Solidification Of Al-Si Alloys Research Articles

Simulation of gas porosity formation and interaction with dendrite and eutectic structures during solidification of Al-Si alloys

A two-dimensional (2-D) multi-phase cellular automaton (CA)-finite difference (FD)-lattice Boltzmann (LB) coupled model is developed to simulate the hydrogen pore formation during dendritic and eutectic solidification of hypoeutectic Al-Si alloys. In this model, the dendrite and eutectic structures are simulated using a CA technique. The solute diffusion is solved by the FD method. The nucleation, growth, and motion behavior of gas pore, and hydrogen transport are simulated by a LB Shen-Chen scheme. It is found that the pores can grow fully but compete in the primary dendrite growth stage. Conversely, pores newly nucleated in residual inter-dendrite liquid during the eutectic solidification stage grow restrictedly and independently. Pore shape severely deformed by the compression of the surrounding solid structures. Moreover, the effects of the initial Si composition, initial hydrogen concentration and cooling rate on the pore formation are qualitatively and quantitatively studied. It is found that the initial Si composition significantly impacts on the pore size and distribution, but little effect on the porosity percentage which decreases markedly with the decreasing initial hydrogen concentration and increasing cooling rate. Comparison reveals that the simulated microstructure morphological characteristics and the predicted porosity percentage agree well with the experimental observation reported in the literature.

Neutron/X-ray diffraction study of crystalline texture and residual stresses of directional solidified Al–Si alloys at different cooling rates

Neutron/X-ray diffraction study of crystalline texture and residual stresses of directional solidified Al–Si alloys at different cooling rates

Growth Mechanism of Eutectic Si in Super-Gravity Solidified Al-Si Alloy during Annealing

Herein, we report that the influence of annealing time on the microstructure and mechanical properties of Al-14.5Si alloys solidified under a super-gravity field. The results indicate that the coarsening of metastable eutectic Si and formation of precipitated Si could be observed at the early stage of annealing. A slight increase in yield strength and tensile strength could be observed in the sample annealed for 0.25 h, which can be ascribed to the formation of precipitated Si with limited size during the early stage of annealing. The intensified diffusion of Si atoms during annealing led to the coarsening and coalescence of the eutectic Si, as well as the coarsening of precipitated Si with further extension of the annealing time.

Tailoring the Void Space of a Silicon Anode for High‐Capacity and Low‐Expansion Lithium Storage

Si anodes have been attracting attention because they offer the critical advantages of being derived from abundant sources and exhibiting a high theoretical capacity. However, Si anodes have not been used in practical applications of Li‐ion batteries because of the unavoidable large volume changes that occur during charging and discharging. Herein, rather than considering the active material, the structural design of the Si anode is focused, developing a material whose volume does not expand even after the Li insertion process. The Si anode is fabricated by leaching Al from a rapidly solidified Al–Si alloy ribbon using a single‐roller melt‐spinning process. Si anode ribbons derived from 50Al–50Si alloy consisting of microscale primary Si and nanoscale eutectic Si show a low‐volume expansion when Li is inserted to a capacity of 3000 mAh g−1. Scanning electron microscopy observations show that the formed pores suppress the expansion of the electrode. At a rate of C/10, the electrode alone, without a binder or conductive carbon additive, exhibits good cycling stability; the initial capacity of 2070 mAh g−1 slightly decreases to 1970 mAh g−1 after 10 cycles. These results open a new platform for designing low‐expansion Si anodes for high‐capacity energy‐storage devices.

