Hypereutectic Al-Si Alloys Research Articles

Controlling Segregation Behavior of Primary Si in Hypereutectic Al-Si Alloy by Electromagnetic Stirring

Controlling the segregation behavior of primary Si in the solidification process of hypereutectic Al-Si alloy is crucial for enhancing the design ability of the solidification structure. To explore the separation condition and morphological evolution of primary Si in detail, a series of experiments concerning the coupling effect of a temperature field and electromagnetic stirring on the segregation behavior of primary Si were carried out. Experimental results show that the temperature field and fluid flow in the melt are two key points for controlling the segregation behavior of primary Si. The establishment of a temperature gradient in the Al-Si melt is a precondition for realizing the separation of primary Si. On the basis of the temperature gradient, the electromagnetic stirring can further strengthen the separation effect for primary Si, forming a Si-rich layer with 65~70 wt.% Si content. The formation of the Si-rich layer is a continuous growth process of primary Si by absorbing Si atoms from Al-Si melt with the help of electromagnetic stirring. The separation technology for primary Si is proposed to realize the segregation control of primary Si, which not only broadens the application of Al-Si alloys in the functionally gradient composites but also provides a low-cost supply strategy of Si raw materials for the solar photovoltaic industry.

Effects of prior ECAP process on the dynamic impact behaviors of hypereutectic Al-Si alloy

The dynamic mechanical responses of as-cast and ECAPed hypereutectic Al-Si alloy subjected to high strain rate compression by split Hopkinson pressure bar (SHPB) system were investigated. The result shows that the dynamic yield strength of the investigated alloy first decreases with the first ECAP pass, followed by an increase in strength with further increase in ECAP passes. Increasing the number of ECAP passes also results in the refinement of both grains and deleterious crack-nucleating particles and in turn, results in the decrease in susceptibility to thermo-mechanical instability and formation of adiabatic shear bands (ASBs). The activation of dynamic recovery (DRV) and dynamic recrystallization (DRX) by increasing the number of ECAP passes were found to significantly suppress dynamic mechanical damage.

Optimization of Tool Life and Surface Roughness for Hypereutectic Al – Si Alloys in Face Milling

The automotive industry is looking to adopt environmentally-friendly machining processes for automotive components. This study intends to investigate the machining parameters affecting the machinability of hypereutectic Al-Si alloys in the context of surface roughness and tool life via a DLC coated face milling cutter inserted under dry cutting conditions. The machining parameters used in this study were cutting speeds of 250 m/min and 350 m/min, feed rates of 0.02 mm/tooth and 0.04 mm/tooth, and a constant depth of cut of 0.3 mm. The orthogonal full factorial (2³) method was used for the experimental trials. A commercial software called Minitab 17 was used to generate the analysis of variance (ANOVA) and the mathematical prediction model for each machining response. The experimental results confirmed that an excellent surface finish was achieved with a value of as low as 0.140 µm, while the highest value for tool life of 105.47 minutes was realized with face milled aluminium alloy A390. From the analyses, it was confirmed that the feed rate is the most significant machining factor affecting surface roughness, while in the case of tool life; cutting speed is the most influential machining factor. The main effect plot showed that the optimum cutting condition for realizing low surface roughness and longer tool life is at 250 m/min, a feed rate of 0.02 mm/tooth, and radial depth of cut 12.5 mm. The prediction model for surface roughness and tool life was developed and reported low percentage errors.

Understanding the formation mechanism of supersonic atmospheric plasma sprayed in-situ hypereutectic Al-25 wt%Si coating with nanostructured coupled eutectic: From powder, in-flight droplet, splat to coating

A comprehensive study on the formation mechanism of supersonic atmospheric plasma sprayed hypereutectic Al-25 wt%Si coating was performed by investigating microstructural evolution from powder, in-flight droplet, splat, to coating. Results show that feedstock Al-20 wt%Si powder exhibits sub-micron eutectic and micro-sized primary Si. Elements loss, surface oxidation, breakup and chemical diffusion occur when in-flight droplets are exposed to plasma jet. The splat exhibits an ideal structure of nano-sized coupled eutectic. The coating consists of featureless (~84.7 vol%) and ultrafine (~14.2 vol%) zones. Four Si morphologies coexist in the coating from scale of angstrom to sub-micron: Si atoms/clusters in supersaturated solid solution; round/rod-shape Si precipitations (~6.23 nm); nano-Si particulates (~34.37 nm) in featureless zone; and sub-micron Si particulates (~107.32 nm) in ultrafine zone. Further, physical/chemical changes mechanisms of in-flight droplets, rapid solidification mechanisms of splat and coating, formation mechanisms of featureless and ultrafine zones, precipitation and growth mechanisms of Si phase are extensively elucidated. Accordingly, the coating demonstrates ultra-high hardness (~259.8 HV0.1, better than previously reported values of hypereutectic Al-Si alloys with similar silicon content), and superior wear resistance. The unique microstructure with solid solution and grain refinement strengthening mechanisms, and in-situ fabricating higher Si-content Al-Si alloys contribute to the exceptional mechanical properties.

