EP3908682B1 - Pb-free cu-zn alloy - Google Patents

Description

The invention relates to a Pb-free Cu-Zn alloy, particularly for making alloy products used under lubricated conditions. An alloy product made from such an alloy is also described.

The special brass CuZn37Mn3Al2PbSi (CW713R) described in the material data sheet (status 2005) of the German Copper Institute is an alloy that has been used extensively for many years and is characterized by high wear resistance and good hot workability. This material has high strength values and average machinability and has good corrosion resistance. For this reason, this alloy is used for structural parts in mechanical engineering, for synchronizer rings and valve guide tubes in automobile construction, as well as for a range of plain bearing elements and hot-pressed parts. This means that alloy products made from this alloy are used under lubricated conditions. Possible applications relate to permanent immersion in oil or the supply of lubricant through channel and groove systems provided for this purpose. Synchronizer rings are in an oil environment. The same can apply to plain bearing elements, which can also only be lubricated with oil. This alloy is also used to manufacture components used in hydraulics, such as distributor plates. This previously known alloy has the following composition (data in % by weight): Cu: 57.0-59.0%, Mn: 1.5-3.0%, Al: 1.3-2.3%, Si : 0.3 - 1.3%, remainder zinc plus unavoidable impurities. Admissible admixtures are tolerated (in percent by weight): Ni: max. 1.0%, Fe: max. 1.0%, Sn: max. 0.4%, Pb: 0.2 - 0.8 %.

As can be seen from the material description given above, this previously known alloy contains Pb. This element is responsible for machinability and, due to its incorporation in tribological layers the running-in behavior as well as friction and wear in sliding applications.

The special brass alloy CW713R is characterized by versatile application properties, such as high wear and cavitation resistance, compatibility with lubricants and sufficient mechanical properties, especially with regard to the strength and toughness of the alloy product. These also include good machinability. The element Pb is introduced into brass alloys to achieve the desired machinability.

For health reasons and because of environmental aspects, efforts have recently been made to design brass alloys without lead. Whenever possible, efforts are made not to have to do without the properties brought about by the element Pb in the alloy.

DE 10 2005 017 574 A1 describes a wear-resistant brass alloy for synchronizer rings with an optional lead content. The composition (in percent by weight) is 57.5 - 59% copper, 2 - 3.5% manganese, 1 - 3% aluminum, 0.9 - 1.5% silicon, 0.15 - 0.4 % iron, 0 - 1% lead, 0 - 1% nickel, 0 - 0.5% tin and balance zinc.

WO 2014/152619 A1 discloses a brass alloy for turbochargers with the following composition, optionally containing lead (data in % by weight): 57-60% copper, 1.5-3.0% manganese, 1.3-2.3% aluminum, 0.5 - 2.0% silicon, 0 - 1% nickel, 0 - 1% iron, 0 - 0.4% tin, 0 - 0.1% lead and the balance zinc.

Calls for sliding applications JP S56-127741 A a brass alloy with the following composition (in % by weight): 54 - 66% copper, 1.0 - 5.0% manganese, 1.0 - 5.0% aluminum, 0.2 - 1.5% silicon , 0.5 - 4.0% nickel, 0.1 - 2.0% iron, 0.2 - 2.0% tin and the balance zinc.

Proceeding from this discussed prior art, the object of the invention is to propose a Pb-free Cu—Zn alloy that is fundamentally suitable for an application or use for which the alloy CuZn37Mn3Al2PbSi described above for the prior art was also suitable. It would be desirable if the mechanical strength properties were even improved compared to this previously known special brass alloy, but without having to accept losses in terms of cold and hot workability and machinability.

• Cu: 57 - 59%,
• Mn: 1,7 - 2,7 %,
• Al: 1,3 - 2,2 %,
• Si: 0,4 - 1,0 %,
• Ni: 0,4 - 0,85 %,
• Fe: 0,3 - 0,7 %,
• Sn: 0,15 - 0,4 %,
• Rest Zn nebst unvermeidbaren Verunreinigungen mit 0,05 Gew.-% je Element, wobei die Summe der unvermeidbaren Verunreinigungen 0,15 Gew.-% nicht überschreitet.

This problem is solved by a Pb-free Cu-Zn alloy with the following composition (in percent by weight):

• Cu: 57 - 59%,
• Mn: 1.7 - 2.7%,
• Al: 1.3 - 2.2%,
• Si: 0.4 - 1.0%,
• Ni: 0.4 - 0.85%,
• Fe: 0.3 - 0.7%,
• Sn: 0.15 - 0.4%,
• Balance Zn plus unavoidable impurities with 0.05% by weight per element, the sum of the unavoidable impurities not exceeding 0.15% by weight.

