The present invention relates to metal powders which are suitable to be employed in 3D printing processes as well as a process fort the production of said powders.

3D printing refers to processes in which material is joined or solidified under computer control to create a three-dimensional object, with material being added together (such as liquid molecules or powder grains being fused together). 3D printing is used in both rapid prototyping and additive manufacturing (AM). Objects can be of almost any shape or geometry and typically are produced using digital model data from a 3D model or another electronic data source such as an Additive Manufacturing File (AMF) file (usually in sequential layers). There are many different technologies, like stereolithography (STL) or fused deposit modeling (FDM). Thus, unlike material removed from a stock in the conventional machining process, i.e. subtractive manufacturing, 3D printing or AM builds a three- dimensional object from computer-aided design (CAD) model or AMF file, usually by successively adding material layer by layer.

In the current scenario, 3D printing or AM has been used in manufacturing, medical, industry and sociocultural sectors which facilitate 3D printing or AM to become successful commercial technology.

One application field for devices generated via 3D printing is the medical sector where not only the precise manufacturing of surgical instruments but also custom made medical devices are of high demand. Virtual planning of surgery and guidance using 3D printed, personalized instruments have been applied to many areas of surgery including total joint replacement and craniomaxillofacial reconstruction with great success. One example of this is the bioresorbable trachial splint to treat newborns with tracheobronchomalacia developed at the University of Michigan. The hearing aid and dental industries are expected to be the biggest area of future development using the custom 3D printing technology.

Patient-matched implants were a natural extension of this work, leading to truly personalized implants that fit one unique individual. The use of additive manufacturing for serialized production of orthopedic implants (metals) is also increasing due to the ability to efficiently create porous surface structures that facilitate osseointegration. However, in spite of the ongoing development in the field, there are still issues that need to be addressed, in particular concerning the production of patient- matched implants.

Orthopedic implant materials are exposed to high mechanical loadings. Although conventional materials based on stainless steel or cobalt-chromium alloys display adequate mechanical strength, their employment raises toxicological concerns due to release of toxic or allergenic elements resulting in inflammatory reactions in the adjacent tissue. Metals and metal alloys based on titanium, tantalum and niobium show a higher biocompatibility with appropriate mechanical properties for avoiding stress-shielding and consecutive implant loosing. However, most of the metal powders available suffer from the drawback that so far their employment as materials in 3D printing processes did not result in products with the expected quality.

US 2016/0074942 discloses a method for producing substantially spherical metal powders. The method includes providing a particulate source metal including a primary particulate and having an average starting particle size; mixing the particulate source metal with a binder and an optional solvent to form a slurry; granulating the slurry to form substantially spherical granules, wherein each granule comprises an agglomeration of particulate source metal; debinding the granules at a debinding temperature to reduce the binder content of the granules forming the debinded granules; at least partially sintering the debinded granules at a sintering temperature such that particles within each granule fuse together to form partially or fully sintered granules; and recovering the sintered granules to form the substantially spherical metal powder.

WO 2017/048199 refers to a titanium-tantalum alloy having a titanium content ranging from 10 wt.-% to 70 wt.-% and wherein the alloy has a body centered cubic structure. Further, a method of forming a titanium-tantalum alloy is described, the method comprising the steps of: (a) slicing a 3D CAD model of a part to be formed into a plurality of 2D image layers; (b) preparing a homogenous powder mixture of titanium powder and tantalum powder; (c) dispensing a layer of the powder mixture onto a processing bed; (d) performing powder bed fusion of the layer of the powder mixture according to one of the 2D image layers in one of: a vacuum environment and an inert gas environment; and performing steps (c) and (d) for each of the plurality of 2D image layers in succession. However, despite the efforts made, there is still the need for metal powders suitable for 3D printing that address the following issues:

The employment of metal powders in 3D printing process requires the powders to have certain properties. For example, the powders have to be flowable in order to generate homogenous and pore-free articles. Further, usually a certain particle size distribution is required which may change depending on the particular 3D process used. The particle size distribution required for powders being employed in selective laser melting processes may thus be different from those mend to be employed in electron beam melting processes. In addition, low contents of oxygen, nitrogen and hydrogen in the powders are favorable to improve the mechanical properties of the later article. Last but not least, the employed powder should possess sufficient adsorption for laser light of distinct wave lengths or electron beams to allow for a sufficient melting which is required for the production of mechanically stable and pore-free articles. Accordingly, there is still the need for powders which are especially designed for the employment in 3D printing processes as well as processes for producing said powders.