Modification and Refinement of Al-23Si Alloy Processed by Addition of Nano-Metal-Phosphate

This research investigates the effect of Nano-Metal-Phosphate on Si particles through solidification in hypereutectic Al-Si alloys. Which means the merge of modification and refinement with the addition (0.01-0.03) % of Nano-Metal-Phosphate produced by a stir casting. The process synchronizing effects of Nano-Metal-Phosphate that relates to the AlP and Al2O3 formation. These include the following, the effect of Al2O3 at the interfaced between the Al phase and Si particles for obstacle the growth and enhance the refinement of the primary Si and the impact of AlP act as sites of heterogeneous nucleation for eutectic Si. Microstructures analysis revealed that both the fundamental and eutectic Si particles were significantly modified and refined. The primary Si grains were refined from star frames to polygon forms, and their edges were more regular. The platelet eutectic Si grains were also modified into the delicate fibrous forms. Resulting in significant ductility enhancement that could have great potential for numerous applications. Besides, studies microstructure and corrosion behaviour by optical and scanning electron microscopy, XRD and electrochemical cyclic polarization.

Tool wear characteristics in machining of hypereutectic Al-Si alloys by cemented carbide tool

Tool wear characteristics in machining of hypereutectic Al-Si alloys by cemented carbide tool

Effect of Ultrasonic Treatment on the Microstructure and Mechanical Properties of Hypereutectic Al-Si Alloy) A392(

Hypereutectic Al–Si alloy (A392) was treated with ultrasonic vibration (UV) induced to the melt through a cylindrical resin–bonded sand mold. This work aims at studying the influence of UV at the different pouring temperatures of A392 alloy during solidification process on the microstructures and mechanical properties. The microstructures of as-cast samples were examined by using optical microscopy (OM) and scanning electron microscopy (SEM). The mechanical properties were investigated by tensile and hardness tests. The results showed clearly that with applied UV at optimum pouring temperature (640 oC), nearly fine uniform and polyhedral shape of the primary Si were obtained. Improvement of tensile properties and hardness of the investigated alloy modified with UV is related to the modification and refinement of primary Si particles. The fracture surfaces of prepared alloy without and with UV were also investigated.

Formation mechanism and conditions of fine primary silicon being uniformly distributed on single αAl matrix in Al-Si alloys

In this study, solidification microstructure of fine primary silicon being uniformly distributed on single αAl matrix was obtained when adding Cu–9 wt%P alloy to an Al–10 wt%Si alloy melt or an Al–20 wt%Si alloy melt, stirring at 200 r/min for 12 min and cooling at the cooling rate of 16.3 K·s−1. The change law of silicon phase size with cooling rate and modified treatment parameters was determined by examining the solidification microstructures. It was found that ultrafine AlP was easily swallowed by primary silicon when the cooling rate was slow. The formation mechanism of superfine AlP particles, the growth process of αAl phase around primary silicon and the disappearance process of eutectic silicon were researched under intense stirring of the melt during the Cu–9 wt%P alloy modification. Moreover, the conditions under which fine primary silicon was uniformly distributed in single αAl matrix in hypereutectic Al-Si alloy were put forwarded. Finally, the formation mechanism of the solidification microstructure without eutectic Si in hypereutectic Al-Si alloy was analyzed.

