US3014824A

US3014824A - Rolling magnesium alloy

Info

Publication number: US3014824A
Application number: US855521A
Authority: US
Inventors: Sidney L Couling; Carl M Zvanut
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1959-11-27
Filing date: 1959-11-27
Publication date: 1961-12-26
Anticipated expiration: 1978-12-26
Also published as: DE1433141A1

Description

nited States Pater 3,014,824 Patented Dec. 26, 1961 3,014,824 RGLLHNG MAGl lllSlUll/i ALLGY Sidney L. Qeuiing, Midland, Mich, and Carl M. Zvannt, Alton, Iii. assignors to The Dow ilhemieal Company, Midland, Mich, a corporation of Delaware No Drawing. Filed Nov. 27, 1959, der. No. 855,521 6 Claims. (Cl. 148--1l.5)

The invention relates to improvements in methods of rolling magnesium-base alloys and more particularly concerns a method of producing magnesium-base alloy in rolled sheet form exhibiting improved bend ductility.

For the purposes of the specification and claims a magnesium alloy containing at least 75 weight percent of magnesium is hereinafter referred to as a magnesium-base alloy.

At one time the use of magnesium-base alloys, of the type which harden on being worked at temperatures below about the minimum recrystallization temperature, was limited by the inability of such alloys in rolled sheet form to take relatively sharp bends without fracture. While U.S. Patent No. 2,029,728 provides a method of producing a rolled product having improved bend ductility to that previously known, technological advances have created a demand for a rolled sheet metal which in addition to excellent bend ductility exhibits good mechanical strength properties and drawability without orangepeel-like surface defects. Such a rolled sheet metal is not obtainable by the method of the aforesaid US. patent or any other known method.

It is accordingly one of the objects of the invention to produce from magnesium-base alloys rolled sheet, plate, profiled shapes, or like products which possess a degree of bend ductility permitting the same to be readily bent radically, as over a short radius, at ordinary temperatures without fracture.

' Another object of the invention is to provide a method of preparing from magnesium-base alloys rolled sheet...

plate, profiled shapes, or like products which not only possesses excellent bend ductility but exhibit good mechanical strength properties and are capable of being drawn without surface roughening (orange-peel eilect) of the metal.

The term bend ductility is here employed to designate that quality of the rolled metal by virtue of which such changes in shape may be made at room temperature without fracture. It has been found that the minimum radius of a 90 degree bend which may be produced at room temperature without fracture also gives a convenient measure of the bend ductility. In employing this test the radius is expressed in terms of the thickness of the specimen. I

In following the method of US. Patent No. 2,029,728, an ingot or rolling slab of magnesium-base alloy is first hot rolled at a temperature between about 800 and 400 F. to obtain a desired reduction in thickness. The rolled metal is then annealed ata temperature between 500 and 900 F. for from 15 minutes to 2 hours and following such rolling and annealin'gsteps the'metal is stressed by an alternate bending and straightening operation without substantially changing its thickness. The so-fiexed metal is then given a final annealing by maintaining it at atemperature between about 500 and 900 F. for from 30 minutes to 2 hours. This procedure has the disadvantage that 'so-produced metal sheet having a good bend ductility is subject to excessive grain growth during the final annealing step, or during subsequent heating if the metal is to be subjected to bending or drawing operations at an elevated temperature. Excessive grain growth is largely responsible for lower mechanical strength properties and poor drawability of rolled sheet metal.

The invention is predicated upon the discovery'that upon bringing magnesium-base alloy to the partially annealed condition rather than to the fully annealed condition, prior to an alternate bending and straightening operation and thereafter annealing the so-stressed metal a bend-ductile product is obtained which possesses good physical and mechanical properties. The invention then consists in the improvedcombination of steps hereinafter more fully described and particularly pointed out in the claims.

ln-carrying out the invention themetal is first rolledto the desired thickness by any suitable procedure known to the art. For example, the rolling may be carried out at a temperaturebetween about 300 to 900 F. so as to produce a reduction in thickness at each pass of usually 15 to 30 percent or whatever the metal will stand without fracture, the metal being reheated from time to time between passes if necessary. The metal after the ultimate pass may be cut into lengths or coiled about a mandrel.

