IL45569A - Thermal treatment of articles composed of aluminum based alloys - Google Patents

Description

rmioi'oa O'SDHDH o-»ssna 'Bin s»o pan niDDiao Ihermal treatment of articles composed of aluminum based alloys ALUMINUM OOMPANT OF AMERICA C. 43617 The present invention relates to a method of thermally treating articles containing an alloy based on aluminum.

The precipitation hardened condition of aluminum alloy 7075 f referred to as the T6 condition of alloy 7075 , has not given sufficient resistance to corrosion under certain service conditions, The T73 temper improves the resistance of precipitation hardened 7075 alloy to stress corrosion cracking^ although it decreases strength significantly vis-a-vis the T6 condition.

According to the present invention there is provided a method of thermally treating an article composed of an alloy consisting essentially of aluminum, l\. to Q% zinc, 1. 5 to 3 · magnesium, 1 to 2 * 5% copper, and at least one element consisting of 0 c 0 to 0 o 3 chromium, 0.1 to 0 . 5$ manganese, or 0 . 05 to 0 , 3$ zirconium; which method includes the steps of solution heat treating the article, then precipitation hardening the article at 175 to 325°F, then subjecting the article to a heat treatment for a time and at a temperature within the perimeter ABCD of Figure 4, and then again precipitation hardening at 175 to 325°F.

In the accompanying drawings : Figures 1-3 are transmission electron micrographs of sections in a plate of aluminum alloy 7075 . The distance equivalent to 0 d micron is indicated on the micrographs.

The metal surfaces reproduced in the micrographs all were perpendicular to the direction of rolling of the plate. ^ Figure and stress relieved condition referred to as the W51 condition.

Figure 2 shows the prior art precipitation hardened condition referred to as the T6 condition.

Figure 3 shows the prior art stress corrosion cracking resistant condition referred to as the T73 condition.

Figure H is a graph showing characteristics of the invention.

The alloys in the present invention have a composition containing *J to 85? zinc, 1.5 to 3.55? magnesium, 1 to 2.55? copper, and at least one element consisting of chromium at 0.05 to 0.3%, manganese at 0.1 to 0.5%, or zirconium at 0.05 to 0.351. The balance of the composition is essentially aluminum.

Alloys designated 7075 by the aluminum industry are preferred for the present invention and have a composition containing 5·1 to 6.15? zinc, 2.1 to 2-952 magnesium, 1.2 to 2.05? copper, 0.18 to 0.35% chromium, 0.305? maximum manganese, 0.¾05S maximum silicon, 0.505? maximum iron, 0.205? maximum titanium, others each 0.055? maximum and others total 0.155? maximum, balance aluminum.

The alloys used in the present invention may also contain one or more of the group of grain refining elements including titanium at 0.01 to 0.25? and boron at 0.0005 to 0.0025?. These elements serve to produce a fine grain size in the cast form of the alloy. This is generally advantageous to mechanical properties.

In addition, it may be helpful to add 0.001 to 0.0055? beryllium for the purpose of minimizing oxidation at times when the alloy is molten.

Up to 0.5$ iron can be tolerated, and the silicon should not exceed 0.-$, in order to avoid the formation of any substantial amount of the intermetallic compound Mg2Sii A preferred heat treatment according to the present invention for obtaining improved stress-corrosion resistance is to immerse alloy, as above defined, in the precipitation-hardened, T6 condition into molten metal for a time and temperature within the perimeter of the quadrilateral EPGH irj Figure i|, then precipitation harden again, In its broader aspects, a T6 condition may be obtained by precipitation hardening solution heat treated alloy at 175 to 325°P. Typical conditions may be: a. For alloys containing less than 7 « 5$ zinc, heating a solution heat treated article to 200 to 275°F and holding for a period of 5 to 30 hours; b, For alloys containing more than 7 * zinc, heating a solution heat treated article to 175 to 275°F and holding for a period of 3 to 30 hours 0 A usual practice for obtaining the T6 condition is obtained by heating a specimen for 2I4. hours at 250°F in a circulatory-air furnace.

