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The aim of this study is to identify the deformation behavior of a high-pressure die-cast Silafont®-36 aluminum alloy in an overaged T7 condition in relation to the significant microstructural changes caused by the overaging heat treatment. The T7 aluminum alloy exhibited a distinctive composite-like microstructure, consisting of coarser α-Al grains and larger spherical Si particles embedded in the matrix, in contrast to its as-cast and T4 counterparts. The softer α-Al matrix of the T7 alloy as revealed by the nanohardness and microhardness led to improved ductility and longer strain-controlled fatigue life at higher strain amplitudes, despite the lower strength. Cyclic stabilization was observed to be a salient feature of the T7 alloy, stemming from the equilibrium between cyclic hardening and cyclic softening. Strain-energy density-based model, by incorporating a material parameter called 'intrinsic fatigue toughness', could be used to predict the fatigue life of the alloy. [ABSTRACT FROM AUTHOR]

To evaluate the role of Fe-rich intermetallics on crack initiation, two fully modified Al-Si alloys, one containing plate-like β-Fe and the second containing primary α-Fe intermetallics, were investigated by in-situ tensile testing in the scanning electron microscope. In the first alloy, large plate-like β-Fe intermetallics oriented parallel to the test direction were the first to crack at an elongation of about 1.8%. More transversely oriented intermetallics caused crack initiation in the matrix which linked up with the final fracture. In the second alloy, the cracking of α-Fe intermetallics initiated at an elongation of about 0.9%. It is concluded that large α-Fe intermetallics crack first and that clusters of α-Fe are the most potent crack initiation sites. [ABSTRACT FROM AUTHOR]

Chromium (Cr) is rarely added to Al–Si die cast alloys as it has a strong tendency to induce the formation of coarse-sized sludge particles. In the present study, high pressure die-cast samples made of Al-7wt.%Si–Cr–Mg alloys with varied Fe contents (0.11 wt%, 0.26 wt% and 0.32 wt%) were tensile tested under as-cast and T5 heat-treated conditions, in comparison to those made of Al-7wt.% Si–Mn–Mg alloy with a constant Fe content (0.11 wt%). The results demonstrated that under the as-cast condition, the Al–Si–Cr alloys with higher Fe contents can achieve comparable tensile properties as the Al–Si–Mn alloy with strictly controlled Fe level. However, after a T5 heat treatment (205 °C for 60 min), the increment in yield strength is less for the Al–Si–Cr alloys in relative to the Al–Si–Mn alloy, regardless of Fe contents. Scanning electron and scanning transmission electron microscopic characterizations were conducted on the as-cast and heat-treated microstructures, aiming to understand the undermined ageing response caused by Cr addition. • Ageing-induced increment in tensile yield strength is less for Al–Si–Cr–Mg alloys in comparison to Al–Si–Mn–Mg alloy. • Ageing-induced increment in tensile yield strength is mainly determined by the precipitation behavior at sub-surface location. • The intervention of Cr solute atoms on nucleation process of Si nanoprecipitate at sub-surface location undermined ageing response of Al–Si–Cr–Mg alloys. [ABSTRACT FROM AUTHOR]

The quasi-static and dynamic deformation behaviours of high-pressure die-casting (HPDC) Al7Si0.2 Mg alloy were studied in this work. Competing effects among porosities, α-Al(Fe/Mn)Si phase, eutectic phase and α-Al dendrites on strain concentration and crack propagation at different strain rates were investigated using X-ray computed tomography (CT), scanning electron microscopy (SEM), in-situ electron backscattering diffraction (EBSD), and high-resolution digital image correlation (HR-DIC). The results indicate that yield strength and ductility of Al7Si0.2 Mg alloy were improved at high strain rates compare to quasi-static loading. Weak zones, including porosities and particle interfaces, primarily contributed to deformation heterogeneity under quasi-static tension, whereas both weak zones and Al matrix played a role under dynamic loading. Moreover, crack propagation was hardly observed on porosities during dynamic deformation, thereby reducing the sensitivity of ductility to porosity. The slip and rotation of α-Al dendrites became more pronounced during dynamic deformation. As a result, the area fraction of fractured eutectic phase increased dramatically and surpassed the area fraction of α-Al(Fe/Mn)Si phase, which became the dominant factor of crack propagation during dynamic deformation. [ABSTRACT FROM AUTHOR]

