US2879191A - Method of producing heat treated copper zirconium alloys and articles formed thereof - Google Patents

Description

March 24, 1959 P. w. NIPPERT ETAL 2,879,191

METHOD OF PRODUCING HEAT TREATED'COPPER ZIRCONIUM ALLQYS AND ARTICLES FORMED THEREOF Filed June 25. 1958 INVENTOR. PAUL- w. NIPPERT ATTORNEYS yway m Unite States Patent METHOD OF PRODUCING- HEAT TREATED- COPPER ZIRCONIUM ALLOYS ARTI- CLES FORMED THEREOF Paul W. Nipper-t, Worthington, and Allen" W. Hodge, Columbus, Ohio, assignorsto TherNippert Electric Products Company, Columbus; Ohio, acorporation .of.

Ohio

Application June 23, 1958, Serial No. 744,313

8 Claims. (Cl. 148-415) This invention relates to copper zirconiumalloys, and

stronger in a transverse-to-cold-working' direction than in a parallel-to-cold working direction.

Silver-bearing copper alloys and chromium-copper alloys have been found to possess good electrical characteristics when used to fabricate electrical conductors forv use at elevated temperatures'but'such'alloys, when subjected to cold working, are not strengthenedina trans? verse-to-cold-working direction, as compared to a parallel-to-cold-working direction, andhence do not possess the above mentioned unique physical characteristic ofthe novel copper-zirconium alloy of the'presentinvention. Moreover, copper-zirconium alloys, producediprior to the novel method of the present invention, are like the.

above mentioned silver-bearing'copper andchromitimcopper alloys in that such copper-zirconium alloys do not become stronger in a transverse-to-cold-working di rection than in a parallel-to-cold-working direction.

In addition to the above, the novel copper zirconium alloy of the present inventionpossesses'another unusual physical characteristic in. that'it isnot notch sensitive. That is, a notched electricalconductor, such as "acommutator segment, formed fromthe copper-zirconium alloy of the present invention; is not weakened by the' presenceof the notches. In contrast,flnotchedelectrical conductor formed of silver-bearing copper or chromiumcopper alloys possess great notch sensitivity in that the conductor is very materially weakened by the presence of the notches.

It is therefore an object ofthe present invention to.

provide a novel copper-zirconium alloy, and method for producing same, which alloy is stronger in a transverseto-cold-working direction as comparedto a parallel-tocold-working direction.

It is another object of the present invention to provide a novel copper-zirconium alloy, and method for producing same which alloy can be.used to. form notched articles that are not weakened by the presence-of the notches.

It is another object of thepresent invention to provide a novel copper-zirconiumalloy having high. strength and high electrical conductivity at elevated temperatures, and

a novel method for producing said alloy whereby the graingrowthand hence, grain size ofthewalloycanbe 2,879,19l Patented Mar. 24, 1959 ice 2 selectively varied without materially reducing said high strength and high electrical conductivity;

It is another object of'the present invention'to providean improved electrical conductor, for useat elevatedtem peratures, which conductor is formed;by cold working and is stronger in the-transverse-to-cold workingdirec tion than in the parallel to-cold-working-direction:

It is still another object-of the present inventionzrto provide an improved electrical conductor, foruse'at ele vated temperatures, which conductor is'provided notches but not weakened by the presence thereof.

Further objects and advantagesof the present invention will be apparent from the following description, reference being had to the accompanying drawingsawherein; a; preferred form of embodimentof-the invention-is clearly shown.

In the drawing:

Figure 1 is a perspective view of 'a typicalcommutator formedof segments each of which segments constitutes an electrical conductor fabricated according to the present invention;

Figure 2 is a perspective view of a rolled-plateformed" from the novel copper-zirconium alloy 'of' the-present invention and according to the novel method thereof; Transversely disposed specimens for demonstrating-the advantages of the presentinvention are cut from the rolled platein the'manner designated by dotted delineation thereon;

Figure 3 is a perspective view of a fir'sttype -of testi specimen formed from the novel copper zirconium all'oy of the present invention, saidspecimen havirigbeencut froma plate of the type illustrated in Figure-'2," and Figure- 4 is a' perspective view of'a'secondtype-"of test specimen formed from the novel copper=zirconium alloy of the present invention-, said specimen'having been cut from 'a plate of the type illustrated in Figure '2.

