A lead brass alloy and a method of making the same

Abstract

The application discloses a kind of lead brass alloys, the mass percentage composition of the lead brass is Cu: 62-65%, Pb: 2.4-3%, Fe: 0.03-0.1%, Al: 0.001-0.1%, Cr: 0.01-0.2%, Ni: 0.07-0.5%, the remainder is Zn and inevitable impurities.The lead brass alloy has high cutting performance, and can realize micro machining of small parts.The application also discloses a preparation method of the lead brass alloy.The process flow of the preparation method of the lead brass includes smelting, horizontal continuous casting, homogenizing annealing, extrusion, intermediate disc drawing, bottom annealing and combined drawing.The ingredients and smelting are carried out according to the mass percentage of each element of the lead brass alloy.The temperature of the homogenizing annealing is 560-700 DEG C, and the holding time is 3-6h.

Description

Technical Field

[0001] This invention belongs to the field of brass technology, specifically relating to a leaded brass alloy and its preparation method. Background Technology

[0002] With the continuous development of modern science and technology, micro-machining technology for micro-parts is becoming increasingly important in national defense, aerospace, medicine, and microelectronics industries. The cutting tools used in micro-machining have very small diameters, generally between 0.1 and 10 mm. Compared with conventional cutting, during micro-machining, the workpiece material undergoes severe elastic-plastic deformation under the friction, extrusion, or plowing action of the cutting edge.

[0003] Factors such as minute tool runout and chip interruption, which are negligible in conventional cutting processes, can have a significant adverse impact in micromachining, leading to changes in part accuracy. Therefore, micromachining places extremely stringent requirements on materials, equipment, cutting tools, and machining processes. Even slight errors during machining can cause workpiece errors to exceed tolerance ranges, requiring remachining or scrapping, thus increasing processing costs.

[0004] In leaded brass, the Pb element exists independently at the grain boundaries or within the solid solution, and has the function of lubrication and reducing friction. Therefore, leaded brass has excellent cutting performance, the chips are easily broken, and the workpiece surface is smooth, making it suitable for machining parts on automatic high-speed lathes.

[0005] The trend towards precision and miniaturization of parts has driven the development of micromachining technology, which places increasingly higher demands on the cutting performance of materials. Currently, leaded brass, which has excellent cutting performance, can no longer meet the requirements of precision and miniaturization in micromachining. This is mainly manifested in problems such as tiny burrs on the surface of precision and miniaturized parts, inconsistent dimensional accuracy, and easy breakage of drill bits when drilling micro-holes.

[0006] The machinability of leaded brass is directly proportional to its Pb content. However, when the Pb content in leaded brass exceeds 3%, it no longer significantly improves the machinability of copper. On the contrary, due to the softness and low melting point of Pb particles, it reduces the mechanical properties of the alloy. Therefore, simply increasing the Pb content cannot improve the machinability of leaded brass.

[0007] From a materials perspective, the control of part accuracy during precision machining is inseparable from the microstructure and crystal structure of the material. This is because during precision machining, the elastic and plastic deformations of the tool and workpiece surfaces are random, and the amount of deformation is often uneven, making it difficult to control accuracy.

[0008] Therefore, it is urgent to improve the uneven deformation during machining by changing the microstructure of leaded brass alloy materials, so that the elastic and plastic deformation of each micro-area of ​​the leaded brass alloy material is uniform during machining, thereby ensuring that the precision of the parts remains consistent and realizing the micro-machining of micro-sized parts. Summary of the Invention

[0009] This invention provides a leaded brass alloy with high machinability, enabling the micro-machining of small parts.

[0010] The present invention provides a leaded brass alloy, wherein the leaded brass has the following mass percentage composition: Cu: 62-65%, Pb: 2.4-3%, Fe: 0.03-0.1%, Al: 0.001-0.1%, Cr: 0.01-0.2%, Ni: 0.07-0.5%, with the balance being Zn and unavoidable impurities.

