Coarse Silicon Particles Research Articles

Effect of fine silicon powders addition on improving nitridation of coarse silicon powders

AbstractThe fabrication of large silicon nitride particles via direct nitride is time‐consuming. In this work, the effect of adding fine silicon powders (5 μm) on improving the nitridation of coarse silicon powders (50, 150 μm) at 1450°C in a thermogravimetric analyzer was investigated. The addition of fine silicon particles prevents the coalescence of the melted coarse silicon particles, due to them converting to silicon nitride particles more quickly. Less time is required to complete the nitridation of coarse silicon as the amount of fine silicon increases. The nitridation of coarse silicon (150 μm) is completed in less than 23 min at 1450°C with adding 50% fine silicon. The nitridation products consist of particles similar in size to the raw silicon particles.

Comparative Study of Microstructure and Properties of Hypoeutectic Al-Si Alloys Being Cast With and Without Melt Thermal Treatment

This study investigated the microstructure and mechanical properties of several hypoeutectic Al-xSi (x = 0, 1, 3, 5, 7, 9) alloys prepared with and without melt thermal treatment. The overall study showed a trend of effective utilization of grain refiner until the transition point (3 wt.% Si) and the slope of graph between grain size vs. wt.% of Si remain similar for the samples prepared with or without chemical refinement. This also indicates that the role of Si in decreasing grain size is more dominating than the poisoning of Ti above transition point. Melt thermal-treated alloys exhibited less significant changes in grain size, especially below the transition point, but significant changes in their mechanical properties. The melt thermal treatment also causes partial refinement of eutectic Si particles by which the coarse silicon particles are broken up into smaller pieces and this has neither been observed in unrefined alloys nor refined alloys.

Combinatorial synchrotron diffraction-constitutive modelling-crystal plasticity simulation framework for direct metal laser sintered AlSi10Mg alloy

Combinatorial synchrotron and electron backscatter diffraction experiments coupled with constitutive modelling and fast Fourier transform crystal plasticity simulations have been employed to capture the mechanical behaviour of direct metal laser sintered AlSi10Mg alloy for different microstructures obtained by heat treatment. The microstructure of the as-printed sample with Al-Si eutectic cellular network transforms to coarse silicon particles on solutionising treatment at 510 °C for 1 h along with decrease in dislocation density that contributes to a decrease in strength and increase in ductility post solution heat treatment. Ageing of the solutionised specimen at 160 °C for 6 h leads to reprecipitation of silicon and improvement in strength with little change in ductility. The deformation behaviour at different length scales is captured by synchrotron diffraction and EBSD, while constitutive modelling using the Kocks-Mecking approach provides a mechanistic perspective on strain hardening of the alloy. Crystal plasticity simulations using fast Fourier transform solver are able to capture stress and strain hot spots in the as printed cellular and post solution treated non-cellular microstructures and provide information on the stress and strain evolution in the microstructure. A high throughput experimental and computational modelling protocol is established in the present investigation that can help rapidly screen different microstructures to establish processing-microstructure-mechanical property paradigm in direct metal laser sintered AlSi10Mg.

Comparative Study on Microstructure and Corrosion Resistance of Al-Si Alloy Cast from Sand Mold and Binder Jetting Mold

This investigation is focused on the corrosion evaluation of an as-cast Al-Si alloy, obtained by two different casting methods: traditional sand casting and three-printing casting, using a binder jetted mold. The experimental results are discussed in terms of chemical composition, microstructure, hardness, and corrosion behavior of two different casting parts. The microstructure and composition of the sample before and after the corrosion tests was analyzed using light microscopy (OM), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (DRX). The corrosion of the two processed castings was analyzed using anodic polarization (PA) test and electrochemical impedance spectroscopy (EIS) in an aerated solution of 3.5% by weight NaCl, similar to the seawater environment. After the corrosion process, the samples were analyzed by inductively coupled plasma/optical emission spectrometry (ICP/OES); the composition was used to determine the chloride solution after immersion times. The sample processed by binder jetting mold showed higher corrosion resistance with nobler potentials, lower corrosion densities, higher polarization resistance, and more stable passive layers than the sample processed by sand casting. This improvement of corrosion resistance could be related to the presence of coarse silicon particles, which decrease of cathodic/anodic ratio and the number of micro-galvanic couples, and the lower amount of intermetallic β Al-Fe-Si phase observed in cast alloy solidified in binder jetting mold.