The rolled metal is then partially annealed in such a. manner as to retain in the sheet part of the strain hardening which had been induced therein by the rolling operation. The so-annealed sheet should be in the condition of being at least 5 percent strain hardened. Or, in another manner of defining the degree of strain hardening retained, the metal is annealed until it is in a condition in whichit exhibits strength properties, such as a tensile yield strength, which in numerical value is intermediate between the properties of otherwise identical alloy sheet which is (l) in the fully annealed condition and (2) in the fully strain hardened condition. Preferably the degree of strain hardening is such that the tensile yield strength of the sheet is larger than that for a fully an.- nealed sheet by about 20 to 60 percent of the numerical difference between fully annealed and fully strain hardened sheet formed of the same alloy.

To accomplish a suitable amount of annealing the rolled metal is heated at temperatures in the range of 300 to 500 F. for from 30 minutes to 2 hours or more. At the lower temperatures of the annealing range a longer time is required than at the higher temperatures. As an example we have found that the magnesium-base alloy having the ASTM designation AZ3l which contains nominally 3 percent by weight of aluminum, 1 percent by weight of zinc, 0.4 percent by weight of manganese, the balance magnesium, is satisfactorily annealed by bringing it to a temperature of 350 F. and maintaining it at that temperature for an hour.

In another manner of preparing the alloy sheet for the subsequent forming operation the metal may be brought to the partially annealed state, as hereinabove defined, by strain hardening annealed or soft temper metal, rather than by partially annealing strain hardened metal. The metal is thus brought .to the said partially annealed state by "rolling the metal in the final pass or passes through the rolls of a mill under suitable conditions of temperature and roll pressure, aswell understood in the .art, to leave in or impart to the metal the desired amount of strain hardening.

Following such rollingand annealing steps the metal is stressed by alternate bending and straightening or like stressing operation without substantially changing its thickness. The radius of curvature or bend'whichrnay 'be'produc'ed in this'treatment without fracture depends upon the bend ductility of the annealed rolled metal. This is usuallyisutficient'to permitbending about a radius of from 4 to times the thickness of the rolled metal. The amount of alternate bending and straightening effective for'the purpose of producing low bend sheet in this treatment depends upon the thickness of the metal and the radius of bendsior curvature'produced in'ben'cling the same. From two to six'bendin'g and straighteningopera- 'tions in which the radius of curvature producedis relato conventional practice. In each case the sheet was then annealed to bring it to the partially annealed condition. The partially annealed sheet was then subjected to alternate bending and straightening in conventional roller leveler apparatus and given a final anneal. The gauge of the sheet used, annealing temperatures, the number of bend reversals and severity of bends and resulting bend ductility are all listed in Table I.

By way of comparison a sheet of magnesium-base alloy 10 having the same composition was treated according to the method of U.S. Patent No. 2,029,728. The condition and results of the comparison tests are listed in Table I as test No. 26.

