US3490955A

US3490955A - Aluminum base alloys and process for obtaining same

Info

Publication number: US3490955A
Application number: US610973A
Authority: US
Inventors: Joseph Winter; Alan J Goldman; William C Setzer
Original assignee: Olin Corp
Current assignee: Olin Corp
Priority date: 1967-01-23
Filing date: 1967-01-23
Publication date: 1970-01-20
Anticipated expiration: 1987-01-20
Also published as: DE1608766C3; NL6800952A; FR1562063A; SE407947B; NO122456B; DE1608766A1; DE1608766B2; CH509412A; GB1192281A; JPS512049B1

Description

Jan. 20, 1970 wm-r R ET AL 3,490,955

ALUMINUM BASE ALLOYS AND PROCESS FOR OBTAINING SAME Filed Jan. 23, 1967 INVENTORS.

JOSEPH WINTE'R ALAN J. GOLDMAN WILL/AM C. SETZER A TTO/PNE V United States Patent U.S. Cl. 148-115 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to aluminum base alloys having high strength prepared by working at a temperature below 450 F., holding at from 250 to 650 F. and working again at a temperature below 450 F.

The present invention relates to a process for the preparation of high strength aluminum base alloys. In particular the present invention resides in a process, an an alloy produced thereby, for the preparation of alumlnum base alloys having strengths considerably higher than conventionally, even with the introduction of severe amounts of cold work.

It is naturally highly desirable to conveniently obtain optimum high strengths in aluminum base alloys, especially in those common, inexpensive, commercially available aluminum base alloys. Various processes are generally known for increasing the strengths of aluminum base alloys. Frequently these processes are expensive and cumbersome or characterized by a plurality of process steps inconvenient and expensive to utilize. In addition, conventional processes are frequently characterized by critically defined process conditions which makes the process inconvenient to operate on a commercial scale. Furtherm re, processes for increasing the strength of aluminum base alloys are frequently selective based on particular alloying ingredients present in the alloy and are not often utilizable over a wide range of aluminum base alloys.

In addition to the foregoing, processes for increasing the strength of aluminum base alloys still frequently leave much to be desired with respect to the ultimate strength obtained. In addition, conventional processes often increase the strength of the aluminum base alloy with attendant losses'of other desirable physical properties, thereby often improving one property with an attendant degradation of another.

Three standard ways to increase physical properties are by alloying, cold working, and second phase precipitation effects. Each of these adversely affects some other mechanical or physical property. For example: alloying is invariably associated with decrease in conductivity; cold working decreases the ductility or elongation; and precipitation hardening will decrease toughness and increase notch sensitivity and decrease corrosion resistance. Also, each of these generally has other deleterious effects.

It is, therefore, an object of the present invention to provide a process for preparing aluminum base alloys having improved strength characteristics.

It is an additional object of the present invention to provide an improved alloy and process as aforesaid which is inexpensive and convenient and readily feasible on a commercial scale.

It is a still further object of the present invention to provide an improved alloy and process as aforesaid which attains greatly improved strength characteristics without inordinate loss of desirable physical properties, for example, electrical properties and finishing characteristics.

3,490,955 Patented Jan. 20, 1970 Additional objects and advantages of the present invention will appear hereinafter.

In accordance with the present invention, it has now been found that the foregoing objects and advantages may be readily attained and an improved alloy and process conveniently provided.

The process of the present invention comprises:

(A) Providing an aluminum base alloy containing from 0.05 to 1.0% iron, from 0.05 to 1.0% silicon, at least one material selected from the group consisting of less than 10.0% magnesium, less than 3.0% manganese, less than 1.0% copper, less than 0.5% chromium, less than 0.5 zinc, less than 0.5 zirconium, less than 0.5 titanium, less than 0.1% boron, others less than 0.5 each, total less than 1.5%, balance essentially aluminum;

(B) Working said alloy, preferably by rolling or drawing, at a temperature below 450 F., with a total reduction in excess of 20%;

(C) Holding said alloy at a temperature of from 250 to 650 F. for a period of time no greater than defined in the following formula: T (8.95+log t)=5,700, wherein T is temperature in degrees Kelvin and t is the maximum time in minutes at temperature T, so that there is no recrystallization throughout the matrix and so that there is less than 10% loss in yield and tensile strength; and

(D) Repeating step (B), preferably repeating steps (B) and (C), preferably a plurality of times.

