Fe-cr-al powder for additive manufacturing

Abstract

The present disclosure relates to an iron-chromium-aluminum (Fe-Cr-Al) powder suitable for additive manufacturing and an additive manufacturing method. The present disclosure also relates to an additively manufactured object.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a powder suitable for additive manufacturing. More specifically, the present disclosure relates to an iron-chromium-aluminum (Fe-Cr-Al) powder having a specific chemical composition for use in an additive manufacturing process. Furthermore, the present disclosure relates to a method of manufacturing a three-dimensional object using an additive manufacturing process and said Fe-Cr-Al powder. Moreover, the present disclosure relates to an additively manufactured object comprising said Fe-Cr-Al powder. BACKGROUND

[0002] Additive manufacturing is defined as a process of joining materials, layer by layer, to build objects from a three-dimensional data model. Metal-based additive manufacturing enables the layer-by-layer manufacturing of near-net-shape metal components with complex geometries, without the constraints of traditional manufacturing methods.

[0003] The use of iron-chromium-aluminum (Fe-Cr-Al) powder containing objects in electrically heated and high temperature applications is attractive. However, one of the problems with objects made from these powders using additive manufacturing processes is that the objects tend to crack during and after manufacturing. In additive manufacturing processes such as selective laser melting (SLM), electron beam melting (EBM) and direct energy deposition (DED), re-solidification is dominated by epitaxial growth of crystals from the previous solidified layer. The solidified material will mainly consist of large columnar grains with a large extension in the build direction. Such coarse and elongated structures make iron-chromium-aluminum objects brittle at low temperatures. The process of layer-by-layer melting and solidification also creates high thermal stresses in the built object. As a result, the manufactured three-dimensional objects tend to crack during and after manufacturing due to the columnar structure and residual stresses. One reason for this can be that the Fe-Cr-Al powder compositions used in additive manufacturing are based on conventional compositions, i.e. these compositions are still tailored for conventional manufacturing methods. Therefore, these compositions can not be suitable for inducing a directed thermal gradient for epitaxial growth during additive manufacturing, which can lead to a severely textured microstructure associated with anisotropic structural properties and cracking. Consequently, manufacturing complex structures in these Fe-Cr-Al powders can be both difficult and complex.

[0004] The document CN110125383 discloses a ferritic Fe-Cr-Al powder composition, wherein the Fe-Cr-Al powder composition comprises in weight %: Cr 18 to 34, Al 4 to 6, Si < 0.5, Ti < 0.5, Y < 1, Zr < 0.5, balance iron. However, even though the powder composition is disclosed and it is mentioned that it can be used in an additive manufacturing process, no actual additively manufactured product is disclosed.

[0005] Therefore, there is still a need in the art for a ferritic Fe-Cr-Al alloy powder having a chemical composition specifically suitable for additive manufacturing providing crack-free objects.

[0006] The present disclosure aims to solve or at least reduce the above-mentioned problems. SUMMARY

[0007] Therefore, the present disclosure provides a ferritic iron-chromium-aluminum (Fe-Cr-Al) powder composition optimized for additive manufacturing of three-dimensional objects.

[0008] The Fe-Cr-Al powder according to the present disclosure is characterized in that the powder has the following composition (in wt%):

[0009]

[0010] the balance of iron and unavoidable impurities;

[0011] wherein TiN is present as a modifying agent.

[0012] In the present disclosure, TiN is present as a modifying agent in the Fe-Cr-Al powder. It has been shown that the modifying agent will provide many advantages during the additive manufacturing process and in the objects manufactured therefrom. In particular, the TiN modifying agent will cause grain refinement and will also provide a nearly isotropic grain structure, as will be further disclosed below.

