Al Si-based Alloys Research Articles

Density and surface tension measurements of molten Al–Si based alloys

AbstractThis study is part of a series of studies aimed at measuring the thermophysical properties of molten phase change material-type metallic thermal energy storage materials near 873 K (600°C). The target material is Al–Si based alloys. First, as a feasibility study, density measurements of the molten state of three Al–Si binary alloys (Al–12.2Si, Al–50Si and Al–90Si in atomic%) were performed. A highly accurate non-contact density measurement method based on the static magnetic field superposition electromagnetic levitation (EML) method was employed as the density measurement method. The validity of this experimental method was confirmed, and density of molten Al–Si base alloys (ADC12 and Al–5.9mass%Si–1.6mass%Fe) were measured as a function of temperature with an expanded uncertainty of 1.2%. In addition, the surface tension of the alloys was measured by the droplet oscillation method using the EML technique. The surface tension was successfully obtained as a function of temperature with expanded uncertainty of 2.3%.

Dy–Al–Si System: Experimental Study of the Liquid–Solid Phase Equilibria in the Al-Rich Corner

The Dy–Al–Si ternary system has been experimentally studied, as the effect of the dysprosium addition on the constitution and topology of the liquidus surface, focusing on the (Al) rich part. The system has been investigated in a composition range of up to about 58 at% silicon. The alloys constitution and the liquidus surface projection have been determined by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS), X-ray powder diffraction (XRPD), and differential thermal analysis (DTA). This work is part of a research framework on the properties and solid–liquid phase equilibria of the R Al–Si (R: rare earth) systems. These data, along with the ternary systems isothermal section, are needed to outline the design, plan, and development of new Al–Si-based alloys. In the Dy–Al–Si system, four primary crystallization fields have been experimentally detected: (Si), DyAlxSi(2−x) (orthorhombic form), Dy2Al3Si2 (Τ2), and DyAl(3−x)Six. The following three invariant equilibria have been identified: at 566 °C the ternary eutectic L ⇆ DyAl2Si2 + (Al) + (Si), at 630 °C the U1 L+ DyAl3 ⇆ Dy2Al3Si2 + (Al), and at 562 °C the U2: L+ Dy2Al3Si2 ⇆ DyAl2Si2 +(Al) reactions. A comparison with other known R Al–Si systems has been conducted.

Effects of Si and Sr elements on solidification microstructure and thermal conductivity of Al\u2013Si-based alloys

Effects of Si and Sr on solidification microstructure and thermal conductivity of Al–Si binary alloys and Al–9Si–Sr ternary were investigated, respectively, with a special focus on the relationship between solidification microstructure and thermal conductivity. It was found that (i) in Al–Si binary alloys, with increasing Si content, α-Al grain size increases and then decreases when Si content is over 7 wt%, while the percentage of eutectic Si continuously increases, which significantly decreases the thermal conductivity and electrical conductivity, and (ii) in Al–9Si–Sr ternary alloys, the presence of Sr has no significant effect on α-Al grain, but effectively modifies eutectic Si and significantly improves the thermal and electrical conductivity. On this basis, two theoretical calculation models [the Maxwell model and the Hashin–Shtrikman (H–S) model] were used to elucidate the relationship between solidification microstructure and thermal conductivity. Compared with the Maxwell model, the H–S model fits better with the measured values. The obtained results are very helpful to the precise composition control during alloy design and recycling of Al–Si-based alloys with the aim to further improve the thermal conductivity of Al–Si-based alloys.Graphical abstract

Enhancing strength-ductility synergy and mechanisms of Al-based composites by size-tunable in-situ TiB2 particles with specific spatial distribution

The increase in strength usually accompanies by the sacrifice of ductility in the composites. This work proposed a strategy of design and synthesis of in-situ TiB2 particles to effectively tailor the microstructures and to enhance the mechanical performance of Al–Si-based composites. The tuning mechanisms for size and morphology of TiB2 particles were investigated by combustion synthesis in the Al–Ti–B reaction system. The nano/submicron-sized TiB2 particles with desirable morphology were then specially selected to construct high-performance Al–Si-based composites. Thanks to the strong interface bonding with a low crystallographic mismatch, TiB2 particles significantly refined the primary α-Al dendrites, eutectic Si and θ’ precipitates in the composites, which were 79.2%, 51.9% and 37.6% respectively smaller than those of the matrix. Numerical modeling results suggested that submicron-sized TiB2 particles were more likely to be engulfed or serve as heterogeneous sites while nano-sized TiB2 particles would be repulsed to the solid/liquid interface to physically restrict the growth of α-Al dendrites. The strength-ductility trade-off dilemma was broken therefore superior mechanical properties were obtained in the composites. This work provides a novel perspective for manipulating Al–Si-based alloys in terms of avoiding poisoning and achieving microstructural refinement and outstanding strength-ductility synergy.

