METHOD OF MAKING A POWDER FOR ADDITIVE MANUFACTURING

Abstract

A method of making a ready-to-print cermet or cemented carbide powder of sintered granules includes the step of sintering the granules in a carburizing atmosphere. Due to the carburizing atmosphere, the granules will have less binder phase on the surface and will not sinter together and will be easy to deagglomerate and the granules will maintain its spherical shape. A powder made according to the method, as well as the use of the powder in an additive manufacturing process is provided.

Claims

1. A method of making a ready-to-print cermet or cemented carbide powder having sintered granules, the method comprising the steps of: forming a slurry including hard constituents, binder metal, organic binder and a solvent; from the slurry, forming unsintered granules including hard constituents, binder metal and organic binder; loading the unsintered granules in a furnace; sintering the granules at a temperature between 1200 and T.sub.s? C., where T.sub.s is the highest temperature where the binder is still in a solid state for a specific cermet or cemented carbide composition, in a carburizing atmosphere to form sintered granules; and subjecting the sintered granules to a deagglomeration step whereby a powder of granules is formed.

2. The method according to claim 1, wherein the carburizing atmosphere is provided by one or more carburizing gases selected from CO, CO.sub.2 and CH.sub.4.

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

4. The method according to claim 1, wherein the ready-to-print powder is a cemented carbide powder, and wherein the hard constituents include at least 50 wt % WC grains.

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

6. The method according to claim 1, method according to claim 1, wherein the sintering temperature is between 1250 and 1300? C.

7. The method according to claim 1, wherein the metallic binder is Co and the sintering temperature is between 1260 and 1290? C.

8. The method according to claim 1, wherein the ready-to-print cermet or cemented carbide powder including sintered granules are subjected to one or more sieving steps.

9. A ready-to-print cermet or cemented carbide powder comprising sintered granules made according to the method of claim 1.

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

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

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

13. A use of the ready-to-print cermet or cemented carbide powder according to claim 9 in an additive manufacturing process for making a cermet or cemented carbide body.

14. The use of the ready-to-print cermet or cemented carbide powder according to claim 13, wherein the additive manufacturing process is binder jetting.

Description

FIGURES

[0051] FIG. 1 shows a SEM Image of a sintered granule made according to the present invention.

[0052] FIG. 2 shows a SEM Image of a sintered granule made according to the prior art.

[0053] FIG. 3 shows a LOM image of a cross section of a granule.

[0054] FIG. 4 shows the different steps where step A is forming unsintered granules, step B is loading the unsintered granules into a furnace, step C is sintering the unsintered granules in a carburizing atmosphere and step D is subjecting the sintered granules to a deagglomeration step.

EXAMPLE 1 (INVENTION)

[0055] Already milled and spray dried ready-to-press (RTP) powders was used as raw material comprising WC, metallic binder and carbides of Cr and when present Ta and Nb. The composition of the different powders based on the elements are shown in Table 1 where the balance is carbon. The WC grain size (FSSS) in Invention 1 and 2 are 0.80 ?m and for Invention 3 the WC grain size (FSSS) was 1.5 ?m. In addition to the powder raw materials, the RTP powders also contained 2 wt % PEG, not included in the dry powder weight.

[0056] A new slurry was made by dissolving the RTP powder into a solvent of ethanol and water in a ratio of 78/22 and by adding 2 wt % extra PEG. The slurry was spray dried to small granules with a D50 approx. 25 ?m.

[0057] The unsintered granules were placed in a sintering furnace on a graphite tray coated with yttrium oxide and was first subjected to a debinding step in hydrogen atmosphere which included a stepwise increase in temperature to 500? C. for a time of 170 min. Then the temperature was increased in a Ar atmosphere (200 mBar partial pressure) to the maximum sintering temperature of 1275? C. where CO and Ar were introduced at a 1:1 flow ratio at 250 mbar partial pressure. The temperature and CO/Ar atmosphere was maintained for 60 min. After that, the granules were cooled down to room temperature by free cooling.

[0058] The sintering resulted in a cake of sintered granules which was subjected to a deagglomeration in a ball mill using 100 kg cylpebs with 30 kg powder. The ball milled was ran dry without any milling liquid. The deagglomeration time was between 10 to 40 min depending on sample.

[0059] The powder were then sieved at 63 ?m.

[0060] The resulting cemented carbide powder comprising sintered granules is herein denoted Invention 1-3.

[0061] A SEM image of a sintered granule of Invention 1 is shown in FIG. 1.

TABLE-US-00001 TABLE 1 Sample W [wt %] Co [wt %] Cr [wt %] Ta [wt %] Nb [wt %] Invention 1 84.1 10 0.43 0 0 Invention 2 81.4 13 0.56 0 0 Invention 3 81.1 12 0 1.35 0.15

[0062] The particle size of the sintered granules was measured by Laser diffraction of the dry granules in an interval of 0.5 and 175 ?m. The results are shown in Table 2.

TABLE-US-00002 TABLE 2 Sample D10 D50 D90 Invention 1 7.8 18.4 32.1 Invention 2 8.4 18.7 32.6 Invention 3 8.5 19.7 35.5

EXAMPLE 2

[0063] The same unsintered granules as used in Invention 1 from Example 1 were sintered in the same way as in Example 1 but at different maximum temperatures using a mix of CO and H.sub.2 atmosphere (in a ratio of 1:1 total partial pressure 250 mBar) (same for all 3 samples). In the end of the cycle 50 bar (Ar) high pressure was applied. The samples are called Invention 1a-c.

[0064] The porosity was measured by image analysis on a LOM (Light optic microscope) image (2000 magnification) on a cross section of a granule using the software Image J. An average of approx. 50 granules was analyzed per powder sample. The powder sintered at 1300? ? C. was noticeable more difficult to deagglomerate and thus the granules were deformed to a higher extent compared to 1250 and 1275? C.

TABLE-US-00003 TABLE 3 Invention 1a Invention 1b Invention 1c Sintering Temp. (? C.) 1250 1275 1300 Porosity [area %] 11.1 3.1 0.42

EXAMPLE 3 (COMPARATIVE)

[0065] Sintered granules were made from a powder having the same composition as Invention 1 in Example 1 according to the process described in US 2017/0072469 using graphite powder as a physical barrier. The graphite powder was removed after sintering. A SEM image of a sintered granule is shown in FIG. 2.

EXAMPLE 4 (PRINTING)

[0066] The sintered granules, Invention 2 from Example 1, was used to print sample cubes (15?15?6 mm). The cubes were produced through binder jet printing. The printing was done using a ExOne Innovent+ with a layer thickness of 50 ?m during the printing. Saturation during the printing was set to 80%. The cubes were then sintered with a high pressure sintering cycle with a maximum temperature 1410? C. for 90 minutes. In the end of the cycle 50 bar high pressure was applied. All physical properties were within the specification for this substrate if would have been made using conventional methods i.e. compaction instead of printing as forming step.

TABLE-US-00004 TABLE 4 Hc Com Density Porosity Granules [kA/m] [%] [g/cm.sup.3] A B C Invention 2 16.3 11.6 14.10 02 00 00