The effects of Gd and/or Zr additions on the grain refinement and eutectic Si modification of A356 cast alloys have been systematically investigated. The results showed that a separate addition of Gd or Zr had almost no effect on eutectic Si modification, while except for an appreciable grain refinement, a combined addition of Gd and Zr significantly refined the eutectic Si and changed its morphology from plate-like to fibrous. As compared with Gd, Zr and Sr separate modification, the Gd and Zr combined addition produced fewer twins but more nanosized precipitates within eutectic Si. The excellent modification effect was attributed to the increase of constitutional undercooling and nanosized precipitates inhibited the growth of eutectic Si during solidification. The Gd and Zr combined addition obviously improved the mechanical properties of A356 cast alloy due to grain refinement and eutectic Si modification. Optimal tensile properties were achieved in the 0.4Gd+0.5Zr modified alloy. A high amount of Zr addition resulted in the formation of coarse Al 5 Si 2 Zr 3 phase and the decrease of ductility. [ABSTRACT FROM AUTHOR]

In this study the micro-deformation mechanisms in an Al–Si cast alloy under stepwise compressive loading up to a cumulated plastic strain of 9.7% are examined via detailed EBSD and slip trace analyses. The presence of heterogeneous microstructural features in the alloy like eutectic structure and second-phase particles significantly affects slip trace characteristics. Four types of slip traces (namely, bowed, 'C'-type curved, 'S'-type curved, and flat slip traces) present in primary α-Al grains are identified, which unveil the nature of dislocation motion in the alloy and reflect the extent of deformation difficulty associated with α-Al grain orientations and eutectic Si particles. Crystallographic factors like lattice rotations, misorientation angles, active slip systems, and Schmid factors are jointly analyzed for the first time to understand the deformation behavior of the heterogeneous alloy. [ABSTRACT FROM AUTHOR]

The variation of tensile properties of A356alloy at 20 °C~−60 °C has been investigated. the distribution of dislocation slip bands near the fracture surface was studied by scanning electron microscopy SEM. Dislocation distribution was observed by transmission electron microscope TEM. The elastic stress field near the Mg 2 Si phase at low temperature was analyzed by ANSYS. When the temperature is decreased from 20 °C to −60 °C, the tensile strength and yield strength of A356 alloy increase and the elongation decreases. Under the effect of cold shrink, the elastic stress produced by the alloy matrix hinders the movement of the dislocation. In low temperature environment, the edge dislocation whose excess half atom facing toward Mg 2 Si experiences a repulsing force. On the contrary it experiences a gravitational force. [ABSTRACT FROM AUTHOR]

The effect of stress concentration on the dislocation motion, the Si particles and the crack propagation path in A356 alloy at the temperature of 20 °C to −60 °C was analyzed by scanning electron microscope and optical microscope using a series of notched tensile specimens and normal tensile specimens. The results show that the sensitivity of A356 alloy to the stress concentration increases, the tensile strength and yield strength of normal specimens and notched specimens increase, and the elongation shows a decreasing trend with the decrease of test temperature from 20 °C to −60 °C. The yield strength is not affected by the notch, and the tensile strength is sensitive to the stress concentration. Stress concentration leads to a large number of dislocation generation. Local plastic deformation occurred in the stress concentration region during the tensile process firstly. With the stress concentration in the aluminum matrix between the Si phase and the crack further increasing, the distribution of cracks along the Si phase leads to the cracking of aluminum matrix particle. [ABSTRACT FROM AUTHOR]

