Hypereutectic Al-Si Research Articles

The fabrication of functional gradient hypereutectic Al-Si composites by liquid-solid separation technology

A liquid-solid separation technology (LSS) is applied to fabricate near-net shape packaging shells made of hypereutectic Al-Si composites with gradient distributions of the microstructure and properties. This new technology solves the problem of the formation of coarse flake primary Si, which will seriously deteriorate the mechanical properties of the materials, and unavoidable with conventional casting method. The microstructure of the hypereutectic Al-Si composite were characterized using scanning electron microscope, energy dispersive spectroscopy and transmission electron microscope. In order to eliminate the influence of crystal defects and stress, the effects of annealing treatment on the thermal expansion coefficient and thermal conductivity of the composites were also investigated. The results show that the volume fraction of Si decreases along a gradient from the bottom centre of the packaging shell, which houses the chips, to the top of the packaging wall, which is welded with the cover materials. The bottom of the shell has a CTE of 11.7 × 10−6 K−1, TC of 124.3 W m-1 K-1, and primary Si content of 53.7%. The top of the shell has a CTE of 18.4 × 10−6 K−1, TC of 151.2 W m-1 K-1, and primary Si content of 23.3%. Such a distribution enables a good matching of the thermal expansion behaviour of the packaging shell with the chips, which have typically low CTE, while avoiding the issue of poor soldering performance of Al/Si alloys. In addition, it reduces the internal stress of the shell due to integrated forming technology.

Effect of Fe, Ni, and Cr on the corrosion behaviour of hyper-eutectic Al-Si automotive alloy under different pH conditions

Effect of Fe, Ni and Cr on the corrosion behaviour of hyper-eutectic Al-Si automotive alloy was studied. The test of corrosion behaviour at different environmental pH 1, 3, 5, 7, 9, 11 and 13 was performed using conventional gravimetric measurements and complemented by resistivity, optical micrograph, scanning electron microscopy (SEM) and X-ray analyser (EDX) investigations. The highest corrosion rate was observed at pH 13 followed by pH 1, while in the pH range of 3.0 to 11, there is a high protection of surface due to formation of stable surface oxide film. The highest corrosion rate at pH 13 is due to presence of sodium hydroxide in the solution in which the surface oxide film is soluble. At pH 1, however, high corrosion rate can be attributed to dissolution of Al due to the surface attack by aggressive chloride ions. Presence of Fe, Ni and Cr in hyper-eutectic Al-Si automotive alloy has significant effect on the corrosion rate at both environmental pH values. Resistivity of alloy surfaces initially decreases at pH 1 and pH 13 due to formation of thin films. The SEM images of corroded samples immersed in pH 1 solution clearly show pores due to uniform degradation of the alloy. In pH 13 solution, however, the corrosion layer looks more packed and impermeable.

Debinding and Sintering of an Injection-Moulded Hypereutectic Al⁻Si Alloy.

Hypereutectic Al–Si (20 wt.%) alloy parts were fabricated by employing a powder injection moulding (PIM) technique with a developed multi-component binder system composed of high-density polyethylene (35 wt.%), carnauba wax (62 wt.%) and stearic acid (3 wt.%). The feedstocks contained 83 wt.% metal powders. The debinding process was carried out by a combination of solvent extraction and thermal decomposition. The effects of solvent debinding variables such as kind of solvents, debinding temperatures and time, and the bulk surface area to volume ratios on the debinding process were investigated. Thermal debinding and the subsequent sintering process were carried out in a heating sequence under a nitrogen atmosphere. The influences of sintering temperature and sintering time on the mechanical properties and structure were considered. Under the optimal sintering condition, sintering at 550 °C for 3 h, the final sintering parts were free of distortion and exhibited good mechanical properties. Relative sintered density, Brinell hardness, and tensile strength were ~95.5%, 58 HBW and ~154, respectively.

A novel method of coating ex-situ SiC particles with in-situ SiC interlayer in Al-Si-C alloy

For SiCp/Al composites, the stability of the interface between SiC particles (SiCp) and matrix is crucial. A novel method, in-situ coating of ex-situ SiCp, is developed in hypereutectic Al-Si alloys within this research. The microstructure, growth mechanism and strengthening behavior of the ex-situ SiCp with in-situ SiC coating were studied. Microstructure observation shows that due to the formation of in-situ coating, the interfacial bonding of ex-situ SiCp and matrix is improved. Further research reveals the epitaxial growth character of the in-situ SiC coating which has the same structure with ex-situ SiCp. The ex-situ SiCp with in-situ SiC coating is easily introduced into complex Al-Si alloys and the tensile strength of the alloy at both room temperature and 350 °C is improved remarkably.

