Technical field of the invention

The present invention relates to the field of additive manufacturing, and more particularly to a method for additive manufacturing of a tailored three- dimensional structure by local synthesis of a target alloy.

Background

Powder metallurgy (PM) is a technology for large-scale manufacturing of precision parts with a complex shape. Powder manufacturing route gives the highest utilization of starting raw material and also the lowest energy requirement per kilogram in comparison with other structural component manufacturing routes. Therefore, powder metallurgy can be described as the green technology of tomorrow, contributing significantly to the sustainable industrial development due to the complex technology giving at the same time a high productivity and the possibility to produce materials (hard-metals, tools steels, etc) not possible by other methods.

Additive Manufacturing (AM) is one of the most rapidly increasing

manufacturing technologies these days, where material in powder form may be utilized as raw material. Additive Manufacturing enables fast, flexible and at the same time cost-efficient production of parts with complex three dimensional shape directly from CAD data, by selective melting of powder layer by layer to a desired shape by controlling the shape and volume of the melting pool.

However, metallurgical and technical complexity of the local melting of powder and a need for defect-free solidification of the melting pool results in a very narrow range of the alloys that can be produced. It is costly and complicated to develop process parameters, such as power and spot size of the energy source, scan speed and strategy, preheating and surrounding atmosphere conditions, for the selective melting of a certain alloy as each alloy varies in melting temperature, thermal conductivity, melt viscosity, etc. Hardware for additive manufacturing sets strict requirements on the powder properties, e.g. powder size distribution, flowability, purity, etc. Hence, only a limited number of prealloyed powders of high purity and with optimized size distribution and mechanical properties may be used in additive manufacturing today. Hence there is a need in the art for improved ways of additive manufacturing allowing for more flexibility in alloy design and for improved properties of the products to be manufactured thereby.

Summary of the invention

The present invention is based on the idea to utilize the advantages of mixing of particles, e.g. particles of different elements, on a local level to bring flexibility to the alloy design. The present invention relates to a generic method to create alloys directly in a melting machine by local alloy synthesis in a melted granule comprising a heterogeneous target alloy composition. By the term "local" is herein meant at the place the alloy is desired to be used.

According to an aspect of the present invention, a method for additive manufacturing of a tailored three-dimensional structure by local synthesis of a target alloy is provided. The target alloy comprises at least a first metallic element and a second element. The first metallic element is not the same as the second element. The method comprises the steps of:

a) mixing at least particles comprising the first metallic element and

particles comprising the second element with a liquid medium to form a suspension, the particles having a particle size within the range of from 0.05 pm to 20 μπτι,

b) subjecting the suspension to freeze granulation to form a granule

comprising the target alloy composition having a granule size within the range of from 25 m to 1000 pm,

c) providing a solid substrate,

d) locally synthesising the target alloy on the solid substrate by melting the granule comprising the target alloy composition and solidifying the melted granule, and

e) optionally, repeating step d), thereby forming a tailored three- dimensional layer-by-layer structure comprising the target alloy.

Typically, the steps a) to d), and optionally e), takes place in a single machine. The present invention allows for improved flexibility in alloy design as alloys of any composition can be produced by mixing and forming granules of particles in the proportions corresponding to a desired alloy composition. In this way, each granule will roughly represent the desired alloy composition on the level of the granule. The present invention providing a method of producing granules of a desired and evenly distributed target alloy composition prior to melting allows for better homogeneity of the target alloy. By the term alloy is herein meant a solid solution between metals, or an intimate mixture of several phases of metals and optionally non-metals (such as oxides, carbides, nitrides, borides or phosphides). According to one example metals constitue at least 75 %, or at least 85 %, or at least 90 %, or at least 95 %, or at least 97 % of the alloy.

A granule comprising the target alloy composition is defined as a granule including the elements of the target alloy in the same proportions (expressed as atomic percentages) as the target alloy itself. Herein, the granules comprising the target alloy composition typically have a granule size within the range of from 25 μιτι to 1000 μιτι, such as in the range of from 50 μιτι to 200 μιτι. By the term "granule size" is herein meant the average diameter of the granule. The granule size may be measured by sieving, by laser diffraction or by optical measurement in a microscope. Granules provide advantages over smaller particles in that they are easier to handle, bring less health hazards and have superior flow characteristics.

