Abstract:
A Cu—Zn—Si alloy includes, in % by weight, 70 to 80% of copper, 1 to 5% of silicon, 0.0001 to 0.5% of boron, up to 0.2% of phosphorus and/or up to 0.2% of arsenic, a remainder of zinc, plus inevitable impurities. Products using the alloy and processes for producing the alloy are also provided. The alloy is distinguished by an improved resistance to oxidation and by uniform mechanical properties.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This is a continuing application, under 35 U.S.C. §120, of copending International Application No. PCT/EP2005/005238, filed May 13, 2005, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German Patent Application 10 2004 049 468.1, filed Oct. 11, 2004; the prior applications are herewith incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION  
     Field of the Invention  
       [0002]     The invention relates to a copper-zinc-silicon alloy and to products using the alloy and processes for producing such a copper-zinc-silicon alloy.  
         [0003]     A most important requirement of copper-zinc-silicon alloys is that they be resistant to dezincification and be able to be machined. Heretofore, good machining properties of brass alloys of that type has been realized by the addition of lead, as is described, for example, in European Patent Application EP 1 045 041 A1. Recently, however, lead-free brass alloys with good machining properties have been developed, as is described, for example, in European Patent Application EP 1 038 981 A1 and German Patent DE 103 08 778 B3, corresponding to U.S. Patent Application Publication No. 2004/0234411 A1. Both lead-free and lead-containing Cu—Zn—Si alloys have a tendency to be oxidized and form a layer of scale at temperatures between 300° C. and 800° C. That layer of scale is only loosely bonded to the metal and can easily become detached therefrom, so that it is then dispersed through the production facilities, with the result that the layer has a disruptive contaminating effect. The production facilities are expensive to clean, making production costs high. A further drawback of the known Cu—Zn—Si alloys is that the mechanical properties of the material change over long workpieces, since the material lacks homogeneity.  
       SUMMARY OF THE INVENTION  
       [0004]     It is accordingly an object of the invention to provide a copper-zinc-silicon alloy, products using the alloy and processes for producing the alloy, which overcome the hereinafore-mentioned disadvantages of the heretofore-known products and processes of this general type and in which the copper-zinc-silicon or brass alloy is improved in terms of its homogeneity and, furthermore, is less prone to the formation of scale.  
         [0005]     With the foregoing and other objects in view there is provided, in accordance with the invention, a Cu—Zn—Si alloy. The alloy comprises, in % by weight, 70 to 80% of copper; 1 to 5% of silicon; 0.0001 to 0.5% of boron; 0 to 0.2% of phosphorus and/or arsenic; a remainder of zinc; and inevitable impurities.  
         [0006]     The copper content is between 70 and 80%, since copper contents of below 70% or above 80% would have an adverse effect on the machining properties of the alloy. The same applies if the silicon concentration departs from the indicated range of 1% to 5%. The boron concentration in the alloy is between 0.0001 and 0.5%. Surprisingly, it has now been found that the addition of boron within the concentration range claimed on one hand, reduces the formation of scale and on the other hand, significantly improves the bonding of the remaining scale to the material. Furthermore, it is also surprising that the addition of boron improves the homogeneity of the microstructure and thereby prevents fluctuations in the mechanical properties. Phosphorus and arsenic may each be present in the alloy in a concentration of up to 0.2%, and can be substituted for one another. Phosphorus and arsenic have a beneficial effect on the formation of the initial cast microstructure and the corrosion properties, and furthermore improve the flow properties of the melt and reduce the susceptibility to stress corrosion cracking. The remaining main component of the alloy is zinc.  
         [0007]     In addition to the advantages listed above of avoiding easily detached layers of scale which increase production costs, improving the mechanical properties, and furthermore good machining properties and shaping properties in combination with a high resistance to corrosion, the resistance to dezincification and stress corrosion cracking are also particularly pronounced in the invention. Dezincification tests carried out in accordance with ISO 6509 yield dezincification depths of only up to 26 μm.  
         [0008]     With the objects of the invention in view, there are also provided products using a copper-zinc-silicon alloy of this type, for electrical engineering components, sanitary-ware components, vessels for transporting or storing liquids or gases, torsionally loaded components, recyclable components, drop-forged components, semi-finished products, strips, sheets, profiled sections, plates, or as a wrought, rolled or cast alloy.  
         [0009]     The Cu—Zn—Si alloy is used for contacts, pins or securing elements in electrical engineering, for example as stationary contacts or fixed contacts, including clamping and plug connections or plug-in contacts.  
         [0010]     The alloy has a high resistance to corrosion with respect to liquid and gaseous media. Moreover, it is extremely resistant to dezincification and stress corrosion cracking. Consequently, the alloy is particularly suitable for use for vessels for transporting or storing liquids or gases, in particular for vessels used in refrigeration or for pipes, water fittings, valve extensions, pipe connectors and valves in sanitary-ware.  