[0011] Cu: The Cu content affects the ratio of α and β phases in the brass structure. When the Cu content is less than 62%, the proportion of β phase increases, which increases the hardness and brittleness of brass. However, when the Cu content exceeds 65%, the structure at room temperature is a single-phase α phase, and only a small amount of β phase appears in the structure at high temperatures above 650℃. Due to the presence of Pb, it is very easy to crack during hot working due to the hot brittleness of Pb.

[0012] Pb: Pb is distributed as free particles in brass at grain boundaries or within grains, which improves the machinability of brass but reduces its plasticity. The morphology, quantity and distribution ratio of the Pb phase also affect the machinability of leaded brass. In the leaded brass provided by this invention, the Pb content is maintained in a relatively high proportion range of 2.4-3%.

[0013] Fe: Fe has very low solubility in brass and exists mainly in the form of hard particles, which act as nucleation sites. Therefore, an appropriate amount of Fe can control the grain size of brass. In the brass of this invention, the Fe content is controlled at 0.03-0.1%. However, if the Fe content exceeds 0.1%, the size of the hard particles will increase, which will affect the machinability.

[0014] Cr: The role of Cr is to hinder grain growth during alloy annealing, control the grain size of the alloy, and improve the hardness of the alloy after annealing. When the Cr content exceeds 0.2%, it has a more significant effect on reducing the plasticity of the alloy. In the brass provided by this invention, the Cr content is controlled at 0.01-0.2%.

[0015] Ni: The atomic radius of Ni is 0.125 nm, and that of Cu is 0.128 nm. Both Cu and Ni are FCC structure crystals and are infinitely miscible. Ni can hinder the aggregation and growth of the Pb phase and promote the dispersed distribution of Pb particles. However, if the Ni content exceeds 0.5%, the inhibition of Pb phase aggregation and growth will actually worsen. In the brass of this invention, the Ni content is controlled at 0.07-0.5%.

[0016] Preferably, the microstructure of the leaded brass comprises an α phase, a β phase, and a Pb phase, wherein the α phase is the matrix phase, and the β and Pb phases are distributed within the α phase. The average size of the Pb phase is ≤1.5μm, the maximum size of the Pb phase is ≤3μm, and the number of Pb phases per unit area is 23,000-32,000 / mm². 2 The Pb particles provided by this invention are relatively fine and uniformly distributed at grain boundaries and within grains, so that during processing, they are cut into powder form, making it easy to remove chips when drilling small holes.

[0017] Preferably, the β phase accounts for 0.1-10% of the area of ​​the leaded brass. Since the solubility of Pb in the β phase is 42 times that of the α phase, a certain proportion of β phase is present in the room temperature structure, and a higher proportion of β phase will appear in the high temperature structure. An appropriate amount of β phase can play a role in encapsulating the Pb phase and reducing the thermal brittleness of Pb.

[0018] Preferably, the grain size of the α phase is ≤10μm, and the number of α phase grains per unit area is 1500-3400 / mm. 2 .

[0019] Since Pb particles are more abundant at grain boundaries than within grains, this invention controls grain size to achieve a larger total grain boundary area and a more uniform distribution of Pb particles. Furthermore, smaller grain size results in higher strength and hardness of the leaded brass alloy provided by this invention, a greater number of grains per unit volume, and a greater number of grains participating in deformation. This leads to better resistance to plastic deformation caused by cutting tools on the machined surface of the leaded brass alloy provided by this invention, thereby making the dimensional accuracy of the leaded brass alloy less prone to change during cutting and reducing the dimensional tolerance fluctuation range.

[0020] Preferably, the lead-brass alloy provided by the present invention has a tensile strength Rm of 400-520 MPa and a yield strength Rp. 0.2 Strength: 320-450 MPa, elongation A50: 12-20%, Vickers hardness HV5: 120-150. Besides microstructure, the mechanical properties of a material directly affect its machinability. Low strength and hardness, coupled with good plasticity, make the material prone to deformation during machining. Furthermore, due to its softness, copper chips easily stick to the cutting tool, affecting the machining accuracy of the parts. Conversely, high strength and hardness, with poor plasticity, result in high cutting forces, high cutting temperatures, and rapid tool wear, also affecting the machining accuracy of the parts.

