CA1071901A - Aluminum iron cobalt silicon alloy and method of preparation thereof - Google Patents

Abstract

ABSTRACT
This disclosure relates to a method of continuously preparing a heat-resistant aluminum base alloy electrical conductor that has a minimum electrical conduc-tivity of sixty-one percent (61%) IACS, improved properties of ultimate tensile strength, yield strength and elongation as compared with conventionally-prepared aluminum alloy electrical conductors, and superior thermal stability to continously-processed aluminum alloy electrical conductors.
The conductor is prepared from an alloy melt containing from 0.30 to 1.30 weight percent iron, from 0.20 to 1.60 weight percent cobalt, from 0.48 to 0.88 weight percent silicon with the balance of aluminum containing trace elements selected from the group consisting of copper, manganese, magnesium, titanium, vanadium and zinc wherein the individual concen-trations of said trace elements do not exceed 0.05 weight percent and the total concentration of said elements does not exceed 0.15 weight percent.

Description

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BACKGROUND OF THE INVENTION

This invention relates to an aluminum alloy suitable for~use in fabricating~an electrical conductor and more particularly concerns an aluminum alloy suitable for fabricating an electrical conductor for use in applications in which the electrical conductor is sub\~]ected to high temperatures for extended periods of time`land therefore must have good thermal stability.
The use of various aluminum alloys, convention-ally referred to as EC, to fabricate electrical conductors is well established in the art. Such alloys characteristi- ~-cally have conductivities of at least`61 percent of the International Annealed Co~per Standard, hereina~ter referred to as IACS and chemical constituents consisting of a sub-stantial amount of pure aluminum and small amounts of impuri-ties such as iron, silicon, vanadium, copper, manganese, magnesium, zinc, boron and titanium. The iron content is generally l~ss than ,0.30 percent and the silicon content is generally less than 0.15 percent. The physical properties of electrical conductors Xabricated from prior aluminum alloys have proven less than satisfactory for many applica-tions which require that the electrical conductor used have a high degree o~ thermal stability. Generally desirable .
thermal stability has been obtainable only at less than desirable tensile s~rength, elongation or conductivity.
For example, it is generally accepted that industrial purity aluminum has a recrystallization tempera-ture of from about 300F to about 662F (150C to 350C). It is also accepted tl~at such aluminum has a very low resis-:' :

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tance to heat and undergoes a softening phenomenon at atemperature of ,rom about 212F to about 392F (lOO~C to 200C)~ Much work has been done in the past to improve the heat resistance of aluminum, however the majority of alloys developed which have acceptable electrical conductivity undergo a significant loss of strength upon being exposed to temperatures of from about 300F to about 392F (150C to 200C) for several hours. Such alloys usually retain only from about 60 percent to about 80 percent of their original tensile strength and elongation after exposure to tempera-tures in this range for several hours.
Thus, it becomes apparent that a need has arisen within the electrical inclustry or an aluminum alloy ~rom which electrical conductors might be abricated which will have both improved thermal stability and tensile strength and acceptable conductivity, elongation and yield strength.
In the past aluminum alloys and rod for the~
fabrication of wirejhave been manu~actured for commercial use by a plurality of separate steps which include casting an aluminum alloy ingot, reheating the ingot to a tempera- -ture which would permit hot rolling of the cast ingot into redraw rod, solutionizing the rod and watex ~uenching the -~
rod before cold drawing the rod into wire. After drawing the wire fabricated by the aforementioned procedure is generally annealed in order to obtain acceptable tensile strength. Although wire produced by the aforementioned techniques has acceptable tensile strength, it is difficult and in fact almost impossible to produce an aluminum alloy .