Herein, the particles, e.g. particles of various elements, typically have a particle size within the range of from 0.05 μιτι to 20 μιτι, such as in the range of from 0.1 μιτι to 5 μιτι, e.g. from 0.1 μιτι to 1 μιτι. By the term "particle size" is herein meant the average diameter of the particle. The particle size may be measured by laser diffraction methods or by optical or scanning electron microscope, or by a combination of scanning electron microscopy, light optical microscopy and image analysis. The particles of elements may be formed by conventional techniques known to the person skilled in the art, e.g. by gas atomization, chemical or electrochemical methods, etc. The particles comprising the first metallic element may e.g. be selected among particles consisting of the first metallic element and unavoidable impurities, particles consisting of a compound comprising the first metallic element and particles consisting of an alloy comprising the first metallic element. The particles comprising the second element may e.g. be selected among particles consisting of the second element and unavoidable impurities, particles consisting of a compound comprising the second element and particles consisting of an alloy comprising the second element. Typically, the particles may consist of a single element. However, in practice it is difficult to keep them 100 % pure, hence, for example an iron particle may also comprise some carbon and some oxygen. As an example, the oxygen may be considered as an impurity, while the carbon may have a function in an alloy to be at least partly composed thereof. In some cases, the oxygen may not be considered as an impurity but as a relevant component in for example oxide dispersion strengthened alloys.

The particles of elements of the target alloy composition are mixed into a liquid medium to form a suspension. The liquid medium may be inert and typically does not react with the particles of metallic elements. The liquid medium may be an organic liquid medium, such as an organic liquid medium selected among cyclohexane and tert-amyl alcohol. Alternatively, the liquid medium may be water. Preferably, the liquid medium may freeze and sublimate properly, thereby allowing the homogeneity of the suspension to remain in the granules. The organic liquid medium may have a freezing temperature being higher than the condenser temperature of a freeze drier used in the freeze granulation process.

The particles of elements and the liquid medium, and optionally any dispersant and/or organic binder and/or pH buffer, may be mixed by means of e.g. a stirrer mixer (such as of propeller type) or a ball mill, or alternatively by agitated milling (such as attrition milling) or by ultrasound. Any other suitable technique for mixing known to the skilled person in the art may be used. The mixing typically takes place in room temperature at atmospheric pressure. The mixing serves to break down undesired agglomerates, that may form in the suspension, into single particles, enabling a more or less homogeneous suspension to be formed. Undesired agglomerates may further be avoided by addition of a dispersant and/or a pH buffer to the suspension. The granules are herein formed by freeze granulation. Alternative means of granulation to consider may be e.g. mechanical granulation or spray drying. Freeze granulation is preferred since granules with high homogeneity (e.g. compared to spray drying) may be produced. High homogenity is enabled as a homogenous mixture of the suspension may be preserved in the granule. Another advantage of freeze granulation is that free flowing spherical shapes may be produced. Freeze granulation is preferably performed in an inert atmosphere, which reduces the risk of oxidation and eliminates the risk of fire or explosion due to inflammable solvent. Freeze granulation is applicable on particles of metallic elements sensitive to oxidation as it may include suspension in an organic liquid medium and spraying in a nitrogen

atmosphere. Freeze granulation also enables efficient recovery of liquid medium.

Freeze granulation may be performed by spraying droplets of the suspension into a container of liquid nitrogen (or any other suitable liquid having a low boiling point). Typically, the liquid nitrogen is stirred enabling granules of desired flow characteristics to be formed. Sprayed droplets of the suspension may be formed by e.g. a two-fluid nozzle (typically including use of nitrogen gas), a pressure nozzle, a rotating nozzle or an ultrasound nozzle.

Alternatively, the suspension may be poured into the container of liquid nitrogen (or any other suitable liquid having a low boiling point). As sprayed droplets of the suspension hit the liquid nitrogen, granules are formed by solidification and do not grow larger and the particles inside the granule stay in place. Consequently, phase segregation may be avoided.