Heat input is one of the most important process parameters during additive manufacturing (AM). It is of great significance to understand the effect of heat input on the microstructure and nanomechanical properties, as well as the underlying mechanisms. Wire-arc additive manufactured (WAAM-ed) Al 4047 alloys under different heat inputs were produced and studied in this work. The as-manufactured Al alloys showed hypoeutectic microstructure that consisted of primary Al (α-Al) dendrite and ultrafine Al–Si eutectic. The effect of heat input on hardness and strain rate sensitivity (SRS) were investigated through nanoindentation. The nanohardness decreased with the increasing heat input, in accordance with the trend of yield strength and microhardness in the previous studies, in which the mechanism was usually explained by the grain growth model and Hall-Petch relationship. This work suggests a distinct mechanism regarding the effect of heat input on nanohardness, which is the enhanced solid solution strengthening produced by lower heat input. In addition, the heat input had little effect on the SRS and activation volume. It is hoped that this study leads to new insights into the understanding of the relation between heat input and nanomechanical properties, and further benefits to improve the targeted mechanical properties and engineering applications of the AM-ed materials. • The WAAM-ed Al 4047 alloys under different heat inputs were studied. • The effect of heat input on hardness and SRS were investigated by nanoindentation. • The hardness of both α-Al and Al–Si eutectic decreased with the heat input. • The heat input had little effect on the SRS. [ABSTRACT FROM AUTHOR]

This study demonstrates the evolution of microstructure and mechanical properties of a hypoeutectic Al–7Si–0.4Mg (A356) alloy processed by accumulative back extrusion (ABE) at temperatures ranging from 200 to 500 °C. ABE processing is one of the new severe plastic deformation techniques enabling one to produce relatively large ultrafine-grained materials in a cylindrical shape. One complete pass of ABE was estimated to introduce a reasonably homogeneous effective strain of ~3 as calculated by finite element analysis. Microstructural observation showed that globular α-Al primary phase was subdivided into fine substructures and Si particles having a fibrous shape were fragmented and spheroidized within the eutectic constituent through ABE processing. There was no evidence of homogeneous distribution of the fine Si particles in the α-Al phase after ABE. Mechanical testing at room temperature showed that both yield strength and ultimate tensile strength of the A356 alloy dramatically increased through ABE, especially at lower processing temperatures, as compared with the as-cast condition whereas there was no significant reduction in ductility at all processing temperatures. The experimental results were discussed with emphasis on the microstructure evolution involving dynamic recrystallization and deformation behavior including strengthening mechanisms and strain hardening in the Al–Si alloy. [ABSTRACT FROM AUTHOR]

The aim of this study was to determine cyclic deformation behavior of a high-pressure die-cast Silafont®-36 automotive alloy with heterogeneous microstructures, focusing on the effect of strain ratio (R ε) ranging from negative infinity (-∞) to +0.5 at a constant strain amplitude of 0.4%. Pronounced cyclic hardening behavior was observed from the stress-strain hysteresis loops at different strain ratios. The degree of cyclic hardening, evaluated via both the traditional method and a newly-proposed slope method, increased with decreasing strain ratio. Fatigue life of the alloy was slightly lower at positive strain ratios, with its peak emerging at zero mean strain (or R ε = −1). In the negative range of strain ratios (R ε <−1), the fatigue life increased slightly with increasing strain ratio, which was mainly related to the plastic strain amplitude. A new power-law based equation was proposed to describe the change of mean stress relaxation at negative strain ratios (R ε <−1), along with a polynomial equation for depicting the situation at zero and positive strain ratios. Strain-energy density analyses corroborated that the intrinsic fatigue toughness was a material constant since it was not affected by strain ratio. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, AlSi8Cu3 alloy chips were consolidated via HPT at an applied pressure 8GPa for 10 revolutions at room temperature. The microstructure and hardness of the HPT consolidated chips were investigated and compared with those of the HPT processed solid AlSi8Cu3 alloy samples and cold compacted chip samples. The HPT process successfully resulted in fully densified bulk samples with a higher microhardness due to the finer Al matrix grains and Si particles sizes with more grain boundaries with high angles and more homogeneous deformation than those in the HPT processed solid AlSi8Cu3 alloy samples due to the higher imposed total strain. [Copyright &y& Elsevier]

