WO2015086585A1

WO2015086585A1 - Method for producing a three-dimensionally structured object by sintering and sintering method therefor

Info

Publication number: WO2015086585A1
Application number: PCT/EP2014/077026
Authority: WO
Inventors: Jens GÜNSTER; Jürgen Heinrich; Thomas MÜHLER
Original assignee: Bundesrepublik Deutschland, Vertreten Durch Den Bundesminister Für Wirtschaft Und Energie, Dieser Vertreten Durch Den Präsidenten Der Bundesanstalt Für Materialforschung Und -Prüfung (Bam); Technische Universität Clausthal
Priority date: 2013-12-13
Filing date: 2014-12-09
Publication date: 2015-06-18
Also published as: DE102013114003A1; DE102013114003B4

Abstract

The invention relates to a method for producing a three-dimensionally structured object by sintering, said method having the steps: providing a three-dimensional green body (7) which is at least semi-transparent to a laser beam (6a, 6b, 6c); and focussing the energy of at least one selected laser beam (6a, - 6b, 6c) onto selected points (9) in the three-dimensional green body (7) in order to carry out laser sintering, the at least one laser beam (6a, 6b, 6c) penetrating the volume of the three-dimensional green body (7).

Description

The invention relates to a method for sintering a three-dimensional structured object.

The invention further relates to a sintering apparatus for the selective laser sintering of a three-dimensional green body for the production of a three-dimensional structured object.

The construction of three-dimensional structures by local sintering of a green body is known per se. Here, the thermally activated sintering of particles of the green body, in particular of ceramic particles of a ceramic green body is utilized. The activation energy required for sintering is coupled by means of a laser to the points of the green body to be sintered.

Conventionally, in this case, the structure of the green body is carried out in layers such that the surface of the green body are each sintered in a two-dimensional plane at selected points by directing a laser beam to the selected points on the surface. In this way, the structured object is formed in layers. Thus, from DE 1 01 28 664 A1 a method and an apparatus for the production of ceramic moldings by sintering selected locations a ceramic material with a laser beam known. Here, at least one layer of a liquid suspension or plastic mass is applied, which is then dried and sintered at selected locations with a laser beam to form the shaped body.

A similar process is described in US 4,863,538 A1. Again, a laser beam is directed by means of a deflection mirror on the surface of a layered green body. DE 1 97 30 742 A1 discloses a method for producing a shaped body, in which a laser beam is moved with an XY scanner. The laser power is adjusted so that the grains of the starting powder are at least partially smashed, so that a mechanical bond is effected by toothing of the powder grains. Again, the shaped body is produced in layers by selective laser sintering.

DE 1 0 2004 01 2 682 A1 also discloses a method and a device for the layered production of three-dimensional objects with a movable device for the layered application of a powdered substrate on a working platform, a device movable in the XY plane for applying an absorber Materials as well as with a laser. The laser energy is not supplied directly to the substrates to be connected, but via an absorber, which absorbs the energy and releases it in the form of heat energy to the surrounding substrate.

Proceeding from this, it is an object of the present invention to provide an improved method for sintering a three-dimensional structured object. The object is achieved by the method having the features of claim 1 and by the sintering device having the features of claim 7. Advantageous embodiments are described in the subclaims.

It is proposed to provide a three-dimensional green body which is at least partially transparent to a laser beam. In this case, the energy of at least one selected laser beam is focused on selected points in the three-dimensional green body for laser sintering, wherein the at least one laser beam penetrates into the volume of the three-dimensional green body. In contrast to the conventional methods, in which the laser sintering is always performed on the surface of a layered green body, the present invention teaches a laser sintering in the volume of a three-dimensional green body. This must be at least partially transparent to the laser beam. The laser beam thus penetrates into the interior of the volume of the three-dimensional green body, so that radiation energy is absorbed in the irradiated volume. By focusing the laser beams within the volume of the three-dimensional green body, these points in the volume space of the three-dimensional green body can then be heated above the required sintering temperature for the time required for sintering.