Unavoidable impurities in the alloy are permitted at 0.05% by weight per element, the sum of the unavoidable impurities not exceeding 0.15% by weight.

This alloy is characterized above all by the selection of the alloying elements Ni, Fe and Sn, as well as by the claimed content of these elements in the alloy composition in relation to the other alloying elements, above all Mn, Al and Si. This balanced alloy composition ensures particularly good properties of the alloy product in terms of cold and hot workability, machinability, strength and wear resistance, the latter especially under lubricated conditions. This result is surprising since Bi is used as a Pb substitute in other special brass alloys, but the alloy according to the invention does not use Bi. While the previously known alloy CuZn37Mn3Al2PbSi although it has good hot workability, the subject matter of the claimed alloy not only has particularly good hot workability, but also good cold workability. The latter was not the case with the previously known alloy. Of interest with this alloy is that it is suitable for making forgings. If the forgings are then subjected to stress-relief annealing, which is carried out in a temperature range between 300° C. and 450° C., this measure can increase the proportion of embedded α-solid solution to 10-15%. In order to achieve the desired properties, annealing in a temperature range of 350 to 380°C is sufficient in many cases. This increased proportion of α-mixed crystals is the reason for the improved cold workability. Without such an annealing step, the alloy structure contains less than 3-5% α-mixed crystals. The same advantages of stress-relief annealing also apply to extruded products, whereby a structure with an α-mixed crystal content of 10 - 15% can also be achieved through the thermal treatment mentioned.

The strength values achievable with this alloy and the surprisingly significantly better cavitation resistance compared to comparison alloys were not foreseeable for the people involved in the development of this alloy. The alloy products forged from the alloy according to the invention have a 0.2% yield strength between 330 and 350 MPa, which is significantly higher than was usual with forgings of the alloy CuZn37Mn3Al2PbSi (values of 230 to 300 MPa). The tensile strength of alloy products made from the alloy according to the invention is 600 to 640 MPa. In the case of the previously known alloy CuZn37Mn3Al2PbSi, the tensile strength values are usually between 590 and 670 MPa. Slightly higher tensile strength values can also be achieved with special treatments.

• Cu: 57,5 - 58,5 %,
• Mn: 2,0 - 2,5 %,
• Al: 1,5-2,0 %,
• Si: 0,50 - 0,70 %,
• Ni: 0,50 - 0,70 %,
• Fe: 0,5 - 0,55 %,
• Sn: 0,20 - 0,35 %.

Studies have shown that the interaction of the elements Ni, Fe and Sn with each other, but also with Mn, Al and Si and in connection with the formation of intermetallic phases leads to particularly good results when the Mn content is reduced to 1.9 - 2 .6%, the Al content of 1.4 - 2.1%, the Ni content of 0.45 - 0.75% and the Fe content of 0.3 - 0.6% is controlled. It has been found to be particularly suitable for the desired purposes with a special characteristic of good cold and hot workability, machinability, strength and wear resistance if the alloy composition is selected as follows (details in % by weight):

• Cu: 57.5 - 58.5%,
• Mn: 2.0 - 2.5%,
• Al: 1.5-2.0%,
• Si: 0.50 - 0.70%,
• Ni: 0.50 - 0.70%,
• Fe: 0.5 - 0.55%,
• Sn: 0.20 - 0.35%.

The special properties of an alloy product made from this alloy are based on the fact that the Si content is preferably not less than the Ni content. Furthermore, the Sn content of the alloy is preferably adjusted in such a way that it is at most only 50% of the Ni content or only at most 50% of the Si content. The Ni content is preferably not less than the Si content, deviations of up to 0.075% being tolerated. The Fe content also plays a role in interaction with the other elements. Preferably, the Fe content is less than the Ni content by about 0.05% to 0.1% by weight.

The special properties described above of an alloy product made from this alloy arise both in the case of forged products and in the case of extruded products.

examples

A number of alloys from the alloy according to the invention were cast, then extruded and parts thereof subjected to a subsequent forging step. In parallel, a comparison sample of the material CW713R was produced in the same way. Below are two samples according to the invention with regard to their alloy composition - samples 1 and 2 - and the composition of a comparison sample (CW713R) are shown as an example: Cu Zn sn feet Mn no Al si pb sample 1 58.4 rest 0.26 0.46 2.1 0.52 1.67 0.52 0 sample 2 58.0 rest 0.23 0.46 2:13 0.54 1.55 0.6 0 CW713R 58.1 rest 0.15 0.35 2.2 0.32 1.6 0.7 0.7

After casting (continuous casting), blocks were sawn and then bars with a diameter of 50 mm and a length of 20 m were pressed from the blocks. The extrusion temperature of the examined series of samples was between 685°C and 710°C. The extrusion temperature of the described samples was around 700°C. The resulting structure is very homogeneous over the pressed bar, both in the longitudinal direction and in the transverse direction of the pressed bar over its entire length. The only thing that can be observed is that the grain size decreases somewhat from the start of pressing to the end of pressing, as is usually observed in extrusion. The structure consists almost exclusively of β-phase with intercalated intermetallic compounds (mixed silicides, which are adjusted in the pressing direction). The proportion of intermetallic compounds is around 3 - 4%.

