D e s c r i p t i o n

The present invention relates to a high-entropy alloy for use as a base material in a laser powder bed fusion process (LPBF), a method for producing a component by means of laser powder bed fusion of a powdered base material in the form of a high-entropy alloy, and a component producible in an additive manufacturing process by laser powder bed fusion of a powdered base material in the form of a high-entropy alloy.

Processes and systems for additive manufacturing of components by powder bed fusion of a powdery base material are known from the prior art. According to the known processes, components are produced layer by layer by selective melting of the powdered base materials using a high-energy beam, e.g. a laser or electron beam. The individual layers typically have thicknesses between 10 and 150 pm. During the application process, a layer of the powdered base material is usually distributed on a platform that can be moved in the vertical axis (defined as the z-axis). After the powdered base material is applied, a focused high energy beam is moved in the xy-plane over the powdered base material. By moving the high energy beam, the beam selectively melts the layer of powdered base material. After melting, the material hardens into the desired shape. The process is repeated layer by layer until the component is finished. In this way, it is possible to produce even complex three-dimensional structures in a simple, fast and cost-effective way.

The disadvantage of the known processes for manufacturing components using a powder bed fusion process is in particular that the selection of materials that can be used is still very limited. For example, there are hardly any materials known for use in a powder bed fusion process that can be used to manufacture particularly hard, durable and wear-resistant components.

It is therefore the object of the present invention to at least partially overcome the disadvantages described above. In particular, it is the object of the present invention to provide a base material for use in a powder bed fusion process as well as a method for producing a component by means of powder bed fusion, which guarantees the generation of particularly hard, durable and wear-resistant components in a simple, fast and cost-effective manner.

The above-mentioned problem is solved by a high-entropy alloy having the features of claim 1 , a method having the features of claim 9 and a component having the features of claim 18. Further features and details of the invention result from the subclaims, the description and the drawings. Features and details described in connection with the high-entropy alloy according to the invention naturally also apply in connection with the method according to the invention and the component according to the invention, and vice versa in each case, so that mutual reference is or can always be made with regard to the disclosure of the individual aspects of the invention.

According to the invention, a high-entropy alloy is provided for use as a base material in a laser powder bed fusion process. The high-entropy alloy according to the invention comprises the elements Al, Cr, Fe and Ni, wherein the proportion of Al in the high-entropy alloy is < 8.5% by weight.

According to the invention, the proposed high entropy alloy is thus designed to enable a suitable starting material for the production of stable, hard and wear-resistant components via a powder bed fusion process, in particular for the production of corrosion-resistant and high-temperature-stable components, due to the intended combination of the elements Al, Cr, Fe and Ni and a proportion of Al in the high entropy alloy of < 8.5 wt.%. The use of a powder bed melting process provided for in the invention also enables even complex shapes and structures to be produced in a simple, cost-effective and reproducible manner.

According to the invention, it was recognised that in a high entropy alloy comprising the elements Al, Cr, Fe and Ni, in particular a limitation of the Al content to < 8.5 wt.% greatly increases the stability of a component which is produced from the high entropy alloy in a powder bed fusion process. In the context of the invention, a high-entropy alloy may be understood to mean in particular an alloy comprising at least 4 different elements. According to the invention, a base material for use in a powder bed melting process can further preferably be understood as a powdery starting material which can be selectively melted via the use of a laser beam or electron beam. It is understood that instead of a laser beam in a powder bedbased laser beam melting process, an electron beam of an electron beam generator can also be used.

Here it can be advantageously provided according to the invention that the proportion of Fe and/or Ni in the high entropy alloy is greater than the proportion of Al and/or Cr. It is understood that here, too, the proportion can be understood as the mass proportion (in % by weight in relation to the total mass of the alloy).

It has been shown to be particularly advantageous if the high entropy alloy is in the form Al _x CrFe2Ni2, where x is preferably < 1 , where in particular 0.7 < x < 0.9. Such a composition is not only advantageous in terms of processability, but in particular in terms of the hardness, stability and durability of a component to be produced.

Hereby, the subject high entropy alloy may have a mixed crystal structure at a temperature of < 1280 °C, preferably a mixed BCC/FCC crystal structure.

Similarly, the high entropy alloy may have a pure FCC crystal structure at a temperature of > 1280 °C, which is preferably responsible for the high stability, durability and wear resistance.