Effect of Primary Silicon Refinement on Mechanical and Wear Properties of a Hypereutectic Al-Si Alloy

This study explores the effect of primary silicon refinement of hypereutectic Al-Si alloy on the mechanical and wear properties. Refinement has been carried out by purging N2 into hypereutectic Al-Si alloy melt for different period of time. Hypereutectic Al-Si alloy with 16.2 wt.% of Si was taken for the study and purged by N2 gas for different time. Hypereutectic alloy under different conditions was characterised for various properties. X-ray diffractometer (XRD) analysis doesn’t show formation of any nitride but optical microscopy (OM) and scanning-electron microscopy (SEM) results show modified/refined morphology of silicon. Mechanical and dry sliding wear properties were evaluated at ambient temperature and results reveal improvement in these properties. Worn surfaces were also studied under SEM and 3D-profilometer for surface analysis. Results indicate that at low load/sliding velocity, wear is oxidative/mild in nature, whereas, oxidative-metallic/severe wear is observed at high load/sliding velocity.

Enhanced Impact Toughness at Ambient Temperatures of Ultrafine-Grained Al-26 wt.% Si Alloy Produced by Equal-Channel Angular Pressing

Overcoming general brittleness of hypereutectic Al-Si alloys is in urgent need for expanding their application in automotive, aerospace and construction industries. A unique phenomenon was observed that bulk ultrafine-grained Al-26 wt.% Si alloy, produced by severe plastic deformation via equal-channel angular pressing, exhibited higher toughness at the impact temperature of − 196 ~ 100 °C than the coarse-grained casting alloy. The improvement in impact toughness at all testing temperatures was mainly due to the homogeneous ultrafine-grained structure with the breakage of brittle primary silicon crystals, which generated more and deeper fracture dimples that consumed much higher fracture energy. It indicates the advantage of bulk ultrafine-grained Al-Si alloys and spurs their application interest at various ambient temperatures.

Study on Microstructure and Mechanical Properties of Hypereutectic Al–18Si Alloy Modified with Al–3B

An hypereutectic Al–18Si alloy was modified via an Al–3B master alloy. The effect of the added Al–3B and the modification temperature on the microstructure, tensile fracture morphologies, and mechanical properties of the alloy were investigated using an optical microscope, Image–Pro Plus 6.0, a scanning electron microscope, and a universal testing machine. The results show that the size of the primary Si and its fraction decreased at first, and then increased as an additional amount of Al–3B was added. When the added Al–3B reached 0.2 wt %, the fraction of the primary Si in the Al–18Si alloy decreased with an increase in temperature. Compared with the unmodified Al–18Si alloy, the tensile strength and elongation of the alloy modified at 850 °C with 0.2 wt % Al–3B increased by 25% and 81%, respectively. The tensile fracture of the modified Al–18Si alloy exhibited partial ductile fracture characteristics, but there were more areas with ductile characteristics compared with that of the unmodified Al–18Si alloy.

Effects of Heat Treatment on the Microstructures and High Temperature Mechanical Properties of Hypereutectic Al–14Si–Cu–Mg Alloy Manufactured by Liquid Phase Sintering Process

Hypereutectic Al–Si alloy is an aluminum alloy containing at least 12.6 wt.% Si. It is necessary to evenly control the primary Si particle size and distribution in hypereutectic Al–Si alloy. In order to achieve this, there have been attempts to manufacture hypereutectic Al–Si alloy through a liquid phase sintering. This study investigated the microstructures and high temperature mechanical properties of hypereutectic Al–14Si–Cu–Mg alloy manufactured by liquid phase sintering process and changes in them after T6 heat treatment. Microstructural observation identified large amounts of small primary Si particles evenly distributed in the matrix, and small amounts of various precipitation phases were found in grain interiors and grain boundaries. After T6 heat treatment, the primary Si particle size and shape did not change significantly, but the size and distribution of CuAl2 (θ) and AlCuMgSi (Q) changed. Hardness tests measured 97.36 HV after sintering and 142.5 HV after heat treatment. Compression tests were performed from room temperature to 300 °C. The results represented that yield strength was greater after heat treatment (RT ~ 300 °C: 351 ~ 93 MPa) than after sintering (RT ~ 300 °C: 210 ~ 89 MPa). Fracture surface analysis identified cracks developing mostly along the interface between the primary Si particles and the matrix with some differences among temperature conditions. In addition, brittle fracture mode was found after T6 heat treatment.

