CA1037843A - Manufacture of wrought brass alloy with special properties - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An improved ternary brass alloy includes silicon as the additive material. The composition limits of the constituents of the brass are fixed by two factors: (1) the Ms or Martensite transformation temperature, and (2) the brass must be a totally beta phase above 454° C. The composition may be betatized, that is, heated at approximately 800° C. and quenched at a fast rate so that the beta phase is retained in order to provide material having a shape memory effect, a low spring back coefficient and super elastic properties. A continuous betatizing and quenching process may be utilized in the manufacture of strip and sheet products. Alternatively, the composition may be hot worked between 600° and 700° C. and then cold worked to provide a material having a low spring back coefficient.

Description

1037~43 This invention relates to a method for manufacture of an improved brass alloy and its fabrication process, and in particular to a ternary brass alloy which may be fabricated into strip, sheet, or wire and which has a low spring back coefficient.
Utilization of various alloying elements in a brass (copper-zinc) system has been suggested heretofore for the purpose of obtaining certain desired characteristics in the system. Edmunds in United States Patent No.

2,394,673, for example, suggested the addition of 1.2% silicon to a 70/30 cartridge brass to provide resistance to season cracking. Gne of the present inventors has investigated numerous ternary beta brass alloys and has re-ported his findings in the Transactions of the Metallurgical Society of AIME, Volume 230 (1964), Page 267; Volume 230, Page 1662 (1964); Volume 236, Page 1532 (1966); Volume 239, Page 756; Volume 239, Page 1668 (1967); and etallur~ical Transactions, Volume 1, Page 251 (1970).
These cited references report studies relating to the Martensitic transformations of various ternary brass alloys. A determination was made of the Martensitic transformation temperature as a function of the percentage of the third element content in such alloys. In addition, an investigation of the reaction of such alloys to stresses and temperature changes were reported in these various publications. These publications thus serve as a `
part of the background upon which a reservoir of knowledge has been developed by the present inventors. Subsequent developmental work to that reported in these various publications has resulted in the development of the presently claimed invention. This subsequent developmental work has resulted in the discovery of a copper-zinc-silicon ternary alloy ant its fabrication process, j the alloy having an exceptionally low and unexpected spring back coefficient. ~ ~

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In a principal aspect then, the present invention provides a method for manufacture of an improved brass alloy having a low spring back co-efficient, said alloy consisting essentially of 0.25% to 3.00% by weight silicon ~0.56 to 6.6 atomic %), 25.00% to 40.00% by weight zinc (23.5 to 40.5, : -atomic %) and the balance copper in a mixed alpha plus beta phase of about 50% ~ 20% by weight alpha phase and the balance beta phase, comprising the -steps of: combining said constituents in a substantially homogeneous mixture, and (a) hot working said constituents at a temperature between 600 and 700C ~
and then cold working said alloy at the temperature of normal use; or (b) ~ -working said alloy by rolling methods to form the alloy and provide a poly-crystalline worked product.
The composition comprises a stable beta phase above 454C. and has a Martensitic transformation temperature defined by the formula MS~280K-80ZnK-120SiK.
In another aspect, the invention provides an improved brass alloy having a low spring back coefficient, consisting essentially of: 0.25% to

3.00% by weight silicon ~0.56 to 6.6 atomic %); 25.00% to 40.00% by weight zinc (23.5% to 40.5 atomic %); and the balance copper in a homogeneous, polycrystalline worked structure of 50% 1 20% by weight alpha phase and the `;
balance beta phase material.
The present invention provides an improvet ternary brass alloy `
having a low spring back coefficient. The invention further provides an improved brass alloy which is easily fabricated, particularly in the strip, sheet and wire forms. Such an alloy may thus be utilized for the manufacture ~ -of springs, particularly for electrical applications.
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The present invention provides a ternary brass alloy which exhibits a low spring back coefficient. It is a particular advantage of the alloy that it may be fabricated into a desired shape by means of presently available commercial fabrication operations. At the same time, the alloy maintains a low spring back coefficient, that is, a spring back coefficient which is lower than the coefficient of competitive materials presently available for commercial applications, e.g. beryllium copper, phosphor bronz, caTtridge brass and Alloy 194.
In a detailed description which follows, reference will be made to the drawings comprised of the following FIGURES:
FIGURE 1 is a portion of the isothermal section at 482C. of the copper silicon zinc ternary phase diagram;
FIGURE 2 is a portion of the isothermal section at 600C. of the copper silicon zinc ternary phase diagram;
; FIGURE 3 is a portion of the isothermal section at 760C. of the copper silicon zinc ternary phase diagram;
FIGURE 4 is a portion of the isothermal section at 847C. of the copper silicon zinc ternary phase diagram;
FIGURE 5 is a schematic representation of a typical fabrication ;
schedule illustrating the process by which the product of the present invention is manufactured;
FIGURE 6 is a schematic representation of means for providing the betatizing and quenching operation in accordance with our Application Serial No. 285,008 which is divided out of this application.