The present invention addresses said need by providing a metal powder suitable for 3D printing as well as a process for the production of said powders.

A first object of the present invention is therefore a metal powder suitable for use in 3D printing processes, characterized in that the particles of the metal powder have an average aspect ratio y _A from 0.7 to 1, preferably from 0.8 to 1, more preferably from 0.9 to 1, even more preferably from 0.95 to 1, with y _A = x _{Feret, m in} /x _{Feret, max} .

The Feret diameter is a measure of an object size along a specified direction. In general, it can be defined as the distance between two parallel planes restricting the object perpendicular to that direction. This measure is used in the analysis of particle sizes, for example in microscopy, where it is applied to projections of a three-dimensional (3D) object on a 2D plane. In such cases, the Feret diameter is defined as the distance between two parallel tangential lines rather than planes (see Figure 1). With respect to the present invention, y _A is defined as the ratio of the minimal Feret diameter of a particle in mm and the maximum Feret diameter of the respective particle in mm. The average aspect ratio y _A within the meaning of the present invention refers to the distribution of aspect ratios of the particles of the powder, determined by statistical analysis of scanning electron microscopy (SEM).

It was surprisingly found that particles with an average aspect ratio within the claimed range are especially suitable for employment in 3D printing process and possess a good flowability.

Metals such as tantalum (Ta), titanium (Ti) and niobium (Nb) are favorable materials for the production of orthopedic implants due to their low toxicity, high biocompatibility and high mechanical stability of the generated objects. Therefore, an embodiment of the present invention is preferred, wherein the powder comprises or consists of a metal that is selected from the group consisting of tantalum, titanium, niobium and alloys thereof. In an especially preferred embodiment, the powder according to the invention comprises an alloy of titanium and niobium. In a particular preferred embodiment, the alloy further comprises tantalum. In an alternatively preferred embodiment, the powder comprises an alloy of tantalum and niobium.

The composition of the powder may be adapted according to the requirements of the specific case. In a preferred embodiment, the inventive metal powder comprises an alloy powder which comprises Ti in an amount of 50 to 75 wt.-%, preferably 57 to 61 wt.-%, based on the total weight of the powder.

In a further preferred embodiment, the inventive powder comprises an alloy powder which comprises Nb in an amount of 25 to 50 wt.-%, preferably 39 to 43 wt.-%, based on the total weight of the powder.

In an especially preferred embodiment, the powder comprises an alloy powder which comprises Ti in an amount of 50 to 75 wt.-%, preferably 57 to 61 wt.-% and Nb in an amount of 25 to 50 wt.-%, preferably 39 to 43 wt.-%, based on the total weight of the powder, respectively.

Further preferred is an embodiment of the present invention wherein the powder comprises an alloy of Ti, Nb and Ta. In such a case, it is preferred that the amount of Ta is 2 to 20 wt.-%, preferably 2 to 15 wt.-%, especially 2 to 6 wt.-%, based on the total weight of the powder. Preferably the alloy comprises Ti in an amount of 50 to 75 wt.-%, more preferably 55 to 70 wt%, even more preferably 55 to 61 wt.-%, Nb in an amount of 25 to 50 wt.-%, preferably 27 to 43 wt.-%, adding up to 100 wt.-% based on the total weight of the powder, respectively. It has been found that the mechanical properties, such as the elasticity of objects, in particular orthopedic implants, can be much improved if the content of certain impurities is kept as low as possible. In a preferred embodiment, the level of oxygen in the inventive powder is less than 3000 ppm, especially less than 1500 ppm and in particular less than 1000 ppm, more particular less than 500 ppm, even more particular less than 300 ppm, ppm referring to the mass of the powder. The level of nitrogen in the inventive powder is preferably less than 200 ppm, especially less than 100 ppm, in particular less than 50 ppm, even more particular less than 30 ppm.