According to another preferred embodiment of the invention, the alloy is solution heat treated, then precipitation hardened at a temperature of 175 to 325°F, then subjected to a time and temperature within the perimeter ABCD,- more preferably EFGH, and then again precipitation hardened for a time of 2 to 30 hours at a temperature of 270 to 320°F.

The article of J, T, Staley et al. entitled "Heat Treating Characteristics of High Strength Al-Zn-Mg-Cu Alloys With and Without Silver Additions" appearing at pages 191 to 199 in the January, 1972 issue of Metallurgical Transactions, published by ASIVAIME, shows that solution heat treat qJS^ich rate, the lapse of time between the solution heat treat quench and the beginning of heating for precipitation hardening, and the heating rate for precipitation hardening may affect the maximum yield strength obtainable in 7075 aluminum alloys.

It is intended that, within the concepts of the present invention, the teachings of Staley et al. be used in the present invention for optimizing results. Thus, it may be advantageous for increasing strength to immerse specimens, which have had their solution heat treatment quench, for example, 1-1/2 years ago, into molten Wood's metal according to the invention.

Referring now to Figures 1 to 3» transmission electron micrographs of various microstruetures important for consideration of the present invention are presented. All of Figures 1 to 3 were taken from a single l/ij.-inch thick 7075 aluminum alloy plate of composition A in Table I. Figures 1 to 3 are microstructures of prior art conditions of 7075 aluminum. In Figure 1, an example of the W l solution heat treated condition is given. A W5l solution heat treated micro-structure is obtained in 7075 aluminum alloy plate by heating to 900°F and then quenching in water at room temperature.

The plate material is then stretched to from 1-1/2 to 3 permanent set for stress relief. This gives the micro-structure shown in Figure 1, including E-phase particles of Al-Mg-Cr precipitate, matrix regions R of single phase aluminum solid-solution material, grain boundaries B and dislocations D. The mottling effect appearing in the matrix region of Figure 1 is an artifact of the action of the thinning solution used in preparing thinned material for transition electron microscopy.

Table I.

Composition of Alloys, in Weight-^.

Figure 2 shows the 7075 alloy material of Figure 1 after it has been brought to the T6, in particular the T65l, temper by heating 1 material in a circulatory-air furnace for 2l\. hours at 250°F, E-phase remains substantially un-changed. Dislocations D and a grain boundary B are shown. Now in the matrix there has appeared many small black dots ; these are referred to as G.P, zones and are clusterings of magnesium and zinc atoms generally in the ratio two zinc atoms for each magnesium atom, Figure 3 shows a specimen taken from the same plate of Figures 1 and 2 in the T73 condition, which is produced from W l material by heating in circulatory-air furnaces for, first, 2l| hours at 250°F and, second, 8 hours at 350°F. Grain boundary precipitate 10 has appeared, and the G.P. zones have grown to greater size. The G.P. zones have begun to exhibit crystallinity by giving rise to X-ray diffraction patterns and are referred to by those in the art as M' and M phase.

Solution potential studies Indicate that the M' and M phases contain some copper atoms. It is believed that the G.P. zones progress toward crystallinity by becoming first ' phase, which is still partially coherent with the matrix crystal structure. The M' phase then changes to phase, which has a crystal structure different from the matrix. It is believed also that the progression through the M' phase to the M phase makes the original G.P. zones increasingly anodic with respect to the matrix and that the resulting anodic particulate matter in the matrix protects against stress-corrosion crack-ing.

Further illustrative of the present invention are the following examples.

Examples 1 to 8 For each example, two tensile blanks of dimensions 3/8 inch by 3/8 inch by 2-1/2 inches were cut from a single lot of 2-1/2 inch thick 7075-T651 (metallurgical history as described for Figure 2) alloy plate such that their lengths were in the short-transverse derection, i.e., in the direction perpendicular to the surface of the plate.

Table II Times and Temperatures in Wood's Metal for Examples 1 to 8 and the Coordinates of Points A to H.