Microstructural Refinement and Corrosion Resistance Improvement of Heat-Treated A356 Alloy Processed by Equal Channel Angular Pressing

The microstructure refinement, hardness and corrosion resistance of heat-treated A356 aluminium alloy processed by equal-channel angular pressing (ECAP) were investigated. ECAP was carried out at room temperature using a mold, with a channel angle of 120° via route A. Results of the investigation confirm that the flaky coarse silicon particles were effectively fragmented from 4.22 to 0.761 μm and the grain size reduced from 171 to 40 μm after four passes of heat-treated as-cast using ECAP process. ECAP processing increases the hardness of heat-treated as-cast alloy from 61 Hv to 125 HV after four passes. Heat-treated A356 alloy shows enhanced corrosion resistance from 0.0424 to 0.00149 mmy-1, after four passes. In this research, ECAP processing has been shown to improve the hardness and corrosion resistance of as-cast A356 alloy.

Effect of Equal Channel Angular Pressing Processing Routes on Corrosion Resistance and Hardness of Heat Treated A356 Alloy

Equal channel angular pressing (ECAP) via routes A and Bc was used to process heat treated A356 alloy. The samples produced a range of microstructures in order to investigate the effect of straining on its corrosion behavior in 3.5 wt. % NaCl solution and hardness. The ECAP was conducted at room temperature in a mold with channel angle of 120°. The results show that brittle coarse silicon particles were effectively fragmented into smaller size in the Al-rich matrix after the processing. The hardness and corrosion resistance test showed improvement after T5 and T6 heat treatment, preferably to T6 due to wholly changed in silicon particles morphology than T5. The hardness increased with ECAP processing from 60.66 Hv to 1133.47, 124.91 Hv after three passes route Bc and four passes route A, respectively. The evaluation of corrosion resistance of the alloy showed improvement from 0.0424 to 0.00173, 0.00149 mmy-1 after three passes route Bc and four passes route A, respectively. In this research, both strength and corrosion resistance are improved by ECAP processing of A356 alloy using both routes.

Effect of Heat Treatments on the Anodizing Behavior of Additive Manufactured AlSi10Mg

In this work, the effect of heat treatments on the anodizing behavior of additive manufactured AlSi10Mg material is studied. Specimens are subjected to artificial aging or stress release heat treatments, followed by galvanostatic anodizing in sulfuric acid. Optical and scanning electron microscopy are employed to study the microstructure and the oxide layer characteristics. Upon applying the heat treatments, the microstructure shows a gradual evolution from a continuous 3D silicon network toward separate coarse silicon particles in an aluminum matrix. The morphology of the silicon phase is found to be a deciding factor as it determines the fraction of silicon that is oxidized during anodizing. This in turn is found to have a direct impact on the anodizing efficiency and oxide layer uniformity. Thicker and more uniform oxide layers are obtained for the additive manufacture AlSi10Mg specimens after the silicon network is broken up due to the heat treatments.

Solidification Behavior and Mechanical Properties of Al-Si-Mg Alloy with Ti Addition

The solidification behavior, microstructural evolution and mechanical properties of Al-Si-Mg foundry alloy with different Ti additions were investigated in the present study. The solidification behavior of those A357 alloys was analyzed through thermal analysis. The microstructures were examined by optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The results showed that the addition of Ti could refine grains of A357 as-cast alloy due to a good restriction on the grain growth, but Ti could not refine secondary dendrite arm spacing (SDAS), thus mechanical properties of the A357 as-cast alloy did not improved significantly. After T6 heat treatment, the microstructure with α-Al dendrites with the Al-Si eutectics at interdendritic space was replaced by a homogeneous α-Al matrix with a nonuniform dispersion of discrete, spheroidizing and coarse silicon particles. Hence, compared with the as-cast alloys, both of the strength and ductility of the T6 treated alloys are significantly improved, and an optimal combination of strength and elongation of the A357 alloy can be achieved by the 0.8 wt.% Ti addition after T6 heat treatment.