space relation to the rolls in the other. The plane of one group of rolls is set at an angle to the other so that the metal being passed between the groups of rolls is alternately bent in opposite directions with progressively de- TABLE 1 Longitudinal evaluation bends of AZ31B as a function of longitudinal bend severity and number of reversals Ratio, Ratio, Mini- Minimum Severity Bend mum Bend Metal Bend Preanneal b Number Applied, Ratio Radius to Test N o. Gage Radius to Temperaof Bend Radius Metal Thiek Inch Metal ture, F. Passes v to Metal ness Alter Thick Thickness Final Anneal d ness 040 3.0 350 2 4 77 .040 3. 350 2 S .77 040 3. 0 350 2 16 77 040 3. 0 350 2 31 2. 0 040 3. 5 350 4 8 0. 75 125 3. 5 350 2 4 75 125 3. 5 350 2 8 75 125 3. 5 350 2 16 75 125 3. 5 350 2 32 2. 75 072 3. 0 350 4 16 43 125 2. 75 350 1 1.0 125 2. 75 350 2 10 02 125 2. 75 350 3 l0 62 125 2. 75 350 1 18 02 125 2. 75 350 2 18 50 125 2. 75 350 5 1S 50 125 2. 75 350 10 18 1. 0 125 2. 75 350 1 31 1. 5 125 2. 75 350 2 31 1 125 2. 75 350 5 31 75 125 2. 75 350 10 31 1. 125 2. 75 350 1 56 2-. 5 125 2. 75 350 2 56 2. 0 125 2. 75 350 5 56 1. 5 25-. 125 2. 75 350 10 56 1. 0 26 (Blank) 06s 3. 5 600 2 16 1. 5

n Determination made on specimen of rolled sheet alter specimen was annealed to O temper condition.

b Annealing step before alternate bending and straightening operation.

0 Alternate bending and straightening carried out at room temperature.

' Final anneal carried out at 600 F. for 1 hour.

creasing curvature until it finally emerges substantially flat and straight. The number of reversals obtained by a given group of rolls alternately spaced is generally given by the expression N -2 in which N equals the number of rolls employed.

It is preferable to carry out the stressing operation at room temperature to 95 F.) although any temperature between 40 and 275 F. may be used, since at such temperatures no consequential change in the thickness of the metal is produced by the treatment.

The stressed metal is then annealed to the fully soft condition as by employing conventional heat treating times and temperatures. As an example, magnesium-base Further examples of the method of the invention were carried out to illustrate the effect of alternately bending and straightening magnesium-base alloy in rolled sheet form in the direction of prior rolling as compared to alternately bending and straightening the sheet in a direction normal to the prior direction of rollin A number of specimens of 0.040 inch thick sheet formed of magnesium-base alloy having the ASTM designation AZ31 were annealed at 350 F. for one hour. These specimens were subjected to alternate bending and straightening according to the process of the invention and then annealed at 600 F. for one hour. The conditions of sostressing the metal and the resulting bend ductility in alloy having the ASTM designation A231 may be annealed at about 600 F. for one hour.

To illustrate the practice of the invention a magnesiumbase alloy having the nominal composition by weight of 3 percent of aluminum, 1 percent of zinc, the balance magnesium, Was cast into a rolling billet and rolled into sheets of various thicknesses at temperatures between about 400 to 900 F. in a number of passes according both the longitudinal and transverse directions of rolling are listed in Table 2. A specimen of the same metal sheet when fully annealed exhibited a bend ductility such that the ratio of the minimum bend obtainable without cracking to the thickness of the sheet was 3 when tested in the direction in which the sheet was rolled and 3.5 when the sheet was tested in a direction transverse to the direction of rolling.

Longitudinal and transverse evaluation bends of .040"

AZ31B as a function of reverse bend severity and direction of reverse bending Bend ductility=ratio of minimum radius bend without fracturing to thickness of sheet.

hL iirection=evaluation test applied in longitudinal direction of rolling s ce h'I tdirection=evaluation'test applied in transverse direction of rolling ee In still a further comparison, the method of the invention and the method of U.S. Patent 2,029,728 were each applied to 0.064 inch thick sheet formed of magnesium-base alloy which was 'in the H24 (ASTM designation) hard temper condition. The average grain diameter ofthe metal sheeet in the as rolled-H24 temper condition after each annealing step in both methods was determined and are reported in Table 4. In both methods the rolled sheet was subjected'to two reverse bends at a severity siich that the ratio of the radius of the bend applied to the metal sheet thickness was 18 to 1. The results of the test show that the average grain diameter of sheet treated according tothe present invention is not substantially increased, especially as compared to the known method in which the metal is snbject to hetero geneous grain growth.

