Hypoeutectic Al-Si Alloys Research Articles

Effect of Solidification Processing Parameters and Silicon Content on the Dendritic Spacing and Hardness in Hypoeutectic Al-Si Alloys

Aluminium with silicon as alloying element, form a class of material providing the most significant part of all material casting manufactured materials. These alloys have a wide range of applications in the automotive and aerospace industries. It is widely recognized that moderate addition of silicon to aluminum, significantly improves the resulting mechanical properties. Hypoeutectic Al-Si alloys were used in the present experimental study to investigate the solidification parameters effect and Si 3-5 wt% addition on the microstructural features and resulting hardness in a vertical directional solidification system. The hypoeutectic alloys were directionally solidified under transient heat flow conditions in a range of cooling rates from 0.3 to 4 oC/s. Characterization analyses by optical microscopy indicate clearly that secondary dendritic arm spacing (λ2) increases significantly with the decrease in solidification speed (SP) and cooling rate (Ṫ). On the other hand, experimental growth laws relating the secondary dendritic arm spacing (λ2) to the solidification speed (SP) and cooling rate (Ṫ) indicate that Si addition from 3 wt% to 5 wt% has induced a thickening effect leading to increase in λ2 . While, experimental results have shown that the resulting hardness increase as solidification processing parameters (SP and Ṫ ) increase. Results of Vickers hardness test for Al-5 wt% Si alloy has hardness increase of 18 % from that of Al-3 wt% Si alloy with mean values of 26 and 22 HV, respectively.

Effect of Structural Modification on Corrosion Behavior of Hypo Eutectic Al-Si Alloy

Al-Si eutectic cast alloys are widely used in aeronautical and automobile industries where significantly high strength, toughness and wear resistance are required. This class of cast alloys exhibit relatively low corrosion resistance in brine environments. The mechanical properties of the alloy system mainly depend upon the shape of Si rich eutectic phase, which mainly has acicular geometry. In present research, the effect of modified microstructure of 12 wt. % Si-Al alloy on corrosion behavior was studied. The needle like Si rich eutectic phase was modified to disperse spherical structure using rare earth metal halides. The corrosion rate and pitting behavior of modified and unmodified alloy were evaluated in 3.5% NaCl solution by general corrosion for calculated time. It was observed that the corrosion rate and pitting tendency of modified alloy had been appreciably reduced as compare to unmodified alloy. The improvement of corrosion properties were the attributes of changed morphology and distribution of Si rich eutectic phase.

Microstructures, mechanical properties, and corrosion behavior of novel high-thermal-conductivity hypoeutectic Al-Si alloys prepared by rheological high pressure die-casting and high pressure die-casting

The microstructures, mechanical properties, thermal conductivities, and corrosion behavior of novel high-thermal-conductivity hypoeutectic Al-Si alloys, prepared by rheological high-pressure die-casting (Rheo-HPDC) and HPDC, were investigated and compared. The microstructure of the Rheo-HPDC alloy, including the primary α-Al (α1), secondary α-Al (α2-Al), β-Al5FeSi, and Si phases, was significantly more refined than that of the HPDC alloy. Compared to the HPDC alloy, the Rheo-HPDC alloy exhibited superior mechanical properties and thermal conductivity, and its ultimate tensile strength (UTS), yield strength (YS), elongation, hardness, and thermal conductivity were improved by 27%, 21%, 72%, 10% and 8%, respectively. Pitting corrosion of the Rheo-HPDC alloy was observed to originate at the interfaces between the Fe-rich intermetallics and the α2-Al particles. Corrosion was observed to propagate in the eutectic areas upon prolonged immersion in 3.5 wt% NaCl solution. Electrochemical testing, scanning Kelvin probe (SKP) measurements, and corrosion-morphology analyses reveal that the Rheo-HPDC alloy has better corrosion resistance, primarily due to the decrease in the potential difference between the Fe-rich intermetallics and the matrix, the refinement of the eutectic Si and intermetallics, and the decrease in the areal ratio of eutectic Si to α2-Al.

