US20260028700A1 - Casting material made of a copper-zinc alloy, method for producing a cast product and cast part - Google Patents

Abstract

A casting material made of a copper-zinc alloy with the composition in wt. %: Cu: 58.0 to 66.0%, Si: 0.15 bis 1,2%, P: 0.20 to 0.38%, Sn: up to 0.5%, Al: up to 0.05%, Fe: up to 0.3%, Ni: up to 0.38, Pb: up to 0.25%, Bi: up to 0.1% Te, Se, In, each up to 0.1%, B: up to 0.01%, with the rest being Zn and unavoidable impurities. The alloy has α-phase, β-phase and phosphide particles. The proportion of β-phase in the sum of the α-phase and β-phase is 20 vol. % and max. 70 vol. %. In an area of 21000 μm² there are 20 to 300 phosphide particles with an equivalent diameter of 0.5 to 1 μm, 30 to 120 phosphide particles with an equivalent diameter of 1 to 2 μm, and 20 to 100 phosphide particles with an equivalent diameter of 2 to 5 μm.

Description

• The invention relates to a casting material composed of a copper-zinc alloy, to a process for producing a cast product and to a casting.
• Copper-zinc casting materials (also called “casting alloys”) consisting of α phase and β phase and having about 1% by weight of lead, for example CuZn39Pb1Al—B (CB757S), have excellent castability, very good machinability and additionally good polishability. Lead-containing copper-zinc casting alloys are used in a multitude of applications in the sanitary sector, specifically for water taps and fittings in the drinking water sector.
• The basis of the positive effect of lead in copper-zinc alloys is that lead is in elemental form as particles in the microstructure, and these particles act as chip breakers. 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. In addition, 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. Furthermore, it is known that lead in copper-zinc alloys results in distinct grain refining. This is favorable for polishability especially in the case of casting material. Furthermore, lead is inexpensive.
• However, lead is damaging to the environment. Lead accumulates in the human body when extremely small amounts are ingested and can lead to damage to health. Therefore, the EU, the USA, China and other states have reduced the limits in copper alloys ever further, and there is a drive to replace lead-containing brass with reduced-led or led-free machinable copper alloys. Limits are defined in EU directives, for example the Directive 2011/65/EU (RoHS), which stipulates 1000 ppm (0.1%) of Pb as the upper limit. In order to ensure good machinability of the material even in the case of such low lead contents, various alloy elements are proposed as an alternative to lead.
• It is known from a multitude of publications that bismuth (Bi) can be used as an alternative to lead in order to improve machinability. In order to alleviate the problem of film formation by Bi along grain boundaries and associated proneness to stress cracking and thermal cracking, it is proposed that further elements be included in the alloy. In this regard, reference is made in particular to documents KR 10 0 555 854 B1, KR 10 2006 096 877 A, JP 2005 290 475 A, JP 2014 122 427 A and JP 2006 083 443 A. Nevertheless, Bi is undesirable since it is firstly a metal of low availability that exists only in limited volumes, and secondly leads to hot brittleness in the material cycles of the copper alloys.
• In addition, 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. Owing to the formation of an α, β and possibly γ microstructure, the alloys have good machinability. The Al content results in formation of Al phosphides, which are unwanted. Although the γ 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 replacement of lead by phosphorus that forms brittle phosphides in the alloy is also described in document EP 3 992 321 A1 for a casting alloy with Cu at 58.5% to 65.0% by weight, Si at 0.40% to 1.40% by weight, P at 0.003% to 0.19% by weight, Pb at 0.002% to 0.25% by weight, balance: zinc and further optional elements. The addition of 0.003% to 0.19% by weight of P for formation of phosphides and of 0.4% to 1.4% by weight of Si for hardening of the α-and β-crystallites leads here to a casting material of good machinability.
• It is an object of the invention to provide a casting material composed of a copper-zinc alloy having excellent machinability, good mechanical properties, a small grain size and hence good polishability, a low tendency to form cavities and a minimum content of alloy constituents of environmental concern. It is a further object of the invention to specify a process for producing a cast product and a casting.
• The invention is described by the features of claim 1 with regard to a casting material composed of a copper-zinc alloy, and by the features of claim 7 with regard to a production process. The further dependent claims relate to advantageous embodiments and developments of the invention.
• The invention relates to a casting material composed of a copper-zinc alloy having the following composition in % by weight:
• • Cu: 58.0% to 66.0%,
  • Si: 0.15% to 1.2%,
  • P: 0.20% to 0.38%,
  • Sn: optionally up to 0.5%,
  • Al: optionally up to 0.05%,
  • Fe: optionally up to 0.3%,
  • Ni: optionally up to 0.3%,
  • Pb: optionally up to 0.25%, preferably up to 0.10%,
  • Bi: optionally up to 0.1%,
  • Te, Se, In each optionally up to 0.1%,
  • B: optionally up to 0.01%,
  • balance: Zn and unavoidable impurities,