[0013] The present disclosure also provides a method of manufacturing a three-dimensional object using an additive manufacturing method and a Fe-Cr-Al powder composition as defined above or below. It has surprisingly been found that by using an additive manufacturing method and the Fe-Cr-Al powder of the present invention, crack-free objects having complex designs and geometries will be obtained in a cost- and time-efficient manner. In particular, it has been found that the TiN modifying agent will refine the solidification structure during the additive manufacturing method, resulting in objects having improved material quality, in particular due to the more equiaxed as-solidified grain structure.

[0014] The present disclosure also relates to crack-free additive manufactured objects obtained by using a Fe-Cr-Al powder as defined above or below (also containing the same alloying elements in the same range as the powder). The crack-free objects, which can have complex designs and geometries, will perform well in high-temperature applications. It has surprisingly been found that the TiN modifying agent in the Fe-Cr-Al powder will limit cracking behavior during the manufacturing process by promoting nucleation, which in turn leads to break-up of columnar structures, resulting in objects having improved properties. Crack-free means that no cracks can be observed macroscopically and microscopically. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1a - d shows SEM micrographs of Fe-Cr-Al powder particles with different compositions;

[0016] Figure 2a - b shows printed cubes composed of different Fe-Cr-Al powder compositions;

[0017] Figure 3a - b shows EBSD micrographs of printed cubes composed of different Fe-Cr-Al powder compositions. DETAILED DESCRIPTION

[0018] The present disclosure relates to a Fe-Cr-Al powder characterized in that the powder has the following composition (in wt%):

[0019]

[0020] the balance of iron and unavoidable impurities;

[0021] wherein TiN is present as a modifying agent.

[0022] The alloying elements of the powder according to the present disclosure will now be described in more detail. The terms “wt%” and “wt%” are used interchangeably. Furthermore, the list of properties or contributions mentioned for a particular element should not be seen as exhaustive.

[0023] Iron (Fe)

[0024] The main role of iron in the Fe-Cr-Al powder is to balance the powder composition or the composition of the alloying elements of the object.

[0025] Chromium (Cr) 12.0 wt% to 25.0 wt%

[0026] Chromium is an important element as it improves the corrosion resistance of the resulting object and increases its tensile strength and yield strength. Furthermore, chromium facilitates the formation of an Al2O3 layer on the final object by a so-called third element effect, i.e. by forming chromium oxides during the transient oxidation stage. Too low amounts of chromium would result in a loss of corrosion resistance. Therefore, chromium should be present in an amount of at least 12.0 wt.%, such as at least 15.0 wt.%, such as at least 20.0 wt.%. Too much chromium would make alpha to alpha' decomposition and 475°C embrittlement possible and would also result in an increased solid solution hardening effect on the ferrite structure. Therefore, the maximum amount of chromium is set to 25.0 wt.%, such as a maximum of 24.0 wt.%, such as a maximum of 23.50 wt.%, such as a maximum of 23.0 wt.%, such as a maximum of 22.50 wt.%, such as a maximum of 22.0 wt.%, such as a maximum of 21.50 wt.%. According to embodiments, the amount of chromium is between 12.0 wt.% and 25.0 wt.%, such as between 18.00 wt.% and 24.0 wt.%, such as between 20.0 wt.% and 23.50 wt.%.

[0027] Aluminium (Al) 3.50 wt.% to 6.50 wt.%

[0028] Aluminium is an important element as aluminium, when exposed to oxygen at high temperatures, forms a dense and thin layer of Al2O3 on the surface of the manufactured object, which will protect the underlying surface from further oxidation. Furthermore, aluminium increases the electrical resistivity. At too low amounts of aluminium, the ability to form an Al2O3 layer will be lost, which will decrease the electrical resistivity. Therefore, aluminium should be present in an amount of at least 3.50 wt.%, such as at least 4.00 wt.%, such as at least 4.50 wt.%, such as at least 4.80 wt.%. Too high amounts of aluminium would result in low temperature brittleness and would also increase the formation of unwanted brittle aluminium compounds. Therefore, aluminium is set to a maximum of 6.50 wt.%, such as a maximum of 6.00 wt.%, such as a maximum of 5.50 wt.%, such as a maximum of 5.40 wt.%, such as a maximum of 5.30 wt.%, such as a maximum of 5.20 wt.%. According to embodiments of the present disclosure, the amount of aluminium is between 3.50 wt.% and 6.50 wt.%, such as between 4.00 wt.% and 5.50 wt.%, such as between 4.50 wt.% and 5.50 wt.%.