Microhardness and corrosion properties of hypoeutectic Al–7Si alloy processed by high-pressure torsion

Abstract High-pressure torsion (HPT) was used to produce hypoeutectic Al–7Si alloy samples having a range of microstructures to investigate the effect of the grain refinement on its corrosion behavior in 3.5 wt.% NaCl solution for the first time. Optical microscopy measurements reveal that with the HPT processing increased from 1/4 to 10 revolutions under an applied pressure of 6.0 GPa, brittle coarse silicon particles and intermetallic phases were effectively broken into ultrafine-grained particles and redistributed homogeneously into the Al-rich matrix. Open-circuit potential and polarization curves results exhibit that corrosion resistance of the Al–7Si alloy in NaCl solution was significantly enhanced upon high torsion strains, with corrosion rate reduced from 7.41 μm y −1 for the as-received sample to 1.68 μm y −1 for the 10-turn processed sample. Electrochemical impedance spectroscopy analysis combined with characterization of the corroded samples using scanning electron microscopy and energy dispersive X-ray spectroscopy indicates that the enhancement in corrosion performance of the Al–7Si alloy is due to the breakage of coarse silicon particles and intermetallic phases, the microstructure homogeneity and the increased HPT-induced active sites. It is demonstrated that microstructure refinement through HPT processing can significantly improve both microhardness and corrosion properties of the Al–7Si alloy.

Effect of the Near-Net Shape Forming on Silicon Morphology in an Al–Si Functionally Graded Material Generated by the Centrifugal Method

A work toward practical usage of hypereutectic Al–25 mass% Si alloy, which exhibits superior properties, as a functionally graded material (FGM) was done. The Al–Si FGM, which is based on the concept of overcoming the limitations imposed by the presence of a hard silicon phase in an aluminum matrix, was generated by a vacuum centrifugal method as a thick-walled tube. Grain coarsening, which is the primary disadvantage of the centrifugal method, was observed. The fraction of silicon phase in the tube unexpectedly varied from greater than 60 mass% at the outer surface to 15 mass% at the inner surface because of the greater density of molten silicon compared to that of the eutectic melt. Thus, the outer region of the tube was lighter than the inner region after solidification. FGM billets for near-net shape forming were machined from the thick-walled tube and were formed into an Al–Si FGM cup using a backward extruding. The products of the FGM cup were successfully manufactured in the temperature range from 853 K (580 oC) to 863 K (590 oC) through visco-plastic deformation. The fraction of silicon phase in the FGM cup varied from greater than 70 mass% Si at the formed cup bottom region to less than 15 mass% Si at the cup wall region. Coarse silicon particles were refined irrespective of the pre-existence of elongated spindle-shaped particles under some experimental conditions. The optimum operating conditions were inferred to be high-speed operation at approximately 853 K (580 °C), which was just above the melting point of the eutectic Al–Si alloy.

Effect of silicon content on microstructure of Al-Si/SiCp composite layer cladded on A380 Al alloy by TIG welding process

Tungsten inert gas (TIG) welding was applied to creating a composite layer on the surface of cast aluminum alloy A380. Different mixtures of Al, Si and SiC powders mixed with sodium silicate solution were pasted on substrates. Surface melting was conducted by TIG welding to produce Al-SiC layer on the surface. Microstructural evolution was investigated by X-ray diffractometry (XRD), optical and scanning electron microscopy (SEM) and elemental microanalysis (EDS). Properties of clad layers were studied by microhardness and sliding wear testing. The results showed a uniform distribution of SiC particles in dendritic aluminum matrix. Addition of excess silicon caused the formation of eutectic crystals and coarse silicon particles in the clad layer which resulted in higher hardness and wear resistance of clad layers.

Structural, mechanical and tribological characterization of Zn25Al alloys with Si and Sr addition

The ZA-27 alloy is a zinc–aluminium casting alloy that has been frequently used as the material for sleeves of plain bearings. It has good physical, mechanical and tribological properties. However, one of the major disadvantages is its dimensional instability over a period of time (ageing). To overcome this, copper in the alloy may be replaced with silicon. Coarsening of silicon particles can be controlled by a suitable addition of strontium. In this paper, the commercial ZA-27 alloy and six different Zn25Al alloys (with 1 and 3wt.% silicon; and with 0, 0.03 and 0.05wt.% strontium) were obtained by casting in the preheated steel mould. Casting of the alloys was carried out at a laboratory level. In the alloys containing silicon, a finer dendritic structure was noticed compared to the structure of the commercial ZA-27 alloy. The addition of strontium influenced the size and distribution of primary silicon particles. Needle-like particles of eutectic silicon were changed into the fibrous ones. The presence of silicon and strontium did not significantly affect mechanical properties of the obtained Zn25Al alloys compared to mechanical properties of the commercial ZA-27 alloy. Wear rate of the alloys containing silicon was lower than that of the ZA-27 alloy. The addition of strontium further lowers the wear rate and slightly increases the coefficient of friction.

