Other metallic elements that undergo peritectic decomposition in a tin brass alloy, such as cobalt, iridium, niobium, vanadium and molybdenum may substitute for from a portion to all of the iron.
Direct chill cast alloys containing from 1% to 4%, by weight of tin, from 0.8% to 4% of iron, from an amount effective to enhance iron initiated grain refinement to 20% of zinc and the remainder copper and inevitable impurities are readily hot worked. The zinc addition further increases the strength of the alloy and improves the bend formability in the “good way”, perpendicular to the longitudinal axis of a rolled strip.
[0001] This patent application is a continuation-in-Part of U.S. patent application Ser. No. 08/844,478 entitled “Iron Modified Tin Brass” by D. R. Brauer et al. that was filed on Apr. 18, 1997.
[0002] 1. Field of the Invention
[0003] This invention relates to copper alloys having high strength, good formability and relatively high electrical conductivity. More particularly, grain refinement of a tin brass is obtained by a controlled addition of iron, cobalt or other element that initiates a peritectic reaction during solidification.
[0004] 2. Description of Related Art
[0005] Throughout this patent application, all percentages are given in weight percent unless otherwise specified.
[0006] Commercial tin brasses are copper alloys containing from 0.35%-4% tin, up to 0.35% phosphorous, from 49% to 96% copper and the balance zinc. The alloys are designated by the Copper Development Association (CDA) as copper alloys C40400 through C49080.
[0007] One commercial tin brass is a copper alloy designated as C42500. The alloy has the composition 87%-90% of copper, 1.5%-3.0% of tin, a maximum of 0.05% of iron, a maximum of 0.35% phosphorous and the balance zinc. Among the products formed from this alloy are electrical switch springs, terminals, connectors, fuse clips, pen clips and weather stripping.
[0008] The
[0009] It is known to increase the yield strength of certain copper alloys through controlled additions of iron. For example, commonly owned U.S. patent application Ser. No. 08/591,065 entitled “Iron Modified Phosphor-Bronze” by Caron et al. that was filed on Feb. 9, 1996, discloses the addition of 1.65%-4.0% of iron to phosphor bronze. The Caron et al. alloy has an electrical conductivity in excess of 30% IACS and an ultimate tensile strength in excess of 95 ksi.
[0010] U.S. patent application Ser. No. 08/591,065 is incorporated by reference in its entirety herein.
[0011] Japanese patent application number 57-68061 by Furukawa Metal Industries Company, Ltd. discloses a copper alloy containing 0.5%-3.0%, each, of zinc, tin and iron. It is disclosed that iron increases the strength and heat resistance of the alloy.
[0012] While the benefit of an iron addition to phosphor-bronze is known, iron causes problems for the alloy. The electrical conductivity of the alloy is degraded and processing of the alloy is impacted by the formation of stringers. Stringers form when the alloy contains more than a critical iron content, which content is dependent on the alloy composition. The stringers originate when properitectic iron particles precipitate from liquid prior to solidification and elongate during mechanical deformation. Stringers are detrimental because they affect the surface appearance of the alloy and can degrade the formability characteristics.
[0013] In high copper (in excess of 85% Cu) tin brasses, the maximum permissible iron content, as an impurity, is typically 0.05%. This is because iron is known to reduce electrical conductivity and, through the formation of stringers, deteriorate the bend properties.
[0014] Other metallic additions to the alloy that induce the formation of a peritectic phase during solidification may substitute for the iron, either in whole or in part. One particular addition is cobalt, while other suitable additions include vanadium, niobium, iridium and molybdenum.
[0015] There exists, therefore, a need for an iron modified tin brass alloy that does not suffer from the stated disadvantages of reduced electrical conductivity and stringer formation.
[0016] Accordingly, it is an object of the invention to provide a tin brass alloy having increased strength. It is a feature of the invention that the increased strength is achieved by an addition of controlled amounts of a combination of iron and zinc. It is another feature of the invention that by processing the alloy according to a specified sequence of steps, a fine microstructure is retained in the wrought alloy.
[0017] Among the advantages of the alloy of the invention are that the yield strength is increased without a degradation in electrical conductivity. The microstructure of a refined as-cast alloy, grain size less than 100 microns, and a wrought alloy, grain size of about 5-20 microns, is fine grain. Still another advantage is that the electrical conductivity is about equal to that of copper alloy C42500 with a significant increase in yield strength.
[0018] In accordance with the invention, there is provided a copper alloy. This alloy consists essentially of from 1% to 4% by weight of tin, from 0.8% to 4.0% by weight of iron, from an amount effective to enhance iron initiated grain refinement to 20% by weight of zinc, up to 0.4% by weight of phosphorus and the remainder is copper, as well as inevitable impurities.
[0019] The grain refined alloy has an average as-cast grain size of less than 100 microns and an average grain size after processing of between about 5 and 20 microns.
