Morphology Of Primary Silicon Research Articles

Effect of Sr on the microstructure and mechanical properties of hypereutectic Al-Si alloy

The hypereutectic aluminum silicon alloy (Si: 20wt.%) was modified by Al-10%Sr modifier. The results show that Sr has a significant refining effect on both primary and eutectic silicon in aluminum silicon alloy (Si: 20wt.%). When the additional amount of Sr reaches 0.4%, the grain size is refined from 80-100 μm without modification to 30-50 μm after modification. The morphology of primary silicon has changed from a large plate-like shape to a deformed fine fishbone shape, while eutectic silicon has changed from a long needle shape to a point shape. The X-ray diffraction analysis results indicate that the Al2Si2Sr phase was generated in the modified alloy. The scanning electron microscopy analysis results show that after 0.4% Sr modification, a white triangular-shaped Al2Si2Sr phase appears in the aluminum silicon alloy (Si: 20wt.%) sample, which is the Al-Si-Sr ternary phase analyzed by XRD testing. The tensile testing results show that the Sr modification additional amount is 0.4%, and the tensile strength of aluminum silicon alloy (Si: 20wt.%) has been improved by about 11% from 162 MPa without modification to 200 MPa after modification.

The effect of phosphorus on solidification behaviour of undercooled Al\u201370\xa0wt.%Si alloys

Effect of refining element phosphorus (P) on the morphology of the primary silicon in the Al–70 wt.%Si alloy was investigated via the electromagnetic levitation (EML) technique. The morphology and microstructure were analyzed by using high-speed video (HSV) and scanning electron microscopy (SEM). It was found that the morphology of primary silicon transformed from dendrites with several branches to blocky shape, and then to equiaxed grains in Al–70 wt.%Si and Al–70 wt.%Si–1.0 wt.%P alloys with increasing of undercooling. The nucleation number and nucleation rate increased exponentially with the increase of undercooling for both alloys. Finally, the growth velocity of primary silicon was discussed in combination with classical theory.

Microstructure and mechanical properties of hypereutectic aluminium‐silicon alloy annealing at liquid‐solid region

AbstractThe microstructure and mechanical properties of hypereutectic Al−Si alloys containing 16 wt.%, 21 wt.% and 30 wt.% Si (AlSi16, AlSi21 and AlSi30) have been investigated in “as‐cast” and “annealed at the liquid‐solid region”. The results show that the morphologies of primary Silicon in AlSi30 changed hardly after being annealed at 587 °C for different time. The morphology of eutectic silicon in all Al−Si alloys changed from flake‐like to granular after being annealed at 587 °C for 5 min to 15 min. With the increase of silicon content, the average size of the granular eutectic silicon decreased. The spheroidized effect for the eutectic silicon in AlSi21 was the best. After being annealed for 15 min, the tensile strength of AlSi16, AlSi21 and AlSi30 alloys decreased by 10.4 %, 11.7 % and 11.5 %, respectively. Their elongation increased 61.1 %, 50.0 % and 33.3 %, respectively. The comprehensive mechanical properties of AlSi16 and AlSi21 alloys were improved markedly after being annealed at 587 °C for 15 min.

Effect of cooling rates on T6 Treatment of B390 Aluminium-Silicon Hypereutectic alloys

This research aimed to study an effect of cooling rates on T6 treatment process of B390 aluminium hypereutectic alloy. B390 casting samples were casted with pouring temperature of 710°C and solidified in three different cooling rates of 33.33, 28.60 and 22.22°C/s, respectively using three metal moulds. After that samples were subjected to T6 treatment: solution treated at 510°C for 30 min and aged at 200°C at various times. However, after ageing, hardness values of as-casted samples reduced with increasing cooling rate. It was found that the specimen cooled with the highest cooling rate exhibited the highest hardness. Peak hardness values of samples cooled with cooling rate of 33.33, 28.60 and 22.22°C/s after ageing obtained from ageing time of 3, 6 and 8 hour, respectively. Furthermore, the result showed that morphology of primary silicon, eutectic silicon and Ali5(Mn, Fe)3Si2 phase presented in the aged specimen cooled with the highest cooling rate exhibited more globular, finer and distributed more evenly compared with the slower cooled samples. It can be concluded that rapid cooling rate increases concentration of a-solid solution resulted in shorter aging time.