In the context of the production of stable components for use in high-temperature applications, it may be provided according to the invention that the high-entropy alloy is stable at temperatures of > 900°C also for longer times of > 50 h, preferably > 75 h, in particular > 100 h. In the context of the invention, stable can be understood to mean in particular that at a constant temperature, at least after a start-up time of a few minutes, there is no change in the phase, the crystal structure or the composition or the like.

Within the scope of a targeted adaptation of the properties of a component that can be produced using the present high-entropy alloy, it can also be provided in accordance with the invention that further elements are present in addition to Al, Cr, Fe and Ni, the proportion of the further elements in the high-entropy alloy preferably being < 20 wt.%, in particular < 10 wt.%. The further elements can preferably be subgroup elements of the periodic table, e.g. Ti, V, Nb, Mn or Co.

With regard to ensuring a particularly homogeneous layer structure, it may further be advantageous if the high entropy alloy has a mean particle size of < 100 pm, preferably of < 50 pm.

It is also an object of the invention to provide a method for manufacturing a component by means of powder bed fusion (in particular in a LPBF-process) of a powdered base material in the form of a high entropy alloy, in particular a high entropy alloy described above. Here, the method comprises the steps of providing a powder bed of base material by means of a spreading device and moving a focused laser beam of a laser (or an electron beam of an electron beam generator) over the powder bed of the base material along a predetermined movement path for melting the base material in the region of the movement path. Thus, the method according to the present invention has the same advantages as already described in detail with respect to the high entropy alloy according to the present invention. In the course of carrying out the present method, it can thereby preferably be provided that a powder bed of powdered base material is applied to a base surface, such as a base plate or the like, which can preferably be moved in the z-direction (perpendicular to the plane of a base surface). Melting of the base material in the region of the movement path can be performed by heating the base material to its melting temperature by means of a laser (or an electron beam generator) with a corresponding energy density. In the context of the invention, powder bed fusion is preferably understood as a process in which material powder is melted and fused together by means of a laser (or an electron beam). In the context of the invention, a powder bed can preferably be understood as a powdery material with a certain degree of filling which is applied to a surface. With regard to a fast, exact and reproducible production of even complex structures, it can be advantageously provided that the focussed laser beam (or the electron beam) is guided partly along a non-linear movement path and partly along a linear movement path, whereby the movement in the form of a non-linear movement path preferably serves to produce the contour of the component and the movement in the form of a linear movement path serves to produce the inner structure of the component. According to the invention, a non-linear movement path can preferably be understood as a movement path which does not consist exclusively of straight lines, but for example of individual interconnected or non-connected wavy lines or geometric figures. The non-linear movement path can advantageously be formed in this case in the form of at least one path of individual identical, overlapping patterns, wherein the smallest and/or largest diameter of the patterns is greater than twice the diameter of the focused laser beam for melting the base material (or the electron beam).

For the manufacture of a 3-dimensional component, it can further be provided that the steps of providing a powder bed of base material with the aid of a spreading device and moving a focused laser beam of a laser (or an electron beam of an electron beam generator) over the powder bed of the base material are carried out cyclically repeatedly one after the other, wherein a renewed provision of a powder bed of base material with the aid of a spreading device preferably only takes place when the molten base material has hardened. In this way, the layered structure of the component can be ensured. In particular, it is advisable to wait for the material to harden in order to ensure a homogeneous layer structure. Before reapplying a powder bed, it can preferably be provided that a base surface or base plate, on which the powder bed and the melted and solidified base material are arranged, is slightly lowered, in particular by exactly one layer thickness of the base material.

Within the scope of a simple and fast production of components with the lowest possible surface roughness, it can be advantageously provided according to the invention that the diameter of the focused laser beam for melting the base material is between 0.05 and 0.5 mm.

In order to enable the rapid production of a component with a layer structure that is as homogeneous as possible, it can also be provided that the layer thickness of the powder bed of the base material is 0.05 to 0.5 mm.

In order to avoid impurities or contamination of the layers of the component which can be produced by means of the present process, it can be further provided in an advantageous manner according to the invention that the process is carried out in an inert gas atmosphere, preferably nitrogen or argon being used as the inert gas, the inert gas being supplied in particular at a positive pressure of 0.1 to 0.5 MPa. Likewise, it can also be provided that the process is carried out under vacuum conditions, e.g. at pressures of less than 100 mbar, preferably less than 10 mbar.

