Process for producing Cu-base alloys

Abstract

A processing method for certain copper base alloys is described which reduces or eliminates the tendency for blister formation during annealing of alloy strip. The problem involves the formation of internal voids and subsequent migration and expansion of hydrogen within these voids, and the solution to the problem includes annealing under carefully controlled conditions of temperature and metal thickness so as to reduce the hydrogen level followed by a controlled deformation which heals the internal defects.

Description

BACKGROUND OF THE INVENTION

Copper base alloys are widely used in industry and are characterized by high formability, good conductivity and pleasing appearance. A high percentage of all copper base alloys are utilized in the form of strip or sheet. The method of producing strip or sheet to final gauge usually involves alternate steps of deformation and annealing. It is often found in certain alloys that annealing after deformation, particularly at thinner gauges, produces undesirable blistering. These blisters are gas filled defects which become apparent when the alloy is heated. As the temperature is raised, gas pressure inside the defect increases, thus expanding and deforming the surrounding metal which has a low yield strength because of the elevated temperature. This problem is particularly common in CDA Alloy 638 which contains 2.5 to 3.1% aluminum, 1.5 to 2.1% silicon, 0.25 to 0.55% cobalt, balance essentiallly copper. Unless otherwise noted, all percentages in this application are weight percentages.

SUMMARY OF THE INVENTION

The present invention comprises a process for the production of copper strip which results in a blister free product. The process is a comparatively simple one which can be applied using standard equipment commonly available in a commercial copper alloy production facility. The process of the present invention includes a hot rolling step followed by a diffusion annealing step performed under carefully controlled conditions. The diffusion anneal step reduces the hydrogen content of the alloy without permitting blister formation. Following the diffusion anneal the alloy is cold worked according to a particular schedule. This cold working operation welds shut the internal defects so that blistering will not occur during subsequent annealing operations. The present invention is broadly applicable to a wide range of copper alloys but is particularly useful in connection with the production of CDA Alloy 638.

It is an object of the present invention to provide a production method for producing high quality copper alloy strip.

It is a further object of the present invention to provide a processing technique which minimizes blister formation in copper alloys.

Further objects will become apparent when the following description of the preferred embodiments and claims are considered.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a process for producing blister free copper alloy sheet or strip through the use of a process which includes the steps of casting, hot working, diffusion annealing, cold rolling and optionally a further annealing step. The following description will provide detailed parameters for each of the steps in the process of the present invention.

The casting of the alloy may be performed using any process which will produce a sound ingot. It is preferred, however, to use a process in which a minimum surface area of molten metal is exposed to the atmosphere during casting. For this reason it is preferred to use D.C. casting.

Regardless of precautions taken, a certain amount of hydrogen will be present within the metal if the casting operation is performed in a normal atmosphere. Hydrogen pickup can occur from moisture or dirt in the charge materials, moisture and impurities in the flux or melt cover, moisture in the air and moisture or dirt in the mold. As an approximation molten copper alloys can hold four times as much hydrogen as solidified copper alloys at similar temperatures. Thus, it is common that solidified copper alloys contain more hydrogen than would be present under equilibrium conditions.

The ingot is then hot worked, usually by rolling, using an appropriate hot working temperature. In the case of CDA Alloy 638 which contains 2.5 to 3.5% aluminum, 1.5 to 2.1% silicon, 0.25 to 0.55% cobalt, balance essentially copper, an appropriate hot working temperature is from 800.degree.to 920.degree.C, preferably 850.degree.to 900.degree.C. In general, the hot working temperature will be from 0.7 to 0.95 T.sub.m where T.sub.m is the absolute melting point of the alloy. During the initial stages of hot working internal cracking occurs and it is to these internal cracks which dissolved hydrogen may diffuse and subsequently cause blisters. Hydrogen is present in the metal itself in dissociated or atomic form. Hydrogen in internal defects will combine to form molecular hydrogen, H.sub.2. Molecular hydrogen is essentially insoluble in copper alloys and will not diffuse through copper alloys. It is desirable to hot work more than 50 percent since partial healing or bonding of these internal cracks occurs. As increased deformation occurs, some of the defects heal as their surfaces bond together. It is preferred that the hot working reduction be from 75 to 95 percent since material made with reductions of this order of magnitude has fewer internal defects than material made with lower reduction. Complete healing of internal cracks is not possible because of the presence of hydrogen within the defect which interferes with the complete bonding of the internal crack surfaces. The final gauge after hot working must be from 0.200 to 0.750 inch and is preferably from 0.300 to 0.550 inch. The importance of this requirement will be made clear in a subsequent paragraph.