Fluidity and Hot Cracking Susceptibility of A356 Alloys with Sc Additions

A356 with scandium (Sc) addition provides interesting results beyond costs. For the practical use of Sc, the effects of Sc on castability must be considered. Fluidity and hot cracking are important factors defining the castability of aluminum casting alloys. In the present work, the influence of Sc addition on the castability of A356 hypoeutectic Al–Si alloy was investigated, which was evaluated through fluidity and hot cracking susceptibility. The fluidity of the alloys was studied by measuring the total volume of solidified aluminum in a multi-channel mold. The hot cracking susceptibility of the alloys was evaluated by using a constrained-rod casting mold test. The results of the fluidity and hot cracking susceptibility test were supported by microstructural analysis. The results indicate that 0.2 wt% Sc addition significantly increases the fluidity of A356 alloy, due to the grain refinement and eutectic Si modification by changing the solidification mode. However, the fluidity slightly decreases when the Sc content increases to 0.4 wt% due to the formation of primary Al2Si2Sc intermetallic phase. The hot cracking of A356 alloy was completely diminished when Sc was added to the alloy.

Relevance of electrical current distribution to the forced flow and grain refinement in solidified Al-Si hypoeutectic alloy

Significant grain refinement in cast metals can be achieved through the application of electric currents during the solidification process. The present paper investigates the distribution of electric currents on the grain size of solidified Al-7wt.%Si alloy under the application of electric current with constant parameters flowing through two parallel electrodes into the melt within a cylindrical mould. The distribution of electric current was controlled by applying an electrical insulation material coating, boron nitride (NB), to the sidewall of the electrodes. Experimental results showed that the employment of these insulated electrodes can reduce grain size in comparison with the reference case of electrodes without BN coating. Flow measurements were performed in Ga-20wt.%In-12wt.%Sn liquid metal. Higher intensity forced flow occurred when the sidewall of the electrodes was insulated. In order to understand the underlying mechanism behind the stronger forced flow, corresponding numerical simulations were performed to reveal the distributions of the electric current, magnetic field, Lorentz force, and the resultant forced flow. The results achieved indicate that the mechanism of grain refinement driven by electric current is dendrite fragmentation induced by forced flow. In addition, a novel approach to enhance the grain refinement without additional input of current energy was developed.

Impression creep properties of hypoeutectic Al-Si alloys with scandium additions

The present work pertains to an improvement of high temperature stability of age hardenable hypoeutectic Al-Si alloy. Addition of scandium (Sc) was attempted to modify the microstructure and to improve the high temperature stability of the hypoeutectic Al-Si alloy. The high temperature stability was determined by impression creep techniques. The tests were performed at temperature 250°C and applied stresses in the range of 160 to 200 MPa for dwell times up to 3000 seconds. The results showed that the creep strength of the hypoeutectic Al-Si alloys was improved. For all loads, hypoeutectic Al-Si alloy with 0.4 wt% Sc had the lowest creep rates and thus the highest creep resistance among all materials tested. This may be attributed to the possible microstructural changes, solute enrichment of the matrix and pinning of the grain boundaries by the fine precipitates.

Fabrication and characterization of hypoeutectic open-cell Al-Si foams using gravity die casting and squeeze casting

Gravity die casting and squeeze casting are the techniques used for the fabrication of hypoeutectic open-cell Al-Si foams which are characterized and studied for their energy absorbing quality in compression tests. The effect of different amounts of sodium chloride (NaCl) (up to 56 vol.%) as a space holder in the casting of aluminum foam on the morphology, density, porosity, compressive and energy absorption properties of aluminum foam was studied. The hypoeutectic Al-Si alloy with NaCl particles as a space holder was used to fabricate the aluminum foam using gravity die casting and squeeze casting. The hypoeutectic open-cell Al-Si foams produced by squeeze casting showed smaller pore size, better pore distribution, higher porosity, good compressive strength and greater energy absorption energy compared to that of gravity die casting. The hypoeutectic open-cell Al-Si foams with 44 vol.% NaCl using squeeze casting showed the best properties among all foams due to its moderate and well-distributed porosity.

The Mechanical Properties of Different alloys in friction stir processing: A Review

This review paper deals with the consequences of Friction Stir Processing on different alloys like Mg-4Y-3Nd(WE43), Mg-ZrSiO4-N2O3, Al-Si hypoeutectic A356 alloy, 5210 steel (WC-12% CO coated). These effects can be on mechanical properties like strength, hardness, machinability, tensile strength. FSP leads to grain growth and refinement of grains into fine structure. FSP is carried out by FSP tool at different speeds and angles which results in difference of properties in the friction stir processed alloy. FSP can be widely used in metals such as aluminium and magnesium that are lightweight and can be used extensively in various applications; but due to certain lack in strength and hardness they are not used in their original form and require certain processing to obtain desired properties.

