EP4117845A1

EP4117845A1 - System for controlling the temperature of the construction space in powder bed fusion-based additive manufacturing installations

Info

Publication number: EP4117845A1
Application number: EP21711532.8A
Authority: EP
Inventors: Jan GIERSE; Florian HENGSBACH; Dominik AHLERS; Mirko Schaper; Thomas Tröster; Thorsten Marten
Original assignee: Addition GmbH
Current assignee: Addition GmbH
Priority date: 2020-03-10
Filing date: 2021-03-10
Publication date: 2023-01-18
Also published as: WO2021180766A1; DE102020106516A1

Abstract

The present invention relates to a sintering device for the additive manufacturing of metallic workpieces using a powder bed, comprising at least the components: a) irradiation unit; b) metal powder feed; c) vertically movable build platform, the build platform forming the base of the powder bed; d) process chamber walls delimiting the powder bed to the sides, the process chamber walls being formed from metal and being individually and locally selectively heatable and coolable as a function of the height from the base of the build platform, wherein a plurality of temperature sensors are arranged in or on the process chamber walls vertically spaced apart from one another. The present invention further relates to the use of a device according to the invention for the additive manufacture of metallic workpieces.

Description

System for temperature control of the installation space of powder bed-based additive manufacturing systems

The present invention relates to a sintering device for the additive manufacturing of metallic workpieces by means of a powder bed, at least comprising the components: a) irradiation unit; b) feed for metal powder; c) construction platform designed to be movable in height, the construction platform forming the bottom of the powder bed; d) the process chamber walls delimiting the powder bed to the sides, the process chamber walls made of metal and designed to be individually heatable and coolable as a function of the height from the floor of the building platform, with a plurality of height-spaced temperature sensors being arranged in or on the process chamber walls . The present invention also relates to the use of a device according to the invention for the additive manufacturing of metallic workpieces.

The additive manufacturing of workpieces has become more and more important in manufacturing technology in recent years. Additive manufacturing is used to manufacture workpieces by sequentially adding substances, usually in layers. Well-known manufacturing processes, such as milling, cutting or turning, work out the shape of the workpiece by removing substance from a larger blank. What speaks in favor of additive manufacturing in the industrial Elmfeld is that a high degree of design freedom is also made possible for demanding applications and complex geometries. It can Individual pieces can be manufactured at economically justifiable costs, which ultimately saves storage and tool costs.

In principle, a large number of different substances can be processed additively. For example, as part of 3D printing, plastics are heated above their melting temperature and extruded in layers or at points from a nozzle to form the molded body. Metals can also be processed additively. Metals can, for example, be joined to workpieces in the form of a powder in a powder bed using thermal processes. In general, the use of powder-bed-based additive manufacturing technologies enables the production of highly complex geometries, also based on the application of material in layers. This process can also be referred to as multilayer micro-welding. During production, a high-power laser or electron beam exposes the contour of the component in the powder bed belonging to the current layer and briefly melts the affected area locally. After each layer, the powder bed is lowered further, a new powder layer is applied, smoothed and locally melted again until the component is completed.

Currently, this process can only be used to produce components made of easily weldable materials. Across all alloys, however, additively manufactured components have both a crystallographic and a morphological preferred direction.

A large number of brittle or particularly hard materials, such as high-carbon steel, can only be processed with difficulty using additive manufacturing, since the components produced usually have micro and / or macro cracks directly after additive manufacturing. Among other things, this can be attributed to the fact that during processing, rapid heating and cooling in the melts and in the surrounding region induce large internal stresses, which result in undesired solidification, melting and embrittlement cracks. Ultimately you can these cracked components are not used or only with difficulty in practice, which greatly limits the usability of the components produced.

A wide variety of approaches to additive manufacturing using laser sintering can also be found in the patent literature.

For example, EP 1 762122 B2 discloses a radiant heater for heating a construction material in a laser sintering device with a planar heat radiating element, with power connections being provided on the heat radiating element so that current can be sent through the heat radiating element in the planar direction for operation as a resistance heating element, characterized in that the heat radiating element consists of a material with a low thermal inertia and that the heat radiating element consists of a material which at a temperature of 20 ° C has a thermal diffusivity of more than about 1.5 * 10 ⁴ m ² / s, the heat radiating element at least in one section is designed in the form of a meander-like surface web.

