Method, use and apparatus for producing a single-crystalline work piece

Abstract

A method for producing or repairing a three-dimensional work piece, the method comprising the following steps: providing at least one substrate (15); depositing a first layer of a raw material powder onto the substrate (15); and irradiating selected areas of the deposited raw material powder layer with an electromagnetic or particle radiation beam (22) in a site selective manner in accordance with an irradiation pattern which corresponds to a geometry of at least part of a layer of the three-dimensional work piece to be produced, wherein the irradiation is controlled so as to produce a metallurgical bond between the substrate (15) and the raw material powder layer deposited thereon. Moreover, a use and apparatus are likewise disclosed.

Claims

1. A method for producing or repairing a three-dimensional metallic work piece having a substantially single crystalline microstructure, the method comprising the following steps: providing at least one substantially single-crystalline substrate; depositing a first layer of a metallic raw material powder onto the substrate; and irradiating selected areas of the deposited raw material powder layer with an electromagnetic or particle radiation beam in a site selective manner in accordance with an irradiation pattern which corresponds to a geometry of at least part of a layer of the three-dimensional work piece to be produced, wherein the irradiation is controlled so as to produce a metallurgical bond between the substrate and the raw material powder layer deposited thereon, and the irradiation is controlled so that the following applies: a remelting rate of a remelting within a plane of a presently irradiated raw material powder layer Rx fulfils the following condition: Rx>0.3, with Rx=((Wdy)/W), W being a melt pool width and dy being a distance between adjacent irradiation sites of the raw material powder layer, wherein the distance dy is defined by adjacent scan vectors along which respective irradiation sites are arranged, and wherein the remelting results from radiation beam diameters of the radiation beam overlapping each other when being moved along the adjacent scan vectors.

2. The method according to claim 1, wherein after completing irradiation of the first raw material powder layer, multiple sequences of depositing and irradiating subsequent raw material powder layers are performed, to successively build up the work piece along a build axis.

3. The method according to claim 1, wherein a remelting rate along the build axis Rz fulfils the following condition: Rz>0.3, with Rz=((Dlz)/D), lz being the layer thickness of the presently irradiated raw material powder layer and D being a melt pool depth occurring as a result of the irradiation.

4. The method according to claim 3, further comprising the step of: adjusting the crystal orientation of the single-crystalline substrate so as to correspond to the build axis.

5. The method according to claim 1, further comprising the step of: adjusting a crystal orientation of the single-crystalline substrate and a grain growth direction in the layer of the three-dimensional work piece occurring upon irradiating said layer so as to correspond to one another.

6. The method according to claim 1, wherein at least one of the following parameters is used for controlling the irradiation: a beam size, a defocusing state and/or a beam profile of the electromagnetic or particle radiation beam, an exposure time of the selected areas of a deposited raw material powder to the electromagnetic or particle radiation beam, the irradiation pattern, a speed of moving an irradiation site across a deposited raw material powder layer, and an energy input of the electromagnetic or particle radiation beam into the selected areas of the raw material powder layer applied onto the substrate.

7. The method according to claim 6, wherein the at least one parameter is constant between at least some of the subsequent raw material powder layers.

8. The method according to claim 1, wherein the irradiation is controlled in dependence on the crystallization behavior of the raw material powder in such a manner that single-crystalline layers of the three-dimensional work piece are produced.

9. The method according to claim 1, wherein the substrate fulfils at least one of the following conditions: the substrate covers at least 0.1%, at least 10%, at least 25%, at least 50%, at least 75% or 100% of a build area that is available for depositing raw material powder layer thereon in order to produce the work piece; the substrate is configured as a substantially planar member and, for example, defines a rectangular plane; a thickness of the substrate along the build axis is not more than 1000 mm, not more than 200 mm, not more than 100 mm, not more than 50 mm or not more than 10 mm, the substrate is a single crystal work piece which, for example, needs to be repaired.

10. The method according to claim 1, further comprising the step of separating the produced work piece from the substrate and, optionally, re-using the substrate for the production of a further work piece.

11. The method according to claim 1, further comprising the step of pre-heating a deposited raw material powder layer prior to irradiating it for producing a work piece layer.