Data-driven modeling of wetting angle and corrosion resistance of hypereutectic cast Aluminum-Silicon alloys based on physical and chemical properties of surface

In the present study, a data-driven approach was applied to study the effect of silicon percentage, surface roughness, elapsed time, and water droplet size on contact angle (CA) of hypereutectic cast Al-Si alloys. In addition, corrosion resistance of the alloys was studied as a function of silicon content and the surface roughness. A factorial design was utilized for the design of experiment, and statistical analysis, including General Linear Model (GLM) and Polynomial Regression, were performed for the interpretation of the CA values. Wenzel and Cassie-Baxter contact angles were also calculated and compared with the corresponding measured contact angles to study which regime was followed. The statistical analysis results show that surface roughness, droplet size, and elapsed time are significant factors in the variation of contact angle. In addition, CA was observed to increase with surface roughness, droplet size, and elapsed time. It was observed that the experimental CA values follow a quasi Cassie-Baxter regime in which CA increases by increasing surface roughness. Si % turned out to be a statistically non-significant factor. It is possibly due to a trade-off between the increase in CA as a result of a change in surface chemistry and decrease in CA because of the smoother surface of Si compared to the eutectic phase in a given surface finish.

Crystallization Behavior and Properties of Hypereutectic Al-Si Alloys with Different Iron Content

Crystallization Behavior and Properties of Hypereutectic Al-Si Alloys with Different Iron Content

Effect of Strontium Modification on the Morphology of Primary Si in Hypereutectic Al-Si Alloys

In the present study, hypereutectic Al-Si alloys were modified by strontium and the effect of the strontium content and holding time on the morphology of primary Si was investigated. Primary Si is modified to imperfect octahedron with primary dendrites or secondary dendrites when the amount of Sr exceeds 0.04wt.%. With extending the holding time, there is no significant difference with the morphology of primary Si. However, the size of primary Si decreases remarkably when the holding time prolongs to 120min. Further analysis reveals that the Sr content has a considerable impact on the morphology of primary Si due to growth mechanism influence, while the modifying time mainly influences the size of primary Si.

Effects of Bi addition on Si features, tensile properties and wear resistance of hypereutectic Al-15Si alloy

Al-Si alloys are very important in the automotive industry. This is thanks to their high strength to weight ratio, suitable fluidity, good corrosion resistance and high productivity. The shape of the eutectic Silicon (Si) has been widely studied since it is considered an index of quality regarding many application properties. Understanding the effects of alloy additions on the morphology of Si has drawn researcher’s attention. One of these elements of interest is Bismuth (Bi), which may be part of the composition of Al-Si alloys either as an addictive or a contamination. Hence, the microstructure aspects related to the presence of Bi are important requiring further consideration. In the present study, Al-15wt% Si hypereutectic alloy is modified with 1.0 wt% Bi. The goal is to compare the effect of this addition on the morphology of the phases forming the microstructure and on tensile/wear properties. In order to pursue that comparison, previously available data for the binary Al-15wt% Si alloy will be used. Various samples characterized by distinct cooling rates were generated by transient directional solidification of the ternary Al-Si-Bi alloy. The experimental variations of the eutectic spacing will be compared to each other based on their trends. It was found that the addition of minor Bi content (∼1%) permitted tensile strength and ductility of 195 MPa and 14%. Furthermore, wear resistance was improved thanks to Bi. The reasons for that will be outlined.

Growth kinetics of primary Si particles in hypereutectic Al-Si alloys under the influence of P inoculation: Experiments and modelling

A quantitative study on the growth kinetics of primary Si particles (PSPs), especially under the effect of P inoculation, during isothermal melt solidification of hypereutectic Al-Si-Cu alloys has been realized by using a unique in-situ micro-focus X-radiography method. The morphology evolution has been captured by in-situ observation and the growth velocity has been measured. It is found that P inoculation reduces the branching ability and the growth velocity of PSPs. As a result, the morphologies were changed from star-like, interconnecting multi-plate shapes into more compact, block-shaped and individual plate-shaped by P addition. A post-situ crystallographic study of PSPs by electron backscatter diffraction (EBSD) shows that the growth of plate-shaped PSPs in both alloys is through the twin-plane reentrant-edge mechanism, while block-shaped particles in the P inoculated alloy do not show any preferential growth. A simple growth model has been developed to simulate the growth kinetics of plate-shaped Si particles. A comparison between the simulated and measured growth velocity indicates that growth of plate-like particles is close to the diffusion-controlled lengthening kinetics at high undercooling while the effect of P addition in reducing growth velocity of PSP plates is due to the lower nucleation undercooling and stronger solute impingement effect caused by nucleation of high density PSPs.

