Binary Al-Si Alloys Research Articles

Effect of friction stir processing on grain refinement and superplastic properties of binary Al-8 wt.% Si to Al-30 wt.% Si alloys

ABSTRACT As-cast binary Al–Si alloys containing ∼8, 12, 20, and 30 wt.% Si, representing the hypoeutectic, eutectic (12 wt.% Si), and hypereutectic compositions, were subjected to severe plastic deformation by friction stir processing (FSP) for grain refinement to 2.3–2.7 μm by dynamic recrystallization. Tensile tests conducted by differential strain rate test technique over the strain rates of 10−4 – 10−2 s−1 and test temperatures in the range of ∼840–530 K led to strain rate sensitivity index (m) varying from ∼0.04 to 0.40 depending on temperature, strain rate, and alloy composition. The constant initial strain rate tests conducted at 10−4 s−1 and 840 K exhibited a maximum elongation of 250% in the hypereutectic Al–20Si alloy, but the same increased linearly with m irrespective of alloy composition. Generally, with increasing Si content, the activation energy for deformation increased from 104.7 ± 14.4 kJ/mol for eutectic Al–12Si to 310.4 ± 32.3 kJ/mol for hypereutectic Al–30Si, which increased further to 572 ± 148 kJ/mol over the higher temperature range of 840–800 K. Analysis of observed deformation and microstructure behaviour supports the occurrence of superplasticity, whereby the accommodation of grain boundary sliding by grain boundary migration led to enhanced grain growth or else the local high-stress concentration at the particle-matrix interface led to cavity formation. There was no evidence of dynamic recrystallization during high-temperature tensile deformation but the flow softening observed is ascribed to the occurrence of concurrent cavitation.

Experimental constitutive relationships for high-temperature deformation of as-cast Al–Si binary alloys

ABSTRACT As-cast binary Al–Si alloys containing ∼2 to 30 wt.% Si were deformed by differential strain rate and differential temperature test techniques at 600–840 K and strain rates 10−4–10−2 s−1 to find the parameters of the constitutive relationships for high-temperature deformation. The maximum strain rate sensitivity index (m) of flow stress was found to be 0.28 in the eutectic Al-12Si alloy. The Arrhenius plot for activation energy for deformation (Q) exhibits bilinear behaviour, with Q being greater at higher test temperatures than at lower temperatures. The magnitude of Q varies as a function of Si content: 95.7 ± 9.8 kJ/mol for Al-2Si to 311.4 ± 98.1 kJ/mol for Al-12Si alloy. Also, the constant initial strain rate tests performed at selected strain rates and temperatures revealed flow hardening at small strains followed by flow softening. Deformation of Al–Si alloys is suggested to be controlled by the dislocation climb mechanism through the participation of both the grain interior and grain boundaries by consideration of effective stress instead of applied stress.

Effect of low-temperature annealing on electric resistivity and mechanical anisotropy of Al-Si binary alloy fabricated by laser-powder bed fusion

Al-12mass%Si binary alloy was fabricated by laser-powder bed fusion (L-PBF). The effect of low temperature annealing (LTA) less than 573 K on electrical and mechanical properties, microstructure, and residual stress of L-PBF Al-12mass%Si was investigated. Due to LTA, electrical resistivity at 77 K and 293 K changed from 34 nΩm down to 10 nΩm and from 68 nΩm down to 45 nΩm, respectively. The lattice constant of α-Al phase increased around 0.0005 nm by LTA according to X-ray diffraction. The change was associated with the decrease in the concentration of solid-solution Si of around 3.4 mass% by electrical resistivity measurement and 3 mass% by X-ray diffraction, when the annealing temperature was 473 K. Whereas, Vickers hardness increases around 145 HV compared with that for the as-built value of 141 HV, and then, decreases down to 126 HV and saturates. TEM/EDS observations confirmed the precipitation of fine Si within the α-Al phase. The decrease in annealing temperature increases the time to reach the Vickers hardness peak. 0.2% proof stress obtained by compression tests shows anisotropy, which can be attributed to the anisotropy on residual stress, which still exists after LTA.