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1 ~7 ~ 9 ~ 1 wire having high thermal stability and acceptable elongation and electrical conductivity using this technique because the procedure inherently produces a structure which çontains elements in solution because all the alloying elements are not removed from solution by the quenching steps and because large precipitates are formed if the alloy is processed a~
high temperatures. The cell structures o-f aluminum alloy wire fabricated from base metal so processed is unstable thereby promoting the formation of large cells when the wire is subjected to any heat treatment thereby leading to a inished produc~ which has either poor thermal stability or poor physical and poor electrical properties~
In Canadian patent 967,405, issued Mar 13, 1975, entitled Aluminum Alloy Used for Electrical Conductors and Other Articles, and Method o ~aking the Same, there is disclosed an improved alu~inum allo~ electrical conductor j prepared by continuously casting an alloy consisting essen~-ially o from 0 Z0 to 1~60 ~eight percent cobalt, from 0.3~ :
to 1.30 ~eight percent .iron, $rom ~9~50 to 97~0 weight percent aluminum, and optionall~ up to 2.n weight percent .
of at least one additional alloying element selected ~rom a speciied group ~hich included silicon to ~orm a c~ntinuous aluminum alloy bar, h.ot-~orking the bar to form continuous rod ~hich is subsequently drawn into ~ire ~ith~u~
intermediate a~neals, and therea*er ~lnally~ annealed ~hile .
this product yielded impro~ed proper~ies o~ increased ultimate - elongation, benda~i1ity and ~atigue resistance ~hen compared with the conYentional EC allo~, it still did not yield a suficient degree o thermal stability to permit use at elevated temperatures for extended periods o~ ~ime~
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Therefore it becomes apparent that there remains a need within the electrical industry for an efficient and economical method of fabricating an aluminum alloy and an aluminum alloy rod from which an electrical conductor having high thermal stability and acceptable physical ana electrical properties can be fabricated.

STATE~EMT OF THE INVENTION

Thexe is provided in accordance with the inven-tion an aluminum alloy electrical conductor for use in appli-cations which require electrical conductors with high thermal stability and which has an increased tensile strength when compared to prior heat-resistank aluminum alloys.
In the broadest aspect of this invention the electrical conductor is manufactured from an aluminum alloy containing fxom 0.30 weight percent iron, from 0.20 to 1060 weight percent cobalt, from 0.48 to 0.88 weight percçnt silicon with the balance of the alloy consisting of aluminum containing trace elements selected from the group consisting of copper, manganese, magnesium, titanium, vanadium and zinc when the concentrations of the individual trace elements do not exceed 0.05 weight percent and the total trace elements concentration do not exceed 0.15 weight percent.
More specifically, the invention resides in the method of preparing a heat-resistant aluminum alloy ~;~
electrical conductor comprising the steps of:

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(a) alloying from 0.30 to 1.30 weight percent iron, from 0.20 to 1.60 weight percent cobalt, at least one additional alloying element, and the balance of aluminum;

(b) casting the alloy in a moving mold formed between a groove in the periphery of a rotating casting wheel and a metal belt lying adjacent to said groove for a portion of its length to form a continuous aluminum base alloy bar; and (c) hot-rolling the continuous bar :
substantially immediately after casting while the bar is .
in substantially that condition as cast to form a con- :
tinuous rod;