Effect of yttrium addition on the microstructures and mechanical properties of hypereutectic Al-20Si alloy

The effects of Y additions (0.2, 0.4, 0.6, 0.8 and 1.0 wt%) on the microstructure and mechanical properties of hypereutectic Al-20Si alloy have been studied in the present work. The as-cast specimens were examined by scanning electron microscope (SEM) equipped with energy spectrometer (EDS), electron probe micro-analysis (EPMA) and X-ray diffraction (XRD). The results demonstrated that the morphology of primary Si could be refined from coarse irregular star-like and plate-like shape to fine block-like when the addition contents of Y increased to 0.8%. The average size of primary Si reduced by 62.9% from 89 µm to 33 µm and the aspect ratio decreased by 38.6% from 2.07 to 1.27. In addition, the eutectic Si structure was modified from coarse needle / flake to fine coral fibrous structure and partial particles. With the refinement and homogenization of Si phases, the optimal elongation (El) increased by 22.5% from 1.13% to 1.36% when the Y content was 0.6%. However, the ultimate tensile strength (UTS) enhanced by 47.9% from 94 MPa to 139 MPa when the addition content of Y was up to 0.8%. Moreover, the refinement and modification mechanism of Y on Al-20Si alloy was also discussed.

Effect of alumina on grain refinement of Al-Si hypereutectic alloys

The size, volume fraction and distribution of primary as well as eutectic silicon affect the mechanical properties of the Al-Si hypereutectic alloys. It is very difficult for the simultaneous refinement and modification of primary and secondary Si particles in hypereutectic Al-Si alloys through traditional processes. This paper explores the role of γ-Al2O3 nanoparticles on Si particles in the course of solidification in hypereutectic Al-Si alloys at particular pouring temperature. The present study involves incorporation of varying contents dispersed γ-Al2O3 nanoparticles into a molten base metal during stir casting and followed by solidification. It has been reported that the synthesized composites having good interfacial bonding (wetting) between the dispersed phase and the liquid matrix was achieved in order to provide improved mechanical properties of the composite. The cast product of hypereutectic Al-16Si alloy with the reinforcement of nanoparticles, illustrated a significant improvement in both wear behaviour and hardness. The dry sliding wear test has been performed on a group of specimens with varying parameters (different loads and sliding velocities) in a pin on disc wear testing machine. Moreover, the wear rate and specific wear rate also affected in different load and different sliding velocities. However in XRD analysis of the samples, the enhancement of wear resistance as well as hardness was due to the formation of brittle phases like SiO2, Al2O3 and Al-rich intermetallic compounds. The hardness value of the materials increases nearly 6% in addition to increase in the density of only 0.8%. As per literature, the large plate eutectic Si particles were modified in to the fine core particles and it acquires enough potential for various applications.

Microscopical Characterization of Cast Hypereutectic Al-Si Alloys Reinforced with Graphene Nanosheets

This paper illustrates the effects of stirring and graphene nanosheet (GNS) addition on the microstructure and mechanical behaviour of 393 hypereutectic Al-Si alloys used in the diesel engine pistons. Two processing routes were applied to fabricate hypereutectic Al-Si alloys: The first route mainly depends on stirring Al-Si alloys for 12 minutes at 400 rpm. The second one involves stirring Al-Si alloys for 2 minutes and then adding graphene nanosheets into the vortex, and consequently continue stirring for 10 minutes at 400 rpm. Results show that the distribution of the primary silicon was improved significantly in stir casting at both the edge and the center of the samples with relative reduction of 34% and 37% in the average particle size respectively. This average primary silicon size was further reduced by 17% with the addition of 1 wt% GNSs. GNS embedding into Al-Si alloy matrix resulted in remarkable increase in hardness values of the nanocomposites compared to the cast alloy.

