ATOMIZED PICOSCALE COMPOSITION ALUMINUM ALLOY AND METHOD THEREOF

Abstract

The invention is a process for manufacturing a nano aluminum/alumina metal matrix composite and composition produced therefrom. The process is characterized by providing an aluminum powder having a natural oxide formation layer and an aluminum oxide content between about 0.1 and about 4.5 wt. % and a specific surface area of from about 0.3 and about 5 m.sup.2/g, hot working the aluminum powder, and forming a superfine grained matrix aluminum alloy. Simultaneously there is formed in situ a substantially uniform distribution of nano particles of alumina. The alloy has a substantially linear property/temperature profile, such that physical properties such as strength are substantially maintained even at temperatures of 250° C. and above.

Claims

1. A process for manufacturing a nano aluminum/alumina metal matrix composite, comprising the steps of: a) providing a powder blend of an aluminum powder and boron carbide, the aluminum powder having a natural oxide formation layer and an aluminum oxide content between 0.1 and 4.5 wt. % and a specific surface area of between 0.3 m.sup.2/g and 5.0 m.sup.2/g; b) hot working the powder blend, and forming thereby a superfine grained matrix aluminum alloy; and c) simultaneously forming in situ a substantially uniform distribution of nano particles of alumina throughout said alloy by redistributing said aluminum oxide; wherein said alloy has a substantially linear property/temperature profile; and wherein the aluminum powder has a particle size of less than 30 μm.

2. The process as claimed in claim 1, wherein said superfine matrix aluminum alloy has a transversal grain size of 200 nm.

3. The process as claimed in claim 1, wherein the step of hot working is carried out at a temperature less than the melting point of said alloy.

4. The process as claimed in claim 1, wherein the aluminum powder is atomized with a natural occurring oxide layer thickness of between 3-7 nm regardless of the type of atomization gas and alloy type during the powder atomization manufacturing process.

5. The process as claimed in claim 1, wherein the process utilizes atomized aluminum powder with a particle size distribution (PSD) of 100% powder less than 30 μm and a d50 between 1 and 20 μm regardless of the atomization gas or alloy composition used in forming said powder.

6. The process as claimed in claim 1, wherein said process is free of mechanical alloying.

7. A powder blend comprising: aluminum powder; and boron carbide; wherein the aluminum powder has a particle size distribution of 100% less than 30 μm and with a d50 between 1 and 20 μm and an oxide content between 0.1% and 4.5% and a specific surface area of between 0.3 m.sup.2/g and 5.0 m.sup.2/g.

8. The powder blend according to claim 7, wherein the powder blend may further comprise a ceramic particulate selected from silicon carbide, titanium oxide, titanium dioxide, titanium boride, titanium diboride, silicon, silicon oxide, and silicon dioxide.

9. A nano composite alloy comprising: aluminum matrix; and boron carbide; wherein the aluminum matrix comprises aluminum having a transversal grain size of 200 nm and evenly distributed nanoscale aluminum oxide particles having a particle size thickness of 3 nm to 7 nm.

10. The nano composite according to claim 9, wherein the nano composite alloy further comprises a ceramic particulate selected from silicon carbide, titanium oxide, titanium dioxide, titanium boride, titanium diboride, silicon, silicon oxide, and silicon dioxide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] For a fuller understanding of the invention, the following detailed description of various embodiments should be read in conjunction with the drawings, wherein:

[0029] FIG. 1 is a prior art graph of oxide thickness vs. type of atomization gas from “Metals Hand Book Ninth Edition Volume 7—Powder Metallurgy”;

[0030] FIGS. 2(a), 2(b) and 2(c) are TEM photomicrographs relating to the effect of 1 μm, 10 μm and <400 μm powder size, respectively, on microstructure (extruded@350° C. billet temperature”, R=11:1);

[0031] FIG. 3 is a TEM photomicrograph relating to the induced work effect to homogenize distribution of fine distorted oxides;

[0032] FIG. 4 is a graph of the bad correlation between d50 and specific surface area;

[0033] FIG. 5 is a graph of the correlation between mechanical properties and specific surface area;

[0034] FIGS. 6(a) and 6(b) are a table and graph, respectively, of the correlation between mechanical properties and specific surface area;

[0035] FIG. 7 is a graph of a typical particle size distribution of a HTA atomized aluminum powder;

[0036] FIG. 8 is a SEM photograph of a HTA atomized aluminum powder;

[0037] FIG. 9 is a TEM photograph of compacted (CIP) HTA atomized aluminum powder;