characterized in that said at least one -~
additional alloying element is silicon in a range of from 0.48 to 0.88 weight percent with the balance of aluminum containing trace elements selected from the group consisting of copper, manganese, magnesium, titanium, vana~ium and zinc wherein the individual concentrations o said trace elements do not exceed 0~05 weight percent ~ :
and the total concentrations of said trace elements does not exceed 0.15 weight percent, and wherein said cast bar :
contains iron-aluminum-silicon-cobalt intermetallic pre-cipitates that are broken-up and evenly dispersed through- ~ .
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out the aluminum ~atrix during the hot-rolling operation thus forming precipitate particles havi.ng a diameter of less than one micron when ~easured along the transverse axis of said particles.
It should be apparent that while the aforementioned Canadian Patent 967,405, of which this application is a -selection Patent application thereon, contemplates that silicon could be selected from the list of optional alloying elements in an amount up to 2.0 weight percent, there was no suggestion therein that improved heat-resisting properties could be achieved by selecting the silicon in t~e specific range of 0.48 - 0.88 in accordance with the invention, nor in controlling the trace impuritles in amount~ taught by the invention.
In one preferred embodiment o the invention the iron is from 0.30 to 0.95 weight percent and the cobalt ~J -'~5 ~ ' ~ ~ 7 ~ 9 ~.
is rom Q.20 to 0.50 ~e~g~t percent. ~n ano~er Rre.~erred em~odiment o~ t~e invention the iron ~s from 0. 30 to O ~ 55 we~ght ~ercent and the co~lt ~s ~rom Q. 5a to Q,8~ weight percent. ~n yet a furt~er Rref~rred e~Bod~ment o~ t~.e in~ent~on the ~ron from Q,55 to 1.3a ~e~ght percent and the cobalt ~s from 0 8Q to 1.60 ~eig~t percent~
`DETArLED DESCRrPTrON OF TH~ INVE~TION
. . . _ . _ For the.purpose of clar~ty~, the following terminology~ used In t~îs ~pRlxcat~on ~s explained as ollows:
Rod - A solid ~roduct th.at is long in relation to its cross section. Rod normally has a cross section o:E
between 3 inches and 0.375 inches.
Wire - ~ solid wrough.t product that is long in relation to its cross section, ~hich is square or rectangular with sharp or rounded corners or edges or is round, a regular hexagon, or a regular octagon, and whose diameter OT greatest peTpendicular distance between parallel faces is between 0.374 inches and 0.0031 inches.
The aluminum alloy~ electrical conductor of th.is invention is prepared by initially melting and alloying aluminum with the necessary amounts o iron, cobalt, silicon .
and oth~r co~sti~utents to provide the requisite alloy for processing. Typical impurities or trace elements also pre~
sent wlthin the melt, but only in trace quantities such as less than 0.05 weight percent each with a total content o~
.,r trace impurities generall~ not exceeding 0.15 weight percent.
O course~ when adjust~ng the amounts o~ trace elements due ~ ;
cons.ideration must ~e gi~en to the conductivity o the inal alloy~s~nce some trace element$ afect conduct~vit~ more ~ .

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severely than others. The typical trace elements include vanadium, manganese, magnesium, ~inc, boron and titanium.
If the content of the titanium is relatively high (but still quite low compared to the aluminum, iron and silicon content), small amounts of boron may be added to tie up the excess -titanium and keep it from reducing the conductivity of the wire.
Silicon, iron and cobalt are the major con-stituents added to the melt to produce the alloy of the present invention. Normally, about 0.48 weight percent sili-con, about 0.47 weight percent iron and about 0.50 weight percent cobalt are added to the typical aluminum component used to prepare the present alloy. Of course, the scope of the present invention includes the addition of more o~ less silicon and iron together with the adjustrnent of the content of all alloying constituents.
After alloying, the melted aluminum composition is continuously cast into a continuous bar. The bar is thlen hot-worked in substantially that condition in which it is received from the casting machine. A typical hot-working operation compxises rolling the cast bar in a rolling mill substantially immediately after being cast into a bar.
One example of a continuous casting and rolling operation capable of producing continuous rod as specified in this application is as follows: -A continuous casting machine serves as a means for solidifying the molten aluminum alloy metal to provide a cast bar that is conveyed in substantially the condition in which it is solidified from the continuouscastin~ machine ' . :.