    where the proportion of unavoidable impurities is less than 0.2% by weight. The alloy has a microstructure composed of α phase, β phase and phosphide particles. The phosphide particles contain or preferably are copper-and/or zinc-containing phosphides. The proportion of the β phase in the sum total of α phase and β phase is at least 20% by volume, preferably at least 40% by volume, and not more than 70% by volume, preferably not more than 60% by volume. Silicon is present both in the α phase and in the β phase. In an area of 21 000 μm², there are 20 to 300 phosphide particles having an equivalent diameter of 0.5 to 1 μm, 30 to 120 phosphide particles having an equivalent diameter of 1 to 2 μm, and 20 to 100 phosphide particles having an equivalent diameter of 2 to 5 μm. The proportion by volume of the β phase and the proportions of Si and P are chosen such that the alloy meets the condition
• $92.7249-0.473254·\left[\mathrm{beta}\right]-80.6378·\left[\mathrm{Si}\right]-142\text{.65}·\left[P\right]+279.309·\left[\mathrm{Si}\right]·\left[P\right]<40$
• where [beta] denotes the proportion of the β phase in % by volume, [Si] the proportion of silicon in % by weight, and [P] the proportion of phosphorus in % by weight.
• 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 material. For this purpose, Si and P are specifically added to the alloy, and the proportion by volume of the β phase is adjusted so as firstly to result in favorable machining properties and secondly to refine the cast microstructure of the original base matrix composed of β phase. Furthermore, the process regime in the casting operation is chosen so as to result in the desired properties.
• A small β grain size is advantageous for good polishability. Therefore, the β phase must be in fine-grain form in the cast state. It has been found that, surprisingly, with increasing P content, distinct grain refining of the cast microstructure of the original base matrix composed of β phase occurs. In order to achieve sufficient grain refining of the cast microstructure, the addition of at least 0.20% by weight of P is necessary. This is similar to the effect of 1% to 3% by weight of Pb on the grain refining of α-β-brass. In the primary crystallization of the β crystallites, the residual melt becomes enriched with P and hence leads to a subdivision and hence grain refining of the β phase. Solidification forms a eutectic composed of phosphide and β phase. In addition to grain refining of the base matrix composed of β phase, grain refining of the a crystallites is observed.
• In the case of a P content of at least 0.20% by weight, phosphide particles are present both in the α phase and in the β phase in the casting material. The alloy preferably contains at least 0.22% by weight of P. In the case of a P content of more than 0.38% by weight, coarse phosphides are formed in the cast state in that individual phosphides coagulate and form long network-like forms. These coarse phosphides wet the grain boundaries and reduce ductility. For use as mechanical components, for example as drinking water valves or connections, the material needs to have high strength, corresponding to a high hardness.
• However, coarse unwanted phosphides can occur even in the case of P contents of below 0.38% by weight when the cooling rate is too low in the casting of the alloy, for example in the case of casting into a stationary mold. The necessary high cooling rates are achieved, for example, in Tammann casting of small 25×55×160 mm blocks into a steel mold. This achieves the effect that, in the case of a P content of 0.20% to 0.38% by weight, the phosphide particles are in globular and finely distributed form in the microstructure even in the cast state. These act as separation sites in machining and promote chip breaking. In order to suitably adjust the phosphide particles, within a temperature range from 550° C. to 350° C., the cooling rate on solidification has to be at least 20° C. per minute (° C./min), preferably at least 30° C. per minute, and at most 60° C. per minute, preferably at most 50° C. per minute. The distribution of the phosphides and the grain size of the β phase are therefore determined not only by the chemical composition of the alloy but also by the conditions in the casting operation. The characteristics of the phosphides in the cast state are thus like a fingerprint left on the product by the particular process regime. The distribution of the phosphides in the cast state can be characterized as follows: In an area of 21 000 μm², there are 20 to 300 phosphide particles having an equivalent diameter of 0.5 to 1 μm, 30 to 120 phosphide particles having an equivalent diameter of 1 to 2 μm, and 20 to 100 phosphide particles having an equivalent diameter of 2 to 5 μm. 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 μm has an equivalent diameter of not more than 2 μm. Considering the cooling conditions, the alloy is also suitable for continuous casting and shape casting, for example in the form of sand casting or mold casting.
• Brittle microstructure constituents are advantageous for machinability of the material that act as separation sites in the machining operation and hence promote chip breaking. The β phase is brittle and promotes machinability. An increase in the proportion of β phase can be achieved by an increase in the Zn content and/or by inclusion of silicon in the alloy, since silicon stabilizes the B phase. It has additionally been found to be advantageous for good machinability when the ductility of the α phase is reduced. This is possible by the alloying and incorporation of silicon into the α phase, and by means of finely distributed phosphides in the α phase. Therefore, the Si content in the alloy must be at least 0.15% by weight. The above-described phosphide particles are particles that act as separation sites on machining and promote chip breaking. A P content of at least 0.20% by weight not only improves machinability but in particular refines the grain. In addition, a small optional fraction of Pb has an advantageous effect on machinability.