Influence of direct electric current on solidification process of Al–Si alloy

The effect of direct electric current on redistribution and microstructure of primary silicon in Al–Si melt during the solidification refining process was studied. Electric current generates a Joule heat, which slows down the cooling rate of the furnace. The precipitated area of primary silicon decreases from 100% to 36.67% with the increase of current density from 0A/m2 to 1.5×106 A/m2. Furthermore, direct current causes a convection leading to the migration of primary silicon in the opposite direction of electric field, which forms the accumulation area. The morphologies of primary silicon and the redistribution of B impurity were investigated under the effect of electric current.

Solidification of the Undercooled Al-Si Alloy Containing 1.0 PctRE

Al-80 pctSi-1.0 pctRE alloy was levitated and melted using the electromagnetic levitation facility in combination with a laser heating unit. The growth morphologies of primary silicon were observed using a high-speed video, and the microstructure was analyzed by the scanning electron microscopy. The morphologies of primary silicon at low, intermediate, and high undercooling are dendrites, fragmented bulks and granular grains, and equiaxed grains, respectively. In addition, the growth velocities of primary silicon were measured, which were consistent with the theoretical prediction. The microstructure refinements of primary silicon played a dominant role in its large microhardness, which increased with the increase of undercooling. Moreover, the hardening effect of dendritic structure was stronger than that of equiaxed grain.

Microstructure and wear behavior of in-situ hypereutectic Al–high Si alloys produced by selective laser melting

In this study, almost dense hypereutectic Al-50 Si alloys were fabricated in situ by selective laser melting (SLM) from the mixture of Al and Si powders under argon atmosphere. The effects of laser power on the microstructure and mechanical properties were determined regarding the size and morphology of primary Si phase (proeutectic Si). The phase analysis shows that the main phase of SLM-processed Al-Si alloys consists of Al and Si for all samples. Additionally, due to the vaporization behavior of Al, the Si content increases after melting process. The primary Si phase presents a smaller size (about 5μm) than that in the conventional produced hypereutectic Al-Si alloys. Moreover, while the laser power increases, the spherical morphology of primary silicon turns into irregular shape. Besides, the maximum value of microhardness (188 Hv) was detected for the samples obtained at laser power of 320W. The sample obtained at laser power of 350W shows the lowest wear rate (5.5×10−4mm3N−1m−1).

Morphologies of Primary Silicon in Hypereutectic Al-Si Alloys: Phase-Field Simulation Supported by Key Experiments

We realized a three-dimensional visualization of the morphology evolution and the growth behavior of the octahedral primary silicon in hypereutectic Al-20wtpctSi alloy during solidification in a real length scale by utilizing the phase-field simulation coupled with CALPHAD databases, and supported by key experiments. Moreover, through two-dimensional cut of the octahedral primary silicon at random angles, different morphologies observed in experiments, including triangle, square, trapezoid, rhombic, pentagon, and hexagon, were well reproduced.

Influence of Neodymium on Wear Resistance of Hypereutectic Al-Si Alloy

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.

Modification of Hypereutectic Al-20%Si Alloy by Neodymium

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.

Effect of Fe on the Microstructure and the Thermal Storage Performances of High-Silicon Aluminum Alloy

The influence of Fe on microstructure and the expansion coefficient and thermal conductivity of Al-30 wt.% Si alloy was studied. Results show that the primary silicon morphology and size changed significantly after joining the Fe, by angular blocky primary silicon sheet or plate into small pieces and then into a polygonal large lump, edge and angle are passivated. As the content of Fe is 0.3 wt.%, material expansion coefficient and high thermal conductivity. Later, with the increase of the content of Fe, alloy point defect and line defect, surface defect and large area defect increase, the thermal conductivity of materials and inflation performance declined. When the content of Fe is 0.1 wt.%, materials with high thermal conductivity and low thermal expansion coefficient, the heat storage performance impact is minimal.

