HEAT-RESISTANT ALUMINUM ALLOY AND PROCESS FOR ITS PRODUCTION

Abstract: A heat-resistant aluminium alloy, which contains 5 to 10 wt % of iron, 0.5 to 3 wt % of molybdenum, 0.5 to 3 wt % of at least one element selected from among chromium, zicronium and vandium, with the sum of iron, molybdenum, chrom­ ium, zicronium vanadium being 6 to 16 wt %, and the balance of substantially aluminum, and which has a tensile strength at 300°C of 26kg/mm² or more. This can be produced by preparing an aluminum alloy powder of the above-­ described formulation according to the melt atomizing process, and solidifying and molding the product at temperatures of 400 to 580°C.

Description: The present invention relates to a heat resistant aluminum ally which can be used in automobile engine parts and aircraft parts. Hitherto, as heat resistant aluminum alloys, 2218, 2219, 2618 and the like are known. However, the aluminum alloys are required to have much better heat resistance in accordance with the technological progress. To provide highly heat resistant aluminum alloys, it is proposed to improve high-temperature strength by the addition of a considerable amount of a transition metal such as iron, which has been avoided since it deteriorates the properties of the alloys, to the alloys. Since such kinds of aluminum alloys contain a considerable amount of the transition metal such as iron, they cannot be produced by the conventional casting method. Therefore, they are produced by consolidating the alloy powder prepared by the melt atomizing method. In fact, such aluminum alloys have improved heat resistance as reported in "Advanced in Powder Technology"; "Advanced P/M Aluminum Alloys", 213-214, 1981: ASM Material Science Seminor published by AMERICAN SOCIETY FOR METALS, and some of them are reported to have tensile strength of about 300 N/mm² (30 kg/mm²) at 300°C. However, such alloys cannot be prepared with good reproducibility and have not been practically used. For parts which are exposed to high temperatures such as the aircraft engine parts, impellers of gas turbine engines and engine pistons, materials are required to have a tensile strength of 250 N/mm² (25 kg/mm²) or larger, preferably larger than 300 N/mm² (30 kg/mm²)at 300°C. However, none of the already developed heat resistant aluminum alloys satisfies this requirement practically. JP-A-60 234936 discloses a heat resistant aluminum alloy having a tensile strength at high temperatures. The same is true for JP-A-61 48551. The known alloys disclose the addition of Mo, Ni, Zr and Ti to aluminum, molybdenum and iron. Chromium, which is added according to the present invention is not disclosed in said documents. The object of the present invention is to provide a heat resistant aluminum alloy which has high tensile strength of larger than 255 N/mm² (26 kg/mm²) at high temperature of 300°C. The heat resistant aluminum alloy of the present having a tensile strength of larger than 255 N/mm² (26 kg/mm²) at 300°C comprises 5 to 10% by weight of iron, 0.5 to 3% by weight of molybdenum and the balance being aluminum and unavoidable impurities, wherein it further contains 0.5 to 3% by weight of chromium. The heat resistant aluminum alloy of the present invention can be produced from powder of an aluminum alloy comprising 5 to 10 % by weight of iron, 0.5 to 3 % by weight of molybdenum, 0.5 to 3 % by weight of chromium, and the balance being aluminum and unavoidable impurities. The powder is produced by a melt atomizing method and hot plastic working at a temperature of 400 to 580°C. The "melt atomizing method" herein used includes all the methods which comprise spraying metal alloy melt having a desired alloy composition and solidifying it quickly, and an air atomizing method, a gas atomizing method, a centrifugal atomizing method and a rotational roll atomizing method are exemplified. As the method for hot plastic working the aluminum alloy powder, extrusion, forging, hot press and HIP are exemplified. As described above, in the conventional techniques for improving the heat resistance of the aluminum alloy, the transition metal element such as iron is added to the aluminum alloy so as to increase the strength of matrix and also to form a thermally stable intermetallic compound. According to the study by the present inventors, it was found that the transition metals have two kinds of effects on the strength of the alluminum alloys. One is the increase of strength of the aluminum alloy and another is the maintenance and improvement of the thermal stability of the aluminum alloys. The combination of these two kinds of the transition elements will provide the aluminum alloy with much higher heat resistance. It is generally known that, among the additive elements, the addition of iron gives the largest strength and heat resistance among the transition elements. The iron atoms form the intermetallic compound: Al₃Fe, the precipitates are dispersed in the matrix, and then increases the strength of the matrix. Since said intermetallic compounds have a large heat resistance, it effectively improves the high-temperature strength of the alloy. Molybdenum contributes to the improvement of the strength of the alloy. Molybdenum forms an Al-Fe-Mo base compound, which is uniformly dispersed in the matrix and improves not only the strength of the matrix but also the heat resistance of the alloy. To further increase the thermal stability chromium is added to the aluminum alloy, and metallic compounds consisting of these elements are precipitated on grain boundaries and will inhibit the excessive growth of crystal grains and the dispersed Al-Fe-Mo compound at high temperatures. Thereby, they significantly improve the high-temperature strength of the aluminum alloy. The content of chromium in the aluminum alloy is 0.5 to 3% by weight. If chromium is contained in an amount of less than 0.5% by, the strength and heat resistance of the aluminum alloy are insufficiently improved. If chromium is used in an amount of larger than 3% by weight, plastic working of the aluminum alloy becomes difficult and toughness and ductility of the aluminum alloy are greatly decreased so that the aluminum alloy cannot be practically used, although the strength and heat resistance of the aluminum alloy are highly improved. The aluminum alloy powder produced by the melt atomizing method is more fine and homogeneous and has less segregation than those produced by casting. However, when the cooling rate during solidification is less than 10² °C/sec., the additive elements iron, molybdenum, and chromium are segregated to make the structure uniform, so that the compacting and plastic deformation of the aluminum alloy powder become difficult. Even if the compacting is possible, the strength and elongation of the aluminum alloy are undesirably decreased. During hot plastic working of the aluminum alloy powder for consolidating, at a temperature of lower than 400°C, deformation resistance is too high to produce alloys with sufficient strength, while at a temperature of higher than 580°C, the precipitates and the crystal grains grow so large that a sufficient high-temperature strength of the aluminum alloy is not achieved. The present invention will be illustrated by following Examples. Examples Aluminum alloy powder having the composition of Table 1 was prepared by the air atomizing method and screened to 149 µm (100 mesh) or less. The alloy powder was consolidated by zone hot plastic working at the temperature specified in Table 1. In extrusion, the alloy was consolidated at an extrusion ratio of 13 to form a rod of 21 mm in diameter. Sample No. Composition (wt%) Molding method Temperature (°C) 1 Al-8Fe-2Mo-2Cr Extrusion 480 2 Al-8Fe-2Mo-2Cr Forging 520 3 Al-8Fe-2Mo-2Cr HIP, 4000 kg/cm² 550 4 Al-8Fe-2Mo-2Cr HIP, 2000 kg/cm² 500 5* Al-10Fe-5Mo-5Cr Extrusion 550 6* Al-4Fe-0.2Mo-0.2Cr Extrusion 450 7* Al-8Fe-2Mo-2Cr Extrusion 620 8* Al-8Fe-2Mo Extrusion 480 9* Al-8Fe-1Zr Extrusion 480 10* Al-8Fe-3Cr Extrusion 480 11* Al-10Fe-2Cr Extrusion 500 Note: Sample Nos. with * were the comparative examples. With the produced aluminum alloy, tensile strength at room temperature and at 300°C was measured. The results are shown in Table 2. The tensile strength at 300°C was measured after keeping the aluminum alloy sample at 300°C for 100 hours. Sample No. Tensile strength N/mm² (Kg/mm²) Remarks At room temp. At 300°C 1 570 (57) 330 (33) Elongation, 2.3 % 2 560 (56) 290 (29) 3 550 (55) 270 (27) 4 550 (55) 280 (28) 5* 580 (58) 340 (34) Elongation less than 1 % Extrusion clog at 500°C 6* 420 (42) 220 (22) 7* 520 (52) 240 (24) 8* 400 (40) 210 (21) 9* 310 (31) 160 (16) 10* 360 (36) 210 (21) 11* 430 (43) 240 (24) 12* 520 (52) 260 (26) 13* 450 (45) 290 (29) Note: Sample Nos. with * were the comparative examples. Sample Nos. 12 and 13 had the compositions of Al-8Fe-3.4Ce and Al-8Fe-2Mo, respectively and corresponded to the conventional alloys disclosed in Advanced P/M Aluminum Alloys, pages 213-214. The tensile strength of the aluminum alloys of examples according to the present invention was larger than 500N/mm² (50 kg/mm²)at room temperature and larger than 255N/mm² (26 kg/mm²)at 300°C. From these results, it is understood that the aluminum alloys of the present invention have excellent high-temperature strength. On the contrary, the aluminum alloys of the comparative examples had inferior high-temperature strength in particular. Although Sample No. 5 had good strength, it had poor plastic workability and considerably small elongation so that it cannot be processed practically. According to the present invention, there is provided a heat resistant aluminum alloy having not only the good room temperature strength but also such excellent high-temperature strength that its tensile strength exceeds 255N/mm² 26 kg/mm²)at 300°C. Therefore, the heat resistant aluminum alloy can be used for the production of automobile engine parts, aircraft parts, gas turbine engine parts, etc, which are conventionally produced from iron base alloys or nickel base alloys, whereby it is possible to make those parts light and highly efficient.

