Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C9/00—Alloys based on copper
  • C22C9/04—Alloys based on copper with zinc as the next major constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/08—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of copper or alloys based thereon

Definitions

• the invention relates to a wrought copper-zinc alloy for production of a semifinished product in wire, pipe or bar form, to a semifinished product produced from a wrought copper-zinc alloy, and to a process for production of such a semifinished product.
• a wrought copper-zinc alloy generally means a wrought material composed of a copper-zinc alloy.
• Copper-zinc alloys with 3% to 5% by weight of lead have excellent machinability and additionally very good hot and cold formability.
• Lead-containing copper-zinc alloys are therefore used in a multitude of applications, for example for connections and components in the automotive industry, in building technology, in mechanical engineering, in electrical appliances and in electronic components, in telecommunications and as fittings in water installations.
• lead in elemental form as particles in the microstructure, and these particles act as chip breakers.
• lead In the course of machining, lead is in the form of a liquid phase as a result of the significant local deformation in the workpiece and the resultant local increase in temperature. Since the lead is not able to absorb stresses in the liquid state, this leads to a concentration of stress in the load-bearing, weakened matrix and hence to easier chip breakage.
• lead is also incorporated into the tribological layer between material and workpiece in the course of machining and hence leads to effective lubrication and hence to a reduction in friction and wear. Moreover, because of its low solubility, lead barely makes any contribution to electrical conductivity.
• document EP 2 194 150 B1 discloses copper-zinc alloys containing 0.1% to 1.5% by weight of Si, 0.03% to 0.4% by weight of Al, 0.01% to 0.36% by weight of P, 0.05% to 0.5% by weight of Sn, and 0.001% to 0.05% by weight of rare earths.
• the alloys Owing to the formation of an ⁇ , ⁇ and possibly ⁇ microstructure, the alloys have good machinability.
• the Al fraction results in formation of aluminum phosphides. Although the ⁇ phase and aluminum phosphides improve chip formation, they worsen the service life of the tool. Moreover, the proportion of rare earths is likely to lead to embrittlement of the microstructure.
• the alloys are used for castings and hot pressings.
• the invention relates to a wrought copper-zinc alloy for production of a semifinished product in wire, tube or bar form, having the following composition in % by weight:
• Cu 58.0% to 63.0%, Si: 0.04% to 0.32%, P: 0.05% to 0.20%, Sn: optionally up to 0.25%, Al: optionally up to 0.10%, preferably up to 0.05%, Fe: optionally up to 0.30%, preferably up to 0.10%, Ni: optionally up to 0.30%, Pb: optionally up to 0.25%, preferably up to 0.10%, Te, Se, In each optionally up to 0.10%, Bi: not more than 0.009%,
• the invention proceeds from the consideration of reducing the proportions of Pb in the copper-zinc alloy as far as possible without worsening the machinability of the alloy.
• Si and P are specifically added to the alloy, and the proportion of the ⁇ phase is adjusted so as to give favorable machining properties on the one hand, especially in the case of drilling, and high conductivity, and on the other hand not to worsen the hot and cold formability of the alloy, and such that the semifinished product produced from the alloy has excellent straightness.
• the process regime especially in the casting and hot forming operations, is chosen so as to result in the desired properties.
• a globular ⁇ phase is a prerequisite for good straightness and trueness to scale of the semifinished product.
• ⁇ phase forms from the ⁇ phase after hot forming. Therefore, the ⁇ phase must be in fine-grain form in the cast state. It has been found that, surprisingly, a fine-grain ⁇ phase in the cast state is favored by homogeneously finely distributed copper- and/or zinc-containing phosphides. In the primary crystallization of the ⁇ crystallites, the residual melt becomes enriched with P and hence leads to a subdivision of the ⁇ phase. Solidification forms a eutectic composed of phosphide and ⁇ phase.
• grain refining of the a crystallites is observed.
• This grain refining of the cast microstructure by P facilitates hot forming, continues into the microstructure after hot forming, and consequently leads to grain refining in the final state.
• phosphide particles are present in the ⁇ phase in the final state.
• ductility of the alloy is low.
