Hypoeutectic Al-Si Alloys Research Articles

Microstructure and Hardness of an Al–8 wt%Si–2.5 wt%Bi Alloy Subjected to Solidification Cooling Rates from 0.1 to 800 K s−1

This work explores the effect of the addition of bismuth (Bi) to Al–8 wt%Si alloys. Bi in Aluminum based alloys works as a self‐lubricating agent, improving machining and wear properties. As Bi is a soft material, it is essential to evaluate how it affects microstructural features and the resulting properties of Al–8 wt%Si alloys. Herein, this hypoeutectic alloy is modified by the addition of 2.5 wt%Bi and subjected to three solidification techniques: differential scanning calorimetry, transient directional solidification, and impulse atomization. Thus, this work investigates the effect of Bi in samples solidified under a wide range of cooling rates. Of specific interest is how Bi modifies the eutectic silicon morphology and alloy hardness compared with a hypoeutectic Al–10 wt%Si alloy from the literature. The silicon (Si) morphology of Al–8 wt%Si–2.5 wt%Bi transitions from flaky (coarse) to fibrous (fine) at a critical cooling rate of 1100 K s−1. Through the combination of Bi addition and processing through impulse atomization, the ternary Al–Si–Bi alloy achieves improvements in hardness of up to 20% compared to Al–10 wt%Si. This is despite having a coarser eutectic microstructure than the binary hypoeutectic Al–Si alloy. This is due to Bi modifying the morphology of the eutectic Si.

Equiaxed grain formation by intrinsic heterogeneous nucleation via rapid heating and cooling in additive manufacturing of aluminum-silicon hypoeutectic alloy

The high strength of Al-Si hypoeutectic alloys additively manufactured by powder-bed fusion is of great scientific interest. To date, the mechanism of grain refinement near the fusion line, which contradicts conventional Hunt’s columnar–equiaxed transition criteria, remains to be elucidated. Here we present the first report on the mechanism of grain refinement. When a laser was irradiated on cast Al-Si alloy consisting of coarse α-Al grain and α-Al/Si eutectic regions, grain refinement occurred only near the eutectic regions. This strongly suggests that the Si phase is crucial for grain refinement. Multi-phase-field simulation revealed that rapid heating due to the laser irradiation results in unmelted Si particles even above the liquidus temperature and that the particles act as heterogeneous nucleation sites during the subsequent re-solidification. These results suggest the feasibility of a novel inoculant-free grain refinement that is applicable to eutectic alloys comprising phases with a significant melting point difference. • The grain refinement near fusion line in Al-Si alloy irradiated with laser was investigated by experiment and multi-phase-field simulation. • The heating is so rapid that Si particles remain unmelted and act as heterogeneous nucleation sites in the subsequent cooling. • Inoculant-free grain refinement is suggested to be possible and applicable to eutectic alloys comprising phases with a large gap of melting point.

Modification of eutectic Si in hypoeutectic Al-Si alloy with novel Al-3Ti-4.35La master alloy

Grain refinement and eutectic Si modification of hypoeutectic Al-Si alloys are crucial to improving their mechanical properties. In this paper, the effects of a novel Al-3Ti-4.35La master alloy on the morphology of eutectic Si and mechanical properties of hypoeutectic Al-7Si alloy were studied, and the modification mechanism on eutectic Si was revealed. The results show that the addition of 0.2 wt% Al-3Ti-4.35La master alloy can transform the eutectic Si phase from the coarse plate and needle to fine fiber and partial granular structure. After modification, the ultimate tensile strength and elongation of the Al-7Si alloy reached 178.3 MPa and 12.0%, which are enhanced by 15.6% and 106.9%, respectively, over the unmodified alloy. A detailed analysis revealed that, on one hand, the addition of Al-3Ti-4.35La master alloy can reduce the nucleation temperature of eutectic Si in Al-7Si, shorten the eutectic reaction time, and inhibit its growth. On the other hand, the morphology of eutectic Si can also be affected by altering its growth behavior. The twinning density in the eutectic Si after the modification increases significantly while the growth direction is anisotropic. The modification mechanism of the Al-3Ti-4.35La master alloy on eutectic Si is believed to be consistent with the impurity-induced twinning mechanism (IIT). This work provides important insights into alloy modifications by rare-earth master alloys.

