Copper-based alloys, processes for producing the same, and products formed therefrom

Abstract

Processes are provided that include providing a copper-manganese alloy containing copper and manganese and having an amount of manganese that is at least 32 weight percent and not more than 40 weight percent of a combined total amount of the copper and manganese in the copper-manganese alloy, and casting the copper-manganese alloy by multidirectional solidification to produce a product in the form of a casting. The copper-manganese alloy has a composition sufficiently near the congruent melting point of the CuMn alloy system to sufficiently avoid dendritic growth during the multidirectional solidification of the copper-manganese alloy to avoid the formation of microporosity attributable to dendritic growth. The product has a cast microstructure having a cellular and/or planar solidification structure free of dendritic growth and having multidirectional columnar grains.

Claims

1. A process comprising: combining copper and ferromanganese to form a copper-manganese alloy, the ferromanganese containing about 75 to 80 weight percent manganese with the balance carbon, iron, and incidental impurities, the copper-manganese alloy containing copper and manganese and having an amount of manganese that is at least 32 weight percent and not more than 40 weight percent of a combined total amount of the copper and manganese in the copper-manganese alloy; and casting the copper-manganese alloy by multidirectional solidification at an uncontrolled solidification rate to produce a product in the form of a casting, the copper-manganese alloy having a composition sufficiently near the congruent melting point of the CuMn alloy system to sufficiently avoid dendritic growth during the multidirectional solidification of the copper-manganese alloy to avoid the formation of microporosity attributable to dendritic growth, the product having a cast microstructure having a cellular and/or planar solidification structure and having multidirectional columnar grains.

2. The process according to claim 1, wherein the copper-manganese alloy contains at least 32 weight percent and not more than 36 weight percent manganese.

3. The process according to claim 1, wherein the copper-manganese alloy further contains one or more of iron, nickel, aluminum, silicon, tin, and lead.

4. The process according to claim 1, wherein the copper-manganese alloy does not contain lead.

5. The process according to claim 1, wherein the product is free of microporosity.

6. The process according to claim 1, wherein the product is a plumbing valve or fitting.

7. A process comprising: combining copper and ferromanganese as a source of manganese to form a copper-manganese alloy, the ferromanganese containing about 75 to 80 weight percent manganese with the balance carbon, iron, and incidental impurities, the copper-manganese alloy containing copper and manganese and having an amount of manganese that is at least 32 weight percent and not more than 40 weight percent of a combined total amount of the copper and manganese in the copper-manganese alloy; and casting the copper-manganese alloy by multidirectional solidification to produce a product in the form of a casting, the copper-manganese alloy having a composition sufficiently near the congruent melting point of the CuMn alloy system to sufficiently avoid dendritic growth during the multidirectional solidification of the copper-manganese alloy to avoid the formation of microporosity attributable to dendritic growth, the product having a cast microstructure having a cellular and/or planar solidification structure and having multidirectional columnar grains.

8. The process according to claim 7, wherein the ferromanganese is solid ferromanganese, the copper has not been deoxidized, and the combining step comprises generating heat by oxidation of the carbon in the ferromanganese to melt and dissolve the ferromanganese.

9. The process according to claim 1, wherein the copper-manganese alloy contains iron.

10. The process according to claim 7, wherein the copper-manganese alloy contains at least 32 weight percent and not more than 36 weight percent manganese.

11. The process according to claim 7, wherein the copper-manganese alloy further contains one or more of nickel, aluminum, silicon, tin, and lead.

12. The process according to claim 7, wherein the copper-manganese alloy does not contain lead.

13. The process according to claim 7, wherein the product is a plumbing valve or fitting.

14. The process according to claim 7, wherein the product is free of microporosity.

15. The process according to claim 1, wherein the casting step is performed in a steel mold.

16. The process according to claim 15, wherein the casting step includes melting the copper-manganese alloy to form a melt and pouring the melt into the steel mold, wherein the temperature of the melt decreases toward an ambient temperature established by the steel mold.

17. The process according to claim 1, wherein the casting step includes melting the copper-manganese alloy to form a melt and pouring the melt into a mold, wherein the temperature of the melt decreases toward an ambient temperature established by the mold.

18. The process according to claim 1, wherein heat is generated by oxidation of carbon in the ferromanganese to melt and dissolve the ferromanganese.