• The machinability of the material is consequently determined by the combined selection of the parameters of β phase, Si and P, and an optionally present small proportion of Pb. In the proposed casting material composed of a copper-zinc alloy, the proportion of the β phase in the sum total of α phase and β phase is at least 20% by volume, preferably at least 35% by volume, more preferably at least 40% by volume. A high proportion of the β phase has an adverse effect on ductility. Therefore, the proportion of the β phase is not more than 70% by volume, preferably not more than 60% by volume. The Si content of the alloy is 0.15% to 1.2% by weight, the P content 0.20% to 0.38% by weight. In addition, there may be up to 0.25% by weight of Pb, preferably up to not more than 0.10% by weight of Pb. At the same time, the proportion of the β phase and the proportions of Si and P are chosen such that the alloy meets the condition
• $92.7249-0.473254·\left[\mathrm{beta}\right]-80.6378·\left[\mathrm{Si}\right]-142\text{.65}·\left[P\right]+279.309·\left[\mathrm{Si}\right]·\left[P\right]<40$
• where [beta] denotes the proportion of the β phase in % by volume, [Si] the proportion of silicon in % by weight, and [P] the proportion of phosphorus in % by weight. This relation quantitatively describes the effect of the parameters of β phase, Si and P on the machining properties of the alloy, and the interaction of these parameters with one another. For example, it is thus possible to compensate for a small proportion of β phase with a higher proportion of silicon and/or phosphorus within the scope of the specification of the composition of the alloy, and vice versa.
• The Cu content of the alloy is 58.0% to 66.0% by weight. In the case of a Cu content of less than 58.0% by weight, ductility of the alloy is low. In the case of a Cu content of more than 66.0%, the zinc content in the alloy is too small to achieve good machinability.
• In addition, the composition of the alloy may preferably be chosen such that the Si/P ratio is at least 0.6, more preferably at least 0.9. In a further preferred embodiment, the composition of the alloy may be chosen such that the sum total of Si and P is at least 0.58% by weight, more preferably at least 0.64% by weight. Both the aforementioned measures contribute both independently and in combination to fulfilment of the above-described relation and hence to achievement of favorable machining properties.
• The optional Sn and Al elements promote formation of the phase. In the case of a Sn content of more than 0.5% by weight, further tin-containing phases may form, which can have an adverse effect on the properties of the alloy. The proportion of tin should preferably be not more than 0.3% by weight, more preferably not more than 0.2% by weight. In addition, aluminum forms aluminum phosphides with phosphorus. However, these are unwanted, and therefore the Al content should not exceed 0.05% by weight.
• Iron leads to grain refining of the microstructure. Moreover, iron forms hard phosphides and silicides, which on machining have an adverse effect on the service life of the tools and polishability. Therefore, the proportion of iron must be not more than 0.3% by weight, preferably not more than 0.1% by weight.
• Nickel promotes the formation of the α phase. Moreover, nickel forms phosphides, which do not have an advantageous effect on machinability. Therefore, the proportion of nickel must be not more than 0.3% by weight, preferably not more than 0.1% 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. In amounts of not more than 0.1% by weight, Bi has no adverse effect on the alloy. Therefore, up to 0.1% by weight of Bi in the alloy is tolerated. Preferably, the proportion of Bi is less than 0.015% by weight.
• The elements Te, Se and In can have an advantageous effect on the machinability of the alloy. In amounts of not more than 0.1% by weight each, they have no adverse effect on the alloy. Therefore, up to 0.1% by weight each of Te, Se and In in the alloy is tolerated.
• An optional proportion of up to 0.01% by weight of boron contributes to grain refining.
• The balance of the alloy composition consists of zinc and unavoidable impurities. In order to avoid uncontrollable influences of the impurities on the properties of the alloy, the proportion of these impurities is not more than 0.2% by weight. In particular, the proportions of Mn and Mg should each be not more than 0.1% by weight, more preferably each not more than 0.05% by weight, because these elements can form phosphides that can compete with the copper-and/or zinc-containing phosphides.
• In a preferred configuration of the invention, 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.
• Advantageously, the ratio of the proportions by weight of P and the sum total of Fe and Ni may be more than 2.0, i.e. P/(Fe+Ni)>2.0. This achieves the effect that predominantly the copper-and/or zinc-containing phosphides that are favorable for the machining properties are formed. The formation of iron phosphides or nickel phosphides is suppressed.
