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aluminium silicon alloy (aluminium silicon alloy) is a forging and casting alloy with aluminum and silicon as the main components. The general silicon content is 11%, while adding a small amount of copper, iron and nickel to improve the strength. The density is about 2.6 ~ 2.7g/cm3, and the thermal conductivity is about 101 ~ 126W/(m·ºC). Young's modulus is 71.0GPa, the impact value is about 7 ~ 8.5J, and the fatigue limit is ±45MPa.

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Because of its light weight, good thermal conductivity, strength, hardness and corrosion resistance, AI-Si alloy is widely used in the automobile industry and machine manufacturing to make some parts used under sliding friction conditions.

l-Si alloy is a very important industrial alloy, widely used in aviation, transportation, construction, automobile and other important industries, is also used in the manufacture of low and medium strength of the shape of complex castings, such as cover plate, motor housing, bracket, and also used as brazing solder. The alloy is a typical eutectic alloy with simple phase diagram and no intermediate compounds. It has the advantages of good casting performance, high strength and low price. Aluminum is the third main group element, and silicon is the semiconductor element, the solid solubility of each other is very small. [2]

Performance and use

When the silicon content is low (such as 0.7), Al-Si alloy has good ductility and is commonly used as deformation alloy. When the silicon content is higher (such as 7%), the aluminum-silicon alloy melt is better filled and is often used as casting alloy. When the silicon content exceeds the Al-Si eutectic point (silicon content is 12.6%) and the silicon particle content is up to 14.5% ~ 25%, the comprehensive mechanical properties can be improved by adding a certain amount of Ni, CU, Mg and other elements. They can be used in car engines instead of cast iron cylinders with significant weight reduction. The aluminum silicon alloy used as cylinder can be treated by electrochemical process to etch the surface aluminum and retain the primary silicon particle embedded in the matrix inside the cylinder wall, and its abrasion resistance and abrasion resistance can be significantly improved. Among them, the alloy with 11% ~ 13% silicon content is one of the best piston materials for its light weight, low expansion coefficient and high corrosion resistance.

Electrothermal production

The metallurgical temperature of aluminum silicon alloy produced by electric heating is about ºC. In the process of metallurgy, alumina and silicon oxide are liquid, which are generally reduced by carbonaceous reducing agent. Generally, the highest temperature of mineral furnace can be reached is about ~ºC.
 

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Al-si binary alloy has a simple eutectic phase diagram, as shown in the "Al+Si Binary Phase Diagram". At room temperature, there are only α (Al) and p (Si) phases. The properties of α (Al) and β (Si) are similar to those of pure silicon. The Si content of eutectic alloy is 12.6%. The microstructure of subeutectic alloy is composed of α (Al) + eutectic crystal (α+p), and that of hypereutectic alloy is composed of β (Si) + eutectic crystal (α+β). Due to the introduction of trace phosphorus into crystalline silicon, even 10ppm of phosphorus to AlP is sufficient to produce primary silicon in a 9% Si subeutectic alloy and cause the eutectic silicon to form coarse plates.
 

With the increase of silicon content, the crystallization temperature interval decreases, the eutectic crystal increases, and the fluidity increases. When the shrinkage rate of silicon is very small, the linear shrinkage rate of the alloy also decreases, and the hot cracking tendency decreases accordingly. The latent heat of crystallization of silicon is large, and the fluidity is still higher than that of eutectic alloy until the content of Si reaches 20%. When the Si content is 16% ~ 18%, the liquidity peak.

Although eutectic AI-Si binary alloy has excellent casting properties, it can only be used for high-speed cooling casting methods such as die casting and extrusion casting because of its poor mechanical properties. For sand casting, gypsum casting and other casting methods with slow cooling rate, metamorphic treatment must be carried out to refine eutectic silicon in order to obtain adequate mechanical properties.

The modification process of refining eutectic silicon cannot also refine primary silicon. For hypereutectic alloys with a large amount of primary silicon, phosphorus must be added to refine primary silicon to improve mechanical properties.

The silicon content has an effect on wear resistance, corrosion resistance, linear expansion coefficient, density and conductivity of Al-Si binary alloy. With the increase of silicon content, the wear, corrosion, linear expansion coefficient, density and conductivity all decrease linearly.