19. A process comprising: combining copper that has not been deoxidized and solid ferromanganese containing carbon to form a copper-manganese alloy containing copper and manganese and having an amount of manganese that is at least 32 weight percent and not more than 40 weight percent of a combined total amount of the copper and manganese in the copper-manganese alloy, wherein heat is generated by oxidation of the carbon in the ferromanganese to melt and dissolve the ferromanganese; melting the copper-manganese alloy to form a melt; pouring the melt into a multidirectional solidification mold to cast a product via multidirectional solidification in the form of a casting, wherein the temperature of the melt decreases toward an ambient temperature established by the mold, the copper-manganese alloy having a composition sufficiently near the congruent melting point of the CuMn alloy system to sufficiently avoid dendritic growth during the multidirectional solidification of the copper-manganese alloy to avoid the formation of microporosity attributable to dendritic growth, the product having a cast microstructure having a cellular and/or planar solidification structure and having multidirectional columnar grains.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 represents the equilibrium phase diagram of the binary CuMn system.

(2) FIGS. 2 through 5 are scanned images of microphotographs of CuMn alloys that were investigated and evidence the onset of non-planar (cellular) growth during solidification but the absence of dendritic growth and structures.

(3) FIG. 6 schematically represents a large-scale CuMn alloy production process in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) The present invention provides a class of copper-manganese alloys based around the congruent melting composition of the CuMn binary system, which is believed to be 34.61.4 weight percent (about 382 atomic percent) manganese and has a melting temperature of about 870 C. In preferred embodiments, the copper-manganese alloys are lead-free, offer high castability for shape casting, and contain sufficiently minimal chemical segregation and microporosity when cast to eliminate the need for lead or other elements to fill the microporosity.

(5) In investigations leading to the present invention, a small heat of a binary CuMn containing 32 weight percent manganese was produced by induction melting in air in a graphite crucible by combining 99.9% pure copper and 99.9% pure electrolytic manganese. After casting the alloy in a small steel ingot mold, metallographic sections of the resulting ingot showed a distinctive brown color and attractive luster when polished. FIG. 2 is an optical micrograph of the binary CuMn alloy following etching, and reveals that the alloy contains a mildly cellular solidification structure, believed to evidence the onset of cellular solidification from planar solidification but without the onset of dendritic solidification. The micrograph also indicates the presence of small amounts of second phase particles that were not identified. Notably, solidification shrinkage microporosity is not present in the as-cast structure, evidencing that the alloy was sufficiently at or near the congruent melting composition of the CuMn binary system to avoid dendritic growth during solidification. More specifically, solidification avoided the onset of non-planar (dendritic) growth and therefore resulted in the mildly cellular solidification structure. The cellular structure (instead of a purely planar structure) was concluded to be the result of the alloy composition likely being on the low-Mn side of the congruent composition, which is believed to be 34.61.4 weight percent.

(6) The above results were particularly notable because conventional wisdom is that only slight amounts of solute in a pure metal will render the solidification under typical conditions to be dendritic due to constitutional supercooling (CS). With increasing CS there is a transition from planar to cellular to dendritic solid/liquid interface growth morphology. As a result of the very wide freezing ranges associated with copper-based casting alloys that contain additions of metals having low melting points relative to copper, non-planar dendritic growth promotes chemical segregation and increases the tendency for microporosity. In this and subsequent investigations, it was concluded that the disadvantages of chemical segregation and microporosity could be avoided with copper-manganese alloys whose manganese contents are sufficiently near the congruent melting composition of the CuMn binary system, preferably at least 33.2 to not more than 36 (34.61.4) weight percent manganese, to achieve a purely planar growth, possibly cellular growth, or the onset of cellular growth from planar growth. Notably, commercial alloys that exhibit planar or clearly cellular growth during solidification are not believed to exist, or in any event are not common.

(7) On the basis of the above, additional CuMn compositions were prepared and cast. FIG. 3 is an optical micrograph of a binary CuMn alloy containing 36 weight percent manganese. The polished unetched specimen reveals that the alloy did not contain microporosity. FIG. 4 contains an optical micrograph of a specimen of the same alloy 36% Mn alloy. The specimen was etched, and the micrograph is at a higher magnification than FIG. 3 to reveal a cellular solidification structure. FIG. 5 is another micrograph of the same 36% Mn alloy following etch, but at a higher magnification to reveal the cellular solidification structure more clearly. The micrographs of FIGS. 4 and 5 were concluded to evidence that the 36% Mn alloy was on the high side of the congruent point, and that dendritic growth can be avoided by limiting the manganese content of the copper alloy to levels of at least 32 weight percent to not more than 36 weight percent, and that manganese contents below 32 weight percent and above 36 weight percent would undesirably lead to dendritic growth as well as chemical segregation and microporosity associated therewith in copper alloys. While a range of at least 32 weight percent to not more than 36 weight percent is believed to be preferred, more broadly the invention can encompass manganese contents that sufficiently, though not necessarily completely, avoid dendritic growth during solidification to avoid microporosity that would form as a result of dendritic growth. For this purpose, on the basis of constitutional supercooling criteria, it is believed that manganese contents of as low as 25 weight percent and as high as 40 weight percent may be tolerable.

