Aluminum-Based Composite Product of High Strength and Toughness

This invention relates to high strength aluminum products, and particularly to methods for increasing the toughness of such products without substantial loss of strength.

High strength aluminum alloys and composites are required in certain applications, notably the aircraft industry where the combination of high strength, high stiffness and low density is particularly important. High strength is generally achieved in aluminum alloys by combinations of copper, zinc and magnesium, and high stiffness is generally achieved by metal matrix composites such as those formed by the addition of silicon carbide, boron carbide or aluminum oxide particles to an aluminum matrix. Recently, aluminum-lithium alloys containing 2.0-2.8% lithium by weight have been developed. These alloys possess a lower density and higher elastic modulus than conventional non-lithium-containing alloys.

The preparation and properties of aluminum-based alloys containing lithium are widely disclosed, notably in J. Stone & Company, British Pat. No. 787,665 (Dec. 11, 1957); Ger. Offen. No. 2,305,248 (National Research Institute for Metals, Tokyo, Jan. 24, 1974); Raclot, U.S. Pat. No. 3,343,948 (Sept. 26, 1967); and Peel et al., British Pat. No. 2,115,836 (Sept. 14, 1983). Powder metallury techniques involving the blending of powdered constituents have been disclosed for a variety of purposes, notably by Fujitsu, Ltd., Japanese Pat. No. 53-75107 (1976); Giorgi et al., U.S. Pat. No. 3,713,898 (Jan. 30, 1973); and Reen, U.S. Pat. No. 3,713,817 (Jan. 30, 1973).

It is also well known that alloys can be made by mixing elemental powders and heating the mixture to a temperature high enough to cause diffusion to take place and form an alloy of uniform composition. See The Physics of Powder Metallurgy, W. E. Kingston, ed., p. 372, McGraw Hill, New York (1951); and C. G. Goetzel, Treatise on Powder Metallurgy, vol. 11, p. 492, Interscience Publishers Inc., New York (1950). Because of the difficulties inherent in obtaining homogeneity, however, the usual practice in aluminum and other alloy systems is to form an alloy powder directly from a prealloyed melt.

Unfortunately, high strength aluminum materials are frequently characterized by low toughness, as evidenced by impact tests on notched specimens (e.g., Charpy tests) and by fracture toughness tests on fatigue precracked specimens where the critical stress intensity factors are determined.

It has now been discovered that high strength and high toughness can be achieved simultaneously in a single aluminum-based metallic product by dispersing particles of a high toughness aluminum-based metal through a matrix comprised of a high strength aluminum-based metal. The dispersion is most conveniently achieved by powder metallurgy techniques. In some cases, the result is a compromise between strength and toughness. The overall result, however, is a combination of strength and toughness which is a substantial improvement over prior art composites and alloys.

FIG. 1 is a plot of longitudinal tensile properties as a function of aging temperature for edge samples taken from one embodiment of the present invention.

FIG. 2 is a plot similar to FIG. 1, relating however to center samples.

FIG. 3 is a plot of transverse tensile properties as a function of aging temperature for the embodiment of FIG. 1.

FIG. 4 is a plot of Charpy impact values as a function of aging temperature for the embodiment of FIG. 1.

FIG. 5 is a plot of fracture toughness as a function of aging temperature for the embodiment of FIG. 1.

FIG. 6 is a plot of yield strength vs. impact toughness for specimens taken from the center of an extrusion of the embodiment of FIG. 1.

FIG. 7 is a plot similar to FIG. 6 except that the plotted values relate to edge specimens.

FIG. 8 is a plot similar to FIG. 1 for a second embodiment of the present invention, the data taken on center specimens.

FIG. 9 is a plot of longitudinal tensile properties on edge specimens vs. aging temperature for the embodiment of FIG. 8.

FIG. 10 is a plot of transverse tensile properties vs. aging temperature for the embodiment of FIG. 8.

FIG. 11 is a plot of Charpy impact values vs. aging temperature for the embodiment of FIG. 8.

FIG. 12 is a plot of yield strength vs. impact toughness for the embodiment of FIG. 8.

FIG. 13 is a plot of Charpy impact values vs. percent lithium taken from the values in the preceding figures for both embodiments.

The present invention is applicable to high strength aluminum-based metallic materials of a wide range of composition, including both alloys and high strength composites having a yield strength of at least about 30 ksi (thousand pounds per square inch), preferably at least about 50 ksi, when heat treated to the highest level. This includes such alloys as those containing lithium, copper, magnesium or zinc as the primary alloying element, notably alloys of the 2000, 5000, 7000, and 8000 Aluminum Association series. Examples are the alloys 2014, 2018, 2024, 2025, 2090, 2218, 2618, 7001, 7039, 7072, 7075, 7079, 7178 and 8090. The term "primary alloying element" is used herein to designate any element which amounts to about 1% or more by weight of the alloy, preferably 2% or more.

