5 Method and system for generating a three-dimensional workpiece

The present invention relates to a method for producing a three-dimensional workpiece from a nickel-based superalloy powder by a powder bed fusion process.

A nickel-based superalloy is a high temperature material which is able to withstand temperatures close to its melting point. Further, it shows advantageous material characteristics, such as mechanical strength, resistance to thermal creep deformation, good surface stability, and resistance to mechanical degradation and to corrosion or oxidation.

15 Typical nickel-based superalloys contain a high number of different alloying elements. Boron and zirconium are known to have a positive effect on the creep properties of the material.

Selective laser sintering (SLS) is an additive manufacturing (AM) technique that uses a

20 laser as the power source to sinter powdered material, aiming the laser automatically at points in space defined by a 3D model, binding the material together to create a solid structure. It is similar to direct metal laser sintering (DMLS). Both are instantiations of the same concept but differ in technical details. Selective laser melting (SLM) uses a comparable concept, but in SLM the material is fully melted rather than sintered,

25 allowing different properties (crystal structure, porosity, and so on). SLS involves the use of a high power laser (for example, a carbon dioxide laser) to fuse small particles of plastic, metal, ceramic, or glass powders into a mass that has a desired three- dimensional shape. The laser selectively fuses powdered material by scanning crosssections generated from a 3-D digital description of the part (for example from a CAD file or scan data) on the surface of a powder bed. After each cross-section is scanned, the powder bed is lowered by one layer thickness, a new layer of material is applied on top, and the process is repeated until the part is completed.

Selective laser melting (SLM) is a particular rapid prototyping, 3D printing, or Additive

35 Manufacturing (AM) technique designed to use a high power-density laser to melt and

Received at EPO via Web-Form on Apr 04, 2023 fuse metallic powders together. Often SLM is considered to be a subcategory of Selective Laser Sintering (SLS). The SLM process has the ability to fully melt the metal material into a solid 3D-dimensional part unlike SLS. With selective laser melting, thin layers of atomized fine metal powder are evenly distributed using a coating mechanism

5 onto a substrate plate, usually metal, that is fastened to an indexing table that moves in the vertical (Z) axis. This takes place inside a chamber containing a tightly controlled atmosphere of inert gas, such as argon at oxygen levels below 500 parts per million. Once each layer has been distributed, each 2D slice of the part geometry is fused by selectively melting the powder. This is accomplished with a high-power laser beam,

10 usually an ytterbium fiber laser with hundreds of watts. The laser beam is directed in the X and Y directions with two high frequency scanning mirrors. The laser energy is intense enough to permit full melting (welding) of the particles to form solid metal. The process is repeated layer after layer until the part is complete.

Electron-beam additive manufacturing or electron-beam melting (EBM) is a type of additive manufacturing, or 3D printing, for metal parts. The raw material (metal powder or wire) is placed under a vacuum and fused together from heating by an electron beam. This technique is distinct from selective laser sintering as the raw material fuses having completely melted. Metal powders can be consolidated into a solid mass using

20 an electron beam as the heat source. Parts are manufactured by melting metal powder, layer by layer, with an electron beam in a high vacuum. This powder bed method produces fully dense metal parts directly from metal powder with characteristics of the target material. The EBM machine reads data from a 3D CAD model and lays down successive layers of powdered material. These layers are melted together utilizing a

25 computer-controlled electron beam. In this way it builds up the parts. The process takes place under vacuum, which makes it suited to manufacture parts in reactive materials with a high affinity for oxygen, e.g. titanium. The process is known to operate at higher temperatures (up to 1000 °C), which can lead to differences in phase formation though solidification and solid-state phase transformation.

Direct metal laser sintering (DMLS) is an additive manufacturing metal fabrication technology, occasionally referred to as selective laser sintering (SLS) or selective laser melting (SLM), that generates metal prototypes and tools directly from computer aided design (CAD) data. It is unique from SLS or SLM because the process uses a laser to

35 selectively fuse a fine metal powder. DMLS uses a variety of alloys, allowing prototypes

Received at EPO via Web-Form on Apr 04, 2023 to be functional hardware made out of the same material as production components. Since the components are built layer by layer, it is possible to design organic geometries, internal features and challenging passages that could not be cast or otherwise machined. DMLS produces strong, durable metal parts that work well as

5 both functional prototypes or end-use production parts. The DMLS process begins with a 3D CAD model whereby a .stl file is created and sent to the machine’s computer program. The DMLS machine uses a high-powered Yb-fiber optic laser. Inside the build chamber area, there is a material dispensing platform and a built platform along with a recoater blade used to move new powder over the built platform. The technology fuses

10 metal powder into a solid part by melting it locally using the focused laser beam. Parts are built up additively layer by layer, typically using layers 20 micrometers thick.