12. The method according to claim 1, wherein a uni-directional irradiation pattern or a multi-directional irradiation pattern is used.

13. Apparatus for producing or repairing a three-dimensional metallic work piece having a substantially single crystalline microstructure, the apparatus comprising: at least one substantially single-crystalline substrate; a powder application device adapted to depositing a first layer of a metallic raw material powder onto the substrate; and an irradiation device adapted to irradiating selected areas of the deposited raw material powder layer with an electromagnetic or particle radiation beam in a site selective manner in accordance with an irradiation pattern which corresponds to a geometry of a layer of the three-dimensional work piece to be produced, and a control unit adapted to controlling the irradiation device so as to produce a metallurgical bond between the substrate and the raw material powder layer deposited thereon, and the control unit controlling the irradiation device so that the following applies: a remelting rate of a remelting within a plane of a presently irradiated raw material powder layer Rx fulfils the following condition: Rx>0.3, with Rx=((Wdy)/W), W being a melt pool width and dy being a distance between adjacent irradiation sites of the raw material powder layer, wherein the distance dy is defined by adjacent scan vectors along which respective irradiation sites are arranged, wherein the remelting results from radiation beam diameters of the radiation beam overlapping each other when being moved along the adjacent scan vectors.

14. Apparatus according to claim 13, wherein a remelting rate along the build axis Rz fulfils the following condition: Rz>0.3 with Rz=((Dlz)/D), lz being the layer thickness of the presently irradiated raw material powder layer and D being a melt pool depth occurring as a result of the irradiation.

Description

(1) In the following, preferred embodiments of the invention are explained in greater detail with reference to the accompanying schematic drawings, in which:

(2) FIG. 1 shows an apparatus for producing three-dimensional work pieces,

(3) FIG. 2 shows, in schematic form, a v-G-diagram, wherein the solidification or crystal growth velocity v in a metallic melt is plotted against the temperature gradient G in the melt.

(4) FIG. 1 shows an apparatus 10 for producing three-dimensional work pieces by selective laser melting (SLM). The apparatus 10 comprises a process chamber 12 which may be sealed against the ambient atmosphere such that an inert gas atmosphere, for example an Argon atmosphere, may be established within the process chamber 12. A powder application device 14 serves to apply a raw material powder by releasing it above a carrier 16. The carrier 16 is designed to be displaceable in vertical direction so that, with increasing construction height of a work piece, as it is built up in layers from the raw material powder on the carrier 16, the carrier 16 can be moved downwards in the vertical direction (in a negative Z-direction).

(5) On top of the carrier, a single-crystalline substrate 15 is arranged which is movable along with the carrier 16. The powder application device 14 deposits a first raw material powder layer directly onto said substrate 15. Further raw material powder layers may then be subsequently deposited on top of said first raw material powder layer according to known additive layer manufacturing processes and especially according to known SLM processes. The workpiece to be produced may thus be build up in a positive direction along the vertical Z-axis of FIG. 1 which forms a build axis.

(6) The substrate 15 is provided with a uniform crystal orientation which corresponds to the build axis, thus extending along the Z-axis. Other than that, the substrate 15 extends within the X-Y plane of FIG. 1 and completely covers a build area that can be used for generating workpiece layers.

(7) The apparatus 10 further comprises an irradiation device 18 for selectively irradiating laser radiation onto the deposited raw material powder. By means of the irradiation device 18, the deposited raw material powder may be subjected to laser radiation in a site-selective manner in dependence on the desired geometry of the work piece that is to be produced. The irradiation device 18 has a hermetically sealable housing 20. A laser beam 22 provided by a laser source 24 which may, for example, comprise a diode pumped Ytterbium fibre laser emitting laser light at a wavelength of approximately 1070 to 1080 nm is directed into the housing 20 via an opening 26.

(8) The irradiation device 18 further comprises an optical unit 28 for guiding and processing the laser beam 22, the optical unit 28 comprising optical elements such as a beam expander 30 for expanding the laser beam 22, a focusing lens 32 for focusing the laser beam 22 at a focus point and a scanner unit 34. The scanner unit 34 and the focus lens 32 are shown by way of example in the form of a galvanometer scanner and an f-theta object lens. By means of the scanner unit 34, the position of the laser beam 22 can be changed and adapted for moving said beam across a deposited raw material powder layer. For example, the irradiation device 18 may be an irradiation device as described in EP 2 335 848 A1.

