjected by solution heat treatment (500ºC/6h/woda) and aging (200ºC/16h/piec) are presented. In next step the alloy was modified and heated significantly above the Tliq temperature (separately and together). It was found that the increase in the strength properties of the tested alloy after heat treatment compared to alloys without solution heat treatment and aging was due to precipitation hardening. The applied aging treatment of ingots (preceded by solution heat treatment), causes not only increase in concentration in α(Al) solid solution, but also a favorable change of the primary Si crystals morphology. During stereological measurements significant size reduction and change in the morphology of hypereutectic silicon crystals ware found. This effects can be further enhanced by overheating the alloy to a temperature of 920ºC and rapid cooling before casting of the alloy.

Effects of melt mixing and AlTi5BRE10 inter alloy on solidification of microstructure of hypereutectic Al–Si alloy

The solidification of microstructure of hypereutectic Al-20%Si alloy obtained by mixing high-temperature Al-30%Si alloy melt and low-temperature Al-10%Si alloy melt in a mixing ratio of 1 : 1 was investigated by optical microscopy. A certain amount of AlTi5BRE10 inter alloy was added into high-temperature Al-30%Si alloy melt before mixing. The results showed that the primary Si crystals were refined and passivated by the melt mixing process and a certain amount of AlTi5BRE10 inter alloy further refined the primary silicon and α-Al phase. The coarse plates, petals and irregular triangular-shaped primary Si crystals turned into fine block shapes, and the α-Al phases were spherical. After the melt mixing treatment, the microstructure of the hypereutectic Al-20%Si alloy was composed of a primary α-Al phase, eutectic (α + Si) phase, and the primary silicon, which caused the hypereutectic Al-20%Si alloy to exhibit typical hypoeutectic and hypereutectic characteristics.

Si Purity Control and Separation from Solidification of Al–30Si Melt under Pulse Electromagnetic Field

Purification of metallurgical-grade silicon (MG–Si) by a combination of Al-Si solidification refining and electromagnetic oscillating separation and acid leaching collection has been studied. The primary Si crystals and Al-Si alloy in hypereutectic Al-30%Si melt were separated during solidification under the pulse electromagnetic field (PEF). The results show that the Si content in Si-rich layer increases with increasing discharging frequency. The typical metallic impurities (Fe, Ti, and Ca) have removal fraction higher than 99.5%. The removal fractions of the impurities B and P which are more difficult to remove are over 90% and 85%, respectively.

Effect of Ga Addition on Morphology and Recovery of Primary Si During Al–Si Alloy Solidification Refining

Abstract The effect of Ga addition on alloy macrostructure, morphology and recovery rate of primary Si during the Al–Si–Ga alloy solvent refining process of silicon was studied in this work. The addition of Ga to Al–Si alloy could change the morphology of the primary Si. The average plate thickness of the primary Si increases with increase of Ga content. With the increase of Ga content, the average plate length of the primary Si crystals becomes larger when the Ga content is less than 5% in the Al–30%Si–xGa alloy, but becomes smaller when the Ga content exceeds 5%. Al–Si–Ga alloys consist of three types, primary Si, Ga x Al1–x , (α-Al+Si+β-Ga) eutectic. (111) is the preferred growth surface of the plate-like primary Si. The recovery rate of the primary Si increases with the increase of Ga content. When the Ga content increased to 20% in Al–30%Si–xGa alloy, the relative recovery rate of the primary Si increased to 50.41% than that in Al–30%Si alloy.

Refining and nucleating behaviors of AlP on primary phase in Al–Ge and Al–Ge–Si alloys

The present study addresses refining behavior of AlP on primary phases in Al–Ge and Al–Ge–Si alloys. In binary hypereutectic Al–70Ge alloy, the addition of Al–P master alloy leads to a refinement performance for primary Ge. It is found that AlP can act as the nucleation sites for primary Ge particles, and between them the lattice mismatch is 3.85%. Besides, the introduction of AlP in Al–Ge alloy results in an increased undercooling for the precipitation of primary Ge. While for Al–60Ge–5Si alloy, Si and Ge form continuous solid solution, with Si distributing in the center of the primary particles and Ge the outer layer. The primary solid solutions can also be significantly refined by AlP. Moreover, due to the nucleation behavior of AlP particles, primary Si crystals doped with a certain amount of Ge are refined to about 30μm from more than 140μm in Al–20Ge–18Si alloy.