When nickel-based superalloys are processed by one of these additive manufacturing processes the superalloy suffers from hot cracking or microcracking, that means cracks are formed in the microstructure during the additive manufacturing process. The microcracking phenomenon is not fully understood. But the more the superalloy is alloyed with zirconium and/or boron, the more cracking occurs.

There are nickel-based superalloys on the market which are not alloyed with zirconium

20 and boron. Without these two elements the superalloy is printable, that means you can process it by additive manufacturing, but it does not have the desired high creep properties.

Under the name “Inconel 738” a superalloy powder is known which essentially consists

25 of: about 0.09 wt% to about 0.2 wt% carbon, about 8 wt% to about 1 1 wt% cobalt, about 15 wt% to about 18 wt% chromium, about 0.75 wt% to about 2.2 wt% molybdenum, about 1 .8 wt% to about 3 wt% tungsten, about 1 wt% to about 3 wt% tantalum, about 0.5 wt% to about 2 wt% niobium, about 3 wt% to about 4 wt% aluminum, about 3 wt% to about 4 wt% titanium with the total content of aluminum

35 and titanium not exceeding about 7.5 wt%,

Received at EPO via Web-Form on Apr 04, 2023 about 0.005 wt% to about 0.05 wt% boron, about 0.01 wt% to about 0.2 wt% zirconium, not more than 0.05 wt% iron, not more than 0.02 wt% manganese,

5 not more than 0.3 wt% silicon, not more than 0.015 wt% sulfur and the balance nickel.

The abbreviation “wt%” shall mean “percent by weight”.

10 Inconel 738 shows a high mechanical strength and a good thermal creep behaviour. However, due to boron and zirconium content it is hard to process by additive manufacturing.

Therefore, it is an object of the present invention to provide an improved additive manufacturing process which allows to print nickel-based superalloy powder containing boron and/or zirconium, in particular Inconel 738 powder.

This object is achieved by a method according to claim 1 .

20 It is assumed that oxidation of boron and/or zirconium contributes to the hot cracking effect. Therefore, the present invention proposes to carry out the additive manufacturing process in a controlled atmosphere with an extremely low oxygen content and in particular in an atmosphere containing less than 25 vppm oxygen. The low oxygen level will avoid the risk of oxidation of boron (B) and zirconium (Zr) during

25 printing and lead to a crack free material.

The minimum oxygen level depends on the design of the printer and on the design of the process chamber of the printer and especially how tight it is. With respect to the hot cracking issue it would even be preferable to have an oxygen content in the controlled atmosphere of not more than 20 vppm oxygen, not more than 15 vppm oxygen or not more than 10 vppm oxygen.

In the following, the terms “printing” and “additive manufacturing” are used as synonyms unless explicitly stated otherwise.

35

Received at EPO via Web-Form on Apr 04, 2023 A particular embodiment of the above-identified superalloy composition is known as High Carbon Inconel 738 (IN738-HC) and it essentially consists of

- 0.15 wt% to 0.2 wt% carbon,

- 8 wt% to 9 wt% cobalt,

5 15.7 wt% to 16.3 wt% chromium,

1 .5 wt% to 2 wt% molybdenum,

- 2.4 wt% to 2.8 wt% tungsten,

1 .5 wt% to 2 wt% tantalum,

0.6 wt% to 1 .1 wt% niobium,

3.2 wt% to 3.7 wt% aluminum,

3.2 wt% to 3.7 wt% titanium with the total content of aluminum and titanium between 6.5 and 7.2 wt%,

- 0.005 wt% to 0.015 wt% boron,

0.05 wt% to 0.15 wt% zirconium,

15 not more than 0.05 wt% iron, not more than 0.02 wt% manganese, not more than 0.3 wt% silicon, not more than 0.015 wt% sulfur

- and the balance essentially nickel.

Another advantageous superalloy composition is known as Low Carbon Inconel 738

20 (IN-738LC) and it essentially consists of

- 0.09 wt% to 0.13 wt% carbon,

- 8 wt% to 9 wt% cobalt,

15.7 wt% to 16.3 wt% chromium,

1 .5 wt% to 2 wt% molybdenum,

25 - 2.4 wt% to 2.8 wt% tungsten,

1 .5 wt% to 2 wt% tantalum,

0.6 wt% to 1 .1 wt% niobium,

3.2 wt% to 3.7 wt% aluminum,

3.2 wt% to 3.7 wt% titanium with the total content of aluminum and titanium between 6.5 and 7.2 wt%,

- 0.007 wt% to 0.012 wt% boron,

0.03 wt% to 0.08 wt% zirconium, not more than 0.05 wt% iron, not more than 0.02 wt% manganese, not more than 0.3 wt% silicon, not more than 0.015 wt% sulfur

35 - and the balance essentially nickel.

