WO1994004712A1

WO1994004712A1 - Lead-free copper base alloys

Info

Publication number: WO1994004712A1
Application number: PCT/US1993/007557
Authority: WO
Inventors: Thomas D. Nielsen
Original assignee: Nielsen Thomas D
Priority date: 1992-08-14
Filing date: 1993-08-11
Publication date: 1994-03-03
Also published as: AU5005793A

Abstract

A lead-free copper base alloy comprising between about 2-10 wt.% tin; between about 2-10 wt.% zinc; between about 1-3 wt.% bismuth; between about .05-1 wt.% phosphorous, and a balance being essentially copper is provided. The bismuth in this lead-free copper alloy is dispersed in a globular form throughout the grain boundaries of the alloy. The alloy may be economically produced by melting certain grades of scrap materials with bismuth and phosphorous followed by casting the resultant molten bath into ingots or barstock.

Description

LEAD-FREE COPPER BASE ALLOYS

Background of the Invention

The present invention pertains to lead-free copper base metal alloys for use in continuous casting and ingot making, and more particularly to a method for producing lead-free copper base metal alloys wherein bismuth is substituted for the lead constituent and phosphorus is utilized to limit the grain boundary effects of the bismuth.

In the past, lead has been added to copper alloys to improve machinability and ductility. Lead, being virtually insoluble in copper, separates from the alloy matrix thereby forming a free-machining alloy, wherein the particles of lead act as a solid lubricant between the alloy and a tool.

One of the problems associated with a copper alloy having a lead constituent is the toxicity of the lead itself, particularly the toxicity associated with the exposure to lead in foundry waste, foundry environment, machine shop waste, machine shop environment, potable water systems, and food processing systems.

Recently, a lead-free copper alloy was introduced which is produced by incorporating bismuth, rather than lead, into the copper alloy. United States Patent No. 4,879,094 which issued November 7, 1989, describes and claims an alloy containing from 1.5 to 7 wt.% bismuth; from 5 to 15 wt.% zinc; from 1 to 12 wt.% tin, and a balance being essentially copper. Such an alloy is said to possess properties making it suitable for manufacturing components for use in potable water installations.

The principal problem associated with a copper alloy having a bismuth, rather than a lead, constituent is that the addition of bismuth to the copper alloy results in a brittle structure due to the tendency of bismuth to wet the exterior of the copper alloy crystals as shown in Figure 1. As disclosed in United States Patent No. 4,879,094, a minimum of 5 wt.% zinc has been added in the past to limit the grain boundary effects of copper alloys containing bismuth. However, we have found that a zinc content ranging from 5 - 15 wt.% has only a minor beneficial effect on the dewetting of the bismuth crystals from the exterior of the copper alloy crystals. In addition, it should be noted that the addition of zinc in the aforementioned range is rather undesirable from a manufacturing perspective, since dezincification in service renders the alloy virtually useless and a high zinc content presents difficulties in casting and machining.

Incorporation by Reference My U.S. Patent No. 4,315,538 is incorporated by reference herein as background information with respect to the present invention.

Brief Summary of the Invention The present invention contemplates a new and improved lead-free copper base alloy containing bismuth which overcomes the tendency of bismuth to wet the exterior of the copper alloy crystals. We have found that the addition of between about .05-1 wt.% phosphorous to the molten metal virtually limits the grain boundary effects of the bismuth. As shown in Figure 2, the presence of phosphorous in the copper alloy freezes the bismuth in a globular form in the grain boundaries; thus, the bismuth is prohibited from wetting the copper crystal grains.

In accordance with one aspect of the present invention, there is provided a lead-free copper base alloy comprising between about 2-10 wt.% tin; between about 2-10 wt.% zinc; between about 1-3 wt.% bismuth; between about .05-1 wt.% phosphorous, and a balance being essentially copper.

Table 1 is a comparison of the elongation and ultimate strength of two copper alloy samples - one prepared in accordance with Example I of the subject invention, and the other prepared in accordance with Example 5 of United States Patent No. 4,879,094.

TABLE 1

Table 1 clearly demonstrates that an amount of phosphorous ranging between about 0.05-1 wt.% yields a copper alloy having an elongation and tensile strength clearly superior to that of the alloy of the '094 patent.

In the past, as little as 0.02 wt.% residual phosphorous has been utilized in some copper alloy compositions as an anti-oxidant. Notably, in the present invention, the phosphorous is present in excess of the amount required for de- oxidation.

