Al-12Si Alloy Research Articles

Study of Tribological and Mechanical Properties of A356–Nano SiC Composites

In the present investigation, the microstructural, wear, tensile and compressive properties of Al–7Si alloy matrix nano composites have been discussed. It is noted that the composites contain higher porosity level in comparison to the matrix and increasing amount of porosity is observed with the increasing volume fraction of the reinforcement phase in the matrix. The wear sliding test disclosed that the wear resistance of the nano SiC reinforced composites is higher than that of the unreinforced alloy. It is believed that the presence of SiC particles could shield the matrix and silicon phase from directly experiencing the applied load from the counterface. It was revealed that the presence of nano-SiC reinforcement also enhanced the hardness, tensile and compressive yield strength of Al–7Si alloy which can be attributed to small particle size and good distribution of the SiC particles and grain refinement of the matrix. The highest yield strength and UTS was obtained by the composite with 3.5 vol% SiC nano-particles. The results show that the addition of nano-particles reduces the elongation of A356 alloy.

Effect of High-Energy Ball Milling to Powders on Microstructure and Physical Properties of Al-50Si Alloy

Al-50Si can be fabricated with a method in which air-atomization is followed by vacuum canning hot-pressing process. In order to improve the mechanical character and the physical property，the treatment of high-energy milling are used in the pretreatment of the powder of Al-50Si.The microstructure，density ,coefficient of thermal expansion are analyzed by scanning electron microscope(SEM) and thermal physical tester. It was found that the silicon grain was refined, the density and strength was improved when high silicon alloy powders were treated by high-energy ball milling.

Research of the Morphology and Distribution of the Primary Silicon in Al-18Si Alloy under a Gradient High Magnetic Field

A gradient high magnetic field effected significantly the morphology and distribution of the primary silicon grains in Al-18Si alloy. Experimental studies shew that in the gradient high magnetic field the primary silicon phase grains, which are large plate-like or five-star-like in the case of solidification without magnetic field, are accumulated on the top of the specimen and refined remarkably with the morphology of polygonal or nodular shapes when the alloy solidifies from the semisolid state. In the segregated layer of the silicon, the distribution of the silicon grains is homogeneous. The size of the primary silicon grains decreases and the grain number density rises with the increase of the magnetic strength maintaining the magnetization force unchangeable. It seems that the high magnetic field influences the diffusion of silicon. Theoretical models have been proposed to explain the refining and the distribution of the silicon grains.

Grain refinement of pure aluminium and Al–7Si with Al–3B master alloy

The potential of AlB2 particles in the grain refinement of aluminium alloys with and without Si was investigated in an effort to develop potent grain refiners for Al–Si based foundry alloys. The Al–Si alloys are grain refined with the Al–3B alloy, whereas pure aluminium is not. The cast grain size in the former decreases with increasing Si content. The addition of 200 ppm B into the Al–7Si alloy, the basis of most commercial foundry alloys, produces remarkably fine grains with no fading effect. The Al–B system relies on a eutectic reaction L→α (Al)+AlB2 at 659·7°C to offer grain refinement, which occurs predominantly via heterogeneous nucleation of α (Al) grains on AlB2 particles thus made available. The grain refinement starts at a critical Si content enough to depress the liquidus temperature of the alloy sufficiently below the eutectic reaction temperature. The grain refining efficiency improves with increasing Si content owing to a decreasing liquidus temperature, which in turn facilitates the formation of an increasing number of solid nuclei. Alloys with insufficient level of Si to displace the liquidus temperature of the alloy below the eutectic reaction temperature cannot take advantage of this reaction and thus enjoy grain refinement. This is clearly why pure aluminium with a liquidus point above the eutectic reaction temperature is not grain refined with the Al–3B alloy at all.

Al2O3 nanoparticles induced simultaneous refinement and modification of primary and eutectic Si particles in hypereutectic Al–20Si alloy

It is well known that the mechanical properties of hypereutectic Al–Si alloys are affected by the size, volume fraction, and distribution of primary and eutectic Si particles. However, it is very difficult to simultaneously refine and modify Si particles in hypereutectic Al–Si alloys by conventional means. This study investigates an effect of nanoparticles on Si particles during solidification in hypereutectic Al–Si alloys. Various contents of γ-Al2O3 nanoparticles were added in hypereutectic Al–20Si alloy melt and further dispersed through an ultrasonic cavitation based technique. The cast hypereutectic Al–20Si alloy with the nanoparticle addition showed a significant enhancement in both strengths and ductility. The ductility of the cast hypereutectic Al–20Si alloy was increased from 0.37% to 1.72% with an addition of 0.5 wt% γ-Al2O3 nanoparticles. Yield strength and ultimate tensile strength of the nanocomposite also showed an improvement of about 6% and 26%, respectively. Study suggests that γ-Al2O3 nanoparticles effectively induced simultaneous refinement of primary Si and modification of eutectic Si, resulting in superior ductility enhancement that is much higher than that conventional methods can offer. Microstructural analysis with optical and scanning electron microscope (SEM) revealed that the primary Si particles were refined from large star shapes with small features to polygon or blocky shapes with smooth edges and corners. Moreover, the large plate eutectic Si particles were also modified into the fine coralline-like ones. The results could have great potential for numerous applications.