Fabrication and mechanical properties of Al–Si-based alloys by selective laser melting process

ABSTRACT Aluminium (Al)-based alloys have attracted as promising materials for structural applications using metal 3D printing due to their excellent specific mechanical strength, formability, and durability. In this study, highly flowable coarse AlSi10Mg powders with average diameter of 65 µm were prepared and then they were additively manufactured by using the selective laser melting (SLM) process. The relative densities exceeding 99% were consistently obtained from the samples fabricated at the scan speed of 1000–2000 mm·s−1 in the range of laser power from 350 to 390 W. The tensile strengths and elongation of all the samples were characterised. Furthermore, Al alloy processed by coarse AlSi10Mg powders does not show a big change in strength values compared to those fabricated by fine AlSi10Mg powders with 45 µm in diameter. Thus, these results clearly show that the developed coarse AlSi10Mg powders can be effectively utilised for the SLM process.

Anomalous strengthening by supersaturated solid solutions of selectively laser melted Al–Si-based alloys

To identify the dominant contributing factor in the anomalously high strength of Al–Si-based alloys fabricated by selective laser melting (SLM), microstructural characteristics of a SLM-built Al–10Si–0.3 Mg alloy (AlSi10Mg) and their changes upon annealing at elevated temperatures were investigated. The as-built AlSi10Mg alloy exhibits a peculiar microstructure comprising of a number of columnar α-Al (fcc) phase with concentrated Si in solution. Numerous nano-sized particles were observed within the α-Al matrix. At elevated temperatures, a number of Si phase (diamond structure) precipitates consumed the solute Si in the columnar α-Al phase, but the microstructure of the α-Al matrix changed slightly. After annealing at elevated temperatures, the tensile strength of the as-built AlSi10Mg alloy substantially decreased accompanied by a reduction in the strain hardening rate. The supersaturated solid solution of the α-Al phase containing numerous nano-sized particles enhanced the strain hardening, resulting in the anomalous strengthening of the SLM-built AlSi10Mg alloy. The microstructural features were formed due to rapid solidification at an extremely high cooling rate in the SLM process, which provides important insights into controlling the strength of Al–Si-based alloys fabricated by SLM.

Architecture of high-strength aluminum\u2013matrix composites processed by a novel microcasting technique

As important lightweight structural materials, cast aluminum alloys have been largely used in the transportation and aerospace industries. In general, Al–Si-based alloys comprise more than 90% of all castings due to their excellent castability and corrosion resistance. However, even though various reinforcements have been introduced, the strength of these alloys is not that high, which severely limits their use for certain high-performance applications. Here, we report on a new strategy and technology to reinforce Al–Si alloys to increase their yield strength into the ~400–660 MPa range, a level that is 29–113% higher than that of all current cast Al–Si alloys, laboratory or commercial, and comparable to that of many high-strength steels but with ~40% lower density. By introducing continuous Ti–6Al–4V reinforcements into the Al–Si matrix through a novel microcasting process, the yield strength of the resulting alloy can be enhanced to between 4 and 6 times higher than that of the pure Al–Si alloy. The extraordinary reinforcing effect originates from the occurrence of multiscale strengthening mechanisms, including macroscale compound strengthening (the rule of mixtures amended by crack arrest mechanism), mesoscale strain-gradient strengthening, and microscale interface-affected-zone and nanoparticle strengthening. The core principle of our material design is to make all components of the composite fully participate in plastic (compatible) deformation, and thus, continuous reinforcements, instead of discrete reinforced structures (e.g., particles, whiskers, and short fibers), were introduced into the Al–Si alloy. Combined with 3-D printing technology, the present microcasting process can realize strengthening at the designed position by architecting specific reinforcements in the matrix.