Aluminum plasticity is large, cutting need to consume a lot of work, with the increase of silicon, eutectic increase, cutting work can be reduced, but eutectic silicon hardness is high, easy to wear the tool, especially the coarse initial silicon of the eutectic alloy, tool wear is more serious, the surface of the processing is very rough. In order to improve the machinability, besides corresponding modification treatment, refining eutectic silicon and primary silicon, bismuth, lead and other easy machinable elements can be added. For hypereutectic alloys, diamond cutting tools can be used, and the best cutting speed and suitable cutting fluid can be selected to obtain smooth machining surface.

To sum up, in order to take into account various service properties and technological properties of the alloy, the silicon content of Al-Si alloys is generally 7% ~ 12%.

The representative of Al-Si binary alloy is ZL102 alloy, the composition is 10% ~ 13% Si, the rest is aluminum, the metallographic structure is α (Al) + eutectic (α+β) and a small amount of primary silicon. ZL102 alloy has the following characteristics.

1) The strengthening effect of heat treatment is small and the mechanical properties are not high

The solubility of silicon in α (Al) solid solution is 1.65% at 577ºC and decreases to 0.05% at room temperature. However, the strengthening effect of heat treatment is not large, and artificial aging after solid solution treatment can only increase the strength of alloy by 10% ~ 20%, because silicon precipitation and accumulation speed is very fast, even in the process of solid solution treatment may occur solid solution decomposition, silicon particle precipitation, no coherent or semi-coherent transition phase, so generally only annealing to eliminate the internal stress. The mechanical properties of ZL102 are not high.

2) Excellent casting performance

The near-eutectic Al-Si binary alloy has a small crystallization temperature interval and a large crystallization potential heat of silicon, so the fluidity is the crown of the cast aluminum alloy, and the tendency of concentrated shrinkage cavity is large. A reasonable riser should be set to obtain dense castings, which will not cause leakage until the destruction. Silicon reduces the solubility of hydrogen after the solidification of liquid aluminum, and pinholes are easy to be produced if the refining is improper.

3) Good wear resistance, corrosion resistance and heat resistance

Al-si binary alloy has soft phase α (Al) and hard phase silicon, and is a typical wear-resistant structure with good wear resistance. The electron potential difference between α (Al) and eutectic silicon is not much. The surface layer of Al2O3 is dense and has a protective effect on the matrix, so the corrosion resistance is good. The eutectic temperature of ZL102 alloy is 577ºC, which is higher than that of other cast aluminum alloys. There is no phenomenon of dissolution or aggregation of enhanced phase when the temperature rises, so the heat resistance is the best.

4) Metamorphic treatment must be carried out to improve mechanical properties

The mechanical properties before metamorphism are low, and the machining properties are poor, so it is necessary to undergo metamorphic treatment, so that the lamellar eutectic silicon can be changed into fibrous, and the primary crystalline silicon can be eliminated to greatly improve the mechanical properties.

In summary, ZL102 alloy is suitable for die casting or corrosion resistance, wear resistance; Thin wall, complex castings to withstand small and medium loads, such as the frame, bright body, base of various instruments, etc. [3]

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The significance of metamorphic treatment

Cast aluminum-silicon alloy has the advantages of low density, high strength, good wear and heat resistance, small thermal expansion coefficient and so on. It is the most widely used and the most productive alloy in cast aluminum alloy. The typical eutectic phase diagram shows that the mass fraction of eutectic point silicon is 11.7% and the eutectic temperature is 577ºC. The maximum solid solubility of silicon in aluminum is 1.65%, and the solid solubility at room temperature is about 0.05%. The eutectic reaction is: according to the silicon content, Al-Si alloy can be divided into subeutectic, eutectic and hypereutectic alloys. There are acicular eutectic silicon and coarse and complex primary silicon in the structure of conventional casting Al-Si alloy, which worsens the properties of the alloy. In industry, modification treatment is used to change the morphology of silicon phase, so that it can be evenly distributed in the matrix with favorable shape and small size, which has a good effect on improving the performance of cast Al-Si alloy. [4]

Methods and effects of metamorphism

There are many kinds of elements, such as Na, Sr, Ba, Bi, Sb and rare earth element Ce, which can affect the metamorphism of eutectic silicon in Al-Si alloy. Among them, Na is the most significant metamorphism and the most widely used in production. In recent years, Sr metamorphism has also been gradually applied in production.