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to the rolling mill, which serves as a means for hot-formin~
the cast bar into a rod or another hot-formed product in a manner which imparts substantial movement to the cast bar along a plurality of angularly disposed axes.
The continuous casting machine is of conven-tional casting wheel type having a cast:Lng wheel with a casting groo~e partially closed by an endless belt supported by the casting wheel and at least one idler pulley. The casting wheel and the endless belt cooperate to provide a mold into one end of which molten metal is poured to solidify and from the other end of which the cast bar is emitte~ in substantially that condition in which it is solidified.
The rolling mill is o~ conventional type having a plurality of roll stands arranged to hot-form the cast bar by a series of deformations. The continuous casting machine and the rolling mill are positioned relative to each other so that the cast bar enters the rolling mill substantially immediately after solidification in substantially that con-dition in which it was solidified. In this condition the cast bar is at a hot-forming temperature within the range of temperatures for hot-forming the cast bar at the initiation of hot-forming without heating between the casting machine and the rolling mill. In the event that it is desired to more closely control the hot-forming temperature of the cast bar within the conventional range of hot-forming tempera-tures, means for adjusting the temperature of the cast bar -~
may be placed between the continuous casting machine and the rolling mill without departing from the inventive concept disclosed herein.

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The roll stands each include a plurality of rolls which engage the cast bar. The rolls of each roll stand may be two or more in number and arranged diametri cally opposite from one another or arranged at equally ~:
spaced positions about the axis of movement of the cast bar through the rolling mill. The rolls of each roll stand of . .
the rolling mill are rotated at a predetermined speed by a power means such as one or more electric motors and the casting wheel is rotated at a speed generally determined by :
its operating characteristics. The rolling mill serves to ~.
hot-form the cast bar into a rod of a cross-sectional area substantially less than that of the cast bar as it enters the rolling mill.
Thus, it will be understood that w.ith this apparatus cast aluminum rod of an indefinite number o.f. dif-ferent lengths is prepared by simultaneous casting of the molten aluminum alloy and hot-forming or rolling the cast alumimum bar.
The continuous rod produced by the casting and rolling operation is then processed in a reduction operation designed to produce continuous wire of various gauges. The annealed rod (i.e., as rolled to E'. temper) .is drawn through a series of progressively constricted dies, without inter-mediate anneals. to form a continuous wire of desired dia-meter. At the conclusion of this drawing operation, the al-loy wire will have an excessively high tensile ~trength, :
yield strength and unacceptably low ultimate elongation, .
plus a conductivity below that which is industry-accepted as a minimum for the electrical conductor, i.e., 61 percent ::~

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of IACS. The wire is then annealed or partially annealed to obtain the desired tensile strength, ultimate elongation and conductivity and is subsequently cooled. At the conclusion of the annealing operation, it is found that the annealed alloy wire has the properties of acceptable conductivity and improved tensile strength together with unexpectedly im-proved percent ultimate elongation and a surprisingly in-creased thermal stability as specified previously in this application. The annealing operation may be continuous as in resistance annealing, induction annealing, convection an-nealing by continuous furnaces or radiation annealing by con-tinuous furnaces, or preferably, may be batch annealing in a batch ~urnace. Continuous annealing temperatures of from about 900F to about 1200F may be employed with annealing times of from about 0.001 seconds to about one (1.~) second.
Batch annealing temperatures of from about 350F to about 800F may be employed with annealing times o-f from about 8 hours to about 0.5 hours. Generally, however, annealing , times and temperatures may be adjusted to meet the require-ments of the particular overall processing operation so long as the desired tensile strength, elongation, conductiv-ity and thermal stability is achieved.
During the continuous casting of this alloy, a substantial portion of the silicon, iron and cobalt present in the alloy precipitate out of solution as aluminum-iron-silicon-cabalt intermetallic compounds. Included for il~
lustration but not limitation among the aluminum, iron, silicon, cobalt intermediate compounds which precipitate out of solution during casting are Co2Alg, FeA13, ~'eA16, AlgFe2-Si2, A112Fe3Si and other intermetallic compounds having the .

