Production of Aluminum matrix composite powder

Abstract: A process for preparing an aluminium matrix composite powder comprising 1 to 40% by weight of ceramic particles uniformly dispersed in a matrix of aluminium or an aluminium alloy, which comprises atomizing a melt comprising the aluminium or aluminium alloy and ceramic particles and solidifying the atomized melt at a solidification rate of at least 10²K/s.

Description: The present invention relates to a process for preparing an aluminium matrix composite powder in which ceramic particles are very uniformly dispersed. Aluminium and aluminium alloys have excellent properties including light weight, high corrosion resistance and high thermal conductivity. Therefore, they have been widely applied to products which are required to have the above properties, such as aircraft, automobiles and other mechanical components. However, aluminium and aluminium alloys have poor properties such as low strength, especially at a temperature of 200°C or more, high coefficient of thermal expansion and low modulus of rigidity. These defects limit the applications of aluminium and aluminium alloys. For improving the defects of aluminium and aluminium alloys, aluminium matrix composites comprising ceramic particles dispersed in matrices of aluminium or aluminium alloys have been developed. As methods for preparing aluminium matrix composite comprising ceramic particles, three methods are known. The first method comprises impregnating molten aluminium or aluminium alloy into a preform formed from the ceramic particles (JP-A-89/306506). Some composites prepared according to the first method are commercialized. In practice, the reason that the ceramic content should be selected to be relatively high (generally 20% by volume or more) for forming the preform limits the application of the first method. The second method comprises mixing the aluminium or aluminium alloy powder with the ceramic particles under dry conditions (JP-A-91/122201). Although the ceramic content can be suitably selected, the second method is not practically applied, because forming a uniform mixture of the aluminium or aluminium alloy powder with the ceramic particles is technically very difficult. The third method comprises dispersing the ceramic particles in molten aluminium or aluminium alloy (JP-A-89/501489). In the third method, the ceramic content can be suitably selected and the dispersion of the ceramic particles in the molten aluminium or aluminium alloy is relatively uniform as compared with the mixture of the second method. However, the third method is not practically applied, because, as shown in the following Comparative Example, alloying elements and the ceramic particles may segregate near grain boundaries and/or may not be uniformly dispersed due to a slower solidification rate. Thus a product resulting from this composite has poor mechanical properties. Advance Materials & Processes, vol 138, November 1990, no.5, pp 71-73 describes the rapid-solidification processing of various alloys using a melt-spun flake method. JP-A-61/99606 describes the atomization of various alloys containing ceramic particles such as Al-Ca or Al-Zr alloys. The present invention seeks to provide a process for preparing an aluminium matrix composite powder in which a suitable amount of ceramic particles is very uniformly dispersed. The present invention also seeks to provide a process for preparing an aluminium matrix composite powder which can provide a product having improved mechanical properties including strength, modulus of elasticity, ductility and wear resistance. The present invention provides a process for preparing an aluminium alloy matrix composite powder comprising 1 to 40% by weight of ceramic particles uniformly dispersed therein, comprising the steps of: preparing a melt of an aluminium alloy with 1 to 40% by weight of ceramic particles uniformly dispersed therein, and atomizing said melt, characterized in that the melt is an aluminium-silicon alloy comprising 1 to 50% by weight silicon and in that the atomized melt is solidified at a solidification rate of at least 10 2 K/s using pressurized air. The term "ceramic particles" herein means not only ceramic in the form of particles, but also ceramic in the form of fibres, flakes or whiskers. The ceramic content in the aluminium matrix composite powder is 1 to 40% by weight. When it is less than 1% by weight, the improvement in mechanical properties of the product is not satisfactory. On the other hand, when it is above 40% by weight, uniform dispersion of the ceramic particles in the matrix cannot be obtained. The ceramic particles usable in the present invention includes oxides such as Al 2 O 3 , SiO 2 and mullite; carbides such as SiC and TiC; nitrides such as Si 3 N 4 ; and borides such as TiB 2 . Ceramic particles having an average particle size of 1 to 40µm are preferable. When the average particle size is less than 1µm, the ceramic particles tend to aggregate mutually and are hardly dispersed uniformly in the matrix. Ceramic particles having