When the liquid aluminum is treated with a complex salt modifier containing sodium fluoride (e.g., ωN&#;F=45%, ωN&#;Cl=40%, ωKCl=15%) or AI-Sr alloy is added to the liquid aluminum, When the residual Na is ωNa 0.001%-0.003% or Sr is ωSr= 0.01%-0.03%, good metamorphic effect can be obtained, and the eutectic silicon in the alloy structure becomes fibrous, which significantly improves the strength and plasticity of the alloy.

The modification process not only changes the silicon crystal structure but also changes the eutectic degree of the alloy. The Na modification process will make the eutectic point shift to the right, even if the eutectic silicon content increases. Therefore, when the alloy is eutectic composition before treatment, it will become subeutectic composition after treatment.

The modification treatment is widely used in the production of cast aluminum alloy. In fact, the ωSi=5~11% aluminum-silicon alloy is carried out within the crystal composition range. With the increase of the silicon content of the alloy, the modification effect is more significant, and the pre-modification treatment has more significant effect in improving the plasticity of the alloy than in improving the strength.

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The effect of metamorphic treatment is also related to the crystallization subcooling degree of the alloy (cooling rate of the casting). The thicker the casting wall, that is, the slower the cooling, the smaller the effect of metamorphic treatment. This is known as the wall thickness sensitivity of metamorphic treatment. Similarly, the effect of metamorphic treatment is more significant in metal mold casting than in sand mold casting. The sensitivity of wall thickness varies with different metamorphic elements. When Na or Sr is used for modification, the sensitivity of wall thickness is small, while when Sb or Bi is used for modification, the sensitivity of wall thickness is larger, that is, only in thin wall castings or in metal mold casting conditions, there is a significant metamorphism effect.

The metamorphic effect of aluminum-silicon alloy will disappear gradually with the extension of the time after treatment. This is similar to the phenomenon of breeding decay in cast iron. The deterioration of metamorphic effect is caused by the gradual decrease of the residual amount of metamorphic elements with the extension of time. At the temperature of liquid aluminum, the Na or Sr contained in the alloy is either oxidized or destroyed by the action of water in the molding sand, and the rate of disappearance is to some extent related to the chemical activity of the metamorphic elements. In addition, the melting point and relative density of metamorphic elements also affect the vanishing rate. From the perspective of the effective time of metamorphism, Na has the shortest effective time of 30-60min, Sr 6-7h, Ba more than 5h and Sb more than 100h, while Te can maintain its metamorphism almost indefinitely, and no metamorphic decay occurs even after the alloy is remelted.

Although the effective time of sodium metamorphism is short, it will bring inconvenience to production, but its metamorphic effect is the strongest, so it is still widely used in production. Sr has strong metamorphism and long metamorphism. Effective time, but Sr is expensive, and Sr deterioration will increase the suction of the alloy, so it can not replace Na at present.
 

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Data Availability Statement

Not applicable.

Abstract

The paper presents the results of tests of rapid solidification (RS) aluminum alloys with the addition of silicon (5%, 11%, and 20%). Casting by melt-spinning on the surface of an intensively cooled copper cylinder allowed to obtain a metallic material in the form of flakes, which were then consolidated in the process of pressing and direct extrusion. The effect of refinement on structural components after rapid solidification was determined. Rapidly solidified AlSi materials are characterized by a comparable size of Si particles, regardless of the silicon content, and the shape of these particles is close to spheroidal. Not only Si particles are fragmented, but also the Al-Si-Fe phase, which also changed its shape from irregular with sharp edges to regular and spherical. The melt-spinning process resulted in a fine-grained structure compared to materials obtained by gravity-casting and extrusion. The influence of the high-temperature compression test on the mechanical properties of rapidly solidified materials was analyzed, and the results were compared with those of gravity-cast materials. An increase in strength properties was found in the case of the AlSi5 RS alloy by 20%, in the case of AlSi11RS by 25%, and in the case of the alloy containing 20% Si by as much as 86% (tensile test). On the basis of the homogeneity of the particle distribution determined by the SEM method, it was found that rapid solidification is an effective method of increasing the strength properties and improving the plastic properties of Al-Si alloys.

Keywords:

Al-Si aluminum alloy, melt-spinning, shape factor, mechanical properties

1. Introduction

Research on the ways of refining grains of metals and alloys led to the application in industrial practice of methods that ensure grain diameters of several micrometers. Experimental studies indicate a significant improvement in strength properties as a result of grain size reduction [1,2,3].