• the sum total of the proportions of Si and P is preferably not more than 0.45% by weight.
• the cooling rate has to be at least 30° C. per minute (30° C./min), preferably at least 40° C. per minute, and at most 60° C. per minute, preferably at most 50° C. per minute.
• the uniformly finely distributed phosphides that accompany a fine-grain ⁇ phase in the cast state dissolve in the matrix during the hot forming and then reform during the cooling operation in the course of hot forming. In this way, the characteristic distribution of the phosphides in the cast state is ultimately mapped onto the microstructure in the final state.
• the distribution of the phosphides in the final state and the globular form of the ⁇ phase are therefore determined not only by the chemical composition of the alloy but also by the process regime in the casting and in the hot forming.
• the characteristics of the phosphides in the final state are thus like a fingerprint left on the product by the particular process regime.
• the distribution of the phosphides in the final state can be characterized as follows: In an area of 21 000 ⁇ m 2 , there are 7 to 200 phosphide particles having an equivalent diameter of 0.5 to 1 ⁇ m, 4 to 150 phosphide particles having an equivalent diameter of 1 to 2 ⁇ m, and not more than 30 phosphide particles having an equivalent diameter of more than 2 ⁇ m.
• the equivalent diameter of a phosphide particle means the diameter of a circle of equal area to the phosphide particle.
• the predominant portion of the phosphide particles having an equivalent diameter of at least 0.5 ⁇ m has an equivalent diameter of not more than 2 ⁇ m.
• the proportion of the phosphide particles having an equivalent diameter of 0.5 to 2 ⁇ m is preferably at least 70% of the number of all phosphide particles having an equivalent diameter of at least 0.5 ⁇ m. This proportion is more preferably at least 75%.
• the Cu content of the alloy is 58.0% to 63.0% by weight. In the case of a Cu content of less than 58.0% by weight, ductility of the alloy is too low. In the case of a Cu content of more than 63.0% by weight, the zinc content in the alloy is too small to achieve good machinability.
• the ratio of the proportions by weight of P and Al is at least 1.0.
• Aluminum forms aluminum phosphides with phosphorus. However, these are unwanted because they do not improve machinability characteristics. In order that there is a sufficient excess of P available for formation of copper- and/or zinc-containing phosphides, the ratio of the proportions by weight of P and Al must be at least 1.0 in the alloy.
• the optional Sn and Al elements promote formation of the ⁇ phase.
• an Sn content of more than 0.20% by weight there is a deterioration in the machinability properties of the alloy.
• this deterioration can be compensated for by heat treatment up to an Sn content of 0.25% by weight.
• the proportion of tin should preferably be not more than 0.20% by weight, more preferably not more than 0.10% by weight.
• Aluminum forms aluminum phosphides with phosphorus. However, these are unwanted, and therefore the Al content should not exceed 0.10% by weight, preferably 0.05% by weight.
• Nickel promotes formation of the ⁇ phase and hence improves cold formability. Moreover, nickel forms phosphides, which do not have an advantageous effect on machinability. Therefore, the proportion of nickel must be not more than 0.30% by weight, preferably not more than 0.10% by weight.
• the element Bi is present as an impurity in secondary raw materials, for example scrap. It is capable of improving the machinability of the alloy. However, proportions exceeding 0.009% by weight can have an adverse effect on hot formability. Therefore, up to 0.009% by weight of Bi in the alloy is tolerated.
• the balance of the alloy composition consists of zinc and unavoidable impurities.
• the proportion of these impurities is not more than 0.2% by weight.
• the proportions of Mn and Mg should preferably each be not more than 0.1% by weight, more preferably each not more than 0.07% by weight, because these elements can form phosphides that can compete with the copper- and/or zinc-containing phosphides.
• the Pb content in the alloy may be at least 0.02% by weight. Even such a small proportion of Pb improves the machining properties and has a positive effect on grain refining.
• the P content may be not more than 0.15% by weight, more preferably not more than 0.12% by weight. This has a favorable effect on hot formability of the alloy.
• the P/Fe ratio may be at least 1.0.
• Iron forms hard iron phosphides with phosphorus.