an average particle size of above 40µm are also not preferred, because they may act as points from which the occurrence of cracks starts in the product. The matrix in the aluminium matrix composite powder comprises an aluminium-silicon alloy. When a product having heat resistance is desired, one or more of Cu and Mg elements may optionally be added in the matrix. Generally 0.5 to 10% by weight of Cu and/or 0.5 to 10% by weight of Mg are added in the matrix, to improve the strength at a high temperature of up to 150°C. This improvement is considered to be mainly due to a precipitation strengthening or hardening by very fine precipitates. To further improve the heat resistance at a higher temperature, at least one transition metal, including Fe, Ni, Mn, Cr, V, Ti, Mo, Nb, Zr and Y, may be added in the matrix. Generally 0.5 to 15% by weight in total of the transition metals are added in the matrix, to improve the heat resistance at higher temperature above 150°C. This improvement is considered to be mainly due to dispersion strengthening or hardening by intermetallic compounds. The aluminium matrix composite powder is prepared by a rapid solidification method, for example an atomization and a spinning disk atomization. The solidification rate is 10 2 K/s or more, more preferably 10 2 to 10 7 K/s, even more preferably 10 2 to 10 4 K/s. Thus fine primary crystals and fine precipitates are very uniformly dispersed in the matrix. A solidification rate of 10 7 K/sec or more is difficult to achieve in an atomization method. The aluminium matrix composite powder is mainly used for the preparation of consolidated products. Generally, the consolidated product is prepared by subjecting to cold shaping followed by hot working such as a hot extrusion, a hot forging or a hot pressing. Alternatively, the aluminium matrix composite powder can be directly used as a powder for thermal spray coating and an abrasive powder. EXAMPLES The present invention is further described in the following Examples. All percentages referred to are by weight unless otherwise indicated. Example 1 Into a molten aluminium alloy having the composition Al-8Si-2Cu-1Mg, 15% of SiC particles (average particle size = 10µm) were uniformly dispersed with mechanical stirring. The thus prepared melt was subjected to atomization using pressurized air and directly pulverized into an aluminium matrix composite powder. The thus atomized aluminium matrix composite powders contained coarse powders having a particle size of 177 to 350µm and fine powders having a particle size of 44 to 63µm, the average particle size being 35µm. Figs. 1 and 2 are optical microphotographs (x 400) of the resultant atomized composite powders. Figs. 1 and 2 clearly show that the SiC particles were very uniformly dispersed in the matrix of the aluminium alloy. The solidification rate of the melt was estimated to be 10 2 to 10 4 K/s, comparing with the aluminium alloy powder atomized under the same condition. This estimation is supported by Figs. 1 and 2 showing that the precipitates dispersed in the matrix were very fine. After sieving so as to collect the powders having the particles size of 350 µm or less; the atomized composite powders were cold pressed isotropically. A preform (green density=60 to 80 %) was thereby prepared. Then, the preform was heated to 480°C and extruded at an extrusion ratio of 10 so as to obtain an extruded product (theoretical density=100 %). Fig. 3 is an optical microphotograph (x 400) of the resultant extruded product. Fig. 3 clearly shows that the SiC particles were very uniformly dispersed in the matrix of the aluminium alloy. Comparative Example Into a molten aluminium alloy having the composition Al-8Si-2Cu-1Mg, 15 % of SiC particles (average particle size=10 µm) were uniformly dispersed. The thus prepared melt was directly casted. Fig. 4 is the optical microphotograph (x 400) of the resultant casted aluminium matrix composite. Fig. 4 clearly shows that the dispersion of the SiC particles in the matrix was very poor, as compared with that in the atomized composite powder as shown in Figs. 1 to 3. The reason of obtaining the ununiform dispersion is because the solidification rate was slower. The dispersibilities of the extruded product prepared from the atomized composite powders of Example 1 and the casted composite were quantitatively determined. That is, the distance between centers of gravity of closest SiC particles was determined with a picture analyzer "Gazo Hakase" (trade name of Kawasaki Steel Corporation). The determination was conducted on three fields of view, each view being 180 x 230 µm. Each view was selected so that the number of the SiC particles observed is as constant as possible. The result is shown in Table 1. distance between centers of gravity of closest particles (µm) average number of observed particles per field of view 1 view 2 view 3 view average invention 5.72 5.55 5.94 5.74 156 control 3.70 4.17 3.78 3.88 161 The distance between centers of gravity of closest particles in the extruded product of the present invention is