RU2845808C1 - Method of hard-alloy products production of specified shape from granulated powder - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to powder metallurgy, in particular to a method for producing hard-alloy products of a given shape using 3D printing. According to the product drawing, taking into account the workpiece deformation during pressing, a mould is made from plastic with strength of more than 50 MPa and Young's modulus of more than 1 GPa. Mould made of plastic is pressed into steel shell, filled with granulated powder and pressed at the pressure of more than 100 MPa. Moulded workpiece is removed from the mould and sintered.

EFFECT: higher hardness, density and strength of obtained hard-alloy product.

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Description

The invention relates to the field of powder metallurgy, in particular to the production of samples of hard-alloy products of a given shape using 3D printing.

There are studies [1, 2, 3] describing the production of hard alloy products using the methods of selective laser melting, selective laser sintering and electron beam melting.

The resulting products have reduced hardness and strength due to high porosity and changes in chemical composition. High temperatures (>2000°C) are used during alloy sintering, causing changes in composition.

Another approach is to use binder jet 3D printing [4], fused deposition modeling (FDM) [5] or gel 3D printing [6] to produce blanks that are then sintered. It is also proposed to use plastic molds made by FDM [7] for slip casting of hard-alloy and ceramic blanks that are then sintered using conventional methods. Mixtures of hard-alloy powders and special flowable or low-melting materials are used as raw materials.

Due to the absence of pressure, the density of the blanks and sintered samples obtained in this way is lower than the density of blanks and samples obtained using conventional technology.

The closest to the proposed method of manufacturing hard-alloy products is the method of manufacturing high-density hard-alloy parts of a given shape and cutting tools by gel casting into molds made by 3D printing from plastic according to the product drawing [8]. A suspension containing tungsten-cobalt powder is injected into the mold of the part obtained from plastic and then extracted at elevated temperature and pressure. The resulting workpiece is sintered by hot static pressing.

Due to the lack of pressure, the density of the blanks and sintered samples obtained in this way is lower than the density of blanks and samples obtained using conventional technology. The use of hot isostatic pressing leads to an increase in the cost of the sintering process.

The technical result of the proposed method for forming hard-alloy blanks consists in reducing the cost of producing individual hard-alloy products, ensuring their high density, hardness and strength by using pressing to compact the blank in the process of forming hard-alloy mixtures used in mass production in plastic forms enclosed in a steel shell, manufactured on a 3D printer MMPN in accordance with the drawings, with subsequent sintering of products from the obtained blanks in a furnace using the methods usual for these grades of alloys.

The technical result consists in ensuring high technological efficiency by using a mold for pressing, pre-designed taking into account shrinkage and deformation based on a product drawing, manufactured by MMPN using a 3D printer from plastic with sufficient rigidity and strength, and pre-placed in a steel shell to increase rigidity and strength.

The technical result is achieved in that in the method for producing hard-alloy products of a given shape from granulated hard-alloy powder, including developing a mold according to a product drawing taking into account the shrinkage of the workpiece during sintering, manufacturing a mold using 3D printing from plastic and sintering the workpiece, developing a mold from plastic according to a product drawing also taking into account the deformation of the workpiece during pressing, using plastic with a strength of more than 50 MPa and a Young's modulus of more than 1 GPa, and pressing the plastic mold into a steel shell, pressing the granulated hard-alloy powder at a pressure of more than 100 MPa, separating the workpiece from the mold and sintering according to the method used for a given alloy grade.

The ability to form the required sequence of actions performed by the proposed means allows solving the assigned task, determines the novelty, industrial applicability and inventive level of development.

The method for producing hard alloy products of a given shape from granulated powder is shown in the drawings.

Fig. 1 is a general view of a hard-alloy product of a given shape made from granulated powder, example 1; Fig. 2 is a general view of a hard-alloy product of a given shape made from granulated powder, example 2.

The proposed method for producing hard-alloy products of a given shape from granulated powder includes several stages. Development of the mold design based on the drawing of the hard-alloy product, taking into account the deformation of the workpiece during pressing and shrinkage during sintering. The shape of the matrix should allow it to be pressed into a pre-fabricated steel shell of a simple shape manually.

