Patent ID: 12240038

FIG.1shows an apparatus10for producing a three-dimensional work piece by an additive layering process. The apparatus comprises a carrier12and a powder application device14for applying a raw material powder onto the carrier12. The carrier12and the powder application device14are accommodated within a process chamber16which is sealable against the ambient atmosphere. An internal atmosphere is established with a shielding gas supplied by a process gas inlet15, the machine also comprises an process gas outlet not shown. Process gas may be circulated from the outlet to the inlet15, thereby cooled or heated. The carrier12is displaceable in a vertical direction into a built cylinder13so that the carrier12can be moved downwards with increasing construction height of a work piece18, as it is built up in layers from the raw material powder on the carrier12. The carrier can comprise a heater and/or a cooler.

The apparatus10further comprises an irradiation device20for selectively irradiating electromagnetic or particle radiation onto the raw material powder applied onto the carrier12. The irradiation device20comprises a radiation beam source22, in particular a laser beam source, and an optical unit24for guiding and processing a radiation beam emitted by the radiation beam source22. A control device26is provided for controlling the operation of the apparatus10and in particular the operation of the powder application device14and the irradiation device20.

Finally, the apparatus10is equipped with several sensor devices. A first sensor device27is adapted for measuring the temperature of the atmosphere inside the process chamber16. A second sensor device28is adapted to detect the temperature of raw material powder/work piece layer during and after being irradiated with electromagnetic or particle radiation. The sensor device28may, for example, be designed in the form of a suitable camera which is adapted to detect infrared radiation resolved to several locations on the raw material layer. In another exemplary embodiment the sensor device28may be a pyrometer device that may detect a temperature at a specific point inside the process chamber16, e.g. on the raw material layer, or an average temperature over an area inside the process chamber16, e.g. on the raw material layer. A third sensor device29is adapted for detecting radiation emitted from the raw material layer in the focus point of the radiation beam emitted by the radiation beam source22and/or in an area around the focus point. The sensed radiation is guided through the optical unit24to the third sensor device29. In a preferred exemplary embodiment the carrier12comprises an further fourth sensor device not shown for measuring the temperature of the carrier. The apparatus10may comprise further sensor devices, for example for measuring the temperature of a process gas at the process gas inlet15or another location, or for measuring the composition of the process gas inside the process chamber16. It is understood, that this example is not limiting and an apparatus according to the invention may comprise only few of the named sensors or all of them and may comprise further sensors.

During operation of the apparatus10for producing a three-dimensional work piece, a layer of raw material powder is applied onto the carrier12by means of the powder application device14. In order to apply the raw material powder layer, the powder application device14is moved across the carrier12under the control of the control unit26. Then, again under the control of the control unit26, the layer of raw material powder is selectively irradiated with electromagnetic or particle radiation in accordance with a geometry of a corresponding layer of the work piece18to be produced by means of the irradiation device20. The steps of applying a layer of raw material powder onto the carrier12and selectively irradiating the layer of raw material powder with electromagnetic or particle radiation in accordance with a geometry of a corresponding layer of the work piece18to be produced are repeated until the work piece18has reached the desired shape and size.

A scanning time for a respective raw material powder, i.e. a time period from the beginning of the exposure of at least a portion of a respective raw material powder layer to electromagnetic or particle radiation until the beginning of the exposure of a new raw material powder layer applied on top of said layer portion to electromagnetic or particle radiation is defined by the equation:
scanning time (ts)=exposure time (te)+waiting time (tw)+raw material powder application time (tp)

The exposure time is defined as a time period during which the raw material powder layer portion is in fact exposed to electromagnetic or particle radiation. The waiting time is defined as a time period during which the raw material powder layer portion is not exposed to electromagnetic or particle radiation and while no new raw material powder layer is applied on top of said layer. The raw material powder application time is defined as a time period during with a new raw material powder layer is applied on top of said layer portion.

For at least a portion of at least some of the raw material powder/work piece layers, the scanning time from the beginning of the exposure of a respective raw material powder layer portion to electromagnetic or particle radiation until the beginning of the exposure of a new raw material powder layer applied on top of said layer portion to electromagnetic or particle radiation is controlled by means of the control device26so as to not fall below a specific minimum value. Specifically, the exposure time, the waiting time, and the raw material powder application time are controlled such that the scanning time does not fall below the specific minimum value. The specific minimum value of the scanning time, i.e. the minimum scanning time is individually set for said layer portion in dependence on a layer portion specific quality parameter. The layer portion specific quality parameter may vary from layer portion to layer portion. Consequently, also the minimum scanning time may vary from layer portion to layer portion.

With increasing height of the work piece18, heat dissipation from the work piece layer portions after scanning becomes more and more difficult. Thus, during the production of the work piece18, a thermal gradient may develop within the work piece18, i.e. layer portions in an upper part of the work piece18may not cool down as desired during the regular process of scanning the layer portion and applying a new raw material powder layer and top of the scanned layer portion. In the production of big volume parts made of maraging steel 1.2709 this may cause the problem that layer portions in an upper part of the work piece18do not sufficiently cool so as to allow the desired austenite/martensite transformation. In particular, the transformation from austenite to martensite does not take place if the layer portion does not cool below the austenite/martensite transformation temperature, i.e. does not cool below 200° C.