~037843 FIGURE 7 is a micro photograph illustrating the grain size and -structure of the alpha plus beta phases of the copper silicon zinc composition of the present invention taken at 40 magnifications;
FIGURB 8 is a micro photograph of the copper silicon zinc beta phase taken at 40 magnifications;
FIGURE 9 is a typical engineering stress versus strain curve; -FIGURE 10 is a stress versus strain curve for two samples of material incorporating the features of the present invention;
FIGURE 11 is a graph representing a typical improved alloy of the present invention in comparison with other prior art material and plots the ;
effect of cold rolling on the ul~imate strength of said materials;
FIGURE 12 illustrates the effect of cold rolling on yield stress;
FIGURE 13 illustrates the effect of cold rolling on the limit of proportionality;
FIGURE 14 illustrates the effect of cold rolling on the hardness; -FIGURE 15 illustrates the effect of cold rolling on elongation characteristics;
FIGURE 16 is a schematic illustration used to illustrate the definition of the spring back coefficient; and -2a FIGURE 17 is a graphical representation of spring back coefficient for the improved material of the present invention as compared to several prior art materials. -In the description which follows, the improved ternary brass alloy of the present invention will be described in terms of a product derived by means of a process. The unique properties which are the result of the invention require not only a specific composition of materials, but also a -~
unique method of manufacture. Thus, first there will be discussed the require-ments for the composition. This will be followed by a discussion of the ' '.

