Incorporation of nano-size particles into aluminum or other light metals by decoration of micron size particles

Abstract

Powder metallurgy technology is used to form metallic composites with a uniform distribution of nano-meter size particles within the metallic grains. The uniform distribution of the nano-meter particles is achieved by attaching the nano-meter particles to micron sized particles with surface properties capable of attracting the smaller particles, then blending the decorated particles with micron size metal powder. The blended powder is then powder metallurgy processed into billets that are metal-worked to complete the incorporation and uniform distribution of the nano-meter particles into the metallic composite.

Claims

1. A process for manufacturing a metal matrix composite consisting of: placing nano-meter size particles in a polar fluid; deagglomerating the nano-meter size particles in the polar fluid; combining the deagglomerated nano-meter size particles with micron size particles in the polar fluid; high-shear blending the combined nano-meter and micro size particles; evaporating the polar fluid allowing the nano-meter size particles to electrostatically attach to the micron size particles forming a powder of the micron size particles with the nano-meter size particles attached thereto.

2. The process of claim 1 wherein the metal matrix is aluminum.

3. The process of claim 2 wherein the nano-meter size particles are alumina.

4. The process of claim 3 wherein the micron size particles are alumina.

5. The process of claim 3 wherein the micron size particles are aluminum.

6. The process of claim 3 wherein the micron size particles are an aluminum alloy.

7. The process of claim 1 wherein the metal matrix is an aluminum alloy.

8. The process of claim 7 wherein the nano-meter size particles are alumina.

9. The process of claim 8 wherein the micron size particles are alumina.

10. The process of claim 8 wherein the micron size particles are aluminum.

11. The process of claim 8 wherein the micron size particles are an aluminum alloy.

12. The process of claim 1 further comprising processing the powder of micron size particles with nano-meter size particles attached thereto into a billet and then metal working the billet to redistribute the nano-meter size particles within the metal matrix.

13. The process of claim 12 wherein the powder of micron size particles with nano-meter size particles attached thereto is processed into a billet by spark plasma sintering.

14. The process of claim 12 wherein the powder of micron size particles with nano-meter size particles attached thereto is processed into a billet by cold isostatic pressing followed by sintering.

15. The process of claim 12 wherein the powder of micron size particles with nano-meter size particles attached thereto is processed into a billet by vacuum hot pressing.

16. A process for manufacturing a metal matrix composite comprising: providing nano-meter size particles; attaching the nano-meter size particles to micron size particles; combining the micron size particles with nano-meter size particles attached thereto into a metal matrix; wherein the nano-meter size particles are produced in a plasma jet and wherein the micron size particles are introduced into the plasma stream such that the nano-meter size particles electrostatically attach to the micron size particles.

17. The process of claim 16 wherein the metal matrix is aluminum.

18. The process of claim 17 wherein the nano-meter size particles are alumina.

19. The process of claim 18 wherein the micron size particles are alumina.

20. The process of claim 18 wherein the micron size particles are aluminum.

21. The process of claim 18 wherein the micron size particles are an aluminum alloy.

22. The process of claim 16 wherein the metal matrix is an aluminum alloy.

23. The process of claim 22 wherein the nano-meter size particles are alumina.

24. The process of claim 23 wherein the micron size particles are alumina.

25. The process of claim 23 wherein the micron size particles are aluminum.

26. The process of claim 23 wherein the micron size particles are an aluminum alloy.

27. The process of claim 16 further comprising processing the powder of micron size particles with nano-meter size particles attached thereto into a billet and then metal working the billet to redistribute the nano-meter size particles within the metal matrix.

28. The process of claim 27 wherein the powder of micron size particles with nano-meter size particles attached thereto is processed into a billet by spark plasma sintering.

29. The process of claim 27 wherein the powder of micron size particles with nano-meter size particles attached thereto is processed into a billet by cold isostatic pressing followed by sintering.

30. The process of claim 27 wherein the powder of micron size particles with nano-meter size particles attached thereto is processed into a billet by vacuum hot pressing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of a plasma chamber in which nano-meter size particles are formed and attached to micron size particles.

(2) FIG. 2 is a plot of the low strain region of stress-strain curves for standard GA-2-10 composite and of a GA-2-10 composite containing nano-meter size particles formed in accordance with the present invention.