β-Al4.5FeSi: Hierarchical crystal and defect structure: Reconciling experimental and theoretical evidence including the influence of Al vs. Si ordering on the crystal structure

The β-Al4.5FeSi phase occurs as plate-shaped particles in secondary, i. e. recycled, Fe-containing hypoeutectic Al-Si alloys and as a component of the intermetallic reaction layer between Al-Si alloys and steels joints. Unsolved issues of the structure interpretation with consequences at all microstructural scales are addressed based on experimental results and new theoretical evidence from DFT calculations. On top of the previously described polytypism (AB and ABCD polytypes, where the letters refer to the positions of a certain type of double layers perpendicular to the stacking direction), ordering of Al vs. Si atoms adds a further aspect of structural variability. Thus, three different ordered crystal structures of β-Al4.5FeSi with different ordering of Al vs. Si atoms and different stacking sequences (AB or ABCD) could be encountered in the same alloy. The two AB structures can be described in space groups A12/a1 with (DFT-base) lattice parameters a = 6.1667 Å, b = 6.1683 Å, c = 20.779 Å and β = 91.51° and A21/e11 with lattice parameters a = 6.1739 Å, b = 6.1602 Å, c = 20.7840 Å and α = 88.63°. The third structure has ABCD stacking. Unit cell distortions away from a pseudotetragonal orthorhombic metric comply with the symmetry are shown to be induced by the ordering of Al vs. Si atoms. Frequently observed δ-like stacking sequences are discussed as consequence of excess Si in β-Al4.5FeSi. Furthermore, recommendation is given for optimal experimental conditions for the characterization of the type of order and polytype in β-Al4.5FeSi.

Effect of Zn content on microstructure and properties of hypoeutectic and near-eutectic Al-Si alloys

In this research, the microstructure and mechanical properties of permanent mould cast Al-Si (7 wt.% and 12 wt.%) alloys without and with different amounts of Zn (1 wt.%, 2 wt.% and 4 wt.%) were analysed. The Al-xSi-yZn alloys have been prepared through a stir casting process using an Al-15 wt.% Si master alloy at 800 °C. Optical microscopic analyses have revealed variation in the α-Al matrix and Si needles in the alloy with the variation in Zn content. The mechanical properties analysis has shown that the hardness, tensile strength and wear resistance of the alloys increase with the addition of Si but decrease with the addition of Zn, within the limit of the alloying additions of the present research.

Experimental study on rapid evaluation of modification effect of hypoeutectic Al-Si alloy based on melt solidification process resistivity monitoring

Experimental study on rapid evaluation of modification effect of hypoeutectic Al-Si alloy based on melt solidification process resistivity monitoring

Multi-scale investigation of A356 alloy with trace Ce addition processed by laser surface remelting

Laser surface remelting (LSR) has shown great application potential in Al-Si alloy parts (e.g., surface strengthening, surface repairing). However, the microstructural details and properties of LSR-treated hypoeutectic Al-Si alloy remains to be investigated. In this work, the microstructure of an Al-7Si alloy with adding 0.05 wt% Ce fabricated by LSR was studied through multi-scale characterization. Three regions with distinct morphologies after LSR were obtained: i) the remelting zone (RMZ) including an equiaxed grain region and a columnar to equiaxed transition (CET) region; ii) the heat-affected zone (HAZ) with segregation of Si; iii) substrate zone (SZ). Nanoscale eutectic Si particles were observed in RMZ and HAZ. Moreover, it is confirmed for the first time that the Ce atom can absorb on the {111}Si planes and leads to high density of twins and stacking faults. After LSR, the increased microhardness of RMZ is mainly attributed to the precipitation and dislocation strengthening. By further T6 heat treatment, the microhardness of RMZ increases to 126 HV. Dispersed small Si particles decrease the critical stress of crack propagation. Additionally, the high density of twins, stacking faults and Al-rich clusters within Si particle plays a significant role in toughness improvement.