[0020] The above stated objects, features and advantages will become more apparent from the specification and drawings that follow.
[0021]
[0022]
[0023]
[0024]
[0025]
[0026]
[0027]
[0028] The copper alloys of the invention are an iron modified tin brass. The alloys consist essentially of from 1% to 4% of tin, from 0.8% to 4.0% of iron, from 5% to 20% of zinc, up to 0.4% of phosphorus and the remainder is copper along with inevitable impurities. As cast, the grain refined alloy has an average crystalline grain size of less than 100 microns.
[0029] When the alloy is cast by direct chill casting, in preferred embodiments, the tin content is from 1.5% to 2.5% and the iron content is from 1.6% to 2.2%. 1.6% of iron has been found to be a critical minimum to achieve as-cast grain refinement. Most preferably, the iron content is from 1.6% to 1.8%.
[0030] Tin
[0031] Tin increases the strength of the alloys of the invention and also increases the resistance of the alloys to stress relaxation.
[0032] The resistance to stress relaxation is recorded as percent stress remaining after a strip sample is preloaded to 80% of the yield strength in a cantilever mode per ASTM (American Society for Testing and Materials) specifications. The strip is heated to 125° C. for the specified number of hours and retested periodically. The properties were measured at up to 3000 hours at 125° C. The higher the stress remaining, the better the utility of the specified composition for spring applications.
[0033] However, the beneficial increases in strength and resistance to stress relaxation are offset by reduced electrical conductivity as shown in Table 1. Further, tin makes the alloys more difficult to process, particularly during hot processing. When the tin content exceeds 2.5%, the cost of processing the alloy may be prohibitive for certain commercial applications. When the tin content is less than 1.5%, the alloy lacks adequate strength and resistance to stress relaxation for spring applications.
TABLE 1 Electrical Conductivity Composition (% IACS) Yield Strength (ksi) 88.5% Cu 26 75 9.5% Zn 2% Sn 0.2% P 87.6% Cu 21 83 9.5% Zn 2.9% Sn 0.2% P 94.8% Cu 17 102 5% Sn 0.2% P
[0034] Preferably, the tin content of the alloys of the invention is from about 1.2% to about 2.2% and most preferably from about 1.4% to about 1.9%.
[0035] Iron
[0036] Iron refines the microstructure of the as-cast alloy and increases strength. The refined microstructure is characterized by an average grain size of less than 100 microns. Preferably, the average grain size is from 30 to 90 microns and most preferably, from 40 to 70 microns. This refined microstructure facilitates mechanical deformation at elevated temperatures, such as rolling at 850° C.
[0037] When the iron content is less than about 1.6%, the grain refining effect is reduced and coarse crystalline grains, with an average grain size on the order of 600-2000 microns, develop. When the iron content exceeds 2.2%, excessive amount of stringers develop during hot working.
[0038] The effective iron range, 1.6%-2.2%, differs from the iron range of the alloys disclosed in Caron et al. patent application Ser. No. 08/591,065. Caron et al. disclose that grain refinement was not optimized until the iron content exceeded about 2%. The ability to refine the grain structure at lower iron contents in the alloys of the present invention was unexpected and believed due to a phase equilibrium shift due to the inclusion of zinc. To be effective, this phase shift interaction requires a minimum zinc content of about 5%.
[0039] Large stringers, having a length in excess of about 200 microns, are expected to form when the iron content exceeds about 2.2%. The large stringers impact both the appearance of the alloy surface as well as the properties, electrical and chemical, of the surface. The large stringers can change the solderability and electro-platability of the alloy.
[0040] To maximize the grain refinement and strength increase attributable to iron without the detrimental formation of stringers, the iron content should be maintained between about 1.6% and 2.2% and preferably, between about 1.6% and 1.8%.
[0041] Zinc
[0042] The addition of zinc to the alloys of the invention would be expected to provide a moderate increase in strength with some decrease in electrical conductivity. While, as shown in Table 2, this occurred, surprisingly, with a minimum of 5% zinc present, the grain refining capability of the iron addition was significantly enhanced, as illustrated in Table 3.
TABLE 2 Electrical Conductivity Tensile Strength Composition (% IACS) (ksi) 1.8 Sn 33 99 2.2 Fe balance Cu 1.8 Sn 29 99 2.2 Fe 5 Zn balance Cu 1.8 Sn 25 108 2.2 Fe 10 Zn balance Cu
[0043]
TABLE 3 Composition Grain Size 1.9 Fe Coarse 1.8 Sn 0.04 P balance Cu 5 Zn Medium 1.9 Fe 1.8 Sn 0.04 P balance Cu 7.5 Zn Fine 1.9 Fe 1.8 Sn 0.04 P balance Cu 10 Zn Fine 1.9 Fe 1.8 Sn 0.04 P balance Cu 15 Zn Fine 3.3 Co Fine 1.8 Sn 0.04 P balance Cu
[0044] Preferably, the zinc content is from that effective to enhance iron initiated grain refinement to about 20%. More preferably, the zinc content is from about 5% to about 15% and most preferably, the zinc content is from about 8% to about 12%.
