WO2024144655A1

WO2024144655A1 - Method for obtaining spherical silicon phase in aluminium alloys

Info

Publication number: WO2024144655A1
Application number: PCT/TR2023/051622
Authority: WO
Inventors: Hasan HASIRCI
Original assignee: Gazi Universitesi
Current assignee: Gazi Universitesi
Priority date: 2022-12-30
Filing date: 2023-12-21
Publication date: 2024-07-04
Anticipated expiration: 2025-06-30

Abstract

The invention relates to the semi-isothermal heat treatment method with fluctuating/wavy (non-isothermal) temperature application, which ensures that the silicon phase in aluminum alloys becomes spherical and the alloy does not liquefy.

Description

METHOD FOR OBTAINING SPHERICAL SILICON PHASE IN ALUMINIUM ALLOYS

Technical field of the invention

The invention relates to the semi-isothermal heat treatment method that enables the spheroidisation of the lamellar silicon phase in aluminum alloys.

The invention is particularly related to the semi-isothermal heat treatment method with fluctuating/wavy temperature application, which ensures that the silicon phase in aluminum alloys becomes spherical and the alloy does not liquefy.

State of the Art

Aluminum alloys, known as light alloys in the industry, are improved by adding elements such as copper, zinc, silicon, magnesium, manganese, iron, nickel and titanium into aluminum metal, in line with the evolving needs of the properties of aluminum.

With the alloying elements added into aluminum alloys, they improve the properties of the main metal, aluminium. Copper, an alloying element, provides hardness, strength and ease of machining to aluminum and contributes to the increase in density and strength. If copper is added to aluminum more than 12%, it increases the brittleness of aluminum. Nickel, which gives strength and hardness to aluminum by adding it in certain proportions like copper, increases the surface quality of aluminum by increasing its brightness and reflectivity properties. Titanium and boron alloy elements added to aluminum are used as grain reducers and increase tensile strength and ductility. Iron, another alloying element added to aluminum, increases strength and hardness at high temperatures. In addition, the high proportion of iron used to reduce hot cracking resistance and shrinkage in aluminum alloys containing silicon causes the aluminum alloy to have a crystalline and brittle structure. Manganese added with iron to increase castability into aluminum changes the properties of intermetallic compounds, reduces shrinkage and increases the strength of the alloys. The magnesium element used in aluminum alloys is used to ensure that aluminum alloys are suitable for heat treatment. Magnesium increases the fluidity of the molten material, creating an increase in strength in the structure and corrosion resistance.

Silicon, another alloying element added to aluminum, increases the fluidity of aluminum, and enables the production of thin parts. In addition, by lowering the melting point of aluminium, it reduces shrinkage during solidification and increases its strength values, enabling its use in different areas of use. In addition, since silicon has a low density, it also reduces the total weight of the aluminum alloy. In order for silicon to show its properties in the aluminum alloy, it should not remain coarse grained and should not form a lamellar phase with pointed ends. However, since silicon, which has low solubility in aluminum, causes internal stresses in the structure of the aluminum alloy, it increases the brittleness of the material and causes a decrease in its strength.

According to the aluminum-silicon binary phase diagram, which gives the solubility of silicon in aluminum, there is a eutectic point corresponding to 12.6% silicon content. Due to the low solubility of silicon in aluminum, silicon forms precipitates in the aluminum matrix, which contributes significantly to the mechanical properties of the alloy. Eutectic and sub-eutectic aluminium-silicon alloys with near-eutectic composition are widely used in the casting industry due to reasons such as excellent wear and corrosion resistance, low coefficient of thermal expansion, and high strength/weight ratio. However, the increase in the amount of silicon causes a significant decrease in the elongation values of the aluminum-silicon alloy. Therefore, these types of alloys need to be improved. The mechanical properties of eutectic and near-eutectic Al-Si casting alloys depend on microstructural properties rather than the chemical composition of the alloy. The amount and morphology of dendritic a-AI and eutectic Silicon and the presence of other possible intermetallic compounds in the microstructure significantly affect the mechanical properties of the alloy.