Effect of strontium addition on silicon phase and mechanical properties of Zn–27Al–3Si alloy

The evolution of silicon phase and mechanical properties of Zn–27Al–3Si alloy with Sr addition were investigated. The results showed that primary Si crystals were gradually modified from polygonal, plate-like and star-like to larger nodular with increasing the addition contents of Sr. A bimodal distribution of eutectic Si crystals was found. One was the coarse eutectic (CE) Si nucleating and growing from the primary Si crystals, the other was the fine eutectic (FE) Si nucleating at other sites (not primary Si). The average size and aspect ratio of FE Si crystals significantly decreased and whose morphology was transformed from a coarser plate-like structure to a fine fibrous structure with more branches with increasing Sr content. Through electron probe microanalysis mapping (EPMA) and cooling curves, it was concluded that the nodular primary Si crystals nucleated and grew by producing a large undercooling (about 40°C) and preferential attachment of Sr to silicon phase. The mechanical properties were investigated by tensile test with various concentration of Sr. It was found that the ultimate tensile strength (UTS) and elongation (EI) increased by 62.6% and 162%, respectively. The improvements in UTS and EI were attributed to the morphology change of silicon phase as well as the improved feeding solidification shrinkage.

Microstructure and Properties of In Situ Si and Fe-rich Particles Reinforced Al Matrix Composites Assisted with Ultrasonic Vibration

In this research, the in situ Si and Fe-rich particles reinforced Al matrix composites were fabricated by rheocasting (RC) process assisted with ultrasonic vibration (USV). After USV treatment, the polygonal primary Si crystals were refined into particles with average diameter of about 15–23 μm, and the fraction of primary Si declined to about 5.4–6.5 vol%. The coarse plate-like δ-Al4(Fe,Mn)Si2 phase was transformed into fine particles with average diameter of about 17–20 μm, and the fraction of particle-like Fe-bearing particles is about 3.6–5.3 vol%. The ultimate tensile strength of the RC composites increases with the increase of Fe content at 350 °C. The increase of the elevated temperature strength of the composites is mainly attributed to the refinement of δ-Al4(Fe,Mn)Si2 phase and the increase of the volume fraction of the Fe-bearing compounds. Compared with the composites without USV, the RC composites assisted with USV have thinner mechanical mixing layer in wear test, which corresponds to smaller wear rate.

Reinforcement of Hypereutectic Al-Si Alloys with Fe-Bearing Compounds and Si Particles Assisted with Ultrasonic Treatment

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.

Effect of Al-5Ti-C Master Alloy on the Microstructure and Mechanical Properties of Hypereutectic Al-20%Si Alloy.

Al–5Ti–C master alloy was prepared and used to modify hypereutectic Al–20%Si alloy. The microstructure evolution and mechanical properties of hypereutectic Al–20%Si alloy with Al–5Ti–C master alloy additions (0, 0.4, 0.6, 1.0, 1.6 and 2.0 wt%) were investigated. The results show that, Al–5Ti–C master alloy (0.6 wt%, 10 min) can significantly refine both eutectic and primary Si of hypereutectic Al–20%Si alloy. The morphology of the primary Si crystals was significantly refined from a coarse polygonal and star-like shape to a fine polyhedral shape and the grain size of the primary Si was refined from roughly 90–120 μm to 20–50 μm. The eutectic Si phases were modified from a coarse platelet-like/needle-like structure to a fine fibrous structure with discrete particles. The Al–5Ti–C master alloy (0.6 wt%, 30 min) still has a good refinement effect. The ultimate tensile strength (UTS), elongation (El) and Brinell hardness (HB) of Al–20%Si alloy modified by the Al–5Ti–C master alloy (0.6 wt%, 10 min) increased by roughly 65%, 70% and 51%, respectively, due to decreasing the size and changing the morphology on the primary and eutectic Si crystals. The change in mechanical properties corresponds to evolution of the microstructure.

