A laser additive manufacturing method for low-defect cemented carbide

Abstract

A laser additive manufacturing method for low-defect cemented carbide belongs to the field of cemented carbide additive manufacturing technology. WC-Co composite powder is first agglomerated and granulated, then mechanically mixed with the agglomerated and granulated Co powder. Selective laser melting equipment is used to print the mixed powder layer by layer. The aggregated Co in the mixed powder melts and fills the pores between the powder particles, reducing the stress gradient between the metal and ceramic phases during solidification. The result is a printed, low-defect, special-shaped cemented carbide workpiece with excellent overall mechanical properties.

Description

Technical Field

[0001] The present invention belongs to the technical field of cemented carbide additive manufacturing, and specifically relates to a process for preparing a cemented carbide workpiece with no microcracks and few pores in one step by using laser 3D printing. Background Art

[0002] Cemented carbide with WC-Co as the main component is an indispensable tool material in modern manufacturing industry and is known as the "teeth of industry". The traditional process route for manufacturing cemented carbide parts mainly includes two processes: "powder metallurgy forming" and "subtractive manufacturing". That is, the raw material powders such as WC and Co mixed with organic binders are first pressed into a blank with a certain shape, and then sintered at high temperature to form it. Finally, it is ground to make its shape, size and surface quality meet the design requirements. The existing method has the advantages of mature technology and easy large-scale production in the preparation of cemented carbide parts with regular shapes. However, it is difficult to obtain blanks with complex shapes or hollow structures by compression molding, and the hardness of cemented carbide after sintering is extremely high, and further subtractive processing is very difficult. This greatly restricts the application of cemented carbide materials in special-shaped components that require high hardness, high strength and toughness.

[0003] The 3D printing technology developed in recent years can achieve rapid near-net forming of cemented carbide parts with complex shapes and structures. Selective laser melting technology is one of the more commonly used methods for printing special-shaped cemented carbide parts. Its main principle is to use a high-energy laser beam to selectively melt metal or metal-ceramic powder layer by layer according to contour data, and to manufacture three-dimensional solid parts by layer-by-layer powder laying, melting, solidification and accumulation. However, when printing cemented carbide by selective laser melting, due to the significant differences in melting point and thermal expansion coefficient between the ceramic phase WC and the metal phase Co, microcracks, holes and other structural defects are easily generated in the printed parts, making it difficult to obtain high mechanical properties. Although these defects can be removed and the performance of the alloy can be improved through subsequent heat treatment, the increase in processing steps weakens the advantages of the 3D printing process. How to print a cemented carbide workpiece with dense structure and good comprehensive performance in one step is an important challenge facing this field.

[0004] To address the above problems, the present invention innovates in the design of 3D printing powder structure and printing process, and proposes a method for printing and preparing almost defect-free cemented carbide blocks based on the selective laser melting process. That is, the aggregation state of Co in the printed powder is regulated, the stress gradient between the metal-ceramic phase during cooling is reduced, and the fluidity of the molten Co is improved to fully fill the pores between the powder particles, thereby printing and obtaining low-defect cemented carbide parts. Summary of the Invention

[0005] The process flow and principle of the preparation method provided by the present invention are as follows: first, WC-Co composite powder is agglomerated and granulated, then mechanically mixed with the agglomerated and granulated Co powder, and the mixed powder is printed layer by layer using a selective laser melting device. The aggregated Co contained in the mixed powder is melted to fill the pores between the powder particles, thereby reducing the stress gradient between the metal and ceramic phases during solidification, thereby printing a special-shaped cemented carbide workpiece with low defects and good comprehensive mechanical properties.

[0006] The present invention provides a laser additive manufacturing method for low-defect cemented carbide, characterized by comprising the following steps:

[0007] (1) Using tungsten trioxide, cobalt trioxide, carbon black and polyethylene glycol as raw materials, the ingredients are prepared according to the Co content of 3%-10% and the total carbon content of 5.5%-5.9% in the WC-Co composite powder synthesized by the final reaction, and using anhydrous ethanol as the medium. The raw materials are fully wet-ground and dried, and then cold-pressed into cylindrical blanks with a diameter of 20-40 mm and a height of 10-30 mm. The blanks are then placed in a vacuum furnace at 1000-1100°C for chemical reaction. The cylindrical blocks obtained by the reaction are ball-milled in a stirred grinder using anhydrous ethanol as the medium for 10-15 hours, and after drying, WC-Co composite powder with an average particle size of 0.1-0.5 μm is obtained;

[0008] (2) agglomerating and granulating the WC-Co composite powder and pure Co powder with a particle size of less than 1.0 μm using a high-speed centrifugal atomization drying process, and obtaining spherical WC-Co composite powder and spherical Co powder with a particle size of 5-38 μm after sieving;

[0009] (3) The spherical WC-Co composite powder and the spherical Co powder are mechanically stirred in proportion until they are uniformly mixed, wherein the total mass proportion of the Co element in the mixed powder is 10%-15% (including the Co in the WC-Co composite powder and the Co in the spherical Co powder);

[0010] (4) Using the mixed powder as raw material, the selective laser melting process is adopted to perform powder printing layer by layer. The laser beam spot size is 100 μm, the laser power is 200-260 W, the laser scanning rate is 550-700 mm / s, and the powder thickness is 30 μm. According to the above process, cemented carbide parts with low defect density are printed.