The focusing of the energy can z. B. by selectively crossing at least two laser beams in the laser sintering selected points in the three-dimensional green body done. In contrast to the energy of a single laser beam, a sintering temperature required for laser sintering is then reached or exceeded in the intersection of the at least two laser beams.

The selective crossing of laser beams can be carried out relatively simply by computer control by controlling a mirror system for deflecting the laser beams. It is also conceivable, however, for the energy to be focused by rotation of the three-dimensional green body in the beam path of a laser beam such that the radiation power required for local sintering of a point at the intersection of the laser beam with the axis of rotation of the rotating three-dimensional green body is selected as the laser sintering point acts. As a result of the rotation of the green body, the energy of the laser beam penetrating into the green body is distributed in the volume in such a way that the required sintering temperature is not reached over the length of the laser beam. Only at the point of intersection of the laser beam with the axis of rotation of the laser beam remains at the selected point, so that over there is a selective heating of the point for sintering over time.

It is particularly advantageous if particles are added to the three-dimensional green body to modify the absorption coefficient of the green body. This allows the absorption coefficient to be specifically influenced and can be optimized for the partial absorption of the laser energy. In this case, it is particularly advantageous if the particles are selected with respect to the wavelength of the laser beam used and the material of the green body so that an increase in the absorption takes place, so that the particles of the green body are heated.

The mean particle size of the particles of the three-dimensional green body is preferably less than 0.25 times the wavelength of the laser beam used. The mean particle size is preferably less than 0.1 times the wavelength of the laser beam used. This ensures that the laser radiation can penetrate the green body, in particular if the green body consists of a ceramic material which does not absorb the laser radiation per se. Due to the small particle size in relation to the wavelength, it is achieved that the laser radiation can penetrate the green body. A sintering apparatus for selectively sintering a three-dimensional green body to produce a three-dimensional structured object has a platform for supporting the three-dimensional green body, at least one laser beam unit for emitting a laser beam onto the three-dimensional green body, and

a control unit, which is set up for focusing the energy of at least one selected laser beam of the at least one laser beam unit onto selected points in the three-dimensional green body for laser sintering, wherein the at least one laser beam is provided for penetrating into the volume of the three-dimensional green body.

For focusing the energy of the at least one selected laser beam, the sintering device may have at least two laser beam units. These laser beam units and the control unit are then set up for selectively crossing the at least two laser beams of the laser beam units in the points selected for laser sintering in the three-dimensional green body. For this purpose, the laser beam units can for example have a controllable by the beam unit mirror system with which the multiple laser beams are aligned freely to the volume of the green body and thus can be crossed in smallest possible points in the volume of the three-dimensional green body.

Optionally, or optionally in combination with the crossing of the laser beams, the platform for the green body can be rotatably mounted rotatably. The sintering device then has a rotary drive which is coupled to the rotation of the three-dimensional green body with the platform. The control unit is connected to the rotary drive and the at least one laser beam Coupled coupled and set up, for example by appropriate programming, the three-dimensional green body in the beam path of the laser beam over a predetermined sintering time such that the required for local sintering of a point radiation power at the intersection of the laser beam with the axis of rotation of the rotating three-dimensional green body as a selected point to Lasersinterung acts. The exposure of the radiation power then occurs for a sufficient time required for laser sintering at the temperature reached at the selected point.

The invention will be explained in more detail with reference to embodiments. Show it :

Figure 1 - sketch of a first embodiment of a sintering device with two laser beam units and intersecting laser beams focused energy;

Figure 2 - sketch of a second embodiment of a sintering device with rotatable platform and three-dimensional green body on the platform;

FIG. 3 shows a diagram of the particle size distribution over the particle size of an aqueous 5% by mass Si0 ₂ suspension.