Comparative Study of Microstructure and Properties of Hypoeutectic Al-Si Alloys Being Cast With and Without Melt Thermal Treatment

This study investigated the microstructure and mechanical properties of several hypoeutectic Al-xSi (x = 0, 1, 3, 5, 7, 9) alloys prepared with and without melt thermal treatment. The overall study showed a trend of effective utilization of grain refiner until the transition point (3 wt.% Si) and the slope of graph between grain size vs. wt.% of Si remain similar for the samples prepared with or without chemical refinement. This also indicates that the role of Si in decreasing grain size is more dominating than the poisoning of Ti above transition point. Melt thermal-treated alloys exhibited less significant changes in grain size, especially below the transition point, but significant changes in their mechanical properties. The melt thermal treatment also causes partial refinement of eutectic Si particles by which the coarse silicon particles are broken up into smaller pieces and this has neither been observed in unrefined alloys nor refined alloys.

Effect of growth orientation and heat treatment on the corrosion properties of AlSi10Mg alloy produced by additive manufacturing

Hypoeutectic Al–Si alloys are widely used due to its excellent castability, corrosion resistance and mechanical properties. Its corrosion resistance is limited by the presence of coarse Si particles. Additive Manufacturing (AM) processes is a powerful alternative to overcome such problems. Although mechanical properties of samples produced by AM have been extensively researched, corrosion resistance has received much less attention. In the present paper, Selective Laser Melting (SLM) was used to obtain cubes of AlSi10Mg using two different growth strategies. Corrosion properties are compared both the stress relief thermal treatment and the growth strategy for the manufacturing of the alloy.

Influence of Modification on Flow Stress Behavior and Corrosive Properties of a Hypoeutectic Al-Si Alloy

The flow stress behavior and corrosive properties of Al-7Si alloy were studied by thermal compression test, electrochemical test, and electron probe microanalysis. The influences of temperatures, strain rates, strains, and morphology of Si particles on stress-strain curves of Al-7Si alloy were analyzed. The peak stress of unmodified-Al-7Si alloy is higher than that of Sr-modified-Al-7Si alloy at the same deformation conditions, and the phenomenon is obviously relevant to the distribution and morphology of Si particles. The morphology of Si particles of unmodified-undeformed-Al-7Si alloy is a typical thick layer, and that of unmodified-deformed-Al-7Si alloy is broken into relatively small particles and the distribution is relatively even in homogeneous deformation zone II. The distribution of Si particles in Sr-modified-deformed-Al-7Si alloy is obviously more even than undeformed alloy. The effect of a small deformation of 20% on the distribution and morphology of Si particles is obviously smaller than that of a large deformation of 50%. The electrochemical self-corrosion potential of Sr-modified-Al-7Si alloy is higher than that of unmodified-Al-7Si alloy, and it proves that the distribution and morphology of Si particles have a certain influence on the corrosive properties of Al-7Si alloy. That is, the even fine Si particles are more conducive to improving the corrosive properties of Al-7Si alloy.

Solidification Simulation of Al-Si Alloys with Dendrite Tip Undercooling

A novel solution approach is proposed for the numerical simulation of the solidification process of binary Al-Si hypoeutectic alloys during upward and downward solidification modes. Undercooling is always observed during solidification, but the phenomenon could not be considered in the present-day numerical solution. In this approach, the temperature distribution in the mushy zone was used to define the fraction of solid, which enabled the evaluation of the effect of dendrite tip undercooling on the characteristics of the binary alloy solidification. The present numerical algorithm was found to significantly reduce the computation time. Transient temperature distribution and solidification time from the numerical analysis, with consideration of natural convection due to temperature and concentration gradients, have been successfully simulated and validated with experiment results. Numerical results with consideration of dendrite tip undercooling have better agreement with experimental results. The effect of dendrite tip undercooling on the fluid flow (velocity profile), G, R and λ1 for both upward and downward solidification modes of Al-Si alloys have been investigated and discussed. Consideration of undercooling was found to increase G and reduce R in both solidification modes. During downward solidification, considering undercooling significantly increased flow velocity and decreased λ1. The primary dendrite arm spacing could be validated with results from uni-directional solidification experiments only when dendrite tip undercooling was considered.