Effect of Zn addition on primary silicon morphology and B distribution in Si–Al alloy

Si–Al–Zn alloy melts were used for silicon purification by the solvent refining process. The effect of metal Zn addition on alloy macrostructure, primary Si morphology, as well as B distribution in Si–Al alloy during solvent refining process was studied in this work. The three main results were as follows. First, with the increasing of Zn content in Si–Al alloy, the primary silicon changed from slender plate-like to irregular and short, meanwhile the distribution of primary silicon changed from bottom to central and top. Second, the more Zn content in Si–Al alloy, the more inclusions of eutectic melts will be in primary silicon, which results in diffusion difficulty of impurity B in inclusions. Third and last, with increasing Zn content, the refining ratio of B increased gradually and removal fraction of B decreased accordingly.

Effects of Mixing Melt Treatment and Heat-Treatment on the Microstructure and Wear Resistance of Al-22%Si Alloys

The effect of hypereutectic Al-22%Si alloys prepared by means of Controlled Diffusion Solidification (CDS) followed by subsequent heat-treatment on the microstructure and sliding abrasive wear behaviors is investigated. The results show that the primary silicon morphology, size and distribution can be improved by CDS, especially for the case of liquid-liquid mixing. After heat-treatment, the eutectic silicon can be spheroidized. Under the same load, the wear rates for the identically mixed and heat-treated Al-22%Si alloys are lower than those unheat-treatment, and the wear rate of liquid-liquid mixed and heat-treated Al-22%Si alloy reach its minimum. The wear mechanism of traditional cast Al-22%Si alloy includes adhesive wear and fatigue breaking-off, while the liquid-liquid mixing can effectively restrain flaking on the worn surface and avoid the fatigue breaking-off, the wear mechanism is changed into abrasion wear.

Review of Microstructure Evolution in Hypereutectic Al–Si Alloys and its Effect on Wear Properties

Al–Si alloys with silicon content more than 13 % are termed as hypereutectic alloys. In recent years, these alloys have drawn the attention of researchers due to their ability to replace cast iron parts in the transportation industry. The properties of the hypereutectic alloy are greatly dependent on the morphology, size and distribution of primary silicon crystals in the alloy. Mechanical properties of the hypereutectic Al–Si alloy can be improved by the simultaneous refinement and modification of the primary and eutectic silicon and by controlling the solidification parameters. In this paper, the effect of solidification rate and melt treatment on the evolution of microstructure in hypereutectic Al–Si alloys are reviewed. Different types of primary silicon morphology and the conditions for its nucleation and growth are explained. The paper discusses the effect of refinement/modification treatments on the microstructure and properties of the hypereutectic Al-Si alloy. The importance and effect of processing variables and phosphorus refinement on the silicon morphology and wear properties of the alloy is highlighted.

Influence of Overheating Temperature on the Shape of Primary Silicon Crystals in Hypereutectic Al-Si Cast Alloys

The paper presents the results of microscopic examinations of the growth of primary silicon crystals in hypereutectic A390.0 (AlSi17Cu5Mg) silumin. A diagram of the growth of primary silicon crystals in the melt without overheating and superheated to a temperature of 920°C has been presented. From the experimental results obtained on crystal morphologies of primary silicon, different sequences of crystal growth of the primary silicon can be expected. In the silumin without overheating, five-pointed star-shaped particles form, after superheating, the crystals of primary silicon take more compact shapes. This structure provides better mechanical properties and thus increases the area of application of alloys with hypereutectic silicon content.

Effect of Heat Treatment on Microstructure and Mechanical Properties of A390 Alloy

In the present work, the effects of heat treatment on microstructure and mechanical properties of A390 alloy were investigated. The results show that the as-cast microstructure of A390 alloy mainly consists of primary silicon, α-Al, eutectic silicon and Al2Cu phase. The morphology of primary silicon is irregular polygonal block with sharp edges. Eutectic silicon exhibits a coarse plate-like and acicular morphology and the fishbone like Al2Cu phases are gathered at the grain boundary. During solution treatment, eutectic silicon undergoes fragmentation and spheroidization and T6 heat treatment has a profound effect on the dissolution of Cu and Mg. However, the morphology and size of primary silicon changes little. After the aging process, Al2Cu phases are precipited as lamellar morphology. As a result, the mechanical properties the heat treated A390 alloy increases significantly after T6 treatment.

