Description

The invention concerns specific powder mixtures comprising at least a first powder of a steel alloy comprising 4.75 to 5.5 wt.-% Cr, 1.0 to 1.75 wt.-% of Mo and 0.32 to 0.45 wt.-% of C and a second powder of a reinforcement material comprising particles having a diameter of less than 30 pm, wherein the mixture comprises about 0.1 to about 5.0 wt.-% of the second powder. The invention further concerns processes for the manufacture of such powder mixtures, processes and devices for the manufacture of three-dimensional objects, three- dimensional objects prepared by such processes and devices and the use of such a powder mixture for minimizing and/or suppressing the crack formation in a three dimensional object.

State of the art

Direct Metal Laser Sintering (DMLS) is a laser-based rapid prototyping and tooling process by means of which net shape parts are fabricated in a single process. Complex parts can be produced directly from 3D-CAD models by layer-wise solidification of metal powder layers in portions of the layer corresponding to the cross-section of the three-dimensional part in the respective layer. This process is described in detail for example in Juha Kotila et al., Steel-based Metal Powder Blend for Direct Metal Laser Sintering Process, Advances in Powder Metallurgy & Particular Materials - 1999, Vol.2 Part 5, p. 87-93 and in T, Syvanen et al., New Innovations in Direct Metal Laser Sintering Process - A Step Forward in Rapid Prototyping and Manufacturing, Laser Materials Processing, Vol. 87, 1999, p. 68 to 76.

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 example, in EP 1 762 122 Al.

There is a high demand for processing metal materials by additive manufacturing processes such as Direct Metal Laser Sintering, so that rapid manufacturing can be applied to applications where a specific material having well-known properties is required. One important class of materials is steel which is widely used in many products. Many different kinds of steel exist and are commercially available for conventional manufacturing methods, such as casting, forging, machining etc. as referenced in international standards, reference books, manufacturers' catalogues etc.

One example of a well-known conventional hot working tool steel which is frequently used in die-casting is H13 steel. It is alloyed with carbon, silicon, chromium, molybdenum and vanadium and has a comparatively high carbon content (about 0.40%), which is higher than that of most metal materials currently used in additive manufacturing. This high carbon content has the downside that it or its compounds causes cracking in parts built by a DMLS process.

There is thus a need to reduce such cracking from the microstructure and the final parts when H 13 or a comparable steel is used for the manufacture of the parts.

In order to improve the properties of three-dimensional objects, it is known in the art to produce three-dimensional objects from steel and reinforcement particles, for example silicon carbide particles, by means of conventional sintering methods and casting methods. The reinforcement effect, however, is limited, for example, due to the dissolution or segregation of the reinforcement particles in the molten iron during the processing time. It is the object of the invention to provide a metal powder mixture which can be processed by laser sintering or similar additive manufacturing methods using a heat source and whereby the object produced has similar properties compared to that of a H13 steel object produced using a conventional manufacturing method, such as casting. In particular, in the object cracking should be reduced

significantly or completely removed from the microstructure and the final parts to allow for use of the thus produced parts in applications requiring high levels of toughness and resistance to abrasion.

The object is achieved by the powder mixture according to claim 1, the process for the production of a powder mixture according to claim 7, the processes for the manufacture of a three-dimensional object according to claim 8, the three- dimensional objects according to claims 11 and 12, the use according to claim 14 and the device for implementing the process according to claim 8 according to claim 15. Refinements of the invention are specified in the dependent claims. Any feature set forth in the dependent claims as well as any feature set forth in the description of exemplary embodiments of the invention below can be understood as a feature suitable for refining the powder mixture, the process for the production of a powder mixture, the method and device for the manufacture of a three-dimensional object, the three-dimensional object itself and the above specified use.