The content of lithium (Li), sodium (Na), potassium (K) in the inventive powder is preferably less than 80 ppm. It was surprisingly found that the quality of the generated products could be much improved if the content of said elements was kept below the claimed level. In a preferred embodiment, the content of Li in the inventive powder is less than 80 ppm, preferably less than 50 ppm and in particular less than 30 ppm. In another preferred embodiment, the content of Na in the inventive powder is less than 80 ppm, preferably less than 50 ppm and in particular less than 30 ppm. In a preferred embodiment of the invention, the content of K is less than 80 ppm, preferably 50 ppm and in particular less than 30 ppm. In an especially preferred embodiment, the sum of the content of Li, Na and K in the inventive powder is less than 100 ppm, preferably less than 50 ppm.

In order for metal powders to be suitable for employment in 3D printing processes, the powders have to possess a certain flowability as well as a certain tap density. In a preferred embodiment, the inventive powder has a tap density, determined according to ASTM B527, of 40 - 80% of its theoretical density, preferably 60 - 80%.

In case the inventive powder is a Ta, Nb or Ti metal powder or an alloy powder thereof, the powder preferably has a tap density of 1,8 to 13,3 g/cm ³ , preferably 2,7 to 13,3 g/cm ³ . For example, fully dense Tantalum has a density of 16.65 g/cm ³ . Accordingly, in a preferred embodiment, the inventive Ta powder has a tap density of 6,6 to 13,3 g/cm ³ , preferably 10,0 to 13,3 g/cm ³ . In addition, fully dense niobium has a density of 8.58 g/cm ³ . Accordingly, in a preferred embodiment, the inventive Nb powder has a tap density of 3,4 to 6,9 g/cm ³ , preferably 5,1 to 6,9 g/cm ³ . In case the inventive powder comprises a binary alloy, particularly a Ti/Nb- alloy, the tap density is preferably from 2,2 to 5,2 g/cm ³ , preferably 3,3 to 5,2 g/cm ³ . In case the powder comprises a ternary alloy, in particular a Ti/Nb/Ta alloy, the powder preferably has a tap density of 2,3 to 6,5 g/cm ³ , preferably 3,5 to 6,5 g/cm ³ .

In a further preferred embodiment, the inventive powder has a flowability of less than 25 s/50g, especially less than 20 s/50g, even more preferred less than 15 s/50g, determined according to ASTM B213. It was surprisingly found that the inventive powders could be easily applied but still showed an appropriate steadfastness to allow the production of precise and clear edged articles.

As mentioned above, the particle size distribution of the inventive powder may be adapted according to need, in particular in accordance with the specific printing process used. Due to the different processing and requirements on the printed article, the demands with respect to the properties of the powder may differ according to the method used. For example, selective laser melting (SLM) uses fine powders to obtain accurately shaped articles with sufficiently resolved structural features. Electron beam melting (EBM) requires coarser particles, which is mainly a consequence of a repulsion of electrostatically charged particles as a consequence of their interaction with the electron beam. Such repulsion is explicitly pronounced for very fine powders. Laser cladding (LC) requires even coarser powders to avoid overspray and associated substantial powder loss.

In a preferred embodiment the inventive powder has a particle size distribution of a D10 greater than 2 mm, preferably greater than 5 mm, and a D90 of less than 80 mm, preferably less than 70 mm, even more preferably less than 62 mm with a D50 of 20 to 50 mm, preferably 25 to 50 mm, determined according to ASTM B822. In a preferred embodiment the powder fraction obtained by classification by sieving is <63 mm. Such a powder is especially suitable for application in selective laser melting processes (SLM).