longer by about 1.5 times as compared with that in the casted composite of the control. Therefore, the dispersibility of the atomized composite powder is clearly superior to that of the casted composite. Example 2 Into a molten aluminium alloy having the composition Al-9Si-1Mg, 3 % of SiC particles (average particle size=25 µm) were uniformly dispersed. The thus prepared melt was subjected to atomization using pressurized air to obtain atomized aluminium matrix composite powders (average particle size=28 µm). An optical microphotograph showed that the atomized composite powders comprised the SiC particles dispersed very uniformly in the matrix of the aluminium alloy. The extruded product was obtained using the above atomized composite powders according to the procedures described in Example 1. The optical microphotograph showed that in the extruded product the SiC particles were dispersed very uniformly in the matrix of the aluminium alloy. Example 3 Into a molten aluminum alloy having the composition Al-7Si-1Cu-1Mg, 25 % of SiC particles (average particle size=5 µm) were uniformly dispersed. The thus prepared melt was subjected to atomization using pressurized air to obtain atomized aluminium matrix composite powders (average particle size=32 µm). An optical microphotograph showed that the atomized composite powders comprised the SiC particles dispersed very uniformly in the matrix of the aluminium alloy. Example 4 Into a molten aluminium alloy having the composition Al-9Si-1Mg, 10 % of Al 2 O 3 particles (average particle size=10 µm) were uniformly dispersed. The thus prepared melt was subjected to atomization using pressurized air to obtain atomized aluminum matrix composite powders (average particle size=30 µm). An optical microphotograph showed that the atomized composite powders comprised the Al 2 O 3 particles dispersed very uniformly in the matrix of the aluminium alloy. Example 5 Into a molten aluminium alloy having the composition Al-20Si-3Mg, 3 % of SiC particles (average particle size=15 µm) were uniformly dispersed. The thus prepared melt was subjected to atomization using pressurized air to obtain atomized aluminium matrix composite powders (average particle size=28 µm). An optical microphotograph showed that the atomized composite powders comprised the SiC particles dispersed very uniformly in the matrix of the aluminium alloy. An extruded product was obtained using the above atomized composite powders according to the procedures described in Example 1. An optical microphotograph showed that in the extruded product, the SiC particles were dispersed very uniformly in the matrix of the aluminium alloy. Example 6 Into a molten aluminium alloy having the composition Al-1Si-5Cu-2Mg, 25 % of SiC particles (average particle size=5 µm) were uniformly dispersed. The thus prepared melt was subjected to atomization using pressurized air to obtain atomized aluminium matrix composite powders (average particle size=32 µm). An optical microphotograph showed that the atomized composite powders comprised the SiC particles dispersed very uniformly in the matrix of the aluminium alloy. Example 7 Into a molten aluminium alloy having the composition Al-10Si-3Cu-1Ni-1Mg, 20 % of SiC particles (average particle size=25 µm) were uniformly dispersed. Furthermore into a molten aluminium alloy having the same composition, 20 % of SiC particles (average particle size=25 µm) were uniformly dispersed, to which 2 % or 4 % of Fe was added. The thus prepared melts were subjected to atomization using pressurized air to obtain atomized aluminium matrix composite powders (average particle size=38 µm). Optical microphotographs showed that the atomized composite powders comprised the SiC particles dispersed very uniformly in the matrices of the aluminium alloys. Extruded products were obtained using the above atomized composite powders according to the procedures described in Example 1. Fig. 5 is an optical microphotograph (x 400) of the resultant extruded product prepared from the composite powders comprising the SiC particles dispersed in the matrix of the aluminum alloy. Fig. 5 clearly shows that in the extruded product, the SiC particles were dispersed very uniformly in the matrix of the aluminium alloy Al-10Si-3Cu-1Ni-1Mg-2Fe. The other optical microphotographs showed that in the extruded products, the SiC particles were dispersed very uniformly in the matrices of the aluminium alloys. Example 8 Into a molten aluminium alloy having the composition Al-10Si-3Cu-1Ni-1Mg, 20 % of SiC particles (average particle size=25 µm) were uniformly dispersed. Furthermore into a molten aluminium alloy having the same composition, 20 % of SiC particles (average particle size=25 µm) were uniformly dispersed, to which 3 % or 6 % of Ni was further added. The thus prepared melts were subjected to atomization using pressurized air to obtain atomized aluminium matrix composite powders (average particle size=38 µm). Optical microphotographs showed that the atomized composite powders comprised the SiC particles dispersed very uniformly in the