Example No . , Time, Temperature, or Point min. op Table II (continued) The chemical composition of the alloy is as presented for alloy B in Table I. The tensile blanks for each example were immersed in molten Wood's metal of composition 50% bismuth, 25% lead, 12.5% tin and 12.5¾ cadmium. The immersion temperatures and times are presented in tabular form in Table II and are plotted in Figure 4. Following immersion in the molten Wood's metal, the cooled specimens were then precipitation hardened by heating them in a circulatory-air furnace for a time of 24 hours at 250°F. In each of Examples 1 to 8, a tensile blank was machined to a 0.125 inch diameter tensile bar for exposure to 3-1/2$ sodium chloride solution by alternate immersion at a stress level of 42 ksi according to Military Specification MIL-A-22771B. The specimens were held until failure with successive immersions for 10 minutes in the salt solution followed by 50 minutes in air. The number of days until failure under such treatment is provided in Figure 4 above the time-temprature point for each Example. The remaining blank of each example was tested for yield strength. The yield strength data for Examples 1 to 8 are presented in Figure 4, below the time-temprature points, in terms of percentage of a yield strength of 62.3 ksi for the T651 condition.

Further illustrative of the preferred embodiment of the invention wherein the second precipitation hardening step is carried out for 2 to 30 hours at a temperature of 270 and 320°F are the following examples: Examples 9 to 14 o Prcedure was as described for Examples 1 to 8, except that all examples utilized an immersion in molten Wood's metal for 90 seconds at 445°F, before the second precipitation hardening. Other parameters and results were as presented in Table III. Examples 9 to 11 form one group of comparative examples characterized by 3 hours at temperature in the second precipitation hardening step, with Examples 12 to 14 forming a second group charactericed by 24 hours at temperature in the second precipitation hardening step. The superior strength and corrosion resistance obtained when the second precipitation hardening was done for 2 to 30 hours at 270 to 320°F will be apparent from comparison of the examples within the groups.

Table III Parameters and Data for Examples 9 to 14, I Aluminum Alloy 7075-T651 in Molten Wood's M At 445°F, Followed by a Second Precipitati Time (Hours) & Example Temperature (°F) of Tensile Strength, Yi No. Second Precipitation Hardening 9 3hrs./250°F 67.2 10 3hrs./275°P 68. 8 11 3hrs. /300eF 66. 8 12 24hrs/250°F 70. 5 . 13 24hrs./275°F 69. 5 14 24hrs. /300°F 69.6 The following definitions hold herein: a. The term "ksi" is equivalent to kilipounds per square inch. b. Wherever percentages are given, reference is to % by weight, unless indicated otherwise. c. The initials "G.P." stand for Guinier-Preston.

It will be understood that the above description of the present invention is susceptible to various modifications, changes, and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

Claims (2)

1. 45569/2 CLAIMS : 1 ≠ 1 , A method of thermally treating an article composed of an alloy consisting essentially of aluminum, I4. to 8% zinc, 1.5 to 3.5% magnesium, 1 to 2.5% copper, and at least one element selected from the group consisting of 0, 05 to 0.3% chromium, 0. 1 to 0.5% manganese, and 0. 05 to O. yfo zirconium, said method comprising the steps of solution heat treating said article, then precipitation hardening the article at 175 to 325°F, then subjecting said article to a heat treatment for a time and at a temperature within the perimeter ABCD of Figure 4, and then again precipitation hardening at 175 to 325°F.

2. A method according to claim 1, wherein the step of again precipitation hardening is performed at 270 to 320°F for 2 to 30 hours. 3 · A method according to claim 1 or 2 , wherein the step of subjecting is for a time and temperature within the perimeter EPGH of Figure !+. * A method according to any of the preceding claims wherein the step of subjecting comprises immersing said article in liquid having said temperature. 5 · A method according to claim ij., wherein said liquid is molten metal. 6.. A method according to claim 1, of thermally treating an article composed of an aluminum alloy, substantially as hereinbefore described with reference to the Examples, 7· A thermally treated article composed of an aluminum alloy whenever made by the method of any of claims 1 to 6.