(3) FIG. 3 is a plot of stress-strain curves for standard GA-2-10 composite and of a GA-2-10 composite containing nano-meter size particles formed in accordance with the present invention.

(4) FIGS. 4A-4D are Scanning Electron Microscope (SEM) micrographs of material at different stages of manufacturing of nano-meter size alumina reinforced aluminum composites: aluminum particle (FIG. 4A), aluminum particle decorated with nano-meter size alumina particles (FIG. 4B), decorated particles consolidated into a solid (FIG. 4C) and consolidated solid extruded into rod with alumina particles uniformly distributed (FIG. 4D).

(5) FIG. 5A is a SEM micrograph of the tensile fracture surface of 6063 aluminum.

(6) FIG. 5B is a SEM micrograph of the tensile fracture of a 6063 matrix composite containing 5% nano-meter size alumina particles.

DETAILED DESCRIPTION OF THE INVENTION

(7) In the following description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods and devices are omitted so as to not obscure the description of the present invention with unnecessary detail.

(8) One technique for making nano-meter size particles of alumina is to pass micron size particles of alumina through a plasma, vaporize the alumina while the particles are in the plasma hot zone and condense the nano-micron size particles when the vaporized alumina emerges from the hot zone. A representation of this process is shown in FIG. 1, where the particles to be vaporized are introduced into the chamber at the top with Gas 1. The nano-meter sized particles of alumina emerge from the bottom of the plasma and are collected on a lower plate. Proceeding in this way involves using some very costly processes for handling a large population of nano-meter sized particles directly as one attempts to incorporate the nano-meter size alumina particles into an aluminum alloy matrix to generate the composite.

(9) The present invention may use spherical alumina produced in accordance with U.S. Pat. Nos. 8,057,203 and 8,343,394, the disclosures of which are incorporated herein by reference, as a carrier to introduce the nano-meter size alumina particles into an aluminum alloy or other light metal alloy. The nano-meter particles are attached to micron size alumina particles directly after the manufacture of the nano-meter alumina particles. Once the nano-meter alumina particles are attached to micron size alumina or aluminum metal particles, one can use conventional powder metallurgy techniques to introduce the micron size particles, with attached nano-meter alumina particles into an aluminum metal matrix to create the required composite

(10) Another process within the scope of this invention involves adding a secondary process to the plasma generation of nano-meter sized particles. Referring again to FIG. 1, spherical micron sized alumina particles are introduced into the chamber with Gas 2 below the plasma. This is a cooler zone where the nano-meter sized particles are being formed by condensation of the vaporized alumina. The nano-meter sized particles form and are electrostatically attached to the micron size spherical particles, resulting in layers of small particles being attached to the larger particles. The decorated spherical particles then fall to the collection plate and are removed from the chamber at the end of the run.

(11) Other techniques that allow nano-meter size particles to be attached or attracted to micron size particles are also within the scope of this invention. The micron size particles can be irregular alumina particles or aluminum particles that have an alumina shell. It is known that oxides exist on atomized aluminum powder regardless of the type of atomization gas used to manufacture. See, Metals Handbook Ninth Edition Volume 7Powder Metallurgy by Alcoa Labs. The naturally occurring aluminum oxide assists in allowing the additional deliberately added nano-size alumina particles to attach to the aluminum particles. One must also provide an environment where the added nano alumina particles are in a lower energy state when they adhere to the aluminum micron size particles than when they agglomerate together electrostatically.

(12) Among the other techniques for decorating micron size particles with nano-meter size particles is the high shear blending of nano-meter-size particles with micron size particles. The shear action of the blender breaks up agglomerates of the nano-meter size particles and allows individual particles to interact and attach to micron size particles. The static electricity generated by the shear motion of the particles keeps the nano-size particles attached to the micron size particles.

(13) Once the nano-meter size particles are attached to the micron size particles, a composite is made by blending the decorated micron size particles with additional aluminum powders and processing the blended powders into a billet using a standard powder metallurgy process. At the billet stage, the nano-meter size particles are still associated with the micron size particles. The billet must be metal worked to allow shear deformation to redistribute the nano-meter size particles throughout the matrix. An extrusion process is a common metal working operations used for this purpose.