Tb–Al–Si systems

There are presently limited information regarding the liquid–solid equilibria at the (Al)-rich region of the different R–Al–Si systems (R = rare earth elements). These data, along with the ternary systems isothermal section, are needed to outline the design, plan and develop new Al–Si-based alloys. The effect of the lanthanide terbium addition to the Al–Si system, in the Al-rich corner, has been experimentally studied. The constitution of the alloys and the liquidus surface projection have been determined by means of differential thermal analysis, scanning electron microscopy, electron microprobe analysis and X-ray powder diffraction. In the system investigated have been identified five regions of primary crystallization: TbAl(3 − x)Six, Tb2Al3Si2, TbAlxSi2 − x, (Al) and (Si).

Effect of Rare Earth Metals on the Mechanical Properties and Fractography of Al–Si-Based Alloys

The present study was performed on A356 and A413 alloys in non-modified and Sr-modified conditions. To these alloys, pure rare earth (RE) metals, mainly lanthanum (La) and cerium (Ce), were added in the amount of 1 wt% each, individually or combined. The results showed that the addition of 2% RE leads to a significant reduction in the grain size (~ 60%) compared to the base alloy. However, it also leads to precipitation of a large amount of insoluble intermetallics (~ 10% in volume fraction). These intermetallics cause a significant decrease in the alloy ductility and hence deteriorate the tensile properties and impact energies. In addition, Sr may also cause destabilization of the RE intermetallics with possibility of partial dissolution during solution heat treatment. The RE metals have an affinity to react with Sr leading to partial demodification of the eutectic Si. They also react with Cu, diminishing the alloy response to age hardening. The RE intermetallics act as crack initiation sites, leading to premature failure of the sample. The combined occurrence of these various effects complicates the prediction of the alloy behavior.

Mechanical properties enhancement and microstructure study of Al-Si-TiB2 in situ composites

Al–Si alloy-based composite is one of the most promising MMC materials owing to its outstanding mechanical properties, wear and corrosion resistance, low cost and ability to be synthesized via conventional casting routes. Challenges in achieving clean interface between reinforced particles and matrix alloy have been overcome by means of in-situ techniques of fabrication. Present investigation is concerned with synthesizing Al-Si-TiB2 in-situ composites through stir casting route using K2TiF6 and KBF4 halide salts for exothermic salt metal reaction. X-Ray diffraction analysis revealed the existence of TiB2 in the prepared samples. Effect of TiB2in-situ particles in the Al-Si base alloy has been investigated from the results obtained from optical microscopy as well as SEM study and wear analysis with a pin on disc wear testing apparatus. Improved hardness and wear properties were observed with addition of TiB2.

Application of Vertical-Type High-Speed Twin-Roll Casting for Up-Grade Recycling and Clad Sheets Fabrication of Aluminum Alloys

A horizontal-type twin roll casting method has been popular for producing aluminum alloy strips, however, it is characterized by a relatively low productivity (1~6 m/min). In contrast, a vertical-type high-speed twin-roll casting method possesses an extremely high productivity (60~120 m/min (1~2 m/s)) and an excellent heat extraction ability. The rapid cooling effect provided significant microstructure refinement and mechanical properties improvement in various kinds of cast aluminum alloy products. Not only “product to product recycling” but also “up-grade recycling” can be achieved by making good use of these merits. Two examples of application showing the potential of vertical-type high-speed twin roll casting method are presented. (1) Several kinds of Al-Si base alloy were cast into the strips. Not only strength and toughness but also formability was increased in the twin roll cast products. In particular, great improvement in deformability shows the potential of the twin-roll cast aluminum alloy products as substitutes for some wrought aluminum alloy products. (2) The vertical-type tandem twin-roll caster was able to fabricate a clad strip by single step. The A4045/A3003/A4045 aluminum alloy clad sheets produced by the twin-roll casting showed better mechanical properties than the conventional hot-roll bonded clad sheets.

Effect of Heat Treatment on Controlling the Morphology of AlFeSi Phase in A380 Alloy

In most aluminum–silicon-based die casting alloys, iron is considered one of the most harmful elements. The presence of iron in Al–Si-based alloys leads to the precipitation of a series of iron-containing intermetallic phases. Of these, the β-AlFeSi phase is the most detrimental to the toughness of Al–Si-based alloys. Reducing the formation of the β-AlFeSi phase has the potential of significantly improving the ductility of die casting alloys. The purpose of this work is to show how combined use of high-temperature and short-time heat treatment can be an effective way of controlling the morphology and fraction of β-AlFeSi phase in A380 alloy. Experimental results indicate that if heat treatment temperature goes up to 500 °C (932 °F), the higher temperature can reduce formation of β-AlFeSi with shorter heat treatment time. Evidence of this reduction is proved in optical and scanning electron microscopy images showing both the decomposition of β-AlFeSi and the transformation of β to α-AlFeSi.

