NICKEL BASE SUPERALLOY FOR ADDITIVE MANUFACTURING

Abstract

The present invention concerns nickel alloys in powder form comprising at least 40 wt.-% Ni, about 20.0 to 25.0 wt.-% Cr, about 5.0 to 25.0 wt.-% Co and about 1.5 to 5.0 wt.-% Ti, which have a content of B in an amount of less than 40 ppmw. Corresponding alloys have the advantage of providing minimal or no micro-cracks as well as an improved ductility in creep conditions compared to similar alloys having a higher content of B, when the alloys are processed by additive manufacturing to prepare three-dimensional objects. The present invention further concerns processes and devices for the preparation of three-dimensional objects from such nickel alloy powders, processes for the preparation of corresponding nickel alloy powders, three-dimensional objects which are prepared from such nickel alloy powders and the use of such nickel alloy powders to minimize and/or suppress micro-crack formation and/or to provide improved creep ductility.

Claims

1. Nickel alloy in powder form comprising at least 40 wt.-% Ni, about 20.0 to 25.0 wt.-% Cr, about 5.0 to 25.0 wt.-% Co and about 1.5 to 5.0 wt.-% Ti, characterised in that the alloy contains B in an amount of less than 40 ppmw.

2. Nickel alloy in powder form according to claim 1, which comprises about 22.0 to 23.0 wt.-% Cr, about 18.0 to 20.0 wt.-% Co and about 3.0 to 4.5 wt.-% Ti.

3. Nickel alloy in powder form according to claim 1, further comprising one or more of at least 1,5 and/or up to 2.5 wt.-% W, at least 1.5 and/or up to 2.5 wt.-% Al, at least 1.0 and/or up to 1.5 wt.-% Ta and at least 0.8 and/or up to 1.2 wt.-% Nb and optionally up to 0.17 wt.-% C and/or up to 0.12 wt.-% Zr.

4. Nickel alloy in powder form according to claim 1, comprising less than or equal to 35 ppmw B.

5. Nickel alloy in powder form according to claim 1 comprising at least 42 and/or up to 55 wt.-% Ni.

6. Nickel alloy in powder form according to claim 1, wherein the powder has a particle size d50 of from 2 to 200 μm as determined according to ISO 13320 by laser scattering or laser diffraction.

7. Process for the manufacture of a three-dimensional object, comprising providing a nickel alloy in powder form as defined in claim 1, and preparing the object by applying the nickel alloy in powder form layer on layer and selectively solidifying the alloy powder at positions in each layer, which correspond to the cross section of the object in this layer, wherein the positions are scanned with a radiation interaction zone of an energy beam bundle.

8. Process for the manufacture of a three-dimensional object according to claim 7, wherein the nickel alloy in powder form prior to solidifying is heated to a temperature of 100° C. or more.

9. Process for the manufacture of a three-dimensional object according to claim 7, further including a step of subjecting the three-dimensional object initially prepared to a heat treatment, and/or for a time of 3 to 15 h.

10. Process for the preparation of a nickel alloy in powder for use in a process according to claim 7, wherein the nickel alloy is atomized in an appropriate device.

11. Three dimensional object prepared according to the process as described in claim 10, and wherein the three dimensional object comprises or consists of the nickel alloy.

12. Three dimensional object according to claim 11, wherein the three dimensional object is a gas turbine component.

13. Use of a nickel alloy in powder form according to claim 1 for minimizing and/or suppressing crack formation in a three-dimensional object and/or for providing improved ductility in creep conditions of the three-dimensional object, wherein the three-dimensional object is prepared in a process involving the step and layerwise build-up of the three dimensional object by additive manufacturing.

14. Device for implementing a process according to claim 7, wherein the device comprises a radiation source, a process chamber having an open container with a container wall, a support, which is inside the process chamber, wherein process chamber and support are moveable against each other in vertical direction, a storage container and a recoater, which is moveable in horizontal direction, and wherein the storage container is at least partially filled with a nickel alloy in powder form.

Description

[0037] FIG. 1 is a schematic view, partially represented in section, of an exemplary device for the layer-wise manufacture of a three-dimensional object according to an embodiment of the present invention.