[0029] Titanium (Ti) 0.20 wt.% to 1.10 wt.%

[0030] Titanium is an important element as titanium forms TiN together with nitrogen. According to an embodiment, the Ti / N ratio in wt.% should be at least 3.3, such as at least 4.5, due to the molar weights of Ti and N.

[0031] In addition, titanium can also reduce the activity of carbon by forming TiC and can further improve the high temperature creep strength. Too low amounts of Ti can result in insufficient TiN modifier for nucleation of ferrite crystals during solidification in the additive manufacturing process in the inventive powder. Furthermore, in case of too low titanium content, there is a high risk of formation of unwanted chromium carbides and / or brittle aluminum nitrides. Therefore, titanium should be present in an amount of at least 0.20 wt.-%, such as at least 0.25 wt.-%, such as at least 0.30 wt.-%. On the other hand, too high titanium content can have a negative effect on the formation of AI2O3, since TiO2may be formed. For these reasons, the maximum content of Ti is set to 1.10 wt.-%, such as a maximum of 1.00 wt.-%, such as a maximum of 0.90 wt.-%, such as a maximum of 0.8 wt.-%. According to embodiments of the present disclosure, the content of Ti is between 0.20 wt.-% and 0.80 wt.-%, such as between 0.20 wt.-% and 0.70 wt.-%, such as between 0.24 wt.-% and 0.60 wt.-%.

[0032] Nitrogen (N) 0.06 wt.-% to 0.20 wt.-%

[0033] Nitrogen is an important element, since it will form TiN particles together with titanium. In the present disclosure, TiN will act as a modifier and is therefore a desired particle. According to embodiments, the Ti / N ratio in wt.-% should be at least 3.3, such as at least 4.5, due to the molar weight of Ti and N.

[0034] Nitrogen is also an important element, since it enables the precipitation of other metal nitrides, such as ZrN. ZrN will improve the high temperature creep resistance. However, if the nitrogen content is too low, too low amounts of nitrides will be formed. Therefore, nitrogen should be present in an amount of at least 0.06 wt.-%, such as at least 0.07 wt.-%, such as at least 0.08 wt.-%, such as at least 0.09 wt.-%. Furthermore, if the nitrogen content is too high relative to the titanium content, there can be a risk of formation of AlN, which will have a negative effect on the oxidation resistance. For these reasons, the maximum content of N is set to 0.20 wt.-%, such as a maximum of 0.15 wt.-%, such as a maximum of 0.10 wt.-%. According to embodiments of the present disclosure, the content of N is between 0.060 wt.-% and 0.20 wt.-%, such as between 0.07 wt.-% and 0.15 wt.-%, such as between 0.07 wt.-% and 0.12 wt.-%.

[0035] TiN modifier

[0036] The Fe-Cr-Al powder as defined above or below will have a uniform distribution of TiN modifier in the powder. TiN is a desired modifier, which will introduce grain refinement and a more isotropic grain structure in the additively manufactured object.

[0037] It has been shown that by using the Fe-Cr-Al powder of the present disclosure in an additive manufacturing method, the degree of grain boundary alignment will be reduced, which will provide the manufactured object with increased crystallographic diversity.

[0038] Another advantage of the TiN inoculants is that they will provide grain refinement in the obtained object manufactured by additive manufacturing. The resulting grain structure of the object will have a significantly reduced average grain size compared to typical conventionally additive manufactured materials without these TiN inoculants.