Hypereutectic AlSi Alloy: Gathering of 3D Microstructure Data

Hypereutectic and eutectic AlSi-base alloys find frequent application in casting automotive components. The properties of this type of alloy depend significantly on their solidification microstructure, especially the size, shape, and distribution of primary and eutectic silicon. The serial sectioning technique was applied for determining the three-dimensional (3D) microstructure of an Al-18wt.%Si alloy. For clear identification of both the larger primary Si particles grown in the melt and the fine lamellar eutectic Si, a series of two-dimensional equidistant cross sections were metallographically prepared. The microstructure in these cross sections was detected and observed at high resolution using a light microscope. The images were stored in a digital library. The 3D reconstruction of primary Si particles and AlSi eutectic was achieved through the application of various software tools. This provided data about the faceted growth behavior of octahedral Si particles and feathery eutectic Si. The image stack was also imported to hierarchical data format (version 5) (HDF5) open source format, thus, enabling availability of the 3D image data to the wider community. In this way, 3D reconstructions of this kind can contribute to a greater understanding of processing/microstructure property relationships in hypereutectic AlSi alloys.

Surface precipitation on engineering alloys

It is well known that solid-state precipitation forms an important microstructural component of many engineering alloys and is used to help control the bulk mechanical properties. In this contribution, a thermal treatment process is introduced that can induce solid-state precipitation also on the surfaces of engineering alloys, resulting in patterned surfaces. The process requires only thermal treatments and could be integrated into existing thermal or thermomechanical processing schedules for engineering alloys—as a consequence, one of the advantages over existing surface treatment processes is that it is comparatively cheap. Three Al–Si-based alloys are used to demonstrate the key features of the surface precipitation process. Surface area coverage of 30% of Si particles, Si(Ge) core–shell particles, and Mg2Si particles on Al–Si, Al–Si–Ge and AA6061 alloys, respectively, are demonstrated. The kinetics of surface precipitation are studied using in situ hot-stage optical microscopy. The surface precipitation process brings together aspects of surface science and physical metallurgy and represents a fertile field for collaboration between these disciplines and for the investigation of surface precipitation, as a potential future surface treatment. The ability to cheaply pattern the surfaces of engineering alloys with compounds that may themselves exhibit interesting functional properties highlights an opportunity for alloy designers and functional materials researchers to collaborate for the long-term goal of functionalizing structural engineering alloys.

Three Dimensional Morphology of Mn Rich Intermetallics in AlSi Alloys Investigated with X-Ray Tomography

Elementary Mn has a great importance as neutralizer of Fe intermetallics like β-Al5FeSi, which have detrimental effect on mechanical characteristics of AlSi alloys. Presence of Mn in AlSi alloys causes the formation of other intermetallic phases. To understand the effect of solidification conditions and fluid flow on the microstructure of AlSi-based alloys and the addition of Mn leading to Mn-based intermetallics, Al-5 wt pct Si 0.2/0.4/1.0 wt pct Mn alloys have been directionally solidified under defined thermal (gradient 3 K/mm, solidification velocity 0.02-0.12 mm/s) and fluid flow (rotating magnetic field 6 mT) conditions. The primary Al-phase and Mn-based intermetallic phases were studied using 3D X-ray tomography. The spatial morphology of primary phase and intermetallics were characterized with respect to different fluid flow and solidification conditions. The tomography has showed the 3D complicated structure of Mn phases developed.

Distribution of Fe-rich phases in eutectic grains of Sr-modified Al–10 wt.% Si–0.1 wt.% Fe casting alloy

The addition of Sr to Al–Si-based alloys is known to modify the morphology of the eutectic Si phase and to influence the nucleation of eutectic grains. Understanding the distribution and the morphology of small constituent phases in eutectic grains such as Fe-rich intermetallic phases can further yield an insight into the cellular substructure of eutectic grains formed during solidification. The addition of 200ppm Sr to an Al–10Si–0.1Fe alloy and its influence on the formation of Fe-rich phases was comprehensively studied by several microscopy techniques. Optical microscopy combined with scanning electron microscopy revealed the existence of two types of Fe-rich phases at cell boundaries of eutectic grains. The Fe-rich phases were identified as α-Al14Fe3Si2 and δ-Al4FeSi2 by transmission electron microscopy. Both Fe-rich phases are located at distinct regions in the eutectic grain, namely in the transition region (region 2: α-phase) and the outer region (region 3: δ-phase) of the eutectic grain. The three-dimensional morphology of the eutectic Si phase and the Fe-rich phases at the eutectic cell boundaries was investigated by focused ion beam tomography. The Fe-rich α-phase was found to form concentrated networks with the 3D morphology of “sheets”, whereas the Fe-rich δ-phases exist as thin platelets. The distribution of Fe-rich phases in the cellular substructure of eutectic grains are described on the basis of the evolution of the eutectic solidification front by a qualitative solidification model.

