High Strength Aluminum Extrusion Alloy

The present invention relates to high strength aluminum base alloys and particularly to wrought high strength aluminum base alloys produced in extruded or hot-rolled plate form, which are well adapted for welding operations in further fabrication steps, wherein the strength properties are retained at high values, even exceeding about 40 ksi for the yield strength of extruded products and 30 ksi for hot rolled plate, without any necessity for interposing special heat treatment steps.

The alloy compositions in accordance with this invention have been shown to meet the specified requirements and have furthermore surprisingly provided excellent solutions to the problems and disadvantages consistently associated with previous attempts to use prior art alloy compositions for such purposes. Such attempts were accompanied by inordinate loss of strength properties on welding, and/or a requirement after welding for special heat treatment and artificial aging steps to recover at least part of the lost strength properties, and/or an excessive tendency to undergo weld failures, such as under-bead weld cracks, and/or susceptibility to various types of corrosion, such as stress corrosion or exfoliation corrosion, which might result in excessive failures in service.

Thus, at least one of the foregoing disadvantages, and usually several of them is encountered in attempts to weld previously known high-strength aluminum base alloys which include magnesium, silicon and copper as essential components, as occurs in such attempted use of AA Alloys 6066 and 6351, and of alloy compositions as disclosed in U.S. Pat. Nos. 3,498,221 and 3,935,007 and in British Pat. No. 1,383,895, also described in Journal of Metals (September, 1976), pages 15-18, which in general were formulated to accomplish purposes differing from the present objectives.

Accordingly, it has been a principal object of the present invention to provide improved high strength aluminum base alloy compositions characterized by the capability of being welded readily without undergoing an excessive decrease in strength properties.

A further object has been the provision of such alloy compositions characterized by the capability of being formed by extrusion or by hot-rolling procedures.

Another object has been the provision of such alloy compositions comprising a defined range of magnesium content in conjunction with other essential elements in proportions required to achieve the desired functional characteristics.

A further object has been the provision of such alloys characterized by heat-treatability and natural aging characteristics.

Another object has been to provide such alloy compositions readily suitable for conversion to wrought products.

Further objects and advantages of the present invention will be apparent from the following detailed description.

In accordance with the present invention, it has now been found that the above objects can be advantageously obtained by the provision of alloy compositions consisting essentially of 0.9-1.5% magnesium, 0.4-0.8% silicon, and 0.9-1.5% copper, wherein the copper must not exceed the sum of magnesium and silicon, and the silicon must not exceed the sum of 0.58 .times. percent Mg + 0.25 .times. percent (Mn + Fe). One or more of the group Cr, Mn, and Fe, is usually present, particularly in extrusion alloys, at a content of about 0.05-0.4% and the balance, other than added elements and usual impurities, is essentially aluminum. The added elements may be one or more of the following at the stated weight percentage ranges: 0.01-0.2 zirconium, 0.01-0.2 titanium, 0.01-0.2 vanadium, 0.01-0.4 cobalt, and 0.01-3.5 nickel. As will be discussed later, such additional elements are beneficial in the strengthening and stabilization of the wrought structure induced by hot working, through the formation of fine dispersed intermetallic precipitates. Other elements may be present as impurities in percentages up to about 0.05% each and totalling less than 0.15%, without adversely affecting the desired properties. In a preferred embodiment, the alloys of the present invention may contain 1.0-1.5% Mg, 0.4-0.7% Si, 1.0-1.5% Cu, and 0.2-0.4% of one or more additive elements selected from the group consisting of Mn, Fe and Cr, and the balance essentially aluminum.

Alloys in accordance with this invention have enabled the attainment in articles, after thermal treatments met in welding, of yield strengths of over 30 or 40 ksi, without requiring processing other than natural aging. This represents a major advance over prior art practices and accomplishments, for example as summarized in Aluminum, Volume 3, American Society for Metals 1967), Chapter 12, especially, pages 407-415. In contrast, temper-rolled sheets of Alloy 5456, the highest strength composition in the non-heat-treatable 5000 series of aluminum alloys display a loss in strength properties after welding to values of yield strength and tensile strength characteristic of annealed metal. While certain heat-treatable Al base alloys could be chosen which displayed better retention of high strength values after welding, these gave rise to other problems and disadvantages such as cracked or otherwise unsatisfactory welds, inadequate corrosion resistance, or the need for special heat treatment procedures.