Abstract: The mechanical properties and microstructure of commercial Al–15Si–2.5Cu–0.5Mg (Alumix-231®) alloy was subjected to extrusion (EXTR), hot extrusion into rods at 565°C at an extrusion ratio of 4:1 and high-pressure torsion (HPT) processing, one of the most severe plastic deformation (SPD) techniques, with an anvil rotation speed of 0.5rpm under a quasi-hydrostatic pressure of 5GPa. Afterwards, the microstructures of specimens were systematically investigated using XRD, SEM, EBSD, and TEM. Also, tensile properties of Alumix-231 processed by EXTR and HPT were determined at room temperature. The HPT specimen shows refined microstructure (around average 165nm) having more spherical and homogeneously distributed eutectic silicon second phases in Al metal matrix as compared to EXTR specimen. The hardness values are greater on the surface of the HPT specimen (239.1Hv) as compared to that EXTR specimen (71.2Hv) and increase with distance from the center of HPT specimen (198.3Hv). Tensile strength and ductility of HPT specimen increased approximately 2 times than that of EXTR sample, respectively. The results indicate that the morphological changes in eutectic silicon especially after HPT play a critical role in enhancing or limiting the overall properties of the Alumix-231 material. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

Abstract: Effects of strain rate (10−4–10−2 s−1) on tensile and compressive strength of the Al–Si alloy and Al–Si/graphite composite are investigated. The strain hardening exponent value of the composite was more than that of the alloy for all strain rates during tensile and compressive loading. The yield stress of the composite was more than that of the ultimate tensile strength of the alloy for all strain rates. Tensile and compressive properties of the alloy and composite are dependent on strain rates. The negative strain rate sensitivity was observed for the composite and alloy at lower strain rates during the compression and tension loading respectively. [Copyright &y& Elsevier]

Abstract: Effect of alloying elements including Si, Cu, Ni and Mg with different existing forms on the microhardness of Al–12Si–4Cu–2Ni–2.6Mg (in wt.%) piston alloy was investigated. It is found that microhardness of the alloy reaches to 1.88GPa when all the Cu, Ni, Mg and 3.63wt.% Si dissolved in the α-Al matrix, and increases to 2.02GPa while only the solid-solved Si precipitates to form nanoparticles. The precipitated Si has greater contribution to the alloy''s microhardness than the solid-solved Si in the same level. Furthermore, the precipitation of intermetallics containing Cu, Ni and Mg that disperses in the Al-matrix can further improve the microhardness to 2.09GPa. The hardening efficiency of the intermetallics precipitation of Cu, Mg and Ni on microhardness is less than that of the nanoscale Si precipitates under the same mass fraction. For Cu, Mg and Ni alloying elements, whether dissolving in α-Al matrix or precipitating as nanoparticles, there is no evident difference on improving microhardness of the tested alloy. [Copyright &y& Elsevier]

Abstract: Direct chill (DC) cast Al–12.7Si–0.7Mg alloy without chemical modification was hot extruded and subsequently heat-treated under various conditions. The microstructure and tensile properties of the extruded profile were investigated aiming at understanding the strengthening mechanisms. The results show that the refined microstructure characterized by fine Si particles uniformly distributed in the Al matrix of fine equiaxed grains was promoted by hot extrusion. The alloy after T6 treatment exhibits good ductility and much higher proof strength as well as tensile strength compared to 6063 alloy in general. The strengthening was attributed to be the combining effect of particle, grain boundary and precipitation strengthening. [Copyright &y& Elsevier]

Abstract: In order to assess Al–Si alloy as potential high temperature structural material, the tensile and wear properties of Al–Si alloy was investigated at various temperatures from ambient to 350°C. Al–Si alloy was fabricated by stir casting technique. Microstructural studies showed that reasonably uniform distribution of silicon particles. It was found that the tensile strength (strain rate=10−3 s−1) of alloy was decreasing with increase of temperature. The flow stress also decreased with increase of deformation temperature. The effect of temperature on tensile behaviour of the alloy had been evaluated and fractographs revealed that fracture behaviour of Al–Si alloy changed from brittle to ductile mode with increase of temperature. The increase in wear resistance of Al–Si alloy with increasing temperature was observed from wear tests. The oxidational wear was predominantly occurred during sliding. [Copyright &y& Elsevier]