The granule size and the size distribution thereof may be influenced by nozzle geometry, pumping speed, gas pressure, viscosity and viscoelastic properties of the suspension, and the surface tension of the suspension. A narrow granule size distribution is desirable to enhance the flow properties of the granules (such as dried granules). A too wide granule size distribution may be adjusted by sieving to remove undesired size fractions. After granulation and before melting, the granules may be dried. The drying is preferably performed at a temperature and at a pressure where sublimation of the froozen medium may take place. If water is used a liquid medium, vacuum drying is preferred.

A solid substrate having a large surface area in relation to the granule size of the granules may serve as the basis onto which a tailored three dimensional structure may be manufactured by local synthesis of a target alloy. The substrate may be made of any material suitable as support for 3D printing, cladding and/or laser metal deposition. The substrate typically has a high thermal conductivity, and serves as a heat sink enabling rapid solidification of the melted granule upon formation of the target alloy. A rapid solidification has positive effects with regard to the homogeneity of the target alloy as it suppresses grain growth and helps avoiding segregation effects.

By the term "melting" is herein meant changing from a solid state to a liquid state by application of e.g. heat, such that the temperature in the area to be melted is above the melting temperature of the main component of the solid substance, herein the granule comprising the target alloy composition, to be melted. The main component is defined as the component representing the largest relative atomic percentage of the solid substance, herein

corresponding to the main component of the target alloy. In particular, by the term "melting" is herein not meant sintering.

According to an embodiment, the granules are selectively melted in step d) by means of selective laser melting, preferably by selective laser melting at substantially atmospheric pressure.

By the term "selective melting" is herein meant melting of an area (also called melt pool or melting pool) for example about equal in size as the granule selected to be melted. The size of the melting pool is mainly determined by the technique used and the parameters being applied (e.g. the spot of a laser or of a electron beam). The size of the melting pool may differs largely between different applications, such as thermal spraying, cladding, 3D printing, etc. Typically, the diameter of the melting pool is smaller than 1 cm. A melting pool being close in size with the granule allows for short diffusion distances upon melting and allows for formation of a homogeneous alloy structure. Grain growth in the target alloy is at least partly dependent on the size of the melt pool, as a grain cannot grow larger than the melt pool if the grain grows by liquid transport.

Selective melting may be performed by e.g. selective laser melting, electron beam melting, electric discharge, arc, thermospraying techniques, etc.

Preferably, the selective melting is performed by selective laser melting, e.g. at substantially atmospheric pressure.

According to an embodiment, the target alloy is a metal alloy or a metal- ceramic composite. According to an embodiment, the target alloy consists of a 316L steel, an INCONEL718 steel, a bulk metallic glass, or a high entropy alloy. For instance, the target alloy may be a non-equilibrium alloy, such as a high entropy alloy or a bulk metal glass. Alternatively, the target alloy may be selected among different types of steel, e.g. 316L steel or INCONEL718 steel. Hence, the target alloy may comprise particles of additional elements than the first metallic element and the second element.

Bulk metallic glass may be defined as alloys with critical cooling rates low enough to allow formation of amorphous structure in layers with a thickness of at least 1 millimeter. With regard to bulk metallic glass, high cooling rate provided by fast solidification of the melt pool on massive substrate and high glass-forming ability of the alloy will allow manufacturing of massive 3D shapes, not possible with casting methods employed until now. Further development of cheap Fe-based metallic glasses that requires significantly higher cooling rate can be developed by the same approach.

