AND RELATED MANUFACTURING SET

The present invention relates to a method for manufacturing a three-dimensional object on a building platform by layer-wise solidification of a building material and a manufacturing set.

Methods for manufacturing a three-dimensional object by layer- wise solidification of a building material are also referred to as additive manufacturing methods. They also include rapid pro totyping and rapid tooling methods. Examples for such methods are known under the names "selective laser sintering" and "se lective laser melting". In these methods, thin layers of build ing material in powder form are applied repeatedly and the building material in each layer is selectively solidified at positions corresponding to a cross-section of a three- dimensional object by selective irradiation using a laser beam, i.e. the building material is melted or partially melted at these positions and then it solidifies. A method for producing a three-dimensional object by selective laser sintering or selective laser melting as well as an apparatus for carrying out this method are described, for instance, in EP 1 762 122 A1.

Usually, a building platform serves as a base for the three- dimensional object to be formed by means of additive manufac turing techniques. In doing so, the bottommost layer of the building material is applied on the building platform and selectively solidified such that the solidified material connects to the building platform. EP 0 734 842 A1 suggests to use as a building platform a sintered plate made of the same building material that is used for additively manufacturing the object such that the object connects better to the plate. In some em bodiments, it is necessary to separate the object and the building platform from each other in an additional step follow ing the completion of the object, e.g., by sawing or cutting.

Hence, it is an object of the present invention to provide an alternative, preferably improved, method for manufacturing an object as well as a manufacturing set for carrying out the method according to the invention.

The object is achieved by the method according to claim 1 and the manufacturing set according to claim 15. Refinements of the invention are specified in the dependent claims. Features spec ified in the dependent claims and in the description below that are mentioned with respect to the method may also be realized in the manufacturing set and vice versa.

A method for manufacturing a three-dimensional object according to the invention is a method for manufacturing a three- dimensional object on a building platform by layer-wise solidification of a building material at positions in the respective layer that correspond to the cross-section of the object, the method comprising

(a) applying a layer of the building material on at least a surface portion of the building platform or on a previous layer,

(b) selectively solidifying the building material of the layer at positions that correspond to the cross-section of the object in the layer,

(c) repeating steps (a) and (b) until the object is completed, wherein the surface portion of the building platform comprises a material to which the building material connects while being solidified, and

wherein the material of the surface portion and the building material have substantially different linear thermal expansion coefficients to an extent that after solidification the object and the building platform separate from each other automatical ly or semi-automatically when the temperature of the object and the building platform is reduced after the completion of the obj ect .

The temperature of the object may be reduced, for instance, to room temperature or a temperature close to room temperature.

The expression "room temperature" is used for a temperature in a range that is suitable for human occupancy, preferably a range from 15 °C to 25 °C, in particular a range from 20 °C to 23 °C. More specifically, it is used for the ambient tempera ture lying in that range .

The linear thermal expansion coefficients of the surface por tion and the building material are considered substantially different if the temperature reduction after solidification leads to an automatic or semi-automatic separation of the ob ject and the building platform separate from each other.

The linear thermal expansion coefficient of a material can be measured by known methods. Tabular values of the linear thermal expansion coefficient can be found, e.g., in CRC Handbook of Chemistry and Physics, 70th edition, CRC Press, Boca Raton, Florida, 1989, D-186, D-187, and E-106 to E-110.

As a consequence of the difference of the linear thermal expan sion coefficients, the object and the building platform or at least the surface portion thereof may show a different thermal contraction behavior upon temperature reduction. The resulting stress may either induce spontaneous separation so that neither additional interactions of the user nor any specific separation tool is required for separation, i.e. the separation of the object and the building platform can be achieved automatically.

Or the connection of the object and the building platform may be weakened such that the subsequent separation is facilitated, i.e. the separation of the object and the building platform can be achieved semi-automatically . In other words: an automatic separation is defined by a separation which necessitates no physical interaction with either the object or the building platform at all, whereas a semi-automatic separation is such form of separation in which such physical interaction may be necessary but whereby the separation forces due to the substan tially different linear thermal expansion coefficients amount to at most half of the overall necessary separation force.