FIG. 1 shows a sketch of a first embodiment of a sintering device 1 which has a control unit 2 and a plurality of, for example, two laser beam units 3a, 3b. The laser beam units 3a, 3b each have a laser beam generator (laser) 4 for generating a laser beam 6a, 6b with a predetermined wavelength, which is aligned with a controllable deflection mirror 5 of the respective laser beam unit 3a, 3b. The laser beam units 3a, 3b and in particular the deflecting mirrors 5 can be controlled by the control unit 2 so that the respective laser beam 6a, 6b, which is generated by the laser beam units 3a, 3b, is aligned with a three-dimensional green body 7. The three-dimensional green body 7 is in this case mounted on a platform 8 and positioned there in a stationary manner. By changing the angle of the deflection mirror 5, the laser beams 6a, 6b can optionally be positioned so that the laser beams 6a, 6b meet at selected points 9. In these points of intersection 9, the energy of the laser beams 6a, 6b is then focused so that a predetermined sintering temperature T _S i _n t _e r over the time in which the laser beams 6 a, 6 _b remain in the respective position for sintering the corresponding particles in the point of the three-dimensional green body 7 is exceeded. In this way, in the three-dimensional space arbitrary volume points of the three-dimensional green body 7 are selected and sintered.

The prerequisite for this is that the three-dimensional, preferably ceramic green body 7 for the wavelength of the laser beams 6a, 6b is at least partially transparent, so that the laser beams 6a, 6b can penetrate the volume of the three-dimensional green body 7.

The green body 7 is transparent or partially transparent to the laser radiation used, but has a different optical refractive index than the surrounding medium, which is usually air or a gas. The thus caused diffraction of the laser radiation at the transition into the green body 7 (transition air (gas) - green body), with non-normal incidence of radiation, can be calculated and compensated by the controller. To avoid diffraction of the laser beams at the transition into the green body 7, the laser beams can also be arranged such that the laser beams only impinge perpendicularly on the surface of the green body 7. For movement of the crossing point 9, only the green body 7 is then moved relative to the laser beams 6a, 6b. For this purpose, it may also be expedient to make the green body not cylindrically but with plan, parallel surfaces, as a cube or cuboid.

The irradiation of a ceramic green body 7 with laser light is achieved in that the green body 7 consists of a ceramic material which does not absorb laser radiation. In the illustrated embodiment, this is achieved in that the powder particles which form the green body 7 are small compared to the wavelength of the laser beams 6a, 6b. The particle size here is less than 0.25 of the wavelength and is typically selected in the range from 0.1 to 0.5 and preferably 0.1 of the wavelength, so that the laser radiation can penetrate the green body 7. In principle, lasers with a wavelength of between 0.5 and 4 .mu.m are suitable for the application described. Commercially available laser systems are available for this purpose, which provide sufficient power for local sintering of ceramic particles and emit at wavelengths of 0.5 to about 2.5 μm. Preference is given to using lasers having a wavelength of 0.9 to 2.5 μm, since the particle size of the particles used to construct the green body 7 can thus be about 200 nm (average particle size in the range from about 100 to 300 nm) , The particle size can be processed by conventional ceramic process technology, such as dry pressing, slip casting or slip casting to form compact green bodies 7 with a residual porosity of less than 40% by volume. Smaller particles are generally much more expensive in their processing into ceramic green bodies 7 and can not readily be formed into compact green bodies 7 with a residual porosity of less than 40% by volume.