Figures 1a, 1b show micrographs of sample 1 in the pressed state from the start of pressing ( Figure 1a along the direction of pressing; Figure 1b perpendicular to the pressing direction). Figures 2a, 2b show corresponding microstructure images from the pressing end. In a subsequent step, the samples cut from the pressed rods were thermally relaxed for three hours at 360° C. As a result of the stress-relief annealing, an α-mixed crystal phase was formed in the structure, resulting in a β-mixed crystal-dominated structure with an α-mixed crystal component of about 14%. The proportion of intermetallic phases is around 3%.

Figures 3a, 3b show micrographs of sample 2 after the stress-relief annealing described above.

The above-mentioned structural parameters and the strength values of these samples are shown in the table below: Condition α share [%] IMP share Tensile strenght [%] Elongation at break [%] 0.2% proof stress [MPa] HBW sample 1 press condition < 4 3.7 671 19.4 367 169 sample 2 Pressed condition and annealed 14 3.1 649 22.5 321 162 CW713R press condition 10 3.4 643 16 318 155

The intermetallic phases are denoted by IMP. The hardness HBW was measured as HBW 2.5/62.5.

The microstructure of the comparative sample CW713R in the pressed state is β-phase dominated with a proportion of α-mixed crystal phase of about 10%. The Pb contained in this alloy has a grain-refining effect and serves as a chip breaker. figure 4 shows a micrograph of sample CW713R in the pressed state and after an annealing treatment, corresponding to that of sample 2. The proportion of α-mixed crystal phase is about 40 - 45%.

In a subsequent step for the production of distributor plates, connecting pieces were cut off from the pressed rods as preliminary forging products and these were hot-forged. The forgings in the sample series were forged at temperatures between 635°C and 670°C. Sample 2 and the comparison sample were forged at around 650°C. The resulting microstructure of a pre-product forged in this way for a distributor plate for a hydraulic application is in Figures 5a, 5b shown. Figure 5a shows the microstructure at the edge while Figure 5b shows the microstructure in the core of the forged product.

These images illustrate the very homogeneous structure across the diameter of the forged semi-finished product. This consists almost exclusively of β-phase with incorporated intermetallic phases of a good 3%.

In a subsequent step, samples of this type were annealed for three hours at 360° C. In the course of this annealing process, an α-phase content of around 12% was formed. The proportion of intermetallic phases increased to about 3.7%. The structure of the annealed semi-finished product for manufacturing a distributor plate for hydraulic applications is in Figures 6a, 6b shown ( Figure 6a Edge; Figure 6b Core). The α-phase contained therein is clearly recognizable.

The microstructure parameters and the mechanical strength values for these samples are shown in the table below: Condition α share [%] IMP share Tensile strenght [%] Elongation at break [%] 0.2% proof stress [MPa] HBW sample 2 forged < 0.1 3.1 626 15.6 341 174 sample 2 forged and annealed 12 3.7 624 13.2 340 174 CW713R forged 5 3.5 535 14.5 275 158

When the comparative forged sample (CW713R) is subjected to an annealing process as described above, the proportion of α-phase increases significantly, up to about 40%.

Pipes were also made from the alloy of Sample 2 and that of the comparative alloy (CW713R) by extrusion. Sections were cut from the tubes which were then machined by turning to compare the machinability of the two alloys. In the course of this turning, rings were created. Interestingly, the machinability of the ring made from the alloy according to sample 2 is at least as good as the machinability of the ring made from the comparison alloy. This is remarkable since the sample according to the invention (sample 2), unlike the alloy composition of the comparative sample, does not contain any Pb because the alloying element Pb in the comparative sample is held responsible for the good machinability of this alloy.

The alloy product of the present invention can be drawn directly. Nevertheless, an intermediate anneal before drawing is preferred in order to achieve as stress-free an alloy product as possible. Furthermore, additional investigations with the alloy compositions of samples 1 and 2 for differently adjusted material states have shown that the tensile strength R _m , the 0.2% yield strength, the elongation at break and the hardness HB are also opposite for directly drawn specimens or specimens drawn after an intermediate annealing step Semi-finished product made from the comparison alloy CW713R is significantly increased. The same was the case for both variants of the samples for a material condition after a final stress-relief annealing. This was found in forgings made from the alloy as well as in extruded semi-finished products that were drawn (stretched) after pressing. In both cases, subsequent annealing can be helpful in relieving stresses contained in the respective workpiece.