In the sense of simple, targeted, fast and high-quality production, it is also conceivable that the predetermined movement path is generated on the basis of a CAD data model of the component to be produced. In this respect, the present method can preferably be carried out at least partially automated, in particular fully automated. It is also understood that the present method may be designed as a computer-implemented method, wherein all or individual obligatory or optional steps of the method according to the invention may be implemented by a computer.

According to another aspect of the present invention, a computer program is provided. The computer program comprises instructions which, when the computer program is executed by a computer, cause the computer program to perform the method described in detail above. Thus, the computer program according to the present invention provides the same advantages as described in detail with reference to the method according to the present invention. The computer program may be implemented as computer-readable instruction code in any suitable programming language, such as JAVA or C++. The computer program may be stored on a computer-readable storage medium, such as a data disk, a removable drive, a volatile or nonvolatile memory, or a built-in memory/processor. The instruction code may program a computer or other programmable device such as a controller to perform the desired functions. In addition, the computer program may be made available on a network such as the Internet, from which it may be downloaded by a user as required. The computer program may be in the form of software or in the form of a computer program product using one or more specific electronic circuits, i.e. hardware, or in any mixed form, i.e. using both software and hardware components.

According to another aspect of the present invention, there is provided a storage means on which a computer program is stored, the computer program being configured and arranged to perform a method as described above. Thus, the storage means according to the present invention also provides the advantages described above. By the storage means may be meant a data carrier such as a memory pen on which the computer program is stored. Furthermore, a control unit with a computer program installed thereon is provided, which is configured and designed for carrying out a method as described above. The control unit according to the invention also offers the advantages described above.

In order to further improve the hardness, durability and wear resistance, it may further be advantageous if, before and/or during a providing of a powder bed of base material material and/or moving a focused laser beam of a laser (or an electron beam of an electron beam generator) over the powder bed of the base material, wherein the heat treatment is preferably carried out in the form of heating a plate for receiving the powder bed of base material, wherein the heat treatment is carried out in particular to a temperature of > 300 °C.

Within the scope of a simple generation of a desired Al content, according to the invention, it can also be provided that before and/or during a providing of a powder bed of base material, an adjustment of the high entropy alloy to a desired Al content takes place, wherein the adjustment preferably takes place via an addition of Cr and/or Fe and/or Ni. Preferably, the addition can be carried out by dosed introduction of said elementary metals in powder form. In addition, it is also possible to add further or other elementary metals to adapt the high entropy alloy to a desired Al content.

Another object of the invention is a component which can be produced in an additive manufacturing process by laser powder bed fusion of a powdery base material in the form of a high-entropy alloy, preferably producible by application of a method described above, in particular using a high-entropy alloy described above. Thus the process according to the process has the same advantages as have already been described in detail with respect to the high-entropy alloy according to the invention or with respect to the process according to the invention.

Within the scope of an exact production of even complex three-dimensional structures, it can further be provided that the component has a number of individual layers of > 1000, preferably a number of individual layers of > 1500, in particular a number of individual layers of > 1600 layers.

In the context of an exact production of even complex three-dimensional structures, it may further be provided that the component has a layer thickness of < 0.1 mm, preferably < 0.075 mm, in particular < 0.05 mm. Further advantages, features and details of the invention will be apparent from the following description, in which embodiments of the invention are described in detail with reference to the drawings. The features mentioned in the claims and in the description may individually or in any combination be essential to the invention.

It shows:

Figure 1 A characterisation of a high entropy alloy of the form AICrFe2Ni2 with an Al content of > 8.5 wt.% for use as a base material in a powder bed fusion process,

Figure 2 a presentation of the effects of preheating on the basis of a characterisation of a high-entropy alloy of the form AICrFe2Ni2 with an Al content of > 8.5 wt.% after preheating to a temperature of > 850 °C,

Figure 3 a presentation of simulations for changes in the crystal structure of a high-entropy alloy of the form Al _x CrFe2Ni2 for x > 1 , as a function of the aluminium content and the temperature,

Figure 4 a presentation of simulations for changes in the crystal structure of a high-entropy alloy of the form Al _x CrFe2Ni2 for x < 1 , as a function of the aluminium content and the temperature,

