Patent Application
20120207640
1. Field of the Invention
This invention relates generally to the field of aluminum alloys and, more particularly, to alloys produced by powder metallurgy processes. Such alloys are compatible with ceramic reinforcements used in metal matrix composites.
2. Background Art
The development of a high strength aluminum alloys based on the Al—Zn—Cu—Mg alloy system for many years has focused on three main approaches: (1) heat treatment variation to maximize property combinations, (2) thermal-mechanical treatments and (3) second phase chemistry control.
The present invention provides aluminum alloy products having improved strength and fatigue resistance. The present invention further provides a method of producing improved aluminum alloys by powder metallurgy processes. These alloys are compatible with ceramic reinforcements used in metal matrix composites. These alloys are characterized by high yield strength and elastic modulus at room temperature and are therefore useful in aircraft and other demanding applications.
In one embodiment, an aluminum alloy consists essentially of about 9.0 to 10.3 wt. % zinc, about 2.5 to 3.5 wt. % magnesium, about 1.5 to 3.0 wt. % copper and less than about 0.5 wt. % of any other alloying constituent. The balance consists of aluminum, although it will be understood that there may also be trace amounts of unavoidable incidental impurities. This new alloy has at least about 10% greater yield strength than 7050-T6 aluminum, one of the strongest aluminum alloys currently in wide use for demanding aerospace applications.
In the following description, for purposes of explanation and not limitation, specific numbers, dimensions, materials, etc. are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known speaker components are omitted so as not to obscure the description of the present invention with unnecessary detail.
Aluminum alloys in accordance with embodiments of the present invention are made by powder metallurgy processes, such as vacuum-hot-pressing or cold-isostatic pressing and sintering. The alloys can be made by blending elemental powders with aluminum powders to create the desired alloy. The alloys can also be made by blending aluminum powder with master alloys containing the desired alloy ingredients. The alloy powders can also be made by atomizing a melt with the desired composition. In particular embodiments, the alloy contains aluminum with between 9.0 and 10.3 percent zinc, 2.5 and 3.5 percent Mg, 1.5 and 2.2 percent copper and less than about 0.5 percent (preferably less than about 0.05 percent) of any other alloying constituent. This alloy can be used as the matrix for a particle reinforced composite. The alloy is made with fine powders in order to control the grain size and microstructure of the final product. The maximum particle size for the powder is 44 microns.
The fine powders for this alloy are blended in commercial units that are compatible with fine aluminum alloys. A ceramic powder that will act as a reinforcement can be added to the alloy powder and blended at this time. Ceramic materials suitable for use as the reinforcement phase include silicon carbide, silicon nitride, SiAlON, titanium nitride, titanium carbide, titanium silicide, molybdenum silicide, nickel aluminate, boron carbide, aluminum nitride, aluminum oxide, magnesium oxide, silicon and mixtures thereof. The powders may be isostatically compressed into a cohesive or coherent shape. This can be effected by placing the powders within a bag, such as a rubber or plastic material, which in turn is placed within a hydraulic media for transmitting pressure through the bag to the powder. Pressures are then applied in the range of 5 to 60 psi which compress the powder into a cohesive shape of about 85 to 93% of full density. This isostatic compaction step facilitates handling of the powder. The isostatically compacted material can then be sintered by placing the compact in a vacuum furnace and heating to temperatures of 875° F., preferably 900° or 950° F., while continuing to pull a vacuum down to a pressure level of one torr, preferably 10−1 or 10−2 torr or less (1 torr=1 mm Hg at 0° C.). The density of the sintered billets remain between 90 and 95 percent of the theoretical and must be metal worked by extrusion, forging or rolling in order to develop full density and full properties.
Alternatively, the material can be compacted to substantially full density at relatively high temperatures. This can be effected by placing the powder or compacted material in a hard tool that is placed in a container and evacuating the container at room temperature and heating to temperatures of 675° F., preferably 700° or 850° to 950° F., while continuing to pull a vacuum down to a pressure level of one torr, preferably 10−1 or 10−2 torr or less (1 torr=1 mm Hg at 0° C.). While still in the sealed container, the material is compressed to substantially full density at temperatures of 900° to 950° F.
When referring to substantially full density, it is intended that the compacted billet be substantially free of porosity with a density equal to 95% or more of the theoretical solid density, preferably 98 or 99% or more. It is desired that the vacuum compaction to full density be effected at a minimum temperature greater than 650° F., for instance 675° F. or higher, and preferably at a minimum temperature of 700° F. or higher. The maximum temperature for compaction should not exceed 960° F. After being compacted to substantially full density at elevated temperature and vacuum conditions as just described the billet which can then be shaped such as by forging, rolling, extruding or the like or can be machined into a useful shape. It is preferred that the billet be worked by any amount equivalent to a reduction in cross section of at least 25%, preferably 50 or 60% or more, where practical, since such favors improved elongation properties. Preferred working temperatures range from 550° to 850° F.
After working the product, it is heat treated to the desired condition and quenched. The product is then aged within a temperature range of about 235° F. to 270° F. for about 6 to 60 hours.
Several materials were made with fine powders with different chemical contents. The zinc content was varied from 8.4 to 11 percent, the magnesium content was varied between 2 and 2.9 percent and the copper content was varied between 1.25 and 2 percent. The alloy billets were extruded into 0.625 inch diameter rods from a 3.5 inch container. The rods were cut into sample blanks. The sample blanks were heat treated to a T-6 condition by solution treating at 900° F. for 1 hour, room temperature water quenched and then aged for 24 hours at 250° F. Room temperature tensile tests were conducted on specimens machined into reduced section bars and the results are presented in Table 1. The yield strength goal for the new alloy was set at 20% higher than 7050, or 85 ksi. The data indicate that all of the alloys have yield strengths greater than 85.
The chemical composition limits for alloys in accordance with embodiments of this invention are defined in Table 2.
It will be recognized that the above-described invention may be embodied in other specific forms without departing from the spirit or essential characteristics of the disclosure. Thus, it is understood that the invention is not to be limited by the foregoing illustrative details, but rather is to be defined by the appended claims.
