US3071847A

US3071847A - Metal treatment

Info

Publication number: US3071847A
Application number: US681957A
Authority: US
Inventors: Dorsey A Paul
Original assignee: Kaiser Aluminum and Chemical Corp
Current assignee: Kaiser Aluminum and Chemical Corp
Priority date: 1957-09-04
Filing date: 1957-09-04
Publication date: 1963-01-08
Anticipated expiration: 1980-01-08

Description

. Jan. 8, 1963 D. A. 3,0 8

METAL TREATMENT Filed Sept. 4, 1957 2 Sheets-Sheet 1 WIDE MEDIUM NARROW STEP WIDTH STEP WIDTH STEP WIDTH PLASTIC PLASTIC PLASTIC STRAIN STRAIN STRAIN RESIDUAL RES.- I RES.

ELASTIC ELASTIC STRAIN STRAIN l; -TENSILE lcomn -rcmsu: com --TENSILE I coMP.-

ELASTIC STRAIN DIRECTION OF WORKING INVENT OR DORSEY A. PAUL D. A. PAUL METAL TREATMENT Jan. 8, 1963 2 Sheets-Sheet 2 Filed Sept. 4, 1957 02-88 0400 Hut/N ZOCDQWFQO mmwwpm Jdjgmwm INVENTOR DORSEY A. PAUL ATTORNEY 025M151 mmwEm wmobm mmwzx zk Ed a mmomu I I I n N M nl nl United States Patent ()fifice 3,071,847 Patented Jan. 8, 1963 3,071,847 METAL TREATMENT Dorsey A. Paul, Erie, Pa., assignor to Kaiser Aluminum & Chemical Corporation, Oakland, Calif., a corporation of Delaware Filed Sept. 4, 1957, Ser. No. 681,957 5 Claims. (Cl.29--180) This invention pertains to the forging of metals. More particularly, the invention relates to an improvement in the production of forgings of solution heat treatable aluminum alloys.

During the past few decades, aluminum forgings have found increased usage in the fiabrication of various structures. One factor which has played an important role in the wide spread usage of aluminum forgings is that a forging of light weight yet with high strength may be produced. Hot working of solution heat treatable aluminum alloys followed by a suitable heat treatment will produce forgings with increased tensile, compressive and impact strength, and increased fatigue resistance.

Common types of forgings which have utilized the desirable properties of aluminum base alloys are the closed the forgings and hand forgin-gs. Hand forgings, to which this application is particularly applicable, are generally used where the required number of for-gings is not sufficient to justify the making of a forging die, for example, in the production \of prototypes, experimental designs, etc. Hand forgings are worked to the desired shape in simple open dies that impose little or no lateral confinement to the stock. The stock is worked by manipulation during repeated strokes of a hammer or a hydraulic press. The

hand forging may then be reduced to the finished shape by machining.

In the usual production of forgings of solution heat treatable aluminum alloys, the stock after forging is solu 'tion heated, quenched, and subsequently aged. The

quenching of the forging from the solution heat treat temperature causes residual stresses to be set up in the forging which in turn will cause distortion of the forging during subsequent machining operations. Since the majority of aluminum hand forgings undergo machining to produce the final shape, the residual stresses created by quenching present a major problem.

It is therefore a primary object of this invention to provide a method for eliminating or substantially reducing the residual stresses in forgings of solution heat treat- It is a further object of this invention to provide a method for relieving residual stresses caused by quenching of forging-s of solution heat treatable aluminum alloys by cold forging the forgings a critical amount sub sequent to the quenching operation.

It is also an object of the instant invention to provide a forged article of solution heat treatable alloy wherein the residual stresses caused by quenching have been eliminated or substantially relieved by cold forging.

These and other objects and advantanges of the invention will be apparent from the ensuing description of the invention.

In the accompanyingdrawings, forming a part of the specification:

FIGURE 1 shows schematically the effect of forging step width on the strain patterns of hand forgings of a solution heat treatable aluminum alloy that have been subjected to cold forging according to this invention;

FIGURE 2 is a perspective schematic view of a hydraulic forging press which may be used for carrying out the cold forging of the invention; and

FIGURE 3 depicts diagrammatically the residual stresses present in a hand forging before and after cold forging according to the invention.