Figure 5 High-resolution representations and characterisations of a high-entropy alloy of the form AICrFe2Ni2 with an Al content of < 8.5 wt.% according to the invention,

Figure 6 High-resolution representations and characterisations of a high-entropy alloy of the form AICrFe2Ni2 according to the invention with an Al content of < 8.5 wt.% after heat treatment,

Figure 7 a representation of test bodies for the determination of the tensile strength,

Figure 8 a presentation of the results of the tensile strength tests for three different test bodies, Figure 9 a representation of a pump impeller produced by a method according to the invention with a high entropy alloy according to the invention in the form AICrFe2Ni2 with an Al content of < 8.5 wt.%,

Figure 10 the individual steps of a process according to the invention for producing a component by means of powder bed fusion of a powdery base material in the form of a high entropy alloy.

Figure 1 shows a characterisation of a high entropy alloy of the form AICrFe2Ni2 with an Al content of > 8.5 wt.% for use as a base material in a powder bed fusion process.

Figure 2 shows a presentation of the effects of preheating on the basis of a characterisation of a high-entropy alloy of the form AICrFe2Ni2 with an Al content of > 8.5 wt.% after preheating to a temperature of > 850 °C.

For x = 1 , which means AhCrFe2Ni2 the Aluminum content is 8.5 wt%. A typical powder with this material used for laser powder bed fusion (LPBF) can be characterized as shown in figure 1. This material however, when used in LPBF with typical printer process parameters - as shown in figure 2 - shows processing challenges in terms of the as-built microstructure as well as cracking formation. In more detail a single-phase BBC microstructure is predominant, FCC formation is suppressed which most probably leads to transgranular cracking formation.

As can be seen in figure 2, crack formation can be avoided by preheating the base plate. Unfortunately the temperature necessary is with 850°C pretty high. On the one hand this leads to powder sintering at the powder bed. On the other hand the temperature effect decreases with increasing distance to the powder bed. This results in microstructural inhomogeneity along the built height. Therefore preheating does not seem to be a feasible solution against the cracking problem.

Figure 3 shows a presentation of simulations for changes in the crystal structure of a high-entropy alloy of the form Al _x CrFe2Ni2 for x > 1 , as a function of the aluminium content and the temperature.

One explanation of this monophase BCC formation for x > lean be based on phase diagrams as shown in figure 3. As LPBF is a process far away from equilibrium, the Scheil-Gulliver approach of phase diagrams seems to be more adequate. Assuming that starting from around 1280°C a kind of shock freezing (rapid solidification under extreme high cooling rates) happens the diagram shows that only BCC-B2 phase 3 will be present and FCC phase 4 is supressed. This coincident with the experimental results as measured.

Figure 4 shows a presentation of simulations for changes in the crystal structure of a high-entropy alloy of the form Al _x CrFe2Ni2 for x < 1 (DHEA-01 blend), as a function of the aluminium content and the temperature.

Figure 4 shows in contrast to this for x<1 at 1280°C that according to the Scheil-Gulliver diagram only FCC phase 4 is present. Shock freezing this situation leads to a monophase FCC microstructure which is in good coincidence with the experimental results as shown in figure 5.

Figure 5 shows high-resolution representations and characterisations of a high-entropy alloy of the form AICrFe2Ni2 with an Al content of < 8.5 wt.% according to the invention.

Figure 6 shows high-resolution representations and characterisations of a high-entropy alloy of the form AICrFe2Ni2 according to the invention with an Al content of < 8.5 wt.% after heat treatment.

In order to experimentally verify the above mentioned expectations a prealloyed AhCrFe2Ni2 was blended with Fe and Ni particles in such a way that the aluminum content was decreased down to 7.5 wt%. This resulted in a blended Alo.gCrFe2Ni2 composition. The experimental results as shown in figures 5 and 6 are taken from samples printed with this powder. The tensile strength measurements were as well base on coupons printed with the blended powder as described.

Consulting again figure 4, this time however the phase diagram for equilibrium conditions, it can be seen, that at 950°C both, FCC as well as BCC should be present. The inventors in order to reach such equilibrium conditions heated the printed parts to 950°C for a period of time. Figure 6 shows the result of such heat treatment for one hour as well as for 6 hours. As expected, duplex FCC + BCC microstructures were obtained.

