DE2446933A1 - PROCESS FOR HEAT TREATMENT OF OBJECTS - Google Patents

Description

1 BERLIN Auguete-Viktoria-Strafle Pat. Dr. Ing.Ruschks Pat. Dipl.-Ing. Olaf Ruechke

Telegram address: Quadratur Berlin TELEX: 183786

Dr. RUSCHKE &. PARTNER PATENT LAWYERS

BERLIN - MÖNCHEN

8 MUNICH

PienzenauerstraSe 2 Pat.-Anw. Dipl.-Ing. Hans E. Ruschke

Phone: 089/98 03

Telegram address: Qudadratur Munich TELEX: 522767

Oct. 1, 1974

A 1471

ALUMINUM COMPANY OF AMERICA

ALCOA BUILDING, PITTSBURGH, STATE PENNSYLVANIA,

UNITED STATES.

Process for the heat treatment of objects

The present invention relates to a method for

Heat treatment of objects made of an aluminum alloy. !

The state of precipitation hardening of an aluminum alloy - 7o75, which is referred to as the T6 temper of alloy-7o75, has under certain stress conditions insufficient resistance to corrosion

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ben. The T 73 condition improves the resistance of a precipitation hardened 7o75 alloy to stress corrosion cracking, although it considerably reduces the strength compared to the To state.

In accordance with the present invention there is provided a method of heat treating an article which consists of an alloy consisting essentially of aluminum, 4 to 8 % zinc, 1.5 to 3.5% magnesium, 1 to 2.5% copper and at least consists of an element which comprises from 0.05 to 0.3% chromium , 0.1 to 0.5% manganese or 0.05 to 0.3 % zirconium. The method includes solution heat treating the article, subsequent precipitation hardening of the article at 8o to 163 ⁰ C, then exposing the article to a time and temperature within the perimeter ABCD in Fig. 4, and then again to precipitation hardening at 8o to 163 ° C a.

The invention is explained in more detail below using exemplary embodiments with reference to the accompanying drawings.

In the drawing show:

Figures 1-3 Electron micrographs of sections in a

Aluminum alloy plate-7o75. The 0.1 micron spacing is in the recordings registered. The metal surfaces shown there were all

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perpendicular to the rolling direction of the plate.

Fig. 1 shows a known, solution-annealed and stress-relieved State, which is referred to as the W 51 state,

Fig. 2 shows a known precipitation hardened condition which is called the Τβ-state,

Fig. 3 shows a known stress corrosion crack resistant State, which is referred to as the T 73 state,

Figure 4 is a diagram showing the features of the invention.

The alloys according to the invention have a composition of 4 - 8 % zinc, 1.5 to 3.5% magnesium, 1 - 2.5 % copper and at least 1 element consisting of chromium from 0.05 to 0.3 % manganese from 0 0.1 to 0.5% or zirconium from 0.05 to 0.3 %. The remainder of the composition is essentially aluminum.

The alloys designated by the aluminum industry as 7075 are preferred for the present invention and have a composition of 5.1 to 6.1 % zinc, 2.1 to 2.9 %. Magnesium, 1.2 to 2 % copper, 0.18 to 0.35 % chromium, maximum

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0.3o % manganese, a maximum of 0.4o % silicon, a maximum of 0.5o % iron, a maximum of 0.2o % titanium, others each a maximum of 0.05% and a maximum of 0.15% in total, the remainder being aluminum.

The alloys used according to the invention can also contain one or more of the group of elements that refine the grain, including titanium from 0.1 to 0.2 % and boron; from 0.05 to 0.02 %. These elements create a fine grain size in the cast structure of the alloy. This is generally advantageous for the mechanical properties.

In addition, it may be useful ^{to add o, oo1 to o, oo5 r} /> beryllium to minimize oxidation at time when the alloy is molten.

Iron and silicon are generally considered impurities available.

Up to 0.5 / ο iron can be tolerated and the silicon content should not exceed 0.4% in order to avoid the formation of a substantial amount of the intermetallic compound Si.

A preferred heat treatment according to the invention to achieve improved resistance to stress corrosion is

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means the alloy as defined above in the T6 precipitation hardened state in molten metal to immerse for a time and temperature within the circumference of the rectangle EFGH in Fig. 4 and then again perform precipitation hardening.

A T6 condition can be largely defined by precipitation hardening a solution-annealed alloy at 8o to 163 ° C. Typical conditions can be:

a) for alloys containing less than 7.5 % zinc, heating a solution-annealed object to 93 to 135 C and holding it for a period of 5 to 30 hours;

b) for alloys containing more than 7.5% of zinc, heating a solution heat treated article to 8o to 135 ⁰ C and holding for a period of 3 - 3o hours.