[0039] Another advantage is that the TiN inoculants in the Fe-Cr-Al powder of the present invention can enable control of the solidification conditions during the additive manufacturing process, thus eliminating the need for time-consuming inter-layer conditioning.

[0040] It has been found that by introducing TiN inoculants in the Fe-Cr-Al powder, the solidification structure in additive manufacturing will be refined, as the TiN inoculants will act as nuclei for ferrite crystal formation, thus favoring the formation of a finer grain structure. TiN is thermodynamically stable in the liquid alloy and will form prior to ferrite crystals during solidification, thus acting as an effective nucleation site for ferrite crystals at the temperature of ferrite solidification. Without being bound by any theory, it is believed that the degree of undercooling required for ferrite nucleation on TiN particles will be very low due to the good lattice match and low interfacial energy between the TiN particles and the lattice structure of the ferrite crystals. In addition, the good coherency between TiN and ferrite will also reduce the stresses in the formed object.

[0041] The TiN inoculant size and / or size distribution can determine the degree of undercooling for equiaxial growth. To this end, according to embodiments, the average size of the TiN inoculants is at least 30 nm, such as at least 50 nm, such as at least 100 nm.

[0042] In addition, in order for the TiN inoculants to nucleate and grow, the presence of oxides in the Fe-Cr-Al powder of the present invention, such as corundum, can be advantageous during the high cooling rate solidification conditions.

[0043] Thus, the uniformly and finely dispersed TiN inoculants in the Fe-Cr-Al powder of the present invention will provide a more isotropic and fine-grained solidification structure with a more random crystallographic orientation during the layer-by-layer procedure of additive manufacturing. This will provide a reduced cracking behavior during and / or after additive manufacturing of the Fe-Cr-Al object. The lower residual stresses and less formed columnar grain structure formed during additive manufacturing will enable the additive manufactured object to be more crack-free.

[0044] Zirconium (Zr) 0.05 wt% to 0.20 wt%

[0045] Zirconium is an important element in the powder composition of the present invention, as it will reduce the activity of C and N by forming ZrC or ZrN precipitates. Zirconium will also improve the high temperature creep strength of the manufactured object. Too low a content of Zr will increase the risk of forming unwanted chromium carbides and / or aluminium nitrides. Therefore, the zirconium should be present in an amount of at least 0.05 wt.%, such as at least 0.07 wt.%, such as at least 0.10 wt.%. On the other hand, too high a content of Zr can have a negative effect on the formation of AI2O3. For these reasons, the maximum content of zirconium is set to 0.20 wt.%, such as a maximum of 0.15 wt.%. According to embodiments of the present disclosure, the content of zirconium is between 0.05 wt.% and 0.20 wt.%, such as between 0.07 wt.% and 0.20 wt.%, such as between 0.070 wt.% and 0.10 wt.%.

[0046] Yttrium (Y) 0.02 wt.% to 0.15 wt.%

[0047] The addition of yttrium improves the oxidation resistance of the manufactured object. Too little added yttrium will result in a reduced oxidation resistance. For this reason, the amount of added yttrium must be at least 0.02 wt.%, such as at least 0.04 wt.%, such as 0.05 wt.%, such as 0.06 wt.%. However, if too much yttrium is added, hot shortness will result. As a result, the maximum content of yttrium is set to 0.15 wt.%, such as 0.10 wt.%, such as 0.08 wt.%.

[0048] Carbon (C) < 0.050 wt.%

[0049] Carbon is an element that is not intentionally added, but is an unavoidable element due to powder handling. This element can result in a reduced hot ductility and formation of metallic carbides. Therefore, in order to limit the presence of too many metallic carbide precipitates, the carbon content must be < 0.050 wt.%, such as < 0.040 wt.%, such as < 0.030 wt.%.

[0050] Silicon (Si) < 0.50 wt.%

[0051] The content level of silicon can be up to 0.50 wt.% in order to increase the electrical resistivity and to increase the hot corrosion resistance. However, above this level, the hardness will increase and also brittleness at low temperatures will occur.