• these are undesirable because they worsen the service life of the tools.
• the ratio of the proportions by weight of P and Fe should be at least 1.0.
• the Fe content may be less than 0.10% by weight, and the Ni content may be not more than 0.07% by weight.
• This restriction inhibits the formation of iron phosphides and nickel phosphides compared to the formation of copper- and/or zinc-containing phosphides that are favorable for machining.
• Particularly favorable properties are achieved when the condition that the P/Fe ratio is at least 1.0 is met simultaneously.
• the Si content may be at least 0.23% by weight. This promotes the machining properties of the alloy.
• a Si content of at least 0.23% by weight has a favorable effect on the surface quality of the product.
• the Si content may be not more than 0.15% by weight, preferably not more than 0.12% by weight, more preferably not more than 0.08% by weight. Such a restriction of the Si content has a favorable effect on the electrical conductivity of the alloy.
• the P content may advantageously be not more than 0.10% by weight. This promotes the electrical conductivity of the alloy to a particular degree.
• the Cu content may be not more than 59.5% by weight. This upper limit for the Cu content gives rise to particularly favorable combinations of conductivity, machinability, mechanical properties and processibility.
• the proportions of the elements Cu, Zn, Si, P and Pb may add up to at least 99.75% by weight. This ensures that the properties of the alloy are determined essentially by the alloy elements Cu, Zn, Si, P and Pb, while the effect of other elements plays only a very minor role. Alternatively or else additionally, it may be advantageous to select the composition of the alloy such that the proportions of the elements Cu, Zn, Si, P, Sn and Pb add up to at least 99.85% by weight. Because there is no formation of tin phosphides, it is less critical as alloy constituent than Fe, Ni or Al, for example.
• the wrought copper-tin alloy may have a hardness of at least 120 HV10, preferably at least 150 HV10.
• the wrought copper-tin alloy may have a tensile strength R m Of at least 500 MPa, preferably at least 530 MPa.
• the wrought copper-zinc alloy may have an electrical conductivity of at least 12.5 MS/m, preferably at least 12.7 MS/m, more preferably at least 13.0 MS/m.
• An alloy having a particularly advantageous combination of properties has the following composition in % by weight:
• the invention further provides a semifinished product in wire, pipe or bar form, made from an above-described wrought copper-zinc alloy, and a component produced from such a semifinished product by machining and optional further processing steps.
• the semifinished product may also take the form of a profile.
• a further aspect of the invention relates to a process of producing a semifinished product in wire, pipe or bar form.
• the process comprises the following steps:
• the cast format may optionally be milled and is then hot-pressed at a temperature of 620 to 700° C. Subsequently, the hot-pressed intermediate product is cooled, where the cooling is effected within the temperature range from 550 to 350° C. at a cooling rate of 30 to 60° C. per minute, preferably 40 to 50° C. per minute.
• the defined cooling establishes a favorable ratio of the proportions of ⁇ phase and ⁇ phase, and a favorable particle distribution of copper- and/or zinc-containing phosphides.
• the hot pressing operation may optionally be preceded by a heat treatment for homogenization of the cast product.
• the hot pressing operation may be followed, without further intermediate steps, by pickling and then a cold forming operation.
• the degree of forming is preferably between 3% and 30%. What is meant here by degree of forming is the relative decrease in the cross-sectional area of the product. Because there are no further steps between the hot pressing and cold forming operations except for the pickling operation, this first production process is very favorable.
• the hot pressing operation is followed by a heat treatment between 525 and 625° C., preferably between 550 and 600° C., for a period of 1 to 5 hours, with subsequent cooling at a cooling rate of 20 to 40° C. per minute within a temperature range from 500 to 350° C.
• a heat treatment between 525 and 625° C., preferably between 550 and 600° C., for a period of 1 to 5 hours, with subsequent cooling at a cooling rate of 20 to 40° C. per minute within a temperature range from 500 to 350° C.
• the heat treatment should be effected at about 550° C.
• the heat treatment is followed by the steps of pickling and cold forming, as in the first production process.
• Samples No. 1 to No. 44 were melted in an induction kiln and then cast.