Materials with a submicron structure show high stability, and grain growth is observed in them at a relatively high temperature, often exceeding 0.4&#;0.5 of the melting point, despite the increased diffusivity observed when grain size decreases [4,5,6]. It is hypothesized that the stability of such a structure may be determined by a large number of triple boundary points. They have an inhibiting effect on the migration of boundaries or the precipitation process related to the decrease in the solubility of alloy additions during grain growth [7,8].

In the case of non-ferrous metals, efforts are still being made to improve their properties through fine-grained refinement. The advantages of aluminum as a construction material, together with its lightness, are still a strong incentive to search for effective and economically effective methods of improving its strength properties. The goal of this research is also to obtain materials with a grain size of less than one micrometer [9,10,11].

Increasing the strength properties can be effectively obtained by SPD (Severe Plastic Deformation) methods. These methods assume a very high degree of plastic deformation of the material while limiting the possibility of spontaneous recrystallization. These methods consist of introducing a large number of defects into the material as a result of plastic deformation, which, after spatial reorganization and mutual reaction, are able to create a large number of grain boundaries and, thus, grain refining. SPD methods are based on plastic deformation with a very high proportion of the compressive hydrostatic stress state, which aims to prevent loss of material cohesion [12,13,14,15].

One of the methods of producing fine-grained materials is the method of rapid solidification (RS). With a properly selected chemical composition and a sufficiently high cooling rate, an amorphous material (metallic glass) can be obtained [16,17]. Therefore, in this type of material, the rapid solidification method is used mainly to refine the structural components, which leads to a significant increase in strength properties. In the RS method, the speed of heat dissipation during cooling and the heat released during crystallization significantly affect the speed of crystallization. Therefore, it is best to use thin ribbons cast on an intensively cooled copper cylinder. Another solution is to use metallic powders that can be obtained by spraying liquid metal. During the process, these powders are cooled in a shield of inert gas, e.g., argon. In the case of the Spray Deposition method, it is important that the liquid droplets do not crystallize before they hit the surface of the drum [18,19,20,21].

The melt spinning method can be used to obtain high-strength, corrosion-resistant aluminum alloys. The use of this method in the case of AA alloy resulted in an increase in the tensile strength by 65%, the yield strength by 45%, and the elongation by 14% [22]. The RS process leads to the grain refinement of various materials. It is thought that the RS contributes to the grain refinement of the AZ91 alloy [23]. Rapid-solidification technology can significantly refine Al-Zn-Mg-Cu alloy grains. After extrusion, the tensile strength and elongation of the extruded bar were 466 MPa and 12.9%, respectively. After T6 heat treatment, the tensile strength of the alloy reached 636 MPa, while elongation decreased to 10.5% [24].

Al-Si alloys are among the most commonly used aluminum alloys in automotive applications (e.g., engine components). Silicon significantly affects the strength of Al-Si alloys by transferring the load from the Al matrix to the hard (rigid) Si phase in the microstructure (load capacity). Casting parameters (i.e., solidification rate, element segregation), as well as the size and distribution of microstructural components in Al-Si alloys (i.e., Si particle morphology, intermetallic compounds, spacing between secondary dendrites) have a direct impact on the microstructure, mechanical properties, and behavior of the material in case of failure (or cracking) [25,26,27,28,29].

This paper presents an analysis of cast Al alloys with different silicon contents (5%, 11%, and 20%), which were obtained by two methods: gravity-casting and extrusion, as well as rapid solidification and plastic consolidation in the extrusion process. The choice of methods was dictated by the discrepancies in properties that arise during the production of materials by these methods. These differences result directly from the rapid solidification process. The literature lacks information on the comparison of the properties of this type of alloy and the impact of the melt spinning method on the fragmentation of the silicon phase and obtaining a morphology close to spherical.

2. Materials and Methods

Aluminum alloys with different silicon contents were used for the tests. The chemical composition of the starting materials is shown in . First, these materials ( ) were gravity-cast into an ingot with a diameter of 38.2 mm and a height of 60 mm. The charges were extruded at a temperature of 375 °C using an extrusion speed of 1 mm/s and processing λ = 25 to form rods with a diameter of 8 mm. A 100 T press manufactured by HYDROMET (Bytom, Poland) was used for extrusion.

