Special brass alloys and special brass alloy products

Abstract

To provide a special brass alloy and a special brass alloy product that is produced from the alloy.SOLUTION: Described is a special brass alloy containing 62.5 to 65 wt.% of Cu, 2.0 to 2.4 wt.% of Mn, 0.7 to 0.9 wt.% of Ni, 1.9 to 2.3 wt.% of Al, 0.35 to 0.65 wt.% of Si, 0.3 to 0.6 wt.% of Fe, 18 to 0.4 wt.% of Sn and Cr, either alone or in combination, ≤0.1 wt.% of Pb, the remainder consisting of Zn and inevitable impurities.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a special brass alloy and products made from this special brass alloy. [Background technology]

[0002] Typical friction applications in lubricated environments generally require a low coefficient of friction of the alloys used, which must be adaptable within certain limits for the respective application, in particular for the friction partners, the lubricant used, and the friction conditions such as contact pressure and relative speed. This is especially true for piston bushings, which are subjected to high static and dynamic loads. Furthermore, there are applications involving high relative speeds of the friction partners, for example, for thrust bearings in turbochargers, where alloys are required that, in addition to reducing the heat generation from the friction surfaces, also ensure good heat dissipation.

[0003] Due to frictional forces and oil contact, the accumulated lubricant components on the bearing surface form a tribological layer, and this process requires a uniform and simultaneously high accumulation rate of the lubricant components and their decomposition products to maintain a sufficiently stable absorption layer on the sliding layer.

[0004] Furthermore, suitable bearing materials are further characterized by wide oil compatibility, so that the formation of the tribological layer is largely insensitive to the choice of specific oil additives.A further objective is to identify alloys for friction applications that have good dry-running properties to ensure sufficient service life under dry friction conditions.

[0005] For components that are subject to frictional stresses, it is even more important that the alloys used have sufficient strength. Therefore, a high elastic limit of 0.2% must be present in order to keep the plastic deformation that occurs under load as small as possible. Furthermore, particularly hard alloys with high tensile strength must be identified to increase resistance to abrasive and adhesive stresses. Furthermore, sufficient toughness is required to protect against impact stresses. In this regard, the number of micro-defects and their resulting growth must be reduced. This is related to the requirement to identify alloys with the highest possible fracture toughness, with little internal stress.

[0006] Suitable alloys for components exposed to friction are usually special brasses that contain, in addition to copper and zinc as the main components, alloying additives consisting of at least one of the elements nickel, iron, manganese, aluminum, silicon, titanium, or chromium. Silicon brasses in particular meet the aforementioned requirements, and CuZn31Si1 represents a standard alloy for friction applications such as piston bushings. Furthermore, tin bronzes, which contain nickel, zinc, iron, and manganese in addition to tin and copper, are also known to be used for friction applications or in the mining industry.

[0007] Swiss Patent No. 223580A provides an example of a copper-zinc alloy suitable for mechanical parts exposed to sliding, such as bearings, worm gears, gear wheels, and sliding shoes. It discloses a copper content of 50-70% by weight, with alloying additives of 2-8% by weight aluminum, 0.05-3% by weight silicon, and 0.5-10% by weight manganese, the remainder being zinc. Furthermore, the alloy may contain up to 10% by weight lead and 0.1-5% by weight of one or more elements from the group consisting of iron, nickel, and cobalt. Furthermore, European Patent No. 0407596B1 discloses a special brass alloy containing, in addition to copper, zinc, manganese, aluminum, and silicon, iron, nickel, and cobalt as optional alloying components. Furthermore, a proportion of 0.03-1% by weight of oxygen is provided. Furthermore, German Patent Application No. 1558467A discloses another special brass alloy intended for use in objects subjected to sliding and friction. In addition to copper and a zinc content of up to 45% by weight, manganese, silicon, and tellurium are alloying additives. Furthermore, Fe, Ni, Al, and Be represent additional optional alloying elements. Furthermore, German Patent Nos. 1558817B2 and 59949C1 describe copper alloys with a wide range of compositions that form bearing materials with reduced wear.

[0008] To achieve specific properties of products made from a particular brass alloy, alloys with a variety of alloying elements are used. Therefore, with such components, it is also necessary to maintain a variety of products in inventory and to be particularly adept at handling this wide variety of alloys.