• the composition of the samples is documented in tables 1 to 4.
• Sample No. 10 represents the lead-containing reference alloy CuZn39Pb3. The samples were milled, homogenized for 1 hour and then hot-formed. Samples No. 8 and No. 9 were hot-formed by pressing at 630° C., the other samples by rolling at 650° C. In the course of cooling after hot forming, the cooling rate within the temperature range between 550 and 350° C. was about 40° C. per minute for the rolled samples and about 30° C. per minute for the pressed samples.
• Samples No. 24 to No. 44 after hot forming, were annealed for 3 hours.
• the annealing temperature was about 550° C. for samples No. 26, No. 27 and No. 38 to No. 41, while it was about 600° C. for samples No. 24, No. 25, No. 28 to No. 37 and No. 42 to No. 44.
• the annealing was followed by cooling within the temperature range between 50° and 350° C. at a cooling rate of about 25° C. per minute. Thereafter, samples No. 24 to No. 44 were milled and subsequently cold-formed with a degree of forming of 20%.
• tensile strength R m and elongation at break A were each determined from the tensile test, as were hardness (Vickers hardness HV10) and electrical conductivity A.
• the longitudinal sections of the samples were examined by light microscopy.
• the area proportions of the ⁇ phase and of the ⁇ phase corresponding to the proportions by volume, and the ⁇ grain size were determined therefrom.
• the light microscope images of the unetched samples were used for quantitative determination of the size distribution of the phosphide particles. Image details of dimensions 167 ⁇ m ⁇ 126 ⁇ m (corresponding to an area of 21 000 ⁇ m 2 ) were chosen, and these were evaluated in 1000-fold magnification by means of the ImageJ software.
• the phosphide particles were classified by their equivalent diameter into the categories of 0.5 to 1 ⁇ m, 1 to 2 ⁇ m, and greater than 2 ⁇ m.
• Machinability was assessed in the final state by drilling tests.
• the drilling tests were conducted with an instrumented drill head.
• the drilling tests were conducted with the following parameters:
• Samples No. 1 to No. 9 are inventive samples in the unannealed state.
• the proportion by volume of the ⁇ phase is at least 33% and at most 46%.
• the ⁇ grain size for the rolled samples is at most 24 ⁇ m, and for the pressed samples at most 35 ⁇ m.
• the low ⁇ grain size of sample No. 5 can be attributed to the alloy element Fe.
• Hardness is at least 160 HV10, and tensile strength R m at least 520 MPa. Elongation at break is at least 10%.
• Electrical conductivity for all samples is at least 12.7 MS/m, and for most of the samples at least 13.0 MS/m. For sample No. 9, electrical conductivity is more than 15.5 MS/m.
• Normalized torque is between 0.85 and 1.08. Normalized normal force is between 0.45 and 0.7. The arithmetic average of normalized torque and normalized normal force is always at least 0.75. The swarf form was always assessed as good (2) or average (1).
• Samples No. 10 to No. 23 are comparative samples in the unannealed state.
• the reference sample No. 10 contains 3.3% by weight of lead and shows very good machining properties. Good machining properties are also shown by sample No. 11, containing 2.0% by weight of lead.
• Samples No. 12 and No. 16 document that the machining properties are very poor without lead and without further alloy elements.
• the forces acting on the drill are within the acceptable range, but the swarf form is poor, which can be attributed to the lack of phosphide particles as chip breakers.
• the Cu content and the Si content were increased compared to sample No. 13.
• the forces acting on the drill are not within the acceptable range and the swarf form is poor.
• electrical conductivity is low.
• sample No. 15 the Si content has been increased slightly compared to sample No. 14, and 0.1% by weight of P has been included in the alloy. The forces acting on the drill were thus reduced, and the swarf form was somewhat improved. However, elongation at break and electrical conductivity are low. Sample No. 15, by comparison with sample No. 1, shows a higher Si content with a similar P content and a similar proportion by volume of the ⁇ phase. The forces acting on the drill are similarly good, but the swarf form for sample No. 15 is less favorable and conductivity is poorer.