[0021] On the other hand, the present invention provides a method for preparing the leaded brass alloy, wherein the process flow of the method for preparing the leaded brass includes: smelting → horizontal continuous casting → homogenization annealing → extrusion → intermediate plate drawing → bottom annealing → combined drawing;

[0022] The lead-brass alloy is prepared and smelted according to the mass percentage of each element.

[0023] The homogenization annealing temperature is 560-700℃, and the holding time is 3-6h.

[0024] Because the Cu and Pb elements provided by this invention are high in content, and Pb is insoluble in the α phase, a large amount of Pb elements accumulate at the α phase boundary. During subsequent hot working, this can easily lead to hot brittle fracture. Therefore, the alloy structure after continuous casting is homogenized by annealing before the extrusion process. Homogenization annealing allows a higher Zn content in the β phase to diffuse into the α phase, making the Zn content in the α phase uniform. This facilitates the α→β phase transformation, increases the amount of β phase in the high-temperature structure of the alloy during hot extrusion, and allows more Pb to dissolve in the β phase, thus avoiding hot brittle fracture of Pb.

[0025] Preferably, rare earth elements are added during smelting, with the amount of rare earth elements added being 0.005-0.03% of the mass of the molten copper. Further, the rare earth element is La. Rare earth elements play two roles: firstly, they refine the Pb phase, increasing the number of Pb particles and ensuring their uniform dispersion in the matrix, thereby improving the alloy's machinability; secondly, they improve the alloy's hot plasticity and reduce its tendency to crack during hot extrusion.

[0026] Preferably, the melting is carried out in an induction furnace at a melting temperature of 1000-1100℃. After all the metal has melted and the composition is found to be qualified, it is held at a temperature of 1030-1070℃, which is also the casting temperature. This invention adds an appropriate amount of Cr, which has a higher melting point, to form nucleation particles during the melting process, refining the ingot's crystalline structure. Cr and Ni also increase the recrystallization temperature of the alloy; their synergistic effect hinders recrystallized grain growth, which is beneficial for forming smaller grain sizes.

[0027] Preferably, the cooling water inlet temperature of the horizontal continuous casting is 10-32℃, the cooling water pressure is 0.3-0.6Mpa, the outlet temperature is 18-42℃, the traction speed is 100-400mm / min, and the ingot sawing length is 500-1000mm.

[0028] Preferably, the extrusion speed is 3-8 mm / s. Since a large amount of Pb is distributed at the grain boundaries, the billet is prone to hot brittle fracture under frictional forces during hot extrusion. Therefore, slow extrusion avoids the hot brittle cracking of Pb in high-Cu content leaded brass during the extrusion process. Furthermore, slow extrusion results in uniform metal flow, relatively uniform temperature distribution, uniform dynamic recrystallization structure, and uniform Pb phase distribution. Slow extrusion reduces the extrusion deformation heat of the alloy, preventing excessively high actual dynamic recrystallization and coarse grains during the extrusion process. Additionally, the billet outflow speed is correspondingly slower during slow extrusion, reducing the friction between the high-temperature extruded billet and the wire groove, making the billet surface less prone to tearing.

[0029] More preferably, the extrusion flow rate is 1-3, the extrusion temperature is controlled at 600-700℃, and the extrusion ratio is controlled at 300-550.

[0030] Preferably, the processing rate of the intermediate plate is 18-35%. For the following small-specification products, after pickling the extruded billet, it is stretched to the intermediate specification on a wire drawing machine. The processing rate needs to be controlled between 18% and 35%. If the coiling processing rate is lower than 18%, the deformation of the billet structure will be uneven. The energy stored in different parts due to lattice distortion will vary, resulting in different recrystallization driving forces and uneven recrystallized grain size during under-annealing. As the degree of cold deformation increases, when the coiling processing rate exceeds 35%, the deformation of the billet structure is very sufficient, the original grains of the extruded billet are completely broken, the number of recrystallization nuclei increases significantly, the recrystallized grains during under-annealing are fine, the resistance to cold heading deformation increases, and cracking is more likely to occur.