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~.037t~43 process used once the composition has been made Following thi~, specific examples will be set forth and then a discussion of the numerous advantage~
and properties of the invention will be provided.
Composition Copper, zinc and silicon are the materials utilized in the appro-priate amounts and fabricated by the appropriate process to provide the alloy of the invention The composition must provide a beta phase material above 454C. FIGURES 1 through 4 illustrate four isothermal sections of the tern-ary phase diagram for the copper-silicon-zinc system. As can be seen by an examination of the ternary phase diagrams, the existence of the beta ; :
phase (which is a body-centered cubic phase) is, in part, dependent upon the temperature. That i9, whether or not a beta phase can exist at a particular composition depends upon the temperature of the system. For example, much of the experimentation which will be described below relates to a ~-~
composition which includes 36. 5 atomic percent zinc and 1. 0 atomic percent silicon. At 847C. such a composition i9 in the beta phase as illustrated in . ~ - ;
FIGURE 4. At 7600C., however, as illustrated in FIGURE 3, the material ~ is comprised of the alpha plus beta phases. One may provide a composition ';
that will, however, have a.beta phase at the desired temperature by choosing b"
amounts of constituents as determined by the isothermal diagrams of FIGURES 1 through 4. To recapitulate, the first important consideration, then, is that the material can be maintained entirely in the beta phase at a ~ ~ ;
desired temperature above 454C.
The second important criterion relating to composition is deter-mined by the Marténsitic transformation temperature. Experimental re- .
sults reported in the above-noted background literature have shown that the following formulation is qualitatively determinative of the Martensitic trans- .:
formation temperature of a copper-silicon-zinc alloy:
M8( K):~ 3280-80Zn- 1 20Si where Zn=atomic percent Zinc and Si = atomic percent silicon.
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As will be described below, it has been found desirable to maintain the Martensitic transformation temperature of the alloy of the invention at approximately the same temperature or slightly below the temperature at which the materail being fabricated will be used. Most often, this will be ~-room temperature, for example.
Thus, the composition is chosen by examination of two separate criteria:
1. The composition must provide a continuous beta phase material at the same temperature above 454C. Preferably, the material is a beta phase material when heated above 600C.
2. The Martensitic transformation temperature of the material is at or below the temperature at which the alloy is intended to be used.
As a result of these two criteria, it has been found that the brass ~ -alloy wich includes 25.00% to 40.00% by weight zinc ~23.5 to 40.5 atomic %) ~ ~
and 0.25% to 3.00% by weight silicon (0.56 to 6.6 atomic %) with the re- - -mainder of the alloy being copper will provide the substantial limits of - composition which can be processed within the scope of the invention. It should also be noted, however, that some impurities (fillers) may be included -in the alloy composition up to an amount of approximately 5% appreciably with-2Q out affecting the phenomena associated with the invention. However, the criteria that the composition have a continuous beta phase and an appropriate s temperature are still effective.
Process FIGURE 5 illustrates schematically a fabrication flow chart which ~
outlines the process by which the product of the present invention may be -, ~ . . .
;I fabricated to provide the unusual and desired characteristics described in detail below. There are generally three processes by which the ultimate desired product may be obtained, the first process being the subject of this invention and the second and third being the subject of our Application .. . . .
1 3a Serial No. 285,ao8 divided out of this application. The second and third processes call for "betatization" or "betatizing" the composition.
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Betatizing or betatization is a word used to describe a distinct step in the process of invention of Application Serial No. 285,008. Betatizing or betatization can be defined as the heating or annealing process by which the composition is maintained at an appropriate temperature above 454C. until all of the composition is changed into the beta phase. The term is analogous to austentization which is well known for steels. As with austentization in steel, a time temperature transformation (TTT) diagram can be provided for the alloy of the present invention. In the specific examples below, it can be seen that time temperature transformation relationships for the specific examples are indicated. ~:
As shown in FI W RE 5, the first step for each of the three processes ~ -calls for combining the constituents of the alloy, melting them together and casting them into an ingot. The casting may then be homogenized at approxi~
mately 800C. Homogenization can take place at a lower temperature, for example, 700C. when the alloy may be in the a + ~ phase. Next, the casting ` I
may be hot rolled at between 600 to 800C. The amount of reduction by hot -rolling is as desired. Cold rolling then follows with optional intermediate anneals at 525C., for example, for a period of one-half hour~ The annealing operation serves to recrystalize the alloy and thus other annealing tempera- `
tures are suitable, for example, any temperature in the range of 300 to 700C. It is possible to cold roll and reduce the thickness of the sheet by as much as 70% to 95% without loss to the integrity of the material ant , without intermediate anneals.
; It is now possible to fabricate the material into a desired sheet or rod protuct. This is illustrated by the symbology in FI W RE 5 tesignated by I. The material in this case is composed of the alps (face-centered cubic) and beta phases and the material may be utilized in this form and will incorporate the advantages of the invention. In case I, it is important '~'"' ' . .
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Alternatively, the processes illustrated by II and III and which are in accordance with Application Serial No. 285,008 may be followed. In each instance (II and III) the specific heat treatment utilized to create the desired material is the same. However, in II the material is first fab-ricated into a desired shape, for example, a leaf spring, before heat treat-ing. In III the heat treatment is initially provided and then the material is fabricated into its ultimate desired product shape.
Returning then to the material of II, the material is initially in the alpha plus beta phase since it is derived as described for I. This material is fabricated into the desired shape and then betatized. Typically, the betatization must take place at more than 790C. However, betatization does depend upon the composition of the material and the appropriate tem-perature may be determined by examination of the ternary phase diagram.
Following betatization, the material is rapidly quenched at a rate preferably greater than 500 &. per second. The rapid quenching is necessary in order to retain the beta phase since it is this phase which is associated with the ;-unique characteristics of the present invention.
The process represented by III calls for betatization followed by rapid quenching as described for II. This in turn is followed by fabrication of the ultimate desired product, e.g. a spring. It has been found, in fact, that fabrication following betatization and quenching (process III) enhances the properties of the material.
FIGURE 6 illustrates a means for the continuous betatization heat ` treatment and the rapid quenching required in the association with the i betatization process of strip or sheet. The material is initially rolled into strip or sheet form and heated to its proper betatization temperature in a furnace. The material is then unrolled or discharged from the furnace and -~