Accordingly, in a first aspect the present invention concerns a powder mixture for use in the manufacture of a three-dimensional object by means of an additive manufacturing method, wherein the powder mixture comprises a first powder of steel alloy comprising 4.75 to 5.5 wt.-% Cr, 1.0 to 1.75 wt.-% of Mo and 0.32 to 0.45 wt.-% of C and a second powder of a reinforcement material comprising particles having a particle diameter of less than 30 pm, wherein the mixture comprises about 0.1 to about 5.0 wt.-% of the second powder

A powder mixture is understood as a granular mixture of two or more

components. The powder mixture according to the invention comprises at least a first and a second material. The second material comprises a reinforcement material, which is often a ceramic material. The powder mixture is for use in an additive manufacturing method. The so manufactured three-dimensional objects comprise materials, which are suitable together in a laser bed process. The reinforcement material has been embedded in a matrix material to thus produce a solidified product. Corresponding materials often have improved (mechanical, more weldable, etc.) properties compared to the matrix material and/or the reinforcement material. In the context of the invention, the reinforcement material has found to minimise cracking in three-dimensional objects prepared from H13 steel.

The powder mixture according to the invention is a powder mixture for use in the manufacture of a three-dimensional object by means of an additive

manufacturing method, wherein the powder mixture comprises a first powder and a second powder, wherein the first powder comprises a steel alloy in powder form, wherein the second powder comprises a reinforcement material, preferably a ceramic material, and wherein the powder mixture is adapted to form an object when solidified by means of an electromagnetic and/or a particle radiation in the additive manufacturing method. The powder mixture may comprise further materials. Using this powder mixture, for example improved additive

manufacturing methods, especially additive manufacturing methods leading to three-dimensional objects comprising a material with reduced or no cracking compared to the same object prepared from the pure matrix powder, are provided.

Preferably, the reinforcement material is embedded in the matrix material of the object at least partially in a chemically unmodified form. This means that at least a part of the reinforcement material being comprised by the second material does not undergo a change of its chemical composition prior to being embedded in the matrix.

In the practice of the invention, as stated above, the steel alloy contains 4.75 to 5.5 wt.-% Cr, 1.0 to 1.75 wt.-% of Mo and 0.32 to 0.45 wt.-% of C. Preferably, it further contains 0.8 to 1.25 wt.-% of Si, 0.8 to 1.2 wt.-% of V, 0.2 to 0.6 wt.-% of Mn, p to 0.05 wt.-% of P and 0.05 wt.-% of S.

In a preferred embodiment, the first powder has a particle size distribution with a d50 of 1 pm or more, more preferably 5 pm or more, still more preferably 10 pm or more, and/or 150 pm or less, more preferably 75 pm or less. In addition, or as an alternative, it is preferred in the invention that the first powder has a particle size distribution with a d50 of from of from 20 to 100 pm and preferably 25 pm or more and/or 50pm or less. The d50 designates the size where the amount of the particles by weight, which have a smaller diameter than the size indicated, is 50% of a sample's mass. Conventionally, as well as in the practice of the invention, the particle size distribution is determined by laser scattering or laser diffraction, e.g. according to ISO 13320:2009.

In one preferred aspect of the invention, the particles of the first powder of steel alloy are substantially spherical.

The second powder in the invention is a powder of a reinforcement material. The reinforcement material is not subject to any relevant restrictions, except that it can have the properties of a ceramic reinforcement and should be different from the steel alloy material of the first powder. In a preferred aspect, the

reinforcement material has a melting point which is higher than that of the metal alloy of the first material, to thus ensure that the reinforcement material does not melt under the conditions used for processing the metal powder by DMLS.

Suitable reinforcement materials include e.g. borides, carbides such as tungsten carbide (WC), silicon carbide (SiC) and/or titanium carbide (TiC), nitrides such as titanium nitride (TiN), oxides such as aluminium oxide (AI ₂ O3), silicides and graphite.

Preferably, the reinforcement ceramic material in the powder mixture of the invention includes a ceramic material selected from silicon carbide and/or titanium carbide, more preferably titanium carbide, and even more preferably the reinforcement ceramic material consists of at least 80 wt.-%, and especially at least 90 wt.-% of titanium carbide. In a yet further preferred embodiment, the reinforcement ceramic material in the powder mixture is titanium carbide.