Microscopical Characterization of Cast Hypereutectic Al-Si Alloys Reinforced with Graphene Nanosheets

Microscopical Characterization of Cast Hypereutectic Al-Si Alloys Reinforced with Graphene Nanosheets

Role of scandium additions in primary silicon refinement of hypereutectic Al–20Si alloys

ABSTRACT This study investigates the effect of Sc on the formation of primary silicon during the solidification of hypereutectic Al–20Si alloys. The evolution of the microstructure was studied using thermal analysis. The results show that the addition of 0·2 and 0·4 wt-% Sc suppresses the nucleation of primary silicon due to the formation of ScP particles instead of AlP particles. For large Sc additions, at the centre of the samples, the primary silicon has a star-shaped morphology with thin branches. The addition of Sc decreases the P refinement efficiency in hypereutectic Al–20Si alloys, which may be a result of the formation of a ScP phase. The results suggest that the addition of Sc poisons the nucleant particles of the primary and eutectic silicon.

Joining of carbon fibre reinforced polymer to Al-Si alloy for space applications

The joining of hypereutectic Al-Si alloy heat pipes to CRFP skins has been studied for radiator panel applications. Three different epoxy adhesives (two thermal conductive adhesives and one high-strength structural one) were used and mechanically characterised before and after thermal cycling. Two adhesives gave the most promising results, in terms of mechanical strength (apparent shear ~ 25MPa for 100µm thick lap joints). Thermal cycling (10 cycles between −30°C and +90°C) did not significantly affect the mechanical properties of the joints or the failure propagation mode. The use of the thermal conductive adhesive could also guarantee good thermal conductivity of the designed panel.

Development of Hypereutectic AlSi Alloy Powder Injection Molding Feedstocks by Rheological Analysis

The comprehensive properties of a feedstock have a critical influence on the powder injection molding process. Proper feedstock with homogeneous structure, favorable flow characteristic, and moldability is the prerequisite for obtaining a final part with excellent comprehensive properties. The objective of the present work was to develop an optimal feedstock for fabrication of hypereutectic AlSi (20 wt.%) alloy parts by the powder injection molding technique. For this purpose, micron‐sized hypereutectic AlSi (20 wt.%) alloy powder was mixed with different amounts of a binder which consisted of 35 wt.% high‐density polyethylene, 62 wt.% carnauba wax, and 3 wt.% stearic acid. The binder contents of the feedstocks were in the range from 13 wt.% to 21 wt.%. The influences of binder content, shear rate, and temperature on the rheological behaviors of feedstocks have been investigated via a capillary rheometer. The feedstock with 21 wt.% binder exhibited a variable flow behavior and was culled. The rest of the feedstocks showed a pseudoplastic behavior. Comprehensive analysis of rheological parameters such as the flow behavior index, yield stress, flow activation energy, and the general moldability index, the feedstock with 17 wt.% binder exhibited the best rheological performance and favorable moldability. The molded part with 17 wt.% binder had constant density, good shape retention, and stiffness as well as homogeneous distribution of the powder and binder. After solvent debinding, the debound item showed a homogeneous porous structure which is suitable for the subsequent thermal debinding and sintering processes.

High temperature tensile properties of hypereutectic Al-25Si based alloy

The high temperature tensile deformation of a hypereutectic Al-25Si based alloy fabricated by spray forming and subsequent hot extrusion was investigated. Tensile tests were conducted at various temperatures and strain rates. It was revealed that the ductility of the alloy is sensitive to both the test temperature and strain rate. At a given strain rate, the peak value of elongation was obtained at 500 °C. At 490 °C and above, the peak value of elongation was observed at a strain rate of 1.0 × 10-2 s-1, although the elongation increased with decreasing strain rate at 460 °C and below. The high elongation was exhibited when a high strain rate sensitivity index (m value) was attained and a liquid phase existed during deformation. The liquid phase appeared as a filament-like structure which is aligned with the tensile direction on the fracture surface of the tensile deformed specimen. A higher elongation (>35%) was obtained when the volume fraction of the liquid phase was 0.7%-1.7%. The maximum elongation of 75% was achieved when the volume fraction of the liquid phase was about 1%. The transition of the activation energy was observed at 430 °C when incipient melting occurred.