According to the invention, it is possible, for instance, to separate an object and the building platform from each other in an uncomplicated and rapid manner. Nevertheless, the object may be sufficiently firmly connected to the building platform dur ing the manufacturing process. As a consequence, the overall fabrication process of an object may be facilitated and accel erated. Preferably, the risk of damages of the object and/or the building may be reduced in addition.

Preferably, the building platform is heated during manufactur ing of the object, more preferably to a temperature of at least 100 °C, even more preferably to a temperature of at least

200°C, most preferably to a temperature of at least 400 °C.

This may, for instance, reduce the temperature reduction of the object during its manufacture. An excessive temperature reduc- tion during the manufacture of the object could for instance lead to an unintended separation of the object and the building platform from each other before the object is completed.

Preferably, the absolute value of the difference between the linear thermal expansion coefficient ou of the material of the surface portion of the building platform and the linear thermal expansion coefficient a ₂ of the building material after its so lidification, i.e. | ax - « ₂ | , is larger than 4xlO ⁶ /K, more preferably larger than 6xlCt ⁶ /K, most preferably larger than

8xl0 ^~6 /K. Such values may, for instance, even more facilitate the separation of the object and the building platform from each other.

Preferably, at least the surface portion of the building plat form comprises a metal. More preferably, the metal is iron or an iron-based alloy, most preferably a steel. By using a metal, in particular iron or an iron-based alloy, especially a steel, it is, for instance, possible to achieve a high thermal stabil ity of the building platform, whereas, at the same time, the building platform has an appropriate thermal expansion coeffi cient.

Preferably, the building material is a powder, more preferably a powder including metallic particles. The separation of a metallic object, which may be manufactured from such material, is, for instance, particularly easy according to the invention as compared to the case where conventional techniques such as sawing or cutting are used.

Preferably, the linear thermal expansion coefficient of the ma terial of the building platform is larger than the linear thermal expansion coefficient of the building material. By doing so, it is for instance possible to avoid that the object under goes excessive thermal contraction upon temperature reduction.

In the context of the invention, it is also possible that the linear thermal expansion coefficient of the building material is larger than the linear thermal expansion coefficient of the material of the building platform.

Preferably, the building material is a powder including a re fractory metal or an alloy that is based on at least one re fractory metal. Here, the term "refractory metals" refers to the group of chemical elements consisting of Ti, Zr, Hf , V, Nb, Ta, Cr, Mo, W, Re, Ru, Os, Rh, and Ir. By using such building materials, it is for instance possible to manufacture the ob ject containing a refractory metal, especially tungsten.

The combination of a building platform comprising iron, in par ticular steel, with a building material comprising a refractory metal, in particular tungsten, may lead to particularly good results regarding the separation of the object from the build ing platform.

Preferably, reducing the temperature of the object and the building platform includes actively cooling the object and/or the building platform after the object has been completed, more preferably by exposing them to a cooling gas and/or a cooling liquid. This makes it for instance possible to accelerate the separation of the object and the building platform from each other .

Preferably, the connection between the object and the building platform is configured to break upon cooling under the given material and temperature conditions. This makes it for instance possible to separate the object and the building platform with out any additional interaction of the user and without any spe cific separation tools.

Preferably, the object and the building platform are connected to each other through predefined breaking points. This makes it for instance possible to reduce the forces to be applied for an automatic or semi-automatic separation.

Preferably, the object comprises support structures which are to be removed from the rest of the object after its separation from the building platform. This may for instance facilitate the manufacturing of objects having a finely structured fili gree shape and/or a hollow shape and/or protruding structures.

In a further aspect, the connection between the object and the building platform may be weakened in particular regions, e.g. particular regions of the support structure, to define a par ticular location where the separation is to take place. Such weakening may be achieved, for instance, by forming a notch de fining a predetermined breaking point.