Metallurgical Processing of Al-Si Alloys with Increased Iron Content Using Sodium, Strontium, and Tellurium

This paper presents the results of research focused on the processing of the hypoeutectic Al-Si alloy with an increased iron content of 1.75 wt.%. The physical-metallurgical method was used to eliminate the undesirable effect of the iron rich b-phase on the mechanical properties of the casting material using liquid metal modification by strontium, sodium, and tellurium. The chemical composition of an experimental material didn’t contain any elements for the solid-solution strengthening of a matrix; therefore, the presented results can document the capabilities of modifying the b-phase under a minimum influence on the properties of the basic Al-Si system.

Hypoeutectic Al-Si Alloy Doped with Chromium, Tungsten and Molybdenum Designated for Pressure Die Casting

AbstractStructural modification of hypoeutectic Al-Si alloy EN-AC 46000 alloy influences its microstructure and mechanical properties. In a series of examinations the EN-AC 46000 alloy has been doped with Cr, W and Mo. The study involved the differential thermal analysis (DTA) and the light microscopy structural analysis of the samples cut from pressure die castings and from samples cast into the DTA cup. The DTA and microstructure analyses revealed new phases in the alloy doped with mentioned additives. The strength properties measurements of the alloys doped with Cr, W and Mo showed significant improvement in tensile strength and elongation, while preserving good hardness.

Effect of Sr–Grain Refining–Si Interactions on the Microstructural Characteristics of Al–Si Hypoeutectic Alloys

The present study was performed on A356.2 alloy to explore the link between the effectiveness of different grain refiner practices and strontium (Sr) modification on both the eutectic silicon (Si) particle morphology and the final grain size. To achieve this objective, several types of grain refiners were added to the molten alloy at 750 °C in different concentrations and for various holding times. Strontium was added in the amount of 200 ppm. Thermal analysis combined with microstructure analysis using optical microscopy and electron probe microanalysis was used to assess the results. It was found that Sr has a certain affinity to react with the boron (B) in Al–5%Ti–1%B master alloy which partially reduced the grain refiner effectiveness. The eutectic Si particles maintained a similar degree of modification as in the non-grain refined Sr-modified condition. Al–4%B used with Sr led to the formation of a new intermetallic (SrB6), resulting in a significant decrease in the modification effect of Sr together with an increase in the obtained grain size. This effect increases at higher levels of added Al–4%B. The presence of excess Si leads to a change in the composition of the Al3Ti phase into (Al, Si)2Ti.

Fabrication of lotus type through-holes using the semisolid condition

In this paper, an easy casting method of an ingot with through holes is presented. The Core-Bar-Pulling Method is proposed to improve the disadvantages of the lotus type porous metal. The through holes were formed by pulling core-bars from a semisolid ingot. Holes with diameters ranging from 0.5 to 5 mm and length 50 mm were made in Al–Si alloy ingots. The relationship between temperature and formability of the holes was investigated by using hypo-eutectic Al–Si alloys. The pulling of core-bars and the forming of holes were easy, under the condition that the Si content was less than 6 mass%. This means that the forming of holes in Al–Si alloys is easy under the condition that flow ability at semisolid condition decreases. The core-bar could be pulled at solid condition at the Si content less than 1%. The deformation of the hole was investigated by comparing the hole-size with the core-bar size. There was no tendency among the core-bar pulling-temperature, Si content and hole-deformation. The shape of the hole fabricated at the solid condition was better than that fabricated at the semisolid condition.

Sequential Casting of Functionally Graded Material

Functionally graded materials (FGM) are used for components with specific characteristics required by the considered area of application. In this research, functionally graded material is obtained with sequential casting process of two different aluminum alloys. They are poured into the mould, aiming to obtain within the same component high thermal resistance and mechanical strength on one side and ductility and elongation on the other side.The new casting has high potential, especially in the production of automotive components, e.g., pistons. Usually, piston alloys are eutectic Al-Si alloys, with high percentage of other alloying elements which increases the thermal resistance of the material. However, this high concentration of alloying elements leads to a considerable reduction of the material’s elongation that is not always tolerable. The low ductility can be an issue for the inferior part of the piston that is more subjected to fatigue stress. To increase the elongation, in addition to the alloy used for the manufacturing, a hypoeutectic Al-Si alloy is considered in the sequential casting of the FGM, that in turn gives rise to a superior ductility in the component.The purpose of this research is the optimization of the manufacturing process parameters of a functionally graded material to be used for the production of a more performing element. In particular, the produced piston shows a superior resistance at high temperatures in the area which it is in contact with the gas combustion and, simultaneously exhibits a superior fatigue life on its lateral part.