Microstructure evolution of Al-Si hypoeutectic alloys prepared by controlled diffusion solidification

The possibility of changing the dendritic microstructure associated with the conventional casting processes of hypoeutectic Al-Si alloys to non-dendritic microstructure by using the controlled diffusion solidification process (CDS) has been investigated. The successful CDS process depends on mixing two precursor alloys heated at a superheat condition near their respective liquidus temperature. Experimental work and simulation work using Ansys software were carried out in the present study by employing Al-Si and Al-Cu systems. This study investigates the effect of the content of the two precursor alloys, the mass ratio changing from 2.6 to 8.3, and the superheat of the first precursor alloy on changing the microstructure. The experimental results show that the pure aluminum used as the first precursor alloy needs more undercooling and agitation during the mixing to form the non-dendritic microstructure compared with hypoeutectic Al-Si alloys. Furthermore, mixing pure aluminum with hypereutectic alloy can change the microstructure of hypoeutectic alloys leading to extending the possibility to choose the second precursor alloy. The results also show that a higher mass ratio is preferred when mixing pure aluminum with hypoeutectic alloy. Furthermore, the microstructure of the alloy Al- 6.45Si- 4Cu- 0.5 Mg- 0.66Fe- 0.66 wt%Zn was successfully changed via the CDS process by mixing Al- 7.75Si- 0.79Fe- 0.78Zn- 0.6 wt%Mg at 2 °C superheat into Al-24 wt%Cu at around 5 °C superheat. The simulation results show that lower air bubbles and better distribution of the two precursor alloys happen during the mixing step when using the Al-Cu system.

Inhibition of Carbide Growth by Sr in High-Alloyed White Cast Iron.

Chromium cast irons have gained a well-settled position among wear-resistant materials where a low manufacturing cost is one of the key factors. The wear properties of these alloys are commonly improved by the addition of carbide-forming inoculating elements such as Ti, V, B, etc., allowing the formation of underlays for the precipitation of both M7C3 carbides and austenite. On the other hand, Sr may work as a surface-active element that adsorbs on the surface of the growing crystal, inhibiting its growth. This mechanism may support the M7C3 nucleation process. The experiment was conducted on near-eutectic chromium cast irons with 20% of Cr and 2.5–3% of C. Different amounts of strontium were used as the microstructure modifier. The improvement of carbides’ stereological parameters and collocation resulted in the improvement in functional properties—wear resistance and impact strength without a significant increase in hardness as well as a decrease in carbide phase. Two types of wear studies with a modified pin-on-disc method and tests in reciprocating motion of samples in the metal-mineral system were performed. The results showed that addition modification with Sr can increase the impact strength of the alloy. EDS analysis of the samples provided results similar to hypoeutectic Al-Si alloys modified with strontium.

Microstructure and Microhardness of Directionally Solidified Al-Si Alloys Subjected to an Equal-Channel Angular Pressing Process

The directional solidification technique allows the study of growth of the solid phase, as-cast structure and, finally, its mechanical characteristics as a consequence of thermal parameters. On the other hand, in the last decade, a process called equal-channel angular pressing (ECAP) has emerged as a widely-known technique in fabrication of ultrafine-grained metals and alloys. Applicability of the ECAP technique affords an excellent potential for changing, in a controlled and beneficial manner, the resulting properties of metals and alloys. For this paper, an experimental research has been conducted to study the effects of solidification parameter (cooling rate) on resulting microhardness in hypoeutectic Al-Si alloys, upon use of an ECAP procedure. The influence of cooling on the scale of the dendritic patterns is presented and discussed with recourse to equations. The resulting microhardness variation with position throughout the as-cast materials and cooling rate were characterized by experimental power laws. Results determined after the solidification experiments have revealed microhardness as a function of both cooling rate and position (P) of the as-cast materials to be dependent on alloy composition. In the ECAP process via route C with three passes, “as-solidified” microstructures have been found to be distorted and fragmented during the severe plastic deformation. This deformation imposed on the billets during the ECAP process facilitated obtaining a fine microstructure and high levels of microhardness were observed. However, even with the ECAP process, it was shown that microhardness is strongly dependent of the cooling rates.