The hot worked strip is then annealed under conditions which will permit the diffusion of hydrogen from within the strip to the surface of the strip and then to the surrounding environment. The temperature and metal thickness required are interrelated such that the metal will not yield under the action of the internal gas pressure, but rather will permit the hydrogen which is trapped in the defects to dissociate the diffuse out of the metal. It is most surprising that at the temperatures employed the molecular hydrogen within the defects can dissociate to permit its diffusion out of the void through the metal and to the surrounding environment. This is particularly unusual since at the temperatures involved, hydrogen in the surrounding atmosphere will not dissociate and thus cannot enter the metal. The annealing temperature should fall within the range of 0.4 to 0.7 T.sub.m where T.sub.m is the absolute melting point of the alloy. In the case of CDA Alloy 638 the temperature range is approximately 450.degree. to 650.degree.C. Naturally, the time of the treatment must be selected so as to permit the diffusion of the hydrogen out of the metal. The time limitation is affected by the thickness of the strip which controls the average diffusion distance for the hydrogen. It is further limited by the temperature of the treatment. In general, periods from 1 to 24 hours are appropriate. Increasing the strip thickness requires longer diffusion times for the same temperature, and for strips of the same thickness longer times are required at lower temperatures. It is important for the temperature range contemplated that the strip be no thinner than 0.200 inch since thin strips have less ability to resist the expansion of defects from increased internal hydrogen pressure than do thick strips. It is also important that the length of the diffusion anneal treatment not be any longer than necessary since undesirable changes to the metallurgical microstructure and properties of the alloy may occur. These undesirable changes include changes in the amount and distribution of second phases, depletion of solute elements and/or undesirable increases in grain size.

The efficacy of this diffusion annealing treatment is independent of the furnace atmosphere employed since the atomic hydrogen will recombine at the free surface of the metal and since the molecular hydrogen in the atmosphere cannot diffuse into the alloy. Thus, either reducing, inert, or oxidizing environments are allowable. It is preferred to use conventional reducing atmospheres in order to minimize surface oxidation during this annealing step.

After the diffusion annealing step the strip is cold rolled at least 60 percent and preferably at least 75 percent. This cold rolling operation serves to weld together the internal defects. Reductions of less than 60 percent do not provide adequate bonding of internal defect surfaces. However, if the strip is to be annealed subsequent to this cold rolling step reductions as low as 40% may be satisfactory. Such optional annealing may be carried out at temperatures of from 0.4 to 0.9 T.sub.m for times of from 5 seconds to 24 hours. Optionally, bonding may also be obtained if the rolling operation is performed at temperatures above room temperature.

Following the cold rolling operation the strip may optionally be annealed so as to obtain the desired mechanical properties such as strength and ductility. This annealing operation is desirable in that it will help to remove any vestige of the prior internal defects. Following the optional annealing step, further operations may be performed. If for example it is desired to have a final product having mechanical properties which correspond to those which result from 10 percent cold work, it would be necessary to anneal the material following the first cold rolling step and then cold roll to 10 percent since the first cold rolling step must incorporate a higher amount of deformation.