The powder of reinforcement material in the invention has a (mean) diameter, based on the weight of the material of less than 30 pm, preferably less than 10 pm and even more preferably less than 3 pm and most preferably less than 1 pm. This particle size is conventionally and in the practice of the invention determined by laser scattering or laser diffraction e.g. according to ISO 13320:2009.

Moreover, in one embodiment of the invention it is advantageous that the powder of ceramic material has a particle size which is less than that of the metal powder. Therefore, in a preferred embodiment, the (mean) diameter of the powder of ceramic is at least 2 times smaller than the d50 of the powder of steel alloy, more preferably at least 5 times smaller and even more preferably at least 10 times smaller. The amount of the powder of reinforcement material in the powder mixture should on the one hand be so small, that the overall properties of the steel to be prepared are not negatively affected, which is regularly achieved when the powder mixture comprises not more than 5% of the reinforcement material. On the other hand, the amount of the powder of reinforcement material should be sufficiently high to ensure that the desired properties in a three- dimensional object thus prepared are obtained. As a preferred amount to fulfil this purpose an amount of about 0.15 wt.-% or more, more preferably about 0.20 wt.-% or more and even more preferably about 0.30 wt.-% or more can be mentioned. Particularly suitable upper limits for the amount of the powder of reinforcement material are about 1 wt.-% or less, in particular 0.75 wt.-% or less and even more preferably about 0.50 wt.-% or less.

The term "about" in this application, when used in the context of amounts, indicates the amount as such as well as any amounts having the same round-off. In case of a range specified by the term "about", the range covered is meant to cover any amounts having the same round-off above the range as well as below that range. It is noted in this connection, that any range disclosed herein with the wording "about" is also disclosed as the same range without this wording, even if this is not explicitly stated.

In a preferred embodiment of the invention, the particles of the reinforcement powder are substantially spherical. In another preferred embodiment, the particles of the reinforcement powder are substantially irregular.

The process for the production of a powder mixture according to a second aspect of the invention is a process for the production of a powder mixture for use in the manufacture of a three-dimensional object by means of an additive

manufacturing method, wherein the powder mixture comprises a first powder and a second powder, wherein the first powder comprises a steel alloy as described above, wherein the second powder comprises a reinforcement material, and wherein the powder mixture is adapted to form an object when solidified by means of an electromagnetic and/or a particle radiation in the additive

manufacturing method, wherein the powder mixture is produced by mixing the first powder and the second powder in a predetermined mixing ratio. Using this method, a powder mixture according to the invention can be produced.

Preferably, the mixing is a dry mixing.

A process for the manufacture of a three-dimensional object according to a third aspect of the invention is a process for the manufacture of a three-dimensional object from a powder mixture by selective layer-wise solidification of the powder mixture by means of an electromagnetic radiation and/or a particle radiation at positions that correspond to a cross-section of the object in a respective layer, wherein the powder mixture is a powder mixture for use in the manufacture of a three-dimensional object by means of an additive manufacturing method, wherein the powder mixture comprises a first powder and a second powder, wherein the first powder comprises a steel alloy, wherein the second powder comprises a reinforcement material, and wherein the powder mixture is adapted to form an object when solidified by means of an electromagnetic and/or a particle radiation in the additive manufacturing method. Using this method, for example a three- dimensional object with reduced cracking compared to the same three- dimensional object, which is only prepared with the metal alloy powder can be manufactured.

Preferably, the process for the manufacture of a three-dimensional object comprises the steps:

- providing a powder mixture as defined above, and

- preparing the object by applying the mixture layer on layer and selectively solidifying the mixture, in particular by application of electromagnetic radiation, at positions in each layer, which correspond to the cross section of the object in this layer, wherein the positions are scanned in at least one interaction zone, in particular in a radiation interaction zone of an energy beam bundle.

Without being bound by any theory, it is believed that when the reinforcement particles are evenly distributed in the melt of the metal alloy, they provide solidification points to the cooling melt. In the absence of such solidification points, the solid steel alloy forms large grains that shrink during solidification and as a result tear apart from each other causing cracks. In direct metal laser sintering, the cooling of the melt is much faster than in conventional

manufacturing methods. Thus, the forces created during solidification are greater than in a conventional casting process. Via the addition of the reinforcement powder, the number of solidification points is increased so that the

microstructure has smaller grains. Due to their smaller size, the tension between the grains can be maintained at a lower level, which prevents the grains from tearing away from each other.