Furthermore, WO 2012 104 536 A2 discloses a device for producing or building a metal part by sintering and laser melting, the device comprising a laser beam generator, a means for deflecting the beam in order to scan the surface of the part to be produced, and a sintering pan containing a metal powder that is used to cover the surface of the part and to be melted by the laser beam to thicken the part. The invention is characterized in that it also comprises at least one means for heating powder which is contained in a region of the sintering pan by induction.

Such solutions known from the prior art can still offer further improvement potential, in particular with regard to the possible quality, for example in the form of the tendency to crack, of the workpieces available. It is therefore the object of the present invention to at least partially overcome the disadvantages known from the prior art. It is in particular the object of the present invention to provide an improved device which enables the production of workpieces with few cracks within a powder bed process from materials that are difficult to weld.

The object is achieved by the features of the independent claims, directed to the device according to the invention and the use of the device according to the invention. Preferred embodiments of the invention are specified in the subclaims, in the description or the figures, with further features described or shown in the subclaims or in the description or the figures individually or in any combination may constitute an object of the invention if they result from the context does not clearly indicate the opposite.

The object is achieved according to the invention by means of a sintering device for the additive manufacture of metallic workpieces by means of a powder bed, which at least includes the following components: a) irradiation unit; b) feed for metal powder; c) construction platform designed to be movable in height, the construction platform forming the bottom of the powder bed; d) the process chamber walls delimiting the powder bed at the sides, the process chamber walls being made of metal and designed to be heatable and coolable in a location-selective manner as a function of the height from the floor of the building platform, with several temperature sensors spaced apart in height being arranged in or on the process chamber walls . Surprisingly, it has been shown that macro-crack-free workpieces can be obtained using an additive powder bed manufacturing process using the structure specified above. Without being bound by theory, the use of jacket heaters, through which the powder bed can be preheated in a targeted manner, contributes in particular. The jacket heating also heats the applied metal powder and the components to be manufactured from the jacket surfaces in the construction direction. Thus, a temperature gradient in the direction of construction can be successfully reduced. In addition to jacket heating, it is important to control and monitor the temperature in the powder bed by means of one or more thermal contact sensors or by means of thermography. The sensors for controlling the jacket heating are arranged directly in or on the metallic jacket and can contribute precisely to the active control and monitoring of the preheating temperature of the powder. The temperature of the powder can thus be regulated to a precisely defined value shortly before the actual sintering process, so that this regulation reduces the tendency of the material to crack. In this way, even materials that are difficult to weld, such as high-carbon steels, can be processed using an additive powder bed process. In addition, specific temperature gradients in the powder bed can be set and maintained via the separately controllable heating and cooling, whereby the fine adjustment is significantly improved via the independent heating and cooling. Another advantage of cooling results from the fact that the powder bed can be controlled after completion and the temperature can be reduced in a precise location, which can counteract tension in the finished workpiece and drastically shorten set-up times.

The sintering device according to the invention is a sintering device for the additive manufacturing of metallic workpieces by means of a powder bed. Using the device according to the invention, metallic workpieces can therefore be manufactured by means of an additive manufacturing process. A powder bed process is carried out from the group of additive manufacturing processes. The method per se is known to the person skilled in the art. As part of the process, a layer by layer application of a metal powder and subsequent thermal Machining of the metal powder at the points where the workpiece is to be built, any workpiece geometry can be obtained. Metallic workpieces are workpieces that are predominantly made of metal. For example, the metal content of the workpieces can be greater than or equal to 75, further preferably greater than or equal to 80 and further preferably greater than or equal to 85 percent by weight.