In order to facilitate a comprehensive study aimed at establishing improved alloy compositions for this purpose, a simulated welding test was developed which would accurately indicate the strength properties resulting on the application of the welding procedure. This was accomplished by forming a single pass edge weld on each face of two plate halves 0.25 inch thick of 6061-T6 aluminum alloy, recording time-temperature curves for measured times up to 90 seconds and at a series of distances on each side of the weld. Hardness, tensile strength and yield strength values, and microstructure were determined for these points. This study established that the effects of low energy (corresponding to single pass) MIG welding (by electric arc under inert gas, using filler wire of alloy 5356 at rates of 15 and 30 inches per minute) could be reproduced by immersing a plate of sample alloy, 0.060 inch thick, in molten salt at 750.degree. F. for 10 seconds and cooling in still air, and high energy welding (corresponding to multi-pass or repair conditions) could be reproduced by treatment in molten salt at 750.degree. F. for 20 seconds.

The above simulated welding test was found to accomplish a loss in hardness and strength properties and a change in microstructure corresponding to the changes determined to occur within a zone about 0.3 to 0.4 inch from the weld bead centerline. Thus, the initial tensile strength decreased from about 50 ksi to about 30 in the 10 second treatment and to about 25 in 20 seconds; the yield strength was lowered from 45 to about 20 in 10 seconds and to about 15 in 20 seconds. The study of microstructure established that the above zone, within which the tensile fractures during strength evaluation tests occurred, was characteristic of an overaged region containing coarsened particles of precipitated Mg.sub.2 Si. Neither the welded plates nor the samples treated in molten salt displayed any natural aging after storage, that being precluded by the completeness of the precipitation during the treatment.

The availability of the above-described simulated welding test enabled the completion of a series of screening tests of varied aluminum alloy compositions, the results of which indicated that the desired objectives might well be attainable through the enhancement of aluminum-magnesium-silicon alloys by increasing their initial strength properties, while providing against undue loss of strength during welding, at the same time improving the resistance to over-aging, and through the simultaneous imparting of a natural aging response, which would occur after the welding operation. As substantiated in the following specific examples, the objectives were attained by the compositions specified herein, within the determined ranges of the stated proportions and with strict observance of the maximum permissible limit of silicon in proportion to the content of magnesium, iron and manganese, and providing a copper content not in excess of the sum of magnesium plus silicon.

The stated composition limits, as established by a comprehensive series of experiments, basically are those which have been found to provide the desired high strength and other essential properties, including weldability without the undue loss of strength, and to display satisfactory resistance to stress corrosion and to corrosion by various environments which might be encountered during use.

The effective range of magnesium content is such as to provide increased initial strength properties effected through the presence of finely dispersed Mg.sub.2 Si particles, as well as adequate retention of such properties through the welding cycle. Such effects are not obtained below the specified minimum content of Mg, while amounts of Mg exceeding the maximum are disadvantageous in increasing the tendency toward overaging during the welding treatment, with consequent undue losses in strength properties. Furthermore, the use of over 1.5% Mg in the alloy is disadvantageous, tending to effect a decreased resistance to stress corrosion. However, an excess of Mg in relation to Si is preferred, as tending to inhibit over-aging and to promote the recovery of strength through natural aging.

The useful range of copper was established as between 0.9 and 1.5% as these proportions provided substantial increases in the initial strength properties, particularly in yield strength and tensile strength, increased the retention of strength during the welding operation, and imparted gains in strength through natural aging following welding. These features were not displayed to any substantial extent by compositions containing less Cu than the minimum. At above the maximum of 1.5% Cu, the strength retention effect was less marked, and the tendency toward deteriorating effects due to environmental corrosion was generally increased.

The above effects in the beneficial range appear to be brought about by the introduction of additional phases and the substantially uniform distribution of the fine hardening intermetallic precipitates throughout the metal. A synergistic effect thereof is the precipitation of Mg.sub.2 Si as tiny needles or rods rather than as large plates or grains found to occur in compositions containing insufficient proportions of copper.

The specified range for the optional added elements, particularly manganese, iron, and chromium likewise states the limits within which the most effective initial strength increase and strength retention during welding are obtained, as the use of less than minimal proportions presents no substantial benefit and the presence of proportions higher than the maximum are correspondingly less effective and may introduce disadvantageous tendencies toward decreased corrosion resistance and impaired natural aging benefits.

Similar effects exist with respect to departures from the specified range of silicon content, where it is also critical to observe the limitation that the Si content must not be more than corresponds to the sum of 0.58 .times. Mg content + 0.25 .times. content of (Mn + Fe). This limitation corresponds to the provision of excess magnesium over that required to combine with silicon to form precipitated silicide, which has been indicated to produce the most advantageous combination of desired properties, particularly of high initial strength, retention of strength during welding, and increase in strength by natural aging following the welding procedure. The presence of excess Si has been found to be notably disadvantageous with respect to the latter two of the above features. In contrast, the effect of excess magnesium is most evident under high energy welding conditions, where subsequent natural aging results in the most significant recovery of strength properties.

