HIGH PERFORMANCE ALUMINUM NANOCOMPOSITES

Abstract

The high performance aluminum nanocomposites are formed by a combination of mechanical alloying and Spark Plasma Sintering (SPS) in order to obtain reinforced nanostrutured aluminum alloys, The nanocomposites are formed from aluminum metal reinforced with silicon carbide (SiC) particulates, wherein the SiC particulates have a particle diameter between about 20 and 40 nm. The nanocomposites are prepared by mixing aluminum-based metal, e.g., Al-7Si-0.3Mg, (Al=92.7%, Si-7% and Mg=0.3%), with SiC nanoparticles in a conventional mill to form a uniformly distributed powder, which is then sintered at a temperature of about 500° C. for a period up to about 20 hours to consolidate the silicon carbide particulates in order to obtain the reinforced aluminum metal-based silicon carbide nanocomposite.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to aluminum alloys, and particularly to high performance aluminum nanocomposites that are based on aluminum (Al) reinforced with nano-sized silicon carbide (SiC) particles.

2. Description of the Related Art

The commercially available aluminum-based alloys, in particular Al—Si—Mg alloys, are used extensively in the automotive industry in various locations, such as cylinder blocks, cylinder heads, pistons, and valve lifters. Under normal applications, Al—Si—Mg alloys are processed in both cast and wrought forms. They are age-hardenable and are routinely heat treated to T6 condition to develop adequate strength. Different industrial applications have called for more advanced processing of aluminum-based alloys, especially Al—Si—Mg alloys, because they are being utilized in multiple vital applications. This is primarily due to their improved corrosion resistance and to their high specific strength. These Al—Si—Mg alloys are aggressively replacing steels in various industries in order to have lighter components with improved properties. However, there has been a limit to the possible improvement of their properties using conventional processing routes, and research efforts have been directed to boosting their performance by adding a second phase particle in different compositions and sizes. Also, the conventional Al—Si—Mg alloys used in the automotive industry have limited capabilities, and their properties can't be stretched beyond certain limits. Indeed, most conventional processes that utilize normal consolidation procedures fail to retain the nano-crystalline structure till the final product is reached.

Thus, high performance aluminum nanocomposites solving the aforementioned problems are desired.

SUMMARY OF THE INVENTION

The high performance aluminum nanocomposites are formed by a combination of mechanical alloying and Spark Plasma Sintering (SPS) in order to obtain reinforced nanostructured aluminum alloys. The nanocomposites are formed from aluminum metal reinforced with silicon carbide (SiC) particulates, wherein the SiC particulates have a particle diameter between about 20 and 40 nm. The nanocomposites are prepared by mixing aluminum-based metal, e.g., Al-7Si-0.3 Mg, (Al=92.7%, Si=7% and Mg=0.3%), with SiC nanoparticles in a conventional mill to form a uniformly distributed powder, which is then sintered at a temperature of about 500° C. for a period up to about 20 hours to consolidate the silicon carbide particulates in order to obtain the reinforced aluminum metal-based silicon carbide nanocomposite.

These and other features of the present invention will become readily apparent upon further review of the following specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a scanning electron microscope (SEM) micrograph of Al-7Si-0.3Mg+20 wt. % SiC powder blend milled for 5 hours.

FIG. 1B shows a SEM micrograph of Al-7Si-0.3Mg+20 wt. % SiC powder blend milled for 12 hours.

FIG. 1C shows a SEM micrograph of Al-7Si-0.3Mg+20 wt. % SiC powder blend milled for 20 hours.

FIG. 2 is a transmission electron microscope (TEM) bright field image of a powder sample of an Al—Si—Mg alloy with 5% SiC milled for 20 hours, showing a crystalline shape with a diameter of about 100 nm.

FIG. 3 is a graph showing the reduction of crystallite size of an Al—Si—Mg alloy reinforced with 5% SiC and with 20% SiC as a function of milling time.

FIG. 4 is a graph showing shows the per cent densification as a function of the spark plasma sintering (SPS) temperature for Al—Si—Mg samples reinforced with 0%, 5%, 12%, and 20% SiC, respectively.

FIG. 5 is a graph showing the hardness as a function of the SPS sintering temperature for Al—Si—Mg samples reinforced with 0%, 5%, 12%, and 20% SiC, respectively.