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immediately quenched by cold rolls and a coolant spray. This quench must be fast enough to prevent decomposition of the beta phase into any other phase. The beta phase is retained. The heat treated metal is then wound on a roll as indicated in FIGURE 6.
Typically, using the arrangement of FIGURE 6, the betatizing treatment for strip or sheet on an economical continuous basis (high pro-cessing rate is feasible) is achieved by providing slight tension (normal practice in strip or sheet finishing), and the proper roll pressure on the hot material as it passes through the heat extraction means (cold rolls and cool-10 ant spray). The roll pressure is adjusted to provide efficient heat transferfrom the strip or sheet surfaces to the roll surfaces. The speed of the rolls, the coolant temperature and flow rate, and the takeup tension are then ad-justed to provide the rapid quench of more than 500C. per second to prevent decomposition of the beta phase and to yield flat, high quality strip or sheet.
Example s Example 1 A high purity or commercial copper in the amount of 62. 19 weight percent (62. 5 atomic percent) is combined with 37. 37 weight percent (36. 5 atomic percent) similar grade zinc and 0. 44 plus or minus 0. 10 weight per-20 cent silicon (1.0 atomic percent silicon). The constituents are thoroughlycombined in a crucible, melted together and cast as an ingot. The resulting -casting is then homogenized at approximately 800C. The homogenized ingot is hot rolled at a temperature between 6000 and 700C. and reduced to `, a suitable plate thickness. Following this, the material is cold rolled up to 80% reduction. As an aside, it should be noted that intermediate anneals at 525 for one-half hour may be provided. However, it is important that the final operation in the process be a cold rolling operation. Such cold rolling enhances the desired physical properties of the material which has been fab-ricated. This material may then be fabricated into a leaf spring, for _ 9 _ -~.037~43 example, the properties of which will be described in detail below.
Example 2 :
The identical procedure to that set forth in Example 1 above is followed. However, subsequent to the cold rolling operation, the material is heat treated for one minute at 830C. The material is then quenched at a rate greater than 500 per second and fabricated as desired. Alternatively, the material is fabricated and then heat treated at 830C. and quenched at more than 500C. per second as described. In each instance, the fabricated product may be a leaf spring of the type for example described in Example 1.
In Example 1, the material was a mixture of alpha phase and beta phases. The alpha phase constitutes approximately 60% of the material with the beta phase comprising the remaining 40%. It is possible, however, de_ pending upon the composition and the Martensite transformation temperature to provide for an alpha phase having 50% plus or minus 20% alpha phase and 50% minus or plus 20% beta phase. In any event, the properties described ~-- below will still appear providing the appropriate final cold working operation ~
has been performed following initial hot rolling.
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plus beta of Example 1 and the beta phase material of Example 2. As seen 20 by examination of FIGURE 7, the alpha plus beta material has a fine grain size, whereas the pure beta phase material has a large grain size. In the following discussion, both the alpha plus beta and the beta phase materials will be discussed in regard to various physical properties. Also, it should be noted that the beta phase alloy will exhibit the super elastic, shape-` memory and spring back properties discussed below even though small amounts of the alpha phase may be contained in the beta phase.
Properties .. .
FIGURES 9 through 17 illustrate the properties observed in the alloys of the present invention as compared to the properties observed in , _ 10 - : ~

10371~3 typical prior art materials utilized for substantially the same purposes.
Referring first to FIGURE 9, there is shown a typical engineering stress versus strain curve for a commercial material. The stress ( 6 ) is on the ordinate and strain ~ ~ ) is on the abcissa. The letters P.L. indicate -` the proportional limit, that is, the point on the stress-strain curve at which stress is no longer linearly proportional with strain. The yield point is that point where the strain is offset from the linear portion of the stress--. strain curve by an amount equal to 0.1%. The stress at this point is defined as the yield stress, YOS. The ultimate tensile stress, U.T.S., is the maximum stress which the material may be subjected to prior to fracture and is cor-, related with the ultimate elongation.
- Utilizing these definitions, FIGURE 10 shows a typical stress-strain - curve for the improved materials of the present invention. As indicated on FIGURE 10, the material identified as alpha plus beta (a + ~) was fabricated as set forth in Example 1 above. The material tested for FIGURE 10 was cold ~ rolled with a 60% reduction in thickness. The material indicated by beta (~
- was betatized and quenched as set forth in Example 2 using the process sche- -~ -matically described in FIGURE 6, in accordance with Application Serial No. -~
~; 285,008.

