US 2966732 A
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United States Patent ALUMINUM BASE ALLOY POWDER PRODUCT Raymond J. Towner and John P. Lyle, Jr., New Kensington, PaL, assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Mar. 27, 1958, Ser. No. 724,228
3 Claims. (Cl. 29-182) This invention relates to articles made from aluminum base alloy powders and it is more particularly concerned with those products resulting from heating and working a compacted mass of atomized particles of an aluminum-chromium base alloy.
Heretofore, compressed and sintered bodies of oxidecoated aluminum flake powders have been produced which possess unique strength properties" at elevated temperatures. The particles of oxide distributed throughout the body appear to impart'the unusual strength at elevated temperatures. The production of the oxidecoated flakes is time consuming and consequently expensive. It has now been found, contrary to previous belief, that useful articles can be made from certain types of atomized aluminum alloy powders. As is 'well known, the atomization process involves disintegrating a stream of molten metal with a jet of gas, such as compressed air, or by mechanicalmeans. Very finely divided particles can be produced by this process that will passthrough a standard Tyler 100 mesh screen.
Itis an object of this invention to provide articles having a high strength at elevated temperatures which are made from atomized particles of an aluminum base alloy containing chromium as the principal added alloy component.
Another object is to provide such articles which do not require any preliminary thermal treatment to place them in condition for service at elevated temperautres.
Still another object is to provide an article made from atomized aluminum-chromium alloy powder that does not depend upon the presence of oxide particles to impart strength at elevated temperatures.
These and other objects are achieved by atomizing a substantially iron-free aluminum base alloy containing not less than 70% by weight of aluminum and from 0.5 to 15% by weight of chromium as the principal added alloy component, and subsequently consolidating and working a mass of such atomized particles under the influence of heat and pressure. The resultant articles have a density closely approximating that of the alloy if cast; and in the hot worked condition, they have a tensile strength of not less than 10,000 p.s.i. and a minimum yield strength of 9,000 p.s.i. at 600 F. after a 100-hour. exposure. These tensile and yield strength values are to be compared with those of some conventional wrought heat treated aluminum base alloys that have beenrecommended for service at elevated temperatures. For example, a wrought aluminum base alloy nominally composed of aluminum, 12.2% silicon, 1.1% magnesium, 0.9% nickel and 0.9% copper has in the solution heat treated and age hardened condition a tensile strength of only 5,000 p.s.i. and a yield 6f 3,000 p.s.i. after exposure at 600 F. for 100 hours. Under the same exposure conditions a second well-known aluminum base alloy nominally consisting of aluminum, 4.5% copper, 1.5% magnesium and 0.6% manganese, when worked, solution heat treated and age hardened,
has a tensile strength of 10,000 p.s.i. and a yield strength of 7,500 p.s.i. The aluminum-chromium powder products can be readily worked under the usual hot working conditions of temperature and pressure employed in fabricating conventional aluminum and aluminum base alloy articles. Furthermore, the hot worked product can be cold worked to a limited extent, if desired. The
tion of the particles are larger than 100 mesh (145 microns. opening) and that the majority of the particles will pass through a 200 mesh screen (74 microns opening). The particles produced in this manner generally have an irregular shape but for the most part are substantially equiaxed in dimensions and have as-cast structure. The aluminum-chromium constituent in the alloy is very finely divided as a result of the drastic chill associated with the atomization process. The surface of the particles are, of course, oxidized if the atomization has occurred in air or in some other oxid1zing-atmos-- phere, however, the oxide skin is very thin and the amount of oxide introduced into the final product is too I small to affect the properties thereof to any significant extent.
The chromium content of the alloy should be between 0.5 and 15% by weight, as mentioned above, and preferably within the range of 5 to 10% to obtain the highest strength at elevated temperatures. If less than 0.5% is employed, the minimum strength is not achieved and if more than 15% is present, the worked article has insufiicient ductility and may fracture under applied stresses. Chromium is substantially insoluble in aluminum and what ever small proportion may be dissolved is too small to have any significant effect upon the properties of the atomized particles. It will therefore be ap-' preciated that the matrix of the atomized particles consists of aluminum with a dispersion of finely divided aluminum-chromium constituent distributed throughout the particle. The high strength at elevated temperatures appears to be controlled by the amount of the aluminumchromium constitutent and the fineness of the dispersion.
The alloy may contain the usual impurities associated with aluminum, for example, silicon and iron. Generally, the silicon impurity should not exceed 0.8% and the iron content should not be more than about 1%. Other impurities, such as copper, may also be present in amounts up to 0.5%. In view of the relatively small amount of iron impurity permitted in the alloy, the composition is referred to herein as being substantially ironfree.
