How to prepare powders for additive manufacturing

Abstract

The present invention relates to a method for making a ready-to-print cermet or hard metal powder containing sintered granules, comprising sintering the granules in a carburizing atmosphere. Due to the carburizing atmosphere, the granules have less binder phase on the surface, do not sinter together, are easy to deagglomerate and the granules maintain their spherical shape. The present invention also relates to a powder made according to said method and the use of the powder in an additive manufacturing process.

Description

[Technical field]

[0001] The present invention relates to a method for producing sintered, ready-to-print cermet or hardmetal powders, and also to powders produced by such methods, which are suitable for additive manufacturing, such as 3D printing of cermet and hardmetal bodies. [Background technology]

[0002] Three-dimensional (3D) printing, or additive manufacturing, is a promising manufacturing technique that allows for printing three-dimensional bodies. 3D printing is a promising manufacturing technique because it allows for the production of complex structures and objects that cannot be achieved by traditional manufacturing processes.

[0003] The use of 3D printing to create cermet or cemented carbide bodies is known in the art. Cermet and cemented carbide materials consist of hard constituents of carbides and / or nitrides, such as WC or TiC, in a metallic binder phase, e.g. Co. These materials are useful in highly demanding applications due to their high hardness and high wear resistance combined with high toughness. Examples of areas of application are cutting tools for metal cutting, drill bits for rock drilling and wear parts.

[0004] One of the challenges when using 3D printing to produce cermet and hardmetal materials is to provide objects that have a microstructure, i.e., porosity, minimal Co islands, etc., comparable to traditionally produced cermet and hardmetal materials, i.e., those formed by pressing.

[0005] However, many attempts have been made to find suitable powders that are easy to manufacture, have suitable properties for 3D printing, and also result in a final sintered cermet or hardmetal with excellent quality.

[0006] In the conventional production of cemented carbide bodies using grinding, spray drying, pressing and sintering, the powder used for pressing is not sintered, but for 3D printing, such powder is too brittle and too porous. Therefore, when used for 3D printing, the dry powder in the form of granules is sintered to various degrees. However, this can cause the granules to adhere to each other, as the metal binders on the surfaces of the granules are easily fused together. Therefore, the sintered powder needs to be crushed or crushed using a large force in order to be used for 3D printing. This can result in non-spherical grains after grinding / crushing of the sintered granules, affecting the flowability of the powder.

[0007] One way to prevent the granules from sintering together is to use a lower sintering temperature, thus forming less metal binder on the surface of the granules, but this results in less dense granules with more pores. If the granules are too porous, the 3D printed cemented carbide may still have pores after final sintering.

[0008] Attempts have also been made to prevent granules from sintering together by physically preventing contact between them, for example by adding powders that can act as barriers, but such methods also require several post-sintering cleaning steps to remove the barrier powder before the final powder can be used for 3D printing.

[0009] It is an object of the present invention to obtain a method for producing cermet or hard metal powders for additive manufacturing, which prevents the granules from sintering together.

[0010] The object of the present invention is to obtain a method for producing cermet or hardmetal powders for additive manufacturing that is easy and requires few steps.

[0011] It is an object of the present invention to obtain a method for producing cermet or hard metal powders for additive manufacturing, with suitable porosity and good flowability. [Brief description of the drawings]

[0012] [Figure 1] 1 is an SEM image of sintered granules made according to the present invention. [Diagram 2] 1 is an SEM image of sintered granules made according to the prior art. [Diagram 3] This is a LOM image of a cross section of a granule. [Figure 4] FIG. 1 shows various steps, where step A forms unsintered granules, step B places the unsintered granules in a furnace, step C sinters the unsintered granules in a carburizing atmosphere, and step D subjects the sintered granules to a deagglomeration step. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The present invention relates to a method for producing a powder according to the above, the method comprising: - forming a slurry comprising hard components, a binder metal, an organic binder and a solvent; forming unsintered granules from the slurry, the granules including the hard components, the binder metal and the organic binder; - The unsintered granules are placed in the furnace - 1200°C and T in a carburizing atmosphere to form sintered granules s sintering the granules at a temperature between T s is the highest temperature at which the binder is still in a solid state for a particular cermet or hardmetal composition, 1200 °C and T s sintering the granules at a temperature between - subjecting the sintered granules to a deagglomeration step, whereby a powder of sintered granules is formed. Includes.

[0014] The slurry includes powders forming the hard components, a binder metal, an organic binder, and a solvent.