Received at EPO via Web-Form on Apr 04, 2023 It has been found that during the additive manufacturing process the oxygen concentration of the controlled atmosphere is critical. The oxygen concentration in the controlled atmosphere should be less than 25 ppm by volume (< 25 vppm) or not more

5 than 20 vppm. Typically, the additive manufacturing process is carried out in a process chamber and the controlled atmosphere is provided within that process chamber.

Such low oxygen concentrations of less than 25 vppm are difficult to achieve in normal 3D printers. In many cases the gas analytic system of a common 3D printer is not able to measure and control the oxygen concentration in the process chamber to such low levels. Therefore, according to a preferred embodiment, part of the atmosphere is withdrawn from the process chamber and the oxygen concentration of the gas is determined outside of the process chamber. The atmosphere within the process chamber is then controlled depending on the determined oxygen concentration. For

15 example, a sample of the process chamber atmosphere is withdrawn, sent to a gas analyzer where the sample is analyzed for oxygen traces and other impurities. Depending on the result of the analysis the gas analyzer controls the oxygen level in the process chamber by purging the process chamber with inert gas if required.

20 According to an alternative preferred embodiment part of the atmosphere is withdrawn from the controlled atmosphere and replaced by a gas flow of inert gas. For a powder bed fusion process carried out in a process chamber and the controlled atmosphere being provided in that process chamber, an inert gas flow through the process chamber is provided. The gas flow is set or controlled so that the oxygen concentration in the

25 controlled atmosphere is less than 25 vppm, less than 20 vppm, less than 15 vppm or less than 10 vppm.

Preferably the controlled atmosphere essentially consists of argon or helium or hydrogen or a mixture of argon and helium or a mixture of argon and hydrogen, in any

30 case with residual oxygen of less than 25 vppm. The inventor has recognized that helium and hydrogen have the benefit to decrease the creation of by-products such as spatters and fumes. This has the advantage to reduce the amount of defective parts and to reduce the energy needed to melt the powder leading to less risks of distortion.

Received at EPO via Web-Form on Apr 04, 2023 The invention is especially useful in a laser powder bed process, that is in an additive manufacturing process where a laser is used as heat source for melting the nickel- based superalloy powder. Preferably, the powder bed fusion process is carried out in a process chamber.

5

Laser powder bed fusion (LPBF) is also known as selective laser melting (SLM) or as direct metal laser sintering (DMLS) as discussed in the introductory part above.

Preferably the additive manufacturing process is a Laser Powder Bed Fusion Process comprising the following steps:

- providing the superalloy powder on a built platform in a process chamber,

- melting the powder with a laser, and

- repeating the aforementioned steps

- wherein the atmosphere within the process chamber is controlled so that the

15 oxygen concentration is below 25 vppm.

The invention is explained below with the aid of an embodiment shown in the drawings.

20

Figure 1 schematically shows a process chamber for producing a workpiece by means of additive manufacturing

Figure 2 shows the oxygen pick up of the material depending on the oxygen concentration of the process atmosphere

25

Figure 1 schematically shows a process chamber 1 for producing a workpiece by means of additive manufacturing, in particular by laser powder bed fusion. The finished workpiece is composed of metallurgical layers that are individually produced in succession. The individual metallurgical layers of the workpiece are respectively produced in that the powder is respectively provided for each metallurgical layer and acted upon with a laser beam. This takes place under a controlled gas atmosphere in the process chamber 1 . The gas atmosphere in the process chamber 1 consists, for example, of argon that was introduced into the process chamber 1 prior to the beginning of the additive manufacturing process.

35

Received at EPO via Web-Form on Apr 04, 2023 The powder used to manufacture the workpiece is a nickel-based superalloy, namely Low Carbon Inconel 738 (IN-738 LC) with a composition of:

- 0.09 wt% to 0.13 wt% carbon,

- 8 wt% to 9 wt% cobalt,

5 15.7 wt% to 16.3 wt% chromium,

1 .5 wt% to 2 wt% molybdenum,

- 2.4 wt% to 2.8 wt% tungsten,

1 .5 wt% to 2 wt% tantalum,

0.6 wt% to 1 .1 wt% niobium,

3.2 wt% to 3.7 wt% aluminum,

3.2 wt% to 3.7 wt% titanium with the total content of aluminum and titanium between 6.5 and 7.2 wt%,

- 0.007 wt% to 0.012 wt% boron,

0.03 wt% to 0.08 wt% zirconium,

15 not more than 0.05 wt% iron, not more than 0.02 wt% manganese, not more than 0.3 wt% silicon, not more than 0.015 wt% sulfur

- and the balance essentially nickel.

Such superalloys are difficult to handle in an additive manufacturing process due to the

20 risk of hot cracking. Therefore, the atmosphere in the process chamber is controlled such that the oxygen concentration is less than 25 vppm, preferably less than 20 vppm or less than 10 vppm.