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general formula of AlxFeySiz. Thus, after casting the bar contains a dispersion of fine particles of the above men-tioned intermetallic compound in a supersaturated solid solution matrix. As the bar is rolled in a hot-working operation immediately after casting, the intermetallic particles are broken up and dispersed throughout the matrix thereby inhibiting large cell formation When the rod is drawn to its final size without intermediate anneals and then aged in a final annealing operation, the tensile strength, elongation and thermal stability are increased due to -the small cell size and the additional pinning of dislocations by the preferential precipitation of the aluminum-iron-silicon-cobalt intermetallic compound on the dislocations sites. Therefore, these dislocatian sources must be activa-ted under t:he applied stress o the drawing operation and this causes both the tensile strength, yield strength, elongation and thermal stabillty to be further improved.
The properties of the present aluminum alloy wire are sig nificantly affected by the size of the aluminum-iron-silicon-cobalt part:icles in the matrix. Coarse preclpitates reduce the percent elongation and thermal stability of the wire by enhancin~ neucleation and thus, formation of large cells which, in turn lowers the recrystal~ization temperature of the wire. Fine precipitates improve the percent elongation, tensile strength, conductivity, and thermal stability of the wire by reducing nucleation and increasing the recrystalli-zation temperatureO Grossly coarse precipitates of the iron-aluminum-silicon cobalt intermetallic compound cause the wire to become brittle and generally unuseful. Coarse ;.

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precipitates have a particle size of above one micron, measured along the transverse axis of the particle, and fine precipitates have a particle size of belo~ ~ne micron, measured along the transverse axis of the particle.
~ more complete understanding of the invention will be obtained from the following examples.

Sample alloy specimens were prepared containing 0.47 percent iron, 0.001 percent copper, 0.003 percent manganese, 0.001 percent magnesium, 0.001 percent titanium, 0.001 percent vanadium and 0.015 percent zinc, 0.50 percent cobalt with the balance being aluminum. Each sample also included a silicon content which varled in a range from 0.04 percent to 0.88 percent silicon. The samples containing from 0.48 to 0.88 percent silicon are considered to be in accordance with the invention. All samples were continuously cast into continuous bars and hot-rolled into continuous rod in similar fashion. The samples were then cold drawn through successively constricting dies to yield a wire having a diameter of 0.102 inches. Sections of the wire were col-lected on separate bobbins and batch Eurnace annealed at various temperatures and or various lengths o time to yield sections of varying tensile strengths. Several samples of each section were tested in a device designed to ~easure the ultimate tensile strength of each section, the elongation of each section, the electrical conductivity of each section and the yield tensile strength of each section. The results are tabulated in Tables I and II.

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Thermal Stability Test:
Other samples, prepared as above, were annealed in a batch furnace at 500F for a period of two hours and allowed to cool. After the cooling period, the samples were tested to determine ultimate tensile strenth and yield tensile strength and similar samples were then aged for four hours at 482F to determine the thermal stability of samples having different concentrations. Tbe results are as follows:

TABLE III
Bar No. SiUTS X 103 % Ret.~TS X 10 ~ Ret.
1 .0417.5 96 16.3 96

2 .1416.5 98 14.0 96

3 .1816.3 96 12.8 91

4 .2716.9 94 13.4 87 .3817.9 ~2 14.7 85 6 .4719.0 92 16.0 86 7 .4818.5 94 15.5 91 .
8 .6420.1 93 16.9 92 9 .8822.0 94 17.8 92 As can be readily determined Erom Table III, Bar Nos. 7-9, which are in accordance with this invention, generally have improved.physical properties after being subjected to the thermal stability test than do the samples having lower levels o silicon not in accordance with this invention.
Superior tensile strength, yield strength, ultimate elongation, electrical properties and thermal , ~7~
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stability are obtained when the silicon concentration of the present alloy consists of from about 0.48 to about 0.88 weight percent, an iron concentration of from about 0.30 to about 1.30 weight percent, a cobalt concentration of from about 0.20 to about 1.20 weight percent and the aluminum ~-which makes up the balance of the alloy contains trace elements of the type and amounts previously recited.
While this invention has been described in detail with partlcular references to preferred embodiments ~ ~ -thereof, it will be understood that variations and modifica-tions can be effective within the spirit and scope of the invention as described hereinbefore and as deined .in the appended claims.