         [0011]     The low corrosion rates also ensure that metal leaching, i.e. the property of losing alloying constituents through the action of liquid or gaseous media, is inherently low. In this respect, the material is suitable for application areas which require low pollutant emissions in order to protect the environment. Therefore, the alloy according to the invention can be used in the field of recyclable components.  
         [0012]     The lack of susceptibility to stress corrosion cracking means that the alloy is recommended for use in screwed or clamped connections in which, for technical reasons, high elastic energies are stored. Therefore, the alloy is particularly suitable for all components which are subject to tensile and/or torsional loads, in particular for nuts and bolts. After cold-forming, the material achieves high values for proof stress. Consequently, greater tightening torques can be realized in screw connections which must not be plastically deformed. The yield strength ratio of the Cu—Zn—Si alloy is lower than in the case of free-machining brass. Screw connections which are tightened only once and in the process are deliberately over-extended, therefore achieve particularly high holding forces.  
         [0013]     Possible uses of the Cu—Zn—Si alloy result for starting materials in both tube and strip form. The alloy is also eminently suitable for strips, sheets and plates which can be milled or punched, in particular for keys, engravings, for decorative purposes or for leadframe applications.  
         [0014]     With the objects of the invention in view, there is additionally provided a process for the production of a copper-zinc-silicon alloy. The process comprises conventional continuous casting and hot-rolling at between 600 and 760° C. with subsequent deformation, in particular cold-rolling, preferably with the addition of further annealing and deformation steps.  
         [0015]     With the objects of the invention in view, there is furthermore provided another process for the production of a copper-zinc-silicon alloy. The process comprises conventional continuous casting and extrusion at up to 760° C., preferably between 650 and 680° C., followed by cooling in air.  
         [0016]     In accordance with another feature of the invention, the Cu—Zn—Si alloy includes 75 to 77% of copper, 2.8 to 4% of silicon, 0.001 to 0.1% of boron, and 0.03 to 0.1% of phosphorus and/or arsenic, as well as zinc as a remainder element plus inevitable impurities.  
         [0017]     In accordance with a further feature of the invention, the copper-zinc-silicon alloy includes at least one element, in % by weight, selected from the group consisting of 0.01 to 2.5% of lead, 0.01 to 2% of tin, 0.01 to 0.3% of iron, 0.01 to 0.3% of cobalt, 0.01 to 0.3% of nickel and 0.01 to 0.3% of manganese. The addition of lead has a positive influence on the machining properties.  
         [0018]     In accordance with an added feature of the invention, the copper-zinc-silicon alloy includes at least one element, in % by weight, selected from the group consisting of 0.01 to 0.1% of lead, 0.01 to 0.2% of tin, 0.01 to 0.1% of iron, 0.01 to 0.1% of cobalt, 0.01 to 0.1% of nickel and 0.01 to 0.1% of manganese.  
         [0019]     In accordance with an additional feature of the invention, the Cu—Zn—Si alloy includes at least one element, in % by weight, out of up to 0.5% of silver, up to 0.5% of aluminum, up to 0.5% of magnesium, up to 0.5% of antimony, up to 0.5% of titanium and up to 0.5% of zirconium. The element is preferably selected from the group consisting of 0.01 to 0.1% of silver, 0.01 to 0.1% of aluminum, 0.01 to 0.1% of magnesium, 0.01 to 0.1% of antimony, 0.01 to 0.1% of titanium and 0.01 to 0.1% of zirconium.  
         [0020]     In accordance with a concomitant feature of the invention, the Cu—Zn—Si alloy includes at least one element, in % by weight, selected from the group consisting of up to 0.3% of cadmium, up to 0.3% of chromium, up to 0.3% of selenium, up to 0.3% of tellurium and up to 0.3% of bismuth. The element is preferably selected from the group consisting of 0.01-0.3% of cadmium, 0.01-0.3% of chromium, 0.01-0.3% of selenium, 0.01-0.3% of tellurium and 0.01-0.3% of bismuth.  
         [0021]     Other features which are considered as characteristic for the invention are set forth in the appended claims.  
         [0022]     Although the invention is illustrated and described herein as embodied in a copper-zinc-silicon alloy, products using the alloy and processes for producing the alloy, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
         [0023]     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]      FIGS. 1A, 1B  and  1 C are photographs showing the formation of a layer of scale after annealing for 2 h at 600° C., on a CuZn21Si3P alloy without the addition of boron in  FIG. 1A , on a CuZn21Si3P alloy containing 0.0004% of boron in  FIG. 1B , and on a CuZn21Si3P alloy containing 0.009% of boron in  FIG. 1C ; and  
         [0025]      FIGS. 2A, 2B  and  2 C are photographs showing the formation of a cast microstructure, of a CuZn21Si3P alloy without the addition of boron in  FIG. 2A , of a CuZn21Si3P alloy with 0.0004% of boron in  FIG. 2B , and of a CuZn21Si3P alloy containing 0.009% of boron in  FIG. 2C . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     The CuZn21Si3P alloys on which the exemplary embodiment is based have variations in concentration of the components, with copper amounting to between 75.8 and 76.1%, silicon amounting to between 3.2 and 3.4% and phosphorus amounting to between 0.07 and 0.1%, together with zinc as the remainder plus inevitable impurities. The alloy examples have different boron contents, at 0%, 0.004% and 0.009%. The alloys are produced by continuous casting followed by extrusion at temperatures below 760° C., preferably between 650 and 680° C., followed by rapid cooling.  