• It may be particularly advantageous that the proportions of Fe and Ni add up to not more than 0.1% by weight. This restriction also inhibits the formation of iron phosphides and nickel phosphides compared to the formation of copper-and/or zinc-containing phosphides.
• In a particularly advantageous embodiment of the invention, the P content may be at least 0.26% by weight and at most 0.33% by weight. If the P content is at least 0.26% by weight, a sufficiently large amount of phosphide particles is formed, in order to achieve a particularly fine grain and very good machinability.
• In an advantageous configuration of the invention, the Si content may be at least 0.50% by weight and at most 1.0% by weight. A casting material composed of a copper-zinc alloy having a Si content within this range is notable for excellent machining properties.
• A further aspect of the invention relates to a process for producing a cast product, wherein the process comprises the following steps:
• • a) melting a copper-zinc alloy having a composition as described above,
  • b) casting a cast product with subsequent cooling of the cast product, where the cooling rate within a temperature range from 550 to 350° C. is at least 20° C. per minute and at most 60° C. per minute.
• The process can produce a cast product from an above-described casting material composed of a copper-zinc alloy. The alloy can be melted using Cu cathodes, Zn blocks, brass scrap, Cu—P prealloys and Cu—Si prealloys. The melting is preferably effected in an induction kiln. Subsequently, the melt is cast to a cast product. The cast product is cooled, where the cooling rate within a temperature range from 550° C. to 350° C. is at least 20° C. per minute, preferably at least 30° C. per minute, and at most 60° C. per minute, preferably at most 50° C. per minute. The defined cooling establishes a favorable ratio of the proportions by volume of the α phase and the β phase to one another and a favorable particle distribution of copper-and/or zinc-containing phosphides in the casting material of the cast product.
• With regard to further technical features and advantages of the process of the invention, explicit reference is hereby made to the elucidations in the context of the inventive casting material composed of a copper-zinc alloy and to the working examples.
• A further aspect of the invention relates to a casting made from an above-described casting material. A casting refers to a product, the material of which underwent no further forming after casting and cooling. The production of a casting proceeds from a cast product that is melted and cast by the above-described process. The cast product is machined for shaping purposes. In addition, at least part of the surface may be polished. The casting may optionally be wholly or partly coated. The casting is thus produced by machining and optional further processing steps from an above-described casting material or cast product. Such a casting may, for example, be a connector, a T-piece, part of a valve, a water tap or a water pipe.
• The invention is elucidated in detail by working examples and comparative examples.
• Samples No. 1 to No. 12 were melted in an induction kiln and then cast in molds to small blocks. The cooling rate on solidification was 36° C. per minute (° C./min) within the temperature range from 550° C. to 350° C. The composition of the samples is documented in table 1. Sample No. 5 represents the lead-containing reference alloy CuZn39Pb1Al—B.
• Transverse slices were taken from the cast blocks, and these were used to examine the microstructure by light microscopy. The reported proportions by volume of the α phase and of the β phase are normalized to the sum total of α phase and β phase. Hardness HV was determined. Electrical conductivity was determined by the eddy current method using a probe.
• The grain size of the β grains was determined in accordance with EN ISO 2624. This was done by drawing linear cuts in widthwise direction of the transverse slices (referred to as “vertical”) and determining the number of cut β grains along the linear cuts. The average linear cut length in widthwise direction corresponds to the average β grain diameter in widthwise direction. Analogously, linear cuts were drawn in thickness direction of the transverse slices (referred to as “horizontal”) and the number of cut β grains along these linear cuts was determined. The average linear cut length in thickness direction corresponds to the average β grain diameter in thickness direction.
• 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 μm×126 μm (corresponding to an area of 21 000 μm²) were chosen, and these were evaluated in 1000-fold magnification by means of the ImageJ software. In this way, it was possible to discern individual particles and to determine the equivalent diameter thereof and area thereof. The phosphide particles were classified by their equivalent diameter into the categories of 0.5 to 1 μm, 1 to 2 μm, 2 to 5 μm and—if present—greater than 5 μm.
• Machinability was determined by means of a plane test. This was done using an indexable insert with a contour that promotes chip breaking. The machining depth was 125 μm and the plane speed 35 m/min. During the planing operation, the bending moment that acts on the tool was measured, and this was used to determine the average value of the bending moment. The resultant chips were visually assessed and categorized by chip form. The chip form was assigned a chip form number according to the following list:

• Chip form
  number	Chip form

  0	torn chip, helical chip
  0.5	coiled chip with 1 to 2 turns
  0.75	coiled chip with an arc that forms an angle of
  	270° to 360°
  1.0	coiled chip with an arc that forms an angle of
  	180° to 270°
  1.25	coiled chip with an arc that forms an angle of
  	less than 180°

  
  The chip form number of 1.0 corresponds to the reference alloy CuZn39Pb1Al—B that contains 1% by weight of lead (sample No. 5).
• Machinability of the samples was assessed using the bending moment ascertained in the planing and the shape of the chips. An average bending moment of not more than 37 Nm and chips that correspond to chip form number 1.0 or 1.25 were rated as very favorable.
• The results of the studies are documented in table 1. Samples No. 1 to No. 4 are inventive samples. Samples No. 5 to No. 12 are comparative samples and are labeled (*).
• In addition, an attempt was made to parametrize the measured average bending moment as a function of the proportion by volume of the β phase and the proportions by weight of Si and P. The functional relationship thus ascertained can be presented as follows:
• $f=92.7249-0.473254·\left[\mathrm{beta}\right]-80.6378·\left[\mathrm{Si}\right]-142\text{.65}·\left[P\right]+279.309·\left[\mathrm{Si}\right]·\left[P\right]$
• • where f approximately quantifies the measured bending moment in Nm and where [beta] denotes the proportion of the β phase in % by volume, [Si] the proportion of silicon in % by weight, and [P] the proportion of phosphorus in & by weight. The value of f calculated by this formula is documented in the last column of table 1. Comparison of this value f with the measured bending moment shows very good agreement between the two parameters (with the exception of the two samples No. 6 and No. 7 that have very poor machinability). Inventive samples No. 1 to No. 4, all of which have a measured bending moment of less than 37 Nm, are characterized in that the value of f is less than 40.