In an alternatively preferred embodiment, the inventive powder has a particle size distribution of a D10 greater than 20 mm, preferably greater than 50 mm, even more preferably greater than 65 mm and a D90 of less than 150 mm, preferably less than 120 mm, even more preferably less than 100 mm with a D50 of 40 to 90 mm, preferably 60 to 85 mm, determined according to ASTM B822. In a preferred embodiment the powder fraction obtained by classification by sieving is 63 to 100 mm. Such a powder is especially suitable for application in electron beam melting processes (EBM). In another alternatively preferred embodiment the inventive powder has a particle size distribution of a D10 greater than 50 mm, preferably greater than 80 mm, even more preferably greater than 100 mm and a D90 of less than 240 mm, preferably less than 210 mm, with a D50 of 60 to 150 mm, preferably 100 to 150 mm, determined according to ASTM B822. In a preferred embodiment the powder fraction obtained by classification by sieving is 100 to 300 mm. Such a powder is especially suitable for application in laser cladding processes (LC).

It was surprisingly found that the mechanical stability as well as the homogeneity of elements distribution of an article generated by 3D printing processes can be greatly improved if the particles of the metal powder employed possess a dendritic microstructure. Therefore, an embodiment of the inventive powder is preferred wherein the powder has a dendritic microstructure with local deviation in the chemical composition. Analysis of the inventive by X-ray diffraction surprisingly showed that the dendritic microstructure in the powder, although differing in composition, all had same crystalline structure in contrast to common powders where the local deviation of the composition is usually associated with different crystal structures resulting in more than one phase in the X-ray diffraction whereas only one phase was detected in the case of the inventive powder. This is confirmed by Figure 8, depicting a powder X-ray diffraction diagram of exemplary inventive powders.

Another object of the present invention is a process for the production of the inventive powder. The inventive process comprises the steps of a) Pressing or pressing and sintering the powdery components of the powder to obtain a metal body; b) atomizing the metal body to obtain a metal powder; c) separating of particles having a particle size of less than 2 mm, preferably less than 5 mm, even more preferred less than 10 mm, determined according to ASTM B822 to obtain the inventive metal powder; and d) classification of the particle sizes of the inventive metal powder via screening to obtain the desired particle size distribution.

It was surprisingly found that the process according to the invention allows the production of metal powders which are especially suitable for 3D printing processes. In a preferred embodiment the separating in step c) of the inventive process is realized by sifting, in particular air classification of the powder.

In an alternatively preferred embodiment, the separating in step c) of the inventive process is realized by de-agglomeration in a water bath using ultra sound and subsequent decantation. In an alternatively preferred embodiment, de- agglomeration is realized by stirring in a water bath and subsequent decantation. In another alternatively preferred embodiment, de-agglomeration is realized via high power dispersion in a water bath and subsequent decantation. High power dispersion may, for example, be carried out using an Ultra-Turrax ^® , available from IKA ^® -Werke GmbH & Co. KG, Germany.

In a preferred embodiment, the degree of decantation of the fine particles can be adjusted by manipulation of the zeta-potential of the dispersion. This may, for example be achieved by adjustment of the pH-value. Therefore, an embodiment of the inventive process is preferred wherein the degree of decantation of the particles is adjusted by adjusting the pH of the water bath used in deagglomeration.

It is desirable to obtain a metal powder with a low oxygen content. Therefore, in a preferred embodiment, the inventive process further comprises a step of deoxidation. The deoxidation of the inventive powder is preferably carried out subsequent to step c) of the inventive powder. Preferably, deoxidation is carried out in the presence of a reducing agent, preferably a reducing agent selected from the group consisting of Li, Na, K, Mg and Ca as well as mixtures thereof. In a further preferred embodiment, the inventive powder is subjected to a leaching step after deoxidation to remove any undesired impurities generated during deoxidation. Leaching is preferably conducted using an inorganic acid. In order to adjust the properties of the powder to match the requirements of the specific application, the surface of the powder can be doped. Therefore, an embodiment is preferred wherein the surface of the powder is doped, preferably with a doping agent selected from the group consisting of phosphorous, boron, silicon, yttrium, calcium, magnesium and mixtures thereof. The process of doping is well known to the person skilled in the art. The person skilled in the art is therefore well aware how the doping agent is to be introduced.