(8) Another aspect of the congruent melting behavior of the CuMn alloys of this invention is that the congruent melting temperature (about 873 C.) is substantially lower than pure copper (about 1085 C.) and most commercial copper alloys. The low melting temperature has a beneficial effect on casting fluidity, that is, the ability of the liquid melt to flow and fill fine cavities and thin sections in a casting mold. As the liquid temperature of the melt decreases toward the ambient temperature established by the casting mold, the driving force for heat transfer to the mold and rate of solidification as the metal flows in narrow channels decreases. Casting fluidity, as it is called (not to be confused with fluidity=1/viscosity of a liquid) is quantified by pouring or drawing a liquid melt into a fine mold channel at a melt temperature and measuring the length of flow (filling) that occurs before solidification at the entrance chokes off the flow. Pure metals generally have higher casting fluidity than alloys because dendritic solidification in alloys restricts the flow of liquid more than plane-front solidification. On this basis, the effect of the lower melting temperature of the CuMn alloys of this invention are expected to exhibit higher casting fluidity as compared to highly-dendritic copper casting alloys because the congruent-melting CuMn alloys of this invention will not exhibit additional flow resistance attributable to dendritic solidification. In addition, because the melting temperature is flat (shallow or broad) surrounding the congruent melting temperature, this beneficial effect should substantially (if not completely) accrue for CuMn alloy compositions over the range of 32 to 36 weight percent manganese, and not solely the congruent point composition of 34.6 weight percent manganese.

(9) To achieve optimal combinations of properties for particular applications, it is foreseeable that the CuMn alloys of this invention can be alloyed if the atomic ratio of copper and manganese is maintained to achieve a structure that is a purely planar, purely cellular, or a combination thereof (free of non-planar dendritic growth). Notable examples of additional alloying constituents include iron, nickel, aluminum, silicon, tin and other alloying elements that may benefit copper alloys. For applications in which lead is acceptable, the CuMn alloys may also be alloyed to contain lead.

(10) In particular embodiments in which the CuMn alloy is to be used in shape casting, for example, in the production of plumbing valves and fittings, chemical optimization is based at least in part on castability (microporosity and susceptibility to hot-tearing), corrosion resistance, machinability, mechanical properties, and cost. The CuMn alloys of this invention may also offer certain advantages over existing copper alloys when produced in wrought form, for example, by rolling, drawing or forging.

(11) The invention also contemplates large-scale production processes for producing the CuMn-based alloys. In one particular example, partially refined copper is used and the source of manganese is ferromanganese. As known in the art, ferromanganese is partially refined manganese, for example, containing about 75 to 80 weight percent manganese with the balance primarily carbon (for example, about 7 weight percent) and iron (in other words, the balance incidental impurities). Ferromanganese is used in the production of the vast majority of steels, and its cost is far less than copper. The final step in refining primary copper is deoxidation, usually by reaction with carbon or hydrocarbon. In the particular example disclosed herein, solid ferromanganese is reacted with primary copper that has not been deoxidized, and the heat generated by oxidation of the carbon in the ferromanganese is employed to melt and dissolve the ferromanganese. Such a process is schematically represented in FIG. 6. The process thus utilizes an intermediate product of the existing large-scale copper refining technology in a manner that enables a low-cost method of alloying manganese and copper. A consequence of this particular process is that the resulting alloy contains a small amount of iron from the ferromanganese.

(12) Though both cast and wrought forms of the CuMn alloys are envisioned for plumbing applications, other forms and applications of the CuMn alloys are also foreseeable. Furthermore, thermomechanical processing and heat treatment can be adapted to develop higher strengths in these alloys for structural applications. It is also foreseeable that the alloys may have appeal for decorative applications on the basis of the intrinsic distinctive brown color of the pure binary alloy.

(13) While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.