Usually the T6 condition is obtained by heating a sample in an air-circulating oven at 121 ° C for 24 hours.

According to another preferred embodiment of the invention, the alloy is solution heat treated, then at a Precipitation hardened temperature of 80 to 163 ° C, then a time and temperature within the ABCD range preferably EFGH suspended and then again for a period of 2 -

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Precipitation hardened for hours at a temperature of 132 to 100 ° C.

The "article by JT Staley," Heat Treating Characteristics Of High Strength Al-Zn-Mg-Cn-Alloys with and without Silver Additions, "which appeared on pages 191-199 in the January 1972 issue of" Metallurgical Transactions, published by ASM / AIME, shows that the rate of heating / quenching, the time lapse between heating / quenching and the start of heating for precipitation hardening, and the heating rate for precipitation hardening can affect the maximum yield strength attainable in 7075 aluminum alloys. It is within the gist of the invention that it is intended to use the teachings of Staley to optimize results. Thus, for the purpose of improving the strength, it can be advantageous to immerse samples which have undergone their solution heat treatment and quenching treatment, for example 11/2 years earlier, in molten wood metal according to the invention.

Referring to Figures 1 to 3, there are shown electron images of various microstructures which are essential to the practice of the invention.

All Figs. 1-3 were made from a 6.3 mm thick 7075-AIu-

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Minium alloy plate of Composition A taken in Table 1. Figures 1 through 3 are known state microstructures of a 7075 alloy. 1 shows an example of the W51 condition (solution annealed). A W51 microstructure is obtained in a 7o75 aluminum alloy sheet by heating to 482 ⁰ C and then quenched in water at room temperature. The sheet material is then stretched to 1.5 to 3% permanent elongation for the purpose of stress relief. This gives the microstructure shown in Fig. 1, which has E-phase particles of Al-Mg-Cr precipitate, matrix regions R of single-phase aluminum solid solution, grain boundaries B, and dislocations D. The veining effect which appears in the matrix area in Fig. 1 is subject to the action of the diluting solution which has been used to prepare diluted material for super microscopy.

Table I.

Composition of the alloys in% by weight.

element alloy A. B. Cu 1.45 1.63 Fe 0.19 o.3o Si o.o9. 0.12. Mn o.o2 o.o7 Mg 2.4o 2.48 Zn 5.92 5.68 ' Ni 0.00 0.00 Cr 0.18 0.19 Ti o.o2 ο. o5 Be o.oo1 O.OO1.

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Fig. 2 shows the 7o75 alloy according to Fig. 1, having been placed on the T 6, in particular T-651 state by heating the material in a forced air oven W51 for 24 hours at 121 ⁰ C it is1 ;. The Ε phase remains essentially unchanged. Dislocations D and a grain boundary B are shown. Here many small black dots have appeared in the matrix, which are referred to as GT zones and are agglomerations of magnesium and zinc atoms in general in a ratio of 2 zinc atoms per magnesium atom.

3 shows a sample which has been taken from the same plate according to FIGS. 1 and 2 in the T 73 state, which is made from W 51 material by heating in air circulation ovens, initially for 24 hours to 121 ^° C. and then are generated ^{at 177 °} C. for 8 hours. A grain boundary precipitate 10 has appeared and the GP zones have grown to a larger size. The GP zones have begun to demonstrate crystallinity by giving X-ray diffraction patterns and are referred to by those skilled in the art as M ¹ and M phases. Solution potential studies indicate that the M ¹ and M phases contain some copper atoms. It is assumed that the GP zones progress to crystallinity by first becoming the M ¹ phase, which is still partially connected to the matrix crystal structure. The M 'phase then changes to the M phase, which has a crystal structure that of

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the matrix is different. It is also believed that progressing through the M ¹ phase to the M phase renders the original GP zones increasingly anodic with respect to the matrix and that the resulting anodic particles in the matrix protect against stress corrosion cracking.

The invention is further illustrated by the following examples explained.

Examples 1 to 8

For each example, two tension bars of 9.5 mm 9.5 mm χ were used 63.5mm from one piece of 63.5mm thick 7o75-T-651 alloy plate (metallurgical development as described in Fig. 2) cut so that their lengths in the direction of Are short side, i.e. in the direction perpendicular to the surface of the plate.

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- 1ο -

Table II

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Times and temperatures in Woodmetall for examples 1-8 and the coordinates of points A to H.