Reference is next made to Figure l of the-=drawings which illustrates a typical commutator installation- For many years the development of small, high-speed'motors, employing commutators of the same general type as'Fig ure 1, was limited by the low strength of=substantially pure, hard-rolled copper commutators. Investigations showed that the'strength of tough-pitch copper at'high temperatures could be increased considerably Without loss of electric conductivity by small additions of silver: The" addition of 25 to 35 oz. of silver per ton ofcoarsegrained, hard-rolled copper extended the strength" of* commutators to prolonged exposures at'390" F. and sh0rt-' time exposures as high'as'750- F.

Up to World War II, this alloy was able to withstand higher temperatures than the best available insulating, materials. When better insulating'materials' were-developed, however, commutator design was again heldback' by the softening of "copper at elevated temperatures. Experience indicates that 450 F is the highest'operating' temperature permissible for'motors equipped with silvercopper commutators.

Chromium-copper then was investigated" and' found suitable for commutators operating at or above 500 F5 (260 C). Use of this alloy also presented a number' of problems. Chromium-copper is extremely difiicultto cast into good, solid wire bar,- and die wear in drawingis extremely severe due to the undissolved chromium and oxide stringers.

Also, if used in thefully heat-treated: condition, chromium-copper commutator. segments have artendency. to crack at the dovetails. Typical commutator-segments 20; provided with dovetails 22 are-illustrated inFigure =1. Tou prevent cracking of segments formedof chromium-copper; it is necessary to heat-treat the bars and .lowerrtheirzhardness below. the brittlestage. This operation :is:very.'crit.-.. ical since it is necessary to ,exceedjhe minimunrtrecrystallization temperature for a period of time that must be determined individually for each batch of copper and each bar size. It is also necessary to check the bar hardness before and after heat treatment. The loss of chromium-copper from both over-softening and from poor original casting runs from 10 percent to as high as 80 percent of the total metal processed.

Commutator segments are subject to stresses from the forces exerted by the clamping or V-ring 24, Figure 1, and from the centrifugal forces generated by high speed rotation, Increasing temperatures decrease the ability of the segments to withstand these stresses. In extreme cases, the increased temperature causes the metal to deform and the segments to be thrown out. Even segment movement of a few ten-thousandths of an inch will cause excessive brush wear. If the metal remains hard, but loses strength and ductility, the segments may crack. Failure of any segment, either by excessive deformation or by cracking, will cause complete motor failure.

During motor rotation, the stresses set up by centrifugal force are transverse to the direction in which the commutator segment 20 is rolled, such direction of cold working being diagrammatically represented by an arrow 25 in Figure 1. Such stresses are also across the dovetail 22 that constitutes part of the commutator design. These centrifugal forces may be augmented by slight distortions resulting from the heavy compression forces exerted on the ends of the segments 20 by the locking rings 24. Since the thermal expansion of copper is greater than that of steel, the assembly compression forces become greater when the temperature rises. In general, cracks in segments of chromium-copper cornmutators invariably. appear to have been tensile failures in the transverse-to-rolling direction.

The novel copper-zirconium alloy of the present invention does not possess the above mentioned disadvantages and hence is highly desirable for the fabrication of superior electrical conductors for use at relatively high temperatures.

It has been discovered that the superior alloy of the present invention can be produced as follows:

A percentage of from .01 to .15 zirconium and the balance electrolytically refined copper are alloyed in a suitable manner.

Electrolytically refined copper suitable forpracticing the present invention, as employed in the claims herein, is defined in ASTM Standards for Electrolytic Copper, 1955 edition, part 2, paragraph B-224-52 as copper having 100 percent conductivity at 20 degrees centigrade and again at paragraph B5-43 as 99.900 copper and silver (silver being counted as copper) having a resistivity of .15328 international ohm per meter-gram at 20 degrees centigrade.