[0011] The technical features and advantages of the method of the present invention are mainly as follows: (1) The present invention directly heats the powder based on the selective laser melting process, which can realize one-time high-precision forming of cemented carbide parts and obtain a crack-free and almost hole-free microstructure; (2) The Co in the aggregated state partially replaces the Co uniformly distributed in the WC-Co composite powder. Under appropriate printing process parameters, the melting of the aggregated Co can effectively fill the pores between the powder particles and reduce the stress gradient between the metal and ceramic phases during solidification, thereby preparing special-shaped cemented carbide parts with low defects and good comprehensive mechanical properties; (3) The combination of 3D printing powder structure design and laser printing process proposed in the present invention solves the technical problem that it is difficult to remove microstructure defects such as pores and cracks in metal-ceramic composite materials during printing. The method is highly operational and easy to scale up. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 The scanning electron microscope morphologies of the spherical WC-Co powder and spherical Co powder prepared in the present invention; wherein, (a) is the scanning electron microscope morphology of the spherical WC-Co powder prepared in Example 1, and (b) is the scanning electron microscope morphology of the spherical Co powder prepared in Example 1;

[0013] Figure 2 The scanning electron microscope microstructures of the cemented carbide workpieces prepared by printing according to the present invention are shown; wherein, (a) is the scanning electron microscope microstructure of the cemented carbide workpiece prepared by printing according to Example 1, (b) is the scanning electron microscope microstructure of the cemented carbide workpiece prepared by printing according to Example 2, and (c) is the scanning electron microscope microstructure of the cemented carbide workpiece prepared by printing according to Example 3;

[0014] Table 1 shows the density and microhardness of the cemented carbide workpieces prepared by printing according to the present invention. DETAILED DESCRIPTION

[0015] The present invention will be further described below with reference to the examples, but the present invention is not limited to the following examples.

[0016] Example 1

[0017] Tungsten trioxide, cobalt trioxide, carbon black and polyethylene glycol are used as raw materials, and the WC-Co composite powder synthesized by the final reaction has a Co content of 10% and a total carbon content of 5.5%. The raw materials are fully wet-ground and dried with anhydrous ethanol as the medium, and then cold-pressed into a cylindrical blank with a diameter of 40 mm and a height of 10 mm. The cylindrical blank is then placed in a vacuum furnace at 1100°C for a chemical reaction, and the cylindrical block obtained by the reaction is ball-milled for 15 hours and dried with anhydrous ethanol as the medium to obtain a WC-Co composite powder with an average particle size of 0.1-0.5 μm. The WC-Co composite powder is dried using a high-speed centrifugal atomization process. and pure Co powder with a particle size of less than 1.0 micron are agglomerated and granulated respectively, and spherical WC-Co composite powder and spherical Co powder with a particle size of 5-38 μm are obtained after sieving; the spherical WC-Co composite powder and spherical Co powder are fully mechanically stirred in proportion until they are evenly mixed, wherein the total mass proportion of Co element in the mixed powder is 15%; the above mixed powder is used as raw material, and the selective laser melting process is adopted for layer-by-layer printing, with a laser beam spot size of 100 μm, a laser power of 260 W, and a laser scanning rate of 700 mm / s. According to the above process, the designed defect-free cemented carbide parts are printed. Figure 1 (a) and (b) are the scanning electron microscope morphologies of the spherical WC-Co powder and spherical Co powder prepared in Example 1, respectively. Figure 2 (a) is the scanning electron microscope microstructure of the cemented carbide workpiece obtained by printing in Example 1. The density and microhardness of the cemented carbide workpiece prepared by printing in Example 1 are listed in Table 1.

