Refinement Of Eutectic Si Research Articles

Research on compression-compression fatigue properties of carbon nanotubes reinforced closed-cell aluminum matrix composite foams by reinforcement content design

As the requirement of compressive fatigue loading on the mechanical properties of aluminium foams increase rapidly, the limited fatigue performance of them significantly restricts their engineering applications. In this study, the compression-compression fatigue properties of carbon nanotubes (CNTs) reinforced closed-cell Al-Si composite foams (C-ASCFs) were investigated. The fatigue strength of C-ASCFs with 0.50 wt% CNTs increased by 54% than that of Al-Si foams, where the CNTs contributed to decrease the crack expansion rate, transfer the fatigue load and hence change the fatigue failure mode, causing a combination of plastic and brittle deformation in them. Meanwhile, the appropriate incorporation of CNTs decreased pore nucleation threshold, optimized the pore morphology, and promoted the formation and refinement of eutectic Si. Such inhibited the probability of fatigue cracks initiation, resulting in the enhancement of fatigue life. This work provides theoretical and practical basis of design strategy of Al-Si composite foams and broadens their application prospect.

Effect of carbon nanotubes on the mechanical properties of cast Al–Si–Cu alloys

To enhance the mechanical properties of cast Al–Si–Cu alloys (ADC12), 0.3% of carbon nanotubes (CNTs) was added. The cast ADC12–0.3%CNT samples were synthesised via heated mould continuous casting for the creation of fine grains, and the artificial aging process was conducted at 173°C for 13 h to obtain precipitation hardening. The reinforcement and refinement of eutectic Si and precipitates (Al2Cu and Mg2Si) were achieved by adding CNTs, which leads to a high tensile strength of about 450 MPa and a relatively high ductility of about 10%. The obtained tensile strength and 0.2% proof stress are more than twice as high as those of the conventional cast ADC12 alloy. The refinement of the microstructures could be due to the interruption of their growth caused by the CNTs.

Improving the mechanical properties of the B319 aluminum alloy by addition of cerium

The effect of cerium (Ce) addition (0.1, 0.3, 0.5, and 1.0 wt%) on the ambient and high-temperature mechanical properties of a commercially available B319 powertrain aluminum alloy was investigated. In addition, to characterize the effects that Ce has on the microstructure of the B319 alloy, field emission scanning electron microscopy, optical microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy and X-ray micro computed tomography were employed. Statistical significance of the results was evaluated using the single-factor ANOVA analysis. The microstructural analyses revealed that Q-AlCuMgSi, α-Al, Al2Cu, eutectic silicon (Si), and Al15(Fe,Mn)3Si2 were present in all of the B319 and B319 + Ce specimens. However, the addition of Ce led to the formation of Al3Ce4Si6 and AlCeSi2 as well as a refinement of eutectic Si. It was found that, addition of 0.1 wt% Ce to the B319 alloy considerably increased the yield and tensile strength at ambient temperature (~10% and ~9%, respectively) and at 250 °C (~14% and ~7%, respectively) as well as the alloy's % elongation and micro hardness (~18–60% and 11%, respectively). However, it was observed that further addition of Ce (up to 1.0 wt %) results in a slight decrease in the materials tensile strength, as compared to the B319 + 0.1% Ce sample. This is presumed to be attributed to the escalating amount of porosity and needle-like Ce bearing intermetallics that were observed in the higher Ce content samples. The results from this study provide the automotive industry with a new cost-effective method of improving the mechanical properties of their currently used powertrain alloys, allowing for greater efficiency and improved performance.

Super-gravity field assisted homogeneous distribution of sub-micron eutectic Si in Al–Si alloy

We reported an investigation of super-gravity field assisted homogenization and refinement of eutectic Si in Al–Si alloy. Our results indicated that the super-gravity could intensify the convection and greatly promote the refining effect of the conventional modifier during solidification. The orientation relationship of the eutectic Si with respect to Al matrix is adopted [1(_) 12]Al//[1(_) 12]Si. The high density of sub-micron eutectic Si (∼0.5 μm in length, ∼0.2 μm in width) could be achieved via the combination of the sodium salt modifier and super-gravity field, which resulted in the significant enhancement of the Vickers hardness (∼52%).