Qualification of uniform fatigue damage tolerance law for additively manufactured and cast Al-Si alloys

The demand on lightweight structures and materials are steadily growing in automotive and aerospace industries. Hypo-eutectic Al-Si alloys are promising candidates due to its high specific material strength and corrosion resistance. Near net shape manufacturing by additive manufacturing (AM) or castings allow a high resource efficiency. The complex geometries lead to a wide range of structural features due to process-induced variations in local cooling rate. Therefore, the relationships between structural characteristics (e.g. dendrites, grain, eutectic, porosity, lack of fusion) and fatigue behavior have to be understood to enable a lightweight and reliable design of Al-Si alloys. In this study, the effect of process and process-induced structural characteristics on stress-strain behavior (quasi-static, cyclic) and high cycle fatigue behavior were characterized. The specimens were processed by laser-based powder bed fusion (PBF-LB) and sand casting (SC) with variation in porosity and dendrite arm spacing. Hereby, the age-hardenable Al-Si alloys AlSi10Mg (AM, SC) and AlSi7Mg (SC) were investigated. AM batches were tested in as-built condition. All casting batches got a HIP densification and T6 heat treatment. The fatigue results were analyzed based on nominal stresses according to Woehler (S-N curve) and local stress intensity based on linear-elastic fracture mechanics (stress intensity factor) as well as elastic-plastic fracture mechanics (J integral). J integral based Shiozawa diagrams allowed a sufficient and uniform fatigue damage tolerance (FDT) assessment for all AM and SC alloys. Hereby, the FDT of Al-Si alloys were dominated by defects (size, position) and cyclic stress-strain behavior (yield strength, cyclic hardening).

Prediction of grain-size transition during solidification of hypoeutectic Al-Si alloys by an improved three-dimensional sharp-interface model

Control of grain size during solidification is an important issue according to the desired usage properties. In this study, both microstructure characterizations and numerical simulations are performed for as-cast hypoeutectic Al-Si alloys to understand the mechanism of grain-size transition with initial Si contents. An improved three-dimensional (3D) sharp-interface model is developed that couples cellular automaton (CA) approach with a deterministic mesh-anisotropy reduction (DMAR) algorithm. The improved sharp-interface model is able to accurately calculate interface curvature and reproduce reasonable dendrite morphology within a wide range of cooling rates. It is found that grain size first decreases with increasing the initial Si content to 3 wt% and then increases with further Si additions. The nucleation undercooling significantly increases with increasing the Si content from 3 wt% to 10 wt% due to a Si-poisoning effect. The grain-size transition is mainly determined by the variations in the nucleation undercooling. Analysis using the Interdependence model also supports that a wide nucleation-free-zone is formed during solidification of Al-10 wt% Si alloy induced by a large nucleation barrier and a decreased growth velocity.

Achieving high strength and superior ductility in Al–Si alloy by cold rolling and friction stir processing

In the current work, the influence of heat treatment, rolling, and friction stir processing (FSP) on the microstructure, hardness, tensile behavior, and fracture mechanism of hypoeutectic Al–Si alloy were investigated. The FSP led to a reduction in the aspect ratio and increased the size and sphericity of particles. Also, fine silicon and Fe-rich particles were uniformly distributed in the matrix. With increasing the rotational speed, the size and aspect ratio of particles was decreased, and their sphericity was increased. The best morphology of Si particles (size of 2.23 µm, aspect ratio of 1.91, and sphericity of 0.682) had been obtained at the rotational speed of 800 rpm. The microhardness profiles of the FSPed samples were roughly homogeneous. After the cold rolling, the yield strength (YS), ultimate tensile strength (UTS), and energy absorption (EA) were enhanced to 288.7 MPa, 296.0 MPa, and 18.5 J/cm3, however, the total elongation (TE) was reduced to 6%. Interestingly, the YS, UTS, TE, and EA of all FSPed samples were much higher than that of the initial material. The 800–120 sample had the best tensile properties with YS, UTS, TE, and EA of 116.7 MPa, 235 MPa, 27.2%, and 57.9 J/cm3, representing 162%, 43%, 263%, and 395% improvement, respectively. The results of the fractography exhibited that the preferential cracks propagated perpendicularly to the loading direction along the Al/Si and Al/Fe-rich interfaces in the initial and heat-treated samples. In contrast, the crack planes of the rolled and FSPed samples were oriented around 45° with respect to the loading direction. The fracture of the initial, heat-treated, and rolled samples was dominated by cleavage mode. In contrast, the FSPed samples revealed a full ductile fracture.