Claims: A heat resistant aluminum alloy having a tensile strength of larger than 255 N/mm² (26 kg/mm²) at 300°C which comprises 5 to 10% by weight of iron, 0.5 to 3% by weight of molybdenum and the balance being aluminum and unavoidable impurities, wherein it further contains 0.5 to 3% by weight of chromium. A process for producing the heat resistant aluminum alloy of Claim 1, which comprises preparing a powder of an aluminum alloy comprising 5 to 10% by weight of iron, 0.5 to 3% by weight of molybdenum, 0.5 to 3% by weight of chromium, and the balance being aluminum and unavoidable impurities by a melt atomizing method and hot plastic working said aluminum alloy powder at a temperature of 400 to 580°C. The process according to Claim 2, wherein the cooling rate in the solidification of the aluminum alloy powder is not less than 10² °C/sec.

Inventors: ODANI Y C O ITAMI WORKS OF SUM, TAKEDA Y C O ITAMI WORKS OF SU, ODANI,Y. C/O ITAMI WORKS OF SUMITOMO ELECT.IND.LTD, TAKEDA,Y. C/O ITAMI WORKS OF SUMITOMO ELEC.IND.LTD
Applicants: ALUMINUM POWDER MET RES ASS, RESEARCH ASSOCIATION OF ALUMINUM POWDER METALLURGY
Application year: 1988
Main classification: C22C21
IPC codes: C22C1, C22C21