Similar reference characters denote corresponding features consistently throughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The high performance aluminum nanocomposites are formed by a combination of mechanical alloying and Spark Plasma Sintering (SPS) in order to obtain reinforced nanostructured aluminum alloys. The nanocomposites are formed from aluminum metal reinforced with silicon carbide (SiC) particulates, wherein the SiC particulates have a particle diameter between about 20 and 40 nm. The nanocomposites are prepared by mixing aluminum-based metal, e.g., Al-7Si-0.3Mg, (Al=92.7%, Si=7% and Mg=0.3%) or Al-12Si-0.3Mg, with SiC nanoparticles in a conventional mill to form a uniformly distributed powder, which is then sintered at a temperature of about 500° C. for a period up to about 20 hours to consolidate the silicon carbide particulates in order to obtain the reinforced aluminum metal-based silicon carbide nanocomposite.

As used herein the term “mechanical alloying” refers to a solid state powder processing technique involving repeated cold welding, fracturing and re-welding of powder particles in a high energy ball mill, which may be a planetary ball mill. As used herein, the term “spark plasma sintering (SPS)” refers to a sintering technique in which a pulsed DC current directly passes through the die having the powder so that the powder is consolidated with the application of high temperature.

Nano-silicon carbide (SiC-β) particles of sizes between 20-40 nm and with high purity (<99%) are used in order to reinforce two conventional aluminum-based alloys, namely, Al-7Si-0.3Mg (Al=92.7%, Si=7% and Mg=0.3%) and Al-12Si-0.3Mg (Al=87.7%, Si=12%, Mg=0.3%)(all percentages by weight). The aluminum-based alloys were obtained in powder form and had particle sizes averaging 40 nanometers.

The nanoeomposites may be mixed together through milling the two constituents (SiC and aluminum-based alloys) in a planetary mechanical mill (Fritsch Pulverisette 5) at different parameters. Typically, the nano-sized silicon carbide (SiC) particles were added to various compositions at 5%, 12% and 20% by weight and then milled at 200 rpm with the aluminum-based alloys for different periods of time up to a maximum of 20 hours. FIGS. 1A-1C display the scanning electron microscope (SEM) micrographs of Al-7Si-0.3Mg+20 wt % SiC powder blend-milled for 5 hours, 12 hours, and 20 hours, respectively. The distribution of the augmentation was evaluated at different milling times in order to ensure optimum distribution that can facilitate the maximum improvement in properties. Energy Dispersive Spectroscopy (EDS) indicated that the reinforcement phase was distributed uniformly using mechanical milling, especially at prolonged milling time of 20 hours. Direct transmission electron microscopy (TEM) of the samples milled for 20 hours shows that the final crystalline size was about 100 nm. FIG. 2 shows a TEM bright field image of a powder sample with 5% SiC milled for 20 hours, indicating a crystallite with a diameter of about 100 nm. Additionally, FIG. 3 shows the reduction of crystallite size as milling progresses. It is apparent that the addition of higher percentages of SiC further refined the structure of the mixture.

After the powders were processed using mechanical alloying, they were subjected to consolidation via spark plasma sintering (SPS) so that the final aluminum alloy product could be obtained. The use of SPS proved to be an excellent choice as the consolidated materials didn't lose their developed nanostructure due to the heat associated with consolidation, and this is primarily due to the fact that SPS induces massive heating in a very short period of time, which doesn't allow for grain growth to take place. Consolidation of the powder was carried out using three different temperatures of 400° C., 450° C., and 500° C. respectively in order to ascertain the optimum processing conditions. FIG. 4 shows the variation of densification for SiC/Al-7Si-0.3Mg alloy against the SPS sintering temperature, and FIG. 5 shows the hardness against the SPS sintering temperatures and silicon carbide concentration. The SiC/Al-7Si-0.3Mg alloy has an average hardness as measured by Vickers indenters of not less than 40. Typically, the hardness is in the range of 40 to 70. The densification and the hardness parameters increase with a concomitant increase in the sintering temperature.

The process for making the nanocomposite resulted in the best alloy after about 20 hours of milling and sintering at about 500° C. The milling process of the invention allows for attaining a crystallite size below 100 nm. The best performing alloy was found to be the alloy containing 20% of nano-SiC consolidated at 500° C. The improvement in properties was due to the retention of the nanostructure, as the use of SPS proved to be useful in this regard. The above results were obtained with the Al-7Si-0.3 Mg aluminum alloy. Results obtained for the Al-12Si-0.3Mg aluminum alloy exhibited a more or less similar trend.