1) Manufacturing a mold using 3D printing by the fusion deposition method with 100% filling from plastic with satisfactory strength (>50 MPa) and rigidity (Young's modulus of at least GPA), and pressing it into a steel shell.

2) Pressing of granulated hard alloy powder mixture into a plastic mold pressed into a steel shell.

3) Separation of the workpiece from the plastic mold and sintering of the product under conditions corresponding to the alloy grade.

Development of a plastic mold

The development of a plastic mold model is carried out in the automated design system based on the drawing of the hard-alloy part planned for production. When designing a plastic mold, it is necessary to take into account the dimensions of the resulting workpiece in order to compensate for the shrinkage of the workpiece during sintering and the elastic deformation of the plastic mold. To do this, it is necessary to know the relative density of the workpiece obtained during pressing under specified conditions. The external dimensions of the plastic mold matrix must correspond to the steel shell used. A steel cylinder with a hole in which the plastic mold matrix will be placed can be used as a shell.

Making a mold from plastic

At the second stage, a mold is made using MMPN. For this, inexpensive plastic is used, providing sufficient strength, precision and surface quality. The steel shell allows the use of polylactide or acrylonitrile-butadiene-styrene, the compressive strength of which is more than 50 MPa, Young's modulus - more than 1 GPa. To achieve maximum strength of plastic, it is necessary to use 100% filling of the mold during printing.

Pressing the workpiece

The resulting form is placed in a steel shell by pressing to increase rigidity and strength. To simplify the subsequent separation of the blank from the mold, a release agent based on stearic acid can be applied to the matrix and punch. Then, a pre-prepared granulated hard-alloy powder mixture is poured into the mold and the blank is pressed at a pressure of at least 100 MPa to achieve the required density of the blank. To improve the compressibility of products of a given shape, the content of plasticizer in the original mixture can be increased. It should be taken into account that, due to the presence of a steel shell and a volumetric stress state, the punch of the plastic mold can withstand pressure inside the matrix that exceeds the compressive strength limit of this plastic by 1.5-2 times. Next, the blank is pressed out and separated from the punch.

Sintering of the workpiece

The blank is sintered using the standard method suitable for the grade of hard alloy used. At the initial stage of sintering, the plasticizer is removed, and at the final stage of sintering, the products are compacted to the required density. No changes in the sintering process are required to sinter products from the resulting blanks. After sintering, mechanical processing may be required to achieve the specified dimensions and surface quality of the products.

Example 1

To prepare hard-alloy bars used for strength determination, with dimensions of 20×6.5×5.25 mm, made of WC-15Co hard alloy, a plastic mold was developed, the model of which is shown in Fig. 1. It was calculated that the cross-sectional dimensions of the mold matrix made of polylactide with a Young's modulus of 1600 MPa will increase by 8.7% as a result of mold deformation during pressing of the hard-alloy mixture in it at a pressure of 140 MPa. The relative density of the workpiece after pressing at a pressure of 140 MPa and removal of the plasticizer, according to the test results, is 55%. Consequently, the linear shrinkage of the sample during sintering to 100% density will be (100% - (100% - 55%) ^1/3 ) = 23.4%. Assuming uniform distribution of the workpiece density and linear deformation during pressing, we obtain a linear shrinkage of the sample equal to (100%+8.7%)*(100%-23.4%)=16.7%. Thus, to obtain a workpiece with a cross-section of 20×6.5, it is necessary to increase the length and width of the matrix and punch of the plastic mold by (16.7%/(100%-16.7%))=20.1%. As a result, the length and width of the matrix and punch were 24.0 and 7.8 mm, respectively.

A plastic mold with the specified cross-section dimensions (Fig. 2) was manufactured according to the MMPN model using an undeclared Flashforge Dreamer 3D printer (manufactured by Flashforge, China). Polylactide (PLA), which is one of the most accessible plastics used for 3D printing, was used as a polymer for the MMPN. The printing conditions were selected in such a way as to ensure the greatest hardness, strength, and rigidity of the plastic mold parts with minimal manufacturing time. The filling was 100%, the first layer thickness was 0.27 mm, the thickness of the remaining layers was 0.15 mm. The printing temperature was 205°C.