Table 1

The second set was cast in the process of rapid solidification (RS) using the melt spinning method, pressed, and hot extruded. The materials were inductively melted in a graphite crucible in an argon protective atmosphere and then cast onto a copper rotating cylinder rotating at a circumferential speed of 10 m/s. This technique allows for an alloy cooling rate of 106 K/s. Rapidly solidified ribbons with a thickness of 30&#;70 µm and a width of about 2.5 mm were compacted into a briquette with a diameter of 38 mm and a height of 10 mm. The rapid solidified ribbons and briquette for AlSi5 RS is shown in .

RS compaction of the ribbons was carried out at ambient temperature on the KHPES 100 Georg KIRSTEN D- Kello press under a pressure of 100 bar. Six briquettes were made for each rapid-solidified material, which constituted the charge for the extrusion process. The extrusion process was carried out with the above-mentioned parameters.

Samples were taken from the extruded rods, and microsections were prepared for microstructure observation. The samples were ground on abrasive papers using 240&#;800 grits (paper grits) and then polished with diamond pastes (DP-Suspension P from Struers) with grits of 9, 3, and 1 µm. Finishing polishing was carried out using a colloidal suspension of silica OP-S from Struers. Grinding and polishing were performed on a RotoPol 11 device (manufactured by Struers, Copenhagen, Denmark). Microstructure observations were performed using an Olympus GX51 light microscope (Olympus Inverted Metallurgical Microscope, Olympus, Tokyo, Japan) and a Hitachi SU-70 scanning electron microscope (Hitachi High-Technologies Corporation, Tokyo, Japan). EDS spectroscopy was used for the study of an element&#;s distribution. The shape ratio was calculated using ImageJ (Rockville, Bethesda, MD, USA). Circularity was used as the shape factor. The static tensile test was carried out at an ambient temperature in accordance with PN-EN ISO -1:-05 [30] using a Zwick Roel Z050 testing machine (manufactured by ZwickRoell Group, Ulm, Germany). From the beginning, middle, and end of each rod, cylindrical samples were taken and made in order to determine the mechanical properties in the tensile test. Samples with a diameter of 6 mm and a length of the measuring base of 30 mm were deformed at a speed of 8 × 10&#;3 s&#;1. The high-temperature compression test was carried out using the MTS 880 testing machine (MTS Systems Corporation, Eden Prairie, MN, USA). Samples with dimensions of 8 mm in diameter and 11 mm in length were used for the compression test. The samples were cut directly from the rod after the extrusion process. The Vickers hardness measurement was carried out with a load of 19.61 N using a Shimadzu HMV-2 T device microhardness tester (Shimadzu Corporation, Kyoto, Japan).

4. Conclusions

Rapid solidification by the melt-spinning method is an effective method of fragmentating structural components in Al-Si alloys. The advantage of these materials is the homogeneity and high stability of the morphology of the precipitates in a wide temperature range, which allows the use of this type of material in the production of products that can be used at elevated temperatures.

Rapidly solidified Al-Si materials are characterized by a comparable size of Si particles, regardless of the silicon content, and the shape of these particles is close to spheroidal. Not only Si particles are fragmented, but also the Al-Si-Fe phase, which also changed its shape from irregular with sharp edges to regular and spherical.

The mechanical properties of materials obtained by combining the technologies of rapid crystallization and plastic consolidation in the extrusion process are significantly higher in comparison to gravity-cast and extruded materials. An increase in strength properties was found in the case of the AlSi5 RS alloy by 20%, in the case of AlSi11RS by 25%, and in the case of the alloy containing 20% Si by as much as 86% in relation to gravity-cast and extruded alloys.

Funding Statement

Financial support under contract 16.16.180.006 and grant POIR.01.01.01-00-/19 is kindly acknowledged.

Author Contributions

Conceptualization, P.N. and M.W.; methodology, P.N., T.S., M.W., and M.W.; validation, P.N. and M.W.; formal analysis, P.N., T.S., and M.W.; investigation, P.N. and M.W.; resources, P.N.; data curation, T.S., P.N., and M.W.; writing&#;original draft preparation, P.N. and T.S.; writing&#;review and editing, M.W.; visualization, P.N.; supervision, T.S.; funding acquisition, P.N. and M.W. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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