High entropy alloys may be defined as solid solution alloys comprising more than five principal elements in equal or near equal atomic percent. With regard to high entropy alloys, high cooling rate provided by fast solidification of the melt pool on massive substrate and high diffusion rates in the liquid state allow manufacturing of massive 3D shapes with homogeneous microstructure, not possible with casting methods employed until now. Further development of alloy with improved mechanical and corrosion resistance at high temperatures can be easily performed by the same approach through tailoring of the alloy composition. According to an embodiment, the first metallic element is selected among nickel, chromium, molybdenum, niobium, titanium, aluminum, iron, zirconium, copper, cobalt, vanadium, manganese, tungsten, calcium and magnesium, and the second element is selected among nickel, chromium, molybdenum, niobium, titanium, aluminum, iron, zirconium, copper, cobalt, vanadium, manganese, tungsten, calcium, magnesium, oxygen, nitrogen, carbon, boron, silicon and phosphorus, provided that the first metallic element is not the same as the second element. By the term "element" is herein meant a substance consisting of atoms of the same atomic number.

According to an embodiment, the particles of at least the first metallic element, the particles of the second element and the granule of the target alloy composition are not pre-alloyed or sintered. The present invention differs from and provides advantages over the conventional methods of mechanical blending during sintering as well as of pre-alloying by for example gas atomization or water atomization, with regard to e.g. alloy design and alloy homogeneity. Additional advantages that may be provided by the present invention are cost efficiency, as particles produced by chemical or

electrochemical techniques may replace particles produced by gas

atomization at least to a certain extent. By not being limited to particles produced by gas atomization, it is also possible to produce a number of modern alloys that require significantly higher cooling rates (such as > 10 000 K per second) than in gas atomization to reach a required microstructure, as e.g. amorphous metals, metallic glasses, high-entropy alloys, etc.

According to an embodiment, the target alloy is firstly formed upon the local synthesis in step d). Suppressed grain growth and less segregation of the target alloy may be provided as the desired target alloy is firstly formed on top of the substrate or on top of a preceding layer of target alloy formed on the substrate in the present invention.

According to an embodiment, the suspension further comprises a dispersant and/or an organic binder. The suspension may further comprise at least one of a dispersant and an organic binder. A dispersant serves to reduce the attractive forces between particles and allows for low viscosity of the suspension. An organic binder serves to keep the granule together. A single additive may function as both dispersant and organic binder. Examples of dispersants are non-aqueous dispersants, such as cationic polymeric surfactants, e.g. Hypermer KD3™ (Croda). Examples of organic binders are paraffin wax-based organic binders, low molecular polyethylene

polypropylene copolymer Licocene™ (Clariant) or polyethylene glycol. The organic binder preferably has the ability to break down and evaporate during melting of a granule formed from a suspension comprising the organic binder.

According to an embodiment, the suspension further comprises a pH buffer. If water is used as liquid medium, also a pH regulating agent, such as a pH buffer, may be added to the suspension. If pH regulation is used, the pH may typically be adjusted such that the particles have a high surface charge, i.e. to a pH about or below the zero point of charge.

According to an embodiment, the additive manufacturing comprises 3D printing, laser metal deposition or cladding. The additive manufacturing may comprise electron beam melting.

Brief description of the drawings

Figure 1 schematically shows the steps of an embodiment of the method according to the present invention. The components are shown in a partly cross-sectional, partly perspective view.

Detailed description of the invention

The present invention relating to a method for additive manufacturing of a tailored three-dimensional structure by local synthesis of a target alloy will now be described in more detail. The method for additive manufacturing may for instance be applied in 3D printing, laser metal deposition, cladding, etc.

The target alloy of the present invention comprises at least a first metallic element and a second element. The target alloy may be a metal alloy or a metal-ceramic composite, for example 316L steel, INCONEL718 steel, bulk metallic glass, high entropy alloy, etc.

The method for additive manufacturing of a tailored three-dimensional structure by local synthesis of a target alloy comprises the steps according to claim 1 . An embodiment of the method is schematically shown in Figure 1 . As shown in step 100 of Figure 1 , particles of at least the first metallic element 10 and the second element 20 are provided. The particles have not been e.g. pre-alloyed or sintered, and typically each of them do only (or mainly) consist of a single element. The particles are provided in proportions corresponding to the composition (expressed in atomic percentages) of the desired target alloy to be produced. The particles of the at least first metallic element and the second element may independently be selected among e.g. nickel, chromium, molybdenum, niobium, titanium, aluminum, iron, zirconium, copper, cobalt, vanadium, manganese, tungsten, calcium and magnesium, provided that the first metallic element and the second metallic element are not the same metallic element. The second element may further be selected among carbon, oxygen, nitrogen, silicon, phosphorus or boron. The particles 10, 20 have a particle size within the range of from 0.05 μιτι to 20 μιτι, typically from 0.1 μιτι to 5 μιτι, preferably from 0.1 μιτι to 1 μιτι.