It will be understood that refined copper that is refined by means other than electrolytic means can be used in practicing the present invention provided such refined copper has an electrical conductivity equal to the electrical conductivity of electrolytically refined copper.

The resulting alloy is next solution annealed at a temperature between 1300 degrees and 1900 degrees Fahrenheit for from five to sixty minutes. The solution annealed alloy is then quenched to produce a rapid reduction in temperature after which it is cold worked, preferably to the extent that a fifty percent reduction in crosssectional area is produced by such cold working.

A preferred composition for practicing the method of the present invention consists of an alloy of substantially .08 zirconium and the balance copper.

At solution annealing temperatures below 1300 degrees Fahrenheit the strength developed subsequently by the copper-zirconium alloy is too low for eificient utilization of the potential of the invention. At substantially 1400 the transverse-to-cold working direction is realized.-

I As stillanother unusual and useful characteristic, the

alloy of the present invention can be solution annealed at various temperatures, throughout the previously mentioned range of 1300 degrees to 1900 degrees Fahrenheit, so as to selectively control grain growth to produce the desired grain size in the finished work. In solution annealing the alloy of the present invention at the various temperatures it has been found that the high strength and high electrical conductivity of the present alloy are not materially affected. Since grain growth is a function of time, as well as temperature, the time at temperature can also be varied throughout a corresponding broad range to control grain growth without materially affecting the above mentioned characteristics. To the knowledge of the present inventors no other copper base alloy can be solution annealed at various temperatures throughout a range but must be treated at certain time-temperature relationship in order to obtain the desired strength and electrical conductivity characteristics. Hence the flexibility in controlling grain size is not present. It will therefore be understood that the above described solution annealing range of the alloy of the present invention is particularly useful where large batches of the alloy are to be uniformly heated throughout thus enabling the central portions of the batches to be gradually brought up to a selected low solution annealing temperature, say 1300 degrees-1500 degrees Fahrenheit, without excessive heating and hence excessive grain growth occurring in the surface portions of the batches.

It has further been discovered that the electrical conductivity of the novel copper-zirconium alloy of the present invention can be increased to the high value of about ninety-five percent of the electrical conductivity of pure copper. This increase in electrical conductivity may be effected by aging the alloy in a furnace either after the previous mentioned solution anneal and quench steps, or after the previously mentioned cold-working step. Maximum increases in electrical conductivity are realized when the alloy is aged at between 450 degrees and 950 degrees Fahrenheit and preferably for a period of between 60 and minutes.

To evaluate the strength of the present alloy, both tensile and creep-rupture tests were performed. In the tensile test, the metal was loaded at a comparatively rapid rate to failure.v In the creep-rupture test, delayed failure is brought about at lower load levels.

Because some metals exhibit great sensitivity to notches, accidental scratches or a designed notch such as thread on a bolt, both notched specimens 27 and unnotched specimens 28 were used for each test, see Figures 3 and 4. Specimens for each test were taken from both the parallel-to-rolling and transverse-to-rolling directions. Such transversely disposed specimens are indicated at 28-A and 28-B, respectively, in Figure 2,

'with such specimens being cut from a rolled plate 31.

The direction of rolling or cold working of plate 31 is diagrammatically indicated by an arrow 25.

Notched bars were used in the rupture tests to indicate the properties of the metal that could be expected in a commutator.

Some mechanical properties of the present novel alloy, both at room temperature and at 550 F. (299 C.), are shown in Table I set forth at the end of the present specification. These properties were determined after the alloy had been given the proper amount of cold work and heat-treatment to produce the hardness noted. For comparison, the properties at 550 F. of silver-bearing copper and of chromium-copper at the usual hardness levels are shown in Table II. The electrical conductivity of the three commutator alloys is shown in Table III.