Example number Time, temperature or period. min. ⁰ F 1 0.5 475 2 1.0 475 - 3 1.5 445 4th 4.0 445. 5 7.0 4oo 6th 15.0 4oo 7th 7.0 375 8th 15.0 375 A. 2o.o 36 ο B. 0.2 5oo C. 1.0 5oo D. 15o.o 36o E. 2o.o 38o F. 0.8 48o G 1.2 48o H 4o.o 38o

50 9 8 18 / 0-7 A 7

The chemical composition of the alloy is as shown for Alloy B in Table I. The tensile specimens for each example were immersed in molten Wood metal having a composition of 50% bismuth, 25% lead, 12.5% tin, and 12.5% cadmium. The immersion temperatures and times are tabulated in Table II and shown in FIG. 4. After immersion in the molten Wood's metal, the cooled samples were then precipitation hardened by being heated in a forced air oven for a period of from 24 hours to 121 ⁰ C. In each of Examples 1 to 8, a tensile specimen was worked in a tensile rod with a diameter of 3.18 mm in order to add it to a 3.5% sodium chloride solution by means of alternating immersion at a tension level of 42, ksi according to the "Military Specification MIL-A -22771B ". The samples were held until fracture with successive immersions for 10 minutes in the saline solution and then for 50 minutes in air. The number of days to break under such treatment is shown in Figure 4 above the time-temperature point for each example. The remaining sample from each example was tested for yield strength. The yield point data for Examples 1 to 8 are shown in FIG. 4 below the time-temperature points in% values of a yield point of 62.3 ksi for the T 651 state.

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Further examples of a preferred embodiment according to the invention follow, with the second precipitation hardening stage is carried out for 2 to 30 hours at a temperature of 132 and 160 ° C.

Examples 9-14

The procedure was as described for Examples 1 to 8 with except that all samples were immersed in a molten Wood metal for 90 seconds prior to the second precipitation hardening were immersed at 23o ° C. Other parameters and results are as given in Table III. Examples 9 to 11 form a group of comparative examples, which are brought to temperature in 3 hours of the second precipitation hardness level, with the Examples 12 to 14 form a second group, which in the second precipitation hardening stage for 24 hours Temperature were maintained. The superior strength and corrosion resistance achieved when the second Precipitation hardening is carried out for 2 to 30 hours at 132 to 160 ° C., results from comparisons of the Examples within the groups. '

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cn ο CD

oo oo

Table III

Parameters and data for Examples 9-14 including an immersion an aluminum alloy 7o75-T651 in molten wood metal during

9o seconds at 23o ° C, then a second precipitation hardness level,

Example no. Time (hours) &
Temperature (F)
the second exit
see hardening Breaking strength Stretch limit
ksi Days to > Break 9 3 hours / 25o ° F 67.2 58.5 42 ksi
Load level 35 ksi
Load level 1o 3 hours / 275 ° F 68.8 58.8 27 8o 11 3 hours / 3oo ° F 66.8 58.1 43 62 12th 24h / 25o ° F 7o.5 62.9 6o 84 pounds 13th 24 hours / 275 ⁰ F 69.5 61. ο 49 5o 14 ' 24 hours / 3oo ° F 69.6 61.4 47 61 56 63 > TvT »

CP) <D CO LJ

The following definitions were given:

a) The term "ksi" is equivalent to kilopounds per inch;

b) if percentages are given, these are percentages by weight, unless otherwise stated;

c) the letters "G.T." stand for Guinier-Preston.

The description of the invention given above can be modified and changed in various ways without However, to move away from the essence of the invention.

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Claims (5)

Claims 1.) A method for heat treating an object made of an aluminum alloy, characterized in that an object is treated which is composed of an alloy consisting essentially of aluminum, 4 to 8 % zinc, 1.5 to 3.5 # .Magnesium , 1 to 2.5 % copper and at least one element consisting of 0.05 to 0.3 % chromium , 0.1 to 0.5% manganese or 0.05 to 0.5 % zircon, so that the object is solution annealed and in that this object is subjected to a time and a temperature within the perimeter ABCD in Fig. 4 and is subsequently precipitation-hardened again at 8o to 163 ⁰ C is then precipitation hardened at 8o to 163 ⁰ C. 2.) The method according to claim 1, characterized in that the repeated precipitation hardening at a temperature from 132 to 160 ° C for 2 to 30 hours. 3.) The method according to claim 1 or 2, characterized in that the exposure for a time and a temperature takes place within the scope EFGH in FIG. 4. 509818/0747 2U6933 4.) Method according to one of the preceding claims,
characterized in that said exposure comprises immersing the article in a liquid at that temperature. 5.) The method according to claim 4, characterized in that the liquid is a molten metal. 509818/0747 Blank page