[0009] JP 2001-355029 A discloses a synchronizer ring, a component designed for temporarily varying friction loads. The component is manufactured by casting a blank, hot extruding a tube, cutting out a ring, hot forging, and then machining. The synchronizer ring is made from a special brass alloy containing 62.46% Cu, 30.8% Cn, and the balance 0.053% Cr, Zn.

[0010] Weber et al.: "New Pb-free Copper Material for Slide Bearing Applications in Combustion Engines and Transmissions," Metall: Fachzeitschrift fue Metallurgie; Technik, Wissenschaft, Wirtschaft, GDMB-Verlag, Volume 63, No. 11, pages 564-567 (November 1, 2009), discloses slide bearings made from a special brass alloy with the following composition: 58% Cu, 0.5% Pb, 1% Si, 2% Mn, 0.5% Ni, 0.5% Fe, 2% Al, balance Zn.

[0011] European Patent No. 3269835B1 discloses a special brass alloy characterized by high strength, improved wear resistance under frictional loads, and good dry-running properties in the absence of lubrication. This special brass alloy contains 60-62 wt.% Cu, 2.1-2.5 wt.% Mn, 0.2-0.6 wt.% Ni, 2.9-3.1 wt.% Al, 0.35-0.65 wt.% Si, ≤0.1 wt.% Fe, ≤0.1 wt.% Sn, ≤0.1 wt.% Pb, ≤0.1 wt.% Zn, and the balance consisting of unavoidable impurities. Products made from this special brass alloy are characterized by elongated intermetallic phases in their structure, which impart high mechanical wear resistance to the alloy product. During extrusion, these intermetallic phases tend to result in good longitudinal alignment. Here, the elongated formation of intermetallic phases, mainly Mn-silicides, serves to protect the matrix from wear-generating stresses. Therefore, the grain size of the β-phase or β-dominated matrix is ​​between 100 and 300 μm. Here, the products made from this alloy have a relatively coarse grain, which can be advantageous for chip fracture in metal cutting processes applied to semi-finished products made from this alloy.

[0012] While this known alloy has beneficial properties, the resulting relatively coarse grains are sometimes considered a disadvantage due to the potential for grain boundary crack formation during cold forming. While semi-finished products made from this alloy can be easily mechanically processed, if a special brass alloy product is expected to have only a low surface roughness, it would be desirable to have a smaller residual surface roughness resulting from mechanical processing to avoid further processing efforts resulting from subsequent polishing or similar surface treatments. Furthermore, these special brass alloy products or semi-finished products have been found to undergo relatively large thermal relaxations. As a result, the process window that must be observed under certain circumstances, for example, for thermal relaxation, must be kept very narrow. Additionally, for products exposed to higher temperatures, particularly large temperature variations, this process of thermal relaxation of this alloy can lead to a decrease in strength. Summary of the Invention

[0013] Therefore, proceeding from this discussed prior art, it is a fundamental object of the present invention to provide a special brass alloy as well as special brass alloy products made therefrom, which is similar in type to the alloy previously disclosed in EP 3269835 B1, but which is improved in terms of grain size and thermal relaxation behavior.

[0014] According to the invention, this object is achieved by a special brass alloy having: 62.5-65 wt% Cu, 2.0 to 2.4 wt.% Mn, 0.7 to 0.9 wt.% Ni, 1.9 to 2.3 wt.% Al, 0.35 to 0.65 wt.% Si, 0.3 to 0.6 wt.% Fe, 0.18 to 0.4 wt. % Sn and Cr, alone or in combination; ≦0.1 wt.% Pb, The remainder consists of Zn and unavoidable impurities.

[0015] This special brass alloy is characterized by a highly homogeneous and fine-grained microstructure, which is already formed in the initial, initially formed semi-finished product (cast or extruded preform). The average grain size is 40-150 μm. Casting the alloy results in an even finer grain size. A further special feature is that even in the semi-finished product, this special brass alloy possesses an α-β structure, with the α phase either embedded in the β phase in a lattice or strip-like manner or permeating the β phase. In this way, the α-phase grains connect the grains of the β matrix, so that the grain network remains intensively interpenetrated by the α phase and thus remains preserved even under cold-forming stresses. Interestingly, this structure is formed in this alloy even in extrusion preforms without a preferred orientation. Therefore, it is interesting to note that the same or nearly the same structure can be observed in both the longitudinal and transverse directions of the pressed joint. As a result, in contrast to the alloys previously disclosed in EP 3269835 B1, the grain boundaries of the structure of special brass alloy products, which may indeed be semi-finished products, produced from this special brass alloy do not exhibit weak points, and their structure, typically with an α phase fraction clearly less than 10%, contains an overwhelmingly predominant β phase, of which the α phase is located at most in the grain boundary regions or within the β phase. This is why the new alloy is less susceptible to grain boundary cracking during cold forming. Furthermore, when such special brass products are used at high temperatures or under the influence of temperature changes, in addition to the fine grain size actually achieved, strength loss is avoided or significantly reduced compared to the aforementioned alloys.

