EP3581667B1 - Moulded parts made from a corrosion resistant and machinable alloy - Google Patents

Description

The present invention relates to the use of a copper alloy for the production of molded parts and a method for the production of molded parts.

State of the art

Water is a valuable raw material and indispensable for daily use. The drinking water must therefore be microbiologically constituted when it is taken from the supply system in such a way that subsequent consumption does not lead to any illness in humans. In order to achieve this, high demands are placed on materials that come into direct contact with drinking water. Copper is the noblest commodity and is considered an indispensable material for water-bearing systems in industry and technology. Because copper has bacteriostatic properties and also offers excellent corrosion resistance. Copper also shows positive properties in terms of shape. Copper casting alloys are easy to cast, and the material is particularly valued for plasto-mechanical shaping due to its high strength and toughness.

However, it is precisely this plastic deformability that causes problems in machining. Here, homogeneous copper materials tend to form long chips. This type of chip impedes the work process during fully automated turning or drilling and leads to heavy wear on the cutting edges of the tool. The chip formation of the copper is often the limiting factor in mechanical processing and therefore has a direct influence on the cost-effectiveness of the workpieces.

Gunmetal is one of the cast copper alloys and is characterized by the combination of good castability with optimal machinability and high strength. Due to its good corrosion resistance, gunmetal is particularly suitable for water-bearing systems such as fittings and sanitary technology. Common gunmetal alloys contain tin to increase strength and corrosion resistance. Zinc is added as an inexpensive substitute for copper. In order to be able to process the products made of gunmetal economically at all, the heavy metal lead is added, which acts as a chip breaker in the alloy and enables machining on CNC machines and conventional lathes.

If the drinking water stagnates for a long time in commercially available lead-containing fittings, there is a possibility that lead will be released into the tap water through metal ion migration. High concentrations of lead are considered harmful to health. For this reason, ever more stringent requirements are being imposed worldwide on the lead content of materials that come into contact with drinking water. In Germany, too, the lead content has been reduced to 10 µg lead/l since December 1st, 2013 via the statutory drinking water ordinance. The pressure to further reduce lead levels in drinking water has increased worldwide and will continue to do so. Legal requirements from the USA require that lead levels in Copper alloys must not exceed an average lead content of 0.25%, regardless of the actual lead concentration in drinking water.

The ideal gunmetal would be free of lead and other questionable substances, with the same or better economy in production and without impairing the corrosion resistance, the high mechanical strength and the good workability.

The EP 2290114 A1 describes a lead-free gunmetal alloy containing 4 to 6% by weight tin, 4 to 6% by weight zinc and less than 0.25% by weight lead. With this alloy, lead-free components can be manufactured using casting processes. Subsequent mechanical processing to create the functional surfaces of these components is not taken into account. Without lead, the specified composition shows a homogeneous α-MK structure, which tends to form long chips and cannot be machined economically. Due to the process, the assumed casting process also requires more material to produce the molded part than alternative forming processes. The US 2012/0082588 A1 , the EP 2 241 643 A1 , the EP 3 225 707 A1 and the US 9,181,606 B2 disclose copper alloys.

The describes a forming process of a lead-free gunmetal alloy EP 2 872 660 B1 . A method is described for preconditioning a gunmetal alloy with 2 to 8% by weight tin, 2.5 to 13% by weight zinc and less than 0.25% by weight lead, which is suitable for hot pressing and at the end of the hot pressing process shows a homogeneous structure. Hot forming enables the economical production of molded parts with little use of material. Although the process sequence up to the shaping of the blank is explained, the subsequent machining process required to carve out the functional surfaces of the components is not taken into account here either. Due to the chemical composition and the subsequent hot forming, a homogeneous structure is formed and here too, due to the lack of a chip breaker, long chips can be expected during machining, which makes economical processing of the components more difficult.

The EP 1 801 250 A1 describes low-migration components made from a copper alloy that has a relatively high proportion of Si, in addition to lower but significant proportions of Mn, Al and Zr. Similar copper alloys are also in the WO 2007/068470 A1 disclosed.

The JP 2013-199699 A discloses a copper alloy containing 0.5 to 11.0% by mass of Sn, 0.03 to 0.70% by mass of P and 0.02 to 1.0% by mass of S, the balance being Cu and unavoidable impurities . A sulfide is dispersed in the copper alloy, the mean diameter of the sulfide being 0.1 to 10 µm and the area ratio of the sulfide being 0.1 to 10%.