         [0027]     All of the alloys have an excellent resistance to dezincification. A dezincification test carried out in accordance with ISO 6509 reveals dezincification depths of only less than 26 μm.  
         [0028]     Referring now to the figures of the drawings in detail and first, particularly, to  FIGS. 1A, 1B  and  1 C thereof, it is seen that if CuZn21Si3P alloys are exposed to temperatures of 300-800° C., for example during hot-working, scale is formed, and this scale can easily become detached and contaminate the production facilities. An extensively scaled surface of a boron-free CuZn21Si3P alloy is illustrated in  FIG. 1A . The surface of the specimen appears predominantly grey in  FIG. 1A . This grey color reveals the scaled surface of the CuZn21Si3P alloy. Only a few individual bright spots without any regular distribution are visible on the surface of the alloy. By contrast, the CuZn21Si3P alloy with a boron content of 0.0004% in  FIG. 1B  has a very much greater number of white spots on the surface of the alloy than the boron-free alloy. These white spots represent bright metallic regions of the alloy.  
         [0029]     These bright metallic regions, i.e. regions without any scale, are distributed uniformly over the surface of the alloy. The proportion of the surface on which scale has formed is considerably reduced and the remaining scale is more securely bonded to the metal than in the case of the boron-free alloy.  FIG. 1C  illustrates a CuZn21Si3P alloy containing 0.009% of boron. This figure clearly reveals that the number of bright metallic surfaces, i.e. of white spots, has increased further. In some areas, there are relatively large continuous regions of bright metallic material, and the figure also reveals a very regular distribution on the surface of the alloy. The proportion of the surface on which scale has formed has decreased further, and the remaining scale is securely bonded to the metal. Therefore, it has surprisingly emerged that low boron concentrations of 0.0001-0.5% restrict the formation of scale on Cu—Zn—Si alloys and at the same time considerably increase the bonding of the scale to the metal, with the result that undesirable contamination of the production facilities is avoided.  
         [0030]     A similar result was also found for Cu—Zn—Si—P alloys with different lead contents, such as for example 0.01%, 0.05%, 0.1% or 2.5%.  
         [0031]     In addition to reducing the susceptibility to scaling of Cu—Zn—Si alloys, boron also has a positive effect on mechanical properties, since boron makes the microstructure of the alloy more homogeneous. This change to the microstructure of the alloy is illustrated in  FIGS. 2A, 2B  and  2 C as a function of the boron concentrations. Whereas a CuZn21Si3P alloy without the addition of boron has a coarse, inhomogeneous microstructure as seen in  FIG. 2A , a CuZn21Si3P alloy containing 0.0004% of boron has a significantly more homogeneous microstructure which already has very uniform grain sizes as seen in  FIG. 2B . A further increase in the boron content to 0.009% results in an even more uniform CuZn21Si3P alloy of even greater homogeneity, in which the grains of the microstructure can no longer be seen by the naked eye, according to  FIG. 2C .  
         [0032]     In addition to optical changes to the microstructure, the addition of boron also has beneficial effects on mechanical properties. This is particularly apparent on rods which have been extruded from Cu—Zn—Si alloys. In order to determine the mechanical properties, samples were taken at the start and the end of such rods. The tensile strength of a rod made from a CuZn21Si3P alloy without the addition of boron differs by more than 60 N/mm 2  at the start of the rod as compared to the end of the rod. A corresponding alloy with a boron content of 0.0004%, by contrast, has a tensile strength difference of only less than 40 N/mm 2  between the start and end of the rod. If 0.009% of boron is added to a CuZn21Si3P alloy, the difference in the tensile strength between the start and end of the rod is less than 5 N/mm 2 .  
         [0033]     Therefore, the material has identical mechanical properties throughout. Accordingly, a uniform strength is achieved over the entire extruded length. The reason for this is the grain-refining action of boron.  
         [0034]     The table below reveals the relationship between the boron content of a Cu—Zn—Si alloy and the increasing homogeneity of the alloy microstructure or the decreasing strength differences within an extruded workpiece.  
                                               Tensile strength       Alloy   Position   in N/mm 2                     CuZn21Si3P   Start of extrusion   514           End of extrusion   578       CuZn21Si3P containing   Start of extrusion   507       0.0004% of boron   End of extrusion   545       CuZn21Si3P containing   Start of extrusion   508       0.009% of boron   End of extrusion   512