[0045] Peritectic Reaction for Cast Grain Refinement
[0046] It is believed that the grain refining effectiveness of the iron addition is due to the iron separating from the melt first, during solidification, as numerous, relatively fine, dendritically shaped particles of fcc (face centered cubic) gamma iron. With continued cooling, these properitectic iron particles effectively nucleate cast grains of the alloy via the peritectic solidification reaction:
[0047] effectively raising the nucleation rate, in turn resulting in cast grain refinement.
[0048] Other metallic elements that undergo a similar peritectic decomposition reaction with elemental or intermetallic properitectic particles in a tin brass may also be used, subject to the proviso that the peritectic composition does not require such a large amount of the addition that the desirable properties of the tin brass, such as processing capability, conductivity or bend formability, are severely degraded.
[0049] Cobalt is a suitable substitute for either a portion, or all, of the iron as shown in Table 4.
TABLE 4 Composition Grain Size 10 Zn Coarse 2.7 Co 1.8 Sn 0.04 P balance Cu 10 Zn Coarse 3.0 Co 1.8 Sn 0.04 P balance Cu 10 Zn Fine 3.3 Co 1.8 Sn 0.04 P balance Cu
[0050] From Table 4, the cobalt content, when used as the primary grain refiner, should be in excess of about 3.0%. Preferably, the cobalt content is between about 3.2% and 4.4% and most preferably from between 3.2% and 3.6%. Excessive amounts of cobalt should be avoided because coarse stringers of excess properitectic cobalt particles may occur and degrade alloy properties.
[0051] Cobalt may be added as a partial substitute for iron. Cobalt less effectively refines the grain structure of the alloys of the invention and the substitution should satisfy the equation:
[0052] M is between 0.45 and 0.65, and preferably from 0.5 to 0.6. Most preferably, the substitution is in the higher range, about 0.6 for lower contents of cobalt and about 0.5 for higher contents of cobalt with an approximate delineation between the lower contents and the higher contents being a 2% cobalt.
[0053] Other suitable properitectic particle formers include iridium in an amount of from about 10% to about 20% and preferably in an amount of from about 11% to 15%; niobium in an amount of from about 0.01% to about 5% and preferably in an amount of from about 0.1% to about 1%; vanadium in an amount of from about 0.01% to about 5% and preferably in an amount of from about 0.1% to about 1%; and molybdenum in an amount of from about 0.5% to about 5% and preferably in an amount of from about 1% to about 3%.
[0054] One or more of these other peritectic reaction initiators may substitute, in whole or in part, for cobalt or iron.
[0055] Other additions
[0056] Phosphorous is added to the alloy for conventional reasons, to prevent the formation of copper oxide or tin oxide precipitates and to promote the formation of iron phosphides. Phosphorous causes problems with the processing of the alloy, particularly with hot rolling. It is believed that the iron addition counters the detrimental impact of phosphorous. At least a minimal amount of iron must be present to counteract the impact of the phosphorous.
[0057] A suitable phosphorous content is any amount up to about 0.4%. A preferred phosphorous content is from about 0.03% to 0.3%.
[0058] Other elements that remain in solution when the copper alloy solidifies may be present in amounts of up to 20% and may substitute, at a 1:1 atomic ratio, for either a portion, or all, of the zinc. The preferred ranges of these solid-state soluble elements are those specified for zinc. Among the preferred elements are manganese and aluminum.
[0059] Less preferred are additions of elements that affect the properties of the alloy. Although, less preferred, additions such as nickel, magnesium, beryllium, silicon, zirconium, titanium, chromium and mixtures thereof may be included.
[0060] For example, nickel additions severely reduce electrical conductivity. As a result, the less preferred additions are preferably present in an amount of less than about 0.4% and most preferably, in an amount of less than about 0.2%. Most preferably, the sum of all less preferred alloying additions is less than about 0.5%.
[0061] Processing
[0062] The alloys of the invention are preferably processed according to the flow chart illustrated in
[0063] The hot rolling reduction is, typically, by thickness, up to 98% and preferably, from about 80% to about 95%. The hot rolling may be in a single pass or in multiple passes, provided that the temperature of the ingot is maintained at above 650° C.
[0064] After hot rolling
[0065] The strip is then annealed
[0066] The strip may also be strip annealed, such as for example, at a temperature of from about 600° C. to about 950° C. for from 0.5 minute to 10 minutes.