Revealing the effect of Si content on the microstructure and properties of Al–Si alloy by experiments and phase-field simulation

The grain size of aluminum alloy is closely related to solute content and solidification conditions. This study investigated the influence of Si content on the solidification microstructure and properties of the Al–Si binary alloy by experiments and phase field simulation. The experimental results indicated that the microstructure of Al–Si alloy changes from coarse columnar grains to uniformly distributed equiaxed grains as Si content increases from 0 to 7 wt%, and the average grain size of Al–2Si is the smallest (2.2 mm). Meanwhile, the tensile strength of Al–Si alloys increases from 56.9 MPa to 111 MPa with increasing Si content, whereas the ductility decreases from 30% to 2.5%. Moreover, high casting temperature will produce coarse columnar crystals, low casting temperature forms equiaxed crystals, the optimal casting temperature in this study is 30 °C above the liquidus. Phase field simulations reveal that the evolution of Al–Si microstructures is governed by two dimensionless parameters: the initial temperature (T0), related to casting temperature and solidification interval, and the latent heat (K1), connected to latent heat, specific heat, and solidification interval. For a certain alloy composition (Al–2Si), the grain size exhibits a non-monotonic trend, decreasing initially and then increasing as T0 increases, with an optimal T0 value of 1.42. When the Si content varies between 2 and 7 wt%, the average grain sizes decrease as K1 increases, with the maximum K1 value observed at 1.38 for Al–2Si.

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%.

Combined effect of Sr-addition and pressure induced solidification on eutectic-Si morphology and mechanical properties of squeeze cast Al-Si binary alloy

Combined effect of Sr-addition and pressure induced solidification on eutectic-Si morphology and mechanical properties of squeeze cast Al-Si binary alloy

Systematic Investigation of Silicon Content Effects on the PEO Coatings’ Properties on Al–Si Binary Alloys in Silicate-Based and Tungstate-Containing Electrolytes

The present study evaluates the effect of a substrate’s silicon content on the coatings’ morphology, structure, and properties in an electrolyte containing sodium tungstate on Al–xSi alloys. The PEO-coated samples demonstrated the structure of the pancake and crater with irregular micro-cracks and micro-pores. The incorporation of the element W in the coatings decreased the surface roughness and porosity due to increasing the electrolyte conductivity and, hence, decreasing the breakdown voltage and intensity of micro-discharges. The friction coefficient of all the coatings stayed constant during the wear test due to the contact of the ball with the inner layer with less porosity and higher micro-hardness. The track depth to the thickness ratio of the coatings was found to be 0.60–0.75 for those without additive coatings and 0.55–0.65 for those with additive coatings. As a result, the higher wear rates, volume losses, track widths, and depths in the specimens with a lower silicon content of the substrate were found due to the greater thickness of the coating and the outer sublayer for both specimens with and without Na2WO4. The coatings formed using the W-containing electrolyte showed a higher micro-hardness and a lower wear rate in comparison with the coatings produced in the silicate-based electrolyte.

Effects of Remelting on Variations in Composition, Microstructure, and Hardness Property of Binary Al-Si Alloys

Effects of Remelting on Variations in Composition, Microstructure, and Hardness Property of Binary Al-Si Alloys

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

Effects of silicon content on electrical resistivity and thermal conductivity of Al-Si binary alloys.

Effects of silicon content on electrical resistivity and thermal conductivity of Al-Si binary alloys.

A case study of machinability evaluation of a gravity cast hypereutectic Al-Si binary MMC in dry turning

Hard Si particles when dispersed widely throughout a eutectic α-Al matrix forms hypereutectic Al-Si binary alloy which is actually a simple in situ metal matrix composite (MMC). Hence, like other Al MMCs, they offer amalgamation of high strength-to-weight ratio and excellent wear resistance properties with high specific elastic modulus, exceptional heat-resistance and low thermal expansion and hence, are used in many industries. Machining of such materials like all other Al alloys is a challenging task mostly because of formation of built-up edges (BUEs) and long continuous chips. A case study of machinability evaluation of such a hypereutectic material with excess Si contents manufactured by gravity cast is presented. The study has been accomplished through investigation and analysis while carrying out turning experiments on a lathe by coated carbide tool inserts. The Taguchi orthogonal array is used for design and Response Surface Methodology has been employed for regression analysis and optimization.

Influence of excess silicon phase on the thermal expansion behaviours of Al-Si binary casting alloys

Al-Si casting alloys are the most commonly used materials for piston alloys. The coefficient of thermal expansion is the key property of a piston material for improving the overall performance and service life of an engine. In the present study, the relationship between the morphology of the excess silicon phase and the coefficient of thermal expansion of Al-Si binary casting alloys was discussed. Optical and scanning electron microscopy were utilized to observe the morphology of the excess silicon phase in the Al-Si binary casting alloys before and after solution aging treatment. The results showed that the morphology of the excess silicon phase significantly influenced the coefficient of thermal expansion of the Al-Si binary casting alloys. After solution aging treatment, the coefficient of thermal expansion of the Al-Si binary casting alloys increased due to the rounding and granulating of the excess silicon phase precipitated during the casting process and decreased due to the precipitation of the finely dispersed Si phase in the α-Al matrix. The change in the coefficient of thermal expansion depended on which of the two kinds of morphological transformation of the excess silicon phase is dominant.