Figure 7 shows a representation of test bodies for the determination of the tensile strength. One set of test bodies 10 (in the form of coupons) was printed with the Alo.gCrFe2Ni2 powder. In order to be able to compare, another set of test bodies 12 (in the form of coupons) from superduplex 1.4517 steel were used, produced by conventional casting, subsequent heat treatment process and extracted from the cast block with wire electro discharge machining (W-EDM).

Additional surface grinding was applied on the surface of the superduplex coupons to reduce the surface roughness caused by WEDM.

Figure 8 shows the results of the respective tensile measurements.

Figure 9 shows a representation of a pump impeller produced by a method according to the invention with a high entropy alloy according to the invention in the form AICrFe2Ni2 with an Al content of < 8.5 wt.%. The pump impeller according to figure 9 has a surface 14 as build, a lattice structure weight reduction concept 16, a minimum overhang angle 18 of 45° and a minimum support structure use 20. The pump impeller according to figure 9 has 1665 layers with a thickness of approximately 0.03 mm resulting in a component height of approximately 5 cm.

In summary, in the approach according to the present invention the powder may be blended in such a way that starting from the prealloyed AICrFe2Ni2, fine Fe and Ni particles are added thereby ending up with relatively decreased Al-content x<1.

The LPBF printability of Al _x CrFe2Ni2 high entropy alloys were investigated. The investigations were carried out with prealloyed powder and elemental additions. For x > 1 LPBF solidification resulted in BCC as-built microstructure and cracks. For x < 1 LPBF solidification resulted in FCC as-built microstructure and crack free material.

The LPBF processing and tensile properties of DHEA-01 blend was investigated (see in particular figures 4 - 6 and figure 8). In order to do so the original prealloyed powder was modified by powder blending. A crack free complex demonstrator component was printed. Heat treatment leads to additional flexibility. Duplex FCC+BCC structures could be refined in such a way. Mechanical testing revealed improved tensile strength and significant strain hardening compared to reference superduplex steel. It should be pointed out here that the present invention is not limited to blended powders. It is as well possible to realize fully prealloyed powder with the optimized powder composition. A possible range for x would be: 0.7 < x < 0.9. In other words x> 1 results into a brittle component, whereas x < 0.7 results into a component which is too soft for most of the applications. In the example the aluminum percentage was decreased by adding Fe as well as Ni, keeping the relation Fe/Ni constant. It is however as well possible to add Fe only or to add Ni only or to add both in a specific ratio deviating from one. Another possibility would be to decrease the Al percentage by adding Chromium, increasing the Chromium content. Any combination of all these additions is possible as well. In addition it is as well possible to add some other suitable material to decrease the aluminum percentage. Further investigations would be necessary in this context.

As mentioned before, the microstructure property relation can be tailored with specific heat treatments.

As could be shown, the alloy is sigma free and is resistant to embrittlement at high temperatures (outperforming the reference material, duplex steel, which is prone to embrittlement at temperatures above 300°C). The addition of Aluminum in the FeCrNi system to enhance mechanical strength and provide lighter material.

Figure 10 shows the individual steps of a process according to the invention for producing a component by means of powder bed fusion of a powdery base material in the form of a high entropy alloy.

The process according to figure 10 shows carrying out a heat treatment 100, preferably being carried out in the form of heating a plate for receiving a powder bed of base material, the heat treatment 100 being carried out in particular to a temperature of > 300°C.

After the heat treatment step an adaptation 200 of the high entropy alloy to a desired Al content takes place, wherein the adaptation 200 preferably takes place via an addition of Cr and/or Fe and/or Ni.

After the adaption 200 of the high entropy alloy to a desired Al content a providing 300 of a powder bed of base material by means of a spreading device takes place, before finally a moving 400 of a focused laser beam of a laser over the powder bed of base material along a predetermined movement path is carried out in order to melt the base material in the region of the movement path.

Li st of refere nce s i q ns

1 Liquid phase

2 BCC_A2 phase

3 BCC_B2 phase

4 FCC phase

5 FCC_L12 phase

6 sigma phase

10 test body

12 test body

14 surface

16 weight reduction concept

18 overhang angle

20 support structure use

100 Heat treatment

200 Adjusting the high entropy alloy to a desired Al content

300 Providing a powder bed of base material

400 Moving a focused laser beam of a laser over the powder bed of base material