The three-dimensional object may be an object of a single material (i.e., a material resulting from the processing of the powder mixture as described above) or an object of different materials. If the three-dimensional object is an object of different materials, this object can be produced, for example, by applying the powder mixture of the invention, for example, to a base body or pre-form of the other material.

In the process of the third aspect, by changing the temperature at which the three-dimensional object is prepared together with the non-melting reinforcement particles in the steel alloy matrix the cracking in the final microstructure of steel alloy can be reduced or completely removed. Thus, in the context of the inventive process, it may be expedient if the powder mixture of the invention is preheated via heating of the building platform to which the powder mixture is applied prior to selective solidification, with preheating to a temperature of at least 120°C being preferred, preheating to a temperature of at least 140°C being more preferred, and preheating to a temperature of at least 155°C may be specified as still more preferred. On the other hand, preheating to very high temperatures places considerable demands on the apparatus for producing the three- dimensional objects, i.e. at least to the container in which the three-dimensional object is formed, so that in one embodiment a maximum temperature for the preheating of at most 600°C and preferably at most 500°C can be specified.

For the inventive process, it is further preferred that the individual layers, which are subsequently subjected at least in part to treatment with electromagnetic radiation, are applied at a thickness of 10 pm or more, preferably 20 pm or more and more preferably 30 pm or more. Alternatively or cumulatively, the layers are applied at a thickness of preferably 100 pm or less, more preferably 80 pm or less and even more preferably 60 pm or less. In a most preferred embodiment the thickness, in which the layers are applied is in the range of 35 to 50 pm.

For the inventive process, it is further preferred that the amount of energy introduced into a given defined volume of the powder mixture is such that the reinforcement material is not completely dissolved during the time in which the energy is applied to the defined volume of the powder mixture. More preferably, the amount of energy introduced into a given defined volume of the powder mixture is such that the reinforcement material of the powder mixture is dissolved to 70 wt% or less, preferably 50 wt% or less, more preferably 30 wt% or less, even more preferably 5 wt% or less during the time in which the energy is applied to the defined volume of the powder mixture.

The three-dimensional object according to a fourth aspect of the invention is a three dimensional object manufactured from a powder mixture by selective layer- wise solidification of the powder mixture by means of an electromagnetic and/or particle radiation at positions that correspond to a cross-section of the object in a respective layer, wherein the powder mixture is a powder mixture for use in the manufacture of a three-dimensional object by means of an additive

manufacturing method, wherein the powder mixture comprises a first powder and a second powder, wherein the first powder comprises a steel alloy in powder form, wherein the second powder comprises a reinforcement material, and wherein the powder mixture is adapted to form an object when solidified by means of electromagnetic and/or particle radiation in the additive manufacturing method. The three-dimensional object has, for example, reduced cracking compared to the same three-dimensional object, which is only prepared with the metal alloy powder.

The three-dimensional object according to the invention in a fifth aspect of the invention is a three-dimensional object, which is constituted of a steel alloy comprising 4.75 to 5.5 wt.-% Cr, 1.0 to 1.75 wt.-% of Mo and 0.32 to 0.45 wt.-% of C as a matrix comprising particles of a reinforcement material having a particle diameter of less than 30 pm, wherein the reinforcement material accounts for 0.1 to about 5.0 wt.-% of the three dimensional object. The contents of the individual metals and reinforcement material in the object can be calculated mathematically from the contents of the metal in the precursor alloy and its content in the three-dimensional object, as well as from the content of the reinforcement material in the mixture used for the fabrication of the three dimensional object.

For specific embodiments of the steel alloy and the reinforcement material in the above three-dimensional object, reference is made to the above preferred embodiments which have been described in connection with the inventive powder mixtures.

The amount of reinforcement material in the above three-dimensional object can be determined by microscopic measurement of the area occupied by the reinforcement material in a transversal section through the three-dimensional object vs. the area occupied by the metal alloy.