Nowadays, heat treatments are applied to disperse the silicon phase within the aluminum alloy and prevent it from forming a pointed phase in the microstructure. Said heat treatments comprise the process steps of adding to a solution, sudden cooling (quenching) and aging. In the step of adding to a solution, waiting for a certain period of time at a fixed temperature value, usually between 450 and 575°C, is required. The aim of the process of adding to a solution is to dissolve the alloying elements in the alloy and disperse them more easily within the alloy, thus creating microstructural shapes that support strength. For this reason, the alloy cools suddenly immediately (quenching process) after being put into solution. Aging is applied to increase the strength of the alloy, which becomes brittle with sudden cooling. The aging process occurs by keeping the alloy at a constant temperature much lower than the solution temperature for a certain period of time. If the temperature applied during the aging process is at room temperature, it is called natural aging. If a hardness that cannot be achieved by natural aging is desired, it is called artificial aging when it is in an oven at a temperature higher than room temperature. The aging process is applied to determine the hardness level of the material by separating and precipitating the alloying elements from the solid solution. In alloys containing silicon, despite the heat treatment steps applied due to the low solubility of silicon, the pointed structures formed by silicon in the microstructure can form again and reduce the strength.

In the state of the art, there are various heat treatment methods developed for aluminium alloys. Some patent applications for these heat treatment methods are given below.

The patent file numbered "TR200701976" is in the state of the art was examined. In the summary of the invention that is the subject of the application, the information that reads: “The invention relates to a method for improving the mechanical properties of Aluminum-Silicon alloys. To put it in more detail, the invention relates to a heat treatment method for improving the material softness of objects having a maximum meltable phase fraction, preferably made of enriched or purified Aluminum-Silicon alloy, optionally a casting alloy or kneading alloy containing other alloying elements and/or polluting elements, wherein these objects are subjected to an annealing process with subsequent aging/maturing.

The patent document numbered "CN1 14369775A" in the state of the art was reviewed. In the invention that is the subject of the application, the heat treatment process of a cast aluminum alloy hydraulic disc brake is mentioned. In said invention, it has been explained that in the process step of adding to a solution, heating to 500-515 °C in 30 to 40 minutes, waiting at this temperature for 35 to 45 minutes, and heating to 525-540 °C in 10-20 minutes and keeping it at this temperature for 5-7 hours will be performed. The aim of the invention

The most important aim of the invention is to ensure that the silicon phase in aluminum alloys has a spherical shape. In this way, it ensures that the strength of aluminum alloys is increased.

Another aim of the invention is to realize the single-stage process of adding to a solution, which exists in the state of the art, in three fluctuating process steps/wavy manner showing increasing, decreasing and increasing temperature, unlike the isothermal process step-by-step solution taking in the known state of the art. The number of these increases and decreases varies depending on the material chemical composition, section thickness and part shape. In this way, the spheroidisation of the silicon phase is ensured and the liquefaction of the alloy is prevented.

Description of the drawings

FIGURE -1 is the representative view of the heat treatment chart in the state of the art.

FIGURE -2 is the representative view of the heat treatment chart that is the subject of the invention.

FIGURE -3 is the representative view of the details of the heat treatment that is the subject of the invention.

Reference numbers

100. Alloy

110. Silicon Phase

T_o. Initial Temperature

Ti. Temperature of Adding to Solution

T₂. Aging Temperature C. Adding to Solution

Si. First Temperature Increasing Step

52. Temperature Decreasing Step

53. Second Temperature Increasing Step

A. Sudden Cooling (Quenching)

Y. Aging

D. Diffusion Movement

Description of the invention

The invention relates to a semi-isothermal heat treatment method that aims to improve mechanical properties by spheroidising the silicon phase (1 10) with the lamellar microstructure found in aluminum alloys (100).

The heat treatment method of the invention increases the mechanical properties of the alloy (100) by causing a change in the shape of the silicon phase (1 10) in the alloy (100) with a diffusion mechanism with fluctuating/wavy temperature via the first temperature increasing step (Si) depending on time, then the temperature decreasing step (S2) and then the second temperature increasing step (S3) again.

During the temperature increasing step (Si), which is the first process step of the timedependent isothermal heat treatment method of the invention, the heat given around the silicon phases (1 10) increases the entropy and a diffusion movement (D) occurs from the silicon phase (1 10) to the alloy (100). With this movement, the silicon phase (1 10) in the lamellar structure dissolves in the alloy (100). This diffusion movement (D), which occurs with the first temperature increasing step (Si), occurs faster since the pointed ends of the lamellar structure of the silicon phase (110) are high-energy regions, and the silicon phase (1 10) loses its thin and long lamellar structure and becomes more elliptically spherical. The spherical silicon phase (1 10) increases the strength of the alloy (100). The first temperature increasing step (Si) is followed by the temperature decreasing step (S2) in which the temperature decreases. In the temperature decreasing step (S2), diffusion movement occurs from the alloy (100) to the silicon phase (1 10). The aim of the temperature-decreasing step (S2) is to prevent the liquefaction of the alloy (100) and to prevent the phases within the alloy (100) that are likely to melt from liquefying. The temperature decreasing step (S2) is followed by the second temperature increasing step (S3), in which the temperature increases again. By means of the second temperature increasing step (S3), the diffusion movement (D) of the silicon phase (1 10) within the alloy is equalised and the silicon phase (1 10) is ensured to precipitate in a spherical manner within the alloy (100) without melting the alloy (100). Additionally, in case there is a silicon phase (1 10) whose spheroidisation has not been completed in the first temperature increasing step (S-i), the silicon phase (110) whose spheroidisation has not been completed is ensured to complete its spheroidisation. In order for the silicon phase (1 10) to maintain its spherical shape, sudden cooling (A) is performed and then the brittleness occurring in the alloy is eliminated by artificial aging (Y).