• TABLE 1
  Composition and structural properties of the samples

  											Phos-	Phos-	Phos-
  											phides	phides	phides
  							α	β	β grain size		0.5-1	1-2	2-5
  						Other	phase	phase	vertical/	β grain	μm per	μm per	μm per
  Sample	Cu	Zn	Si	P	Pb	elements	% by	% by	horizontal	form	21 000	21 000	21 000
  No.	% by wt.	% by wt.	% by wt.	% by wt.	% by wt.	% by wt.	vol.	vol.	μm	—	μm2	μm2	μm2
  1	61.29	37.773	0.57	0.31	0.053	Fe: 0.004	62	38	527/506	globular	137	49	41
  2	60.456	38.447	0.65	0.369	0.068	Fe: 0.004	43	57	134/158	globular	160	72	40
  3	59.619	39.484	0.587	0.297	0.001	Fe: 0.007	45	55	611/617	globular	59	54	68
  4	59.597	39.882	0.264	0.249		Fe: 0.004	59	41	645/781	globular	38	48	58
  5 (*)	60.123	37.978	0.0017	<0.0005	1.104	Al: 0.683	48	52	68/96	globular	17	1	0
  						Fe: 0.103
  						B: 0.0013
  6 (*)	59.075	40.914	0.0012	0.001	0.001	Fe: 0.0013	56	44	1420/4564	columnar	2	0	0
  7 (*)	59.093	40.898	0.0011	<0.0005	0.001	Fe: 0.0012	59	41	1140/4152	columnar	3	3	1
  8 (*)	62.505	36.952	0.532		0.000	Fe: 0.0065	70	30	938/2355	columnar	3	1	0
  9 (*)	62.469	36.828	0.568	0.126	0.000	Fe: 0.0072	74	26	825/1061	globular	43	39	13
  10 (*)	61.387	38.059	0.544	0.00055	0.000	Fe: 0.0065	64	36	884/2645	columnar	3	0	0
  11 (*)	61.287	38.083	0.551	0.067	0.000	Fe: 0.0076	62	38	728/2051	columnar	65	23	0
  12 (*)	60.777	38.663	0.277	0.276	0.000	Fe: 0.0043	78	22	605/856	globular	97	88	56

  Composition and technological properties of the samples

  						Other		El.		Chip form
  Sample	Cu	Zn	Si	P	Pb	elements	Hardness	conductivity	Torque	number	f
  No.	% by wt.	% by wt.	% by wt.	% by wt.	% by wt.	% by wt.	HV10	MS/m	Nm	—	Nm
  1	61.29	37.773	0.57	0.31	0.053	Fe: 0.004	128	10.83	34.9	1.25	33.9
  2	60.456	38.447	0.65	0.369	0.068	Fe: 0.004	133	10.97	27.7	1.25	27.7
  3	59.619	39.484	0.587	0.297	0.001	Fe: 0.007	134	11.3	26.7	1.25	25.7
  4	59.597	39.882	0.264	0.249		Fe: 0.004	111	13.18	36.5	1.25	34.9
  5 (*)	60.123	37.978	0.0017	<0.0005	1.104	Al: 0.683	106	14.82	24.4	1.0
  						Fe: 0.103
  						B: 0.0013
  6 (*)	59.075	40.914	0.0012	0.001	0.001	Fe: 0.0013	95	17.74	47.9	0	71.7
  7 (*)	59.093	40.898	0.0011	<0.0005	0.001	Fe: 0.0012	91	17.76	96.8	0	73.2
  8 (*)	62.505	36.952	0.532		0.000	Fe: 0.0065	104	11.75	37.1	1.25	35.6
  9 (*)	62.469	36.828	0.568	0.126	0.000	Fe: 0.0072	106	10.71	31.7	1.25	36.6
  10 (*)	61.387	38.059	0.544	0.00055	0.000	Fe: 0.0065	112	12.02	33.5	1.25	31.8
  11 (*)	61.287	38.083	0.551	0.067	0.000	Fe: 0.0076	113	11.59	31.8	1.0	31.1
  12 (*)	60.777	38.663	0.277	0.276	0.000	Fe: 0.0043	109	12.39	42.5	1.0	42.0