Structural Modification of Sand Cast Eutectic Al-Si Alloys with Sulfur/Sodium and Its Effect on Mechanical Properties

In the present study, the structural modification of sand cast Al-12wt%Si alloy with sulfur/sodium and its effect on mechanical properties were investigated. Different addition levels of sulfur and sodium were used to modify and produce castings of the same shape and size from the alloy. The results indicated that the addition of sodium or sulfur to eutectic Al-Si alloy can modify the Al-Si eutectic morphology from needle-like eutectic silicon structure to fine-scale eutectic silicon structure with significant improvement in mechanical properties of the alloy. The optimum levels of modification by sodium flux (60% NaF and 40% NaCl) and sulfur were found to be 0.6% - 1.0% and 0.02% - 0.05% of the weight of the alloy respectively. The alloy modified with 0.6% Na flux had the best mechanical properties closely followed by the one modified with 0.02% sulfur. Over modification of the alloy with sodium produced over modification band which consisted of aluminum dendrites and coarse silicon particles in the microstructure of the alloy. Increase in concentration of sulfur decreased the degree of fineness of the eutectic silicon structure with significant decrease in mechanical properties of the alloy and this is suggested to be as a result of the presence of a brittle sulfur compound at the grain interfaces of the alloy.

Microstructural Characteristic and Mechanical Behavior of Nodular Silicon Hypereutectic Al-Si Alloys

The microstructure and mechanical properties of Al-Si-Cu-Mg alloys containing 12 wt.% to 30 wt.% Si are discussed. The eutectic and primary silicon particles are nodulized by a designed modification practice followed by a solution heat treatment of 6 h to 8 h at 510°C to 520°C. Metallographic analysis was used to measure structural characteristics of the Si-rich structures. Spheroidization of silicon phase leads to an increase in tensile strength and ductility of alloys at room temperature and 300°C compared with commercial Al-Si alloy. Increasing Si concentration causes the ultimate tensile strength and elongation at room temperature to fall due to the appearance of coarse silicon particles, but the ultimate tensile strength at 300°C remains unchanged.

Microstructural refinement of a cast hypereutectic Al–30Si alloy by friction stir processing

Hypereutectic Al-30 wt.% Si alloy was subjected to friction stir processing (FSP) to modify the cast microstructure. FSP reduces the size of undesirable coarse silicon particles, eliminates porosities, and homogenizes and refines the cast microstructure. This paper demonstrates the effect of two pass overlap friction stir processing on microstructural refinement of Al–30Si alloy, which delineates significant reduction in size and aspect ratio of silicon particles from average 200 to 2 µm and 4.93 to 1.75 µm respectively. The stir zone of two pass overlap FSP exhibits relatively homogeneous Si particle distribution. Increase in frequency of silicon particles less than 1 µm was also observed in two pass FSP stir zones. Hardness in stir zones was measured to be 75 Hv after first pass and the same changed to 85 Hv respectively after second pass. Further uniform microhardness was observed in the FSP stir zone which was not the case in as-cast Al–30Si microstructure.

Thixoforming of Hypereutectic Al/Si Automotive Pistons

For hypereutectic Al/Si alloys, one of the advantages of thixoforming in comparison with casting routes is the relatively short processing times at high temperatures and hence limited coarsening of the Si phase. Coarse silicon particles give poor mechanical properties. Here two hypereutectic Al/Si alloys (magneto-hydro-dynamically (MHD) stirred A390 from Pechiney and an extruded A390 alloy from Showa in Japan) have been thixoformed to form pistons. Opening up the die entrance to the full width of the crown made the flow into the die more uniform and helped to reduce the tendency for large pores to form due to swirling of the slurry. Die heating reduced cold shuts in the skirt (thin section) of the piston. Placing inserts into the die to make holes for the piston pins (i.e. having an obstacle in the thicker regions) evened up the flow between the thick and the thin regions in the die. Massive pores experienced in earlier shots were then eliminated. Placing ceramic material in the die entrance considerably reduced the shrinkage porosity in the crown. The use of the Showa alloy, where the globular semisolid microstructure is achieved by a solid state deformation route rather than MHD, gave reduced shrinkage porosity and eliminated macrosegregation of the eutectic and the silicon. Computer modeling has aided optimization of the die.