For the three-dimensional object of either of the above fourth and fifth aspects, it is preferred that they have a relative density of 98% or more, preferably 99% or more and more preferably 99.5 % or more, wherein the relative density is defined as the ratio of the measured density and the theoretical density. The theoretical density is the density which can be calculated from the density of the bulk materials used to prepare the three-dimensional object (basically metal alloy and reinforcement material) and their respective ratios in the three-dimensional object. The measured density is the density of the three-dimensional object as determined by the Archimedes Principle according to ISO 3369:2006.

In a sixth aspect, the present invention concerns the use of a powder mixture as described above for minimizing and/or suppressing crack formation of in a three- dimensional object, wherein the three-dimensional object is prepared in a process involving the step- and layerwise build-up of the three-dimensional object by additive manufacturing, preferably by laser sintering or laser melting.

Finally, in a seventh aspect the present invention concerns a device for

implementing a process as described above in the third aspect, wherein the device comprises a laser sintering or laser melting device, a process chamber having an open container with a container wall, a support, which is inside the process chamber, wherein open container and support are moveable against each other in vertical direction, a storage container and a recoater, which is moveable in horizontal direction, and wherein the storage container is at least partially filled with a powder mixture as described in the first aspect. Other features and embodiments of the invention are provided in the following description of an exemplary embodiment taking account of the appended figures.

Figure 1 is a schematic view, partially represented in section, of a device for the layer-wise manufacture of a three-dimensional object according to an

embodiment of the present invention.

Figure 2 is a SEM picture of a H13 steel alloy powder in admixture with an irregular TiC powder with a d50 of 1,4 pm

Figure 3 is a micrograph of a test body prepared from pure H13 steel alloy powder at a platform temperature of 200°C

Figure 4 is a micrograph of a test body prepared from H13 steel alloy powder and 0.4% TiC powder (d50 of 1,4 pm) at a platform temperature of 165°C

Figure 5 is a set of micrographs of a test body prepared from H13 steel alloy powder and 0.4% TiC powder (d50 of 1,4 pm) at a platform temperature of 200°C

The device represented in Figure 1 is a laser sintering or laser melting apparatus 1 for the manufacture of a three-dimensional object 2. The apparatus 1 contains a process chamber 3 having a chamber wall 4. A container 5 being open at the top and having a container wall 6 is arranged in the process chamber 3. The opening at the top of the container 5 defines a working plane 7. The portion of the working plane 7 lying within the opening of the container 5, which can be used for building up the object 2, is referred to as building area 8. Arranged in the container 5, there is a support 10, which can be moved in a vertical direction V, and on which a base plate 11 which closes the container 5 toward the bottom and therefore forms the base of the container 5 is attached. The base plate 11 may be a plate which is formed separately from the support 10 and is fastened on the support 10, or may be formed so as to be integral with the support 10. A building platform 12 on which the object 2 is built may also be attached to the base plate 11. However, the object 2 may also be built on the base plate 11, which then itself serves as the building platform. In Figure 1, the object 2 to be manufactured is shown in an intermediate state. It consists of a plurality of solidified layers and is surrounded by building material 13 which remains unsolidified. 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, for example an electron beam. The apparatus 1 also comprises a recoater 16, which is movable in a horizontal direction H, for applying layers of building material 15 within the building area 8. Optionally, a radiation heater 17 for heating the applied building material 15, e.g. an infrared heater, may be arranged in the process chamber.

The device in Figure 1 furthermore contains an irradiation device 20 having a laser 21, which generates a laser beam 22 that is deflected by means of a deflecting device 23 and focused onto the working plane 7 by means of a focusing device 24 via an entrance window 25, which is arranged at the top side of the process chamber 3 in the chamber wall 4.