In accordance with the present invention it has been found that the foregoing process results in a surprising improvement in strengths, even in the common aluminum alloys, and even with the introduction of thermal treatments after severe amounts of cold working. For example,

tensile properties have been reproducibly obtained in excess of 53,000 p.s.i. for aluminum alloy 3003, in excess of 45,000 p.s.i. for aluminum alloy 5005, in exess of 35,000 p.s.i. for aluminum alloy 1100 and in excess of 35,000 p.s.i. for EC grade aluminum. Throughout the present specification, aluminum alloy numbers represent Aluminum Association designations. This is particularly surprising since normally thermal treatments after cold working result in a considerable decrease in yield and tensile strengths in order to increase ductility.

In general, the present invention is broadly applicable to a wide range of aluminum base alloys as stated above, including high purity aluminum, and significant improvement is obtained with all these materials. It is preferred, however, that the aluminum base alloy contain less than 99.5% aluminum and naturally that certain additional elements be present in the alloy. This is reflected in the following which shows the permissible and preferred amounts of additional elements wherein all percentages are percentages by weight: Silicon from 0.05 to 1.0%, preferably from 0.3 to 0.7%; iron from 0.05 to 1.0%, preferably from 0.4 to 0.8%. In addition to iron and silicon, the alloy must contain at least one of the following materials: copper from 0 to 1.0%, preferably from 0.1 to 0.3%; manganese from 0 to 3.0%, preferably from 0 to 1.6%; magnesium from 0 to 10.0%, preferably from 0 to 5.0%; chromium from 0 to 0.5 preferably from 0 to 0.2%; zinc from 0 to 0.5 preferably from 0 to 0.3%; zirconium from 0 to 0.5 preferably 0 to 0.3%; boron from 0 to 0.1%, preferably from 0 to 0.05%; titanium from 0 to 0.5 preferably from 0 to 0.2%, others each less than 0.5%, total less than 1.5%, preferably each less than 0.05%, total less than 0.15%. Particularly preferred alloys include aluminum alloy 5005, 3003, 1100, EC grade aluminum, superpurity aluminum, etc. In general, the preferred alloys are those of the 1000 series, 3000 series and 5000 series.

In accordance with the present invention the aluminum base alloys may be cast in any desired manner. The particular method of casting is not critical and any commercial method may be employed, such as Direct Chill or Tilt Mold casting. The alloys may also be hot rolled to plate form in a conventional manner.

After casting it is preferred in accordance with the present invention to provide a homogenization or solutionizing treatment. This solutionizing treatment should be performed at a temperature above 850 F. and preferably above 950 'F. and the ingot should be held at temperature for a minimum of 4 hours. After the solutionizing step, the ingot should be rapidly cooled to below 450 F. and preferably rapidly cooled to below 250 F. at a rate of above 400 F. per hour.

In accordance with the present invention, if desired, the solutionizing step may be combined with the casting operation, i.e., in the casting operation the material may be held at the requisite temperature for the requisite period of time followed by rapidly cooling.

The purpose of the solutionizing step is as follows. When the aluminum base alloy contains alloying additions as indicated hereinabove, the solutionizing step followed by rapid cooling puts as much of these ma terials into solution as possible. Thus, the solute elements or alloying additions are in solid solution, preferably to the maximum degree, in the aluminum or solvent matrix. This is, as stated hereinabove, a preferred operation.

In accordance with the present invention, the next step is the critical working operation. The preferred type of working is naturally rolling and the present specification will be particularly directed to this form of initial working. It should be understood, however, that other types of working are contemplated, especially in later steps, such as drawing, swaging, etc.