[0016] It was surprising to realize that such a significant difference in structure, and therefore the advantages achieved with this alloy, could be realized by a relatively small change in the alloy composition compared to what is known from EP 3269835 B1. It was not foreseeable that a slight change in the zinc equivalent of this particular brass alloy compared to what is known from EP 3269835 B1 to achieve slightly more alpha phase would result in such a significant change in the particular brass alloy product. Therefore, it is the resulting unexpected interaction of the elements involved in the construction of the alloy already during the initial forming (casting or extrusion) that results in the beneficial properties of the particular brass alloy or the products or semi-finished products made therefrom. This also involves low thermal relaxation, so that a correspondingly large process window can be allocated for thermal relaxation.

[0017] It is noteworthy that, as already specified, when this particular brass alloy is extruded to produce semi-finished products, no differences in the structure of the core and marginal zones of the pressed product can be detected. Interestingly, the α phase that penetrates the β phase in a lattice-like manner during both longitudinal and transverse elongation of the pressed product is formed in a manner that does not differ significantly in any case with respect to the orientation of its longitudinal axis, and therefore the α phase that penetrates the β phase does not have a preferred direction in the pressed product. Therefore, such preforms can be processed without having to take into account the preferred structural elongation. Thus, for example, sections of extruded connecting pieces intended for forging can be formed independently of the extrusion direction. Furthermore, it is particularly advantageous that the intermetallic compounds (silicides) embedded in the α-β matrix are rather rounded, spherical and at most only slightly elongated in nature, in contrast to the elongated nature of the special brass alloy known from EP 3269835 B1, and therefore have no preferred orientation and are at most only slightly elongated, and as a result do not break during forming.

[0018] The elements Sn and Cr, which affect grain refinement, are present in the alloy, either alone or in combination, in amounts of 0.18 to 0.4 wt.%. According to one embodiment, the alloy contains only Sn and no Cr. The Sn content is preferably 0.2 to 0.3 wt.%. In another design, the alloy contains no Sn, but preferably 0.2 to 0.27 wt.% Cr. Amounts of these elements exceeding 0.4 wt.% do not result in significant improvements. In addition to Sn and Cr, the Fe content also contributes to grain refinement. The use of Sn also has beneficial properties related to the formation of a passivation layer on the surface of special brass alloy products manufactured from the alloy, thereby improving their tribological properties.

[0019] The semi-finished products made from this special brass alloy are characterized by good cold formability. Therefore, it is possible to manufacture products such as slide shoes, which require a considerable forming speed. For example, in the case of slide shoes, the flange must be able to be provided on the semi-finished product after forging. Despite this good cold formability, the material is sufficiently hard to meet the requirements of such slide shoes during use, especially the required settling time.

[0020] The thermal relaxation resistance is achieved by the fine grained structure and the aforementioned α-β matrix. In this respect, another useful effect that can be observed is that the grains of the matrix are unordered and therefore have no preferred orientation.

[0021] It is also worth noting that this alloy has a reduced electrical conductivity of about 10% compared to the alloy known from EP 3269835 B1, and accordingly, corrosion resistance is improved in this way, since lower corrosion currents can flow.

[0022] The α-phase content of the cast or extruded preform is approximately 40-60%. In the comparative alloys according to EP 3269835 B1, the α-phase content in this alloy state is only up to 10%, but is usually significantly less than 10%, so the presence of such an α-phase content in the cast or extruded preform is unexpected. The approximately equal content of α and β phases within the aforementioned limits represents a good starting point for reducing or increasing the α-phase content by heat treatment processes, such as annealing, as a function of the desired structural composition of the final product. Heat treatment of the alloy product at low temperatures, typically between 270°C and 290°C, for 4.5 to 6 hours significantly reduces the α-phase content, i.e., to 20% to 25% with these heat treatment parameters. Heat treatment at higher temperatures, for example, between 435°C and 460°C for approximately 2.5 to 3.5 hours, results in an increase in the α-phase content. In this way, it is possible to produce special brass alloy products with an alpha phase content of 70-75%, within which the actual desired alpha phase content in the final product can be individually adjusted independently of the initial formation.