Microstructural Evaluation of Semi-Solid and Thixoformed Parts of LM28 Aluminum Alloy

Attractive physical and mechanical properties of aluminum alloys make them very interesting for the automotive industry. The commercial way for manufacturing LM28 alloy is die-casting, but this process encounters several problems such as shrinkage and gas porosities. Their good mechanical properties and high resistance to wear are because of the presence of hard primary silicon particles distributed in the matrix. Therefore, the size and morphology of primary silicon and also the structure of α-Al particles in hypereutectic Al–Si alloys influence the mechanical properties of the alloys. In this research, a new process of manufacturing of this alloy has been developed using LM28 feedstock produced through cooling slope casting. The feedstocks produced via cooling slope casting had a partial globular structure that contained globules, rosettes and dendrites of α-Al. These feedstocks were thixoformed under three different pressures. The primary dendrites and rosettes changed to globular structure. The microstructure of thixoformed parts contained α-Al globules, small primary Si particles dispersed between these globules, and Al-Si eutectic phase. The mechanism of the formation of α-Al globules by this process was explained. Microstructures of as cast specimens, feedstocks produced via cooling slope, specimens that were heat treated in the semi-solid temperature and thixoformed specimens were studied with optical microscope and image analysis. The morphological changes during these processes were interpreted.

Effect of Nd on microstructure and wear resistance of hypereutectic Al–20%Si alloy

In this paper, pure Nd was adopted to modify hypereutectic aluminum–silicon alloy (Al–20%Si). The morphology of eutectic and primary silicon phases was analyzed by OM and SEM. OM and SEM results show that pure Nd (0.3 wt.%) can significantly refine both eutectic and primary silicon of hypereutectic Al–20%Si alloy. Morphology of primary silicon was transformed from star-shaped and irregular morphology to fine polyhedral and grain size of primary silicon was refined from 80–120 μm to 20–50 μm. TEM results show that a new needle shape ternary phase (AlSi x Nd y ) forms in modified alloy. XRD results show that three little unknown diffraction peaks appear after Nd modification and it is induced that they should be diffraction peaks of ternary AlSi x Nd y intermetallic phase in the modified alloy by analyzing both TEM and XRD results. Friction and wear resistance tests show that friction coefficient of Al–20%Si alloy decreases after Nd modification. Wear resistance of Al–20%Si alloy after 0.3 wt.%Nd modification was significantly improved as compared to the initial sample. The improvement of wear resistance was mainly attributed to change of morphology, size and distribution of eutectic silicon and primary silicon after Nd modification. The dominant wear mechanism for 0.3 wt.%Nd modified alloy was abrasive wear, adhesive wear and oxidative wear mechanism, but wear mechanism for unmodified alloy was abrasive wear and adhesive wear mechanism.

Effect of intensive shearing on morphology of primary silicon and properties of hypereutectic Al–Si alloy

Al–17Si–0·01Sr alloy melt was intensively sheared in the liquid state using the melt conditioning by advanced shear technology (MCAST) process before high pressure die casting (HPDC). The resultant microstructure and mechanical properties were examined and compared with those of the same alloy produced by the conventional HPDC process. It was found that intensive melt shearing can result in fine and equiaxed primary silicon particles uniformly distributed in the aluminium matrix, giving rise to much improved ductility and wear resistance.

Morphologies of primary silicon in hypereutectic Al–Si alloys with melt overheating temperature and cooling rate

The morphologies of primary silicon were studied in detail with different cooling rates and melt overheating temperatures. According to the present results, the morphologies of primary silicon are a strong function of the solidification conditions such as cooling rate and melt overheating temperature. With the elevation of melt overheating temperature, the morphologies of primary silicon will change from star-like and other irregular primary silicon to octahedral primary silicon and the size of primary silicon will gradually decrease. On the other hand, it can be safe to deduce that the Si–Si cluster of larger size should be beneficial for the formation of the nucleus of star-like and other irregular primary silicon and the Si–Si cluster of smaller size should be beneficial for the formation of the nuclei of octahedral primary silicon. With increasing cooling rate, the size of primary silicon will gradually decrease. If the cooling rate is lower than a certain (or critical) cooling rate, the cooling rate will have hardly effect on the morphologies of primary silicon in the solid state. If the cooling rate is higher than a certain (or critical) cooling rate, the cooling rate will play an important role in determining the morphologies of primary silicon in the solid state.