Furthermore, cavitation investigations were undertaken with the forged and annealed sample 2. For this purpose, surfaces of test specimens obtained from sample 2 were first ground with a grain size of 1000 mesh and a cavitation test was then carried out on them in accordance with ASTM G32 in distilled water. It has been shown that the highly estimated cavitation resistance of the comparison alloy CW713R could be significantly increased again. This reduction in the cavitation tendency in water indicates that alloy products with the composition according to the invention have improved durability even under high dynamic loading in a lubricant environment, such as occurs, for example, in cylinder liners of axial piston pumps. Such cylinder liners are made from extruded and then cold-drawn (stretched) semi-finished products. Cylinder liners for such applications are therefore particularly suitable for production from the alloy according to the invention.

Claims (14)

1. Pb-free Cu-Zn alloy for producing alloy products used under lubric conditions with the following composition (values in % by weight):

   Cu: 57-59 %

   Mn: 1.7 - 2.7 %

   Al: 1.3 - 2.2 %

   Si: 0.4 - 1.0 %

   Ni: 0.4 - 0.85 %

   Fe: 0.3 - 0.7 %

   Sn: 0.15 - 0.4 %

   the remainder being Zn as well as unavoidable impurities with 0.05 % by weight per element, wherein the sum total of the unavoidable impurities does not exceed 0.15 % by weight.
2. Pb-free Cu-Zn alloy according to claim 1, characterised by:

   Mn: 1.9 - 2.6 %

   Al: 1.4 - 2.1 %

   Ni: 0.45 - 0.75 %

   Fe: 0.3 - 0.6 %
3. Pb-free Cu-Zn alloy according to claim 2, characterised by:

   Cu: 57.5 - 58.5 %

   Mn: 2.0 - 2.5 %

   Al: 1.5 - 2.0 %

   Si: 0.50 - 0.70 %

   Ni: 0.50 - 0.70 %

   Fe: 0.35 - 0.55 %

   Sn: 0.20 - 0.35 %
4. Pb-free Cu-Zn alloy according to any one of claims 1 to 3, characterised in that the Si content is not smaller than the Ni content.
5. Pb-free Cu-Zn alloy according to any one of claims 1 to 4, characterised in that the Sn content amounts to a maximum of 50% of the Ni content and a maximum of 50% of the Si content.
6. Pb-free Cu-Zn alloy according to any one of claims 1 to 5, characterised in that the Fe content is less by 0.05% to 0.1% than that of the Ni content.
7. Lubricating product, produced from a Pb-free Cu-Zn alloy according to any one of claims 1 to 6, characterised in that the lubricating product produced from the alloy comprises a β structure and an embedded α-mixed crystal proportion of less than 5% and a proportion of intermetallic phases of 2.5 - 4.5 %.
8. Extruded product, produced from a Pb-free Cu-Zn alloy according to any one of claims 1 to 6, characterised in that the extruded product produced from the alloy comprises a β structure and an embedded α-mixed crystal proportion of less than 5% and a proportion of intermetallic phases of 2.5 - 4.5 %.
9. Alloy product according to any one of claims 7 or 8, characterised in that the alloy product is thermically stress-relieved by an annealing process, and, by means of this process the proportion of the α-mixed crystals is raised to 10 - 30%, in particular to 10 - 15%, and a proportion of intermetallic phases of 3 - 5% is formed.
10. Alloy product according to any one of claims 7 to 9, characterised in that the hardness of the alloy product amounts to 160 - 190 HBW 2.5/62.5, in particular 170 - 185 HBW 2.5/62.5.
11. Alloy product according to any one of claims 7 to 10, characterised in that the alloy product exhibits a 0.2% elongation limit of between 300 and 400 MPa, in particular between 300 and 350 MPa, and a tensile strength of 600 - 700 MPa, in particular of 600 - 640 MPa.
12. Alloy product according to claim 7 and 11, characterised in that the alloy product exhibits an elongation after fracture of between 10 - 30%, in particular 13 - 20%.
13. Alloy product according to claim 8 and 11, characterised in that the alloy product exhibits an elongation after fracture of between 10 - 16%.
14. Alloy product according to any one of claims 7 to 13, characterised in that the electrical conductivity of the alloy product amounts to between 9 and 11 MS/m, in particular between 9.3 and 10.0 MS/m.

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