Various mechanical stress relieving methods, that is, the reduction of residual stresses by mechanical methods at moderate temperatures, have been known in the art. The reduction of residual stresses is accomplished by mechanically introducing stresses that cause plastic flow. A commonly used method of stress relieving involves stressing the metal stock beyond the yield point by pressing, drawing or stretching. This procedure is frequently combined with a straightening operation. Although pressing, drawing or stretching will reduce some of the residual stresses present in the metal, it is not as effective as the cold forging method with which this invention is concerned. Also, stress relieving methods involving stretching etc. are not readily adaptable for use with some fabricated parts, such as hand forgings.

The method of this invention involves the elimination or substantial reduction of residual quenching stresses in forgings by a cold forging operation which produces a non-uniform plastic strain across the thickness of the stock, the plastic strain being highest toward the center and zero at the" surfaces. In the cold forging operation,

two important factors control the character of the plastic strain pattern. The ratio of stock thickness or height to forging step width may alter the shape of the plastic strain pattern, while the percentage reduction could change about 2.0 to 2.5. The reduction of the stock thickness by the cold forging operation should be from about 1% to 4%, preferably 2%.

The residual elastic strains or stresses after springback are tensile at the surface and compressive at the center. Both the plastic strains and elastic springb-ack strains are shown schematically in FIGURE 1, together with the effect of forging step width upon the strain pattern. It willbe noted from FIGURE 1 that the residual elastic stress pattern can be varied from one with a sharp peak at the center to one with a W shape, the latter being the result of a narrow forging step width. This contrasts with a stretching operation which produces a uniform plastic strain across the thickness. -In a stretching operation after release of a load, there is no residual elastic stress afiter spring-back 'if' there were none before stretch.

The residual stress caused by quenching is compressive at the surface and tensile at the center; that is, opposite in sign to that caused by cold forging. Consequently, by use of the proper forging step width relative to the thickness of the stock, it is possible to shape the curve of residual stress caused by cold forging to conform very closely to that caused by quenching, but opposite in sign; The magnitude of the residual stress should then be adjusted by the correct amount of percentage reduction. By cold forging it is possible to reverse the internal stress pattern caused by quenching. In a stretching operation for stress relief, it is only possible to approach Zero stress by the maximum possible stretch. The sign of the stress can never be reversed. The cold forging method of stress relief permits a wider latitude of control of the final stress pattern.

The following examples, which are not intended to limit the invention, are illustrative of the eflicacy of the instant invention in relieving residual stresses caused by quenching of hand forgings of solution heat treatable aluminum alloys by cold forging the said hand forgings a critical amount subsequent to the quenching step.

Example I A hand forging of aluminum alloy 7075 (having a nominal composition of 1.6% copper, 2.5% magnesium, 5.6% zinc, 0.3% chromium, balance aluminum and normal impurities), 3.31 inches by 9 inches by 21 inches, was sawed into 2 equal sections, each section measuring 3.31 inches by 9 inches by 10.5 inches. Both sections were solution heat treated at 865 F. for 6 hours and quenched in water at 140 F. After quenching, one piece was reduced 3.1% in thickness by cold forging on a hydraulic press as shown in FIGURE 2. The apparatus generally comprised a top plate '1 with a top die or tool engaged therewith by means of die key 9 and a base plate 3 with a bottom die or tool 7' engaged therewith by means of die key 11. In FIGURE 2 the hand forging is designated as F and the direction of working is shown by an arrow. The

working surfaces

15 and 17 of top die 5 and bottom die 7 respectively are shown beveled along their long dimensions. The width of the die or tool which is used in determining the ratio of stock thickness to forging step width is the width of the working

surfaces

15 and 17 exclusive of the beveled surfaces. Stop or gauge blocks 13 are disposed between top die 5 and bottom die 7 for controlling the reduction of thickness of the stock. 7

The width of the dies used in this example was 6 inches and the original thickness of the forging was 3.31 inches. The stop or gauge blocks were 3.19 inches in length. Allowances of 0.004 inch per inch of thickness were made for the expected elastic spring-back. There was a 0.5 inch overlap of the width of the dies between successive forging steps; therefore the forging step width was 6-0.5 or 5.5 inches. The ratio, then, of the thickness of the hand forging to the forging step width was 3.31/5.5 or 0.6. After the one piece had been cold worked, both pieces were artificially aged for 24 hours at 250 F.