Consequently, work piece layer portions that do not undergo the austenite/martensite transformation during build-up of the work piece18, only transform after the work piece18has been completed. This, however, may cause dimensional deviations over the height of the work piece18. In particular, the volume change involved with the austenite/martensite transformation may cause an enlarged width of the work piece18in an upper part of the work piece18due to the inability of the material to expand in the vertical direction when the phase transformation happens only after the completion of the work piece18.

In order to address this problem, in the apparatus10described herein, upon producing the work piece18from maraging steel 1.2709, a first layer portion specific quality parameter which is used by the control unit26for controlling the scanning time is indicative of a temperature of a respective layer portion at the end of the scanning time. In particular, the specific minimum value of the scanning time is set such that the respective layer portion has a desired crystallographic structure, namely a martensitic structure, at the end of the scanning time. In the exemplary embodiment described herein, this is achieved by ensuring that the temperature of the respective layer portion at the end of the scanning time does not exceed 200° C.

The first layer portion specific quality parameter and/or the minimum scanning time is/are determined, for a at least portion of at least some of the layers of the work piece to be produced, prior to starting the production of the three-dimensional work piece. Specifically, the first layer portion specific quality parameter, which is indicative of a temperature of a respective layer portion at the end of the scanning time, and the minimum scanning time are determined prior to starting the production of the three-dimensional work piece, for each layer portion, by means of a computer-aided simulation as shown inFIGS.2aandb.

As becomes apparent fromFIG.2a, the temperature of the raw material powder/work piece layer portions at the end of an envisaged scanning time which results from the geometry of the work piece layer portions to be produced and the envisaged operating parameters of the irradiation device such as the scan speed, the spot size and the power of the irradiation beam increases with increasing vertical height of the work piece18. In an upper part of the work piece18the temperatures rise up to 272° C. and thus well the above austenite/martensite transformation temperature of 200° C. Consequently, these work piece layer portions undergo the austenite/martensite transformation only after the work piece18has been completed. The volume change involved with the austenite/martensite transformation thus causes an enlarged width of the work piece18in an upper part of the work piece18as shown inFIG.3a.

FIG.2bshows the temperatures of the raw material powder/work piece layer portions at the end of a scanning time which, while taking into consideration the first layer portion specific quality parameter, is controlled so as to not fall below a layer portion specific minimum scanning time. If the scanning time is controlled so as to be long enough, the layer portions in the upper part of the work piece18have enough time to cool down to temperatures below 156° C. Consequently, each of the layer portions undergoes the austenite/martensite transformation already during the production of the work piece18allowing the volume change involved with the austenite/martensite transformation to take place in all directions (i.e. also in the vertical direction). A continuous width of the work piece18can thus can be achieved as shown inFIG.3b.

In the exemplary embodiment described herein, wherein the minimum scanning time, upon taking into consideration the varying heat dissipation with increasing vertical height of the work piece18is determined such that the first layer portion specific quality parameter, i.e. the temperature of a raw material powder/work piece layer portion at the end of the scanning time, does not exceed 200° C., the control device26adapts the scanning time to the minimum scanning time by simply prolonging the waiting time while keeping the exposure time constant (for a work piece with constant exposure area). It is, however, also conceivable for the control device26to adapt, i.e. to both the exposure time and the waiting time in order to ensure that the scanning time does not fall below the minimum scanning time.

Further, while in the exemplary embodiment described herein, the layer portion specific quality parameter and the minimum scanning time are determined by means of a computer-aided simulation prior to starting the production of the work piece18, it is also conceivable to determine the layer portion specific quality parameter and/or the minimum scanning time in situ during the production of the three-dimensional work piece. For example, the sensor device28may be used to measure the temperature of the raw material powder/work piece layer portions during production of the work piece18, e.g. either resolved locally or with an average value over the area. The control device26then may determine a suitable minimum scanning time which ensures that the temperature does not exceed 200° C. and adapt the current scanning time accordingly.

In another exemplary embodiment, the layer portion specific quality parameter and/or the minimum scanning time are determined in situ during the production of the three-dimensional work piece by the control device26in a closed loop control manner. This means the control device26may prolong the minimum scanning time, e.g. by prolonging the waiting time, in intervals, determining the current temperature in every interval, and stopping the waiting time when the determined temperature falls below a predetermined threshold value, e.g. 200° C.

Quality issues in the work piece18to be produced may also result from abrupt changes in exposure area and hence exposure time in portions of adjacent layers as shown in the lower discontinuous curve ofFIG.4. Therefore, in the apparatus10described herein, upon producing the work piece18, a second layer portion specific quality parameter which is used by the control unit26for controlling the scanning time is indicative of an abrupt exposure area change between at least a portion of a respective layer and at least a portion of an adjacent layer. The second layer portion specific quality parameter may easily be determined prior to the start of the production of the work piece18based on the geometry data of the work piece to be produced. Thus “critical” layer regions with layer portions showing abrupt exposure area changes and hence abrupt exposure time changes relative to at least portions of neighboring layers can easily be identified.

In order to avoid abrupt exposure area/exposure time changes between adjacent layer portions, the specific minimum value of the scanning time is set such that a difference in the scanning time between adjacent layer portions does not exceed a predetermined maximum value. This is in particular achieved by selecting a suitable exposure time while keeping the waiting time constant. Consequently, the control unit26controls the scanning time such that, in the “critical” layer region, the exposure time is continuously increased and continuously decreased instead of changed in in an abrupt manner as indicated by the upper continuous curve inFIG.4.