Cellular automaton simulation and experimental validation of eutectic transformation during solidification of Al-Si alloys

The morphology of eutectic silicon in solidification microstructure is critical to the performance of Al-Si-based alloys. Simulating eutectic Si phase formation has been a challenge in ICME (integrated computational materials engineering) based design and manufacturing of solidification products of Al-Si-based alloys. In this study, our previous three-dimensional (3-D) cellular automaton (CA) model for α-Al dendritic growth was extended to include eutectic (α-Al + Si) transformation in multi-dendrite domains, providing a complete solidification simulation of critically important Al-Si based alloys. The quantitative results of the Si phase in the eutectic microstructure were experimentally validated using scanning electron microscopy and deep etching techniques. The simulation results show a good agreement with the experimental observations and calculations by the Scheil model and lever rule. This 3-D CA model is useful for predicting and optimizing the solidification microstructure including eutectic transformation during solidification processing such as casting, potentially welding, and additive manufacturing.

Microstructure and Microhardness of Directionally Solidified Al-Si Alloys Subjected to an Equal-Channel Angular Pressing Process

The directional solidification technique allows the study of growth of the solid phase, as-cast structure and, finally, its mechanical characteristics as a consequence of thermal parameters. On the other hand, in the last decade, a process called equal-channel angular pressing (ECAP) has emerged as a widely-known technique in fabrication of ultrafine-grained metals and alloys. Applicability of the ECAP technique affords an excellent potential for changing, in a controlled and beneficial manner, the resulting properties of metals and alloys. For this paper, an experimental research has been conducted to study the effects of solidification parameter (cooling rate) on resulting microhardness in hypoeutectic Al-Si alloys, upon use of an ECAP procedure. The influence of cooling on the scale of the dendritic patterns is presented and discussed with recourse to equations. The resulting microhardness variation with position throughout the as-cast materials and cooling rate were characterized by experimental power laws. Results determined after the solidification experiments have revealed microhardness as a function of both cooling rate and position (P) of the as-cast materials to be dependent on alloy composition. In the ECAP process via route C with three passes, “as-solidified” microstructures have been found to be distorted and fragmented during the severe plastic deformation. This deformation imposed on the billets during the ECAP process facilitated obtaining a fine microstructure and high levels of microhardness were observed. However, even with the ECAP process, it was shown that microhardness is strongly dependent of the cooling rates.

Effect of Si Content on the Morphology Evolution of the Si Primary Dendrites in Al-Si Alloy Solvent Refining Process

Solvent refining with Al-Si alloy is a promising purification method for the production of solar-grade silicon (SoG-Si) feedstock owing to the advantages of low production cost and high impurity removal efficiency. In this process, larger refined Si primary dendrites should be easily collected after acid leaching, which is favorable to recovery, thereby reducing the production cost. Hence, the growth behavior of the precipitated Si crystal must be investigated systematically. In the present work, the morphology evolution of solidified Al-Si alloys with a wide range of Si content (30 ~ 70 wt%) was analyzed. The typical plate-like Si primary dendrites grown following the twin plane re-entrance edge (TPRE) mechanism formed in all alloy compositions. As increasing the Si content from 30 wt% to 50 wt%, the Si primary dendrites underwent a coarsening process attributed to the preferred growth along with < 211 > and < 111 > directions, leading to an increase in the experimental recovery rate. However, the preferred growth along < 211 > direction was inhibited when the Si content is higher than 55 wt%. Moreover, the broken effect originating from grain collision and thermal stress on the Si primary dendrites was enhanced by further increasing the Si content, resulting in a decrease in the experimental recovery rate. Therefore, the optimum composition is determined as Al-50 ~ 55 wt% Si for solvent refining solution, based on the cost reduction consideration.

3D X‐Ray Diffraction Characterization of Grain Growth and Recrystallization in Rolled Braze Clad Aluminum Sheet

Braze clad on aluminum (Al) sheets has enabled fast and convenient brazing assembly of complex heat exchangers. However, there are details in the brazing process that are not fully understood. Herein, 3D X‐ray diffraction (3DXRD) is used to investigate the grain position, size, and orientation before and after controlled atmosphere brazing (CAB). The outcomes are presented as maps of center‐of‐mass positions with relative grain size distribution and color‐coded grain orientations. The results show that, for braze clad Al sheets exposed to CAB simulation, it is possible to distinguish grains from the solidified AlSi alloy from those in the core Al alloy. It is also possible to distinguish new grains obtained through recrystallization during CAB. Hence, the study shows that stretching of the rolled Al sheet by 6% provides enough stored energy in the core material so that recrystallization occurs during CAB and, in addition, provides conditions for AlSi alloy grain growth into the core material. While the phenomenon is well known, it is poorly understood for processes in connection with brazing of mechanically formed Al alloy components in heat exchanger assemblies, and these results demonstrate the potential for gaining deeper insights through 3DXRD.