The material is worked or rolled at a temperature below 450 F. with a total reduction in excess of 20%. It is preferred to roll at a temperature below 200 F. The material may be rolled in one or more passes with the amount of reduction per pass not being critical. In general, it is preferred to take a plurality of smaller reductions rather than one large reduction. In general, each pass should take at least a 15% reduction. Large reductions may be taken, if desired, for example, reductions in excess of 99% may be taken, e.g., in wire form. Throughout the present specification, the term reduction means total reduction in area.

After the rolling or working step the material is critically held at from 250 to 650 F. for a period of time no greater than defined in the following formula:

T (8.95-I-log t) =5,700

wherein T is any given temperature within the foregoing temperature range in degrees Kelvin and t is the maximum time in minutes at temperature T. The minimum time at temperature is not particularly critical, but should be at least one second. Naturally, the higher the temperature within the foregoing temperature range, the shorter the maximum holding time and the lower the temperature the longer the maximum holding time. It is preferred to operate in the temperature range of from 250 to 450 F. Examples of maximum allowable times determined in accordance with the foregoing formula are: approximately 400 hours at 300 F.; approximately 16 hours at 400 F.; and 2 minutes at 650 F.

As indicated above, after the rolling or working step the material is critically held at from 250 F. to 650 F. for no longer than the time determined by the foregoing empirical equation for which the constants were determined experimentally. It is interesting to note that changing the form of this equation to l/t=exp (Q/RT) gives a value of Q, the activation energy, that is slightly lower than is required for recrystallization in aluminum. This indicates that the initiation of recrystallization is the upper limit for the thermal treatment.

Subsequent to the'thermal treatment, the material is worked or rolled again at a temperature below 450 F. with a total reduction of at least 20% in the same manner as indicated hereinabove. This second rolling or working step may be the final step, or may, and preferably is,

' then followed by an additional thermal treatment at from 250 to 650 F. as indicated hereinabove.

Cold working after a low temperature thermal treatment is unusual in the fabrication of wrought aluminum structures inasmuch as low temperature treatment or partial annealing are normally introduced to stabilize the structure or lower the strength to desired levels in order to meet specific properties. In fact, the H2X and H3X standards of the Aluminum Association specifies work hardening and partial annealing or work hardening and then stabilizing. In accordance with the present invention, however, a stabilizing or partial annealing as a preparatory step for subsequent cold working provides the significant mechanical property increase of the present invention.

It is preferred to repeat the rolling and thermal treatment steps a plurality of times, preferably from 3 to 5 times. In accordance with the present invention, the final step in the process may be either the rolling or working step or the thermal treatment step upon particular requirements.

A modification of the present invention includes the following. If desired, the rolling step may be performed within the thermal treatment range. Thus, where one rolls at a temperature of from 250 to 450 F. and holds the material at temperature one may efi'etcively combine the working or rolling step with the thermal treatment step and thereby avoid a separate thermal treatment step.

An additional modification includes the following: The final step may optionally be the holding step of the present invention at from 250 to 650 F. but for a longer period of time than permitted by the foregoing formula, so that there is no recrystallization throughout the matrix but there is less than 25% loss in yield and tensile strength. This would result in yield and tensile strengths still greatly superior than normally obtained, but the ductility would be increased.

In accordance with the present invention the first rolling operation or the first deformation forms a cellular sub-grain structure. That is, the microstructure of the alloy is characterized by grains within grains. The thermal treatment step tends to stabilize the sub-grain walls by migrating solute atoms towards the sub-grain walls. The second deformation forms more sub-grain walls within the sub-grain structure, thereby incrementally refining the sub-grain size finer and finer as deformation and thermal treatment steps are repeated.

Thus, the improved alloys of the present invention are characterized by greatly improved strength characteristics and ultra fine sub-grain structure with the sub-grain.