[0038] FIG. 2 is a depiction of the dimensions of test bodies prepared from an inventive and conventional IN-939 nickel alloy, which were used to determine the stress and strain characteristics of the materials

[0039] The device represented in FIG. 1 is a laser sintering or laser melting apparatus 1 for the manufacture of a three-dimensional object 2. The apparatus 1 contains a process chamber 3 having a chamber wall 4. A container 5 being open at the top and having a container wall 6 is arranged in the process chamber 3. The opening at the top of the container 5 defines a working plane 7. The portion of the working plane 7 lying within the opening of the container 5, which can be used for building up the object 2, is referred to as building area 8. Arranged in the container 5, there is a support 10, which can be moved in a vertical direction V, and on which a base plate 11 which closes the container 5 toward the bottom and therefore forms the base of the container 5 is attached. The base plate 11 may be a plate which is formed separately from the support 10 and is fastened on the support 10, or may be formed so as to be integral with the support 10. A building platform 12 on which the object 2 is built may also be attached to the base plate 11. However, the object 2 may also be built on the base plate 11, which then itself serves as the building platform.

[0040] In FIG. 1, the object 2 to be manufactured is shown in an intermediate state. It consists of a plurality of solidified layers and is surrounded by building material 13 which remains unsolidified. The apparatus 1 furthermore contains a storage container 14 for building material 15 in powder form, which can be solidified by electromagnetic radiation, for example a laser, and/or particle radiation, for example an electron beam. The apparatus 1 also comprises a recoater 16, which is movable in a horizontal direction H, for applying layers of building material 15 within the building area 8. Optionally, a radiation heater 17 for heating the applied building material 15, e.g. an infrared heater, may be arranged in the process chamber.

[0041] The device in FIG. 1 furthermore contains an irradiation device 20 having a laser 21, which generates a laser beam 22 that is deflected by means of a deflecting device 23 and focused onto the working plane 7 by means of a focusing device 24 via an entrance window 25, which is arranged at the top side of the process chamber 3 in the chamber wall 4.

[0042] The device in FIG. 1 furthermore contains a control unit 29, by means of which the individual component parts of the apparatus 1 are controlled in a coordinated manner for carrying out a method for the manufacture of a three-dimensional object. The control unit 29 may contain a CPU, the operation of which is controlled by a computer program (software). During operation of the apparatus 1, the following steps are repeatedly carried out: For each layer, the support 10 is lowered by a height which preferably corresponds to the desired thickness of the layer of the building material 15. The recoater 16 is moved to the storage container 14, from which it receives an amount of building material 15 that is sufficient for the application of at least one layer. The recoater 16 is then moved over the building area 8 and applies a thin layer of the building material 15 in powder form on the base plate 11 or on the building platform 12 or on a previously applied layer. The layer is applied at least across the cross-section of the object 2, preferably across the entire building area 8. Optionally, the building material 15 is heated to an operation temperature by means of at least one radiation heater 17. The cross-section of the object 2 to be manufactured is then scanned by the laser beam 22 in order to selectively solidify this area of the applied layer. These steps are carried out until the object 2 is completed. The object 2 can then be removed from the container 5.

[0043] According to the invention, a nickel alloy in powder form is used as building material 15.

[0044] According to the embodiments described below, the nickel alloy powder is processed by the direct metal laser sintering (DMLS) method. In the selective laser sintering or selective laser melting method small portions of a whole volume of powder required for manufacturing an object are heated up simultaneously to a temperature which allows a sintering and/or melting of these portions. This way of manufacturing an object can typically be characterized as a continuous and/or—on a micro-level—frequently gradual process, whereby the object is acquired through a multitude of heating cycles of small powder volumes. Solidification of these small powder portions is carried through selectively, i.e. at selected positions of a powder reservoir, which positions correspond to portions of an object to be manufactured. As in selective laser sintering or selective laser melting the process of solidification is usually carried through layer by layer, where the solidified powder in each layer is identical with a cross-section of the object that is to be built. Due to the small volume or mass of powder which is solidified in a given time span, e.g. 1 mm.sup.3 per second or less, and due to conditions in a process chamber of such additive manufacturing machines, which can favour a rapid cool-down below a critical temperature, the material normally solidifies quickly after heating.

[0045] In conventional sintering and casting methods one and the same portion of building material is heated up to a required temperature at the same time. A whole portion of material required to generate an object is cast into a mould in a liquid form. This volume of building material is therefore held above a temperature level required for melting or sintering for a much longer time compared to the selective laser sintering or selective laser melting method. Large volumes of hot material lead to a low cooling rate and a slow solidification process of the building material after heating. In other words, selective laser sintering or selective laser melting methods can be differentiated from conventional sintering and casting methods by processing of smaller volumes of building material, faster heat cycles and less need for heating up build material with high tolerances for avoiding a premature solidification of the material. These can be counted among the reasons why the amount of energy introduced into the building material for reaching the required temperatures can be controlled more accurately in selective laser sintering or selective laser melting methods. These conditions allow for setting an upper limit of energy input into the powder portions to be processed, which determines a temperature generated in the powder portions, more precisely, that is lower and closer to the melting point of the respective material than in conventional sintering or casting methods.