[0038] FIG. 10 is a graph of the linear property/temperature profile; and

[0039] FIGS. 11(a) and 11(b) are TEM photomicrographs relating to the importance of the extrusion temperature.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0040] In carrying out the invention, the first step is selection of aluminum powder size. The present invention focuses on the particle size distribution (PSD) of the atomized aluminum powder which is not used for conventional powder metal technology. In fact the trend in aluminum P/M industry is to use coarser fractions of the PSD—typical in the d50 size of 50 .mu.m-400 μm range because of atomization productivity, recovery, lower cost, superior die fill or uniform pack density and the desire to have low oxide powder. Most commercial applications seek to reduce the oxide content especially in the press and sinter near-net-shape aluminum P/M parts for automotive and other high volume applications. Manufacturers of powder and end-users want the lower oxide aluminum powder since it is extremely difficult to perform liquid phase sintering and obtain a metallurgical particle to particle bond which is necessary to obtain theoretical densities and high mechanical properties with acceptable ductility values with oxide on the powder grain boundaries. The prior grain boundary oxide network results in low fracture toughness, low strength, and marginal ductility. Efforts have been made to reduce the alumina oxide but this oxide coating on the aluminum powder is extremely stable in all environments and is not soluble in any solvent. This fact leads the press and sinter near-net-shape industry and the high performance aerospace industry aluminum PM industry to purchase low oxide powder material.

[0041] In total contrast to the above noted industry criteria, the present invention employs superfine aluminum powder PSD (by industrial definition a PSD <30 μm) which results in alumina oxide content in the 0.1-4.5 w/o range, which is the oppose side of the spectrum.

[0042] The invention includes taking the superfine powder and hot working the material below the recrystallization temperature of the alloy which further reduces the transverse grain size by a factor of 10 to a typical grain size of e.g. about 200 nm. The effect of the starting powder particle size is illustrated in FIG. 2 which shows the effect of 1 μm, 10 μm, and <400 μm aluminum powder extruded at 350° C. The hot work operation evenly distributes nanoscale alumina oxide particles (the former 3-7 nm oxide skin of the aluminum powder) uniformly throughout the microstructure as illustrated in FIG. 3 and circled in the micrograph. This ultra fine grain size and the nanoscale alumina particles combination results in a dual strengthening mechanism. The nanoscale alumina oxide particles pin the grain boundaries and inhibit grain growth to maintain the elevated mechanical property improvement of the composite matrix material. In certain embodiments, the oxide is redistributed into uniformly dispersed nano alumina particles intermixed with inter-metallic compounds. In certain embodiments, the inter-metallic compounds have a particle size of from about 2 to about 3 μm.

[0043] It has been found that increasing the alumina oxide content of one specific type of powder by 50% does not result in higher mechanical properties compared to the original powder. Increasing the oxide content by 100% or more may result in problems during consolidation process. During powder treatment to increase the alumina oxide content only the thickness of the oxide layer can be increased which results in bigger dispersoids in the matrix after hot working.

[0044] To increase the strengthening mechanism of grain boundary pinning, which is the designated positioning of nano-scaled dispersoids (alumina particles, the former oxide layer of the starting powder) along the grain boundaries of the microstructure, it is desirable to bring more fine particles into the structure. This can be realized by using a finer starting powder, or a powder with a higher specific surface area.

[0045] By considering the particle size distribution together with the specific surface area of the starting powders, the mechanical properties of the hot worked material can be predicted. Powders with a higher specific surface will generally result in better mechanical properties compared to powders with a lower specific surface area. As can be seen in FIG. 4 powder sample #9 has roughly the same specific surface area as powder sample #5, although the PSD of sample #9 is much coarser than the PSD of sample #5. The mechanical properties correlate with the specific surface area, not with the PSD of the powders (FIG. 5). This figure shows UTS vs particle size distribution and specific surface area (test results of mechanical properties obtained on test specimen containing 9% of boron carbide particulate). Mechanical properties (UTS) correlate with BET not with the d50.

EXAMPLES

[0046] Different powders with specific surface areas in the range between 0.3-5.0 m.sup.2/g were hot worked by extrusion at 400° C. into rods with a diameter of 6 mm which had been used for the production of tests specimen for tensile tests. The results are shown in the table and chart of FIGS. 6(a) and 6(b), respectively. This demonstrates that the finer the particle distribution (the higher the surface area) the better the mechanical properties. Powders were produced via gas atomization using confined nozzle systems and classified to required PSD via air classification. Afterwards, compacts were produced, by extrusion@400° C., R 11:1. High temperature tensile tests were made after 30 min. soak time@testing temperature.