(14) Nano alumina particles produced by either condensation of particles from a plasma or produced by thermal decomposition of organo-metallic compounds can be deagglomerated or separated by being placed in a polar fluid at room temperature and exposed to high shear mixing such as produced by Ross Series 700 Ultra-High Shear Mixers. One such polar fluid is isopropyl alcohol that is compatible with both the nano alumina and the aluminum powder. The free nano alumina particles are then attached to aluminum alloy micron sized powders by combining the two types of powder in at room temperature in a vessel filled with a polar fluid such as isopropyl alcohol. The combined mixture is blended with a V blender using an intensifier bar for 20 minutes. The polar fluid is evaporated from the final blend and the nano alumina particles are found to be attached to the aluminum powder by static electricity. The powder is then processed into billets. The billets are then metal worked to incorporate and scatter the nano alumina particles within the matrix alloy.

(15) In a first example, the process described herein was used to make a composite with GA-2 matrix alloy with 10 volume percent of the decorated micron size spherical alumina particles, GA-2-10D. The nano-meter size particles made up an estimated 3 to 5percent of the total alumina added. Therefore, the composite that was made contained between 0.3 and 0.5 percent by volume of the nm-size particles. The billet size was 25 mm diameter by 13 mm long. The billet was made by heating the powder with the passage of electric current through the powder and applying a pressure of approximately 9 bars once the powder reached the desired temperature, 480 C. to 510 C. The process was done in a vacuum. The process is referred to as spark plasma sintering (SPS). The billet then had a 12.5 mm diameter extrusion plug machined from the center. This plug was warm extruded into a 5.6 mm diameter rod. This is an extrusion area ratio of 5.16:1. This is a low extrusion ratio but will convert the powder metallurgy billet into a wrought rod, and incorporate the nano-meter size particles into the matrix grains.

(16) In order to assess the success of incorporating the nano meter oxide particles into the aluminum, we ran tensile tests at 300 degree C. on nano-meter particle containing composite and standard GA-2-10 composites made by the SPS process. At this temperature all the strengthening due to heat treatment induced precipitates in the GA-2 alloy will be eliminated due to over-aging of the precipitates. The strengthening brought about by the nano-meter size particles will remain. Identical tests were also conducted on standard GA-2-10 composite samples that were powder metallurgy processed by cold isostatic pressing (CIP) followed by sintering and then extruded from a 89 mm billet to a 15.9 mm rod, an extrusion area ratio of 36:1. Stress-strain curves generated by each of the three types of metals are shown in FIGS. 2 and 3. FIG. 2 is the stress strain behavior of the materials at low strain, up to 2 percent. FIG. 3 is the stress strain behavior of the materials up to ten percent strain. Both of these figures show that the standard SPS material and the CIP/Sinter materials behave in a similar manner. These materials have a proportional limit at about 60 MPa. Above this stress the standard materials yield and the stress increases by work hardening to a yield stress of around 80 MPa. The material that contains nano-meter particles has a stress-strain curve that is similar to the other materials up to the 60 MPa proportional limit. Above 60 MPa, the nano-meter containing material work hardens at a higher rate to a yield stress of about 120 MPa. At 80 MPa for the standard composite and 120 MPa for the nano-meter containing material the samples undergo creep deformation until failure occurs at about 10 percent elongation as shown in FIG. 3.

(17) In a second example, the process described herein was used to make a composite with a 6063 Aluminum matrix, 0.7 Mg, 0.4 Si. This matrix alloy was mixed with isopropyl alcohol, then sufficient nano alumina particles that have been commercially processed into a colloidal suspension are added to the aluminum-alcohol blend. The combined blends are processed in a blender at high speed for 3 minutes and the blend is dried. The composites were made containing 1.5%, 5% and 10% nano meter size particles respectively. These composites were processed by the SPS technique described earlier. The billet 25 mm diameter by 13 mm thick then had a 12.5 mm diameter extrusion plug machined from the center. This plug was warm extruded into a 5.6 mm diameter rod. This is an extrusion area ratio of 5.16:1. This is a low extrusion ratio but will convert the powder metallurgy billet into a wrought rod, and incorporate the nano-meter size particles into the matrix grains as we have shown earlier in the GA2-20 metal tested at 300 C. The microstructure of the aluminum composite at different stages of the processing is contained in FIG. 4. FIG. 4 is a series of Scanning Electron Microscope, SEM, micrographs of material at different stages of manufacturing of nano-meter size alumina reinforced aluminum composites. FIG. 4A is an aluminum particle. FIG. 4B is an aluminum particle decorated with nano-meter size alumina particles. FIG. 4C is a consolidated solid with decorated particles forming rings around prior particle boundaries. FIG. 4D is the consolidated solid extruded into rod with alumina particles uniformly distributed.