[0031] Preferably, the bottom annealing temperature is 350-420℃, and the holding time is 4-8 hours. Further, the annealed wire rod is pickled to remove the surface oxide scale.

[0032] The leaded brass of this invention requires a low-temperature, slow annealing process. The annealing temperature is 350-420℃, and it is positively correlated with the Cr content. When the Cr content is in the lower-middle range of 0.01-0.1%, the annealing temperature is 350-380℃; when the Cr content is in the upper-middle range of 0.1-0.2%, the annealing temperature is 380-420℃. The temperature should be increased from room temperature, and after reaching the set temperature, the holding time should be extended to 4-8 hours. This is to avoid the Pb phase from easily fusing and growing during high-temperature annealing. Lower-temperature annealing allows the Pb phase to remain as fine spherical structures within the extruded structure. If the annealing temperature is below 350℃, the recrystallization process is very slow, and recrystallization is difficult to complete. When the annealing temperature exceeds 420℃, the average grain size will exceed 10μm. If the holding time is less than 4 hours, the uniformity of the recrystallized grains is poor; if the holding time exceeds 8 hours, the grains gradually grow and easily exceed 10μm. Within the range provided by this invention, the higher the Cr and Ni content, the smaller the recrystallized grains.

[0033] Preferably, the combined drawing process has a processing rate of 9-20%. For the above small-sized products, the billet is directly drawn into the finished product. According to the mechanical performance requirements of the finished product, the processing rate is controlled between 9-20%. If the processing rate of the finished product is less than 9%, the tensile strength and hardness of the material are low; if the processing rate of the finished product exceeds 20%, the hardness of the material is high, and it is easy to crack during processing.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] This invention improves machinability by adding a high proportion of Cu and Pb, resulting in a greater distribution of free Pb particles at grain boundaries and within grains. It also controls the grain size of the leaded brass by adding a small amount of Cr, increasing the number of grains per unit volume. This results in a greater number of grains participating in deformation during machining, resisting the impact of the tool on the machined surface and making plastic deformation controllable. Consequently, the dimensional accuracy of the leaded brass alloy provided by this invention is less affected during machining, with a smaller range of dimensional tolerance fluctuations. Furthermore, the addition of an appropriate amount of Ni, working synergistically with a small amount of Cr, inhibits the aggregation and growth of the Pb phase at grain boundaries and within smaller grains, promoting the dispersed distribution of Pb particles. This results in powdery chips during machining, making chip removal easier when drilling small holes. Attached Figure Description

[0036] Figure 1 The morphology and distribution of the Pb phase under a 500X microscope provided in Embodiment 1 of the present invention;

[0037] Figure 2 The photographs show the morphology and distribution of the Pb phase under a 500X microscope, which are provided in Comparative Example 1 of this invention.

[0038] Figure 3 Metallographic images provided in Embodiment 1 of the present invention;

[0039] Figure 4 The metallographic image provided for Comparative Example 1 of this invention. Detailed Implementation

[0040] The present invention will be further described in detail below with reference to the embodiments.

[0041] This invention provides 5 embodiments and 5 comparative examples, and the specific components are shown in Table 1.

[0042] The preparation steps of the example are as follows:

[0043] 1) Smelting: Prepare the ingredients according to the required composition, and the smelting temperature is 1000~1090℃.

[0044] 2) Horizontal continuous casting: Ingot specifications are Φ145~254mm, casting temperature is 1020~1090℃, cooling water inlet temperature is 10~32℃, cooling water pressure is 0.3~0.6Mpa, outlet water temperature is 18~42℃, traction speed is 100-400mm / min, and ingot sawing length is 500~1000mm. The ingot homogenization annealing temperature is 560-700℃, and the holding time is 3-6h.

[0045] 3) Extrusion: The extruded billet specifications are Φ4-16mm, the extrusion flow rate is 1~3, the extrusion temperature is 600~700℃, and the extrusion ratio is 440~580.

[0046] 4) Intermediate plate pulling: The processing rate is 15-25%.

[0047] 5) Annealing with residual heat: The annealing temperature is 350-420℃, starting from room temperature and heating up, and the holding time is 4-8 hours.