TABLE 4 Grain-Size Diameters AZ31B (.064 Gage) 'As a further comparison of the method of the invention with the method used heretofore magnesium-base allo-y having the 'ASTM designation AZ3l was rolled by conventional methods to a thickness of 0.064 inch. The physical properties of the metal as rolled were determined. A specimen or the so-ro-lled sheet was partially annealed according to the present invention and then further stressed and annealed to complete the method of the invention. Physical properties of the sheet were determined after each of the annealing steps. The said physical properties are listed in Table 3.

Another specimen of the same 0.064 inch thick rolled sheet was treated according to the method of US. Patent No. 2,029,728. The physical properties of the so-treated sheet were similarly determined after each annealing step. These test results are also listed in Table 3. The results show that the physical strength properties of the alloy sheet processed according to the method of the invention do not ditfer substantially from the properties of the same alloy sheet processed according to the said patented method, but the bend ductility is greatly improved.

Among the advantages of the invention are the improvement of bend ductility of treatedsheet without substantial loss of physical properties, the retention of a homogeneous grain'structure having substantially the average grain diameter exhibited by the sheet asrolled priorto alternate bending and straightening, and the improvement ofbend ductility in so treated sheet in both the longitudinal-and transverse directions of rolling.

What is claimed is:

1. The method of producing a bend-ductile rolled prodnot from a magnesium-base alloy which comprises: providing magnesium-base alloy in rolled sheet form; bringing the rolled metal to the partially annealed condition; alternately bending and straightening the partially annealed metal at a temperature between 40 and 275 P. so as to stress the metal Without substantially changing its thickness; and annealing the same; said partially annealed condition of the metal being characterized by the metal having a tensile yield strength larger than that of the same metal in the fully annealed condition by about 20 to percent of the numerical difierence between the TABLE 3 Physical Properties Ratio of Bend Ra- Sarnple dius to Method UTS TYS CYS Percent Metal E Thickness .064 Gage Sheet as Rolled 44. 3 33. 8 28.0 8.7 8 Annealed 1 Hr. at- 600 F 37. 8 23v 0 15.0 21.0 3. 5 Alternately Bent and Sir tened, 2 17.8. 2,029,728.

Reversals (160 annealed 1 Hr. at 000 F 35. 4 l3. 3 14. 0 25. 2 1. 5 .064 (gage SlSieet Arfiealed llHruBat g- 36.0 26. 0 19.0 16. 0 4. 8 .064 wa e heet ternate y en an straightened, 3 reversals (3025) An- Present Invention nealed 1 Hr. at 600 F 35.0 15.0 15.0 20.0 0. 8

Severity of bend applied, ratio of radius of bend applied to thickness of metal sheet.

7 tensile yield strength of the fully annealed and that of the fully strain hardened metal.

2. The method of producing a bend-ductile rolled prodnot from a magnesium-base alloy which comprises: rolling the metal at a temperature between 850 and 400 P. so as to produce a reduction in thickness; heating the metal at a temperature in the range of 300 to 500 F. for a time sufiicient that the tensile yield strength of the metal is about midway between the tensile yield strength of the fully annealed metal and that of fully strain hardened metal; alternately bending and straightening the rolled metal at a temperature between 40 and 275 F. so as to stress the metal without substantially changing its thickness; and annealing the same.

3. The method of producing a bend-ductile rolled product from a magnesium-base alloy which comprises: rolling the metal at a temperature substantially above the temperature at which the metal strain hardens on working so as to produce a reduction in thickness; further rolling the metal to a predetermined final thickness at a temperature at which the metal is strain hardened on working and under conditions of roll pressure that the so'rolled metal is in the partially annealed condition as it leaves the rolls of the mill; alternately bending and straightening the so-rolled metal at a temperature between 40 and 275 F. so as tostress the metal without sub stantially changing its thickness; and annealing the same; said partially annealed condition of the metal being characterized by the metal having a tensile yield strength larger than that of the fully annealed metal by about 20 to 60 percent of the numerical difierence between the tensile yield strength of the fully annealed and that of the fully strain hardened metal.