Modification of hypoeutectic Al-Si alloys

Modification of the Al-Si eutectic microstructure has been studied by using the entrained droplet technique in a range of hypoeutectic Al-Si alloys doped with different levels of phosphorus and sodium. Differential scanning calorimetry has been used to investigate the kinetics of silicon nucleation during eutectic solidification, and transmission and scanning transmission electron microscopy has been used to investigate the eutectic microstructure and microchemistry. The Al-Si eutectic microstructure can be modified from coarse-faceted silicon particles nucleating at low undercooling to clusters of fine-scale silicon particles nucleating at high undercooling by either increasing alloy purity, adding sodium, or increasing cooling rate during solidification. The unmodified coarse silicon particles are heterogeneously nucleated on pre-existing AIP particles. Increasing alloy purity removes phosphorus and prevents the formation of AIP; adding sodium leads to the formation of Na3P in preference to AIP; and increasing cooling rate allows insufficient time for the precipitation of AIP from low levels of dissolved phosphorus. At a cooling rate of ∼ 10 K min−1, the transition from an unmodified to a modified Al-Si eutectic microstructure takes place when phosphorus levels fall below 0.25–2 p.p.m., and when sodium levels increase to 80–850 p.p.m.

Strontium modification of aluminium alloy castings in the expendable pattern casting process

The influence of strontium modification on microstructure, pore characteristics and tensile properties of Al-Si-Cu castings produced by the expendable pattern casting (EPC) process has been investigated. Test castings have been produced at strontium concentrations varying from 0.0004%–0.03%. The castings have been analysed by microscopic and image analysis techniques. The results indicate that optimum strontium concentrations are between 0.01% and 0.015%. A coarsening of silicon particles is observed at strontium concentrations greater than 0.015%–0.02%. The addition of strontium to the melt also refines the iron- and copper-bearing phases in the casting. The amount of bulk porosity, average pore size and tensile properties of modified EPC castings are comparable to green sand castings. EPC castings are prone to surface porosity. Strontium modification has a significant effect on the pore distributions in EPC castings. The overall porosity and the maximum pore size increase with strontium modification.

The coarsening of silicon particles in a submicron-grained layer-deposited Al-12wt% Si alloy

A submicron-grained Al-12wt% Si alloy with a uniform distribution of fine silicon particles was produced by a layer-deposition process and subsequent hot working. The grain structure was characterized by high angle grain boundaries with a random spectrum of orientation. The average sizes of both grain and silicon particle were about 0.4 μm in the as-rolled state. The grain refinement came from the dynamic recrystallization on silicon particles. The grains did not show significant growth in annealing even at 493 °C for 24 hours, whereas silicon particles grew at a remarkable rate. The coarsening phenomenon of silicon particles at elevated temperatures was studied systematically by annealing the specimens at 372 °C, 433 °C, 472 °C and 493 °C respectively for different periods. Results showed that the volume of a silicon particle increased linearly with annealing time. The activation energy for coarsening was calculated by Arrhenius equation to be 54 kcal/mole which is almost triple the activation energy for silicon diffusion in aluminum matrix. Because of the high energy value, the coarsening should not be diffusion-controlled but interface-controlled. In addition, the size distribution profile of silicon particles in aluminum matrix after annealing also accounts for the interface-controlled kinetics.

急冷凝固Ａｌ‐Ｓｉ合金粉末のセミソリッドダイキャスティングにおける組織と硬さの変化

The atomized Al-12 and 17mass%Si alloy powders were precompacted in a cylindrical die and heated at eutectic temperature for various periods. The volume fraction of solid phase in the slurry was controlled by heating period. When the small quantities of liquid phase was formed, the density reaches to over 99% of the calculated value by applied compressive stress of low level. However, if the volume fraction of solid phase decreases to 50%, the large voids form in the center of products. The size of silicon particles crystallized from the liquid phase becomes large with decreasing of the volume fraction of solid, but this coarsening of silicon particles is suppressed by pressing continuously before the generation of liquid phase. The bonding strength between the powder particles increases by the continuous pressing.