[0023] These useful wear-resistant properties of products made from this alloy are already reflected in their hardness. After extrusion, the semi-finished products have a hardness between 135 and 145HB [2.5 / 62.5]. By heat treatment after forming, the hardness can be increased to values ​​above 160HB if the workpieces are heat treated at the higher temperatures and for shorter treatment times mentioned above.

[0024] This particular brass alloy preferably contains 63-64 wt% Cu, 2.1-2.2 wt% Mn, 2.0-2.2 wt% Al, and 0.4-0.5 wt% Fe.

[0025] The invention will now be described by way of example embodiments with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0026] [Figure 1]10a-d are photomicrographs of as-pressed extruded samples made from a first special brass alloy according to the present invention. [Figure 2] 2 is a photomicrograph of a sample of the alloy of FIG. 1 in polished longitudinal and transverse sections compared with a photomicrograph of a second special brass alloy according to the present invention; [Figure 3] FIG. 1 is a detailed view of a polished longitudinal section of a first special brass alloy according to the present invention juxtaposed with a comparative sample. [Figure 4] FIG. 1 is a detailed view of a polished longitudinal section of a second special brass alloy according to the present invention juxtaposed with a comparative sample. [Figure 5] 1 is a photomicrograph of a slide shoe made from a first special brass alloy according to the present invention. [Figure 6] 1 is a photomicrograph of a turned bushing made from a first special brass alloy according to the present invention. [Figure 7] FIG. 10 shows an image of the microstructure of a retaining segment formed by forging from a first special brass alloy according to the present invention after forging. [Figure 8] FIG. 1 shows a microstructural image of a retaining segment formed by forging from a first special brass alloy according to the present invention after heat treatment (annealing) downstream of forging. [Figure 9] FIG. 10 shows an image of the microstructure of a slide shoe formed by forging from the first special brass alloy according to the present invention after forging. [Figure 10] FIG. 1 shows an image of the microstructure of a slide shoe formed by forging from a first special brass alloy according to the present invention after heat treatment (annealing) downstream of forging. [Example]

[0027] Samples were prepared from two special brass alloys according to the invention and a comparative alloy and were subsequently extruded at about 700° C. The composition of sample V of the comparative alloy and of the two samples E1, E2 made from the special brass alloy according to the invention are reproduced below (specifications in weight %):

[0028] TIFF0007787125000001.tif79170

[0029] The comparative alloy is a special brass alloy described in the example embodiment of EP 3269835 B1. Sample E1 is a first special brass alloy according to the present invention and represents a Sn-containing variant of the special brass alloy according to the present invention. Figure 1 shows micrographs of this alloy in the pressed state, with the pressed connection removed in the longitudinal direction (Figures 1a and 1b) and transverse directions (Figures 1c and 1d). Figures 1a and 1c show the removal from the core, while Figures 1b and 1d show the removal from the radial margin zone. Samples 1a and 1b show the removal from the longitudinal direction of the pressed connection, whereas samples 1c and 1d show the removal from the transverse direction. It is noteworthy that the microstructure is homogeneous from the core to the margin and in both the longitudinal and transverse directions. Furthermore, these micrographs show the typical α-β structure of this alloy, with the α phase (bright grains) interpenetrating the β grains in the form of lattices or strips.

[0030] This special microstructure of the pressed connections is also evident in the photomicrographs shown in Figure 2, which are shown at a low magnification. In this figure, sample E1 is shown on the left, and sample E2 is shown with its structure on the right. Sample E2 is a Cr-containing variant of the special brass alloy according to the invention. The upper pair of images in each case shows the structure in the longitudinal extension of the pressed connections (preforms). The lower pair of images in Figure 2 shows the structure in the transverse direction. These photomicrographs also reveal the formation of an interestingly highly homogeneous structure in both the longitudinal and transverse directions of samples E1 and E2. In these photomicrographs, the α phase is again the lighter component.

[0031] 3 and 4 show in each case a juxtaposed micrograph of sample E1 juxtaposed with a micrograph of sample V (FIG. 3), and a second sample E2 according to the invention juxtaposed with a micrograph of a sample of comparative alloy V (FIG. 4). These juxtapositions reveal a significantly different structural formation of the special brass alloy according to the invention compared to the comparative alloy. While the comparative alloy in the worked "pressed joint" shows exclusively β-phase, the alloy according to the invention shows β-phase interpenetrated by α-phase, with the α-phase grains extending beyond the grain boundaries of adjacent β-phase grains.