Further, also a liquid medium 30, such as water or an organic liquid medium such as tertamyl alcohol or cyclohexane, is provided. The liquid medium is chosen such that it may suitably form a suspension with the particles upon mixing. Preferably, it may also suitably freeze and sublimate upon freeze granulation. Optionally, also at least one of a dispersant, a pH buffer and an organic binder may also be provided .

Thereafter, as shown in step 101 of Figure 1 , the particles of at least the first metallic element 10 and the second element 20 are mixed with the liquid medium 30 (and optionally, at least one of a dispersant, a pH buffer and an organic binder) to form a suspension 40. The mixing may be performed by any suitable means of mixing, e.g. a ball mill or a propeller, during several hours, e.g. 12 hours, typically at room temperature. Preferably, the mixing takes place until a homogeneous suspension is obtained.

In step 102 of Figure 1 , the suspension 40 is subjected to freeze granulation. Granules 50 of the composition (expressed in atomic percentages) of the desired target alloy are formed. The granules 50 have a granule size within the range of from 25 μιτι to 1000 μιτι, typically from 50 μιτι to 200 μιτι.

Upon manufacturing of a tailored three-dimensional structure comprising the target alloy, a solid substrate 60 is provided. As shown in step 103 of Figure 1 , granules 50 may be applied onto the solid substrate. The way of application of granules to the substrate may differ depending on the type of application, e.g. cladding, 3D printing or laser metal deposition. The granules may before instance be deposited on the substrate by spreading with methods such as gravity flow, vibrations combine with gravity or transport in a carrier gas.

As an example, the granules 50 may be applied onto the solid substrate 60 in a layer, such as a monolayer, 70 as shown in step 103 of Figure 1 .

Thereafter, energy required to melt the granules of the selected area which is intended to be made up of the target alloy, i.e. the area corresponding to the so-called melting pool, is applied onto said selected area. The energy enabling melting, such as selective melting, may be applied by means of e.g. selective laser melting or electron beam melting, preferably by selective laser melting at substantially atmospheric pressure. Upon melting the particles of the granule, corresponding to the desired composition of the target alloy, are contacted and a local synthesis of the target alloy is initiated. The solid substrate 60 is serving as a heat sink and consequently, the target alloy 80 is finally formed upon solidification. Solidification is typically advantageously accelerated as heat from the melted granule is taken up by the substrate. The target alloy covering the selected area of the substrate constitutes a first tailored layer of the target alloy, as shown in step 104 of Figure 1 . As further shown in step 104 of Figure 1 , the target alloy 80 is solely formed in the area subjected to melting. Hence, an area not receiving the energy required to melt the granule will typically not be covered by any target alloy. As shown in Figure 1 , the area subjected to melting (the melting pool) may correspond to the cross-sectional area of one or more granules 50.

In alternative embodiments, the selected area intended to be made of the target alloy, i.e. the area corresponding to the so-called melting pool, may be seletively heated prior to application of granules. In such embodiments, granules will melt only in the heated selected area, and consequently the effect will be the same as in the prior embodiment. The step of melting and solidification, i.e. of local synthesis of the target alloy, may be repeated such as to form a tailored three-dimensional layer-by-layer structure comprising the target alloy. In step 105 of Figure 1 , a selectively melted first layer, partly comprising granules and partly comprising the target alloy, is covered by a second layer, e.g. a monolayer, 90 of granules available for melting and local synthesis of the target alloy of a second tailored layer of the target alloy.