Effect of Chemical Modification of Al-Si Alloys on Thermal Diffusivity and Contact Heat Transfer at the Casting–Chill Interface

Abstract The heat flow during the unidirectional downward solidification of Al-7Si and Al-12Si alloys was analyzed using thermal analysis technique and inverse modeling. Chills instrumented with thermocouples were brought into contact with a small pool of liquid metal so as to minimize the effect of convection caused by pouring and temperature gradients. Modification melt treatment resulted in an increase in the cooling rate of the solidifying casting near the casting–chill interfacial region. The corresponding interfacial heat flux transients were also found to be higher. The thermal diffusivities of alloys were measured using a laser pulse technique and were found to be higher for modified alloys. However, the increase in the heat flux transients was attributed mainly to the improvement in the casting–chill interfacial thermal contact condition brought about by the decrease in the surface tension of the liquid metal upon the addition of sodium.

Modifying agent selection for Al-7Si alloy by Miedema model

To determine the modifying agents for Al-7Si alloys, microstructure observation and mixing enthalpy analysis using Miedema model for Al-7Si alloy with additions of different rare earth elements were performed, and the effects of rare earth elements on the modification of eutectic silicon morphology were investigated. The results of mixing enthalpy analysis show that these four rare earth elements, La, Sm, Pr, and Ce, which have the large negative mixing enthalpies with Si, can be selected as modifying agents for eutectic silicon morphology. The element with the largest negative mixing enthalpy is Ce. Furthermore, the microstructures indicate that these four elements can effectively modify the eutectic (α)Al-Si crystals in Al-7Si alloy, and the most effective one is also Ce. Differential scanning calorimetry (DSC) results show that the eutectic temperature depressions due to the additions of modifying agents are the important reasons for the modification of eutectic (α)Al-Si crystals.

Effect of Microstructure on Wear Characteristics of Spray Cast Al-Si Alloys

In the present study, the effect of microstructure on the wear characteristics of spray cast Al-15Si and Al-20Si alloys has been investigated and compared with that of vertical axis centrifugal cast and chill cast alloys. The spray cast alloys have been subjected to Hot Isostatic Pressing for porosity reduction. HIPing has reduced the porosity in the spray cast Al-15Si and Al-20Si alloys from 19% to 8% and 19% to 2% respectively. The microstructures of spray cast alloys consisted of finely divided globular shaped Si particles with sizes varying from 2–10 µm. The Si particles have been distributed uniformly throughout the Al matrix. On the contrary coarse and segregated microstructures have been observed in vertical axis centrifugal cast and chill cast alloys. The sizes of primary and eutectic Si in vertical axis centrifugal cast alloys are smaller than that in chill cast alloys. The wear rates are the lowest for spray cast alloys and highest for chill cast alloys over a wide range of sliding velocity. The reasons for better wear resistance of spray cast alloys have been discussed in the light of the microstructural features evolved during spray casting.

Dry Sliding Wear Behaviour Of Spray Formed ZrSiO4 Reinforced Al-Si-Sn Alloy

The wear behaviour of Al-12Si alloy and Al-12Si-Sn/ZrSiO4 composite prepared by spray forming technique has been investigated under dry sliding conditions at different loads and temperatures. The wear rate of spray formed composite was significantly lower than that of as cast Al-12Si alloy. Paricle size of ZrSiO4 was varied from 53 to 105 µm and the amount of Sn was taken 5 and 10 %. Smaller particles of ZrSiO4 were able to reduce the wear rate up to more extent as compared to that of bigger particles in the same matrix. On increasing the amount of Sn the wear rate decreases.

Effect of Sc addition on the microstructure and mechanical properties of as-atomized and extruded Al–20Si alloys

The microstructure evolution and mechanical properties of Al–20Si alloy with Sc addition were investigated after as-atomized and hot-extruded conditions. Microstructural and spectroscopic analyses demonstrate that Sc is enriched at the Si/Al interface. It could be observed that the presence of Sc refined the size of the primary and eutectic Si, without significantly affecting the shape. A considerable improvement in tensile strength, compressive strength, and elongation was observed due to addition in Sc percentage into the composition. The correlation between the microstructure and mechanical behavior strongly indicates that the addition of Sc modifies the Si particles by impurity induced twinning (IIT) mechanisms.