• Sample No. 18 differs from sample No. 17 by 0.38% by weight of P. This essentially leads to a distinct improvement in swarf form. However, elongation at break is very low.
• Sample No. 19 aside from Cu and Zn, contains only 0.11% by weight of P. Although this favors the swarf form, the forces acting on the drill are unsatisfactory. In the case of sample No. 22, by comparison with sample No. 19, the proportion of P was increased to 0.29% by weight, with consequent worsening of the normal force.
• Sample No. 20 differs from sample No. 15 essentially by a somewhat smaller P content. This leads to an improvement in elongation at break. But electrical conductivity is not at the desired level owing to the high Si content.
• sample No. 2 which contains no tin the inclusion of tin in the alloy leads to moderate worsening of the forces acting on the drill. Almost exclusively phosphide particles having a diameter of 0.5 to 1 ⁇ m are observed. Electrical conductivity is barely affected by 0.22% by weight of Sn.
• Sample No. 23 contains 0.29% by weight of Si and 0.30% by weight of P. Similarly to sample No. 18, the swarf form is good, but elongation at break, conductivity and the forces acting in the course of drilling are unsatisfactory.
• Samples No. 24 to No. 31 are inventive samples in the annealed state. Samples No. 26 and No. 27 were annealed at 550° C., while the other samples in table 3 were annealed at 600° C. The proportion by volume of the ⁇ phase is at least 31% and at most 49%. ⁇ grain size is between 25 and 40 ⁇ m, with samples No. 26 and No. 27 having the smallest grain size. Hardness is at least 160 HV10, and tensile strength R m at least 510 MPa. Elongation at break is at least 11.5%. Electrical conductivity for all samples is at least 12.7 MS/m, and for most of the samples at least 13.0 MS/m. Normalized torque is between 0.79 and 1.03.
• Normalized normal force is between 0.56 and 0.71.
• the arithmetic average of normalized torque and normalized normal force is always at least 0.75.
• the swarf form was always assessed as good (2) or average (1).
• sample No. 31 which corresponds to the unannealed sample No. 21 in terms of its composition, annealing achieved a significant improvement of the forces acting on drilling and the swarf form.
• the annealing moved the center of the distribution of the phosphide particles toward larger particles.
• Samples No. 32 to No. 44 are comparative samples in the annealed state.
• the lead-containing samples No. 32 and No. 33 show good machining properties in the annealed state.
• the solely Cu- and Zn-containing samples No. 34 (annealed at 600° C.) and No. 38 (annealed at 550° C.), in the annealed state too, are characterized by poor properties on drilling.
• the silicon-containing but phosphorus-free samples No. 35 and No. 36 which were annealed at 600° C., give a torque on drilling that is more than twice as high as the torque ascertained on reference sample No. 10.
• the greater Si content in the case of sample No. 36 does improve the swarf form, but reduces electrical conductivity.
• the silicon-containing but phosphorus-free sample No. 39 that was annealed at 550° C. has more favorable forces on drilling than samples No. 35 and No. 36. This can be attributed to the much higher proportion of ⁇ phase. However, the swarf form is poor.
• Sample No. 37 which contains 0.58% by weight of Si and 0.10% by weight of P and was annealed at 600° C., has more favorable properties on drilling than sample No. 15 with the same composition in the unannealed state, but electrical conductivity is unsatisfactory owing to the high proportion of Si.
• Samples No. 1 to No. 44 document that it is possible by a specific selection of the elements Si and P to produce alloys that have a favorable combination of properties.
• Si reduces the forces acting on the drill and hence improves the machining properties.
• an Si content of more than 0.32% by weight reduces electrical conductivity.
• a P content of 0.05% to 0.2% by weight promotes chip formation.
• a higher proportion of P in combination with Si leads to a deterioration in ductility and electrical conductivity.
• Alloys having a favorable combination of these properties can be produced even without annealing. By annealing between 550° C. and 600° C. in particular, for some element combinations, the machining properties can be subsequently improved by controlled adjustment of the ⁇ phase content and the phosphide particles.
• Alloys having an above-described composition can also be used as casting alloys for castings.

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