Factors affecting eutectic Si modification in Al-Si hypoeutectic alloy with the addition of Na, Sr, Eu and Yb

Factors affecting eutectic Si modification in Al-Si hypoeutectic alloy with the addition of Na, Sr, Eu and Yb were investigated. The modification effect is determined by element concentration in eutectic Si, which is mainly attributed to the solubility of modifying element M in α-Al and the interaction energy parameter of Al (or Si) and modifying element. A great modification is achieved by M of a limited solubility in α-Al and a moderate interaction energy parameter of Si (or Al) and M. The results are verified by the experimental results reported in the previous literatures. The effect of cooling rate on modification is also discussed in this work.

Effect of Cooling Rate and Modification by Strontium on the Thermal Conductivity of Al-8Si Alloy

Cooling rate plays a critical role in determining the thermal conductivity of Al-Si alloys. Although the effect of morphology and size of Si (changed by heat treatment) on its thermal conductivity has been investigated, the effect of cooling rates on thermal conductivity has not been well studied. In this study, we investigated the microstructure of an Al-8Si (with and without modification by Strontium (Sr)) alloy with cooling rates from 46.2 °C/s to 234 °C/s. It was found that the effect of cooling rate on thermal conductivity of Sr modification and Sr-free samples are opposite from each other. As a result, while the cooling rate increased from 46.2 °C/s to 234 °C/s, the calculated thermal conductivity increased from 145.3 MS/m to 151.5 MS/m for Sr-free Al-8Si alloy, and the calculated thermal conductivity was reduced from 187.5 MS/m to 176.7 MS/m for the Sr-modified Al-8Si alloy. By discussing how thermal conductivity correlates with eutectic silicon morphology and secondary dendrite arm spacing, the relationship between cooling rate and thermal conductivity were explained. This work suggests a new design strategy for improving the thermal conductivity of Al-Si hypoeutectic alloys.

Fabrication of hypoeutectic Al-4Si alloy with high electrical conductivity, high plasticity and medium strength by the dual treatment of Al matrix and eutectic Si microstructure

Hypoeutectic Al-Si alloys exhibit excellent mechanical properties and relatively good electrical conductivity (EC), and they are often used as overhead transmission conductors. But, due to the contradiction between EC and mechanical properties (especially EC and strength), high EC and good mechanical properties are hard to be obtained simultaneously, which limits their applications in the field of electrical engineering. Thus, it is necessary to improve the matching between EC and mechanical properties. In this study, composite boron treatment, Sr modification, homogenization heat treatment and hot extrusion deformation process were done to fabricate a kind of hypoeutectic Al-4Si alloy with high EC, high plasticity and medium strength. Through the process mentioned above, not only Al matrix was purified, but also eutectic Si exhibited good orientation consistent with α-Al. In addition, eutectic Si morphology was modified from large needle/plate-like to fine particles, and its distribution was changed from segregation along grain boundary to uniform distribution. The scattering probability of solid solution TM elements to electrons was significantly reduced, and the number of channels for electron migration in the eutectic Si region was also increased obviously. Meanwhile, the optimization of the microstructure enhanced the mechanical strength and elongation of Al-4Si alloy at the same time. The special extrusion texture also promoted improvement of the electrical and mechanical properties of Al-4Si alloy. Finally, the EC of Al-4Si alloy was improved from 41.8 %IACS to 59.2 %IACS, and the corresponding ultimate tensile strength (UTS) and elongation can reach 206 MPa and 26.7%, respectively. Compared with that as-cast condition, EC, UTS and elongation improved by 41.6%, 60.9% and 136.3%, respectively, which has a certain application prospect in the field of electrical engineering.