Effects of complex modificating technique on microstructure and mechanical properties of hypereutectic Al–Si alloys

In this article, the structure of Al-18Si alloy was modified by thermal-rate treatment technique at 930 °C based on the DSC result. The mechanical properties of Al–18Si alloy were improved remarkable by a complex technique with alloying and thermal-rate treatment. A new treating technique named as complex modificating technique was proposed, and the performance of this technique on Al–18Si–1.5Cu–0.6Mg alloy was investigated. The results show that primary Si can be refined when Al–P master alloy was added into the melt at 770 °C after thermal-rate treatment. Compared with the conventional casting technique by which the melt of alloy was unmodified, better refinement effect can be obtained with the combination of alloying and complex modificating technique: the size of primary Si is decreased from 66 to 16 μm, the tensile strength increased by 75.94% and the brinell hardness by 66.59%. Moreover, the mechanism of the complex modificating technique was also discussed.

A novel fading-resistant Al–3Ti–3B grain refiner for Al–Si alloys

Al–Ti–B refiners with excess-Ti perform adequately for wrought aluminum alloys but inefficiently in the case of foundry alloys. The high content of silicon in the latter, which forms silicides with Ti and severely impairs the refining potency of the nuclei, is known to be responsible for the poor performance. Hence, new grain refiners, such as Al–3B and Al–3Ti–3B master alloys with excess-B have been developed with well documented advantages for Al–Si alloys. It is very desirable to involve TiAl 3 particles in the Al–3Ti–3B master alloy to maximize its grain refining efficiency. However, fading phenomenon is a key drawback for application of the TiAl 3-containing refiners in aluminum foundry. In the present work, new Al–3Ti–3B grain refiners, containing TiB 2, AlB 12 and TiAl 3 particles were developed with an aim to prolong the acting time after inoculation. The results showed that inoculation of Al–7Si alloy with thus meliorated Al–3Ti–3B grain refiner has produced a fine grain structure which was approximately maintained up to 30 min.

Alignment and Permeability of Al-7Si Alloy Directional Solidification with the Application of a Pulsed Magnetic Field

An experiment on the directional solidification of Al-7Si alloys under the influence of a pulsed magnetic field (PMF) is performed. Maximal peak value of PMF intensity up to 0.226 T between two iron cores was generated when a capacitor bank of 120 µF capacitance with initial voltage of 1000 V was triggered to a pair of solenoids. The PMF decreases the temperature gradient ahead of the liquidus front and increases the width of the mushy zone. The convection of symmetrical vortices seems to charge the intersectional growth of primary dendrite stems and the decrease of dendrite arms spacing under a PMF with higher magnetic intensities. Using Darcy's permeability law and Lehmann's model, the order of magnitude of the interdendritic liquid flow velocities with the application of PMF is evaluated. A strong damped effect of the mushy zone in the bulk flow of the melt is revealed.

Influence of Sn Content on the Microstructure and Dry Sliding Wear Behaviour of Hypereutectic Al-20Si Alloy

The influence of Sn content on the microstructure and dry sliding wear behaviour of hypereutectic Al-20Si alloy was investigated. The results show that the b-Sn in the alloys precipitates mainly in the form of strips and blocks on the grain boundaries of α-Al phase or the interface of silicon and α-Al phases. The addition of Sn can improve the wear resistance of the hypereutectic Al-20Si alloy.

Effects of High Density Ultrasonic Field Coupling on the Microstructures and Properties of Al-Si Alloy

The effects of high density ultrasonic field coupling on the microstructures and properties of Al-12Si alloy were investigated. It is shown that when the melt undergoes ultrasonic coupling processing prior to solidification, the nucleation rate of liquid phase can be raised to make α(Al) dendrite transform towards near equiaxed grains, the growth of Si phase is restrained and eutectic Si microstructure is refined due to acoustic streaming effect and thermal mechanism; When the melt undergoes ultrasonic coupling processing during the melt solidification, large degree of supercooling is produced in the liquid phase in the solidification interface front edge to reduce the critical radius of crystal nucleus and critical work of nucleation and break up, rupture by melting and refine the Si phase to improve obviously the strength of Al-12Si alloy due to its cavitation effect, acoustic streaming action and heat undulation; The crushing effect of ultrasonic coupling on Si phase occurs mainly during the crystallizing solidification and threshold sound intensity exists.