Despite the many copper alloys that are now known in the state of the art, there is still a challenge in specifying red brass copper alloys that, on the one hand, do not require the use of lead (Pb), a component that is problematic from an environmental and health point of view, and, on the other hand, enable forming processes without affecting the mechanical characteristics and corrosion resistance suffer. The provision of copper alloys that show good hot workability (ie without a significant drop in mechanical parameters) and are preferably also easy to machine has turned out to be particularly challenging here.

Object of the present invention

Due to the disadvantages of the prior art described above, it is the object of the present invention to provide the use of a copper alloy for the production of molded parts and a method for the production of molded parts which overcomes these disadvantages. In this context, it would be particularly desirable to use a copper alloy which comprises as few components as possible, is lead-free or essentially lead-free and, moreover, can do without expensive metal components and/or metal components which are difficult to mix.

Brief description of the invention

This object is achieved by using a copper alloy according to claim 1 and the method for producing molded parts according to claim 2. Preferred embodiments are specified in the subclaims and the following description.

Detailed description of the invention

The present invention is described below initially with regard to the use of the alloy according to the invention. However, it is clear to the person skilled in the art that the preferred embodiments described in this context can also be applied to the production method described and are also to be regarded as preferred embodiments for these aspects of the invention.

The present invention makes it possible to produce molded parts with high mechanical strength, high dimensional accuracy and high corrosion resistance from a gunmetal alloy, which has a chip breaker in its structure, via hot forming with little use of material, which can then also be economically machined after the hot pressing process can be subjected to. The hot-workable gunmetal alloy does not require any elements such as Al, Si, Pb, Sb, Te, Se, C and Bi in the structure to form a chip breaker and is therefore easily reusable.

The present invention therefore provides the use of a copper alloy which is particularly suitable for the production of molded parts from at least one hot forming process with subsequent machining, which has the following composition in % by weight: Sn: 2 to 4 % Zn: 0.1 to less than 1.5% S: 0.05 to 0.45% pb: less than 0.25% Ni: less than 0.6% Sb: less than 0.2%, optionally further containing phosphorus up to a maximum of 0.06% by weight, B up to a maximum of 0.03% by weight, Zr up to a maximum of 0.03% by weight and unavoidable impurities, and the remainder being Cu.

As already explained above, the alloy used according to the invention contains in particular no elements from the group Al, Si, Sb, Te, Se, C and Bi and in preferred embodiments also no Pb.

• Sn: 2 bis 4 %, in Ausführungsformen 2 bis weniger als 3,5 %, wie 2 bis 3,25%
• Zn: 0,1 bis weniger als 1,5 %
• S: 0, bis 0,45 % und in Ausführungsformen 0,1 bis weniger als 0,25 %, wie 0,1 bis 0,2%
• Ni: weniger als 0,5%, wie von 0 bis 0,4%, von 0 bis 0,25%

Preferred contents of alloy components to be used according to the invention are as follows, these being disclosed and claimed according to the invention individually and in every combination (again in each case in % by weight):

• Sn: 2 to 4%, in embodiments 2 to less than 3.5%, such as 2 to 3.25%
• Zn: 0.1 to less than 1.5%
• S: 0.0 to 0.45% and in embodiments 0.1 to less than 0.25%, such as 0.1 to 0.2%
• Ni: less than 0.5%, such as from 0 to 0.4%, from 0 to 0.25%

The copper content in the alloy is preferably 88% by weight or more, more preferably 90% by weight or more.

It has unexpectedly turned out that the known disadvantages of the prior art can be overcome with the copper alloys disclosed here. In particular, semi-finished products/intermediate products made from the copper alloy can very well be subjected to hot forming. Despite the reduction of strain hardening that frequently occurs during hot forming (typically at temperatures of around 600 to 950°C), the use of the alloy according to the invention makes it possible to produce molded parts (which may then be further processed, for example by machining), which still have excellent mechanical properties and also show no deterioration in corrosion resistance.

Furthermore, it has been shown that the molded parts obtained in this way (that is, after hot forming) can also be further processed in an economical manner, since in particular the undesirable formation of long chips does not occur. It is thus evident that, despite the processes taking place during hot forming, chip-breaking components are still present in the microstructure of the alloy, although the alloy used according to the invention does not have typical chip-breaking components such as Pb or Si. Thus, the present invention provides the use of a copper alloy that has an excellent balance of desired properties. Molded parts can therefore be produced from this alloy, in particular by hot forming, possibly combined with further processing steps as described here, without fearing that the other desired properties of the copper alloy and its suitability for use in hot forming will be compromised.

The alloy used according to the invention can therefore be used advantageously for the production of molded parts, with these production processes including hot forming, possibly combined with other processing processes, for example subsequent machining.
With a view to maintaining the desired properties of the copper alloy described here, the individual alloying components, alone but also in their interaction, enable good and reproducible control of the alloy properties.