Influence of Al-B grain refiner on porosity formation of directionally solidified Al-Si alloys

This work aims to present a perspective for porosity formation in three different alloys: A356, A413 and A380.1 by taking into account the addition of Al-B grain refiners: AlTi5B1 and Al3B. The directional solidification method was used, and microstructural changes of the alloys and its correlation with porosity formation were investigated. Pore size, number of pores, average pore length and distribution of pores were statistically analyzed. Also, external shrinkage was examined, and the volume of external shrinkage was calculated. It was found that there was a relationship between external shrinkage and the size and number of pores. As the size and number of pores internally decrease, external shrinkage increases. Additionally, porosity is decreased in all the three Al-Si alloys when Al-B grain refiners are used. The distribution of pore diameters is low when AlTi5B1 is used. Grain refiners have a different effect on porosity formation of Al-Si alloys with regard to their solidification morphology.

On the effects of particle size and preform porosity on the mechanical properties of reaction-bonded boron carbide infiltrated with Al-Si alloy at 950 °C

The infiltration of boron carbide preforms with Al alloys at relatively low temperature prevents the formation of the undesired Al4C3 phase. In the present study the effect of boron carbide powder particle size on the mechanical properties and phase composition of composites infiltrated with Al-20%Si alloys at 950 °C was investigated. According to XRD analysis, the infiltrated composites contain Al8C7B4, AlB2 and AlB12, as well as non-reacted Al and Si that originated from solidification of Al-Si alloy. The presence of small amounts of SiC was noted in specimens fabricated from fine boron carbide powder. No evidence for the formation of non-desired aluminum carbide phase was obtained. Infiltration of ceramic preforms with virtually the same green density generated composites with an elastic modulus and bending strength that continuously decreased from 270 GPa and 405 MPa to 195 GPa and 345 MPa for powder with 90% particles close to 3 μm and powder with 90% particles close to 180 μm, respectively. These results ambiguously confirm that boron carbide particle size strongly affects mechanical properties of reaction-bonded composites infiltrated with Al-Si alloy at 950 °C and reflect the amount of newly formed ceramic phases appearing during infiltration and the presence of defects at the metal-ceramic interface.

Structural and Mechanical Properties of Directionally Solidified Al-Si Alloys

This review covers research aimed at finding the optimum composition and growth rate to obtain a highly modified Al-Si alloy using directional solidification. Investigations of microstructure and mechanical properties as a function of Si content and growth rate are analyzed. These works show that the composition yielding a eutectic microstructure changes considerably with increasing solidification rate in the range of 102-104 μm/s. The increase in ultimate tensile strength with increasing Si content up to that giving a completely eutectic microstructure is explained by a redistribution of volume content of α-Al and eutectic. The increase in tensile strength with increasing rate is explained by a decrease in microstructural scale accompanying the transformation of flake-to-fiber eutectic microstructure. The optimal fine fiber structure without any primary crystals of Al-Si alloy at a given Si content is obtained at the solidification rate giving a completely eutectic microstructure at that composition. Hypereutectic alloys can be fully modified using rapid cooling at such solidification rate that causes coupled growth of the eutectic for given composition of the alloy. Additional Sr modification results in a super-modified structure, high tensile strength and record high elongation.