Evolution of microstructure and its effect on wear and mechanical properties of spray cast Al–12Si alloy

Abstract In the present study, the microstructural features, mechanical properties and wear characteristics of spray cast Al–12Si alloy have been investigated and compared with that of the chill cast alloy. The spray cast alloy was subjected to hot isostatic pressing for reducing the porosity of the deposit from 26% to 6%. The microstructure of the spray cast and hot isostatically pressed alloy consisted of finely divided globular shaped eutectic Si uniformly distributed in the Al matrix. The fine particles of Si were invariably observed in the bottom and top regions of the spray-deposit in contrast to relatively large size Si particles in the central region. The microstructure of the chill cast alloy was coarse and consisted of an eutectic mixture of α-Al and Si needles. The room temperature tensile tests and hardness measurements showed improved mechanical properties of spray cast alloy over that of the chill cast alloy. In addition, the dry sliding wear test indicated a superior wear resistance of the spray cast alloy over a wide range of sliding velocity. This behavior of the spray cast alloy is discussed in the light of its microstructural modification induced by spray casting.

Inference of optimal speed for sound centrifugal casting of Al-12Si alloys

True centrifugal casting is a standard casting technique for the manufacture of hollow, intricate and sound castings without the use of cores. The molten metal or alloy poured into the rotating mold forms a hollow casting as the centrifugal forces lift the liquid along the mold inner surface. When a mold is rotated at low and very high speeds defects are found in the final castings. Obtaining the critical speed for sound castings should not be a matter of guess or based on experience. The defects in the casting are mainly due to the behavior of the molten metal during the teeming and solidification process. Motion of molten metal at various speeds and its effect during casting are addressed in this paper. Eutectic Al-12Si alloy is taken as an experiment fluid and its performance during various rotational speeds is discussed.

Improved wear resistance of Al–15Si alloy with a high current pulsed electron beam treatment

A hypereutectic Al–15Si alloy (Si 15 wt.%, Al balance) was irradiated by high current pulsed electron beam (HCPEB). The HCPEB treatment causes ultra-rapid heating, melting and cooling at the top surface layer. As a result, the special “halo” microstructure centering on the primary Si phase is formed on the surface due to interdiffusion of Al and Si elements. The composition of the “halo” microstructure is distributed continuously from the center to the edge of the “halo”. Compared to an untreated matrix, the remelted layer underneath the surface presents single contrast because of the compositional homogeneity after HCPEB treatment. The thickness of the remelted layer increases slightly from 4.4 μm (5 pulses) to 5.6 μm (25 pulses). HCPEB treatment broadens and shifts the diffraction peaks of Al and Si. The lattice parameters of Al decreases due to the formation of a supersaturated solid solution of Al in the melted layer. Through analysis of Raman spectra and transmission electron microscopy (TEM), the amorphous Si (a-Si) and nanocrystalline Si are formed in the near-surface region under multiple bombardments of HCPEB. The relative wear resistance of a 15-pulse sample is effectively improved by a factor of 9, which can be attributed to the formation of metastable structures.

Microstructural Features, Wear, and Corrosion Behaviour of Spray Cast Al—Si Alloys

In the present study, the microstructural features, wear characteristics, and corrosion behaviour of spray cast Al—12Si, Al—15Si, and Al—20Si alloys have been investigated. The alloys were spray cast and hot isostatically pressed. Hot isostatic pressing has considerably reduced the porosity in spray cast alloys. The microstructure of spray cast alloys showed fine and globular-shaped Si particles in the Al matrix in contrast to needle-type eutectic Si and blocky-type primary Si in the Al matrix in chill cast alloys. The dry sliding wear tests showed that the wear rates of spray cast alloys are invariably lower and the potentiodynamic polarization tests showed that the corrosion resistance of spray cast alloys is considerably higher than that of chill cast alloys. The high wear and corrosion resistance of spray cast alloys have been discussed in the light of the microstructural modification induced during the spray casting process.

Laser processing of bulk Al–12Si alloy: influence of microstructure on thermal properties

The feasibility of enhancing the thermal conductivity of an alloy via microstructural refinement was examined using Al–12%Si alloy as a model material. Al–12Si alloy samples were fabricated at different process parameters using laser engineered net shaping (LENS™) and the effect of microstructural features on the thermal conductivity was studied and compared with conventionally cast alloy. The large difference in melting points and laser light absorptivities of Si and Al as well as the low melt viscosity of Al–12Si alloy resulted in a very small process window to successfully fabricate bulk Al–12Si alloy samples using LENS™. Comparison of microstructural features of laser-processed samples with cast Al–12Si alloy showed significant refinement in eutectic Si for laser processed samples. Microstructural refinement not only improved the thermal conductivity of Al–12Si alloy but also compensated the detrimental effect of porosity on thermal conductivity. The thermal conductivity of cast alloy varied between 82 and 93 W/mK, which is ∼21–76% lower than the values exhibited by laser-processed samples in the range 103–153 W/mK. The results of LENS™ fabrication, microstructural evolution and thermal properties of Al–12Si alloy bulk structures can be extended to other immiscible alloys and metal matrix composites for a variety of engineering applications.