(18) Tensile samples were machined from the extruded rods and the machined tensile samples were annealed at 480 C. for 2 hours followed by furnace cooling to 120 C in order to remove any residual work hardening and hardening precipitates from the warm extrusion. Room temperature tensile tests were conducted in these composites. Room temperature elastic moduli were measured by ultrasonic velocity measurements. The test data is contained in Table 1. These data demonstrate the increase in elastic modulus and strength brought about by the addition of the nano particles. The strength increase is more significant than the modulus, as expected for the small amount of reinforcement addition.

(19) TABLE-US-00001 Elastic Yield Ultimate Modulus Strength Strength Elongation Material Description (GPa) (MPa) (MPa) (%) 6063 Aluminum- 69.0 89.6 152 33.0 Metals Handbook 6063/1.5% nano 70.7 188.2 218 24.0 6063/5% nano 77.2 221 269 26.0 6063/10% nano 81.4 245 303 18.3

(20) Scanning electron microscopy was carried out on the fracture surfaces of the tensile samples. As seen in FIG. 5A, the fracture surface of the baseline 6063 contains highly deformed ligaments of aluminum. FIG. 5B shows that the fracture surface of the nano-meter particle-containing composite also contains highly deformed ligaments. These ligaments were more segmented and contained many particles less than 100 nano-meter in diameter. These particles are the nano-size particles added during the processing according to this patent.

(21) In a third example, the process described herein may be used to make a composite by using the CIP/Sinter process. Nano-meter decorated micron size particles are blended with an aluminum alloy powder with a total alumina content of 20 volume percent, aluminum alloy content of 80 volume percent. The blended powder is placed in a rubber mold and the powder is compacted to approximately 50 percent theoretical density. The rubber mold is sealed and evacuated by a vacuum pump to approximately 1 Torr. The sealed and evacuated rubber mold is placed in a cold isostatic chamber, a large pressure vessel, and a pressure of approximately 50,000 to 80,000 psi is applied within the pressure vessel. The pressure is applied for several minutes and then removed. This process produces a powder compact that is between 85 and 95 percent of theoretical density. This is necessary so the compacted powder can be outgassed during the sinter operation.

(22) The compacted mixture is then sintered in vacuum, or inert-gas atmosphere. The compacted powder is heated to a sintering temperature that is the highest eutectic melt temperature of the compacted mixture so that sintering of the matrix takes place to form the composite billet. This sintered composite billet has a density that is still approximately that of the starting compacted mixture, between 85% and 95% of the theoretical density, but is sealed by the transient eutectics that are present during the sintering process.

(23) The billet is then heated to approximately 425 C. and then extruded. The extrusion may be a rod or other shape with a ratio of area of the billet divided by the area of the shape of greater than 10 to 1, preferably greater than 20 to 1. After the extrusion process the nm size particles are contained within the aluminum matrix grains.

(24) A fourth method for producing composites is by vacuum hot pressing. Blended powder is placed in a steel die. The steel die can be any desired size and can contain several kilograms of the blended powder. The powder is typically compacted at room temperature to a theoretical density of between 60 and 80 percent of theoretical. The die, powder and punch assembly are placed in a vacuum container and a vacuum of approximately 1 Torr is established. The vacuum container and die assembly are heated to a consolidation temperature, typically between 450 C. and 565 C. Once the temperature of the blended powder is uniform, a pressure is applied to the punch assembly and the composite is consolidated to a density of greater than 95 percent theoretical. The billet is then metal worked to liberate the nm size particles from the surface of the micron size particles and the nm particles are incorporated into the matrix alloy grains.

(25) It will be recognized that the above-described invention may be embodied in other specific forms without departing from the spirit or essential characteristics of the disclosure. Thus, it is understood that the invention is not to be limited by the foregoing illustrative details, but rather is to be defined by the appended claims. In particular, but without limitation, particular embodiments of the invention have been described in the context of aluminum and aluminum alloys. It is to be understood that the invention may also be applied to other metals and alloys.