Fatigue in a heat treatable high silicon containing aluminium alloy

The use of cast aluminium alloys in automobiles contributes to reductions in weight and fuel consumption without impairing the safety for the occupants or the performance of the car. Most of the alloys used are heat treatable hypoeutectic Al-Si alloys, which have the drawback of exhibiting low wear resistance. So industry relies in wear resistant alloys, such as grey iron, for the liners of the combustion chambers in engine blocks, which increase the weight of the engine. Therefore, it is of interest to cast high silicon containing alloys into engine components that are able to resist wear while maintaining the mechanical properties required by the components. This work presents the result of the work carried out in a high silicon containing heat treatable aluminium alloy as it is subjected to high cycle fatigue. The alloy was prepared and cast in ingots designed to promote one dimensional solidification gradient to obtain samples to study the high cycle fatigue. The material was machined into hour-glass specimens that were tested at room temperature in a servohydraulic machine under load control following the stair case method. The results show that the resistance to fatigue depends on the microstructure of the sample, as the fatigue cracks originate in pores close to the surface of the sample and propagate through the eutectic aggregate. The results from this work are compared with those from previously obtained with hypoeutectic alloys.

Hypoeutectic Al-Si Alloy with Cr, V and Mo to Pressure Die Casting

Abstract This paper presents the results of hypoeutectic 226 grade alloy as well as prepared on its basis Al-Si alloy containing Cr, V and Mo. The additives tested were added as AlCr15, AlV10 and AlMo8 master alloys. Alloys tested were poured into DTA sampler as well as using pressure die casting. An amount of Cr, V and Mo additives in alloy poured into DTA sampler comprised within the range approximately 0.05-0.35%. Alloys to pressure die casting contained 0.05-0.20% Cr, V and Mo. The crystallization process was examined using the derivative thermal analysis (DTA). The microstructure of castings made in the DTA sampler as well as castings made with use of pressure die casting were examined. The basic mechanical properties of castings made using pressure die casting were defined too. It has been shown in the DTA curves of Al-Si alloy containing approximately 0.30 and 0.35% Cr, Mo, and V there is an additional thermal effect probably caused by a peritectic crystallization of intermetallic phases containing the aforementioned additives. These phases have a morphology similar to the walled and a relatively large size. The analogous phases also occur in pressure die casting alloys containing 0.10% or more additions of Cr, V and Mo. The appearance of these phases in pressure die casting Al-Si alloys coincides with a decrease in the value of the tensile strength Rm and the elongation A. It has been shown die castings made of Al-Si alloys containing the aforementioned additives have a higher Rm and A than 226 alloy.

Effect of combinative addition of mischmetal and titanium on the microstructure and mechanical properties of hypoeutectic Al-Si alloys used for brazing and/or welding consumables

Effects of mischmetal (RE) and/or Ti modifier on the microstructure including α-Al dendrites, eutectic Si phases and other secondary phases of Al-Si brazing and/or welding alloys were investigated by differential scanning calorimetry (DSC), optical microscopy (OM), scanning electron microscopy (SEM). The DSC results showed that an addition of RE decreased the eutectic temperature and caused supercooling, promoting the nucleation of eutectic Si crystals. In addition, the maximum temperature of the first endothermic peak varied with the different RE contents, which had a good correlation with the microstructural modification of the eutectic Si phase. The α-Al dendrites were well refined by increasing the cooling rate or adding 0.08 wt.% of Ti. When 0.05 wt.% RE was added to the Al-5Si-0.08Ti alloy, the morphology of eutectic Si phase was transformed from coarse platelet to fine fibers and the mechanical properties of the resulting welding rod were well improved. Whereas, when excess RE was added, a large number of β-Fe phases appeared and the aspect ratios of β-Fe phases increased. The morphologies and chemical components of two kinds of RE-containing intermetallic compounds (IMCs) were also discussed.