High strength composites to which the present invention is applicable include a wide range of products wherein aluminum matrices are reinforced with particles, whiskers or fibers of various materials having a high strength or modulus. Examples of the reinforcing phase include boron fibers, B.sub.4 C-coated boron, SiC-coated boron, B.sub.4 C whiskers and particles, SiC whiskers and particles, carbon or graphite fibers, fused silica, alumina, steel, beryllium, tungsten and titanium. The alloys are generally preferred.

The high toughness component of the present invention may be an aluminum-based alloy or composite with an impact toughness of at least about 20 foot-pounds, preferably at least about 50 foot-pounds, or aluminum itself. The term "impact toughness" as used herein designates a value determined by conventional impact techniques, notably the Charpy test technique, a standard procedure established by the American Society for Testing and Materials. Straight aluminum having a maximum impurity level of about 0.5% by weight is preferred. Commercially pure aluminum will generally suffice.

The composite of the present invention may be formed by blending particles of the two components in the desired proportion. The particle size is not critical and may vary over a wide range. In most applications, particles ranging in diameter from about 10 to about 1,000 microns, preferably from about 50 to about 500 microns, or having a volume of about 0.0001 to about 0.01 cubic centimeters each, will provide the best results. It is preferred that the particles of both components have approximately the same size range.

The relative amounts of the components may also vary widely, depending upon the composition of each component and upon the desired properties of the ultimate product. Composites containing from about 2% to about 40% by weight of the high toughness component, preferably from about 5% to about 25% by weight, will generally provide the best results.

The particles themselves may be formed according to conventional techniques, including pulverization, ribbon and splat techniques. Once the powders are formed and sized and appropriate amounts selected, blending is achieved by conventional means.

The blended powders are then consolidated, again by conventional means, to form a billet which can be further processed into the ultimate product. Consolidation may be achieved by unidirectional compaction (including canister techniques), isostatic compaction (both cold and hot), rolling, forging, sintering, or other known methods. Consolidation preferably includes compaction to at least about 85% full density, more preferably at least about 95%. It is particularly preferred that the consolidation and compaction processing steps include the removal of substantially all bound water from the surface of the particles prior to the achievement of full density. This is generally achieved by purging the particle mixture with an inert gas and/or degassing the particles either prior to consolidation or after partial compaction, involving the use of reduced pressure and elevated temperature, preferably not exceeding about 1100.degree. F. (593.degree. C.).

In many cases, the increase in toughness will be accompanied by a loss in strength. In general, the former will more than compensate for the latter, resulting in a product which is improved in overall properties.

The following examples are offered for purposes of illustration, and are intended neither to define nor limit the invention in any manner.

A composite product was prepared as follows.

A powdered aluminum-lithium alloy containing 2.41% Li, 1.21% Cu, 0.73% Mg and 0.11% Zr (designated herein as 1611) was prepared by a conventional powder metallurgy technique, involving melting and combining the component metals at 1700.degree. F. (927.degree. C.) and atomizing the melt in an inert gas. The resulting particles were sized to -100 mesh (U.S. Sieve Series).

The particles were then blended for 2 hours at room temperature in a rotating V-shaped blender with similarly sized particles of commercially pure aluminum (minimum purity 99.5%), the latter comprising 10% of the total mixture. The mixture was then heated to 900.degree. F. (482.degree. C.), degassed and consolidated by compaction to full density in a canister. The billet was then removed from the canister and extruded at 850.degree. F. (454.degree. C.) at a 29-to-1 ratio, followed by solution heat treatment, stretching in the direction of extrusion to a 5% length increase and aging for 16-100 hours. Different samples were aged at different temperatures.

Tensile properties and impact toughness values were then measured on specimens from the samples as well as samples prepared in the identical manner but without the inclusion of the pure aluminum powder. The tensile tests were performed on round specimens 0.25 inch (0.64 cm) in diameter with a gage length of 1.0 inch (2.54 cm), taken from the extrusion edge of the sample, using standard ASTM testing procedures. Longitudinal tests were performed on both center and edge samples, the latter representing the short transverse edges of the extrusion.

Table 1.1 below lists yield strengths and elongations measured in the longitudinal direction for the various aging temperatures, most entries indicating several trials. An average value for each aging temperature is shown graphically in FIG. 1 (edge results) and FIG. 2 (center results), where the 300.degree. F. values are for 16 h aging time.

It is evident from these figures that some loss in strength resulted from incorporating the pure aluminum, while the elongation on the average was approximately unchanged.