Tin acts as a solid solution strengthener in the alloy and thus increases the tensile strength, yield point and hardness, but reduces the elongation at break. Furthermore, tin increases the corrosion resistance, with the corrosion resistance increasing with increasing tin contents. During the production of the blanks for hot forming, it was found that tin causes strong segregations in the structure, which lead to the formation of zone crystals during solidification. At the beginning of the solidification, copper crystals with a lower tin content are precipitated and the residual melt is enriched with a tin content that is above the average content of the alloy. Deviating from the stable copper-tin phase diagram, a (α + δ) - eutectoid can be present at room temperature if the tin content exceeds 7% by weight in the structure, which under equilibrium conditions only occurs at a maximum of 15.8% by weight tin. The possible δ phase crystallizes in the fcc lattice and should therefore be easily deformable, but the phase has a brittle behavior due to its voluminous unit cell of 416 atoms. This complicates the later hot forming process. By a Heat treatment at high temperatures with sufficient time can eliminate the (α + δ) - eutectoid, but heat treatment is associated with high energy consumption.

Furthermore, there is a risk of grain enlargement of the structure during the treatment. This would lead to a reduction in elongation, making the later hot forming process more difficult. With a tin content of 2 to 4% by weight, high mechanical strength with high elongation is guaranteed and the formation of (α + δ) - eutectoids in the cast state is avoided.

Sulfur is almost insoluble in solid copper and the original properties of the material, such as corrosion resistance, are not affected by the addition of sulphur. Due to its insolubility in solid copper, sulfur leads to a constitution behavior that influences the solidification process of copper-tin alloys in a similar way to lead. Unlike lead, however, sulfur is not present in the structure as an element at the end of solidification, but in the form of an intermetallic metal-sulphur compound that is evenly distributed in the structure. It could be recognized that this phase is incoherent and brittle in the microstructure and thus creates a chip-breaking mechanism.

The properties of the sulfides influence the mechanical, plastic behavior of the gunmetal material. The influence is determined via the proportions of the sulfide phases in the material. From a sulfur content of more than 0.6% by weight, the stress-transmitting α-Cu matrix is so severely impaired by the sulfides that a hot pressing process is made very difficult. The sulfur content of 0.05% by weight to 0.45% by weight, particularly preferably 0.1% by weight to 0.45% by weight, ensures that there are sufficient sulfide inclusions in the structure for a chip-breaking To generate mechanism and to ensure a hot forming process.

Zinc is added to the alloy as an economical substitute for copper. It was recognized that there is a close connection between the zinc content and the sulfur content via the point in time and the type of distribution of the sulfide formation. The higher the zinc content, the earlier the sulfide inclusions form in the structure during the solidification of the casting. If the zinc content is more than 5% by weight, the sulphide formation is shifted to temperatures in the range of the solidification temperature of the gunmetal alloy. In this temperature range, there are still high levels of molten material in the cast structure, which are connected to one another in places.

A high zinc content then leads to an early formation of sulfides. These sulphides are inhomogeneous and concentrated in places in the structure and thus make the hot pressing process more difficult due to a local weakening of the α-MK matrix. If the zinc content is low, the formation is shifted to lower temperatures and the sulphides are in former residual melt areas separately and homogeneously distributed. The zinc content of 0.1 to less than 1.5% by weight ensures that sulfide formation at higher temperatures is avoided.

Investigations have shown that the copper alloy used according to the invention has a particular suitability for use in a manufacturing process for molded parts due to its specific composition, this process comprising at least one hot forming step. Due to the special composition of the alloy, further processing steps can also be carried out without any problems after hot forming, for example subsequent machining.

Hot forming according to the invention can be a hot pressing process, for example. According to the invention, however, other hot forming processes are also possible, which are known to those skilled in the art. Before hot forming, for example a hot pressing process, the blank is heated to 600°C to 950°C. Above 600 °C, the yield point is sufficiently low to plastically deform the gunmetal material using a hot forming process. According to the invention, hot working can be carried out at any suitable temperature within the above temperature window, for example at 700 to 900°C. The respective temperature is selected by a person skilled in the art as a function of the type of molding, the desired speed of forming, etc.

It was recognized that the atomic bonds of the sulfides also become weaker in the specified temperature range, so that dislocation movements in these superstructures are facilitated. In this temperature range, the phases lose their brittleness and become malleable and thus do not inhibit the hot forming process. Dynamic recrystallization of the α-MK matrix then takes place immediately after forming, which eliminates the zone solid solution with different tin concentrations that previously existed in the cast state and ensures a homogeneous concentration over the cross-section and thus constant mechanical parameters and corrosion properties.

