Aluminum base alloy powder product

United States Patent Office

2,966,735

Patented Jan. 3, 1961

2,966,735

ALUMINUM BASE ALLOY³ POWDER PRODUCT

Raymond J. Towner and John P. Lyle, Jr., New Kensington, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania

No Drawing. Filed Mar. 27, 1958, Ser. No. 724,231

Claims. (CI. 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 aluminumtitanium 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 oxide coated 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 mechanical means. Very finely divided.particles can be produced by this process which pass through a standard Tyler 100 mesh screen.

It is 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 titanium as the principal added alloy component.

Another object is to provide such articles which; do 40 not require any preliminary thermal treatment to place them in condition for service at elevated temperatures.

Still another object is to provide an article made from atomized aluminum-titanium alloy powder that does not depend upon the presence of oxide particles to impart 45 strength at elevated temperatures.

These and other objects are achieved by atomizing a substantially iron-free aluminum base alloy containing not less than 70 percent by weight of aluminum and from 1.0 to 15% by weight of titanium 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 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 been recommended 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 in the solution heat-treated and agehardened condition has a tensile strength of only 5,000 p.s.i. and a yield strength of 3,000 p.s.i. after an exposure of 100 hours at 600° F. 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-titani- um 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 hot5 worked product can be cold worked to a limited extent, if desired. The fabricated alloy product can be placed in service without any preliminary thermal treatment. The strength of the wrought powder products at elevated temperatures is not influenced to any significant extent by the 10 oxide film which coats the atomized particles.

The atomized alloy particles are preferably prepared by melting the alloy in the desired composition and projecting it through a suitably designed nozzle with the aid of a compressed gas. The atomizing condition should 15 be so adjusted that none or only a small proportion of the particles are larger than 100 mesh (145 microns opening) and that the majority of the particles pass through a 200 mesh screen (74 microns opening). Articles produced in this manner generally have an ir20 regular shape but for the most part are substantially equiaxed in dimensions and have an as-cast structure. The aluminum-titanium constituent in the alloy is very finely divided as the result of the drastic chill associated with the atomization process. The surface of the par25 tides are, of course, oxidized if the atomization has occurred in air or in some other oxidizing atmosphere, however, the oxide skin is very thin and the amount of oxide introduced into the final product is too small to affect the properties thereof to any significant extent.

The titanium content of the alloy should be between 1.0 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 1.0 is employed, the minimum strength is not achieved; and if 35 more than 15% is present, the worked article has insufficient ductility and may fracture under applied stresses. Titanium is substantially insoluble in aluminum and whatever small proportion may be dissolved is too small to have any significant effect upon the properties of the atomized particles. It will, therefore, be appreciated that the matrix of the atomized particles consists of aluminum with a dispersion of finely divided aluminum-titanium constituent distributed throughout the particle. The high strength at elevated temperatures appears to be controlled by the amount of the aluminum-titanium constituent and the fineness of the dispersion.

The alloy may contain the usual impurities associated with aluminum, for example, silicon and iron. Generally, the silicon impurities should not exceed 1% and 50 the iron content should not be more than about 1%. Other impurities, such as copper, may also be present in amount up to 1%. In view of the relatively small amount of iron impurity permitted in the alloy, the composition is referred to herein as being substantially iron55 free.

For some purposes, it may be desirable to add one or more of the elements selected from the group composed of manganese, nickel, cobalt, chromium; vanadium, zirconium, molybdenum and tungsten in amounts of 0.1 to 60 10% by weight of each, the total not exceeding 10%. These elements act as hardeners and, like titanium are substantially insoluble in the aluminum'matrix. To obtain the properties attributable to the aluminum-titanium constituents, the titanium content in the alloy should ex65 ceed the total amount of any added hardener elements.

To make the wrought article from 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, 70 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.

3,866,735 and applying a sufficient pressure thereto for a long enough period of time to cause at least some consolidation on the welding of the atomized particles. Pressures of 200 to 150,000 p.s.i. are satisfactory which are applied at 600° F. after a 100-hour exposure at that temperature. The composition of the alloys and the tensile strength at both room temperature and 600° F. are given in Table 1 below:

TABLE 1

Tensile properties of extruded Al-Ti powder products

<td rowspan="2"> Alloy</td><td rowspan="2"> Percent Ti</td><td colspan="3"> At Boom Temperature</td><td colspan="3"> At 600° Γ.</td>

<td> Tensile Strength, p.s.i.</td><td> Yield Strength, p.s.i.</td><td> Percent Elong.</td><td> Tensile Strength, p.s.i.</td><td> Yield Strength, p.s.i.</td><td> Percent Elong.</td>

<td> A______</td><td> 1.6</td><td> 40,800</td><td> 32,000</td><td> 18.5</td><td> 14,200</td><td> 12,500</td><td> 24.8</td>

<td> B______</td><td> 4.3</td><td> 39,700</td><td> 24,800</td><td> 8</td><td> 13,000</td><td> 11,000</td><td> 8.4</td>

<td> C______</td><td> 7.8</td><td> 39,800</td><td> 26,500</td><td> 6.6</td><td> 15,900</td><td> 13,400</td><td> 6.3</td>

<td> D______</td><td> 4.8+4.2 Cr</td><td> 46,200</td><td> 36,800</td><td> 6</td><td> 21,800</td><td> 18,300</td><td> 5</td>

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 compact 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. for hot working or to obtain adequate workability. This should only be done if more than 3 % titanium 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 and brought to the desired temperature in the compression chamber. Where the powder is charged to a press cylinder, compacted and immediately extruded, it is generally convenient to compress the powder mass against a blind die and then substitute a die with the desired orifice therein. 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 hot working 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-titanium atomized powders were consolidated and the product hot worked. The powders were of a fineness such that approximately 70% passed through a 200 mesh screen and substantially all of the remainder passed through a 100 mesh screen. Each powder was charged to an extrusion press cylinder, preheated to 800° F. and compressed against the 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 to 800° F. After the compact was thus formed, it was ejected from the cylinder, cooled, scalped and reheated for hot working. These compacts were reheated to 850° F., except for the 1.6% Ti alloy which was reheated to 800° F., inserted in an extrusion press cylinder heated to 800° F. and extruded to %” diameter rod. Tensile specimens were cut from the extruded rod and tested without any preliminary thermal treatment, some at room temperature and others

It is apparent from the foregoing that the tensile and yield strengths of the alloys increase with an increase in the titanium content. The tensile properties obtained at 600° F. are considerably higher than those of the two ²⁵ commercial aluminum base alloys referred to hereinabove. It is also significant that the high strengths at 600° F. were obtained without any preliminary thermal treatment of the worked powder product, such as solution heat treatment, which is a definite economic advan³⁰ tage.