The sample weight of 12.5 g of WC-15Co powder granules containing 4% rubber was selected to provide the required volume and weight of the blank. Before pressing, the punches and the inner surface of the plastic mold were treated with a stearic acid-based release agent to facilitate separation of the samples from the mold. Pressing was performed on an IP-250M test press. Before pressing, the maximum pressure that the plastic mold punch could withstand during pressing without noticeable plastic deformation and failure was determined experimentally. The pressing pressure in the steel mold was 140 MPa. After pressing, the samples were pressed out and carefully separated from the punch. The density of the blanks obtained by pressing in plastic molds was 7.98 g/cm ³ . The relative densities of the compacts are 90% based on the theoretical density of the powder and plasticizer mixture of 8.85 g/ ^cm3 . The relative density of the resulting blanks is higher than the density of 20-30% of the blanks obtained by direct 3D printing methods (SSZDP, ZDGP, SLP). This is due to the fact that in 3D printing, the mixture is compacted under the action of gravity, which does not allow filling the pores even when using mixtures with increased fluidity.

After pressing, the samples were sintered in a vacuum furnace at a maximum temperature of 1450°C for 60 minutes. After sintering, the density, mass, geometric dimensions, hardness and strength of the obtained samples were measured. The length (19.8 mm) and width (6.4 mm) of the sample differed from the predicted values within the tolerance (GOST 20019-74). Strength tests according to the standard method ISO 3327:2009 showed that the strength of the sample was 1950 MPa. The relative density is 99.5%, the Vickers hardness is 1150HV.

Due to the high relative density (>99%) and the absence of large defects, the samples obtained by sintering have strength and hardness corresponding to the standard alloy VK15 GOST 3882-74. The density of the obtained samples is higher than the density of all samples obtained directly by 3D printing, so their strength was higher. The samples obtained by pressing into a printed plastic mold retained the imprint of the punch surface on their surface; an example of an imprint on a bar is shown in Fig. 3. The profile of the punch of the plastic mold is completely repeated in the surface profile of the obtained sample. In this case, the dimensions of the sample elements are 15-25% smaller than the dimensions of the same elements in the punch, which is approximately equal to the shrinkage of the sample.

Example 2

To prepare a square cutter SNUM-120408 with dimensions of 12.7×12.7×5 mm from the carbide powder mixture WC-15Co with a plasticizer (4% rubber) used in Example 1, a plastic mold was developed, shown in Fig. 4. The dimensions of the matrix cross-section were increased by 20.1% to 15.2*15.2 in order to take into account the deformation during pressing and shrinkage during sintering. The process of making the mold, pressing the cutter and sintering repeats Example 1. The mass of the sample, taking into account the specified height of the sample, was 9.2 g. After sintering, a square cutter was obtained, a photograph of which is shown in Fig. 5. The characteristics of the obtained sample meet the requirements of GOST 3882-74 for the VK15 alloy for a density of 99.2% (13.95 g/cm ³ ) and a hardness of HRA86.

Conclusion

The proposed method allows to significantly reduce the costs of obtaining individual hard-alloy products of a given shape by using industrial granulated hard-alloy powder mixtures as a result of their pressing in a plastic mold made using an MMPN 3D printer and subsequent sintering under normal conditions.

List of sources

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2. J. Chen et al. Microstructure analysis of high density WC-Co composite prepared by one step selective laser melting, International Journal of Refractory Metals & Hard Materials, vol. 84, Nov 2019, Art no. 104980, doi: 10.1016/j.ijrmhm.2019.104980

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Claims (1)

A method for producing a hard-alloy product of a given shape from granulated hard-alloy powder, including the production of a mold containing a matrix and a punch from plastic by 3D printing, forming and sintering a workpiece, characterized in that said mold is manufactured according to a drawing of the product taking into account the deformation of the workpiece during pressing from plastic having a strength of more than 50 MPa and a Young's modulus of more than 1 GPa, wherein the external dimensions of the matrix provide the possibility of pressing it into a steel shell manually, granulated hard-alloy powder is placed in the matrix, and forming is carried out by pressing at a pressure of more than 100 MPa.