The device comprises an irradiation unit. Energy is fed into the powder bed at a precise location via the irradiation unit. The irradiation takes place at the points where a further layer is to be added to the previously applied workpiece layers. For this purpose, the metal powder in the powder bed is thermally melted at these points by the irradiation unit. The irradiation unit is able to send out high-energy radiation, which leads to welding of the metal powder due to the locally accurate temperature increase. The irradiation unit can comprise, for example, a laser source, a light source or also an electron beam source. Furthermore, the irradiation unit can have additional structures which, for example, contribute to the positioning of the energetic beam on the powder bed. Furthermore, the irradiation unit can also have protective devices, mirrors, lenses or similar structures.

The device comprises a feed for metal powder. The feed for the metal powder serves the purpose of layering metal powder on top of the existing powder bed. The metal powder is thus applied sequentially and the powder bed is irradiated via the irradiation unit between two application processes. The application device is set up so that a certain amount of powder is applied to the powder bed per unit area of the powder bed. The application unit can also be designed to remove excess powder from the powder bed and to store it in one or more storage devices. The device comprises a construction platform designed to be movable in height, the construction platform forming the bottom of the powder bed. The bottom of the powder bed forms a construction platform, which is designed to be movable in height. For example, it is possible for the construction platform to be lowered in the course of manufacture. The construction platform is usually in an elevated position at the beginning of the manufacturing process and is lowered in proportion to the further layer structure by depositing metal powder. This enables a more or less constant distance between the powder bed and the irradiation unit. The construction platform can also have other technical facilities, such as heating, on its underside. The building platform can be lowered using a hydraulic ram mechanism, for example.

The device comprises the process chamber walls delimiting the powder bed on the sides, the process chamber walls being made of metal and configured so that they can be heated and cooled in a location-selective manner as a function of the height from the floor of the building platform, with a plurality of temperature sensors spaced apart in height in or on the process chamber walls are arranged. The process chamber walls or the jacket surfaces can consist of metal if the proportion of a metal in the process chamber walls is greater than or equal to 75 percent by weight. The process chamber walls are the walls that are in direct contact with the powder bed. The process chamber walls can be heated and cooled. This means that different areas of the process chamber walls can each be individually supplied with thermal energy via a heating source. The different areas of the process chamber walls can each also be individually controlled via cooling and / or heating elements. The heating can take place in such a way that in each case a certain height section of the process chamber walls can be tempered by a separate heating device. This results in parallel sections on the process chamber walls, the temperature of which can be set individually. The consequence of this is that the corresponding sections in the powder bed can be individually controlled in terms of their temperature parallel to the building platform. Possible temperature ranges the individual heating are, for example, greater than or equal to 200 ° C., preferably greater than or equal to 400 ° C., greater than or equal to 600 ° C. and greater than or equal to 850 ° C. For the reproducible determination of the temperatures in the different areas of the powder bed, it has been found to be very important that temperature sensors are used to control the powder bed temperatures, which are in direct mechanical contact with the process chamber walls. Accordingly, the temperature sensors can be attached in or directly to the process chamber walls. In this respect, the temperatures can be measured very directly and the entire system can react much more quickly to deviations in the specified temperature profile. For this purpose, each process chamber wall has at least two temperature sensors, the temperature sensors being at different distances from the building platform. According to the invention, each process chamber wall can have at least 5, furthermore preferably 10, and furthermore preferably 15 temperature sensors. In addition to the differences in the distance to the building platform, it is also possible that several temperature sensors are arranged at a specified distance from the building platform. This makes it possible, for example, to record the thermal influence of the distance to the other process chamber walls. The individual temperature sensors can be the same or different in height from one another. It is thus possible, for example, for the temperature sensors to each have the same distance from one another. This distance can be, for example, 1 cm, 5 cm or 10 cm. The distances can preferably be selected as a function of the size of the workpiece or as a function of the thickness of the newly applied powder layers. However, it is also possible for the temperature sensors to have a different distance from one another. For example, the distances in the area of the melting of the metal powder can be selected to be smaller. In this way, the temperatures of the powder bed in the area of the welding can be controlled very precisely. The cooling can take place, for example, via inert gases or liquids, for example in the form of molten solids, with the cooling medium being able to be arranged either between the heater and the building space or between the heater and the environment. Within a preferred aspect of the device, the sintering device can be a laser sintering device. The energy input of the laser can be controlled very precisely and in this combination, a very precise control of the temperature of the powder bed in the area of the welding is advantageously noticeable. The device according to the invention is a laser sintering device in which at least 50% of the thermal energy for melting the powder is provided by a laser.