The effect of the stress relief was determined by sawing the pieces in half parallel to the 9 inch by 10.5 inch faces of the sections and measuring the distortion for both the longitudinal dimensions (direction of working) and the transverse dimensions.

In the case of the unrelieved specimen, the distortion in the longitudinal dimension (10.5 inch dimension) was 0.068 inch or 0.013 inch/inch, and in the transverse dimension (9 inch dimension) the distortion was 0.045 inch or 0.01 inch/inch. In the case of the specimen which was stress relieved according to the invention, there was no measurable warpage or distortion in the longitudinal dimension (10.5 inch dimension) while the distortion in the transverse dimension (9 inch dimension) was only 0.014 inch or 0.003 inch/inch.

Example II Two hand forgings of 7075 aluminum alloy were solution heated at 865 F. for six hours and quenched in water at 140 F. The dimensions of each forging were 3 inches by 10 inches by 36 inches. One of the quenched forgings was reduced, 2% in thickness by cold forging on a hydraulic press shown in FIGURE 2. The forging step width was 1.25 inches. The ratio of the forging thickness to the forging step width was 3/ 1.25 or 2.4. After the cold forging operation performed on the one forging, both forgings were artificially aged for 24 hours at 250 F. FIGURE 3 shows the residual stresses before and after cold forging, as well as the plastic strains produced during cold forging. The left hand diagram of FIGURE 3 shows the measured residual stresses in the forging which had been heat treated, quenched and aged without further work. The right hand diagram shows the internal residual stress pattern through the thickness of the forging which was solution heated, quenched, cold forged and then aged. The center diagram shows the plastic strain pattern across the thickness of the forging effected by the cold forging of this invention. Both the longitudinal and transverse stresses are shown in each diagram. It will be noted that the signs of the stresses in the longitudinal direction have actually been reversed, as well as substantially reduced, and in the transverse direction the stresses have been reduced.

Various changes or modifications may be made without departing from the spirit and scope of the invention and, accordingly, the invention is not to be limited except by the appended claims wherein what is claimed is:

1. A method for producing forged shapes of solution heat treatable aluminum alloys wherein the shape is fabricated by forging, solution heating, quenching and aging, the improvement for substantially reducing residual stresses created by said quenching comprising the step of cold forging the said shape after quenching and prior to aging, said cold forging effecting a small reduction of thickness of said shape, the ratio of the thickness of the shape to the forging step width used in said cold forging step being in the range of about 0.3 to 4.0.

2. A method according to claim 1 wherein the ratio of the thickness of the shape to the forging step width is from about 2.0 to 2.5.

3. A method for producing forged shapes of solution heat treatable aluminum alloys wherein the shape is fabrica'ted by forging, solution heating, quenching and aging, the improvement for substantially reducing residual stresses created by said quenching comprising the step of cold forging the said shape after quenching and prior to aging to a reduction of from 1 to 4% of the total thickness of said shape, the ratio of thickness of said shape to the forging step width used in the cold forging step being from about 0.3 to 4.0.

4. A method according to claim 3 wherein the said reduction is 2% of the total thickness of said shape and the ratio of the thickness of said shape to the forging step width is from about 2.0 to 2.5.

5. A solution heated, quenched and aged forged article of a solution heat treatable aluminum alloy which has been made according to the method of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 1,891,234 Langenberg Dec. 20, 1932 2,454,312 Fritzlen Nov. 23, 1948 2,743,516 Forrester May 1, 1956 2,894,421 Appel July 14, 1959 FOREIGN PATENTS 423,868 Great Britain Feb. 11, 1935 738,070 Great Britain Oct. 5, 1955