The device in Figure 1 furthermore contains a control unit 29, by means of which the individual component parts of the apparatus 1 are controlled in a coordinated manner for carrying out a method for the manufacture of a three-dimensional object. The control unit 29 may contain a CPU, the operation of which is controlled 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 height which preferably corresponds to the desired thickness of the layer of the building material 15. The recoater 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 recoater 16 is then moved over the building area 8 and applies a thin layer of the building material 15 in powder form on the base plate 11 or on the building platform 12 or on a previously applied layer. The layer is applied at least across the cross-section of the object 2, preferably across the entire building area 8. Optionally, the building material 15 is heated to an operation temperature by means of at least one radiation heater 17. The cross-section of the object 2 to be manufactured is then scanned by the laser beam 22 in order to selectively solidify this area of the applied layer. These steps are carried out until the object 2 is completed. The object 2 can then be removed from the container 5. According to the invention, a powder mixture is used as building material 15. The powder mixture comprises a first powder and a second powder. According to the embodiments described below, the first powder comprises a steel alloy in powder form. The second powder comprises a reinforcement material.

According to the embodiments described below, the powder mixture is processed by the direct metal laser sintering (DMLS) method. In the selective laser sintering or selective laser melting method small portions of a whole volume of powder required for manufacturing an object are heated up simultaneously to a temperature which allows a sintering and/or melting of these portions. This way of manufacturing an object can typically be characterized as a continuous and/or - on a micro-level - frequently gradual process, whereby the object is acquired through a multitude of heating cycles of small powder volumes. Solidification of these small powder portions is carried through selectively, i.e. at selected positions of a powder reservoir, which positions correspond to portions of an object to be manufactured. As in selective laser sintering or selective laser melting the process of solidification is usually carried through layer by layer the solidified powder in each layer is identical with a cross-section of the object that is to be built. Due to the small volume or mass of powder which is solidified in a given time span, e.g. 1 mm ³ per second or less, and due to conditions in a process chamber of such additive manufacturing machines, which can favour a rapid cool-down below a critical temperature, the material normally solidifies quickly after heating.

In conventional sintering and casting methods one and the same portion of building material is heated up to a required temperature at the same time. A whole portion of material required to generate an object is cast into a mould in a liquid form. This volume of building material is therefore held above a

temperature level required for melting or sintering for a much longer time compared to the selective laser sintering or selective laser melting method. Large volumes of hot material lead to a low cooling rate and a slow solidification process of the building material after heating. In other words, selective laser sintering or selective laser melting methods can be differentiated from

conventional sintering and casting methods by processing of smaller volumes of building material, faster heat cycles and less need for heating up build material with high tolerances for avoiding a premature solidification of the material. These can be counted among the reasons why the amount of energy introduced into the building material for reaching the required temperatures can be controlled more accurately in selective laser sintering or selective laser melting methods. These conditions allow for setting an upper limit of energy input into the powder portions to be processed, which determines a temperature generated in the powder portions, more precisely, that is lower and closer to the melting point of the respective material than in conventional sintering or casting methods. This advantage makes it possible to minimize common problems of conventional sintering and casting methods. One such phenomenon is dissolution of reinforcement material in a steel melt during manufacturing, especially if a resulting material is thermodynamically unstable. The selective laser sintering or selective laser melting method allows for reducing dissolution by lowering the heating temperatures, for example generated by a laser and/or electron beam, in defined areas of the powder bed and for raising a cooling rate after heating.

Thus, the reinforcing quality of the reinforcement material, i.e. its ability to change (mechanical) properties of an object in a favourable manner, can become much more apparent. The phrase "mechanical properties of an object" is understood in this context as properties which derive from material properties of the object and not from a specific shape and/or geometry of the object.

Mechanical properties of the object can be tensile strength or yield strength, for example.

An object generated from a powder mixture according to the invention may show a change of various mechanical properties, but most notably shows a suppression of crack formation. The inventive method of manufacturing a three-dimensional object thus may provide considerable advantages by improving the mechanical properties compared to an object manufactured without reinforcement material. A comparatively short exposure of the building material or the processed material to high temperatures leads to a minimization of the dissolution of the

reinforcement material in the steel alloy material. Furthermore, chemical reactions of the reinforcement material with the steel alloy material are minimized. This is important as the reaction products are generally brittle. If the layer of the reaction product is thick, a considerable weakening of the material can occur.

In a specific embodiment of the invention, the first material is a H13 grade steel according to ASTM A681. In the following, the present invention is further illustrated by mean of examples, which however should not be construed as limiting the invention thereto in any manner.