Additionally, the method unexpectedly achieves nano-structures of the reinforced aluminum SiC alloys. Generally, bringing materials in the nano-structure regime introduces several improvements into the alloys properties such as improved hardness and strength. The nanostructure of the alloy is reached during the mechanical alloying stage of the milling process, whereas the spark plasma sintering preserves the nano-structures. Most conventional processes that utilize normal consolidation procedures fail to retain the nano-crystalline structure till the end when the final product is reached, while in the present method results in a reinforced alloy material retaining the nano-crystalline structure.

The high performance nanocomposite alloys that are based on aluminum and reinforced with nano-SiC particulates can be useful especially in automotive industry because they provide increased performance in aluminum-based alloys as commonly used in cylinder blocks, cylinder heads and pistons etc. The above method can also be used to scale up production with high quality products, if adequate precautions are being considered.

It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.

Claims

1. A high performance aluminum nanocomposite, comprising an aluminum alloy reinforced with uniformly distributed silicon carbide particles, wherein the silicon carbide particles have a diameter between about 20 and 40 nm.

2. The high performance aluminum nanocomposite of claim 1, wherein the aluminum alloy comprises Al-7Si-0.3Mg, having a composition of about 92.7% by weight of Al, 7% by weight of Si and 0.3% by weight of Mg.

3. The high performance aluminum nanocomposite of claim 1, wherein the aluminum alloy comprises Al-12Si-0.3Mg, having a composition of about 87.7% by weight of Al, 12% by weight of Si and 0.3% by weight of Mg.

4. The high performance aluminum nanocomposite of claim 1, wherein the nanocomposite is nano-crystalline in structure, having a crystalline size of up to 100 nm.

5. The high performance aluminum nanocomposite of claim 1, wherein the nanocomposite has an average hardness as measured by Vickers indenters of not less than 40.

6. The high performance aluminum nanocomposite of claim 1, wherein the silicon carbide nanoparticles comprise between 5 wt % and 20 wt % of the nanocomposite, the balance being the aluminum alloy.

7. A method of making a high performance aluminum nanocomposite, comprising the steps of:

(a) mixing an aluminum-based metal alloy with silicon carbide nanoparticles by mechanical alloying in a ball mill to form a uniformly distributed powder having a nano-crystalline structure;

(b) sintering the powder to consolidate the powder to obtain a reinforced aluminum alloy nanocomposite, whereby the mechanically alloyed powder retains the nano-crystalline structure after sintering.

8. The method of making a high performance aluminum nanocomposite according to claim 7, wherein step (a) comprises adding to the aluminum alloy between 5% and 20% silicon carbide nanoparticles by weight and milling the mixture at 200 rpm for between 5 hours and 20 hours.

9. The method of making a high performance aluminum nanocomposite according to claim 7, wherein the sintering step is carried out at a temperature of between 400° C. and 500° C.

10. The method of making a high performance aluminum nanocomposite according to claim 7, wherein step (a) comprises adding the aluminum alloy in powder form with an average particle size of 40 microns.

11. The method of making a high performance aluminum nanocomposite according to claim 7, wherein the ball mill comprises a planetary ball mill.

12. The method of making a high performance aluminum nanocomposite according to claim 7, wherein the aluminum-based metal alloy comprises 40 micron particles of an alloy of aluminum, silicon, and magnesium

13. The method of making a high performance aluminum nanocomposite according to claim 7, wherein the aluminum-based metal alloy comprises Al-7Si-0.3Mg.

14. The method of making a high performance aluminum nanocomposite according to claim 7, wherein the aluminum-based metal alloy comprises Al-12Si-0.3Mg.

15. A method of making a high performance aluminum nanocomposite, comprising the steps of:

milling a mixture of particles of an Al—Si—Mg alloy having an average particle diameter of 40 μm and particles of silicon carbide having a diameter of between 20 nm and 40 nm in a planetary ball mill for about 20 hours in order to form a reinforced aluminum nanocomposite; and

sintering the reinforced aluminum nanocomposite at a temperature of about 500° C. in order to consolidate the reinforced aluminum nanocomposite while retaining the reinforced aluminum nanocomposite in a nano-crystalline structure.

16. The method of making a high performance aluminum nanocomposite according to claim 15, wherein the silicon carbide particles comprise between 5 wt % and 20 wt % of the mixture.

17. The method of making a high performance aluminum nanocomposite according to claim 15, wherein the Al—Si—Mg alloy comprises Al-7Si-0.3Mg.

18. The method of making a high performance aluminum nanocomposite according to claim 15, wherein the Al—Si—Mg alloy comprises Al-12Si-0.3Mg.