Cu-Ni-Zn-Mn Alloy

Abstract

Precipitation hardened alloy on the basis of copper, zinc, nickel and manganese exhibiting a high strength and ductility with values similar to those of stainless steels in combination with excellent machinability. The inventive alloy family is characterized by fine fibre-like or globular precipitates that emerge during intermediate temperature annealing treatments, which in case of the unleaded variations significantly improves the machinability. The alloy of invention is particularly suited for free machining applications such as the production of pen tips and reservoirs for writing implements of reduced tip dimensions, where conventional Cu—Ni—Zn—Mn alloys fail due to lack of strength and where the corrosion resistance in gel-based inks is insufficient without restriction to other fields of application.

Description

FIELD[0001]The present invention generally relates to wrought Cu—Ni—Zn (nickelsilver) alloys, more particularly to Cu—Ni—Zn—Mn alloys mainly for the use in areas where machining operations are substantial.DESCRIPTION OF RELATED ART[0002]With regard to the current market situation the trend goes from ball point pens typically filled with oil-based inks of relatively high viscosity towards roller-ball pens with inks of lower viscosity. These new lower viscosity inks are mainly water-based gel-inks. Compared to oil-based inks, gel-inks have the advantage of allowing a greater variety of bright colors and can have glitter effects, as they usually contain pigments that sink into the paper. Driven by stylistic arguments and reducing the ink consumption the trend in writing instruments goes towards finer pens, which can be more easily be realized with low viscosity inks, in particular with roller-ball pens. Reducing pen tips to dimensions smaller than 1.6 mm diameters cause stringent conse...

AI Technical Summary

Benefits of technology

[0010]It is part of the aim of this invention to introduce new microstructural design solutions for the alloy, which allows having, even in the absence of lead as a chip breaker, good machinability performances in free-machining operations. This can be solved on the one hand by adjusting the microstructure related to its partitioning of the alpha / beta phases and / or by additions of minor alloying elements forming precipitates with one of the major alloying elements. The minor alloying elements foreseen for this task are Fe, Al, Ca, Sn, P, and Si. Although it is known, that duplex structures, or precipitates favor chip breaking compared to a monophase structure, our multi-path approach is new with regard to the field of application as well as the alloy family of Cu—Ni—Zn—Mn alloys. First mentioned approach in the invention relating to the fine needle like precipitates of beta or beta′ phase in alpha mother grains is conceptually new and can be applied not only to this alloy family, but basically all Cu—Zn alloys, where part of the microstructure is in a metastable condition with respect to the phase transformation. The second approach of using precipitation of supersaturated solutions is a well-known process to increase the strength, but here it fulfills for this specific family of alloys and specific application two tasks: hardening and chip breaking and thus can be considered as novel. Lastly adding Ca as chip breaker has so far not found notation in combination with the fields of applications and the alloy family considered mentioned here.

[0011]Conventionally, there are four different hardening mechanisms known in single phased metals: Precipitation hardening, cold deformation hardening, solid solution strengthening, and grain size strengthening (Hall-Petch strengthing). Industrially mainly the first two mechanisms are of importance. Precipitation hardening is typically used in low-alloyed Cu-alloys where high electrical conductivity paired with moderate strength is requested. Spinoidal decomposition can be regarded as a special variation of precipitation hardening out of a supersaturated solid solution and finds application in Cu-alloys mainly in alloys containing substantial amounts of Sn or Ti. Cold deformation hardening is typically used for increasing the strength in rods, profile and wire products independent of the type of alloys. Solution hardening can be regarded as a side-effect when adding additional elements for improving different properties of the alloys, but is as such not of great relevance. Finally grain size hardening is industrially and technically difficult to control and its hardening contribution becomes evident only at grain sizes smaller than about 10 micrometers, sizes difficult to achieve in industrial production.