'~ 20 FIGURE 11 illustrates the effect of increased reduction in thickness ,,3' during cold rolling on the ultimate tensile strength (U.T.S.) for material of ~ the type set forth in Example 1 above as compared with such prior art materials "'r as 70/30 brass and 98% copper, 2% iron, 0.02% phosphorous, an alloy commonly :~ used for electrical applications as spring applications and known as Alloy 194. Note the increase in the ultimate tensile strength of the alloy of the present invention.
FIGURE 12 illustrates the marked increase in the yield strength of , the + ~ form of alloy made in accordance with the present invention in ' comparison with 70/30 brass and Alloy 194.

~ FIGURE 13 illustrates the effect of cold rolling on the limit of ; 3 ':
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proportionality. That is, the material with a higher tensile strength as it i5 reduced in thickness by greater and greater amounts has a greater pro-portional limit than a material with a lower tensile strength. It can be seen that the alpha plus beta (~+~ ) alloy of the present invention exhibits a dramatic increase in the limit or proportionality over 70/30 brass.
FIGURE 14 illustrates the effect of cold rolling on hardness. It is desirable that the cold rolling make the material harder, however, not so hard that it would tend to become brittle. As can be seen by the diagram of FIGURE 14, the (~ ) alloy of the present invention compares quite favorably with the prior art alloys used for similar purposes.
FIGURE 15 illustrates the relationship between the amount of elongaticn which a material will exhibit or permit as a function of the amount 4:, of reduction in thickness of the material by cold rolling. In the range where ., :
the various materials are most useful, namely, when the temper is desig-nated as spring or extra spring, the alloy of the invention exhibits elongation characteristics which are at least equal or superior to the prior art materials.
- Besides the advantages illustrated and discussed above, perhaps the most important advantage realized by the present invention is the im_ 20 proved or low spring back coefficient exhibited by the material. The spring .' back coefficient is somewhat related to the phenomena known as super elasticity, sometimes observed in various alloys. Super elasticity implies that a material will relact in an elastic and sometimes nonlinear fashion over a very long range of strains of that material. Generally, super - elasticity is associated with cast materials or hot worked ingots as contras-ted with strip, sheet, wire or other wrought materials. That is, it has been found that many materials which are super elastic cannot be easily made : into sheet materials by means of cold rolling or other such similar forming operations. In fact, the inventors know of no sheet formed super elastic .:
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material prior to the present invention. Thus, although the super elasticity phenomena is observed in a cast material, the advantages of that super elastic phenomena cannot be carried over into the rolled material since the rolled material will tend to crack or otherwise become unuseful.
It is thus desirable to provide a material which can be sheet formed and which will exhibit characteristics analogous to thcse observed in cast super elastic materials. Such a sheet material can also be defined as one having a low spring back coefficient. While heretofore there has been no material which was super elastic and which also could be formed into a sheet material and thus have a low spring back coefficient, the material of the present invention has such unique properties. That is, the material of the present invention can be rolled into plate and ultimately into sheet or drawn into thin rod or wire and will exhibit not only super elastic proper-ties, but will exhibit an extremely low spring back coefficient. Since it has been impossible heretofore to form super elastic materials into a sheet or a drawn condition, it is remarkable that the present invention provides a material and the process by which such material can be formed and which when it is formed exhibits properties even more desirable than simple super elastic properties.
As mentioned above, tne means for comparing elastic properties of sheet or wire materials is known as the spring back coefficient.
FIGURE 16 illustrates the definition of this coefficient, K. Materials hav_ ing a low spring back coefficient have the ability upon bending of returning substantially to their original condition. Thus, a material which is per_ fectly elastic and can return completely to its initial position will have a spring back coefficient of 0. A material which will not return to its new position, but will remain at a position to which it is bent, will have a spring back coefficient of 1. Lead is a typical example of a material having a spring back coefficient of 1.