Examples

Example 1 - E316L stainless steel

Particles of metallic elements were mixed to form a target alloy composition of 10 % nickel, 17% chromium , 2 % molybdenum and the rest iron. The typical particle sizes were in the range 0.1 - 5 μιτι. The composition of particles of metallic elements was mixed with an organic liquid medium: cyclohexane, by means of a ball mill at room temperature for 12 hours to form a suspension. A dispersant Hypermer KD3™ and a paraffin wax-based organic binder were also mixed into the suspension.

The suspension was sprayed through a two-fluid nozzle with nitrogen as the carrier gas. The resulting droplets were collected in a container with stirred liquid nitrogen. The frozen droplets, forming granules of a target alloy composition, were transferred to a freeze dryer to remove the liquid medium. The typical granule size of the granules were in the range of 80 - 120 μιτι. The granules, obtained by freeze granulation, were transferred to a machine for selective laser melting. The granules were spread in even layers and melted selectively to form a component layer by layer. The final component comprised a 316L type of steel. Example 2 - INCONEL 718 nickel-based superalloy

Particles of metallic elements were mixed to form a target alloy composition of 55 % nickel, 20% chromium, 3 % molybdenum, 5% niobium, 1 % titan, 1 % aluminium and the rest iron. The typical particle sizes were in the range 0.1 - 5 μιτι. The composition of particles was mixed with an organic liquid medium: cyclohexane, by means of a ball mill at room temperature for 12 hours to form a suspension. A dispersant Hypermer KD3™ and a paraffin wax-based organic binder were also mixed into the suspension. The suspension was sprayed through a two-fluid nozzle with nitrogen as the carrier gas. The resulting droplets were collected in a container with stirred liquid nitrogen. The frozen droplets, forming granules of a target alloy composition, were transferred to a freeze dryer to remove the liquid medium. The typical granule size of the granules were in the range of 80 - 120 μιτι.

The granules, obtained by freeze granulation, were transferred to a machine for selective laser melting. The granules were spread in even layers and melted selectively to form a component layer by layer. The final 3D

component comprised an INCONEL718 type of steel.

Example 3 - Zr-Cu-AI-Ni bulk metallic glass (BMG)

Particles of metallic elements were mixed to form a target alloy composition of 55 % zirconium, 30% copper, 10% aluminum and 5% nickel. The typical particle sizes were in the range 0.1 - 5 μιτι. The composition of particles was mixed with an organic liquid medium: cyclohexane, by means of a ball mill at room temperature for 12 hours to form a suspension. A dispersant Hypermer KD3™ and a paraffin wax-based organic binder were also mixed into the suspension.

The suspension was sprayed through a two-fluid nozzle with nitrogen as the carrier gas. The resulting droplets were collected in a container with stirred liquid nitrogen. The frozen droplets, forming granules of a target alloy composition, were transferred to a freeze dryer to remove the liquid medium. The typical granule size of the granules were in the range of 80 - 120 μιτι.

The granules, obtained by freeze granulation, were transferred to a machine for selective laser melting. The granules were spread in even layers and melted selectively to form a component layer by layer. The final 3D

component is made up of the Zr-Cu-ZI-Ni bulk metallic glass.

Example 4 - CuCoNiCrFeAlx high entropy alloy (HEA)

Particles of metallic elements were mixed to form a target alloy composition of 20 % copper, 20% cobalt, 20% nickel, 20 chromium, 19% iron and 1 % aluminum. The typical particle sizes were in the range 0.1 - 5 μιτι. The composition of particles was mixed with an organic liquid medium: cyclohexane, by means of a ball mill at room temperature for 12 hours to form a suspension. A dispersant Hypermer KD3™ and a paraffin wax-based organic binder were also mixed into the suspension. The suspension was sprayed through a two-fluid nozzle with nitrogen as the carrier gas. The resulting droplets were collected in a container with stirred liquid nitrogen. The frozen droplets, forming granules of a target alloy composition, were transferred to a freeze dryer to remove the liquid medium. The typical granule size of the granules were in the range of 80 - 120 μιτι.

The granules, obtained by freeze granulation, were transferred to a machine for selective laser melting. The granules were spread in even layers and melted selectively to form a component layer by layer. The final 3D component is made up of the CuCoNiCrFeAM high entropy alloy.