(9) In addition, the apparatus 10 comprises a heating device 37 for preheating the raw material powder prior to irradiating the laser beam 22 onto the raw material powder.

(10) Finally, the apparatus 10 comprises a control unit 38 which is adapted to control the operation of the powder application device 14 and the irradiation device 18 in dependence on the crystallization behavior of the raw material powder, in order to tailor the microstructure of a work piece made of said raw material powder by a additive layer construction method. The crystallization behavior of a metallic melt, may be described a v-G-diagram, which, in schematic form, is illustrated in FIG. 2. In the v-G-diagram, a solidification or crystal growth velocity v is plotted against the temperature gradient G, typically on a double logarithmic scale. A v-G-diagram may be measured or calculated for any desired material, as it is well known in the art. In the schematic v-G-diagram of FIG. 2, a curve C separates an areal region of the diagram, wherein the combination of high solidification or crystal growth velocities and low temperature gradients lead to the development of a polycrystalline globulitic microstructure, from an areal region of the diagram, wherein the combination of low solidification or crystal growth velocities and (local) undercooling of the melt due to high temperature gradients result in the formation of dendrites and single crystals. With regard to the present embodiment, these temperature gradients particularly relate to temperature gradients in the negative Z-direction of FIG. 1.

(11) The control unit 38 of the apparatus 10 thus is adapted to control the operation of the powder application device 14 and the irradiation device 18 in such a manner that, in dependence on the raw material powder type, a suitable combination of the solidification or crystal growth velocity and the temperature gradient occurring in the melt produced by irradiating the powder with the laser beam 22 is obtained in order to generate the desired microstructure, and in particular a directionally solidified microstructure comprising substantially dendrites and/or single crystals.

(12) Specifically, the control unit 38 is adapted to control the laser source 24 and the optical unit 28 so as to adjust different operating parameters of the laser source 24 and the optical unit 28 in dependence on the crystallization behavior of the raw material powder, in order to tailor the microstructure of a work piece made of said raw material powder by an additive layer construction method. The operating parameters of the laser source 24 and the optical unit 28 which may be controlled by means of the control unit 38 include a beam size, in particular a beam diameter, of the laser beam 22 irradiated onto the raw material powder applied onto the carrier 16 and a beam profile of a laser beam 22 irradiated onto the raw material powder applied onto the carrier 16.

(13) For example, the beam size of the laser beam 22 may be increased under the control of the control unit 38 in order to promote the occurrence of a low solidification or crystal growth velocity in combination with a high temperature gradient in the melt produced by irradiating the powder and to thus obtain a substantially single crystalline or directionally/dendritically solidified microstructure in the generated work piece. Additionally or alternatively thereto, it is conceivable to promote the occurrence of a low solidification or crystal growth velocity in combination with a high temperature gradient in the melt and thus the formation of a substantially single crystalline or directionally/dendritically solidified microstructure in the generated work piece by changing the beam profile of the laser beam 22.

(14) Further, the operating parameters of the laser source 24 and/or the optical unit 28 which may be controlled by means of the control unit 38 include a moving speed of an irradiation site (presently corresponding to a scan speed) across the deposited raw material powder and/or a radiation or scan pattern of the laser beam 22. For example, the moving speed may be set under the control of the control unit 38 in order to promote the occurrence of a low solidification or crystal growth velocity in combination with a high temperature gradient in the melt produced by irradiating the powder and to thus obtain a substantially single crystalline or directionally/dendritically solidified microstructure in the generated work piece. For doing so, a moving speed between 50-500 mm/s may be chosen. Additionally or alternatively thereto, it is conceivable to promote the occurrence of a low solidification or crystal growth velocity in combination with a high temperature gradient in the melt and thus the formation of a substantially single crystalline or directionally/dendritically solidified microstructure in the generated work piece by setting the distance between hatches along which the laser beam 22 is guided over the powder surface. Said distance may be chosen to be less than a beam diameter of the laser beam 22 or, as a general example, may be less than 1 mm.