The process step adding to solution (C) takes 0.5-6 hours depending on the thickness and chemical composition of the aluminum alloy (100) and the process step of aging (Y) takes between 0.5-10 hours, depending on the thickness and chemical composition of the aluminum alloy (100). The process step of aging (Y) preferably lasts between 2- 10 hours. In the process step adding to solution (C) which comprises the first temperature increasing step (Si), then the temperature decreasing step (S2) and then the second temperature increasing step (S3) again, for the spheroidisation of the silicon phase (110) after the second temperature increasing step (S3), the first temperature increasing step (Si), then the temperature decreasing step (S2) and then the second temperature increasing step (S3) can be repeated.

Figure 1 shows the heat treatment chart applied to aluminum alloys (100) containing the silicon element in the state of the art. In the state of the art, aluminum alloy (100) is heated from the initial temperature (To) to the temperature of adding to solution (T1) is kept at temperature of adding to solution (T1) for a certain period of time in the process step of adding to solution (C), and then cooled by applying sudden cooling (A) (quenching) and then heated to the aging temperature (T2) and then subjected to the process step of aging (Y) in which it is cooled again to the initial temperature by applying the aging temperature (T2) for a certain period of time.

When the semi-isothermal heat treatment that is the subject of the invention in Figure 2 is compared with the state of the art, in the single-step process of adding to solution (C), which is the first process step of the heat treatment in the state of the art, the silicon phase (1 10) in the aluminium alloy (100) becomes spherical by diffusion, while the aluminum alloy (100) liquefies. The liquefaction of the aluminum alloy (100) disrupts the piece integrity of the aluminum alloy (100). When the liquefaction in the aluminum alloy (100) occurs on the surface, it causes porous structures to form on the surface of the aluminum alloy (100), while the partial liquefaction in the aluminum alloy (100) causes grain coarsening and hot tears in the microstructure. In the semiisothermal heat treatment that is the subject of the invention, since the process step of adding to solution (C) takes place in 3 steps with first temperature increasing step (Si), temperature decreasing step (S2) and second temperature increasing step (S3), spheroidisation of the silicon phase (1 10) is ensured while liquefaction of the aluminum alloy (100) is prevented.

In the first temperature increasing step (Si), the alloy (100) is heated from the temperature of adding to solution (T1) to 550°C and then cooled again to the temperature of adding to solution (T1). In the temperature decreasing step (S2), the alloy (100) is cooled from the temperature of adding to solution (T1) to 520°C and then heated again to the temperature of adding to solution (T-i). In the second temperature increasing step (S2), the alloy (100) is heated from the temperature of adding to solution (T-i) to 550°C and then cooled again to the temperature of adding to solution (T-i). After the first temperature increasing step (Si), the temperature decreasing step (S2) and the second temperature increasing step (S3), the sudden cooling (A) (quenching) and aging (Y) steps follow, respectively.

The initial temperature (To) is room temperature, the temperature of adding to solution (T-i) is between 450-550°C, and the aging temperature (T2) is between 150-250°C.

The semi-isothermal heat process of the invention comprises the process steps of: I. Heating the aluminum alloy (100) with a constant temperature increase from the initial temperature (To) to the temperature of adding to solution (T-i),

II. Heating the aluminum alloy (100), which reaches the temperature of adding to solution (Ti), to 550°C and then cooling it to the temperature of adding to solution (T-i),

III. Cooling the aluminum alloy (100) that reaches the temperature of adding to solution (T-i) to 520°C and then heating it to the temperature of adding to solution (Ti),

IV. Heating the aluminium alloy (100), which reaches the temperature of adding to solution (Ti), to 550°C and then cooling it to the temperature of adding to solution (T-i),

V. Returning the aluminium alloy (100), which reaches the temperature of adding to solution (T-i), to its initial temperature (To) as a result of sudden cooling (quenching),