Influence of impurity elements on the nucleation and growth of Si in high purity melt-spun Al–Si-based alloys

The nucleation and growth of Si has been investigated by TEM in a series of high purity melt spun Al–5Si (wt%)-based alloys with a trace addition of Fe and Sr. In the as-melt-spun condition, some twinned Si particles were found to form directly from the liquid along the grain boundary. The addition of Sr into Al–5Si-based alloys promotes the twinning of Si particles on the grain boundary and the formation of Si precipitates in the α-Al matrix. The majority of plate-shaped and truncated pyramid-shaped Si precipitates were also found to nucleate and grow along {111}α-Al planes from supersaturated solid solution in the α-Al matrix. In contrast, controlled slow cooling decreased the amount of Si precipitates, while the size of the Si precipitates increased. The orientation relationship between these Si precipitates and the α-Al matrix still remained cube to cube. The β-Al5FeSi intermetallic was also observed, depending on subsequent controlled cooling.

Simulation of a directional solidification of a binary Al-7wt%Si and a ternary alloy Al-7wt%Si-1wt%Fe under the action of a rotating magnetic field

We propose an approach for the solidification of metallic alloys taking into account the turbulence in the liquid phase. For modeling of the turbulent flow a k-ε model is used. This model is applied for solidification of AlSi-based alloys under the action of rotating electromagnetic field of intensity up to 65mT. Two cases have been considered, namely solidification of a binary Al-7wt%Si and a ternary alloy Al-7wt%Si-1wt%Fe. It is shown that increase of the electromagnetic field leads to the formations of various types of channels.

Improvement of mechanical properties of a cast Al–Si base alloy by friction stir processing

This study demonstrated that friction stir processing (FSP) could result in a significant improvement in tensile properties of AC8A, particularly in ductility. Both tool rotation rate and traversing speed could affect the strength of FSPed specimens, which was related to the characteristics of coarsening, dissolution and re-precipitation of the strengthening particles. High tool rotation rate enhanced the dissolution of β′ and Q′ particles, which increased the potential for re-precipitation as fine particles. Low tool rotation rate in FSP mainly caused precipitate coarsening.

The electrochemical properties of Al–Si–Ni alloys composed of nanocrystal and metallic glass for lithium-ion battery anodes

Melt-spun Al75−XSi25NiX (X = 2, 4, 7, and 10 mol%) alloys were investigated as anode materials for lithium-ion batteries. The Al68Si25Ni7 anode showed a maximum capacity of 840 mA h g−1 at the fifth cycle and maintained 661 mA h g−1 after 40 cycles with a high coulumbic efficiency of 93%. The specific capacity increased as the decrease in the Ni content during the first 20 cycles, but the cycle performance became poorer. For the Al65Si25Ni10 anode, the specific capacity increased slowly as the cycles increased and reached 370 mA h g−1 after 40 cycles. When the Al68Si25Ni7 ribbons were annealed, their initial capacity became higher, but much poorer cycle performance and low coulumbic efficiency occurred. Except Al65Si25Ni10, the AlLi compound could be detected in the anodes after lithiation. However, the capacity faded rapidly due to the formation of excessive AlLi in the Al73Si25Ni2 and annealed Al68Si25Ni7 anodes. The experiments revealed that the as-quenched ribbons consisted of the nanoscaled α-Al, metallic glass and α-Si, and their fractions were dependent on the Ni content. The α-Al was a supersaturated solid solution of Si and Ni in fcc-Al. For the as-quenched Al68Si25Ni7 ribbons, the α-Al grains were embedded in the amorphous matrix. It can be understood that metallic glass can store Li, and the supersaturated solid solution can store Li even more easily compared with other known Al–Si-based alloys. A conclusion can be drawn that the microstructure that the nanoscaled α-Al embedded in the metallic glass matrix is beneficial to improve the structure stability, restrain serious structural evolution, and limit the volume variation and pulverization during electrochemical cycles.