EXAMPLE I In the following examples the following alloys were used. Aluminum alloy 1100; aluminum alloy 3003; aluminum alloy 5005; and super-purity aluminum. All of the alloys were Direct Chill cast and scalped into ingots 1% x 4 x 6".

EXAMPLE 11 In this example Alloy 3003 as cast was cold rolled incrementally from 1.750 to 1.5 to 1.25" to 1.0" to 0.8" to 0.65" to 0.5" to 0.35 to 0.25 to 0.175" to 0.122" to 0.083" to 0.07" to 0.05" to 0.036" to 0.025 to 0.018 to 0.014. After each reduction except the last there was a minute holding step at 400 F. The resulting mate rial had an average yield strength at 0.2% offset of 58,- 200 p.s.i. and an ultimate tensile strength of 63,400 p.s.i. with 2% elongation.

For comparative purposes Alloy 3003 was cold rolled to 0.014" gage and had the following properties: yield strength 0.2% offset of 27,000 p.s.i.; ultimate tensile I strength of 31,000 p.s.i. with 2% elongation.

In accordance with the present invention, FIGURE 1 is a photomicrograph of aluminum alloy 3003 obtained in accordance with the foregoing example at 0.036 gage. FIGURE 2 is a photomicrograph of aluminum alloy 3003 at 0.036" gage, with the alloy prepared in the following manner; the alloy was homogenized at 1100 F., hot rolled starting 950 F. and cold rolled to gage. Both photomicrographs are at a magnification of 30,000 The photomicrographs were prepared by a transmission electron micrograph taken from thin foils prepared by electro-chemical milling of a cold rolled material to a thickness of approximately 2,000 Angstroms.

From examination of the photomicrographs, it can be seen that FIGURE 2 has gross areas of dislocation tangles interspaced with large areas of apparently unworked materials. On the other hand, FIGURE 1, the alloy of the present invention has a series of recognizable, discrete grains of approximately 0.0001 mm. No regions of apparent unworked material are visible. The discrete subgrains are separated by recognizable grain boundary walls.

EXAMPLE III Aluminum alloy 3003 prepared in Example I was heated to 1100 F. and held for 16 hours. It was then water quenched to room temperature in 5 seconds followed by cold rolling incrementally from 1.75" to 1.5" to 1.25" to 0.8 to 0.65" to 0.5" to 0.35" to 0.25" to 0.175" to 0.122 to 0.088". Following this, the material was further cold rolled incrementally, except that after each reduction except the last the material was heated to 400 F. and held for 10 minutes at temperature and water quenched to room temperature. The reductions were as follows: from 0.088" to 0.072 to 0.05" to 0.036" to 0.029" to 0.024" to 0.020" to 0.017" to 0.013". The resultant material had an average yield strength of 48,000 p.s.i. at 0.2% offset and an ultimate tensile strength of 55,400 p.s.i. with 2% elongation. The microstructure was similar to that shown in FIGURE 1. Identical material processed in the same manner without the interanneals had a yield strength of only 38,000 p.s.i. at 2% offset and an ultimate tensile strength of 43,300 p.s.i. with 4% elongation.

EXAMPLE IV Aluminum alloy 5005 prepared in Example I was cold rolled incrementally from 1.75" to 1.5" to 1.25" to 1.0 to 0.8" to 0.65" to 0.5" to 0.35 to 0.25" to 0.175" to 0.122" to 0.085" to 0.06" to 0.042" to 0.03 to 0.022". After the last cold reduction the material was held for 10 minutes at 300 F. followed by an additional cold reduction to 0.018". The resultant material had an average yield strength of 48,900 p.s.i. at 0.02% offset and an ulti mate tensile strength of 49,800 p.s.i. with an elongation of 1%. As a comparison, aluminum alloy 5005 was cold rolled to 0.018" gage and had the following properties: yield strength 28,000 p.s.i. at 0.2% offset; ultimate tensile strength 29,000 p.s.i.; with an elongation of 1%.