[0047] An example of the aluminum particle size used for the development is illustrated in FIG. 7. This graph illustrates PSD and as can be seen, the d50 is about 1.27 μtm with d90 about 2.27 μm, which is extremely fine. Attached is a Scanning Electron Microscope (SEM) photograph (FIG. 8) “Picture of ultra fine atomized Al powder D50-1.2 μm ” and Transmission Electron Micrograph (TEM). See FIG. 9, “Picture of ultra fine atomized Al powder D50-1.3 .mu.m” which illustrates the spherical shape of the powder. As shown therein, the hum marker (SEM) respectively the 0.2 μm marker (TEM) is a reference to verify the particle size of the powder. Since the aluminum powder in the particle size range is considered spherical it is easier to mathematically model and predict the oxide content. When modeling the oxide thickness and comparing the actual value of the oxide by dissolving the matrix alloy, there is good correlation that documents the targeted aluminum oxide content of the invention. Another characteristic of the powder is the very high surface area of the resulting PSD and the oxygen content as an indicator of the total oxide content of the starting raw material. The purchase specification to assure superior performance shall include the alloy chemistry, particle size distribution, surface area, and oxygen content requirements.

[0048] FIG. 10 illustrates the unique linear property/temperature profile of the high temperature nano composite aluminum alloy of the invention. The figure shows UTS (Rm) vs. temperature, 1.27 μm (d50) powder grade, consolidated via direct extrusion@350° C., R=11:1, 30 min. soak time at testing temperature before testing.

[0049] The typical processing route to manufacture the material for this invention is to fill the elastomeric bag with the preferred particle size aluminum powder, place the elastomeric top closure in the mold bag, evacuate the elastomeric mold assembly to remove a air and seal the air tube, cold isostatic press (CIP) using between 25-60,000 psi pressure, dwell for 45 seconds minimum time at pressure, and depressurize the CIP unit back to atmospheric pressure. The elastomeric mold assembly is then removed from the “green” consolidated billet. The billet can be vacuum sintered to remove both the free water and chemically bonded water/moisture which is associated with the oxide surfaces on the atomized aluminum powder. Care must be taken not to overheat the billet or approach the liquid phase sintering temperature in order to prevent grain growth and obtain optimum mechanical properties. The last operation is to hot work the billet to obtain full density, achieve particle to particle bond, and most importantly disperse the nano alumina particles uniformly throughout the microstructure.

[0050] A preferred hot work method is to use conventional extrusion technology to obtain the full density, uniformly dispersed nano particle aluminum/alumina oxide composite microstructure. Direct forging or direct powder compact rolling technology could also be used as a method to remove the oxide from the powder and uniformly disperse the alumina oxide throughout the aluminum metal matrix. It is highly preferred to keep the extrusion temperature below the re-crystallization temperature of the alloy in order to obtain the optimum structure and optimum mechanical properties. FIGS. 11(a) and 11(b) are SEM photo micrographs which illustrate the importance of the extrusion temperature in order to increase the flow stress to mechanically work the material to obtain the desired microstructure. In photo micrograph FIG. 11(a) are visible the uniformly dispersed nano-alumina oxide particles in the newly formed grains. The nano particle alumina oxide particles are visible even inside the grain and at the grain boundaries which typically is done through the mechanical alloying process methods. The second photo micrograph FIG. 11(b) shows the larger grain size and the structure does not exhibit the same degree of work or the nano particles inside the grains.

[0051] To further demonstrate the significance of extrusion temperature in obtaining the desired microstructure for optimum mechanical properties, outlined below are typical mechanical properties of the nano aluminum/alumina composite material at various extrusion temperatures on tensile data at room temperature and 350° C. test temperatures.

TABLE-US-00001 Room Temperature Various Billet Extrusion Temperatures Mechanical Properties 350° C. 400° C. 450° C. 500° C. UTS - Mpa/(KSI) 310 (44.95) 305 (44.25) 290 (42.05) 280 (40.60) Yield - Mpa/(psi) 247 (35.82) 238 (34.51) 227 (32.91) 213 (30.88) Elongation % 9.0% 10.0% 10.0% 10.9% 1100 Aluminum/UTS 124 (18.00) N/A N/A N/A

TABLE-US-00002 350° C. Test Temperature Mechanical Various Billet Extrusion Temperatures Properties 350° C. 400° C. 450° C. 500° C. UTS-Mpa/ 186 (26.97) 160 (23.20) 169 (24.50) 160 (23.20) (KSI) Yield-Mpa/ 156 (22.62) 145 (21.00) 150 (21.75) 150 (21.75) (KSI) Elongation 10.7% 10.4% 9.5% 10.0%

[0052] These are excellent mechanical properties for a 4.5% nano alumina particle reinforced 1100 series superfine grained aluminum material compared with conventional ingot metallurgy 1100 series aluminum technology. Further, these results demonstrate the advantages of the superfine grained microstructure in combination with the small amount of nano particle aluminum/alumina materials compared to various conventional alloys and the concept of adding other ceramic particulate or rapid solidification of super saturated alloy elements in the aluminum matrix.