In accordance with a further aspect of the present invention there is provided a lead-free copper alloy for use in continuous casting and ingot making. The alloy of the present invention can preferably be utilized for potable water system parts made according to the continuous cast process. In addition, the alloy of the present invention may be used for sleeve bearings, food processing machinery sanitary parts, and other categories of copper alloy parts to be wrought and/or machined. Still in accordance with a further aspect of the present invention there is provided an economical method of producing a lead-free copper alloy, which comprises melting certain grades of scrap materials and virgin elemental materials to produce a molten metal bath comprised of between about 2-20 wt.% tin; between about 2-40 wt.% zinc; between about 0-30 wt.% nickel; and, a balance being essentially copper; adding bismuth and phosphorous to the molten metal bath to yield a final bismuth content of about 1 to 3 wt.% in the metal bath and a final phosphorous content of about .05 to 1 wt.% in the metal bath.

Still other benefits and advantages of the invention will become apparent to those of ordinary skill in the art upon a reading and understanding of the following detailed specification.

Description of the Preferred Embodiments of the Invention

The lead-free copper base alloy of the present invention may be prepared by combining virgin elemental materials in a molten metal bath. Alternatively, an economically attractive method may be employed wherein the alloy is prepared by melting certain grades of copper alloys readily available as scrap material to produce a molten metal bath. The resultant molten metal bath is preferably comprised of between about 2-20 wt. % tin; between about 2-40 wt. % zinc; between about 0-30 wt. % nickel; and, a balance being essentially copper. Bismuth and phosphorous are subsequently added to the molten bath in amounts sufficient to yield a final bismuth content of about 1 to 3 wt. % in the bath and a final phosphorous content of about .05 to 1 wt. % in the bath.

Illustrative of the grades of copper alloys which may be employed as scrap material in the practice of the present invention are C50100, C50200, C50500, C50700, C50710, C50715, C50800, C50900, C51000, C51100, C51800, C51900, C52100, and C52400 and C90300. C50100, C50200, C50500, C50700, C50710, C50715, C50800, C50900, C51000, C51100, C51800, C51900, C52100, C52400, and C90300 are designations for copper alloys under the Unified Numbering System (UNS) , a numbering system developed by the Society of Automotive Engineers (SAE) and the American Society for Testing and Materials (ASTM) . The UNS uses a prefix letter, C for coppers and copper alloys, followed by a five digit number. In the UNS numbering system, the coppers and copper alloys are classified according to composition. In fact, an alloy may not be assigned to a designation under the UNS system unless its complete chemical composition is disclosed and the copper or copper alloy is in commercial use. Thus, one skilled in the art can easily identify the compositions of copper alloys designated by C50100, C50200, C50500, C50700, C50710, C50715, C50800, C50900, C51000, C51100, C51800, C51900, C52100, C52400, and C90300. It is not intended to limit the present invention to the foregoing types of grades of copper alloys for other well known grades of copper alloys will be readily apparent to one skilled in the art.

If the copper alloy contains a metal, i.e. nickel, which is more easily wetted by bismuth and soluble in solid copper, the amount of phosphorous to be added to the molten metal must be increased to about 0.3 wt.% to overcome the increased tendency for bismuth to form a continuous grain boundary film. In practice, the present invention is utilized as a molten metal treatment, wherein phosphorous is added to the metal prior to pouring castings. An addition of phosphorous to the liquid metal should preferably result in at least 0.1 wt.% phosphorous in the castings. However, the present invention contemplates an amount of phosphorous as high as 1 wt. % in the castings. With normal air melt induction furnaces, one (1) pound of 15% phosphorous-copper per one hundred (100) pounds of molten metal yields an alloy having the phosphorous composition of the present invention. The amount of phosphorous in the liquid metal may be maintained by covering liquid metal in holding furnaces, ladles and tundishes with a flux composed of one third (1/3) glass, one third (1/3) carbon, and one third (1/3) sodium chloride.

The alloy manufactured according to the above procedure is subsequently continuous cast into barstock. Mechanical properties of the alloy, including tensile strength, yield strength, elongation and hardness are listed in Table 3 for the WRM 844 Bi copper alloy and in Table 5 for the WRM 976 Bi copper alloy.

The tensile strength, yield strength and elongation of the copper alloy specimens were measured in accordance with ASTM standard E8.

The hardness of the copper alloy specimens was determined by means of a Brinell hardness test. The Brinell hardness number (BHN) was calculated from the area of the impression surface in a specimen. The impression surfaced was caused by the penetration of a 10 mm diameter standard hard steel ball under a load of 500 kg acting from 30 to 60 seconds.

Lastly, it should be noted that the terminology "lead- free copper base alloy" as used in the present invention is inclusive of alloys having trace amounts of lead, i.e., lead present as an incidental impurity.

The present invention will now be described in greater detail in the following examples.