The softening temperatures of both cold-worked and aged alloy of the present invention and of commercial chromium-copper, after exposures of /2 hour is about 930 F. (500 C.). Silver-bearing copper (25 oz. per ton) softens from 600 to 625 F. (315 to 330 C.). In service, commutator segments are usually exposed to 3 lower temperatures; cumulative exposuretimes, however, may be much longer.

Rupture tests werev made to determine the strength of the commutator alloys underextended loading times at elevated temperatures,, Table IV. As indicated, the rupture strength of all ofthe, alloys decreased at the elevated temperature. Silver-bearing copperexposed to 550 F. (2889 C.) for 100 hr. or more was permanently damaged and lost much of its strength. Chromiumcopper evidenced great notch sensitivity, although it retained considerable strength in the parallel-to-rolling direction. In the transverse-.to-rolling direction, the rupture strength was approximately thatof softened silverbearing copper. Chromium-copper did not soften permanently.

Quite different results were obtainedwith the ZiI'CO nium-copper alloy of the present invention. First, the. notches strengthened the copper-zirconium alloy bars that were loaded in either direction, transverse or longitudinal. Second, specimens loaded in the transverse-torolling direction were stronger than corresponding speci-, mens loaded in the parallel-,to-rolling direction. Unnotched bars, after 500 hr. ofsustained loading inthe longitudinal direction at 550 F. retained 76 percent of their original tensilestrength. Under each of the conditions of-loading, they were still stronger.

It can be noted also that the time-rate of loss of strength of alloy of the present invention .was very much less than the other two alloys.

Another property. of primary importance is the ability of a commutator alloy to develop and maintain a commutation film. Commutation tests show that brnsh life (both at sea level and at high altitudes) on commutators, fabricated from the alloy of the present invention, is at least equal to the brush life on similar commutators of silver-bearing copper, or chromium-copper.

TABLE I Typical mechanical properties of the zirconium-copper alloy produced according to the present invention Specimens taken in parallel-to-rolling ,direcion:

Hardness, Rockwell B 46 Tensile strength, p.s.i 53, 500 42, 000 Yield strength, 0.2% ofls'et, p.s.L. 50, 500 40, 000 Proportional limit, p.s.i- 19, 000 Modulus of elasticity, p.s 18, 700,000 Elongation, percent in 2 in- 10.6 7. Reduetionin area, percent; 54. 3 Specimens taken in transverse-to-rolling direcion:

Hardness Rockwell B. 66 Tensile strength, p.s.i ,000 Yield strength, 0.2% ofiset, p.s. 41, 500 Elongation, percent in 2 in 6. 5

TABLE '11 Typical mechanical properties of silver-bearing copper and chromium-copper at 550 F. (288 C.)

. .T EI I Electrical conductivity of copper alloys at 68 F. (20 C.)

' Percent International annealed copper standard 100.0 Silver-bearing copper, Rockwell 61 B 97.0 Zirconium-copper alloy of the invention:

Rockwell 68 B 95.8 Chromium-copper, Rockwell 82' B 82.0

TABLE} IV Typical rupture strengths of-commutat0r alloys at550 F. (288 'C'.)'

After 'hr., p.s.i. After 500 hr., p.s.i.

Un- Notched Un- Notched notched: notched Parallel-to-rolllng direction;

Zirconium-copperv alloy of the Invention 34,000 37;500 32,000 35,500 Chromium-copper 36,000, 28,000 32.500 23,500 Silver-bearing copp 17,500, 15,500 10,500 Transverse-to-rolling direction:

Zirconium-copper alloy 7 ofthe invention; 38,000 41,500 36,500 38, 500 Chromium-copper 18,-500 17,000 13,000 12,000 Silver-bearing copper. 20,000 p 15,500 14,000

1 Values extrapolated to 500 hr. from tests running upto 350 hr.

2 Rupture strength approximately;l0;000'.p.s.l. after-210 hr.-, and decreasing rapidly.

a Rupture strength approximately 10,000 p.s.i. after 300 hr., and decreasing rapidly. Rupture strength of silver-bearing copper, uunotched, after 500 hr. is estimated only: Recrystallization and softening of the metal, very noticeable in the notched specimens, might make these apparent values higher than the truevalues.