In a preferred embodiment of the device, the heating of the process chamber walls can be selected from the group consisting of radiation, induction, resistance heaters or combinations thereof. These heating systems have proven to be particularly suitable for reproducible and rapid control of the temperatures of the powder bed. With these types, a very even temperature of the powder bed can be provided, which results in an improved isotropy of the workpieces available.

In a preferred embodiment of the device, the process chamber walls can be heated via a bifilar wound resistance heater located outside the process chamber wall and the process chamber walls can be cooled via an internal liquid cooling. The combination of external bifilar wound resistance heating with internal cooling has surprisingly led to clear advantages in the workpieces available. The bifilar winding of the conductors of the resistance heating can contribute to the compensation of electromagnetic fields of the heating. In addition, the asymmetrical winding of the heating conductors can help minimize the temperature gradient at the level of the heated process wall. Due to the additional shielding of the metal powder by the cooling medium, for example in the form of molten metal, salts or ionic liquids, magnetic inhomogeneities seem to be further weakened, which leads to an even more uniform and weakened magnetic field. contributes to the netic profile in the powder of the installation space. In contrast to external cooling, this configuration can be significantly more advantageous. Such an arrangement is also not obvious, since the thermal load is initially potentially increased by the cooling medium. This is partly correct, but the advantages with regard to the magnetic effects and the equalization of the energy radiation into the powder through a dissipative effect of the cooling medium clearly predominate. In addition, dynamic heating profiles can be established through independent control of the heating and cooling, which can be controlled to a high degree in a spatially resolved manner. In this way, even strongly asymmetrical designs can be produced very homogeneously and free of cracks in the powder bed.

Within a further preferred aspect of the device, the irradiation unit can comprise a laser and a focusable IR radiation unit. A focused top heating unit with IR radiators outside the process chamber in conjunction with a laser has proven to be particularly suitable to avoid undesirable heating of the process space and more specific focus of the amount of energy on the essential powder areas. This combination of IR emitter and laser enables workpieces that are particularly crack-free to be achieved. Without being bound by theory, this can probably be attributed to the fact that an excessively large temperature difference to the powder bed surrounding the laser spot can be avoided over the larger effective area of the IR radiator. The radiation from the IR radiation source can be parallelized via a collimator and the emitters can be positioned outside the chamber by means of one or more plano-convex lenses. The latter can minimize the heating of the build chamber as such. Another advantage can be that the powder layer can be exposed at an angle so that the geometry of the actually elliptical illumination field of the IR radiator can be changed with the aid of a defined lens and, for example, can be formed into a round exposure field. For this purpose, for example, the focusing lenses can be placed in a water-cooled frame, which also acts as a gas-tight seal the building chamber can act. The power of the focusable IR radiation unit can, for example, be in a range of 0-20 W / cm ² .

In a further preferred embodiment of the device, the height-adjustable construction platform can be designed at least in two parts from a lower platform heater and an upper, construction platform, the seal between the platform heater and the construction platform via high-temperature-resistant lamellar rings, which between tween the construction platform and the Platform heating are arranged. This configuration has resulted in a flexible solution for the lower installation space, which at the same time prevents powder material from getting stuck internally and jamming between the two structural parts. By using a lamellar ring, a secure seal between the process wall and the construction platform can be achieved, so that the trickling of metal powder into the gap between the wall and the movable floor platform can be prevented. The lamellar ring seals reliably and better than, for example, the glass fibers that are frequently used, even when moving and under high temperature loads with high contact forces. High-temperature-resistant laminar rings can, for example, be made of Ni-based or made of heat-resistant steel. The ring can preferably have an elliptical shape before assembly. The positioning of the lamellar ring is chosen so that the separate building platform can be easily installed and removed. In addition, this installation point prevents the powder material from getting stuck internally and jamming with the lamellar ring, so that undesired leaks can be avoided. This installation location and the use of a lamellar ring in combination with an inner coating of the process wall are particularly suitable. In particular, these two features can avoid sintering powder material on the process wall. The additional coating reduces the friction of the construction platform and can be high-melting, so that sintering of the powder material on the wall can be further avoided. A blocking or an irregularity in the lifting mechanism can advantageously be avoided in this way. The additional coating can also minimize the fretting of the lamellar ring towards the process wall. Furthermore, a comparatively higher clamping force of the spring ring can be selected, which also ensures reliable sealing of the installation space during the cooling-up and cooling-down phases.