Abstract: Effect of hot extrusion on microstructure and mechanical properties of near eutectic casting Al–12.0%Si–0.2%Mg alloy was investigated by mechanical properties testing and microstructure observation using OM (optical microscope), SEM, and TEM. Dynamical recrystallization of Al matrix, dynamical precipitation of deep sub-micron Si and Mg2Si particles and deformation-induced twinning of Si particle occur during extrusion. The mechanism of dynamical precipitation and recrystallization during hot extrusion for near eutectic Al–Si–Mg alloy is concluded according to microstructure observation. The dynamically precipitated particles are thought to play a Zener pinning role on the growth of recrystallized grains, resultantly leading to a much finer grain size of Al matrix. A good combination of tensile strength (256.3MPa) and elongation (15.0%) when aged for 12h after post-extrusion solution treatment is mainly due to fine aging precipitates and recrystallized fine grain structure. The negligible aging hardening effect of extruded samples is due to dynamical precipitation of sub-micron Si and Mg2Si particles. [Copyright &y& Elsevier]

Abstract: Al–Si–Mg/SiCP foams were prepared by melt processing route with TiH2 as the blowing agent. The effects of foaming temperature (640 and 670°C), SiCP size (12 and 21μm), and volume % (5–20) on liquid foam expansion and subsequent collapse were studied. Irrespective of the particle size, significant collapse was observed in the liquid foam at 670°C. At the lower foaming temperature of 640°C, the liquid foam collapse was low to moderate, with finer and higher (15/20%) contents of SiCp resulting in low collapse and the best overall quality. Preferential segregation of particles to gas/metal interface, postulated by earlier investigators, was not observed in the present case. The presence of SiC within the cell walls induced brittleness, and created irrecoverable damage during the initial phase of compressive deformation. The compressive strength of the foams was lower than theoretical predictions. Minor defects such as porosity, present within the cell walls, might be responsible in lowering of the compressive strength. [ABSTRACT FROM AUTHOR]

Abstract: This study was undertaken to investigate the effect of Fe-rich intermetallics on the wear behavior of SAE F332 piston alloy. The wear behavior of Al–Si alloys was influenced by the presence of hard and brittle β-Al5FeSi intermetallics. It was shown that an increase of iron from 0.15wt% in the base alloy to about 0.7wt% increased the hardness and improved the wear resistance by about 10% under applied loads of 20 and 40N. Addition of iron up to 2.5wt% further increased the hardness, however, it decreased the wear resistance by over 70% under the same applied loads. The negative effect of intermetallics was controlled via combined effects of high cooling rate solidification and strontium modification. The wear resistance of the controlled 1.2wt% Fe alloy was improved by about 50% compared to the unrefined alloy with the same amount of iron under applied loads of 20 and 40N. [Copyright &y& Elsevier]

The wear behaviour of spray formed Al–3.5Cu–10Si–20Pb (wt.%) alloy has been studied in different environmental conditions. The wear tests were carried out in air atmosphere and vacuum, using a pin-on-disc type wear testing equipment. The wear behaviour in air atmosphere clearly indicated the influence of surface oxidation on the mating surfaces. This resulted in the formation of irregular shaped oxide particles having bright contrast in SEM investigation of worn surfaces and debris particles. The nature of variation in wear with sliding distance, both in air atmosphere and vacuum, remained similar showing two distinct wear regimes, viz., running in wear and steady state wear. A relatively lower running in period and a large reduction in the frictional force were observed in vacuum as compared to that obtained in air under same conditions of sliding speed and applied pressure. The wear rate varied from (0.25 to 0.61)×10−12 m3 m−1 for a range of applied pressure of 0.2–1.8MPa and at a constant sliding speed of 1.0ms−1 in vacuum, whereas, the wear rates observed in air atmosphere for the same sliding conditions range between (0.75 and 3.87)×10−12 m3 m−1. The coefficient of friction in air atmosphere varied for low pressures and remained almost constant at 0.25 in high pressure regime, whereas in vacuum it decreased linearly and finally attained a constant value of 0.1. The wear rates for different sliding speeds of 0.5–2.0ms−1 at constant applied pressure of 1.0MPa were compared. The results depicted two different regimes of variation in wear rate with sliding speed in vacuum as well as in air atmosphere. The wear rate increased with sliding speed in vacuum, on the other hand, in air atmosphere the wear rate decreased with increasing sliding speed, reaching a minimum at a critical sliding speed and then increasing with further increase in sliding speed. The coefficient of friction remained almost constant for different sliding speeds for both air and vacuum. However, under vacuum, the value of coefficient of friction was lower than that in air, through the range of sliding speed. The wear behaviour of the spray formed alloy under different conditions is discussed as inferred from the microstructural features of the alloy and the nature of the worn surfaces of the wear test specimen. [Copyright &y& Elsevier]