Boron removal from silicon by hydrogen assistant during the electromagnetic directional solidification of Al Si alloys

Abstract Boron (B) removal from silicon (Si) by using an Ar-20% H2 gas mixture during the Al Si solvent refining was investigated. Electromagnetic directional solidification was employed to enrich the primary Si crystals in the melt and simultaneously drive the produced BxHy gas species to migrate out of the Si enrichment zone. Thermodynamic analysis shows that the formation of BxHy gas species by the reaction of dissolved boron [B] and dissolved hydrogen [H] in Al Si melt is feasible. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) analysis confirmed that the impurity carbon (C), oxygen (O), and B adhered to the wall of the gas cavity, indicating that B removal can be attributed to the migration of gas species. As the flow time of Ar-20% H2 gas mixture increased, the B removal fraction significantly increased, and a maximum value of 96.3% for B removal fraction was achieved when the flow time was 150 min. Finally, an overall process for producing solar grade silicon (SoG-Si) from metallurgical grade silicon (MG-Si) with the combination of Ar H2 refining in Al Si solvent and vacuum directional solidification was proposed.

A comparison of Selective Laser Melting with bulk rapid solidification of AlSi10Mg alloy

In Selective Laser Melting (SLM) layers of atomized powder are spread sequentially on a building platform and melted locally by a laser beam. The melt pool is quenched by the underlying material. SLM of AlSi10Mg alloys results in the development of microstructures consisting of supersaturated primary Al-rich phase surrounded by varied amounts of Al-Si eutectic. The origin of such microstructure is not fully understood. For insight into this issue, this work compares the results of processing AlS10Mgi alloys by SLM and by single-step rapid solidification techniques: Melt Spinning (MS) and Copper Mould Casting (CMC) achieving a range of cooling rates and microstructures which are analysed by means of microscopy, XRD and DSC.The results obtained in these experiments together with the literature available on rapidly solidified Al-Si alloys suggest a correlation among microstructures of the products made with the three techniques. Data on lattice parameter and enthalpy of Si precipitation from primary Al concur in indicating that Si supersaturation scales in the order SLM > CMC > MS. The type and size of microstructural features, i.e. cells, columns, fibrous and lamellar eutectic, reveal the role of solidification conditions (undercooling, recalescence) and precipitation in the solid state for all techniques. Dendrite growth modelling validates the solidification results.

Study of the Boron Distribution and Microstructure of Solidified Al-Si Alloy During the Process of Silicon Purification

AbstractThe Al-Si melts that contain different silicon contents were solidified with a series of cooling rates, and the boron contents in primary silicon phases and eutectic silicon phases were measured and discussed. The results indicate that the boron content in the eutectic silicon phases is higher than that in the primary silicon phases when the cooling rate is constant. When the cooling rate decreases, the boron content in the primary silicon phases decreases, but the boron content in the eutectic silicon phases increases. The microstructure observations of solidified ingots show that there is an interface transition layer beside the primary silicon phase, and the average width of the interface transition layer increases with decreasing cooling rate.

Formation of equiaxed crystal structures in directionally solidified Al-Si alloys using Nb-based heterogeneous nuclei.

The design of chemical compositions containing potent nuclei for the enhancement of heterogeneous nucleation in aluminium, especially cast alloys such as Al-Si alloys, is a matter of importance in order to achieve homogeneous properties in castings with complex geometries. We identified that Al3Nb/NbB2 compounds are effective heterogeneous nuclei and are successfully produced in the form of Al-2Nb-xB (x = 0.5, 1 and 2) master alloys. Our study shows that the inoculation of Al-10Si braze alloy with these compounds effectively promotes the heterogeneous nucleation of primary α-Al crystals and reduces the undercooling needed for solidification to take place. Moreover, we present evidences that these Nb-based compounds prevent the growth of columnar crystals and permit to obtain, for the first time, fine and equiaxed crystals in directionally solidified Al-10Si braze alloy. As a consequence of the potent heterogeneous particles, the size of the α-Al crystals was found to be less dependent on the processing conditions, especially the thermal gradient. Finally, we also demonstrate that the enhanced nucleation leads to the refinement of secondary phases such as eutectic silicon and primary silicon particles.