[0032] For example, the semi-finished product initially formed from the special brass alloy according to the invention in the form of an extruded connecting piece has an α-phase proportion of about 35-55%, in particular between about 40% and about 50%. The α-phase proportion in samples E1 and E2 according to the invention is in each case about 45%. The β-phase forms the remainder. The proportion of intermetallic phases is about 3%.

[0033] In the special brass alloy according to the present invention, the formation of the structure dependent on the alloy described above is used not only for the thermal and mechanical purposes discussed. Instead, the formation of this structure with a homogeneous distribution of α and β phases is suitable for adjusting the proportion of α phase as a function of the requirements imposed on the final product. This can be achieved by heat treatment (annealing). If the semi-finished product is subjected to annealing at a lower temperature but for a longer treatment time, the proportion of α phase decreases. To achieve this, the semi-finished product is treated at a temperature between 260°C and 300°C for 4 to 6 hours, in particular at about 280°C for 5 hours. By carrying out the heat treatment with these parameters, the proportion of α phase can be reduced to below 30%.

[0034] However, the proportion of α-phase in the pressed joint increases if the heat treatment is carried out at a higher temperature for a shorter time. This heat treatment is carried out at a temperature between 430°C and 470°C for 2.5 to 4 hours, in particular at a temperature of about 450°C for 3 hours. After this, the proportion of α-phase can be increased to more than 65%.

[0035] The reduction in the proportion of α-phase leads to a certain increase in the proportion of intermetallic phases, which in turn can be proportionally contained in such annealed products in a proportion of 4.5% to 5.5%.

[0036] The special brass alloy of this invention is particularly suitable for forging press-fit connection segments, and its homogeneous extrusion structure contributes to this. Figure 7 shows the forged connection segment from sample E1. Forging was performed at 700°C. As can be seen in the microstructure image in Figure 7, even after forging, the original structure remains essentially visible and therefore preserved.

[0037] Figure 8 shows the holding segment after heat treatment at 280°C for 5 hours. The heat treatment contributed to further homogenization of the structure. The α-phase content, which was approximately 40% after forging, was reduced to approximately 29% by the heat treatment. Furthermore, the hardness increased from approximately 140 HB [HBW 2.5 / 62.5] to approximately 148 HB.

[0038] A slide shoe was also produced from sample E1 by forging. Figure 9 shows the structure after the forging process, which was carried out at 710 °C. The microstructure established in the pressed joint was also essentially preserved in this sample, despite the forging. The forged slide shoe was then heat-treated at 450 °C for 3 hours. Figure 10 shows that the α-phase fraction increased by the annealing process, from approximately 50% (α-phase fraction in the pressed joint) after forging to approximately 68%. The HB hardness only increased slightly after annealing compared to the hardness after forging, from 155 HB [HBW 2.5 / 62.5] to 159 HB.

[0039] In the comparative alloy, sample V, the matrix of the pressed joint contained an alpha phase fraction of <1%.

[0040] It is interesting to note that the special brass alloy according to the invention has a conductivity that is slightly more than 10% lower than that of the comparative alloy sample V. The conductivity of the comparative alloy is 10.4-10.7 MS / m, whereas the conductivity of the alloy E1 according to the invention is only 8.8 MS / m. This improves the corrosion resistance of this special brass alloy.

[0041] The mechanical properties of samples E1 and E2 can be taken from the following table:

[0042] TIFF0007787125000002.tif32170

Claims

1. 63 to 64 wt.% Cu, 2.0 to 2.4 wt. % Mn, 0.7 to 0.9 wt. % Ni, 2.0 to 2.2 wt. % Al, 0.35 to 0.65 wt.% Si, 0.4 to 0.5 wt. % Fe, 0.2 to 0.3 wt. % Sn or 0.2 to 0.27 wt. % Cr; Contains ≦0.1 wt.% Pb, A special brass alloy with the balance consisting of Zn and unavoidable impurities. A product comprising:

1. An article of manufacture, characterized in that it comprises an α-β mixed crystal matrix having a proportion of α phase between 35% and 55% and a proportion of intermetallic phase between 2% and 5%.

2. The product described in claim 1, characterized in that the special brass alloy contains 2.1 to 2.2 wt. % Mn.

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