Proeutectic Crystallisation of AlSi17Cu5 Alloy after Overheating and Modification with Al-CuP Master Alloy

The influence of strong overheating above Tliq.and modification with phosphorus in the form of CuP master alloy on the crystallisation process of AlSi17Cu5 alloy has been discussed. The study shows that with overheating of an alloy from the 3XX.X series above the melting point Tliq.(about 250÷300°C), the β phase undergoes practically complete dissolution. Then, in the liquid metal, areas with high variations in the content of the dissolved silicon appear. These are numerous microregions enriched with dissolved silicon, in which, due to overheating, partial formation of homogeneous nuclei takes place. This process is further enhanced with the use of a modifier (phosphorus) and, as a consequence of the modification process, additional "substrates" (AlP) arise in molten metal to serve as sites for the heterogeneous nucleation of primary Si crystals. In the vicinity of these microregions, some other sites with lower silicon content are formed, in which the proeutectic crystallisation of α (Al, Me) dendrites takes place. This is confirmed by the ATD thermal analysis diagrams, where the additional exothermic effect is observed after the nucleation and crystallisation of primary Si crystals and before the solidification of a α (Al) β (Si) eutectic.

Effects of rare earth Er addition on microstructure and mechanical properties of hypereutectic Al–20% Si alloy

The microstructure evolution and mechanical properties of hypereutectic Al–20%Si alloy with Er addition were investigated in the article. The as-cast samples were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Microstructural analyses demonstrated that primary Si was significantly refined from coarse polygonal, platelet-like and star-like shape to fine blocky shape, and eutectic Si structure was modified from coarse platelet-like/needle-like structure to fine coral-like fibrous structure as the addition contents of Er is 0.5%. However, the primary and eutectic Si phases became coarser when the level of rare earth Er was up to 0.8%. The mechanical properties were investigated by tensile test with various concentration of Er. It was found that the ultimate tensile strength (UTS) and elongation (El) increased by 72.5% and 72%, respectively, due to decreasing of the size and changing of morphology on primary and eutectic Si crystals, and the change of mechanical properties corresponds to the evolution of microstructure. In addition, the modification mechanism of Er on Al–20%Si alloy was also discussed.

Effect of in situ γ-Al2O3 particles on the microstructure of hypereutectic Al–20%Si alloy

In the present study, the effect of in situ γ-Al2O3 particles on the morphology and size of the primary and eutectic Si phases in hypereutectic Al–20%Si alloy has been investigated. Microstructural analyses demonstrates that in situ γ-Al2O3 particles significantly refined primary Si crystals from coarse polygonal and star-like shape to fine blocky shape with smooth edges and corners. The eutectic Si structure was modified from the coarse acicular structure to fine flake-like morphology that is intermediate between acicular and coral-like fibrous structure. In addition, the modification mechanism of in situ γ-Al2O3 particles on Al–20%Si alloy was also discussed.

Solidification of primary Si in electromagnetically levitated Al-50%Si undercooling melts

Abstract By combination of electromagnetic levitation and copper-plate quenching, deep undercooling and non-equilibrium solidification of Al-50%Si melts were realized. Through repeatedly heating and cooling, a maximum undercooling of 310 K was reached. Several different morphologies of primary Si solidified at different undercoolings were found on the quenched surfaces. With increasing of undercooling, the primary Si crystals solidified gradually changed to fine clumpy crystals from coarse faceted dendrites. When the undercooling reached the maximum value of 310 K, the microstructures contained fine granular crystals. The gain size of primary Si was refined significantly.

Effect of rare earth cerium addition on the microstructure and tensile properties of hypereutectic Al–20%Si alloy

It is well known that the mechanical properties of hypereutectic Al–Si alloys are influenced by the size, morphology and distribution of primary and eutectic Si crystals. In the present work, the microstructure evolution and mechanical properties of hypereutectic Al–20%Si alloy with rare earth cerium (Ce) additions (0, 0.3, 0.5, 0.8 and 1.0wt.%) were investigated. The as-cast samples were characterized by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron probe microanalysis (EPMA) with wavelength dispersive spectroscopic (WDS). The microstructural observation showed that primary Si crystals were significantly refined from coarse polygonal and star-like shape to fine blocky shape with smooth edges and corners, and eutectic Si phases were modified from coarse platelet-like/needle-like structure to fine fibrous structure and discrete particles with increasing the addition contents of rare earth Ce. The mechanical properties were investigated by tensile test with various concentration of Ce. It was found that the ultimate tensile strength (UTS) and elongation (El) increased by 68.2% and 53.1%, respectively, due to decreasing of the size and changing of morphology on primary and eutectic Si crystals.