Preferably, the step of selective solidification is achieved by exposing the building material to an energy source, more pref erably a laser beam and/or an electron beam. This may for in stance provide for a solidification of a great variety of building materials including, e.g., metallic material.

The manufacturing set according to the invention comprises· a) A building platform for an apparatus for manufacturing a three-dimensional object by layer-wise solidification of a building material at positions in the respective layer that correspond to the cross-section of the object, b) the building material itself,

wherein at least a surface portion of the building platform comprises a material to which the building material connects while being solidified, and

wherein the material of the surface portion of the build ing platform and the building material have different lin ear thermal expansion coefficients such that after solidi fication the object and the building platform are separat ed from each other automatically or semi-automatically, when the temperature of the object and the building plat form is reduced after the completion of the object. This manufacturing set provides a set for carrying out the method according to the invention.

Other features and expediencies of the invention may be found in the description of exemplary embodiments with the aid of the appended drawing .

Fig. 1 is a schematic view, partially represented in sec

tion, of an apparatus for layer-wise manufacturing a three-dimensional object using a method according to an embodiment of the present invention.

The apparatus shown in Fig. 1 is a laser sintering or laser melting apparatus 1 for manufacturing of one or more objects 2 (only one object 2 is shown) .

The apparatus 1 includes a process chamber 3 having a chamber wall 4. A container 5 being open at the top and having a con tainer wall 6 is arranged in the process chamber 3. The opening at the top of the container 5 defines a working plane 7, where in the area lying within the opening, which can be used for building up the object 2, is referred to as build area 8. A support 10 which can be moved in a vertical direction V is ar ranged in the process chamber 3. A base plate 11 which closes the container 5 toward the bottom may be fastened to the sup port 10 or may be formed integrally with the support 10.

A building platform 12, on which the object 2 is built, is at tached to the base plate 11. Alternatively, the building plat form 12 may be formed integrally with the base plate 11 or the building platform 12 may at the same time also serve as the base plate 11. Preferably, the building platform 12 is flat. It may, however, in principle have any other shape.

In Fig. 1, the object 2 to be manufactured is shown in an in termediate state. It consists of a plurality of solidified lay ers and is surrounded by non-solidified building material 13.

The apparatus 1 furthermore contains a storage container 14 for building material 15 in powder form, which can be solidified by electromagnetic radiation, for example a laser, and/or particle radiation such as an electron beam. The apparatus 1 also com prises an application device 16 which is also referred to as "recoating device" or "recoater" . The application device 16 is movable in a horizontal direction H for applying layers of building material 15 within the build area 8. Optionally, a ra diation heater 17 for heating the applied building material 15 may be arranged in the process chamber.

The apparatus 1 furthermore contains an irradiation device 20 having a laser 21 which generates a laser beam 22 that is de flected by the use of a deflecting device 23 and focused on the working plane 7 by means of a focusing device 24 via a coupling window 25. The apparatus 1 furthermore contains a control unit 29 for con trolling the individual components of the apparatus 1 for car rying out a method for manufacturing of an object 2. The con trol unit 29 may contain a CPU, the operation of which is con trolled by a computer program (software) .

During operation of the apparatus 1, the following steps are repeatedly carried out: for each layer, the support 10 is lowered by a distance, which preferably corresponds to the desired thickness of one layer of the building material 15. The appli cation device 16 is moved to the storage container 14, from which it receives an amount of building material 15 that is sufficient for the application of at least one layer. The ap plication device 16 is then moved over the build area 8 and ap plies a thin layer of the building material 15 in powder form on the building platform 12 or on a previously applied layer. The application takes place at least over the total cross- section of the object 2, preferably across the entire build area 8. Optionally, the building material 15 is heated to a work ing temperature by means of the radiation heater 17. Also op tionally, the building platform 12 and/or the base plate 11 is heated by means of at least one heater (not shown in Fig. 1) such as an electric heater. The cross-section of the object 2 to be manufactured is then scanned by the laser beam 22 in or der to selectively solidify this area of the applied layer. These steps are carried out repeatedly until the object 2 is completed.