For some purposes, it may be desirable to add one or more elements selected from the group composed of nickel, cobalt, manganese, titanium, vanadium, zirconium, molybdenum, and tungsten in amounts of 0.1 to 10% each, the total not exceeding 10%. These elements act as hardeners and, like chromium, are substantially insoluble in the aluminum matrix. To attain the properties attributable to the aluminum-chromium constituent, the chromium content of the alloy should exceed the total amount of any added hardener elements. To make the wrought article from the atomized powder, the powder may be initially formed into a compact that is subsequently worked or it may be charged directly to a compression chamber such as an extrusion, press cylinder and be extruded therefrom after initial consolidation of the mass. The initial compact may be made by heating the powder to a temperature between 700 and 900 F. and applying a sufiicient pressure thereto for a long enough period of time to cause at least some consolidation and welding of the atomized particles. Pressures of 200 to 150,000 p.s.i. are satisfactory which are applied for varying periods of time from a minute or less to a few hours. Generally, a longer time is required where low pressures are employed. The comfor hot working. The. compact of the Al6.9% Cr alloy was reheated to 800i F., while that of the All0.1% Cr alloy was reheated to 850 F., inserted in an extrusion press cylinder heated to 800 F. and extruded to 11" diameter rod. Tensile specimens were cut from the extruded rod and tested without any preliminary thermal treatment, some at room temperature and others at 600 F. after a 100-hour exposure at that temperature. The composition of the alloys tested and the tensile properties at both room temperature and at 600 F. are given in Table 1 below.
pact may be left in the press cylinder and then extruded, or it may be ejected, cooled, scalped, reheated to the hot working temperature and hot worked. In some cases it may be desirable to reheat the compact to temperatures as high as 1150" F. before hot working in order to obtain adequate workability. This should only be done if more than 3% chromium is present; Where the powder is charged to a compression chamber, it may be initially heated to a temperature between 700 and 900 F. and introduced to the chamber or it may be charged cold and heated within the chamber. Alternatively, it may be heated to an intermediate temperature, then charged to the chamber and brought to the desired temperature. When the powder is to be compacted and immediately extruded, it is generally convenient to compress the powdered mass against a blind die in a press cylinder and then substitute an extrusion die forit to produce the desired extruded shape. Although reference has been made to the extrusion of the powder mass, it is to be understood that it can be subjected to other types of hotworking operations, such as rolling, forging or pressing, providing a suitable compact is initially produced. The hot working is preferably performed within the temperature range of 700 to 900 F.
Our invention'is illustrated by the following examples wherein aluminum-chromium atomized powders were consolidated and the product hot worked. The powders were of a fineness such that approximately 75% passed through a 200 mesh screen and substantially all ofthe remainder passed through a 100 mesh screen. Each alloy powder was charged to an extrusion press cylinder preheated to 800 F. and compressed against a blind die under a pressure of 100,000 p.s.i. for a' period of approximately one minute. In the course of charging and compressing the powder the temperature of the compact reached 700-800 F. After the compact was-thus formed it was either ejected from the cylinder, and subsequently hot worked or it was retained in the cylinder, an extrusion die having a diameter opening'therein substituted for the blind die, and the compacted mass extruded to form a rod in diameter. Only the compact of the Al0.8% Cr alloy was extruded directly after formation; the compacts of the other alloys were ejected from the cylinder, cooled, scalped' and reheated It is apparent from the foregoing that the tensile and yield strengths of the alloys increase with an increase in the chromium content. Moreover, it will be noted that the tensile properties at 600 F. are higher than those of the two commercial aluminum base alloys. referred to hereinabove. It is also significant that the high strength at 600 F. was obtained without any preliminary thermal treatment of the worked powder products, suchv as a solution heat treatment, which is an economic advantage.
Having thus described our invention and certain embodiments thereof, we claim:
1. A hot worked aluminum base alloy powder article free from oxide except as an incidental impurity and having a maximum iron content of 1%, said hot worked alloy powder being formed from atomized powder of aluminum base alloy containing at least by weight of aluminum and from 0.5 to 15% by weight of chromium as the essential component, the amount of said component exceeding the total quantity of any hardening elements present in the alloy, said alloy being substantially free from elements which form a solid solution with aluminum, except as they occur as impurities, said hot worked article being characterized in the as-worked condition by a tensile strength of 600 F. after a hour exposure of not less than 10,000 p.s.i. and a yield strength of not less than 9,000 p.s.i.
2. A hot worked aluminum base alloy powder article according to claim 1 wherein the chromium content is 5 to 10%.
3. A hot worked aluminum base alloy powder article according to claim 1 wherein the alloy also contains at least one hardening element selected from the group consisting of nickel, cobalt, manganese, titanium, vanadium, zirconium, molybdenum, and tungsten in amounts of 0.1 to 10% each by weight, the total not exceeding 10% by weight, the chromium content of said alloy exceeding the amount of hardening elements added thereto.
References Cited in the file of this patent UNITED STATES PATENTS