Solidification behavior, microstructure and silicon twinning of Al-10Si alloys with ytterbium addition

The solidification behavior, microstructure and silicon twinning of Al-10Si alloys with Yb addition were investigated by thermal analysis, optical microscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results indicate that the nucleation temperature, minimum temperature, and growth temperature of Al-10Si alloys decrease with increasing Yb content. The cooling curves of the Yb-modified alloys exhibit marked recalescence. The recalescence of the modified alloy peaks at 2.3 °C when the Yb content is 0.7 wt%. The 3D morphologies of eutectic Si in Yb-modified alloys change from a coarse plate-like structure to a honeycomb structure with many fine fibrous structures. The Al-Si-Yb intermetallic compound is observed in the 1.0 wt% Yb-modified alloy. Meanwhile, XRD analysis and TEM results indicate that the average twin spacing in the Yb-modified alloys is 18–46.2 nm. The average twin spacing of eutectic Si decreases with increasing Yb content. When the Yb content in the modified alloy is increased to 0.7 wt%, the average twin spacing value of eutectic Si reaches to 18 nm, which promotes the formation of twins and refinement of eutectic Si.

Effect of the laser surface treatment on structure and mechanical properties of AlZn10Si8Mg cast alloy

The laser surface melting/hardening is method which provides increasing mechanical properties of secondary (recycled) Al-Si cast alloys for automotive industry. Improvement of mechanical properties and structure of secondary aluminium alloys can often significantly increase the lifetime of casting and reduce costs for fuel and reduction of environmental loading. Self-hardening Al-alloys (Al-Zn-Si-Mg alloys) introduce an innovative class of light Al-alloys. AlZn10Si8Mg alloy is used for engine and vehicle constructions, hydraulic unit and mould making without the need of heat treatment because this alloy is self-hardened. For this study, a laser beam of Nd: YAG laser, BLS 720, with laser power 50 and 80 W, was used. The final microstructure of Al- alloys depend on the laser process parameters. The laser beam action on the surface of the experimental alloy has changed the microstructure of the material. Melting area is alpha-phase with much fine columnar dendrites morphology without the presence of Si-particles and intermetallic phases. In the transition area, grain refinement of eutectic Si (finer and rounder Si particles) as the modify action of the laser was observed. By increasing the laser power the microhardness of surface layers decreases. In the surface layer (after the 80 W laser beam action), cracks due to uneven heat transfer of the material were observed.

Refinement of Eutectic Si in High Purity Al-5Si Alloys with Combined Ca and P Additions

The effects of combined additions of Ca and P on the eutectic Si in a series of high purity Al-5 wt pct Si alloys have been investigated with the entrained droplet technique and complementary sets of conventional castings. Differential scanning calorimetry (DSC) and thermal analysis were used to investigate the eutectic droplet undercooling and the recalescence undercooling, respectively. Optical microscopy, SEM, EPMA, and TEM were employed to characterize the resultant microstructures. It was found that 250 ppm Ca addition to Al-5Si wt pct alloys with higher P contents leads to a significant increase of the eutectic droplet undercooling. For low or moderate cooling rates, the TEM results underline that Ca additions do not promote Si twinning. Thus, a higher twin density cannot be expected in Ca containing Al-Si alloys after, e.g., sand casting. Consequently, a refinement of the eutectic Si from coarse flake-like to fine plate-like structure, rather than a modification of the eutectic Si to a fibrous morphology, was achieved. This strongly indicates that the main purpose of Ca additions is to counteract the coarsening effect of the eutectic Si imposed by higher P concentrations. Significant multiple Si twinning was observed in melt-spun condition; however, this can be attributed to the higher cooling rate. After DSC heating (slow cooling), most of Si twins disappeared. Thus, the well-accepted impurity-induced twinning mechanism may be not valid in the case of Ca addition. The possible refinement mechanisms were discussed in terms of nucleation and growth of eutectic Si. We propose that the pre-eutectic Al2Si2Ca phase and preferential formation of Ca3P2 deactivate impurity particles, most likely AlP, poisoning the nucleation sites for eutectic Si.