4. The method of producing a bend-ductile rolled product from a magnesium-base alloy which comprises: rolling the metal at a temperature and under a roll pressure sufficient that the rolled metal is brought to the partially annealed condition while the metal is reduced in thickness; alternately bending and straightening the so-rolled metal at a temperature between 40 and 275 P. so as to stress the metal without substantially changing its thickness; and annealing the same; said partially annealed condition of the metal being characterized by the metal having a tensile yield strength larger than that of the fully annealed metal by about 20 to 60 percent of the numerical difference between the tensile yield strength of the fully annealed metal and that of the fully strain hardened metal.

5. In a method of producing a bend-ductile rolled product from a magnesium-base alloy in which the metal is rolled at a temperature between 85 0 and 400 P. so as to produce a reduction in thickness, and subsequently stressed by alternately bending and straightening the metal at a temperature between 40 and 275 F. and annealed, the improvement which comprises, in combination therewith, subsequent to said rolling and prior to the said alternate bending and bringing the metal to the partially annealed state, the said partially annealed state of the metal being characterized by the metal having a tensile yield strength larger than that of fully annealed metal by about 20 to percent of the numerical difference between the tensile yield strength of the fully annealed and that of the fully strain hardened metal.

6. In a method of producing a bend-ductile rolled product from a magnesium-base alloy in which the metal is rolled at a temperature between 850 and 400 P. so as to produce a reduction in thickness, and annealed, the improvement which comprises, in combination there with, and prior to said annealing step, subjecting said metal to stressing by alternately bending and straightening the metal at a temperature between 40 and 275 F. while the metal is in the partially annealed state, the said partially annealed state of the metal being characterized by the metal having a tensile yield strengthlarger than that of fully annealed metal by about 20 to 60 percent of the numerical difference between the tensile yield strength of the fully annealed and that of the fully strain hardened metal.

References Cited in the file of this patent UNITED STATES PATENTS.

OTHER REFERENCES Metals Handbook, American Society for Metals, 1948 Edition, page 978,

UNITED STATESPATENT. OFFICE CERTIFICATE OF CORRECTION Patent No. 014,824 December 26 1961 Sidney L. Couling et all,

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should ,read as corrected below.

Column 8, line 12, after "and" insert straightening Signed and sealed this 19th day of May 1962.,

(SEAL) Attest:

Attesting Officer Commissioner of Patents

Claims

1. THE METHOD OF PRODUCING A BEND-DUCTILE ROLLED PRODUCT FROM A MAGNESIUM-BASE ALLOY WHICH COMPRISES: PROVIDING MAGNESIUM-BASE ALLOY IN ROLLED SHEET FORM, BRINGING THE ROLLED METAL TO THE PARTIALLY ANNEALED CONDITION, ALTERNATELY BENDING AND STRAIGHTENING THE PARTIALLY ANNEALED METAL AT A TEMPERATURE BETWEEN 40 AND 275*F. SO AS TO STRESS THE METAL WITHOUT SUBSTANTIALLY CHANGING ITS THICKNESS, AND ANNEALING THE SAME, SAID PARTIALLY ANNEALED CONDITION OF THE METAL BEING CHARACTERIZED BY THE METAL HAVING A TENSILE YIELD STRENGTH LARGER THAN THAT OF THE SAME METAL IN THE FULLY ANNEALED CONDITION BY ABOUT 20 TO 60 PERCENT OF THE NUMERICAL DIFFERENCE BETWEEN THE TENSILE YIELD STRENGTH OF THE FULLY ANNEALED AND THAT OF THE FULLY STRAIN HARDENED METAL.