Effects of the Addition of Mg on the Microstructure and Mechanical Properties of Hypoeutectic Al–7%Si Alloy

It is well known that the mechanical properties of hypoeutectic Al–Si alloys are influenced by the size, morphology and distribution of primary α-Al dendrites, eutectic Si crystals and other intermetallics. In the present work, the effect of Mg addition on the size and morphology of the primary α-Al dendrites and eutectic Si phases in Al–7%Si alloy has been investigated. The microstructure of as-cast samples with different Mg content was characterized by scanning electron microscopy. Microstructural analyses demonstrated that primary α-Al was significantly refined from coarse dendrites to fine equiaxed grain with increasing the amounts of Mg. In addition, the eutectic Si structure was modified from coarse needle-like/platelet-like structure to a fine coral-like fibrous structure. The mechanical properties were investigated by tensile test with various concentrations of Mg. It was found that the ultimate tensile strength improved by 61% with an increase in Mg content. However, the ductility initially increased by 80% with an increase in Mg content from 0.5 to 1.5%, then it decreased with further increasing Mg content up to 2.0 and 2.5%. It is due to precipitation of coarse Chinese script Mg2Si phase that leads to the increase in strength and the decrease in ductility.

The Role of Si and Cu Alloying Elements on the Dendritic Growth and Microhardness in Horizontally Solidified Binary and Multicomponent Aluminum-Based Alloys

Horizontal directional solidification (HDS) experiments were carried out with Al-3wtpctCu, Al-3wtpctSi, and Al- 3wtpctCu-5.5wtpctSi alloys in order to analyze the interrelation between the secondary dendrite arm spacing (λ 2) and microhardness (HV). A water-cooled horizontal directional solidification device was applied. Microstructural characterization has been carried out using traditional techniques of metallography, optical, and SEM microscopy. The ThermoCalc software was used to generate the phase equilibrium diagrams as a function of Cu and Si for the analyzed alloys. The effects of Si and Cu elements on the λ 2 and HV evolution of the hypoeutectic binary Al-Cu and Al-Si alloys have been analyzed as well as the addition of Si in the formation of ternary Al-Cu-Si alloy. The secondary dendrite arm spacing was correlated with local solidification thermal parameters such as growth rate (V L), cooling rate (T R), and local solidification time (t SL). This has allowed to observe that power experimental functions given by λ 2 = Constant (V L)−2/3, λ 2 = Constant (T R)−1/3 and λ 2 = Constant (t SL)1/3 may represent growth laws of λ 2 with corresponding thermal parameters for investigated alloys. Hall–Petch equations have also been used to characterize the dependence of HV with λ 2. A comparative analysis is performed between λ 2 experimental values obtained in this study for Al-3wtpctCu-5.5wtpctSi alloy and the only theoretical model from the literature that has been proposed to predict the λ 2 growth in multicomponent alloys. Comparisons with literature results for upward directional solidification were also performed.

Change of Secondary Dendrite Arm Spacing of Hypoeutectic Al-Si Alloys according to Si Content and Cooling Rate

Change of Secondary Dendrite Arm Spacing of Hypoeutectic Al-Si Alloys according to Si Content and Cooling Rate

Investigation of Phosphorus-Containing Compounds in Aluminium Alloys with Emphasis on the Formation Mechanism

It is well known that the presence of phosphorus in hypoeutectic Al-Si alloys causes coarsening of Al-Si eutectic even when the phosphorus concentration is as low as a few ppm. The reason is that phosphorus reacts with aluminum to form AlP, the particles of which serve as potential nuclei for silicon due to a very similar lattice parameter with Si. The purpose of this study is to investigate the possibilities for reducing the nucleation potency of AlP by changing the alloy chemistry. The experiments include melting and solidification of a number of aluminum alloys under different conditions. After that, the samples are examined using SEM and TEM microscopy to elucidate the presence of phosphorus-containing compounds and to explain their formation mechanism. The experimentally obtained data are discussed on a systematic basis of thermodynamic calculations and compared with the corresponding data from the relevant literature. Particularly, the results reveal that such elements as Mg, Ca and Sr are potentially capable of neutralizing the nucleation activity of AlP although their effect is dependent on the presence of other chemical elements in the aluminium alloy.