Preparation of micro-porous Si particles from Mg2Si powder

Polycrystalline Si powder consisting of micro-porous particles was prepared by heating Mg2Si powder at 600°C for 40h under a pressure of 500Pa. The porous Si particles were composed of primary Si crystals ranging in size from 100nm to 5μm with pores less than 10μm in size. Porous Si particles with similar morphology were also prepared at 700°C for 50Pa and 500Pa for 10h. The formation of the porous Si particles was attributed to Mg evaporation from Mg2Si particles, followed by crystallization of Si.

Effect of Wavelike Sloping Plate Rheocasting on Microstructures of Hypereutectic Al-18 pct Si-5 pct Fe Alloys

To refine and spheroidize the microstructures of hypereutectic Al-Si-Fe alloys, a novel method of wavelike sloping plate (WSP) rheocasting was proposed, and the effect of the WSP rheocasting on the microstructures of hypereutectic Al-18 pct Si-5 pct Fe alloys was investigated. The results reveal that the morphologies of the primary Si crystal, the Al18Si10Fe5, and the Al8Si2Fe phases can be improved by the WSP rheocasting, and various phases tend to be refined and spheroidized with the decrease of the casting temperature. The alloy ingots with excellent microstructures can be obtained when the casting temperature is between 943 K and 953 K (670 °C and 680 °C). During the WSP rheocasting, the crystal nucleus multiplication, inhibited grain growth, and dendrite break-up take place simultaneously, which leads to grain refinement of the alloys.

Refinement of primary silicon of hypereutectic Al–Si alloys by ultrasonic radiation and its application for die castings

The study purpose is to investigate effects of ultrasonic radiation on refinement of primary Si crystals of hypereutectic Al–Si alloys and to examine the refining effect in die casting. The ultrasonic refining effect was examined by using Al–Si alloys without or with addition of phosphorus. The experiments included melting of alloy in a crucible, radiation of ultrasound into the melt at temperatures at least 60 K higher than the Si crystallization temperature, holding of the melt for 20–80 s and pouring of it into a Cu mold by gravity casting. The refining effect was found to be dependent on P content and holding time. The EPMA investigation revealed that the ultrasonic radiation increases the number of AlP particles acting as heterogeneous sites for the primary Si nucleation. The refining effect was significant at the holding time of 20–50 s. Then, based on the above results, the radiation conditions for the die casting were determined. A key point of the conditions was that the refining effect and melt fluidity are kept at the desired level for a period after the radiation. Microstructural investigation showed considerable refinement of primary Si crystals and Al–Fe–Mn–Si compounds. The ultrasonically treated die castings exhibited an improved tensile strength.

Effect of phosphorus modification on the microstructure and mechanical properties of DC cast Al-17.5Si-4.5Cu-1Zn-0.7Mg-0.5Ni alloy

Direct chill (DC) casting of an Al-17.5Si-4.5Cu-1Zn-0.7Mg-0.5Ni alloy with minor additions of Zr, V and Ti was performed and the effect of phosphorus modification on the solidification microstructure and mechanical properties were investigated. The results show that the addition of phosphorus refines the primary Si crystals remarkably but coarsens the eutectic Si in the DC cast billets. In the samples taken in 1/2 radius region of the billets of 100 mm in diameter, the average size of the primary Si particles in the unmodified alloy is 38 μm, while that in the modified alloys range from 13 to 26 μm, depending on the melt treatment time. The AlP particles as heterogeneous nucleation sites for the primary Si crystals were observed in the center of the primary Si particles. In the modified alloy, the achieved ultimate tensile strength is 320MPa and 433MPa respectively in the as-cast state and T6 state. The size of the primary Si particles is critical for the improvement of the mechanical properties of the alloy.