In a preferred embodiment of the device, the temperature sensors can be arranged directly on the outside of the process chamber walls. In addition to arranging the temperature sensors in the process chamber walls, it can also be advantageous that the temperatures are measured by sensors which are located directly on the outer walls of the process chamber walls.

Within a preferred characteristic of the device, the inside of the process chamber walls can be coated non-stick at least at the level of the temperature sensors. Surprisingly, it turned out to be advantageous for the process chamber walls to receive a further coating. The coating prevents the metal powder from sticking to the process chamber walls. The further coating can consist, for example, of a CoCrMo coating or have this.

The use of the device according to the invention for the additive manufacturing of metallic workpieces is also according to the invention. The device according to the invention can be used particularly advantageously in powder bed processes in which metallic workpieces are produced. The metallic workpieces are made from metal powders whose thermal properties are less favorable compared to the thermal properties of other powders. In this respect, the device according to the invention can also be used to improve the properties of the metallic workpieces obtainable for these difficult-to-process metal powders. Crack-free metallic workpieces can result.

Within a preferred embodiment, the device according to the invention can be used to manufacture laser-sintered workpieces, the materials of the laser-sintered Tert workpieces are selected, for example, from the group consisting of high-carbon steel, titanium aluminides, iron aluminides, wear-resistant cobalt alloys, nickel-containing materials. The advantages of the device according to the invention can occur in particular with laser-sintered workpieces which are made from materials that are difficult to weld, such as high-carbon steels. In many cases, high carbon steels can only be thermally welded inadequately due to their composition. According to the prior art, there are usually workpieces which show severe cracking. By means of the device according to the invention presented here, in addition to thermally induced stresses, in particular cracks in additively manufactured components made of the above-mentioned materials can be avoided.

Further advantages and advantageous embodiments of the objects according to the invention who illustrated by the drawings and explained in the following description. It should be noted that the drawings are only of a descriptive nature and are not intended to restrict the invention.

It shows the:

1 schematically shows the structure of a system according to the invention for laser sintering;

2 shows an embodiment of the device according to the invention with external heating and internal cooling;

Fig. 3 shows an inventive embodiment of the device with two-part Ausgestal device of the coolable and heatable construction platform with sealing elements in the form of high-temperature-resistant lamellar rings.

In FIG. 1, a possible structure of a system according to the invention is shown schematically. The structure contains an irradiation unit 1, 2, 3, which in this case consists of a collimator and focusing unit 1, an XY scanner 2 and a protective laser glass 3. stands. The actual laser or electron beam can be generated elsewhere. The laser beam is passed through the irradiation unit and can then be directed through the laser protection glass 3 onto the powder bed 9 in a location-selective manner. The device also has a coater 5, which provides the powder bed 9 with new layers of powder. Excess material can be transported away into the powder overflow container 4 by means of the coater 5. The powder bed 9 is delimited at the bottom by the construction platform 10 and at the sides by the process chamber walls 8. In this case, the process chamber walls 8 have the temperature sensors (not shown). The current temperature profile of the powder bed can be measured by means of the temperature sensors arranged at different heights

9 can be measured. On the outside of the process chamber walls 8, heat sources 7 are arranged which, depending on the height of the building platform 10, can hit the powder bed 9 at different heights with different temperatures. The powder bed 9 is delimited on the sides by insulation 14. For the thermal insulation of the entire structure, a cooling sleeve 6 can be used, which has, for example, water cooling. The wall is also designed to be coolable 16, the cooling being between the heating elements 7 and the powder bed 9. The building platform 10 can for example have a platform heater 11, which together with the building platform