Compositions in accordance with the invention and comparison alloys were melted, fluxed by treatment with chlorine gas for 5 minutes or with a nitrogen-dichlorodifluoromethane mixture for 10 minutes, and cast as 5 pound Durville ingots, using a pouring temperature of 1320.degree. F. The ingots, after homogenization at 930.degree. F. for 24 hours, were cut into 4 inch square sections, 0.75 inch in thickness. These sections were hot rolled at 930.degree. F. in a single pass to a thickness of 0.15 inch and water quenched. Such sections, requiring no solution treatment before aging, could be used to estimate the press quench effect which might be expected in commercial scale extrusions.

A portion of the hot rolled plate was cold rolled to a thickness of 0.060 inch, solution annealed, water quenched, and aged for 18 hours at 320.degree. F. to develop peak aging properties, denoted as -T6 temper.

Another portion of the above hot rolled plate was tested after being aged for 18 hours at 320.degree. F., denoted as -T5 temper.

Tests on Al alloyed with 0.36 to 1.0% Mg and 0.25 to 1.5% Si at -T6 temper, prepared as described above, resulted in measured values of yield strength (Y) -- tensile strength (T) -- elongation (E), respectively, of 12 ksi -- 18 ksi -- 13 initially for an alloy of 0.36% Mg, 0.25% Si, and balance Al, and 4 -- 13 -- 28 after immersion for 10 seconds at 750.degree. F. (simulated welding test). The corresponding values for an alloy of 0.71 Mg, 1.5 Si, and balance Al were 40 -- 44 -- 6 and 14 -- 22 -- 14, respectively. Ternary alloys of these elements in proportions between the above limits yielded intermediate values, with losses after the welding test ranging from 8 to 26 ksi in yield strength and from 5 to 22 ksi in tensile strength. Similar values of strength losses also resulted with similar alloys, each containing a small addition of Sn, Cd, Mn, Co, V, or Cr.

This series also included three comparison Al alloys containing Mg, Si, and Cu, in proportions not in accordance with the present invention, which yielded test results similar to the above, as shown in Table I.

In contrast, the following examples will be seen to substantiate the attainment of the objectives of the present invention by the provision of alloy compositions in accordance therewith.

Alloy A, containing 1.38% Mg, 0.67% Si, 1.41% Cu, 0.39% Mn, balance Al (all percentages being by weight, unless otherwise indicated), tested at -T5 temper, displayed the following tensile properties initially, after 10 seconds at 750.degree. F., after 20 seconds at 750.degree. F., and following natural aging for 2 weeks after each treatment, shown in Table II.

Thus, the simulated low energy welding test caused a substantially smaller loss in tensile properties than resulted in the previous tests. Furthermore, natural aging following the high energy test (20 seconds) resulted in restoring much of the lost strength.

Comparison alloys having the following compositions not in accordance with the invention were subjected at -T5 temper to the same tests as used in the previous example.

Test results were as follows:

In contrast, significantly improved test results were obtained with alloys in accordance with the invention, included in Table IV.

Three commercial alloys were selected for direct comparison with alloys in accordance with the invention, yielding test results, as listed in Table V.

Parallel test results listed in Table VI for three alloys in accordance with the present invention substantiate their significantly superior results.

The comparisons afforded by the above two examples show that preferred alloys in accordance with this invention, after low energy welding and natural aging, are substantially superior to the commercial alloys. After high energy welding and natural aging, the present alloys display over twice the strength of 6351 and have tensile properties comparable to those of alloys 7006 and 7039, but without their operational disadvantages.

This example substantiates the disadvantageous effects which occur when the silicon is present in the alloy in an excess amount, such as to be greater than can be precipitated as a silicide of magnesium or other metal. The alloys listed in Table VII (a) were prepared as in the preceding examples and the test results are summarized in Table VII (b), the "Initial" values having been measured on samples prepared at T5 temper.

Thus, the present invention provides aluminum base alloys of high strength, capable of retaining adequate strength after being subjected to operations at elevated temperatures, as in fusion welding processes, corresponding to retained yield strength of about 40 ksi or higher for extruded products or somewhat less for hot rolled plate. Strong crack-free welds are consistently and readily obtainable with the present alloys and they show excellent formability for conversion to products having good resistance to stress corrosion and other corrosive influences. Accordingly, these alloys are well adapted for use in varied commercial fields, as in automotive vehicle bodies and components, such as for tanks and containers.

The above description and specific examples substantiate the attainment of the specified objectives of this invention in accordance with the alloy compositions and preferred treatment procedures set forth. It will be understood by those skilled in the art that various modifications may at times be employed advantageously in the illustrative examples, within the scope of the appended claims.