EXPERIMENTS TO DETERMINE THE RELATIONSHIP BETWEEN HARDNESS A-ND AGING TIME OF SUPER- SATURATED COPPER ZIRCONIUMALLOYS Reference is made to US. Patent No. 2,097,816 issued to Hensel et al. on November ,2; 1937;

The Hensel patent shjows by'a curve (line 10', Figure 1) how an alloy described as copper-l percent zirconium that has been quenched from 950 'centigrade (1742 degrees Fahrenheit) increases in hardness on artificial aging at 45 0 centigrade (842' degrees Fahrenheit). Points readfrom the curve show approximately the following values:

Aging Time, hours, Hardness,

Rockwell B As quenched 0 (1) (presumably after natural aging) 2:11;---. .II:I Q: 53 4 58 a 58 16 53 Two alloys were prepared'in accordance with'the present invention, using electrolytically refined copper and a copper-zirconium master alloythat was prepared from.-

commercial (Kroll process) zirconium sponge available in 1957. These two zirconium-copper alloys were rolled to bar, annealed at 950 centigrade (1742 degrees Fahrenheit) and aged at 450 centigrade (842degreesFahrenheit). One of these alloys contained 0.22 percent of zirconium, the other 1.0 percent.

The average hardness values obtained from these two.

Hardnsss, Rockwell B Aging Time, hours 0.22% Zr Alloy 1.0% Zr Alloy OOOOJQQH assess When plotted, both sets of data indicate a maximum hardness of about Rockwell B 30. Since the determinations were made at different times, the diiferences are more probably caused by small errors in the Rockwell B readings than by differences in the hardness of the alloys. (The Rockwell B scale is not well adapted to making hardness determinations on so soft a metal.)

There is almost no relationship between the aging response of these-two zirconium alloys made in 1957, which have a maximum hardness of about Rockwell B 30, and the alloy described in U.S. Patent 2,097,816, which had a maximum hardness of about Rockwell B 58. The alloy described in the patent cited showed a vigorous response to age hardening, while the two alloys produced according to the present invention show a relatively feeble response. That these striking differences, which suggest entirely different alloys, are not caused by small differences in the zirconium content is demonstrated by the similar response of the two alloys of widely different composition made in 1957.

Only a conjecture as to the cause of the hardening behavior of the alloy described in US. Patent 2,097,816 can be advanced. Zirconium available at the time the patent was issued (1937) probably was produced by aluminothermic reduction and may have contained important quantities of silicon, titanium, iron, aluminum, and possibly other impurities such as nickel or chromium. Quite possibly the iron, nickel, etc., may have combined with the silicon, the titanium, or even with a part of the zirconium, to form compounds which could be precipitated from solution during aging. This fortuitous hardening produced an alloy suitable for making welding electrodes. n the other hand, the impurities increased the electrical resistance of the unaged alloy to less than 50 percent I.A.C.S. Line 11 of Figure 1 of the patent shows that the conductivity could be increased by aging at 450 centigrade (842 degrees Fahrenheit) to about 73 percent I.A.C.S. at the maximum hardness, and to a maximum of about 78 percent I.A.C.S. by severely over aging (32 hours or more at 450 centigrade (842 degrees Fahrenheit).

Ultra-high quality commutators demand an alloy with good thermal stability and with an electrical conductivity of 90 percent I.A.C.S. or higher. The teachings of US. Patent 2,097,816 do not suggest that such an alloy could be produced from zirconium-copper. Rather, they tend to show that a very high conductivity alloy could not be produced from an alloy of copper with zirconium. Since high-conductivity (more than 90. percent I.A.C.S.), thermally stable alloys have been produced from binary alloys of copper and zirconium, and since these alloys do not show 'the response to artificial aging of the alloy described in the patent cited, it is believed that the alloy described in the patent was not a binary alloy of copper and zirconium, but that it contained other elements fortuitously introduced as impurities. Moreover, it is believed that these impurities were largely responsible for the behavior of-the alloy during artificial aging and for its mechanical and electrical properties.