What is claimed is:

1. A process for preparing high strength wrought aluminum base alloys which comprises:

.(A) providing an aluminum base alloy consisting essentially of from 0.05 to 1.0% iron, from 0.5 to 1.0% silicon, at least one material selected from the group consisting of less than 10.0% magnesium, less than 3.0 manganese, less than 1.0% copper, less than 0.5% chromium, less than 0.5% zinc, less than 0.5% zirconium, less than 0.5% titanium, less than 0.1% boron, balance essentially aluminum;

(B) working said alloy at a temperature below 450 F. with a total reduction in excess of 20%;

(C) holding said alloy at a temperature of from 250 to 650 F. for a period of time of at least one second but no greater than defined in the following formula: T (8.95 +log t)=5,700, wherein T is temperature in degrees Kelvin and t is the maximum time in minutes at. temperature T, so that there is no recrystallization throughout the matrix and so that there is less than 10% loss in yield and tensile strength; and

(D) repeating step (B).

21A process according to claim 1 wherein steps (B) and (C) are repeated.

3. A process according to claim 1 wherein steps (B) and (C) are repeated in a plurality of times.

4. A process according to claim 1 wherein the materials in step (A) are present in the following amounts: silicon from 0.3 to 0.7%, iron from 0.4 to 0.8%, at least one material selected from the group consisting of copper from 0.1 to 0.3%, manganese up to 1.6%, magnesium up to 5.0%, chromium up to 0.2%, zinc up to 0.3%, titanium up to 0.2%, zirconium up to 0.3% and boron up to 0.05%.

5. A process according to claim 1 wherein prior to said working step- (B) the material is homogenized at a temperature above 850 F. for at least 4 hours.

6. A process according to claim 5 wherein after said homogenization step the material is rapidly cooled to below 450 F.

7. A process according to claim 1 wherein step (B) is rolling at a temperature below 200 F.

8. A high strength wrought aluminum base alloy consisting essentially of from 0.05 to 1.0% iron, from 0.05 to 1.0% silicon, at least one material selected from the group consisting of less than 10.0% magnesium, less than 3.0% manganese, less than 1.0% copper, less than 0.5% chromium, less than 0.5% zinc, less than 0.5% zirconium, less than 0.5% titanium, less than 0.1% boron, balance essentially aluminum, said alloy having ultra fine subgrain structure with the sub-grain size being less than 0.0001 mm., with the sub-grains having boundary walls of pinned dislocation tangles.

9. An alloy according to claim 8 wherein the matrix between dislocation tangles consists of individual regions having lower content of alloying additions and low density of dislocations.

10. An alloy according to claim 9 containing from 0.3 to 0.7% silicon, 0.4 to 0.8% iron, at least one material selected from the group consisting of 0.1 to 0.3% copper, up to 1.6% manganese, up to 5.0 magnesium, up to 0.2% chromium, up to 0.3% zinc, up to 0.2% titanium, up to 0.3 zirconium, up to 0.05% boron.

References Cited UNITED STATES PATENTS 2,168,134 8/1939 Pavelka 148-115 3,232,796 2/1966 Anderson 148-115 3,366,476 1/1968 Jagaciak 148-115 RICHARD O. DEAN, Primary Examiner US. Cl. X.R.

33 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,490,955 Dated January 20, 1970 Inventor(s) Joseph Winter, Alan Goldman and William C. Setzer It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

I- In Column 1, line 22, the word "an", first occurrence,

should read --and-.

In Column 2, line 36, the word "exess" should read -excess-.

In Column 4, line 25, after the words "treatment step" insert --depending-;

In Column 4, line 31, the word "effetcively" should read --effectively--.

In Column 5, line 25, after the words "milling of" the word "a" should read --as--;

In Column 5, line 29, the word "interspaced" should read --interspersed--.

In Column 6, line 2, after the word "from", second occurrence, change "0.5" to read --0.05--;

In Column 6, line 5, after the word "than", first occurrence, change "3.0" to read --3,0%--;

In Column 6, line 23, after the words "are repeated" delete the word "in".

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