After the object 2 is completed, the object 2 and the building platform 12 typically have a temperature above the room temper ature due to the energy used for the solidification of the building material 15, e.g. electromagnetic energy provided by a laser 21, and/or due to heating using a separate heating de- vice, for example an electric heater for heating a building platform 12. It is not required that the temperature of the ob ject 2 and the building platform 12 is uniform.

Preferably, the object 2 and the building platform 12 have - at least in the vicinity of the zone where the object 2 and the building platform 12 are connected to each other - a temperature that is higher than the room temperature by at least

75 °C, more preferably at least 175 °C, even more preferably at least 375 °C.

The temperature of the object 2 and the building platform 12 is then reduced. This temperature reduction may take place before and/or during the object and the non-solidified building mate rial 13 are separated from each other and/or after the object and the non-solidified building material 13 have been separated from each other.

The material of the surface portion on which the first layer is applied and the building material have substantially different linear thermal expansion coefficients to an extent that after solidification, the object (2) and the building platform sepa rate from each other automatically or semi-automatically when the temperature of the object and the building platform is re duced after the completion of the object (2) .

According to a first embodiment of the invention, the tempera ture reduction is achieved by free cooling, i.e. by letting the temperature of the object 2 and the building platform 12 approaching the room temperature .

According to a second and preferred embodiment of the invention, temperature reduction includes actively cooling the ob- ject 2 and/or the building platform 12 after the object 2 has been completed, more preferably by exposing the object 2 and the building platform 12 to a cooling gas and/or a cooling liquid, e.g. by a use of a cooling gas flow and/or a cooling liq uid flow.

Upon cooling, the connection between the object 2 and the building platform 12 weakens. In a preferred embodiment of the invention, spontaneous fracture occurs through the thin interface layer between the object 2 and the building platform 12 such that the object 2 and the building platform 12 are auto matically separated from each other. In this case, the connec tion simply breaks upon temperature reduction. The fractured surfaces are substantially smooth.

According to a third embodiment, the object 2 comprises support structures being connected to the building platform 12 during manufacturing of the object. For instance, such support structures may be designed as bracings or stiffenings that are re moved from the rest of the object after the latter has been separated from the building platform 12.

It is possible to use the support structures to connect the object 2 to the building platform 12. In this case, the connec tion is effected only through particular regions where the sup port structures adjoin the building platform 12. Therefore, the effect of weakening the connection of the object 2 to the building platform 12 upon cooling is particularly pronounced.

According to further embodiments of the invention the building material 15 comprises a material selected from the group con sisting of tungsten, molybdenum, Invar, niobium, silicon, tan talum, zirconium, iridium, and titanium; and the building plat- form 12 comprises a material selected from the group consisting of steels, copper, nickel-based superalloys, Inconel alloys, cobalt-based superalloys, brass, bronze, and silver.

In the context of the invention it is also possible to use a building material comprising a material selected from the group consisting of steels, copper, nickel-based superalloys, Inconel alloys, cobalt-based superalloys, brass, bronze, and silver; and a building platform comprising a material selected from the group consisting of tungsten, molybdenum, Invar, niobium, sili con, tantalum, zirconium, iridium, and titanium.

The following values for the linear thermal expansion coefficient are preferred for the following materials: between

10xl0 ^_6 /K and 16xlO ^_s /K for steels, 12xlO ⁶ /K for Inconel alloys, 12xlO ^_6 /K for cobalt-based superalloys, 18xlO ^_6 /K for brass and 18xlO ⁶ /K for bronze.