[0048] 6) Combined drawing: The processing rate is 8-30%, and the straightness of the bar is 0.05-0.2 mm / m.

[0049] The Pb phase distribution in the leaded brass prepared in Example 1 is shown in the photograph. Figure 1 As shown, the black dots are Pb phases, with an average size of only 0.97 μm and a maximum size of 2.45 μm. The number of Pb phases distributed reaches 26,443 per mm. 2 The leaded brass conforms to the requirements of this invention regarding the size and distribution of the Pb phase: average Pb phase size ≤ 1.5 μm, maximum Pb phase size ≤ 3 μm, and the number of Pb phases per unit area is 23,000-32,000 / mm². 2 Metallographic structure of the product, such as Figure 3 As shown, the black spots represent the β phase, which accounts for 7.80% of the total area, has a grain size of 7.5 μm, and a grain count of 2943 pic / mm. 2 The leaded brass conforms to the requirements of this invention regarding the β phase distribution ratio, grain size, and number of grains per unit area: the β phase area ratio is 0.1-10%, the grain size is ≤10μm, and the number of grains is 1500-3400 / mm. 2 .

[0050] 7) Finished product inspection.

[0051] Key process parameters are shown in Tables 2, 3, and 4.

[0052] Comparative Example 1 is HPb63-3 purchased from the market. Bar stock, such as Figure 2 As shown, the Pb phase has a large size, with an average size of 3.6 μm and a maximum size of 4.85 μm; Figure 4 As shown, the α phase has a coarse grain size, reaching 28.6 μm, and a small number of α phase grains, only 852 per mm. 2 The β phase accounts for a relatively large area, reaching 11.30%, which exceeds 10%.

[0053] Comparative Example 2: No Cr element was added. The remaining components and preparation process were the same as in Example 1. The purpose was to compare the effect of not adding Cr element on the alloy microstructure and properties.

[0054] Comparative Example 3: No Ni element was added. The remaining components and preparation process were the same as in Example 1. The purpose was to compare the effect of not adding Ni element on the alloy microstructure and properties.

[0055] Comparative Example 4: The alloy was smelted without the addition of rare earth element La, and the rest was the same as in Example 1. The purpose was to compare the effect of smelting without adding La on the alloy microstructure and properties.

[0056] Comparative Example 5: The annealing temperature was 540°C, and the rest was the same as in Example 1. The purpose was to compare the effects of different annealing temperatures on the alloy microstructure and properties.

[0057] The microstructure of the obtained examples and comparative examples was tested as follows, and the results are recorded in Table 5. In the five examples, the α phase grain size was 6.8-9.1 μm, all ≤10 μm, and the α phase grain number was 2144-2943 / mm. 2 Both are between 23,000 and 32,000 pieces / mm 2 Within this range, the average size of the Pb phase is 0.89–1.40 μm, all ≤1.5 μm; the maximum size of the Pb phase is 2.12–2.91 μm, all ≤3 μm; and the number of Pb phases per unit area is 23,765–26,443 / mm². 2 All are in the range of 23,000-35,000 pieces / mm 2 Within this range, the area proportion of the β phase was 0.69-8.42%, all within the range of 0.1-10%; in the five comparative examples, the grain size of the α phase was 13.7-30.4 μm, all exceeding 10 μm, and the number of α phase grains was 710-1353 / mm. 2 All are below 1500 / mm 2 The average size of the Pb phase ranged from 2.90 to 5.42 μm, all exceeding 1.5 μm. The maximum size of the Pb phase ranged from 4.69 to 7.09 μm, all exceeding 3 μm. The number of Pb phases per unit area was 10,645 to 18,350 per mm. 2 All are below 23,000 per mm 2 The area of ​​the β phase ranges from 9.12% to 13.93%, with some areas exceeding 10%.

[0058] Grain size, grain number, β phase size and area ratio, and Pb phase distribution were observed under a microscope.