The inventive powder may further be manipulated in order to improve the performance in 3D printing processes. Therefore, an embodiment is preferred wherein the inventive powder is further subjected to an acid treatment, the acid preferably being hydrogen fluoride or complex-forming carboxylic acids. It was surprisingly found that the absorption of the laser radiation during the printing process can be improved if the powder is subjected to such an acid treatment. Without being bound by theory, it is believed that the treatment may lead to a roughening of the powder surface, thus increasing the surface absorption of radiation.

In a preferred embodiment, the complex-forming carboxylic acid is selected from the group consisting of carboxylic acid, dicarboxylic acid and alpha hydroxy acid as well as mixtures thereof.

The inventive powder is especially suitable for the application in 3D printing processes. Thus, a further object of the present invention is the use of the inventive powders in additive manufacturing processes. Preferably, the process is selected from the group consisting of selective laser melting (SLM or LBM), electron beam melting (EBM) and laser cladding (LC).

Another object of the present invention is a process for producing a three- dimensional article using the inventive powder, wherein the three-dimensional article is build-up layer by layer.

Yet another object of the present invention is a three-dimensional article obtained by the inventive process and/or comprising the inventive powder. The three- dimensional article is characterized by its favorable properties which make the three-dimensional article especially suitable for medicinal applications. The requirements for articles used for medicinal purposes are widely diversified, ranging from biocompatibility to mechanical strength. It was surprisingly found that three-dimensional articles produced from the powder according to the invention, in particular using 3D printing processes, showed especially advantageous properties. The elastic modulus (Young's modulus, m _E ) was found to be close to the one of natural bones which have an elastic modulus of around 40 GPa. Common materials presently used show a much higher elastic modulus of more than 100 GPa, leading to an inferior compatibility of the implant. It is therefore desirable, to obtain implants with properties much closer to those of the natural bone, an object achieved in the course of the present invention. In a preferred embodiment, the three-dimensional article according to the invention therefore has an elastic modulus m _E of 20 to 100 GPa, preferably 40 to 90 GPa, in particular 40 to 80 GPa, determined according to DIN EN ISO 6892-1.

In a further preferred embodiment, three-dimensional article according to the invention has an ultimate strength (R _m ) of 600 to 1400 MPa, preferably 600 to 1200 MPa, in particular 600 to 699 MPa, determined according to DIN EN ISO 6892-1.

Also preferred is an embodiment, wherein the three-dimensional article according to the invention has a yield strength R _p0.2 of 500 to 1200 MPa, preferably 500 to 1000 MPa, in particular 500 to 699 MPa, determined according to DIN EN ISO 6892-1.

In an especially preferred embodiment, the inventive three-dimensional article has

• an elastic modulus m _E of 20 to 100 GPa, preferably 40 to 90 GPa, in particular 40 to 80 GPa, determined according to DIN EN ISO 6892- 1;

• an ultimate strength R _m of 600 to 1400 MPa, preferably 600 to 1200 MPa, in particular 600 to 699 MPa, determined according to DIN EN ISO 6892-1; and

• a yield strength R _p0.2 of 500 to 1200 MPa, preferably 500 to 1000 MPa, in particular 500 to 699 MPa, determined according to DIN EN ISO 6892-1.

In a further preferred embodiment, the inventive three-dimensional article is characterized by a strain value A _g at ultimate strength R _m of more than 0.5%, preferably more than 1%, in particular more than 4%, determined according to DIN EN ISO 6892-1, respectively.