[0015] By powders forming the hard components is meant in this specification powders selected from carbides, nitrides or carbonitrides of one or more of the elements W, Ta, Ti, Nb, Nb, Cr and V.

[0016] When making a cemented carbide powder, the hard constituents include at least 50 wt% WC grains. The hard constituents may also include carbides or carbonitrides of one or more of Ta, Ti, Nb, Cr, Hf, V, Mo and Zr, such as TiN, TiC and / or TiCN.

[0017] When making a cermet powder, the hard constituents include carbides or carbonitrides of one or more of Ta, Ti, Nb, Cr, Hf, V, Mo, and Zr, such as TiN, TiC, and / or TiCN.

[0018] By metallic binder is meant herein one or more elements selected from Co, Ni and Fe, preferably the metallic binder is Co. The amount of metallic binder in the slurry is suitably between 5 wt% and 14 wt%, preferably between 8 wt% and 14 wt%, based on the dry powder weight. The metallic binder phase content is calculated herein excluding organic binders and solvents.

[0019] The organic binder is preferably selected from Polyethyleglycol (PEG) or wax. The amount of organic binder may be between 1 wt% and 5 wt% based on the powder weight.

[0020] The solvent may be any solvent suitable for making a slurry, preferably a mixture of water and an alcohol.

[0021] The formation of unsintered granules is preferably carried out by using spray drying or any other suitable technique capable of forming spherical granules.

[0022] The unsintered granules are placed into the furnace on trays, preferably graphite trays coated with yttrium oxide to prevent the powder from sticking to the trays.

[0023] The first part of the sintering cycle is the debinding step, where the organic binder is removed. Preferably, this is done at a temperature between 200°C and 550°C, for a period between 5 and 240 minutes. Debinding can also be done in two or more steps, i.e. by increasing the temperature stepwise. The exact debinding process depends on several things, e.g. type of furnace, batch size, type of binder, etc.

[0024] The second part of the sintering, i.e. solid-state sintering, is performed above 1200 °C and T s It is carried out at a temperature below T s is the highest temperature at which the binder is still in a solid state for a particular cermet or hardmetal composition. This temperature is usually shown as a line called the "solidus" in a phase diagram for each particular cermet or hardmetal composition. Solid-state sintering is carried out for a period between 5 and 60 minutes. Preferably, sintering is carried out at a temperature between 1250°C and T s C., more preferably between 1250.degree. C. and 1300.degree. C.

[0025] In determining the exact sintering temperature, the particular cermet or hardmetal composition must be taken into consideration.

[0026] In one embodiment of the present invention, when the metallic binder is Co, sintering is carried out at a temperature between 1250°C and 1300°C, preferably between 1260°C and 1290°C.

[0027] By carburizing atmosphere it is meant herein that carbon is added to the furnace. This can be done in several ways, for example with one or more carbon-containing gases, e.g. CO, CH4, CO2. Some carbon contribution can also come from the furnace if it is a graphite furnace.

[0028] Preferably, the carburizing atmosphere is created by a carbon-containing gas, preferably CO, CO2 and / or CH4. The partial pressure of the carburizing gas should be at least 50 mBar, preferably at least 75 mBar, of the total amount of gas present. The partial pressure of the carburizing gas should be below atmospheric pressure. Other suitable gases that may be present are Ar, H2, etc. The total pressure in the sintering furnace should be below atmospheric pressure.

[0029] After sintering, the granules usually adhere to each other with weak forces. To obtain the final powder, it is necessary to carry out only gentle deagglomeration. Deagglomeration is preferably carried out by gentle grinding in a ball mill without grinding liquid or the like. If smaller amounts of granules are to be deagglomerated, deagglomeration can be carried out by hand using a mortar.

[0030] Because the granules are not sintered together, but instead are only weakly attached to one another, the spherical or rounded shape achieved from the spray drying process may be maintained after deagglomeration.

[0031] Prior to using the sintered granules for 3D printing, the ready-to-print powder containing the sintered granules may be subjected to one or more sieving steps, which may be performed for several purposes, such as to remove any larger fragments that have not been deagglomerated, as well as to achieve the desired granule size.

[0032] The present invention also relates to a ready-to-print cermet or hardmetal powder comprising sintered granules made according to the above method.

[0033] Ready to print, as used herein, means that the powder is ready for use in an additive manufacturing process such as 3D printing.