Abstract: Swirled enthalpy equilibration device (SEED) technology was used to cast semi-solid metal, SSM, billets of A356 Al–Si alloy. The deformation behavior of the SSM billets was studied using parallel plate compression viscometry. The compression tests were carried out at different applied pressures and solid phase morphologies. The rheological studies were conducted assuming the SSM slurry behaving like Newtonian or non-Newtonian fluid. The calculated viscosity numbers confirmed psuedoplastic behavior of the SSM billets and were correlated to the morphology, average aspect ratio, and shear rate to highlight the effect of metallurgical and process parameters on the viscosity and thus “rheological behavior” of SSM alloys. An empirical equation is proposed to correlate viscosity with average aspect ratio and shear rate: This equation is valid for the shear rates less than 0.01s−1. [Copyright &y& Elsevier]

Abstract: To select effective modifying agent for Al–Si alloys, in addition to conventional parameters such as the crystal structure and lattice constant, the mixing enthalpy, which was calculated by the Miedema model, was newly considered. The large negative value of mixing enthalpy was counted as a new parameter. According to the result of calculation, four elements, including Sr, Y, Sm and Gd, which have large negative mixing enthalpies with Si, were selected as modifying agent for eutectic Si crystals. It was observed that these elements result in the modification of the eutectic Si crystals and that the eutectic depression of the modified alloy is larger than 6K. [Copyright &y& Elsevier]

Abstract: Two Al–Si alloys of different silicon content having 10 and 15wt.% were tested by loading at an isothermal temperature of 540°C. To explore the characteristics of the rupture structure, various loads (2–80N) were chosen in this study. The results indicate that the rupture structures are mainly affected by transformation stress under lower load. Besides, the dissolved silicon is trended to distribute along α-Al dendrite boundary, but it is gradually governed by the increased loading-effect and restrain the distribution of eutectic silicon. Based on the both interactions, the two changing curves of rupture time reveal commonly a trend of that initial decrease and then increase, whereas the T R-value of 15 Si is larger than 10 Si. To increase the load will raise generally the hardness of rupture. However, this hardness of 10 Si is higher under 15N-loaded below. The cracking paths were more preferred to proceed along the silicon particles clustered zone. [Copyright &y& Elsevier]

Abstract: Tensile properties of a severely deformed Al–11mass%Si alloy processed from 4 to 32 passes by rotary-die equal-channel angular pressing (RD-ECAP) were investigated at elevated temperatures. Elongation to failure significantly increased with increasing the number of RD-ECAP passes at each test temperature. The sample pressed 32 passes exhibited 34% elongation to failure at 573K, which was five times higher than that of the as-cast alloy. In addition, the RD-ECAP processed (RD-ECAPed) sample exhibited large elongation to failure at 573K even at the high strain rate. The elongation to failure of the sample pressed 32 passes was 28% at the high strain rate of 2.30×10−1 s−1 at 573K. High strain rate superplasticity was obtained in the RD-ECAP processed Al–11mass%Si alloy. The maximum total elongation of 192% was obtained at 5.75×10−1 s−1 at 788K. The strain rate sensitivity (m value) was 0.35. The obtained large elongation was related to the contribution of the grain boundary sliding (GBS). On the other hand, the RD-ECAPed specimen followed by T6 treatment exhibited high tensile strength as well as the un-pressed alloy with T6 treatment, and good plasticity at elevated temperatures, due to the homogenous distribution of the second-phase particles, the equiaxed and small grains. [Copyright &y& Elsevier]