[0053] As mentioned above, one of aspects of this invention is to add a ceramic particulate to the nano aluminum/alumina composite matrix. One of the driving forces to the development of this new technology was the need for a high temperature matrix material to add boron carbide particle to expand the field of application of U.S. Pat. No. 5,965,829. It was a goal to develop a high temperature aluminum boron carbide metal matrix composition material suitable to receive structural credit from the US Nuclear Regulatory Commission for use as a basket design for dry storage of spent nuclear fuel applications. With elevated temperature mechanical properties of the aluminum boron carbide composite, designers can take advantage of the light weight/high thermal heat capacity of aluminum metal matrix composites compared to the industry standard stainless steel basket designs. In Europe, designers typically use boronated stainless steel but the areal density is low, the upper limit for the B10 isotope being 1.6% content, alloy density is high, and the thermal conductivity and thermal heat capacity is low compared to aluminum based composites. The aluminum-based composites of the present invention do not suffer from these shortcomings.

[0054] Another driving force behind the development of an aluminum boron carbide metal matrix higher temperature composite, in addition to the market need for such a material, was the experience with extruding up to 33 wt % boron carbide composite materials in a production environment, including the techniques described in U.S. Pat. No. 6,042,779 entitled “Extrusion Fabrication Process for Discontinuous Carbide Particulate Metal Matrix Composites and Super Hypereutectic Al/Si Alloys,” issued on Mar. 28, 2000 (the '779 patent) and which is hereby incorporated by reference in its entirety. This extrusion technology could allow designers the freedom of design to extrude to net-shape a variety of hollow tube profiles in order to maximize packing density, add flux traps, and lower manufacturing cost.

[0055] A particular use for the addition of ceramic particulate to the nano particle aluminum/alumina high temperature matrix alloy is the addition of nuclear grade boron carbide particulate. All of the tramp elements for the alloy matrix material such as Fe, Zn, Co, Ni, Cr, etc. are held to the same tight restrictions and the boron carbide particulate is readily available in accordance to ASTM C750 as outlined in the above described U.S. Pat. No. 5,965,829. The boron carbide particulate particle size distribution is similar to that outlined in the '829 patent. An exception to the teaching of the '829 patent is the use of high purity aluminum powder with the new particle size distribution as described above.

[0056] The typical manufacturing route for the composite of the invention is first blending the aluminum powder and boron carbide particulate materials, followed by consolidation into billets using CIP plus vacuum sinter technology as outlined in the above referenced patent. In a preferred embodiment, the extrusion is carried out in accordance with the teaching of U.S. Pat. No. 6,042,779 (the '779 patent), which is hereby incorporated by reference in its entirety. Since this is an elevated temperature aluminum metal matrix composite material it was found necessary to change the temperature of the extrusion die, container temperature, and billet temperature in order to maintain the desired properties. In general it is desirable that the die face pressure be increased by about 25% over previously employed standard metal matrix composite materials. In order to overcome the higher flow stress of the nano particle aluminum/alumina composite matrix alloy, the extrusion press must be sized about 25% larger in order to extrude the material. Extrusion die technology is capable of these higher extrusion pressures without experiencing failure of collapse of the extrusion die.

[0057] An example of the new high temperature nano particle aluminum/alumina plus boron carbide at a 9% boron carbide reinforcement level and the resulting typical mechanical properties and physical properties are outlined below.

TABLE-US-00003 Property 25° C. 100° C. 200° C. 300° C. 350° C. Description (70° F.) (212° F.) (392° F.) (572° F.) (662° F.) UTS- MPa/KSI 238/34.5 208/30.2 166/24.4 126/18.3 116/16 Yield - Mpa/KSI 194/28.1 164/23.8 150.21.7 126/18.2 105/15 Elongation % 11% 10% 9.0% 8.0% 8.0% Modulus of  83/12.2  81/11.9  73/10.7 63/9.2   55/7.9 Elasticity MPa/MPSI Thermal 184 185 184 183 Conductivity (W/m-K) Thermal 106 107 106 107 Conductivity (BTU/ft-hr- ° F.) Specific Heat 0.993 1.053 1.099 1.121 J/g-° C. Specific Heat 0.237 0.252 0.269 0.280 (BTU/lb-° F.) Notes: Tensile coupons were machined and tested in accordance in ASTM E8 &ASTM E 21 Thermal conductivity tested in accordance to ASTM E 1225 Specific heat tested in accordance to ASTM E 1461