Although the preceding discussion has been in terms of the production of copper strip or sheet it will be appreciated that the process of the present invention is equally applicable to other material forms such as rod and wire. The process of the present invention is applicable to all copper alloys in which blistering occurs as a result of entrapped hydrogen.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

Claims

What is claimed is:

1. A method for producing blister free copper alloy material using as a starting material a copper alloy which has been hot worked at least 50 percent to a thickness of from 0.200 to 0.750 inch including the steps of:

A. annealing the copper alloy material at a temperature of from 40 to 70 percent of the absolute melting temperature of the alloy for a time of from 1 to 24 hours; and

B. cold working the material at least 60 percent.

2. A method as in claim 1 wherein the starting material contains from 2.5 to 3.1 percent aluminum, from 1.5 to 2.1 percent silicon, from 0.25 to 0.55 percent cobalt, balance essentially copper.

3. A method as in claim 1 wherein the hot worked starting material has been reduced in area by at least 75 percent during hot working.

4. A method as in claim 1 wherein the thickness of the starting material is from 0.300 to 0.500 inch.

5. A method as in claim 1 wherein Step A is performed in a protective reducing atmosphere.

6. A method as in claim 1 wherein the deformation in Step B is at least 75 percent.

7. A method for producing annealed blister free copper alloy material using as a starting material a copper alloy which has been hot worked at least 50 percent to a thickness from 0.200 to 0.750 inch including the steps of:

A. annealing the copper alloy material at a temperature of from 40 to 70 percent of the absolute melting temperature of the alloy for a time of from 1 to 24 hours;

B. cold working the material at least 40 percent at a temperature of less than the temperature used in Step A; and

C. annealing the material.

8. A method as in claim 7 wherein the starting material contains from 2.5 to 3.1 percent aluminum, from 1.5 to 2.1 percent silicon, from 0.25 to 0.55 percent cobalt, balance essentially copper.

9. A method as in claim 7 wherein the hot worked starting material has been reduced in area by at least 75 percent during hot working.

10. A method as in claim 7 wherein the thickness of the starting material is from 0.300 to 0.500 inch.

11. A method as in claim 7 wherein Step A is performed in a protective reducing atmosphere.

12. A method as in claim 7 wherein the deformation as in Step B is at least 75 percent.

13. A method for producing blister free copper alloy material including the steps of:

A. hot working the alloy at least 50 percent to a thickness of from 0.200 to 0.750 inch;

B. annealing the copper material at a temperature of from 40 to 70 percent of the absolute melting temperature of the alloy for a time of from 1 to 24 hours; and

C. cold working the material at least 60 percent.

14. A method as in claim 13 wherein the starting material contains from 2.5 to 3.1 percent aluminum, from 1.5 to 2.1 percent silicon, from 0.25 to 0.55 percent cobalt, balance essentially copper.

15. A method as in claim 13 wherein the material is reduced at least 75 percent during Step A.

16. A method as in claim 13 wherein the thickness of the copper alloy material following Step A is from 0.300 to 0.500 inch.

17. A method as in claim 13 wherein the deformation in Step C is at least 75 percent.

18. A method for producing blister free copper alloy material including the steps of:

A. hot working the alloy at least 50 percent to a thickness of from 0.200 to 0.750 inch;

B. annealing the copper material at a temperature of from 40 to 70 percent of the absolute melting temperature of the alloy for a time of from 1 to 24 hours;

C. cold working the material at least 40 percent; and

D. annealing the material.

19. A method as in claim 18 wherein the starting material contains from .25 to 3.1 percent aluminum, from 1.5 to 2.1 percent silicon, from 0.25 to 0.55 percent cobalt, balance essentially copper.

20. A method as in claim 18 wherein the material is reduced at least 75 percent during Step A.

21. A method as in claim 18 wherein the thickness of the copper alloy material following Step A is from 0.300 to 0.500 inch.

22. A method as in claim 18 wherein the deformation in Step C is at least 75 percent.