A sample of the gas atmosphere is extracted from the manufacturing chamber 1 in the

25 form of a gas stream 2 and fed to an analyzer 3. The analyzer 3 measures certain parameters, such as the oxygen content and other impurities, of the sample in order to determine if the gas atmosphere is still sufficiently inert and the oxygen concentration is below the 25 vppm threshold or below the 10 vppm threshold, respectively. Otherwise, an undesirable formation of microcracks could take place in the workpiece.

Preferably, the analyzer 3 is a unit that includes gas control, gas analytic systems and an electrical control system. The gas analytic system comprises at least one oxygen probe, preferably two different oxygen probes. Further, the analyzer 3 may include a dew point sensor to monitor the dew point of the sample withdrawn from the process

35 chamber 1 . Control valves and pressure sensors hold the pressure in the process chamber on a stable level.

Received at EPO via Web-Form on Apr 04, 2023 The parameters such as, for example, the water vapor content or the oxygen content of the gas stream 2 are measured and compared with a nominal value in the analyzer 3. For the oxygen content the nominal value is the same as the above-mentioned

5 threshold. If the measured parameters lie below the nominal value, i.e. if the water vapor content or the oxygen content lies below the predefined nominal value, the gas stream 2 is completely returned into the manufacturing chamber 1 .

However, if the oxygen content is higher than the nominal value, the gas stream is partially or completely discarded. Instead, an inert gas, in the present example pure argon, is introduced into the process chamber 1 . An automatic inert gas/argon supply replaces part of the atmosphere in the process chamber 1 by pure argon whereby ensuring that the oxygen impurities are reduced. The system controls the oxygen level by purging the process chamber 1 with pure argon if required. The pressure in the

15 process chamber 1 is controlled with a pressure sensor and a vent valve 6. Preferably, the pressure in the process chamber 1 is kept constant.

Figure 2 shows the pick-up of oxygen, nitrogen and hydrogen during the laser powder bed fusion process. The powder used in this process was Low Carbon Inconel 738 (IN-

20 738 LC), a nickel-based superalloy.

In figure 2, from the left to the right, the first group of three bars 10a, 10b, 10c shows the oxygen content of the powder, the second group of three bars 11a, 11b, 11c shows the nitrogen content and the third group of three bars 12a, 12b, 12c shows the

25 hydrogen content. Within each group 10, 11 , 12 the left bar 10a, 11 a, 12a shows the original content of oxygen, nitrogen and hydrogen, respectively, prior to the laser powder bed fusion process, the middle bar 10b, 11b, 12b shows the content after a laser powder bed fusion process with an oxygen concentration in the controlled atmosphere of 20 vppm and the right bar 10c, 11c, 12c shows the content after a laser powder bed fusion process with an oxygen concentration in the controlled atmosphere of 1000 vppm. The oxygen, nitrogen and hydrogen content has been determined from spatter which has been collected after the laser powder bed fusion process has been finished. It is compared to the respective values of the original virgin powder before carrying out the laser powder bed fusion process.

35

It can be seen that the oxygen content of the spatter is considerably higher when the oxygen concentration of the controlled atmosphere is at 1000 vppm than when the

Received at EPO via Web-Form on Apr 04, 2023 oxygen concentration of the controlled atmosphere is controlled to 20 vppm. When the controlled atmosphere is controlled at 20 vppm oxygen, the oxygen content of the material is in average only 20 ppm higher than the oxygen content of the virgin powder. On the other hand, when the oxygen concentration in the controlled atmosphere is at

5 1000 vppm, the oxygen content of the material will increase by as much as 100 ppm. Since the hot cracking effect is correlated to the oxygen content of the material, the inventive use of a controlled atmosphere with a very low oxygen concentration will avoid the risk of oxidation of boron (B) and zirconium (Zr) during printing and lead to a crack free material.

10

It can also be seen that the nitrogen content and the hydrogen content are not or not significantly affected. That means by using the inventive process the oxygen content, the nitrogen content as well as the hydrogen content of the powder will not significantly be affected. After the laser powder bed fusion process the properties of the powder are very similar to the properties of the virgin powder. That means the powder can be reused much more often when the oxygen concentration of the controlled atmosphere is controlled in the inventive manner. Since nickel-based superalloy powder IN738 is expensive, this is another considerable benefit.

20

Received at EPO via Web-Form on Apr 04, 2023 List of Reference Numbers

1 Process chamber

2 sample from process chamber to analyzer

5 3 analyzer

4 sample from analyzer to process chamber

5 argon supply to analyzer and to process chamber

6 vent valve

10a, 10b, 10c oxygen content

11 a, 11b, 11c nitrogen content

12a,12b,12c hydrogen content

Received at EPO via Web-Form on Apr 04, 2023