[0067] The first recrystallization anneal
[0068] The bars are then cold rolled
[0069] The strip is then given a second recrystallization anneal
[0070] The alloys are then cold rolled
[0071] The alloys are then relief annealed
[0072] Following the relief anneal
[0073] The advantages of the alloys of the invention will become more apparent from the examples that follow.
[0074] Copper alloys containing 10.5% zinc, 1.7% tin, 0.04% phosphorous, between 0% and 2.3% iron and the balance copper were prepared according to the process of
[0075] The 0.2% offset yield strength and the tensile strength were measured on a tension testing machine (manufactured by Tinius Olsen, Willow Grove, Pa.).
[0076] As shown in
[0077]
[0078] Copper alloys containing 10.4% zinc, 1.8% iron, 0.04% phosphorous, between 1.8% and 4.0% tin and the balance copper were processed according to
[0079]
[0080] Since the strength increase is monatomic with the amount of tin while the conductivity decreases, the tin content should be a trade-off between desired strength and conductivity.
[0081] Copper alloys containing 1.9% iron, 1.8% tin, 0.04% phosphorous, between 0% and 15% zinc and the balance copper were processed according to
[0082]
[0083]
[0084] Table 5 illustrates a series of alloys processed according to TABLE 5 Elec. Tensile Yield Conduct. Strength Strength Alloy Composition % IACS (ksi) (ksi) A 1.8 Sn 33% 99 96 2.2 Fe 0.06 P balance Cu B 1.8 Sn 29% 99 94 2.2 Fe 0.06 P 5.0 Zn balance Cu C 1.8 Sn 25% 108 101 2.2 Fe 0.06 P 10.0 Zn balance Cu D 4.27 Sn 17% 102 96 0.033 P balance Cu
[0085] Table 5 shows that the addition of 5% zinc did not increase the strength of the alloy and slightly reduced electrical conductivity. A 10% zinc addition had a favorable impact on the strength.
[0086] The benefit of the zinc addition is more apparent in view of Table 6 where the strength to rolling reduction is compared.
TABLE 6 MBR/t MBR/t Alloy % Red. YS TS GW BW A 25 80 83 1.0 1.3 C 25 84 88 0.8 1.6 A 33 83 86 1.0 1.3 C 33 89 94 0.9 2.1 A 58 96 99 1.7 3.9 C 60 96 102 1.6 6.4 A 70 100 104 1.9 6.3 C 70 101 108 1.9 ≧7
[0087] A further benefit of the zinc addition is the improved good way bends achieved with alloy C. Bend formability was measured by bending a 0.5 inch wide strip 180°about a mandrel having a known radius of curvature. The minimum mandrel about which the strip could be bent without cracking or “orange peeling” is the bend formability value. The “good way” bend is made in the plane of the sheet and perpendicular to the longitudinal axis (rolling direction) during thickness reduction of the strip. “Bad way” is parallel to the longitudinal axis. Bend formability is recorded as MBR/t, the minimum bend radius at which cracking or orange peeling in not apparent, divided by the thickness of the strip.
[0088] Usually, an increase in strength is accompanied by a decrease in bend formability. However, with the alloys of the invention, an addition of 10% zinc increases both the strength and the good way bends.
[0089] Alloys of the compositions indicated in Table 7, with the balance being copper, were processed according to Process 1. Table 7 shows the effectiveness of cobalt as a partial substitute for iron in the tin brass alloys of the invention.
TABLE 7 CR 22% CR 65% As-cast (RA) (RA) Grain YS/UTS/EL) YS/UTS/EL Zn Sn Fe Co P Size (ksi/ksi/%) (ksi/ksi/%) 10.4 1.80 1.5 0.5 0.04 fine (83/87/7) (101/108/4) 10.4 1.80 1.78 — 0.04 fine (81/85/11) (102/108/2) 10.4 1.80 1.5 — 0.04 coarse — —
[0090] Table 8 illustrates the magnetic permeability of hot rolled plate when formed from cobalt containing tin brass is higher than the magnetic permeability of the same alloy when an equivalent amount of iron is present, using 0.6Co=Fe as the equivalency relationship.
TABLE 8 Magnetic Permeability As-cast (Hot Rolled Zn Sn Fe Co P Grain Size Plate) 10.2 1.87 2.02 — 0.03 fine 1.05-1.10 10.5 1.80 — 3.3 0.04 fine 1.2
[0091] While described particularly in terms of direct chill casting, the alloys of the invention may be cast by other processes as well. Some of the alternative processes have higher cooling rates such as spray casting and strip casting. The higher cooling rates reduce the size of the properitectic iron particles and are believed to shift the critical maximum iron content to a higher value such as 4%.
[0092] It is apparent that there has been provided in accordance with the invention an iron modified phosphor bronze that fully satisfies the objects, means and advantages set forth hereinabove. While the invention has been described in combination with embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the appended claims.