Table 1.2 lists yield strengths and elongations measured in the transverse direction for the same aging temperatures. Samples from two different locations were taken for each aging temperature, as shown in the table. Averages for each pair are shown graphically in FIG. 3.

Once again, a loss of yield strength is observed while elongation is generally unchanged.

Impact values were determined in the longitudinal direction by Charpy impact tests, using 10 mm square, V-notched specimens at ambient temperature, the notches running transverse to the direction of extrusion. Multiple specimens from both the center and edge of the extruded samples at the extrusion edge were tested. The results are shown in Table 1.3. Averaged values are shown graphically in FIG. 4, where the 300.degree. F. values are for 16 h aging time.

It is clear from these figures that the impact toughness is consistently higher in the samples containing the added unalloyed aluminum.

Fracture toughness values (K.sub.1A) in the short transverse direction were provided by the stress intensity factor measured by applying tension in the short transverse direction at right angles to a machined notch extending into the sample in the extrusion direction. The extrusions used were 0.5 inch (1.3 cm) thick and 1.5 inch (3.8 cm) wide. The stress intensity results at the various aging temperatures (three trials each) are shown in Table 1.4, and the averages depicted graphically in FIG. 5.

The samples containing the added unalloyed aluminum are consistently superior.

Stress corrosion cracking thresholds were determined in the same manner, except that the specimens were subjected to controlled drips of 3.5% aqueous sodium chloride solution during the test, which lasted three weeks. The thresholds at various aging temperatures are shown in Table 1.5.

Again, the results for the samples containing the added unalloyed aluminum are consistently higher.

While the data above indicate an increase in toughness at the expense of strength, FIGS. 6 and 7 demonstrate that the overall result, i.e., the combination of strength and toughness at both center and edge of the extrusion, measured longitudinally, is superior for the product containing the added unalloyed aluminum. The values for the points in these graphs are given in Tables 1.6 and 1.7, each of which cover a range of aging conditions in terms of both temperature and time. The ranges extend from mild conditions through optimum conditions (resulting in peak properties) and beyond into overaging with detrimental effects. Since overaging is both detrimental and wasteful of both energy and processing time, the results plotted for comparison in the figures are those corresponding to aging conditions increasing to and including the optimum but not beyond. In FIG. 6 and Table 1.6, the optimum is generally between 300.degree. F. at 40 hours and 340.degree. F. at 100 hours, whereas in FIG. 7 and Table 1.7, the optimum is 300.degree. F. at 40 hours. The figures show a general improvement in the combination of strength and toughness for both center and edge up to these conditions, for the product containing the unalloyed aluminum.

A composite product was prepared according to the procedure of Example 1, using, however, an aluminumlithium alloy containing 3.49% Li, 1.25% Cu, 0.74% Mg and 0.12% Zr (designated herein as 1614).

The test procedures of Example 1 were applied. Tensile properties measured in the longitudinal direction at the center of the extrusion for different aging temperatures are listed in Table 2.1 below and shown graphically in FIG. 8.

Tensile properties measured in the longitudinal direction at the side edge of the extrusion are listed in Table 2.2 and the averages shown graphically in FIG. 9.

Tensile properties measured in the transverse direction are listed in Table 2.3 and the averages shown graphically in FIG. 10.

Charpy impact test results, following again the procedure of Example 1, are listed in Table 2.4 and the averages shown graphically in FIG. 11.

Collectively, the data in these tables and figures indicate a consistent large improvement in toughness in the samples containing the added unalloyed aluminum, with only a small decrease in strength, and in some cases, no decrease at all. That the overall result is an improvement is confirmed by FIG. 12, which is a plot of data taken from Tables 2.1, 2.2 and 2.4.

To demonstrate that the toughness increase in these alloys is not simply a result of the decreased lithium content when unalloyed aluminum is added, the Charpy impact values are plotted as a function of lithium content in FIG. 13 for the four alloys covered by Examples 1 and 2. These values all represent the data from aging at 250.degree. F. for 16 hours. While toughness does decrease with increase lithium content, the plot demonstrates that at the same lithium level, the products containing the added unalloyed aluminum are tougher than those composed of the straight alloys. This is evidenced by the vertical distance between the dashed and solid lines. Similarly, a given lithium content in a composite product containing added unalloyed aluminum produces the same toughness as a straight alloy with a higher lithium content--compare alloy 1611 with the composite of alloy 1614 and 10% added aluminum (horizontal distance between dashed and solid lines). Plots of the data for the other aging temperatures show the same types of differences.

The foregoing description is offered for illustrative purposes only. Numerous modifications and variations of the procedures and materials described above, while still falling within the spirit and scope of the invention, will be readily apparent to those skilled in the art.