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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of preparing a heat-resistant aluminum base alloy electrical conductor having a minimum conductivity of sixty-one percent (61%) IACS comprising the steps of:
(a) alloying from 0.30 to 1.30 weight percent iron, from 0.20 to 1.60 weight percent cobalt, at least one additional alloying element, and the balance of aluminum;
(b) casting the alloy in a moving mold formed between a groove in the periphery of a rotating casting wheel and a metal belt lying adjacent to said groove for a portion of its length to form a continous aluminum base alloy bar; and (c) hot-rolling the continuous bar sub-stantially immediately after casting while the bar is in substantially that condition as cast to form a continuous rod;
characterized in that said at least one additional alloying element is silicon in a range of from 0.48 to 0.88 weight percent with the balance of aluminum containing trace elements selected from the group con-sisting of copper, manganese, magnesium, titanium, vanadium and zinc wherein the individual concentrations of said trace elements do not exceed 0.05 weight percent and the total concentrations of said trace elements does not exceed 0.15 weight percent, and wherein said cast bar contains iron-aluminum-silicon-cobalt intermetallic precipitates that are broken-up and evenly dispersed throughout the aluminum matrix during the hot-rolling operation thus form-ing precipitate particles haying a diameter of less that one micron when measured along the transverse axis of said particles.

2. A method according to claim 1, charac-terized by maintaining the iron from 0.30 to 0.95 weight percent and the cobalt from 0.20 to 0.50 weight percent.

3. A method according to claim 1, charac-terized by maintaining the iron from 0.30 to 0.55 weight percent and the cobalt from 0.50 to 0.80 weight percent.

4. A method according to claim 1, charac-terized by maintaining the iron from 0.55 to 1.30 weight percent and the cobalt from 0.80 to 1.60 weight percent.

5. A method according to claim 1, 2 or 3 characterized by including the further steps of drawing said continuous rod through a series of wire-drawing dies without any preliminary or intermediate anneals to form a sire, and thereafter annealing to partially annealing said wire.

6. A method according to claim 4 or 5 charac-terized by including the further steps of drawing said continuous rod through a series of wore-drawing dies without any preliminary or intermediate anneals to form a wire, and thereafter annealing to partially annealing said wire.

7. A heat resistant aluminum base alloy electrical conductor having a minimum conductivity of 61%
IACS comprising:
(a) iron with a weight per cent of 0.30 to 1.30;
(b) cobalt with a weight per cent of 0.20 to 1.60 ;
(c) silicon with A weight per cent of 0.48 to 0.88;

(d) the balance aluminum but wherein trace elements are selected from the group comprising of copper, manganese, magnesium, titanium, vanadium and zinc and wherein the individual concentrations of said trace elements do not exceed 0.05 weight per cent and the total con-centrations of trace elements does not exceed 0.15 weight and wherein iron - aluminum - silicone -cobalt intermetalic precipitates have a diameter of less than-one micron when measured along the transverse axis of said particle and the said conductor retains at least 90 per cent of its original ultimate tensile strength after heat aging at a temperature of 482°F for a period of four hours.

8. The conductor as claimed in claim 7 wherein the weight percentages of iron range from 0.30 to 0.95 and of cobalt from 0.20 to 0.50.

9. The conductor as claimed in claim 7 wherein the weight percentages, of iron range from 0.30 to 0.55 and of cobalt from 0.50 to 0.80.

10. The conductor as claimed in claim 7 wherein the weight percentages, of iron range from 0.55 to 1.30 and of cobalt from 0.80 to 1.60.