[0018] Example 2

[0019] Tungsten trioxide, cobalt trioxide, carbon black and polyethylene glycol are used as raw materials, and the WC-Co composite powder synthesized by the final reaction has a Co content of 7% and a total carbon content of 5.7%. The raw materials are fully wet-ground and dried with anhydrous ethanol as a medium, and then cold-pressed into a cylindrical blank with a diameter of 30 mm and a height of 20 mm. The cylindrical blank is then placed in a vacuum furnace at 1050°C for a chemical reaction. The cylindrical block obtained by the reaction is ball-milled for 12 hours and dried with anhydrous ethanol as a medium to obtain a WC-Co composite powder with an average particle size of 0.1-0.5 μm. The WC-Co composite powder and the WC-Co composite powder are dried using a high-speed centrifugal atomization drying process. Pure Co powder with a particle size of less than 1.0 micron was agglomerated and granulated separately, and after screening, spherical WC-Co composite powder and spherical Co powder with a particle size of 5-38 μm were obtained; the spherical WC-Co composite powder and spherical Co powder were fully mechanically stirred in proportion until they were evenly mixed, wherein the total mass proportion of Co element in the mixed powder was 12%; the above mixed powder was used as raw material, and the selective laser melting process was adopted for layer-by-layer printing, with a laser beam spot size of 100 μm, a laser power of 230 W, and a laser scanning rate of 620 mm / s. According to the above process, the designed defect-free cemented carbide parts were printed. Figure 2 (b) is the scanning electron microscope microstructure of the cemented carbide workpiece obtained by printing in Example 2. The density and mechanical properties of the cemented carbide workpiece prepared by printing in Example 2 are listed in Table 1.

[0020] Example 3

[0021] Tungsten trioxide, cobalt trioxide, carbon black and polyethylene glycol are used as raw materials, and the WC-Co composite powder synthesized by the final reaction has a Co content of 3% and a total carbon content of 5.9%. The raw materials are fully wet-ground and dried with anhydrous ethanol as the medium, and then cold-pressed into a cylindrical blank with a diameter of 20 mm and a height of 30 mm. The cylindrical blank is then placed in a vacuum furnace at 1000°C for chemical reaction, and the cylindrical block obtained by the reaction is ball-milled for 10 hours. After drying with anhydrous ethanol as the medium, a WC-Co composite powder with an average particle size of 0.1-0.5 μm is obtained. The WC-Co composite powder and the average particle size of the WC-Co composite powder are dried by a high-speed centrifugal atomization process. Pure Co powder with a particle size of 0.6-1.0 microns was agglomerated and granulated respectively, and after screening, spherical WC-Co composite powder and spherical Co powder with a particle size of 5-38 μm were obtained; the spherical WC-Co composite powder and spherical Co powder were fully mechanically stirred in proportion until they were evenly mixed, wherein the total mass proportion of Co element in the mixed powder was 10%; the above mixed powder was used as raw material, and the selective laser melting process was adopted for layer-by-layer printing, with a laser beam spot size of 100 μm, a laser power of 200 W, and a laser scanning rate of 550 mm / s. According to the above process, the designed defect-free cemented carbide parts were printed. Figure 2(c) is the scanning electron microscope microstructure of the cemented carbide workpiece obtained by printing in Example 3. The density and microhardness of the cemented carbide workpiece prepared by printing in Example 3 are listed in Table 1.

[0022] Table 1

[0023]

Claims

1. A laser additive manufacturing method for low-defect cemented carbide, characterized in that: The following steps are involved: (1) Using tungsten trioxide, cobalt trioxide, carbon black and polyethylene glycol as raw materials, the ingredients are prepared according to the Co content of 3%-10% and the total carbon content of 5.5%-5.9% in the WC-Co composite powder synthesized by the final reaction, and using anhydrous ethanol as the medium. The raw materials are fully wet-ground, dried and then cold-pressed into cylindrical blanks, which are then placed in a vacuum furnace at 1000-1100°C for chemical reaction. The cylindrical blocks obtained by the reaction are ball-milled in a stirred grinder using anhydrous ethanol as the medium for 10-15 hours, and after drying, WC-Co composite powder with an average particle size of 0.1-0.5 μm is obtained; (2) agglomerating and granulating the WC-Co composite powder and pure Co powder with a particle size of less than 1.0 μm using a high-speed centrifugal atomization drying process, and obtaining spherical WC-Co composite powder and spherical Co powder with a particle size of 5-38 μm after sieving; (3) The spherical WC-Co composite powder and the spherical Co powder are mechanically stirred in proportion until they are uniformly mixed; (4) Using the mixed powder as raw material, the selective laser melting process is adopted to perform powder printing layer by layer. The laser beam spot size is 100 μm, the laser power is 200-260 W, the laser scanning rate is 550-700 mm / s, and the powder thickness is 30 μm. According to the above process, cemented carbide parts with low defect density are printed.

2. The laser additive manufacturing method for low-defect cemented carbide according to claim 1, characterized in that: Step (1) The diameter of the cylindrical blank is 20-40 mm and the height is 10-30 mm.

3. The laser additive manufacturing method for low-defect cemented carbide according to claim 1, characterized in that: The total mass proportion of the Co element in the mixed powder is 10%-15%, including the Co in the WC-Co composite powder and the Co in the spherical Co powder.

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