• Samples No. 1 to No. 4 are inventive samples. The proportion by volume of the β phase is at least 38% and at most 57%. β grain size in widthwise direction (“vertically”) is not more than 645 μm, and in thickness direction (“horizontally”) not more than 781 μm. The ratio of β grain size in thickness direction to β grain size in widthwise direction is not more than 1.21. The grains thus have a topology without a preferential direction and are rated as globular. Hardness is at least 110 HV10. The measured bending moment is not more than 36.5 Nm. The form of the chips for all samples corresponds to chip form number 1.25. The form of the chips is thus very favorable.
• Samples No. 5 to No. 12 are comparative samples. Reference sample No. 5 contains 1.1% by weight of lead and is characterized by a very small grain size, a very low bending moment and a good chip form. Samples No. 6 and No. 7, aside from copper and zinc, contain only very small amounts of silicon and phosphorus. For both samples, β grain size is very high, bending moment measured on machining is high and the chip form is poor.
• Samples No. 8 to No. 11 each contain about 0.55% by weight of silicon. Samples No. 8 and No. 10 respectively contain no and only very little phosphorus, while samples No. 9 and No. 11 contain phosphorus in an amount of 0.126% by weight and 0.067% by weight respectively. In the case of samples No. 10 and No. 11, the zinc content is somewhat more than 1% by weight above the zinc content of samples No. 8 and No. 9. The greater zinc content leads to a greater proportion by volume of the β phase. Comparison of samples No. 8 and No. 10 with samples No. 6 and No. 7 shows that the inclusion of about 0.55% by weight of silicon in the alloy distinctly reduces the bending moment acting during machining as a result of hardening of the α phase and of the β phase. The higher proportion by volume of the β phase in the case of samples No. 6 and No. 7 is more than compensated for here by the silicon content in the case of samples No. 8 and No. 10. In addition, bending moment also falls with rising P content. The chip form in the case of samples No. 8 to No. 11 is favorable.
• β grain size in the case of samples No. 8 to No. 11 is lower than in the case of samples No. 6 and No. 7. B grain size tends to decrease with increasing P content. On the other hand, samples No. 8 to No. 11 have a β grain size greater than the β grain size of samples No. 1 to No. 4. Especially in the case of samples No. 8 to No. 11, horizontal β grain size is greater by more than a factor of 1.25, usually even by more than a factor of 2.5, than the vertical β grain size, whereas, in the case of samples No. 1 to No. 4, horizontal β grain size is greater than vertical β grain size at most by a factor of 1.25.
• Sample No. 12, because of the phosphorus content of 0.276% by weight, shows a globular β grain having a vertical β grain size of 605 μm and a horizontal β grain size of 856 μm. However, sample No. 12 has a proportion by volume of the β phase of only 22% and a low Si content of 0.277% by weight, which means that the bending moment acting during machining is increased and the condition
• $92.7249-0.473254·\left[\mathrm{beta}\right]-80.6378·\left[\mathrm{Si}\right]-142\text{.65}·\left[P\right]+279.309·\left[\mathrm{Si}\right]·\left[P\right]<40$
• is not met: The value of f calculated from Si content, P content and the proportion by volume of the β phase is 42.0, and hence is in good agreement with the measured bending moment of 42.5 Nm. Comparison of sample No. 12 with sample No. 4, which has a similar composition, shows that a low Si content has to be compensated for by a higher Zn content in order that the casting material has a sufficient proportion by volume of β phase and hence favorable machining properties. It is thus necessary to choose the alloy composition and the process regime such that the aforementioned condition is met in the interplay of these quantities and parameters. It is not sufficient to consider each of the quantities and parameters mentioned merely individually.
• Samples No. 1 to No. 4 are notable for grains having a topology without a preferential direction, i.e. for globular grains, and having a β grain size of not more than 800 μm, whereas β grain size is in principle greater in the case of samples No. 8 to No. 11. Furthermore, in the case of samples No. 8, No. 10 and No. 11, β grain size in thickness direction (“horizontally”) is much greater than in widthwise direction (“vertically”). Therefore, the grain form in the case of samples No. 8, No. 10 and No. 11, as in the case of samples No. 6 and No. 7, is described as columnar and rated as unfavorable. The cause of the favorable topology and size of the grains in the case of samples No. 1 to No. 4 is the phosphorus content of at least 0.24% by weight in combination with the specifically chosen cooling rate in the temperature range between 550° C. and 350° C. on solidification after casting. The favorable topology and size of the grains in the case of samples No. 1 to No. 4 results firstly in good mold filling on casting and secondly in good polishability of these samples.