[0059] The following performance tests were performed on 6 examples and 6 comparative examples, and the results are recorded in Table 6. Among the measured mechanical properties of the 5 examples, the tensile strength Rm was 465-511 MPa, all within the range of 400-520 MPa, and the yield strength Rp... 0.2The strength ranges from 357 to 410 MPa, all within the range of 320 to 450 MPa; the elongation (A50) is 12.6% to 16.3%, all within the range of 12% to 20%; the hardness (HV5) is 133 to 145, all within the range of 120 to 150. Compared to the previous HPb63-3, the cutting index is 105 to 110, all exceeding 100, indicating superior machinability. Among the five comparative mechanical properties measured, the tensile strength (Rm) is 420 to 476 MPa, also within the range of 400 to 520 MPa, and the yield strength (Rp) is... 0.2 The strength ranges from 302 to 351 MPa, with some exceeding the 320-450 MPa range. The elongation (A50) is 14.8-16.8%, all within the 12-20% range. The hardness (HV5) is 129-137, all within the 120-150 range. Compared to the previous HPb63-3, the cutting index is 94-103, indicating that the cutting performance is inferior to that of the example.

[0060] Tensile strength, yield strength Rp0.2 and elongation: tested in accordance with GB / T228.1~2021 "Metallic materials - Tensile testing - Part 1: Test method at room temperature".

[0061] Hardness HV5: Tested according to GB / T4340.1~2009 "Metallic materials Vickers hardness test - Part 1: Test method".

[0062] Cutting performance: The cutting force was calculated based on the cutting force tester and compared with that of HPb63-3, then determined according to the formula: The cutting performance index was obtained, and the specific data are shown in Table 6.

[0063] Table 1. Components (wt%) of the embodiments and comparative examples of the present invention

[0064]

[0065] Table 2 Casting parameter control of embodiments and comparative examples of the present invention

[0066]

[0067] Table 3. Extrusion process parameter control of embodiments and comparative examples of the present invention.

[0068]

[0069]

[0070] Table 4. Control of downstream processing parameters in the embodiments and comparative examples of the present invention.

[0071]

[0072] Table 5. Microstructure of the embodiments and comparative examples of the present invention.

[0073]

[0074]

[0075] Table 6 Mechanical and cutting properties of embodiments and comparative examples of the present invention

[0076]

Claims

1. A lead-brass alloy, characterized in that, The leaded brass has the following mass percentage composition: Cu: 62.2-65%, Pb: 2.4-3%, Fe: 0.03-0.1%, Al: 0.001-0.1%, Cr: 0.01-0.2%, Ni: 0.07-0.5%, with the balance being Zn and unavoidable impurities; The microstructure of the leaded brass comprises an α phase, a β phase, and a Pb phase. The α phase is the matrix phase, while the β and Pb phases are distributed within the α phase. The average size of the Pb phase is ≤1.5 μm, the maximum size of the Pb phase is ≤3 μm, and the number of Pb phases per unit area is 23,000-32,000 / mm². 2 ; The grain size of the α phase is ≤10μm; The β phase accounts for 0.1-10% of the area of ​​the leaded brass; The number of α-phase grains per unit area is 1500-3400 / mm. 2 .

2. A method for preparing the lead-brass alloy according to claim 1, characterized in that, The process flow of the leaded brass preparation method includes: smelting → horizontal continuous casting → homogenization annealing → extrusion → intermediate plate drawing → bottom annealing → combined drawing; The lead-brass alloy is prepared and smelted according to the mass percentage of each element as described in claim 1. The homogenization annealing temperature is 560-700℃, and the holding time is 3-6h; The annealing temperature is 350-420℃, and the holding time is 4-8h.

3. The method for preparing the lead-brass alloy according to claim 2, characterized in that, The extrusion speed is 3-8 mm / s.

4. The method for preparing the lead-brass alloy according to claim 2, characterized in that, The processing rate of the intermediate plate is 18-35%.

5. The method for preparing the lead-brass alloy according to claim 2, characterized in that, Rare earth elements are added during smelting, with the amount of rare earth elements added being 0.005-0.03% of the mass of the molten copper.

6. The method for preparing the lead-brass alloy according to claim 2, characterized in that, The melting temperature is 1000-1100℃, and the holding temperature is 1030-1070℃.

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