Abstract: It is well documented that applying mechanical vibration to the mold during solidification has a profound effect on the microstructure and mechanical properties of castings. However, it is still not well understood how mechanical vibration change the resulting microstructure. Most of the available studies are qualitative and provide little quantitative information that can be used by the casting industry. In this work, mechanical mold vibration is applied to an Al–Si eutectic (Al–12.5% Si) alloy at a frequency of 100Hz and variable amplitudes in the range of 18–199μm. It is shown that the silicon morphology was strongly influenced by the level of vibration amplitude. Generally, increasing the vibration amplitude tends to reduce the lamellar spacing and change the silicon morphology to become more fibrous. However, exceeding a critical value of vibration amplitude tends to coarsen the silicon. The corresponding changes in mechanical properties are also investigated. It is shown that the maximum elongation is more influenced by vibration than the tensile strength for the range of conditions tested here. [Copyright &y& Elsevier]

Abstract: Solidification of 0.1–1.0mm diameter droplets of Al–50at.%Si hypereutectic alloy was achieved in a 3m drop tube. Phase morphologies of deeply etched sections of the solidified samples with different diameters are examined. With decreasing of the diameter of the droplets, refinement of the primary silicon phase is observed. A tendency for the primary silicon crystals to form granular morphology is found in the smaller samples. Stratified deposits of aluminum in the primary silicon along the silicon plate are evident in bigger samples, but less evident with decreasing of the sample size. Morphologies of Al–Si eutectics change from needle-like to short, blunt and worm-like forms with decreasing of the sample size. Through calculation of the nucleation rates at different sizes of the solidified sample and analyses of the crystal morphologies, grain refinement of the primary silicon can find its origins of copious nucleation during rapid cooling and solute-restriction to crystal growth. [Copyright &y& Elsevier]

In-situ heating in a transmission electron microscope was used to investigate the stress-relief mechanisms occurring in the 3–4 nm thick aluminum-oxide films present on the surfaces of submicron Al–11.6% Si powder particles during melting. Experimental observations show that the aluminum-oxide shell undergoes creep to relieve the high stress generated by the expanding liquid during melting. Surface oxides that contain defects do not undergo uniform creep and are prone to cracking, which causes ejection of some of the pressurized liquid inside the particles through the crack. Calculations were performed to estimate the stresses generated on the aluminum-oxide shell by the expanding liquid and the possible effect of the stress generated by the confining oxide shell on the melting temperature of the Al–Si alloy. The calculations show that the oxide film creeps to avoid the high tangential stress (on the order of 15 GPa) produced by the expanding liquid and that this creep mechanism prevents the oxide from markedly changing the melting behavior of the Al–Si particles. [Copyright &y& Elsevier]

Electrolytic reduction of unpurified bauxite can give an aluminum–silicon alloy with high impurity content. Experiment has shown that the eutectic microstructure in either hypoeutectic or eutectic alloys has the fine fibrous appearance of an impurity-modified alloy produced by conventional methods from pure aluminum and silicon. This self-modification persists after repeated re-melting. The undercooling in solidification of re-melted alloy caused the self-modification inheritance. It is argued that self-modification is associated with the electrolytic process in reduction cell rather than trace elements in alloy. [Copyright &y& Elsevier]

An experimental study has been conducted on an aluminum silicon alloy to evaluate the effects of microstructural damage on the overall ductility of the alloy. The alloy contained two populations of brittle phases, i.e. silicon particles and iron rich intermetallics. The morphology of the silicon second phase was varied from a fine fibrous to a fully spherodized structure. Microstructural damage was introduced by cold rolling. To separate the effect of strain on damage from its effect on work hardening behavior, the aluminum matrix was recrystallized after cold rolling. Damage of the second phase was observed by metallographic observations and quantified by changes in material density. It was observed that the tensile ductility was strongly dependent on the nature of the iron rich intermetallic phase. Cold reduction was found to be highly beneficial to the overall ductility, primarily as a result of damage to the iron rich phase. It was, therefore, concluded that the effect of microstructural damage on ductility requires a careful consideration of the nature of the damage and its effect on subsequent deformation behavior. Finally, density measurements suggested that some of the microstructural damage could be reduced during annealing of the alloy. [Copyright &y& Elsevier]