Formation of Heterostructure with Appearance of α-Al Phase in Hyper- and Eutectic Al-Si Alloys by Solidification at High Rate

Normally, the microstructure of eutectic and hypereutectic Al-Si alloys consists of Al-Si eutectic or Al-Si eutectic + SiI, that is why they have good wear-resistance properties, but their ductility decreases somewhat, which can be improved by the appearance of non-equilibrium α-aluminum grains. In present work, a recrystallization and partial melting (RAP) technique was applied in the preliminary research to investigate the solidification behavior of the eutectics at high cooling rate. The results show that numerous α-Al dendrites appeared instead of the eutectic structure. In the followed researches, the combination of some technological procedures, based on the competitive growth of dendrites and eutectics, such as heterogeneous nucleation and high cooling rate, were applied: A413.0 and A390.0 alloys were melted in resistant furnace and poured into a mold, made of copper at one side and of steel at another, which is connected to the temperature measuring device via four K-class thermocouples. The various pouring regimes: gravity, via 450 tilt cooling slope and injection with Argon gas for 2 min. before pouring, were applied at different temperatures of 680, 650, 6300 C. The results show that the hetero-structure of eutectic and hyper-eutectic Al-Si alloys, consists of non-equilibrium α-Al grains, primary silicon and eutectics was obtained. Grain size varies with cooling rate, with minimum value of 10 μm when the specimen thickness is 5 mm. Non-dendritic grains were achieved with semi-solid treatment (pouring via cooling slope). The elongation of alloy is expected to be enhanced due to the appearance of α-Al grains.

Correlation between the microstructure and machinability in machining Al–(5–25) wt% Si alloys

The machinability of the Al–Si alloys becomes worse with the Si content increasing. This article explored the influence of Si content on the machinability including cutting forces and chip breakability in machining the Al–Si alloys. The change in machinability of the Al–Si alloys with different Si content attributes to the evolution of microstructure. The cutting forces when cutting hypoeutectic Al–Si alloys are higher than the hypereutectic alloys because only Mg2Si ( β-phase) precipitations are formed in the former one versus cooperative precipitation of Al2Cu ( θ-phase) and Mg2Si in the latter. With regard to hypereutectic Al–Si alloys, cutting force components decrease with the increase in Si content due to the formation of brittle primary Si, which induces the initiation of cracks in material removal process. Moreover, the cutting stability of Al–Si alloys is improved with Si contents in the range of 12–18 wt%, while deteriorates for the Si content of 25 wt%. Besides, the chip breakability is improved due to the increase in eutectic silicon in hypoeutectic and eutectic ranges and because of the formation of block-shaped primary Si for hypereutectic alloys. The optimization of Si content is 12–18 wt% with cutting speed not less than 150 m/min.

In-situ X-radiographic study of nucleation and growth behaviour of primary silicon particles during solidification of a hypereutectic Al-Si alloy

Abstract Real-time observation of structure evolution in a hypereutectic Al-Si(-Cu) alloy under near-isothermal melt solidification condition was conducted by using in-situ micro-focus X-radiography. The nucleation, growth rate and morphological development of primary Si particles (PSPs) were studied at different cooling rates. It is found that an increase of cooling rate increases nucleation rate, reduces the crystal growth and extends the nucleation temperature range of primary Si particles, which all help to refine primary Si particles. The minimum nucleation undercooling also increases with increasing cooling rate. Based on the experimental findings, it is proposed that Si atom clusters act as the nuclei of Si crystals. In addition, cooling rate does not alter the branching growth mechanism of PSPs but only shorten the branch length and reduce the development of side plates. Higher cooling rate may increase the peak growth velocity of PSPs due to the higher nucleation undercooling. Further, electron backscatter diffraction (EBSD) characterization of the post solidified in-situ sample confirms that twinning in primary Si occurs in the nucleation stage or during the growth process, and the growth of twined branches and plates follows the twin plane re-entrant edge (TPRE) mechanism. Twinning during growth may facilitate the branching.