[0034] The powder comprises sintered granules having an average diameter (D50) between 15 μm and 30 μm comprising WC and a metal binder, the sintered granules having a porosity between 0.1 vol % and 5 vol %.

[0035] The porosity of a sintered granule can be determined in various ways. One way is to calculate it by first measuring the density of the granule using a pycnometer. The measured density is then compared to the theoretical (or perfect) density for the particular cermet or hardmetal composition. The perfect density can be determined by calculation using suitable software such as Thermocalc, for example, or by measuring it on a porosity-free sintered piece made from a powder with the same composition. The difference is presumed to be the porosity.

[0036] Another way to measure porosity is to use image analysis on SEM or LOM images of cross sections of at least 50 sintered granules. The results are then given in area %, but extrapolated to the same value in vol %.

[0037] Sintered granules made according to the present invention have surface regions that have a metal binder content that is less than the metal binder content in the interior portion of the granule.

[0038] The depletion of the metal binder in the surface region can be seen most easily in SEM images of the granules, where the surface of the granules can be seen to be coated with WC crystals. The metal binder can sometimes be seen at the bottom of the larger gaps between the WC grains, but not on the actual surface of the granules. See, for example, Figure 1.

[0039] Sintered granules usually have a spherical or rounded shape, which is achieved from the spray drying process. Because the granules are not sintered together during sintering, the shape that the granules achieved from spray drying can be maintained.

[0040] By unsintered granules is meant herein compact particles comprising an organic binder, particles of hard components and binder metal.

[0041] By sintered granules it is meant herein compact particles comprising particles of a hard component embedded in a binder metal matrix.

[0042] By cermet is meant herein a material comprising hard constituents in a metallic binder phase, the hard constituents comprising carbides or carbonitrides of one or more of Ta, Ti, Nb, Cr, Hf, V, Mo and Zr, e.g. TiN, TiC and / or TiCN.

[0043] By hard metal powder is meant herein a material comprising a hard component in a metallic binder phase, the hard component comprising at least 50 wt.% WC grains. The hard component may also comprise carbides or carbonitrides of one or more of Ta, Ti, Nb, Cr, Hf, V, Mo and Zr, such as TiN, TiC and / or TiCN.

[0044] The present invention also relates to the use of the above-mentioned ready-to-print powders for producing cermet or hardmetal bodies by additive manufacturing such as 3D printing.

[0045] In one embodiment of the present invention, the three-dimensional printing is binder jetting, which has the advantage that it is a relatively inexpensive three-dimensional printing method.

[0046] 3D printing of cermet or hardmetal pieces is known in the art and the exact printing parameters are up to the skilled artisan to determine based on the type of printing technique used. Typically, the printing step is followed by hardening and powder removal, and then a sintering step in which the final cermet or hardmetal piece is achieved. EXAMPLES

[0047] Example 1 (Invention) Already ground and spray dried ready-to-press (RTP) powders were used as raw materials containing WC, metallic binder, and carbides of Cr and, if present, Ta and Nb. The composition of the different powders based on elements is shown in Table 1, the remainder being carbon. The WC grain size (FSSS) in inventions 1 and 2 was 0.80 μm, and in invention 3 the WC grain size (FSSS) was 1.5 μm. In addition to the powder raw materials, the RTP powders also contained 2 wt% PEG not included in the dry powder weight.

[0048] A new slurry was made by dissolving the RTP powder in a solvent of ethanol and water in a ratio of 78 / 22 and adding 2 wt% additional PEG. The slurry was spray dried into small granules with a D50 of approximately 25 μm.

[0049] The unsintered granules were placed in a sintering furnace on graphite trays coated with yttrium oxide and first subjected to a debinding step in a hydrogen atmosphere, which involved a stepwise increase in temperature to 500° C. for 170 min. The temperature was then increased to a maximum sintering temperature of 1275° C. in an Ar atmosphere (200 mBar partial pressure), where CO and Ar were introduced in a 1:1 flow ratio at 250 mbar partial pressure. The temperature and CO / Ar atmosphere were maintained for 60 min. The granules were then cooled to room temperature by free cooling.

[0050] Sintering resulted in a cake of sintered granules, which was subjected to deagglomeration in a ball mill using 100 kg Silpeb with 30 kg powder. The ball milled material was run without grinding liquid and dried. Deagglomeration times were between 10 and 40 minutes depending on the sample.

[0051] The powder was then sieved at 63 μm.