Unconventional Metallographic Methods for the Structural Characterization of Laser Hardened Al-Zn-Si Cast Alloy

The laser surface hardening is method which provides increased mechanical properties of secondary (recycled) Al-Si cast alloys for automotive industry. Improvement of mechanical properties and structure of secondary aluminium alloys can often significantly increase the lifetime of casting and reduce costs for fuel and reduction of environmental loading. For study was used a laser beam Nd: YAG lasers, BLS 720, on the test samples of secondary cast alloy AlZn10Si8Mg. AlZn10Si8Mg cast alloy are used for engine and vehicle constructions, hydraulic unit and mould making without the need of heat treatment because this alloy is self-hardened. The effect of laser beam was evaluated with the laser power 50 W and 80 W on the surface of samples. The final microstructure of Al-alloys depend on the laser process parameters. The changes of microstructure after laser surface hardening was observed by using classical techniques of etching (standard black-white contrast - etching by 0.5 % HF, 10 % H3PO4and colour contrast - etching by Weck-Al) and deep etching with HCl. Due to the action of laser beam on the surface of the secondary alloy AlZn10Si8Mg there have been changes in the microstructure of the material. Melting area is alpha-phase with much fine columnar dendrites morphology without the presence of Si-particles and intermetallic phases. In the transition area were observed grain refinement of eutectic Si (finer and rounder Si particles) as the modify action of the laser. By increasing the laser power the microhardness of surface layers decreases. In the surface layer (80 W) were observed cracks due to uneven heat transfer of the material.

Effect of Y addition and cooling rate on refinement of eutectic Si in Al–5 wt-%Si alloys

A series of Al–5 wt-%Si alloys with or without 200 ppm Y addition have been produced using conventional casting and melt spinning respectively. The effects of Y addition and cooling rates on the refinement of eutectic Si have been investigated using thermal analysis, differential scanning calorimetry and multiscale microstructure characterisation techniques. In the case of conventional casting, the addition of up to 200 ppm Y was found to cause no modification effect. The eutectic Si presented a plate-like structure rather than a fibrous morphology. No significant Si twinning was observed. In the case of melt spinning, a higher cooling rate caused a much finer eutectic Si and Si twinning. The addition of 200 ppm Y promoted heavier multiply Si twinning. After a continuous cooling from 873 to 673 K, an Al2SiY phase was observed beside or within the Si particle. The refinement mechanism was also discussed.

Effect of material and test parameters on wear behaviour of eutectic Al–Si alloy (LM13)

This paper investigates the effects of contact pressure, sliding speed and alloy condition on dry sliding wear behaviour of LM13 (Al–12Si–0˙8Cu–1˙0Ni–0˙6Mg) alloy. A pin on disc type wear testing machine was used to study the wear behaviour of alloy in untreated, melt and heat treated condition. Dry sliding wear tests were performed at two contact pressures (0˙31 and 0˙93 MPa) and four sliding speeds (0˙2, 0˙5, 0˙9 and 2˙0 m s–1). The wear rate decreased with the increase in sliding speed irrespective of contact pressure and alloy condition used in this work. The untreated alloy exhibited higher wear rate than melt and heat treated alloy. The refinement of eutectic Si by melt treatment and spheroidisation of eutectic Si after heat treatment of LM13 alloy have been attributed to the reduction in wear. Melt treatment and heat treatment of LM13 alloy increased the hardness and tensile strength, while increase in ductility was marginal.