In situ nanocrystals manipulate solidification behavior and microstructures of hypereutectic Al-Si alloys by Zr-based amorphous alloys

In this study, different contents (0.0, 0.1, 0.2, and 0.4 wt.%) of Zr55Cu30Al10Ni5 amorphous alloys (Zr-AA) are used to manipulate hypereutectic Al-Si alloys. The edge-to-edge model shows that there is a perfect lattice matching between the crystallized phases and α-Al and Si phases. The thermal analysis of solidification behavior of hypereutectic Al-Si alloys unmanipulated and manipulated by Zr-AA, indicates that the addition of Zr-AA greatly promotes the nucleation of primary Si and significantly accelerates the process of eutectic reaction. When manipulated by the optimal content of 0.2 wt.% Zr-AA, the sizes of α-Al dendrites and primary Si decrease from 212.46 μm and 19.24 μm to 143.39 μm and 16.47 μm, respectively. The eutectic Si changes from needle-like to rod-like and exhibits a mean size decrease from 20.21 μm to 11.69 μm. Meanwhile, alloys exhibit ultimate tensile strength of 362 MPa and fraction strain of 2.21% after 0.2 wt.% Zr-AA manipulation, which are 23% and 37% higher than those of the unmanipulated alloy (295 MPa and 1.61%). The strengthening mechanisms of hypereutectic Al-Si alloys manipulated by in situ nanocrystals crystallized in the melt are mainly attributed to the fine grain strengthening effect and precipitation strengthening effect. The increase in plasticity is due to the refinement and morphology improvement of Si phases.

Development of high strength suction cast hypereutectic Al–20Si alloy containing gamma alumina and strontium

ABSTRACTIn this research, the hypereutectic Al–20Si alloy, containing an optimised amount of 4 wt-% γ-Al2O3 and 0.1 wt-% strontium, was successfully synthesised through the suction casting process. The cooling rate (563 K s−1) involved during solidification was estimated by measuring secondary dendritic arm spacing. As a result, a significant change in the morphology of silicon was observed and also led to the formation of a small fraction of refined α-Al throughout the microstructure. The work demonstrates that high tensile properties in hypereutectic Al–20Si alloys can be achieved without the use of conventional inoculants (such as phosphorus). The suction cast Al–20Si alloy exhibits remarkably enhanced tensile properties of average ultimate tensile strength of 355 MPa and average ductility of 7.1%.

Effects of solidification thermal parameters and Bi doping on silicon size, morphology and mechanical properties of Al-15wt.% Si-3.2wt.% Bi and Al-18wt.% Si-3.2wt.% Bi alloys

The use of bismuth (Bi) in aluminum (Al) industries is recognized as prime importance because of possible recycling as alloying element in special applications such as those enhancing machinability. The effects of Bi interaction on solidification progress as well as on the resulting microstructure of hypereutectic Al-Si alloys, i.e. the morphology and size of Si crystals are rare information. As such, the present research work investigates the characteristic parameters of eutectic and primary Si with addition of Bi for both ternary Al-15 wt.%Si- 3.2 wt.%Bi and Al-18 wt.%Si- 3.2 wt.%Bi alloys samples directionally solidified (DS) under various cooling rates. The alloy containing 15 wt.%Si showed that Bi acts in the suppression of the growth of primary Si crystals for samples solidified at cooling rates ranging from 2.0 to 31 K/s. For lower rates, a minor fraction of five-fold branched Si was recognized in transverse specimens. A mixture of globular-fibrous eutectic Si is shown to occur for eutectic cooling rates higher than 5 K/s. The Al-18 wt.%Si- 3.2 wt.%Bi alloy is characterized by a mixed structure of eutectic flaky Si and primary angular Si with intersecting spines at nearly 90º all over the entire range of experimental cooling rates (i.e., from 0.4 to 40 K/s). Despite the intricacy of the formed microstructures, it was possible to establish relationships between tensile properties and eutectic Si spacing in order to compare the properties between the tested alloys. Tensile strength and elongation decrease with increasing alloy Si content.

Correlative Tomography – Combining x-ray nanotomography and FIB/SEM serial sectioning to analyze Al-Si cast alloys

Aluminium alloys with silicon as their major alloying element are widely used in die or sand casting of machine or vehicle parts. The microstructure of hypereutectic Al-Si alloys (>12% Si) consist of primary silicon particles and the Al-Si eutectic. Elements such as Cu, Mg and Ni are added in order to form a great variety of different intermetallic phases which improve mechanical properties at elevated temperatures. Size, shape and connectivity of these complexly formed intermetallic phases play a decisive role for the properties and are for that reason of great interest. As the complex shapes and connectivity are not accessible by means of classical 2D techniques, 3D analysis of the microstructure is mandatory. Due to the size of the particles in the ?mrange, only high resolution 3D imaging techniques are suitable. In this work we used 3D X-ray Microscopy as well as FIB/SEM Serial Sectioning to image the microstructure of an AlSi12Cu4Ni2Mg cast alloy. 3D images were taken at the very same sample volume in order to compare information gained from the different techniques.