Example 1: Preparation of test bodies with and without ceramic powder

A powder mixture was prepared by introducing non-melting ceramic TiC particles (d50 value was 1,4pm) into the H13 steel alloy matrix. The amount of ceramic particles added was 0,4 weight percent of the mixture. As the H13 steel alloy,

EOS H13 powder (chemistry according H13 standard) was used.

A SEM picture of the powder mixture is provided in Figure 2.

The powder mixture was subsequently used to prepare test bodies. As a comparison, an identical test body was prepared using only EOS H13 powder.

The test body using only EOS H13 powder was prepared at a platform

temperature of 200°C. A micrograph of the test body thus prepared is provided in Figure 3. As is apparent from the micrograph, the test body had visible cracking.

A second test body using only EOS H13 powder was prepared at a platform temperature of 150°C. A layer thickness of 30 pm was used for the preparation of all test bodies.

A series of test bodies was prepared using powder mixture with a ceramic particle content of either 0.2 wt.-% or 0.4 wt.-%. As in the test body using only EOS H13 powder, the 0.2 wt.-% and 0.4 wt.-% ceramic particle containing powder mixtures were used for the preparation of test bodies at a platform temperature of 200°C. In addition, the 0.4 wt.-% ceramic particle containing powder mixture was also used for the preparation of test bodies at 175°C, 165°C and 150°C. A micrograph of the test body prepared with 0.4 wt.-% of TiC at a platform temperature of 165°C is provided in Figure 4.

The thus prepared test bodies were investigated for microcracks and rated according to the following rating scheme; 0 = clean, 1 = few micro cracks in some samples, 2 = some micro cracks in all samples, 3 = micro cracking in all samples, 4 = macro cracking seen visually, 5 = macro cracking in all samples seen visually. The results of the evaluation of the test bodies prepared is provided in the below table 1.

Table 1:

As is apparent from the Table 1, the formation of cracks can significantly be reduced and even eliminated by the addition of ceramic TiC powder to the H13 steel alloy powder. In the case of 0,2 wt.-% TiC addition the cracks are

significantly reduced compared to the test body prepared with H13 steel alloy powder at a platform temperature of 200°C. With 0.4 wt.-% TiC addition, no cracks were observed for test bodies prepared at platform temperatures of 200°C, 175°C and 165°C and also at 150°C there was a significant reduction of the cracks compared to the test body prepared with H13 steel alloy powder at a platform temperature of 200°C.

In addition, if the platform temperature is lower, the tendency of the formation of cracks is higher. Example 2: preparation of high and low load test bodies

Test bodies at high and low load (i.e. with a platform, on which the test bodies are built, so that the majority of the surface of the platform is covered by test bodies (high load) or with a platform, whereon less test bodies are prepared so that only a minor part of the platform is covered by the test bodies (low load)) were prepared using a powder mixture of H13 steel alloy powder and 0.4 wt.-% TiC (as in example 1). The process temperature for the preparation of the test bodies was 200°C and a layer thickness of 40 pm was used. A picture of the test body thus prepared at high load is provided in Figure 5. As can been seen in this figure, there is no visible cracking in the test body.

Two further test bodies were prepared with the same powder mixture as above at high load and at a platform temperature of 175°C. The first test body was prepared with a layer thickness of 30pm, while the second test body was prepared with a layer thickness of 40pm. When comparing the two test bodies, it was observed that the sample prepared at a layer thickness of 40 pm had less cracking than the sample prepared at a layer thickness of 30 pm. This is believed to be possibly due to the fewer exposure times to the part to heating/cooling cycles, which leads the reduction of cracking.

List of reference signs:

1 laser sintering or laser meting apparatus

2 three-dimensional object

3 process chamber

4 chamber wall

5 container

6 container wall

7 working plane

8 building area

10 support

11 base plate

12 building platform

13 building material

14 storage container

15 building material

16 recoater

17 radiation heater

20 irradiation device

21 laser

22 laser beam

23 deflecting device

24 focusing device

25 entrance window

29 control unit