However, after the deformation process at room temperature, the sulphides are again distributed in the structure and are brittle, which means that they act as chip breakers. It could be determined that even in the case of hot-formed molded parts with a low sulfur content of 0.05% by weight, stick-slipping of the tool occurs during mechanical processing due to changes in the friction conditions between the chip and the tool over time. These changed friction conditions are due to the inhomogeneous microstructure, which consists of a copper-containing α-MK matrix with embedded sulfides after the hot pressing process. Due to the stick-slip shearing bands are created in the chip, which lead to lamellar chips and shearing chips and in the further course of processing when discharged via a Break the chip breaker in the tool. This prevents long chips and enables economical machining.

In order to enable the hot pressing process provided according to the invention, the average grain size present in the cast state should not be more than 2 mm. The necessary measures to ensure such an average grain size are known to the person skilled in the art. Grain refinement is possible, for example, by using chemical additives such as zirconium and boron up to levels of 0.005 to 0.03% by weight or other alternative methods for grain refinement such as electromagnetic stirring, ultrasonic excitation, vibration, blowing in gas or by means of strong supercooling of the Melt during casting.

The copper alloy described above is particularly suitable for use in the production of molded parts, with the production comprising at least one hot forming operation. It is also possible to use it to produce molded parts, in which further processing steps take place after the at least one hot forming, for example subsequent machining. The corresponding manufacturing process is particularly suitable for the manufacture of components, such as media, e.g. gas or water-carrying lines and components to be connected, such as fittings, etc. Molded parts that are particularly in focus are components of domestic plumbing pipe systems, including pipes, fittings, end caps and connectors . The basic process steps for producing such moldings are known to the person skilled in the art and are therefore not described in detail here.

In this context, it is essential to the invention that due to the above-described specific composition of the copper alloy to be used, there is no drop in mechanical parameters and corrosion resistance even after hot forming. In addition, it has been shown that both before and after hot forming, the molded parts obtained can be subjected to other processing without any problems. In particular, machining is possible since the problematic and undesirable formation of long chips does not occur. In this way, a molded part can be produced in an economical manner (since in particular the other desirable properties of the copper alloy, such as good hot workability, inertness to the materials in contact with the workpieces, in particular drinking water, and corrosion resistance, are not impaired). In this context, it should be particularly emphasized that the advantages of the present invention described here are achieved, although the use of the components Pb, Si, etc., which are otherwise often considered necessary in the prior art, is dispensed with.

This unexpected advantage of the copper alloy described herein enables it to be used economically to produce the shaped parts described above.

Example

A molded part for the drinking water installation was produced from lead-free gunmetal in a grain-refined state by means of hot forming with subsequent machining. It turned out that chip breakers were present in the structure of the alloy after the hot pressing process, so that economical, fully automated mechanical processing was possible.

Claims (5)

1. The use of a copper alloy for the manufacture of moulded parts by means of a method with at least one hot forming process, wherein the alloy has the following composition in terms of percent by weight: Sn: 2 to 4% Zn: 0.1 to less than 1.5% S: 0.05 to 0.45% Pb: less than 0.25% Ni: less than 0.6% Sb: less than 0.2% and optionally phosphorus to a maximum of 0.06% by weight, B to a maximum 0.03% by weight, Zr to a maximum of 0.03% by weight, as well as unavoidable impurities, and with the remainder being Cu.
2. Method for the manufacture of moulded parts from a copper alloy, wherein the alloy has the following composition in terms of percent by weight: Sn: 2 to 4% Zn: 0.1 to less than 1.5% S: 0.05 to 0.45% Pb: less than 0.25% Ni: less than 0.6% Sb: less than 0.2% and optionally phosphorus to a maximum of 0.06% by weight, B to a maximum 0.03% by weight, Zr to a maximum of 0.03% by weight, as well as unavoidable impurities, and with the remainder being Cu; wherein the method has the following steps:
   at least one hot forming process of the copper alloy for the manufacture of a moulded part.
3. Use or method according to one of the preceding claims, characterised in that the sulphur content is 0.1 to 0.2% by weight.
4. Use or method according to one of the preceding claims, wherein the alloy does not contain any elements from the group of Al, Si, Sb, Te, Se, C and Bi, and/or wherein the alloy contains no Pb.
5. Use or method according to one of the preceding claims, wherein the copper alloy is suitable for the manufacture of moulded parts by means of a method further comprising machining after the at least one hot forming, wherein the method comprises the further step of machining after the at least one hot forming; or wherein the use comprises machining that follows the hot forming.