10 is designed to be movable in height via a hydraulic ram 12. The stamp 12 can have insulation 13, for example. At the start of the production of a me-metallic workpiece, the construction platform 10 is located in the upper region of the device. A predefined powder layer thickness is sequentially deposited on the building platform 10 via the applicator 5, which is then irradiated with laser light, for example, via the irradiation unit 1, 2, 3. The powder bed 9 is irradiated in a location-selective manner and the metal powder is melted at these points. After the site-selective irradiation, the construction platform 10 is lowered and a new powder layer is applied via the application device 5. In this way, a metallic workpiece is produced additively. FIG. 2 shows an embodiment of the device according to the invention with a focus on the arrangement of the heating elements 7 and cooling elements 15. This figure shows that the heating elements 7 represent a heating element 7 which is external to the installation space 9. The internal cooling 15 is arranged closer to the installation space 9. Both temperature control circuits are integrated into the side walls of the device. The cooling circuit 15 can preferably be operated by means of a liquid temperature control medium, which is preferably able to further scatter magnetic currents which could be caused by the heating. The magnetic currents through an induction heater 7 can be reduced, for example, by arranging the power lines required for generating heat in a bifilar manner in the installation space wall. Through the heating 7 and cooling 15, un different height ranges of the powder bed in the installation space can be controlled very precisely in the temperature distribution. In addition, an improvement in set-up times can be achieved via the cooling system 15. In principle, however, it is also possible for the cooling to be provided via a gas cooling system 16 which, in relation to the heating elements 7, is arranged further away from the installation space. In addition, the figure shows that in addition to the installation space walls, the floor of the device can also be heated and cooled. This temperature control can be achieved both via liquid and gas cooling.

FIG. 3 shows an embodiment of the device according to the invention with a two-part construction platform 17, 18. The lower platform heater 17 of the construction platform can have both cooling and heating means and can use the sealing element 19 in the form of highly temperature-resistant lamellar rings 19 with respect to the actual upper construction platform 18 be sealed. The upper construction platform 18 carries the actual powder bed. In this figure, the side walls 8 of the installation space 9 are also shown.

Claims

1. Sintering device for the additive manufacturing of metallic workpieces by means of a powder bed, at least comprising the components: a) irradiation unit (1, 2, 3); b) feed for metal powder (5); c) construction platform (10) designed to be movable in height, the construction platform (10) forming the bottom of the powder bed; d) the process chamber walls (8) delimiting the powder bed to the sides, characterized in that the process chamber walls (8) are made of metal and can be heated (7) and cooled (15) in a location-selective manner as a function of the height from the floor of the construction platform (10) , wherein in or on the process chamber walls (8) a plurality of height-spaced temperature sensors are arranged.

2. Apparatus according to claim 1, wherein the sintering device is a laser sintering device.

3. Device according to one of the preceding claims, wherein the heating (7) of the process chamber walls (8) via an external, bifilar wound resistance heater (7) in the process chamber wall (8) and the cooling of the process chamber walls (8) via an internal liquid cooling system (15) ) he follows.

4. Device according to one of the preceding claims, wherein the temperature sensors are arranged in the process chamber walls (8).

5. Device according to one of claims 2-4, wherein the irradiation unit (1, 2, 3) comprises a laser and a focusable IR radiation unit.

6. Device according to one of the preceding claims, wherein the height be movable designed construction platform (10) is formed at least in two parts from a lower Plattformhei tongue (17) and an upper construction platform (18), wherein the seal (19) between's platform heating (17) and building platform (18) via high-temperature-resistant Lamel lenringe (19), which are arranged between the building platform (18) and the platform heater (17).

7. Device according to one of the preceding claims, wherein the inside of the process chamber walls (8) is coated non-stick at least at the level of the temperature sensors.

8. Use of a device according to one of claims 1-7 for the additive manufacturing of metallic workpieces.

9. Use according to claim 8 for the production of laser-sintered workpieces, the materials of the laser-sintered workpieces being selected from the group consisting of high-carbon steel, titanium aluminides, iron aluminides, wear-resistant cobalt alloys, nickel-containing materials.