While the form of embodiment of the present invention as herein disclosed constitutes a preferred form, it is to be understood that other 'formsmight be adopted, all coming within the scope of the -claims which follow.

We claim:

1. The steps in the method of producing an improved metal alloy for cold working which alloy possesses greater strength in a direction transverse to the direction of cold working, said method comprising alloying about .01 to about 1.0 percent zirconium and the balance electrolytically refined copper; solution annealing said alloy at a temperature between 1300 degrees and 1740 degrees Fahrenheit; quenching said alloy; and subjecting said alloy to cold working. I

2. The steps in the method of producing an improved metal alloy for cold working which alloy possesses greater strength in a direction transverse to the direction of cold working, said method comprising alloying about .01 to about 1.0 percent zirconium and the balance refined copper having an electrical conductivity equal to that of electrolytically refined copper; solution annealing said alloy at a temperature between 1300 degrees and 1740 degrees Fahrenheit; quenching said alloy; and subjecting said alloy to cold working.

3. The steps in the method of producing an improved metal alloy for cold working which alloy possesses greater strength in a direction transverse to the direction of cold working, said method comprising alloying about .01 to about .15 percent zirconium and the balance refined copper having an electrical conductivity equal to that of electrolytically refined copper; solution annealing said alloy at a temperature of substantially 1400 degrees Fahrenheit; quenching said alloy; and subjecting said alloy to cold working.

4. The steps in the method of producing an improved metal alloy for cold working which alloy possesses greater strength in a direction transverse to the direction of cold working, said method comprising alloying about .08 percent zirconium with the balance refined copper having an electrical conductivity equal to that of electrolytically refined copper; solution annealing said alloy at a temperature between 1300 degree and 1740 degrees Fahrenheit; quenching said alloy; and subjecting said alloy to cold working. 7 I

5. The steps in the method of producing an improved metal alloy for cold working which alloy possesses greater strength in a direction transverse to the direction of cold working, said method comprising alloying about .01 to about .15 percent zirconium and the balance refined copper having an electrical conductivity equal to that of electrolytically refined copper; solution annealing said alloy at a temperature between 1300 degrees and 1740 degrees Fahrenheit; quenching said alloy; and subjecting said alloy to cold working.

6. An electrical conductor made of a cold worked alloy of about .01 to about 1.0 percent zirconium and the balance refined copper having an electrical conductivity equal to that of electrolytically refined copper, said alloy having an electrical conductivity in excess of 90 percent.

7. An electrical conductor having an electrical conductivity greater than percent I.A.C.S. made of a cold worked alloy of about .01 to about .15 percent zirconium and the balance refined copper having an electrical conductivity equal to that of electrolytically refined copper.

8. An electrical conductor made of a cold worked alloy of about .08 percent zirconium and the balance electrolytically refined copper, said alloy having an electrical conductivity in excess of percent.

References Cited in the file of this patent UNITED STATES PATENTS 2,097,816 Hensel et al. a Nov. 2, 1937 2,842,438 Saarivirta et al. --1 July 8, 1958 2,847,303 Pruna Aug. 12, 1958 OTHER REFERENCES Metals Handbook, 1948 edition by American Society for Metals, page 858.

Claims (1)

1. THE STEPS IN THE METHOD OF PRODUCING AN IMPROVED METAL ALLOY FOR COLD WORKING WHICH ALLOY POSSESSES GREATER STRENGTH IN A DIRECTION TRANSVERSE TO THE DIRECTION OF COLD WORKING, SAID METHOD COMPRISING ALLOYING ABOUT .01 TO ABOUT 1.0 PERCENT ZIRCONIUM AND THE BALANCE ELECTROLYTICALLY REFINED COPPER; SOLUTION ANNEALING SAID ALLOY AT A TEMPERATURE BETWEEN 1300 DEGREES AND 1740 DEGREES FAHRENHEIT; QUENCHING SAID ALLOY; AND SUBJECTING SAID ALLOY TO COLD WORKING.

Images