In a specific embodiment of the invention the building platform 12 is made of a steel material. Preferably, the steel is a mild steel and has a linear thermal expansion coefficient of about 12xlO ^_6 /K. While manufacturing the object, the building platform 12 is heated, preferably to a temperature of at least 400 °C. The building material 15 is a powder containing tungsten. Pref erably, powdered tungsten is used as building material 15. The object 2 is manufactured by a selective laser sintering tech nique or a selective laser melting technique. When the building platform 12 and the object 2 are cooled down after the comple tion of the object 2, the object 2 is automatically released from the building platform 12 due to thermal expansion stress.

In a further specific embodiment of the invention, the building platform 12 is made of a mild steel with AISI or SAE grade C45U corresponding to DIN 1.1730. Preferably, the building material 15 is a tungsten powder or molybdenum powder, particularly a powder having a purity of at least 95 wt%, i.e. a powder con taining at least 95 wt% tungsten or molybdenum, respectively.

In a further specific embodiment of the invention, the building platform 12 is made of a stainless steel with AISI or SAE grade 316L corresponding to DIN 1.4404. Preferably, the building ma terial 15 is a tungsten powder or molybdenum powder, particu larly a powder having a purity of at least 95 wt%, i.e. a powder containing at least 95 wt% tungsten or molybdenum, respec tively .

In a further specific embodiment of the invention, the building platform 12 is made of a stainless steel with AISI or SAE grade 304L corresponding to DIN 1.4307. Preferably, the building ma terial 15 is a tungsten powder or molybdenum powder, particu larly a powder having a purity of at least 95 wt%, i.e. a pow der containing at least 95 wt% tungsten or molybdenum, respec tively .

In a further specific embodiment of the invention, the building platform 12 is made of a tin bronze UNS number C70600 corre sponding to DIN 2.0872. Preferably, the building material 15 is a tungsten powder or molybdenum powder, particularly a powder having a purity of at least 95 wt%, i.e. a powder containing at least 95 wt% tungsten or molybdenum, respectively.

According to the present invention, it is possible to select an appropriate combination of the material of the building plat form 12 and the building material 15 such that both have linear thermal expansion coefficients leading to the result that the manufactured object 2 and the building platform 12 separate from each other automatically or semi-automatically upon tem perature reduction. In a situation where one of these materials is predefined, a suitable material can be selected as the other material .

Preferably, the material of the building platform 12 and/or the building material 15 are selected such that the solubility of these materials in each other and/or their tendency to form al loys is low. Preferably, "low solubility" means that the solu bility of the material of the building platform 12 is less than 10 mg, more preferably less than 1 mg per gram of the building material 15 and/or that the solubility of the building material 15 is less than 10 mg, more preferably less than 1 mg per gram of the material of the building platform 12.

For instance, in the case of the use of tungsten and steel as building material 15 and the material of the building platform 12, brittle intermetallics may be formed in the interface layer between the object 2 and the building platform 12 thus facili tating the separation of the object 2 and the building platform 12 from each other.

While the present invention has been described by means of se lective laser sintering or selective laser melting, respective ly, the present invention is not limited to selective laser sintering or selective laser melting. The present invention may be applied to any possible method for producing an object by applying in layers and selectively solidifying a building material. For example, an irradiation device may be used which con tains one or more lasers. The lasers may be gas lasers, solid- state lasers or lasers of any other kind, e.g. laser diodes, especially arrays having VCSEL (Vertical Cavity Surface Emit ting Laser) or any combination thereof. In principle, any irra- diation device by means of which energy may be selectively ap plied on a layer of the building material and suitable for solidifying the building material may be used. This may be a light source different from a laser, an electron beam, or any other suitable energy source or radiation source. The invention may also be applied to selective mask-sintering, in which a mask and an expanded light source are used instead of a de flected laser beam, or to absorption sintering or inhibition sintering. Instead of the introduction of energy, the selective solidification of the applied building material can also be performed by printing, e.g., by application of an adhesive. In general, the invention relates to the production of an object by means of layer-by-layer application and selective solidifi cation of a building material independently of the manner in which the building material is solidified.