Effect of solidification rate on the coarsening behavior of precipitate in rapidly solidified Al-Si alloy

The gas-atomized Al-Si alloy powder with different particle sizes was subjected to isothermal annealing for understanding the effect of solidification rate on precipitation and growth Si crystals. The results show that Si precipitates grew more quickly in the small samples with a large solidification rate due to high interfacial energy. Moreover, these Si crystals had a tendency to form a quasi-spherical shape after annealing at a low temperature or for a short holding time. Coarsening of the Si precipitates during annealing was examined using a LSW equation. Thermal stability of the rapidly solidified alloy was significantly influenced by its original microstructure as a result of high solidification rate. Furthermore, more serious clustering of Si-Si phase was also observed in the small samples, attributed to the rapid coarsening of the Si phases.

The microstructure of the surface layer of magnesium laser alloyed with aluminum and silicon

The surface layer under analysis was formed as a result of diffusion bonding of a thin AlSi20 plate to a magnesium substrate followed by laser melting. Depending on the process parameters, the laser beam melted the AlSi20 plate only or the AlSi20 plate and a layer of the magnesium surface adjacent to it. Two types of microstructure of the remelted layer were thus analyzed. If the melting zone was limited to the AlSi20 plate, the microstructure of the surface layer was typical of a rapidly solidified hypereutectic Al–Si alloy. Since, however, the liquid AlSi20 reacted with the magnesium substrate, the following intermetallic phases formed: Al3Mg2, Mg17Al12 and Mg2Si. The microstructure of the modified surface layer of magnesium was examined using optical, scanning electron and transmission electron microscopy. The analysis of the surface properties of the laser modified magnesium revealed that the thin layer has a microstructure of a rapidly solidified Al–Si alloy offering good protection against corrosion. By contrast, the surface layer containing particles of intermetallic phases was more resistant to abrasion but had lower corrosion resistance than the silumin type layer.

Inhibited cold compactibility of rapidly solidified Al–Si alloy powder with large solidification rate

Rapidly solidified powder with refined microstructure is widely used as raw materials for powder metallurgy route. Understanding and controlling the densification behavior of the powder are important to obtain high quality products. In this work, the effect of solidification rate on cold compactibility is investigated via compaction of the rapidly solidified hypereutectic Al–Si alloy powder with different particle sizes. The deformation capacity of the powder with different particle sizes is investigated by linear compaction equations. Furthermore, the microstructure and fracture surface of the compacts are also examined to understand the powder deformation behavior during compaction. The results indicate that the deformation capacity is significantly affected by the microstructure characteristics and the microhardness of Al matrix controlled by solidification rate (or particle size).

Evaluation of dendrite morphology using fractal dimension and dimensionless perimeter in unidirectionally solidified Al-Si Alloys

The dendrite morphology of unidirectionally solidified Al-Si alloys was evaluated by measuring the fractal dimension and dimensionless perimeter of dendrites. In an unidirectional solidification experiment, columnar crystals grew from a bottom chill and columnar to equiaxed transition (CET) occurred at the upper part of an ingot. Then, equiaxed crystals were formed at the top of the ingot. Different dendrite morphology was observed in longitudinal, transverse and oblique sections, however, the fractal dimension or dimensionless perimiter of the dendrites in the sections with same local solidification time showed same values, and continuously decreased with increase in the local solidification time through columnar, CET and equiaxed regions. It can be considered that the fractal dimension and dimensionless perimiter of dendrites are controlled by local solidification time and irrespective of dendrite morphology. This result demonstrated the potential of the fractal dimension and dimensionless perimiter as a parameter for estimating local solidification time of an ingot in which the measurement of SDAS is difficult.

Effect of a high axial magnetic field on the structure of directionally solidified Al–Si alloys

Abstract