In the described embodiments, the laser beams 6a, 6b are controlled by control by the computer program-based control unit 2 such that at the selected crossing points 9 the predetermined sintering temperature T _sinte r for a certain period of time (holding time) at which the temperature is approximately equal to the sintering temperature T. _sin t _e r is achieved. Here sinter the ceramic particles of the green body 7 at the intersection 9, without the adjacent particles are significantly impaired. This local sintering makes it possible to build up a three-dimensional structure in the volume of the ceramic green body 7 by selectively heating individual crossing points 9 and not the entire volume irradiated by the laser beams 6a, 6b. In this case, the absorption of laser energy leads to heating to the sintering temperature Tsinter- only at the selected points of intersection 9. The minimum size of the volume points in which the heating of the green body 7 leads to local sintering, without sintering directly adjacent volume areas, corresponds to FIG maximum resolution with which three-dimensional structures can be built up. The concentration of the radiation power on the crossing points 9 of the sample is achieved in the illustrated embodiment by crossing a plurality of laser beams 6a, 6b. It is conceivable in the embodiment that even more than two laser beams are used for crossing.

FIG. 2 shows a second embodiment of the sintering apparatus 1 for sintering a three-dimensional structured object from a three-dimensional green body 7. Again, a computer-controlled control unit 2 is provided which drives a single laser beam unit 3c. Again, the laser beam unit 3c has a laser 4 and a deflection mirror 5.

In this illustrated embodiment, the platform 8 is rotatably supported by means of a rotary drive 1 0. The green body deposited on the platform 8 can thus be rotated about an axis of rotation R. During the rotation of the green body 7 about the axis of rotation R, the energy of the laser beam 6c is focused on the point of intersection 9 of the laser beam 6c with the axis of rotation R. In the crossing point 9, the predetermined sintering temperature T _S i _n t _e r is reached for the time required for local sintering (holding time).

In order to be able to locally sinter any intersection points 9 in the volume of the green body 7, the green body 7 or the platform 8 is arranged to be relatively movable relative to the rotary drive 10, so that the axis of rotation R can be displaced with respect to the green body 7. This displaceable or otherwise movable in space device is indicated by the arrows on the platform 8.

In both embodiments, in addition to the selected, locally sintered crossing points 9, in general also other areas of the green body 7 are heated by the laser beams 6a, 6b, 6c. However, the temperature remains Due to the process-related lower radiation power, the temperature of the other regions is far below the sintering temperature T _sin t _e r- Even at temperatures below the sintering temperature T _sinte r, sintering can be initiated. However, this requires a considerably longer holding time. Via a computer control, only the entire volume of the main body 7 in the laser beam path is positioned in such a way that it is possible to construct complex structures by focusing the laser beams 6a, 6b, 6c, which penetrate the volume of the ceramic green body 7. FIG. 3 shows a diagram of the particle size distribution over the particle size in μιη for an aqueous 5 mass% Si0 ₂ suspension. Here, a silicon oxide (SiO ₂ ) powder having a d ₅₀ of 1 86 nm and a particle size distribution shown in FIG. 3 is formed into a cylindrical green body 7 by uniaxial pressing.

In Figure 3, the particle distribution curves for various products are a) 4649 Corning

b) 4650-Corning

c) 4654-Corning

d) 4655 Corning

e) 4651-Wacker

f) 4652-Wacker

g) 4653-Wacker applied.

After the green body 7 has been produced, it is irradiated by a thulium fiber laser at a wavelength of 1940 nm and with a maximum power of 1 00 watts. The green body 7 is for the wavelength of Thulium fiber laser partially transparent. The partial transparency of the green body 7 is adjusted by suitable additives (color bodies such as carbon or organic compounds) in the Si0 ₂ powder. The focus of the laser beam 6c converging through a laser object lies in the volume of the green body 7. The focal point is the point of intersection 9, in which the green body 7 is locally heated to the predetermined sintering temperature T _sin t _e r. In order to achieve heating of the green body 7 to the sintering temperature T _sinte r only at this point, the green body 7 rotates about an axis of rotation R on which the focal point of the laser beam 6c is located. The process variables laser power, focal length of the laser optics and rotational speed of the sample are to be adjusted so that heating of the intersection point 9 to the predetermined sintering temperature Tsinter in the desired partial volume is achieved over the holding time required for local sintering.