[0046] In the following, the present invention is further illustrated by mean of examples, which however should not be construed as limiting the invention thereto in any manner.

EXAMPLE 1: PREPARATION OF TEST BODIES FROM A CONVENTIONAL AND INVENTIVE NI ALLOY

[0047] A nickel alloy with the composition Cr: 22.4 wt.-%; Co: 18.9 wt.-%; Ti: 3.7 wt.-%; W 2.0 wt.-%; Nb 1.1 wt.-%; Al 1.9 wt.-%; Ta 1.3 wt.-%, Zr 0.02 wt.-%; C 0.14 wt.-%; and B: 22 ppmw, balance Ni (inventive, sample A) and a nickel alloy with the composition Cr: 22.79 wt.-%; Co: 19.29 wt.-%; Ti: 3.8 wt.-%; W 2.17 wt.-%; Nb 0.93 wt.-%; Al 1.85 wt.-%; Ta 1.41 wt.-%, Zr 0.03 wt.-%; C 0.15 wt.-%; and B: 48 ppmw (reference, sample B) were used as the starting materials. From the materials, test bodies with the dimensions as shown in FIG. 2 were printed and heat treated at 1160° C. for 4 h, 1000° C. for 6 h, 900° C. for 24 h and finally at 700° C. for 16 h.

[0048] The thus prepared test bodies were investigated for their mechanical properties. Stress testing was performed at 816° C. using an Instron 5982 electromechanical machine equipped with a SF16 furnace and an Instron 7361C extensometer (gauge length 12.5 mm). Temperature was measured by a Type-K thermocouple attached to the specimen and kept within ±2° C. during the tests. The control mode during the testing was at constant speed of the cross head of the machine, the strain was measured with an axial extensometer up to 5% total strain when the extensometer was removed. After 5% total strain the extension of the cross-head was used for the recalculation of the strain and strain rate. For each material, three different nominal strain rates were used: 8×10.sup.−3, 1×10.sup.−5 and 1×10.sup.−6 s.sup.−1.

[0049] In this test series all samples failed before 5% elongation which means that all measured strains correspond to the strain measured by the axial extensometer attached to the samples. While the machine is controlled by a constant cross head speed, the true strain is measured by the axial extensometer. This gave some differences between the true strain rate and the intended nominal strain rate.

[0050] The results of the measurement are provided in table 1 below:

TABLE-US-00001 TABLE 1 Diam- E-mod- Elonga- eter strain rate ulus Rp0.2 Rm tion Sample ID [mm] [1/s] [GPa] [MPa] [MPa] [%] sample A#1 5.22 6.90E−03 135 697 834.5 4.9 sample A#2 5.175 9.30E−06 140 394 481.5 3.4 sample A#3 5.21 8.50E−07 85 300 379 2.3 sample B#1 5.22 7.10E−03 144 698 848.5 4 sample B#1 5.22 8.00E−06 95 402 471.2 1.9 sample B#1 5.17 7.10E−07 100 330 368 1.1

[0051] As is apparent from the above table, the flow stress decreases with decreasing strain rate. This can be attributed to the creep effect during slow strain rate testing. Also, the elongation at fracture decreases with decreasing strain rate, which may be attributable to more accumulated creep damage during tests with lower strain rates. Finally, when comparing the samples A at comparable strain rates, the elongation at fracture is lower for the non-inventive samples B than for the inventive samples A. In micrographs of crosscuts of test bodies prepared from non-inventive samples a number of microcracks could be observed, which were not present in crosscuts of corresponding test bodies prepared from inventive samples.

[0052] On comparison of test bodies prepared from the inventive nickel alloy by additive manufacturing with test bodies which were prepared from conventional IN939 by means of casting, the performance in the stress strain behaviours was found to be comparable.

LIST OF REFERENCE SIGNS

[0053] 1 laser sintering or laser meting apparatus [0054] 2 three-dimensional object [0055] 3 process chamber [0056] 4 chamber wall [0057] 5 container [0058] 6 container wall [0059] 7 working plane [0060] 8 building area [0061] 10 support [0062] 11 base plate [0063] 12 building platform [0064] 13 building material [0065] 14 storage container [0066] 15 building material [0067] 16 recoater [0068] 17 radiation heater [0069] 20 irradiation device [0070] 21 laser [0071] 22 laser beam [0072] 23 deflecting device [0073] 24 focusing device [0074] 25 entrance window [0075] 29 control unit