A Quantitative Study on the Interaction Between Silicon Content and Heat Treatment on Thermal Conductivity of Al-Si Binary Alloys

A Quantitative Study on the Interaction Between Silicon Content and Heat Treatment on Thermal Conductivity of Al-Si Binary Alloys

Composition-dependent solidification cracking of aluminum-silicon alloys during laser powder bed fusion

Consistent manufacturing of volumetrically dense engineering components, free of solidification cracks by laser powder bed fusion (LPBF), has been demonstrated for Al-Si alloys such as AlSi10Mg and Al12Si. The success in LPBF of these alloys is attributed to the near eutectic composition with a small freezing range. To illuminate this observation, cracking susceptibility was examined from Scheil-Gulliver solidification modeling by calculating the hot cracking susceptibility, |dT/dfS1/2|. To validate the findings from hot cracking susceptibility calculations, six binary Al-Si alloys, whose compositions were strategically chosen at hypo-, near-, and hyper-eutectic compositions, were gas atomized into alloy powders, and processed by LPBF. Only Al-Si alloys with 1.0 and 2.0 wt.% Si were found to exhibit cracking, which was predicted by relatively large magnitudes of |dT/dfS1/2|. Either as particles or with a eutectic structure, Si segregation at the intercellular boundaries was observed to define the sub-grain cellular structure. For selected compositions, measurement of the cellular structure allowed for estimation of the cooling rate to be 106 to 107 K•s−1. Excluding the alloys with solidification cracking, an increase in tensile strength and the corresponding decrease in ductility were observed with an increase in Si concentration, which were attributed to the formation of a cellular structure and the amount of Al-Si eutectic found at the intercellular boundaries.

Effects of Alloy Composition and Solid-State Diffusion Kinetics on Powder Bed Fusion Cracking Susceptibility

Laser powder bed fusion (LPBF) has demonstrated its unique ability to produce customized, complex engineering components. However, processing of many commercial Al-alloys by LPBF remains challenging due to the formation of solidification cracking, although they are labelled castable or weldable. In order to elucidate this divergence, solidification cracking susceptibility from the steepness of the solidification curves, specifically |dT/df S 1/2 |, as the fraction solidified nears 1 towards complete solidification, was calculated via Scheil–Gulliver model as a function of solute concentration in simple binary Al-Si, Al-Mg, and Al-Cu systems. Introduction of “diffusion in solid” into Scheil–Gulliver model resulted in a drastic reduction in the cracking susceptibility (i.e., reduction in the magnitude of |dT/df S 1/2 |) and a shift in the maximum |dT/df S 1/2 | to higher concentrations of solute. Overall, the calculated solidification cracking susceptibility correlated well with experimental observation made using LPBF AA5083 (e.g., Al-Mg) and Al-Si binary alloys with varying Si concentration. Cracking susceptibility was found to be highly sensitive to the composition of the alloy, which governs the variation of |dT/df S 1/2 |. Furthermore, experimental observation suggests that the contribution of “diffusion in solids” to reduce the cracking susceptibility can be more significant than what is expected from an instinctive assumption of negligible diffusion and rapid cooling typically associated with LPBF.

Structure and stability of vacancy–solute complexes in Al–Mg–Si alloys

This study investigates the structure and stability of vacancy–solute complexes in Al–Mg–Si alloys using first-principles calculations along with the prediction of initial structures using multiple linear regression analysis. In Al–Mg and Al–Si binary alloys, vacancy–solute complexes become unstable when more than five solute atoms are combined with a vacancy, due to the repulsive interactions between adjacent atom pairs. In Al–Mg–Si alloys, the formation energy of the vacancy–solute complexes continues to decrease as the number of solute atoms increase. The stability of vacancy–solute complexes mainly arises from the Mg–Si bonds formed with the increasing number of solute atoms. The combination of inward displacement of the Mg atoms and outward displacement of the Si atoms accommodates the increase in the number of Mg–Si bonds. Owing to short range repulsive interatomic interactions, Mg–Mg bonds cannot be formed in vacancy–solute complexes of Al–Mg–Si alloys. However, Si–Si bonds are a preferable alternative to enhance stability in vacancy–solute complexes, despite their decreased strength compared with Mg–Si, Al–Mg, and Al–Si bonds. The concentration of Si atoms in the vacancy–solute complexes is always larger than that of Mg atoms at finite temperatures. The stability of the Si-rich composition originates from the difference between the Si–Si and Mg–Mg interactions in the vacancy–solute complexes.