VI. Heating the aluminium alloy (100), which returns to the initial temperature (To) with the process step of sudden cooling (A) (quenching), to the aging temperature (T2),

VII. Keeping the aluminium alloy (100) at aging temperature (T2) for 0.5-10 hours, and

VIII. cooling the aluminium alloy (100) to the initial temperature (To) after the aging process (Y).

Claims

1. A semi-isothermal heat treatment method for spheroidisation of the silicon phase (110) in aluminum alloys (100), comprising the process steps of: i. Heating the aluminum alloy (100) with a constant temperature increase from the initial temperature (To) to the temperature of adding to solution (Ti), ii. Heating the aluminium alloy (100), which reaches the temperature of adding to solution (Ti ), to 550°C and then cooling it to the temperature of adding to solution (T-i), iii. Cooling the aluminum alloy (100) that reaches the temperature of adding to solution (T-i) to 520°C and then heating it to the temperature of adding to solution (T-i), iv. Heating the aluminum alloy (100), which reaches the temperature of adding to solution (Ti ), to 550°C and then cooling it to the temperature of adding to solution (T-i), v. Returning the aluminum alloy (100), which reaches the temperature of adding to solution (T-i), to its initial temperature (To) as a result of sudden cooling (quenching), vi. Heating the aluminium alloy (100), which returns to the initial temperature (To) with the process step of sudden cooling (A) (quenching), to the aging temperature (T2), vii. Keeping the aluminium alloy (100) at aging temperature (T2) for 0.5-10 hours, and viii. cooling the aluminium alloy (100) to the initial temperature (To) after the aging process (Y).

2. A semi-isothermal heat treatment method for spheroidisation of the silicon phase (110) in aluminium alloys (100) according to Claim 1 , wherein in the process step of heating the aluminium alloy (100) which reaches to the temperature of adding to solution (T-i) to 550°C and then cooling it to the temperature of adding to solution (T-i), the first temperature increasing step (Si) is realised.

3. A semi-isothermal heat treatment method for spheroidisation of the silicon phase (110) in aluminum alloys (100) according to Claim 1 , wherein in the process step of cooling the aluminum alloy (100) which reaches to the temperature of adding to solution (T-i) to 520°C and then heating it to the temperature of adding to solution (T-i), the second temperature increasing step

(52) is realised.

4. A semi-isothermal heat treatment method for spheroidisation of the silicon phase (110) in aluminum alloys (100) according to Claim 1 , wherein in the process step of heating the aluminum alloy (100) which reaches to the temperature of adding to solution (T-i) after the temperature decreasing step (S2) occurs to 550°C and then cooling it to the temperature of adding to solution (T-i), the second temperature increasing step (S2) is realized.

5. A semi-isothermal heat treatment method for spheroidisation of the silicon phase (110) in aluminum alloys (100) according to Claim 1 , wherein in the process step of sudden cooling (A) (quenching) the aluminum alloy (100) which reaches to the temperature of adding to solution (T-i) to initial temperature (To), the process of sudden cooling (A) (quenching) is realised.

6. A semi-isothermal heat treatment method for spheroidisation of the silicon phase (110) in aluminium alloys (100) according to Claim 1 , wherein in the process step of keeping the alloy (100) at the aging temperature (T2) until it reaches the desired brittleness point after the process step of sudden cooling (A) (quenching), process steps of aging (Y) is realised.

7. A semi-isothermal heat treatment method for spheroidisation of the silicon phase (1 10) in aluminum alloys (100) according to Claim 1 , wherein the process steps of first temperature increasing (Si), temperature decreasing (S2), second temperature increasing (S3), sudden cooling (A) (quenching) and aging (A) are realised respectively.

8. A semi-isothermal heat treatment method for spheroidisation of the silicon phase (110) in aluminum alloys (100) according to Claim 1 , wherein in the process step of heating the aluminum alloy (100) which reaches to the temperature of adding to solution (T-i) to 550°C and then cooling it to the temperature of adding to solution (T-i), the process steps of first temperature increasing (Si), temperature decreasing (S2), second temperature increasing

(53), steps can be repeated.

9. A semi-isothermal heat treatment method for spheroidisation of the silicon phase (110) in aluminum alloys (100) according to Claim 1 , wherein the initial temperature (To) is the room temperature.

10.A semi-isothermal heat treatment method for spheroidisation of the silicon phase (110) in aluminum alloys (100) according to Claim 1 , wherein the temperature of adding to solution (T-i) is between 450-550°C and aging temperature (T2) is between 150-250°C.