After the so-executed local sintering of the three-dimensional structure, this is dissolved out with water from the unsintered remainder of the green body 7.

According to the particle size distribution in Figure 3, the powder used is tuned to the wavelength of the laser used so that the ceramic green body 7 is partially transparent to the wavelength.

By incorporating color bodies tuned to the wavelength of the laser energy used and adjusting the absorption coefficient for the wavelength, i. As a rule, the described sintering process can be optimized.

Furthermore, it is advantageous if the material of the green body and / or the additives used are selected so that the absorption coefficient of the sintered parts does not change significantly and remains sufficiently transparent. This ensures that these locally sintered points can still be penetrated by laser beams to from the laser source starting crossing points behind the already sintered points chen without the green body 7 must be rotated.

Claims

claims:

1 . Process for sintering a three-dimensional structured object, characterized by

Providing a three-dimensional basic body (7) which is at least partially transparent to a laser beam (6a, 6b, 6c) and focusing the energy of at least one selected laser beam on selected points (9) in the three-dimensional green body (7) for laser sintering, wherein the at least a laser beam (6a, 6b, 6c) penetrates into the volume of the three-dimensional green body (7).

2. The method according to claim 1, characterized by selectively crossing at least two laser beams (6a, 6b) in the laser sintering selected points (9) in the three-dimensional green body (7).

3. The method according to claim 1, characterized by rotation of the three-dimensional green body (7) in the beam path of a laser beam (6c) such that for a local sintering of a point (9) required radiation power at the intersection of the laser beam (6c) with the axis of rotation ( R) of the rotating three-dimensional green body (7) acts as a selected point (9) for laser sintering.

4. The method according to any one of the preceding claims, characterized by admixing of particles to form the three-dimensional green body (7) used to modify the absorption coefficient.

5. The method according to claim 4, characterized in that the admixing of particles for modifying the absorption as a function of the wavelength of the at least one laser beam (6a, 6b, 6c) takes place.

6. The method according to any one of the preceding claims, characterized in that the average particle size of the particles of the three-dimensional green body (7) is less than 0.25 times the wavelength of the laser beam used (6a, 6b, 6c) and preferably smaller than that 0, 1 - times the wavelength of the laser beam used (6a, 6b, 6c).

7. sintering apparatus (1) for selective laser sintering of a three-dimensional green body (7) for producing a three-dimensional structured object, comprising: a) a platform (8) for supporting the three-dimensional green body (7);

b) at least one laser beam unit (3a, 3b, 3c) for emitting a laser beam (6a, 6b, 6c) onto the three-dimensional green body; and c) a control unit (2) for focusing the energy of at least one selected laser beam (6a, 6b, 6c) of the at least one laser beam unit (3a, 3b, 3c) on selected points (9) in the three-dimensional green body (7) for laser sintering is arranged, wherein the at least one laser beam (6a, 6b, 6c) is provided for penetrating into the volume of the three-dimensional green body (7).

8. sintering device (1) according to claim 7, characterized in that the sintering device (1) has at least two laser beam units (3a, 3b) and the laser beam units (3a, 3b) and the control unit (2) are arranged to selectively cross the at least two laser beams (6a, 6b) of the laser beam units (3a, 3b) in the laser sintering selected points (9) in the three-dimensional green body (7) ,

9. sintering device (1) according to claim 7, characterized in that the platform (8) for the green body (7) is rotatably mounted rotatably and the sintering device (1) has a rotary drive (1 0) for rotation of the three-dimensional green body (7) the platform (8) is coupled, wherein the control unit (2) with the rotary drive (1 0) and the at least one laser beam unit (3c) is coupled and arranged, the three-dimensional green body (7) in the beam path of the laser beam (6c) over a predetermined To sintering time to rotate, such that the required for a local sintering of a point (9) radiation power at the intersection of the laser beam (6c) with the axis of rotation (R) of the rotating three-dimensional green body (7) acts as a selected point for laser sintering.