EXAMPLE I 150 pounds of C51000 and 150 pounds of C90300 were melted in a normal air melt induction furnace at about 2040- 2140°F until molten. 294 pounds of copper, and 39 pounds of zinc were then added to the furnace. When the melt was ready to pour and superheated 25°F, 2% pounds of bismuth per 100 pounds in the melter were added to the molten bath. One pound of 15% phosphorus-copper per 100 pounds in the melter was then added to the molten bath. The melt was induction stirred for one minute at which time the metal was poured into a continuous cast tundish. The alloy was continuous cast utilizing the method disclosed in our U.S. Patent No. 4,315,538, incorporated herein by reference, to effect a uniform fine crystal size. The bismuth particles comprising the resulting alloy were quite small and evenly dispersed due to the presence of phosphorus in an amount of .10 wt %.

Table 3 below compares the composition and mechanical properties of our WRM 844 Bi alloy versus the nominal composition and typical mechanical properties of C84400, a copper-based alloy containing lead and typically utilized for load bearing components in plumbing goods. C84400 is the UNS designation corresponding to a copper alloy having the composition shown in Table 3.

TABLE 3

AVERAGE OF 3 SAMPLES TAKEN DURING ONE CAST

Our 844 Bi alloy was seen to be free machining, generating small chips in a turning operation. Average measurements of the lathe motor compared favorably with measurements observed for C84400. EXAMPLE II 150 pounds of C51000 and 150 pounds of C90300 were melted in a normal air melt induction furnace at about 2040- 2140°F until molten. 48 pounds of copper, 99 pounds of nickel, and 33 pounds of zinc were then added to the furnace. When the melt was ready to pour and superheated 25°F, 2% pounds of bismuth per 100 pounds in the melter were added to the molten bath. Two pounds of 15% phosphorus-copper per 100 pounds in the melter was then added to the molten bath. The melt was induction stirred for one minute at which time the metal was poured into a continuous cast tundish. The alloy was continuous cast utilizing the method disclosed in our U.S. Patent No. 4,315,538, incorporated herein by reference, to effect a uniform fine crystal size. The bismuth particles comprising the resulting alloy were quite small and evenly dispersed due to the presence of phosphorus in an amount of .3 wt %.

Table 4 below compares the composition and mechanical properties of our WRM 976 Bi alloy versus the nominal composition and typical mechanical properties of C97600, a copper-based alloy containing lead and typically utilized for food processing equipment parts. C97600 is the UNS designation corresponding to a copper alloy having the composition shown in Table 4.

TABLE 4

AVERAGE OF 3 SAMPLES TAKEN DURING ONE CAST

The invention has been described with reference to preferred and alternate embodiments. Obviously, modifications and alterations will occur to others upon the reading and understanding of this specification. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims

Having thus described the preferred embodiments, the invention is claimed as follows:

1. A lead-free copper alloy for use in continuous-casting and ingot making, which comprises: between about 1 to 3 wt.% bismuth, said bismuth being dispersed in a globular form throughout the grain boundaries of the alloy; between about 2 to 10 wt.% zinc; between about 2 to 10 wt.% tin; between about 0.05 to 1 wt.% phosphorous; and, a balance being essentially copper.

2. A lead-free copper alloy as recited in claim 1, further comprising: between about 0 to 1 wt.% iron; between about 0 to 1 wt.% manganese; and, between about 0 to 2.5 wt.% nickel.

3. An improved lead-free copper alloy of the type containing between about 1 to 3 wt.% bismuth; between about 2 to 10 wt.% zinc; between about 2 to 10 wt.% tin; and a balance being essentially copper, wherein the improvement comprises: between about .05 to 1 wt.% phosphorous.

4. A process for producing a lead-free copper base alloy for continuous-casting and ingot making, which comprises the steps of:

(a) melting metal to produce a molten metal bath comprising between about 2-20 wt. % tin; between about 2-40 wt. % zinc; between about 0-30 wt. % nickel and a balance being essentially copper;

(b) adding bismuth to the molten metal bath in an amount sufficient to yield a final bismuth content of about 1 to 3 wt. % in the metal bath; (c) adding phosphorous to the molten metal bath in an amount sufficient to yield a final phosphorous content of about .05 to 1 wt. % in the metal bath; and,

(d) continuous casting the molten metal bath.

5. A process for producing a lead-free copper base alloy as recited in claim 4, wherein the metal is comprised of at least one grade of copper alloys selected from the group consisting of C50100, C50200, C50500, C50700, C50710, C50715, C50800, C50900, C51000, C51100, C51800, C51900, C52100, and C52400 and C90300.

6. A process for producing a lead-free copper base alloy as recited in claim 4, wherein the bismuth is dispersed in a globular form throughout the grain boundaries of the alloy.