[0012]Similar to duplex steels, brasses or nickel-silver alloys having a certain range of Zn content exhibit a duplex alpha (face-centered cubic, fcc) and beta (body-centered-cubic, bcc) structure, which apart from representing a fifth mode of increasing strength, is also beneficially influencing the machinability, grain size stability and hot workability. Current commercially available leaded Cu—Ni—Zn—Mn alloys range in the Ni content from 5 to 25 wt. %, a Mn 0-7 wt. %, Zn 25-40 wt. % and rest Cu and impurities typically <1 wt. %. According to Guillet's rule [L. Guillet and A. Portevin, Revue de Metallurgie Memoirs XVII, Paris, (1920), 561] Mn shows with a factor of 0.5 only a slight influence towards the beta rich side in the phase diagram, while Ni exhibits a factor of −1.2 keeping the phase diagram on the alpha-rich side, and thus almost in balance for a Mn content of 6 wt. % and Ni content of 12 wt. %. Thus, as a first approximation the complicated 4 component system Cu—Zn—Ni—Mn can in this case be treated as the Cu—Zn binary phase diagram. However, as shown below for more precise estimates on a multicomponent phase diagram, more advanced thermodynamic software tools are required. With increasing Ni and Mn content the strength increases. Typical tensile strength values for cold drawn materials are 700-800 MPa, while in fewer cases values up to 900 MPa can be found for strongly cold drawn wires, however that typically goes at the expense of ductility, so that tensile elongations are limited to −1%.

Problems solved by technology

Reducing pen tips to dimensions smaller than 1.6 mm diameters cause stringent consequences with respect to the strength of the tip material.

Therefore, so far only stainless steel have been used as tip material for the finest tips, while Cu-based alloys are regarded as not being suitable due to their inferior strength.

Another common mistrust of Cu—Ni—Zn alloys compared to stainless steels is the resistance against corrosion in water-based gel inks.

On the other hand, Cu—Ni—Zn alloys of grey color bear significant disadvantages, which are related to the effect of ‘fire cracking’ [H. W. Schlapfer, W. Form Metal Science 13 (1979); H. W. Schlapfer, W. Form Metall, 32, 135 (1978)] that is related to the high internal stresses in the pure mono-phased alpha alloys containing lead.

However, due to the incompatibility of Bismuth with certain Cu-based alloys (high internal stress causing stress corrosion cracking) a replacement of Pb with Bi in die-castings and wrought products is not recommended.

Alloys containing bismuth are also more difficult to recycle, because recycling is done unmixed and so far fully developed recycling does only exist for lead containing copper alloys [Adaptation to Scientific and Technical Progress of Annex II Directive 2000 / 53 / EC; J. Lohse, S. Zangl, R. Groβ, C. O. Gensch, O. Deubzer. Öko-Institut e.V.

Bismuth is industrially rated as less toxic than lead and other neighboring heavy metals, however injection of large doses can cause kidney damage.

Alternatively, silicon has been suggested, as an element addition to favor chip breaking in brasses, but is due to the less-favorable chip form, the absence of a self-lubricating effect causing higher wear damage on tools and the associated difficulty to recycle such chips neither an easy choice for a free-machining Cu-based alloy.

Furthermore the risk of Fe—Si precipitates during casting of brasses containing low Fe concentrations further reduces the machinability.

Unfortunately, the alloy is not an easy to manufacture alloy due to the high sensitivity of forming oxides causing embrittlement.

In addition, in brasses, Te forms brittle ZnTe intermetallics as well results in unfavorable properties.

Graphite containing Cu-alloys are expensive due to high production cost via spray casting technology.

Solution hardening can be regarded as a side-effect when adding additional elements for improving different properties of the alloys, but is as such not of great relevance.

Finally grain size hardening is industrially and technically difficult to control and its hardening contribution becomes evident only at grain sizes smaller than about 10 micrometers, sizes difficult to achieve in industrial production.

Typical tensile strength values for cold drawn materials are 700-800 MPa, while in fewer cases values up to 900 MPa can be found for strongly cold drawn wires, however that typically goes at the expense of ductility, so that tensile elongations are limited to −1%.

Here however, the strengthing is only in part resulting from cold deformation (increase in dislocation density and point defects), but from the precipitation strengthening.

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