This study investigated the active strengthening precipitates existing in the structure of 354-type Al–Si–Cu–Mg cast alloys following the addition of Zr as a primary alloying element in addition to Ni and Mn in different amounts and combinations. The investigation comprised three main parts: the first part simulated the formation and dissolution of strengthening precipitates and their precursors using differential scanning calorimetric (DSC) heating analysis of as-quenched samples, the second part presented transmission electron microscopy (TEM) observations of the fine precipitates existing in the structure of the alloys studied, and the third part examined the mechanical performance (tensile properties and hardness values) of the alloys studied. Interpretation of the DSC heating data in accordance to the available literature showed that the alloys are strengthened primarily by θ-Al2Cu and S–Al2CuMg precipitates and their precursors. Furthermore, some of the copper content was consumed in forming Al–Ni–Cu particles instead of the θ-Al2Cu strengthening phase and its precursors and hence the Ni-free alloys contain higher fractions of the θ-Al2Cu phase and its precursors than the Ni-containing alloys. TEM investigations confirmed the results obtained from the DSC heating analysis regarding the combined strengthening effect of θ-Al 2 Cu and S –Al 2 CuMg precipitates and their precursors as well as the fraction of θ-Al 2 Cu phase in Ni-free and Ni-containing alloys. TEM observations revealed, also, that the alloys studied have a secondary strengthening effect which is exerted by precipitates in the form of Al x (Zr,Ti)Si that formed following the addition of Zr. The Mechanical performance showed to be highly dependent on the chemical composition which controls alloys structure and response to the applied heat treatment and hence the formation of the fine precipitates which are mainly responsible to the strengthening effect. [ABSTRACT FROM AUTHOR]

In this study, hypoeutectic and eutectic Al–Si alloys were processed through rolling and friction stir processing (FSP), and the effects of deformation routes on the microstructures and mechanical properties of Al–Si alloys were studied. FSP led to the fragmentation and uniform distribution of Si phases in the Al matrix and then eliminated the preferential crack propagation channels of Al–Si alloys during tensile tests. The optimized crack propagation path and high work hardening rate led to a tensile elongation of as high as 39% in FSP Al–7Si alloys. Further rolling refined the Al grains and introduced dislocations into the Al–Si alloys. The fine Si phases, tens of nanometers is size, in the FSP samples promoted grain refinement after rolling. Compared with the Al–Si alloys subjected to warm rolling only, the alloys subjected to FSP and warm rolling exhibited higher strength and ductility. FSP alloys with the highest strength were achieved through cold rolling because of high grain boundaries and dislocation-strengthening effects. This work provided a method for fabricating Al–Si alloys with excellent mechanical properties by combining FSP and rolling. [ABSTRACT FROM AUTHOR]

The influence of ε-Al 3 Ni phase on tensile and creep performance of the Al–12Si-0.9Cu-0.8 Mg-(1–4)Ni alloys was investigated at elevated temperature. The Image-Pro Plus was used to characterize the volume fraction, mean particle size, and surface roundness of the ε-Al 3 Ni phase. The results show that the volume fraction of ε-Al 3 Ni phase is in accordance with the thermodynamic calculation and the mean particle size of ε-Al 3 Ni phase increases with Ni element. The ultimate tensile strength at 250 °C increases from 162 MPa to 187 MPa with the volume fraction of ε-Al 3 Ni phase varied from 1.03% to 8.20%. The elongation of the studied alloys shows a reverse trend. The minimum creep rates of the investigated alloys decrease with increasing the volume fraction of ε-Al 3 Ni phase. The decrease in minimum creep rate illustrates that the ε-Al 3 Ni phase is effective in improving the creep resistance of the Al–Si–Cu–Mg–Ni alloy. [ABSTRACT FROM AUTHOR]

The effect of microalloying 0.1 wt% Sc on microstructure and mechanical properties of as-cast Al–12Si (wt.%) alloys was investigated. Sc refines α-Al grain average size from 400 μm to 135 μm and eutectic Si average size from 4.6 μm to 0.7 μm, and modifies Si from plate-like shape to fibers as well. The microaddition of Sc also leads to the formation of coherent nanosized Al 3 Sc precipitation and other Sc containing intermetallic phases with a length <10 μm. The tensile strength increases from 183 MPa of the base alloy to 254 MPa of the Sc-modified alloy, and the elongation increases from 3.5% to 8.3%, respectively. Moreover, this Sc-modified alloy has a retention of 85% strength even after thermal exposure at 350 °C for 160 h. [ABSTRACT FROM AUTHOR]