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The coefficient, K, is mathematically defined as the ratio of the original angular deflection of a sheet of material from the horizontal com-pared with the final angle of deflection once the force imparting the deflec-tion has been re vedO For example, if the original deflection imparted over a mandrel having a radius of Ro is equal to 90 and the final angle ~f is 45 after the material is released, then the spring back coefficient is 0.5.
Referring now to FIGURE 17, there is shown a diagram wherein the spring back coefficient is plotted against the radius of a mandrel about which the spring or formed sheet is bent. As can be seen by examining the graph, the material of the present invention has a drastically improved spring back -coefficient. As illustrated in FIGURE 17, the spring back coefficients for ~ -the ~ material of Application Serial No. 285,008 and a l ~ material of the present invention are significantly lower than competitive type materials utilized for the same purposes. -Another phenomenon observed in the present invention is often termed the "shape memo~y effect". That is, the alloy of the invention may be deformed at a temperature below the Martensite transformation temperature.
Upon heating the alloy above the Martensite transformation temperature, the deformed alloy material will almost resume its original configuration. This has been found particularly true for the beta t~) embodiment described above and claimed in Application Serial No, 285,008. Moreover, if you again cool the material below the transformation temperature, but render no stress, the material will deform to nearly the same shape as the original deformed mat-erial. Upon reheating, the material again returns to its original undeformed shape. Such cycling may be continued.
Other advantages exhibited by the alloys of the present invention include the good machinability, easy tinning and solderability and improved fatigue resistance in comparison with prior art spring materials.

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,, .: . .: . -~037~43 Finally, as set forth below in Table A, there is an indication of the electrical conductivity at 68F. exhibited by the alloy of the present application and of Application Serial No. 285,008 as compared with typical competitive alloys. The conductivity is well within the range of being acceptable, thus indicating utility of the invention for various electrical contactors, etc.
TABLE A
ELECTRICAL CONDUCTIVITY % IACS AT 68F

CuZnSi Alloy (~) ~ 23%
CuZnSi Alloy (l ~ ~) ~ 22%
f before heat treatment 1 25%
Beryllium Copper 10 ~ after heat treatment ~ 50%
70/30 Brass '`~ 28%
Alloy 194 ~ 60%

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Claims (7)

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

1. A method for manufacture of an improved brass alloy having a low spring back coefficient, said alloy consisting essentially of 0.25%
to 3.00% by weight silicon (0.56 to 6.6 atomic %), 25.00% to 40.00% by weight zinc (23.5 to 40.5, atomic %) and the balance copper in a mixed alpha plus beta phase of about 50% 1 20% by weight alpha phase and the balance beta phase, comprising the steps of:
combining said constituents in a substantially-homogeneous mixture, and (a) hot working said constituents at a temperature between 600° and 700° C and then cold working said alloy at the temperature of normal use; or (b) working said alloy by rolling methods to form the alloy and provide a polycrystalline worked product.

2. The process of claim 1(a) wherein said cold working step reduces at least one dimension of said composition up to 95%.

3. The process of claim 1(a) including the final step of fabricating said composition into a finished product.

4. An improved brass alloy having a low spring back coefficient, consisting essentially of:
0.25% to 3.00% by weight silicon (0.56 to 6.6 atomic %);
25.00% to 40.00% by weight zinc (23.5% to 40.5 atomic%);
and the balance copper in a homogeneous, polycrystalline worked structure of 50% ? 20% by weight alpha phase and the balance beta phase material.

5. The alloy of claim 4 wherein said alloy includes no more than 5% by weight impurities.

6. An improved brass alloy having a low spring back coefficient consisting essentially of 37.37 weight percent zinc and 0.44 plus or minus 0.10 weight percent silicon and the balance copper in a homogeneous, poly-crystalline form having about 50% ? 20% by weight alpha phase and the balance beta phase material.

7. An improved brass alloy having a low spring back coefficient, consisting essentially of:
0.25% to 3.00% by weight silicon (0.56 to 6.6 atomic %);
25.00% to 40.00% by weight zinc (23.5% to 40.5% atomic %);
and the balance copper in a homogeneous, polycrystalline worked structure of 50% ? 20% by weight alpha phase and the balance beta phase material, wherein said alloy has been hot worked between 600° and 700° C and subsequently cold worked at the temperature of normal use.