Transient heat flow parameters and microstructure of the upward solidified eutectic Al–Si alloy: a theoretical and experimental study

ABSTRACT In this work, a theoretical and experimental study has been carried out with the eutectic Al12.6Si alloy (wt.%) in order to determine and evaluate the transient heat flow parameters, such as metal/mold interfacial heat transfer coefficient (hi), growth and cooling rates (VL and TR, respectively), during upward solidification. The experimental thermal data were obtained via temperature mapping using a directional solidification device as well as via mathematical model to determine the time-varying hi values given by the general expression hi=at -n . The VL and TR values were determined from the generated simulated and experimental curves. The results showed a good agreement between the simulated and experimental values. The prediction of the microstructure has revealed the formation of irregular eutectic as well as the presence of non-faceted α-Al primary phase, like halos dendrites, and massive faceted primary Si crystals. Mathematical expressions that correlate halos dendrite spacing (λHD) as a function of hi, VL and TR have been proposed. The halo microstructure has been observed when hi, VL and TR values reach 3,608.00 WmoC−1, 0.36 mms−1 and 0.13 oCs−1, respectively. A comparative analysis with other works from literature has been conducted and the results have confirmed that the n exponent assumes values less than 0.5 for non-ferrous alloys (n < 0.5). For the particular case of binary Al–Si alloys it can be less than 0.35, that is, n < 0.35.

Predicting gas and shrinkage porosity in solidification microstructure: A coupled three-dimensional cellular automaton model

Porosity formation during solidification of aluminum-based alloys, due to hydrogen gas and alloy shrinkage, has been a major issue adversely affecting the performance of solidification products such as castings, welds or additively manufactured components. A three-dimensional cellular automaton (CA) model has been developed, for the first time, to couple the predictions of hydrogen-induced gas porosity and shrinkage porosity during solidification microstructure evolution of a binary Al-Si alloy. The CA simulation results are validated under various cooling rates by porosity measurements in an experimental wedge die casting using X-ray micro computed tomography (XMCT) technique. This validated porosity moel provides a critical link in integrated computation materials engineering (ICME) design and manufacturing of solidification products.

Effect of pressure on solidification cracking susceptibility of Al–Si alloys

ABSTRACTAn approach was developed to calculate the crack susceptibility under various levels of pressure, and the corresponding numerical method was presented. The binary Al–Si alloy system was selected for study because the effect of high pressure on its phase diagram has been reported. The results showed a higher pressure can lead to a higher crack susceptibility and shift the most crack susceptible composition to higher solute contents. It was found a higher pressure can increase the effect of back diffusion on the solidification path and hence the crack susceptibility. This study provides a new understanding of the effect of pressure on solidification cracking susceptibility and can be a relevant starting point for studying solidification cracking under high pressures.

Engulfment and distribution of second-phase nanoparticle during dendrite solidification of an Al–Si binary alloy: a simulation study

To achieve optimum strengthening effects of external nanoparticles (NPs), uniform dispersion of NPs in the melt is necessary for the casting manufacturing of metal matrix nanocomposites in which dislocation-based strengthening mechanisms play a significant role. However, the engulfment of nanoparticles within the solidifying grains and the avoidance of pushing them outside the solidification front are always a major challenge. Therefore, the understanding of local interface velocity and interface/particle dynamics during alloy solidification is of significant importance. Existing numerical studies on particle engulfment/pushing do not take into consideration the anisotropy of crystal growth and assume planar solidification interface, and thus they are unable to obtain the nanoparticle distribution in realistic alloy solidification. In this research, we investigate the engulfment/push behavior and the overall distribution of SiO2 nanoparticles in the dendrite solidification of an Al–Si binary alloy. Phase-field method is used to simulate the dendrite growth and to predict local solidification front velocity. In combination with the critical engulfment velocity obtained from a non-steady-state particle/front interaction model, the engulfment/push behavior of the entire solidification domain as well as the final distribution of nanoparticles can be analyzed. It is found that the distribution pattern of NPs obtained from simulation is overall consistent with the limited experimental results in the literature. In addition, the two main dislocation-based strengthening effects, e.g., Orowan bowing and CTE (coefficient of thermal expansion) mismatch strengthening, brought by external nanoparticles are quantitatively predicted. The degree of undercooling (60, 80, and 100 K) and nanoparticle size (10, 20, 30, 40, and 50 nm) are varied to investigate their influence on the NP engulfment behavior and the resulted strengthening effects.