Studies on Relationship between Wear Behaviour and Microstructure of a Hypereutectic Al-Si Alloy

A commercial grade hypereutectic Al-Si alloy (LM-28) has been modified by adding P in the form of Cu-14P master alloy. The microstructures of unmodified and modified were examined under SEM. The mean size of the primary silicon in unmodified alloy is 24.87µm and the same observed in modified alloy containing 0.06% P is 12.53µm. The volumetric wear rate and COF at high temperature have been studied using pin-on-disc wear testing machine. It has been observed that the wear resistance of modified alloys is noticeably improved when compared to the unmodified alloy. The worn surfaces were characterized by confocal microscopy and observed wear mechanisms are explained in the background of their microstructures.

Study on the Anti-Poison Performance of Al-Y-P Master Alloy for Impurity Ca in Aluminum Alloys.

In this article, the anti-poison performance of novel Al–6Y–2P master alloy for impurity Ca in hypereutectic Al–Si alloys was investigated in detail. According to the microstructural analysis, it can be found that the primary Si and eutectic Si particles could be relatively modified and refined. In order to investigate the influence mechanism of Ca on the limited refinement performance of Al–6Y–2P master alloy, types of Al–xSi–2Ca–3Y–1P (x = 0, 6, 12, 18, and 30) alloys were prepared. It is observed that Ca takes the form of more stable Ca3P2 compounds by reacting with YP, and the surface of Ca3P2 particles are unsmooth, and even some have wrinkles in Al Al–2Ca–3Y–1P alloy. With the increase of Si content in Al–xSi–2Ca–3Y–1P (x = 6, 12, 18 and 30) systems, the multi-encapsulation structures, i.e., the phosphide (AlP and YP), hexagonal Al2Si2Ca, the Al3Si2Y2 or primary Si from inside to outside in order were examined.The excapsulation of YP and AlP caused by Al2Si2Ca might be the reason for the limited refinement effect of Al–6Y–2P master alloy for hypereutectic Al–18Si alloys.

Ultrarapid formation of multi-phase reinforced joints of hypereutectic Al-Si alloys via an ultrasound-induced liquid phase method using Sn-51In interlayer

Ultrarapid formation of hypereutectic Al-Si alloys /Sn-51In/ hypereutectic Al-Si alloys joints was achieved within 0.2s via an ultrasound-induced liquid phase method at 180°C in air. Ultrasonic vibration accelerated the element inter-diffusion between aluminium and indium to induce liquid phase. The joints consisted of Si particles, an Al-In solid solution and intermetallic compounds. The migration behaviour of Si particles, which were regarded as tracers, under the influence of ultrasonic vibration was used to investigate the joint formation. As the ultrasonic vibration time increased, the shear strength of the joints increased. The existence of Sn in the interlayer considerably affected the joint shear strength by forming intermetallic compounds.

Effect of load on the tribological properties of hypereutectic Al–Si alloy under boundary lubrication conditions

Researchers reported that the IC engine components (piston, cylinder liner etc) fail due to the friction losses (~45%) and wear losses (~25–40%). So the demand of light weight, low friction and wear resistance alloys increases day by day, which reduces the emission and increases the efficiency of the IC engine. In this connection, tribological tests on hypereutectic Al–25Si alloy were performed using a ball-on-disk configuration under dry and lubricated sliding conditions. Hypereutectic Al–25Si alloy was prepared by rapid solidification process with T6 condition. T6 condition improves the friction, wear and mechanical properties of the alloy. Friction coefficient and wear rate of the alloy was measured under high loads ranging from 100 to 300 N. It was found that the friction coefficient (COF) and wear rate of hypereutectic Al–25Si alloy/steel tribo pair increased with increase in load. Significant reduction in COF and wear rate was accomplished with SAE20W50 engine oil and Si particles act as solid lubricant. Optical microscope, 3D surface profilometer and scanning electron microscope (SEM) coupled with an energy dispersive spectrometer (EDS) were used for characterization the worn surface morphologies. The morphology, size and distribution of high Si particles due to its fabrication process caused the improvements in COF and wear rate under lubricated conditions. Adhesive wear, abrasive wear and plastic deformation acted as the dominant wear mechanism for hypereutectic Al–25Si alloy.