Claims (8)

1. A casting material composed of a copper-zinc alloy having the following composition in % by weight:

Cu: 58.0% to 66.0%,

Si: 0.15% to 1.2%,

P: 0.20% to 0.38%,

Sn: optionally up to 0.5%,

Al: optionally up to 0.05%,

Fe: optionally up to 0.3%,

Ni: optionally up to 0.3%,

Pb: optionally up to 0.25%,

Bi: optionally up to 0.1%,

Te, Se, In each optionally up to 0.1%,

B: optionally up to 0.01%,

balance: Zn and unavoidable impurities,

where the proportion of unavoidable impurities is less than 0.2% by weight,

where the alloy has a microstructure composed of α phase, β phase and phosphide particles, and the proportion of the β phase in the sum total of α phase and β phase is at least 20% by volume and at most 70% by volume, where silicon is present both in the α phase and in the β phase, where, in an area of 21 000 μm², there are 20 to 300 phosphide particles having an equivalent diameter of 0.5 to 1 μm, 30 to 120 phosphide particles having an equivalent diameter of 1 to 2 μm, and 20 to 100 phosphide particles having an equivalent diameter of 2 to 5 μm, and where the proportion of the β phase and the Preliminary Amendment-Page 3 proportions of Si and P are chosen so as to satisfy the condition

$92.7249-0.473254·\left[\mathrm{beta}\right]-80.6378·\left[\mathrm{Si}\right]-142\text{.65}·\left[P\right]+279.309·\left[\mathrm{Si}\right]·\left[P\right]<40$

where [beta] denotes the proportion of the β phase in % by volume, [Si] the proportion of silicon in % by weight, and [P] the proportion of phosphorus in % by weight.

2. The casting material composed of a copper-zinc alloy as claimed in claim 1, wherein the Pb content is at least 0.02% by weight.

3. The casting material composed of a copper-zinc alloy as claimed in claim 1, wherein the ratio of the proportions by weight of P and the sum total of Fe and Ni is more than 2.0.

4. The casting material composed of a copper-zinc alloy as claimed claim 1, wherein the proportions of Fe and Ni add up to not more than 0.1% by weight.

5. The casting material composed of a copper-zinc alloy as claimed in claim 1, wherein the P content is at least 0.26% by weight and at most 0.33% by weight.

6. The casting material composed of a copper-zinc alloy as claimed claim 1, wherein the Si content is at least 0.50% by weight and at most 1.0% by weight.

7. A process for producing a cast product, wherein the process comprises the following steps:

a) melting a copper-zinc alloy having a composition as claimed in claim 1,

b) casting a cast product with subsequent cooling of the cast product, where the cooling rate within a temperature range from 550 to 350° C. is at least 20° C. per minute and at most 60° C. per minute.

8. A casting made from a casting material as claimed in claim 1.