matrices of the aluminium alloys. Extruded products were obtained using the above atomized composite powders according to the procedures described in Example 1. The optical microphotographs showed that in the extruded products, the SiC particles were dispersed very uniformly in the matrices of the aluminium alloys. Test Example 1 The extruded product obtained in Example 1 was worked so as to prepare a specimen having a parallel part (6 x 40 mm) and a total length of 80 mm. As a control, a specimen was prepared similarly from the casted composite obtained in Comparative Example. After subjecting to a T6 treatment, the mechanical properties of each specimen were tested. The results are shown in Table 2. tensile strength (kgf/mm 2 ) 0.2% proof stress (kgf/mm 2 ) elongation (%) Izod impact value (J/cm 2 ) invention 34.8 29.4 5.5 8.59 control 33.8 29.5 0.3 1.35 tensile properties: JIS Z 2241 Izod impact value : JIS Z 2242 As clear from the results in Table 2, the products obtained from the atomized composite powders of the present invention are very superior in ductility and wear impact as compared with the casted composite. Accordingly, the atomized composite powders of the present invention are very useful as industrial materials. Test Example 2 The extruded products obtained in Example 7 were worked so as to prepare specimens, each having a parallel part (6 x 40 mm) and a total length of 80 mm. After subjecting to a T6 treatment, each specimen was kept at 200°C for 100 hours. Then, the mechanical properties of each specimen were tested at 200°C. The results are shown in Table 3. tensile strength (kgf/mm 2 ) 0.2 % proof stress (kgf/mm 2 ) elongation (%) Al-10Si-3Cu-1Ni-1Mg 23.0 20.3 4.9 Al-10Si-3Cu-1Ni-1Mg-2Fe 24.6 21.8 3.6 Al-10Si-3Cu-1Ni-1Mg-4Fe 26.1 24.9 2.2 As clear from the results in Table 3, the tensile strength and 0.2 % proof stress were even more improved with an increase of the Fe content. Test Example 3 The extruded products obtained in Example 8 were worked so as to prepare specimens, each having a parallel part (6 x 40 mm) and a total length of 80 mm. After subjecting to a T6 treatment, each specimen was kept at 200°C for 100 hours. Then, the mechanical properties of each specimen were tested at 200°C. The results are shown in Table 4. tensile strength (kgf/mm 2 ) 0.2 % proof stress (kgf/mm 2 ) elongation (%) Al-10Si-3Cu-1Ni-1Mg 23.0 20.3 4.9 Al-10Si-3Cu-4Ni-1Mg 27.8 26.4 2.3 Al-10Si-3Cu-7Ni-1Mg 32.6 31.1 1.0 As clear from the results in Table 4, the tensile strength and 0.2 % proof stress were even more improved with an increase of the Ni content.

Claims: A process for preparing an aluminium alloy matrix composite powder comprising 1 to 40% by weight of ceramic particles uniformly dispersed therein, comprising the steps of: preparing a melt of an aluminium alloy with 1 to 40% by weight of ceramic particles uniformly dispersed therein, and atomizing said melt, characterized in that the melt is an aluminium-silicon alloy comprising 1 to 50% by weight silicon and in that the atomized melt is solidified at a solidification rate of at least 10 2 K/s using pressurized air. A process according to claim 1 wherein the aluminium matrix composite powder comprises 3 to 25% by weight of ceramic particles. A process according to claim 1 to 2 wherein the ceramic particles comprise at least one of a carbide, oxide, nitride and boride. A process according to claim 3 wherein the ceramic particles comprise a carbide and/or oxide. A process according to any one of the preceding claims wherein the ceramic particles have an average particle size of 1 to 40 µm. A process according to claim 5 wherein the ceramic particles have an average particle size of 5 to 25 µm. A process according to any one of the preceding claims wherein the aluminium-silicon alloy comprises at least one of Cu and Mg. A process according to claim 7 wherein the aluminium-silicon alloy comprises at least one of 0.5 to 10% by weight Cu and 0.5 to 10% by weight Mg. A process according to claim 1 wherein the aluminium-silicon alloy comprises aluminium, 6 to 20% by weight Si and optionally at least one of 0.5 to 5% by weight Cu and 0.5 to 3% by weight Mg. A process according to any one of the preceding claims wherein the aluminium-silicon alloy further comprises at least one transition metal. A process according to claim 10 wherein the aluminium-silicon alloy comprises 0.5 to 15% by weight of at least one transition metal. A process according to any one of the preceding claims wherein the solidification rate is 10 2 to 10 7 K/s. A process according to claim 12 wherein the solidification rate is 10 2 to 10 4 K/s. A process according to any one of the preceding claims which further comprises forming the aluminium matrix composite powder into a consolidated product.

Inventors: KUBO KOHEI, KUSUI JUN, NAGASE FUMIAKI, TANAKA AKIEI, YOKOTE TAKAMASA, KUBO, KOHEI, KUSUI, JUN, NAGASE, FUMIAKI, TANAKA, AKIEI, YOKOTE, TAKAMASA
Applicants: TOYO ALUMINIUM KK, TOYO ALUMINIUM KABUSHIKI KAISHA
Application year: 1992
Main classification: C22C1
IPC codes: C22C1, C22C32