[0052] The resulting cemented carbide powders containing the sintered granules are designated as Inventions 1-3 herein.

[0053] An SEM image of the sintered granules of Invention 1 is shown in FIG. TIFF2024528519000002.tif38170

[0054] The particle size of the sintered granules was measured by laser diffraction of the dry granules at intervals of 0.5 μm and 175 μm, and the results are shown in Table 2. TIFF2024528519000003.tif38170

[0055] Example 2 The same unsintered granules used for invention 1 from example 1 were sintered in the same way as in example 1 but at different maximum temperatures using a mixture of CO and H2 atmospheres (at a total partial pressure of 250 mBar in a 1:1 ratio) (same for all three samples). At the end of the cycle, a high pressure of 50 bar (Ar) was applied. The samples are called invention 1a-c.

[0056] The porosity was measured by image analysis on LOM (optical optical microscope) images (magnification 2000) of the cross-sections of the granules using the software Image J. An average of approximately 50 granules per powder sample was analyzed. Powders sintered at 1300°C were significantly more difficult to deagglomerate and therefore the granules were deformed to a higher extent compared to 1250°C and 1275°C. TIFF2024528519000004.tif29170

[0057] Example 3 (Comparison) Sintered granules were made from powders having the same composition as invention 1 in Example 1, following the process described in US2017 / 0072469 using graphite powder as a physical barrier. After sintering, the graphite powder was removed. An SEM image of the sintered granules is shown in Figure 2.

[0058] Example 4 (Printing) Sample cubes (15x15x6mm) were printed using the sintered granules, invention 2 from example 1. The cubes were produced by binder jet printing. Printing was carried out using ExOne Innovent+ with a layer thickness of 50μm during printing. Saturation during printing was set to 80%. The cubes were then sintered for 90 minutes using a high pressure sintering cycle with a maximum temperature of 1410°C. At the end of the cycle, a high pressure of 50 bar was applied. All physical properties were within the standards for this substrate when made in a conventional way, i.e. using compression instead of printing as the forming step. TIFF2024528519000005.tif31170

Claims

1. forming a slurry comprising hard components, a binder metal, an organic binder and a solvent; - forming unsintered granules comprising hard components, binder metal and organic binder from the slurry; - placing the unsintered granules in a furnace; - T ｓ However, between 1200° C. and T in a carburizing atmosphere, which is the highest temperature at which the binder is still in a solid state for a particular cermet or hardmetal composition. ｓ sintering the granules at a temperature between 0.4° C. to form sintered granules; - subjecting the sintered granules to a deagglomeration step, whereby a powder of granules is formed; A method for making a ready-to-print cermet or hardmetal powder comprising sintered granules, comprising:

2. The carburizing atmosphere is CO, CO ２ and C.H. ４ The method of claim 1 , wherein the carburizing gas is provided by one or more gases selected from:

3. 3. The method of claim 2, wherein the carburizing gas is present at a partial pressure of at least 50 mBar.

4. 10. The method of claim 1, wherein the ready-to-print powder is a cemented carbide powder and the hard component comprises at least 50 wt% WC grains.

5. 10. The method of claim 1, wherein the metallic binder is one or more elements selected from Co, Ni and Fe, and the amount of metallic binder is between 5 wt% and 14 wt% based on the dry powder weight.

6. 2. The method of claim 1, wherein the sintering temperature is between 1250°C and 1300°C.

7. 2. The method of claim 1, wherein the metallic binder is Co and the sintering temperature is between 1260°C and 1290°C.

8. The method of claim 1 , wherein the ready-to-print cermet or hardmetal powder comprising sintered granules is subjected to one or more sieving steps.

9. 9. A ready-to-print cermet or hardmetal powder comprising sintered granules produced according to the method of any one of claims 1 to 8.

10. 10. The ready-to-print cermet or cemented carbide powder of claim 9, wherein the granules have an average particle size between 15 μm and 30 μm.

11. 10. The ready-to-print cermet or cemented carbide powder of claim 9, wherein the porosity of the granules is between 0.1 vol% and 5 vol%.

12. 10. The ready-to-print cermet or cemented carbide powder of claim 9, wherein the powder is a cemented carbide powder.

13. 10. Use of the ready-to-print cermet or hardmetal powder according to claim 9 in an additive manufacturing process for making a cermet or hardmetal body.

14. 14. Use of the ready-to-print cermet or hardmetal powder according to claim 13, wherein the additive manufacturing process is binder jetting.

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