Further preferred is an embodiment of the inventive three-dimensional article wherein the three-dimensional article has a value A30 of strain at fracture of more than 2%, preferably more than 4%, in particular more than 10%, determined according to DIN EN ISO 6892-1, respectively.

Preferably the article is an implant, in particular a medicinal implant, such as dental implant, hip implant, knee implant, shoulder implant, craniofacial implant or spine implant. In another preferred embodiment, the article is for high temperature applications, such as ovens and reactors. The present invention will be explained in more detail with the help of the following examples which are not to be understood as limiting the invention.

Examples:

Several powders were prepared according to steps a) and b) of the described process. The results are summarized in Table 1 :

The obtained powders where subjected to partition step as described in step c) of the inventive process, whereby:

1) sifting/air classification

2) ultrasonic treatment

3) water decantation (at pH 5 to 8)

The powders were de-oxidized subsequently and classified by sieving according the fractions given in Table 2. The results are summarized in Table 2:

The inventive powders of examples 9, 12, 13 and 14 were employed in a selective laser melting process to produce three-dimensional test articles 1, 2, 3 and 4, respectively, the properties of which were tested according to DIN EN ISO 6892- 1 :2016. The respective results are summarized in Table 3, the given values being the average results obtained over a series of tests. m _E : elastic module, determined according to DIN EN ISO 6892-1 : 2016

R _p0.2 : yield strength, determined according to DIN EN ISO 6892-1 : 2016

R _m : ultimate strength, determined according to DIN EN ISO 6892-1 : 2016

F _m : maximum force, determined according to DIN EN ISO 6892-1 : 2016 A _g : strain at R _m , determined according to DIN EN ISO 6892-1 : 2016

A ₃₀ : strain at fracture, determined according to DIN EN ISO 6892-1 : 2016

S ₀ : area of cross section, determined according to DIN EN ISO 6892-1 : 2016

Table 3:

Figure 1 is a graphical representation of the definition of the Ferret diameter as explained above.

Figure 2 shows a SEM image of the inventive powder according to example 2. As can be clearly seen, the particles possess a homogenous sphericity and particle size distribution. Figure 3 shows a SEM image of the powder according to comparative example 4. In contrast to the inventive powder, the common powder shows a high degree of agglomeration, thus being unsuitable for use in 3D printing. Figure 4 shows a SEM (cross section) image of the inventive powder according to example 11 which clearly shows the uniformity of the powder and the absence of any undesired particles with sizes that would render the powder unsuitable for 3D printing processes.

Figure 5 shows an SEM image of the inventive powder according to example 12 after treatment with HF which resulted in a roughening of the surface of the powder.

Figure 6 shows an SEM image/EDX cross section of one powder particle of the inventive powder according to example 11. The image clearly shows the dendritic character of the powder with

1 : Ti 70.8 wt%, Nb 26.3 wt%, Ta 2.9 wt%

2: Ti 72.4 wt%, Nb 25.4 wt%, Ta 2.2 wt%

3: Ti 54.0 wt%, Nb 38.2 wt%, Ta 7.8 wt%

4: Ti 54.6 wt%, Nb 38.0 wt%, Ta 7.4 wt%

Figure 7 shows an SEM image/EDX cross section of one powder particle of the inventive powder according to example 13. The image clearly shows the dendritic character of the powder with

1 : Ti 56.4 wt%, Nb 40.0 wt%, Ta 3.6 wt%

2: Ti 64.4 wt%, Nb 32.1 wt%, Ta 3.4 wt%

3: Ti 67.4 wt%, Nb 31.4 wt%, Ta 1.2 wt%

4: Ti 53.7 wt%, Nb 41.4 wt%, Ta 4.9 wt%

5: Ti 53.5 wt%, Nb 42.0 wt%, Ta 4.5 wt%

Figure 8 shows X-ray diffraction diagrams of the inventive powders according to examples 1 (Nb), 7 (Ti42Nb) and 13 (Ti40Nb4Ta). As can be clearly seen, only one crystallographic phase was identified in each diagram, even for the binary and ternary system.