(15) Finally, a laser power of the laser source 24 may be controlled by means of the control unit 38 in such a manner that, in dependence on the raw material powder type, a suitable combination of the solidification or crystal growth velocity and the temperature gradient occurring in the melt produced by irradiating the powder is obtained in order to generate the desired microstructure. Specifically, the laser power of the laser source 24 may be increased under the control of the control unit 38 in order to promote the occurrence of a low solidification or crystal growth velocity in combination with a high temperature gradient in the melt and thus the formation of a substantially single crystalline or directionally/dendritically solidified microstructure in the generated work piece.

(16) The control unit 38 further is adapted to control the operation of the carrier 16 in connection with an operation of the powder application device 14 so as to adjust a thickness of a raw material powder layer applied onto the carrier 16 in dependence on the crystallization behavior of the raw material powder, in order to tailor the microstructure of a work piece made of said raw material powder by an additive layer construction method. For example, the operation of the carrier 16 may be controlled so as to move by a predetermined amount in the negative Z-direction, said amount corresponding to the thickness of the raw material powder layer being deposited by means of the powder application device 14. Specifically, the thickness of the raw material powder layer applied onto the carrier may be set to a value between 50-250 m, if it is desired to promote the occurrence of a low solidification or crystal growth velocity in combination with a high temperature gradient in the melt and thus the formation of a substantially single crystalline or directionally/dendritically solidified microstructure in the generated work piece.

(17) Moreover, the control unit is adapted to control the heating device 37 so as to adjust a preheating temperature of the raw material powder in dependence on the crystallization behavior of the raw material powder, in order to tailor the microstructure of a work piece made of said raw material powder by an additive layer construction method.

(18) Finally, the control unit 38 is adapted to control any of the above discussed parameters such that a grain growth direction of the irradiated and thereby melted raw material powder layers corresponds to a crystal orientation of the substrate 15. In the present case, this means that the control unit 38 sets the above parameters such that a grain growth direction along the build axis is achieved.

(19) In sum, the single-crystalline substrate 15 thus promotes a single-crystalline microstructure of the workpiece layers produced from the raw material powder deposited thereon. This single-crystalline microstructure and the associated grain growth is maintained when depositing and irradiating subsequent raw material powder layers by means of suitably setting the above-discussed parameters with the control unit 38.

(20) For doing so, a very thin top surface layer of the single-crystalline substrate 15 is melted when irradiating a raw material powder layer being deposited first and directly onto the substrate 15. This way, a metallurgical bond forms between said surface layer of the single-crystalline substrate 15 and the melted powder material of the first layer. Due to the substrate 15 having a preferred crystal orientation, the microstructure of the melted powder material will also epitaxially grow along this orientation to produce and overall single-crystalline work piece. Overall, it is not mandatory that the substrate 15 has a respective single-crystalline microstructure. However, this may allow for a particularly efficient production of a single-crystalline microstructure within the produced workpiece layer.

EXAMPLE 1

(21) A predominantly single-crystalline work piece having a height along the build axis Z of about 10 mm has been generated from the material IN738LC with the device of FIG. 1. This material has been used both for the substrate 15 as well as the raw material powder deposited thereon.

(22) Alternatively, the substrate and powder material may have a different chemical composition form one another, wherein the substrate is preferably single-crystalline.

(23) For producing work pieces based on this powder material and substrate, suitable ranges for the relevant process parameters have been identified. Specifically, the laser power has been set to 500-1000 W, the scan speed has been set to 50-500 mm/s, a hatch distance between adjacent scan vectors has been set to 100-500 m and the layer thickness of the deposited raw material powder has been set to 50-250 m.

(24) Furthermore, these parameters have been set so that a remelting rate Rz along the build axis Z fulfils the following condition: Rz>0.3 and a remelting rate within the plane of a presently irradiated raw material powder layer Rx fulfils the following condition: Rx>0.3. Said remelting rates are determined as discussed above. Moreover, it has been found that good results are still achieved when only setting one of the remelting rates in this manner.

(25) Overall, according to this example, a single-crystalline work piece has been produced with an improved quality and a higher reliability while using the (preferably single-crystalline) substrate 15.