[0052] Tantalum (Ta) < 0.30 wt.%

[0053] Tantalum can optionally be added and, if added, will improve the high temperature creep strength. Tantalum can also reduce the carbon activity by forming TaC precipitates, and therefore the maximum content of tantalum is set to 0.30 wt.%.

[0054] Hafnium (Hf) < 0.30 wt.%

[0055] Hafnium can optionally be added. The addition of hafnium will improve the high temperature creep strength. However, hafnium can reduce the activity of carbon by forming HfC precipitates. Therefore, the maximum hafnium content is set to < 0.30 wt.-%.

[0056] Manganese (Mn) < 0.40 wt.-%

[0057] Manganese can be present as an impurity. Manganese can interfere with the formation of the AI2O3 layer, thereby negatively affecting the oxidation resistance. Therefore, the maximum content of manganese is < 0.40 wt.-%, for example < 0.20 wt.-%.

[0058] Nickel (Ni) < 0.60 wt.-%

[0059] Nickel can be present as an impurity. However, nickel can increase the hardness and the brittleness at low temperatures. Therefore, the maximum content of nickel is < 0.60 wt.-%, for example < 0.5 wt.-%.

[0060] Oxygen (O) < 600 ppm

[0061] Oxygen can be present in the form of oxides. The maximum content allowed is < 600 ppm.

[0062] According to an embodiment, the powder can further comprise a minor amount of one or more of the following impurity elements, for example but not limited to: magnesium (Mg), cerium (Ce), calcium (Ca), phosphorous (P), tungsten (W), cobalt (Co), sulfur (S), molybdenum (Mo), niobium (Nb), vanadium (V) and copper (Cu), in an amount of at most 0.2 wt.-%.

[0063] Furthermore, the Fe-Cr-Al powder as defined above or below can comprise the alloying elements mentioned herein within any of the ranges mentioned herein. According to one embodiment, the inventive powder consists of all the alloying elements mentioned herein within any of the ranges mentioned herein.

[0064] Furthermore, the additively manufactured object as defined above or below can comprise or consist of the alloying elements of the Fe-Cr-Al powder as defined above or below within any of the ranges mentioned herein.

[0065] The Fe-Cr-Al powder as defined above or below can be manufactured by different methods.

[0066] For example, but not limited to:

[0067] - directly by gas atomization;

[0068] - heating a powder comprising all alloying elements within the ranges mentioned above or below but with a low nitrogen content in a nitrogen rich atmosphere, i.e. nitriding the powder;

[0069] - mixing a powder comprising all alloying elements within the ranges mentioned above or below but with a low nitrogen content with a powder containing finer particles of less stable nitrides;

[0070] - mixing fine / small particles of TiN with the Fe-Cr-Al powder such that the obtained powder has the same alloying element composition as defined above or below.

[0071] According to embodiments, the Fe-Cr-Al powder particles have a (average) size of less than 200 pm, such as less than 120 pm, such as less than 100 pm, suitable for use in additive manufacturing methods.

[0072] According to embodiments, the Fe-Cr-Al powder size distribution can be selected from 4 pm to 200 pm, such as 10 pm to 120 pm, such as 10 pm to 90 pm.

[0073] The present disclosure also relates to a method of manufacturing a three-dimensional object using an additive manufacturing method and a Fe-Cr-Al powder composition as defined above or below.

[0074] According to embodiments, the additive manufacturing method is selected from a powder bed fusion method or a direct energy deposition (DED) method.

[0075] In a powder bed fusion additive manufacturing method, a layer of powder is selectively melted using e.g. a high power laser. Due to the small interaction volume and the melt pool, the cooling rate during the process is very high. As a result, the microstructure will be very different compared to a wrought or cast object using the same powder composition.

[0076] During a powder bed fusion additive manufacturing method, the TiN refiner in the Fe-Cr-Al powder will be present in the melt before solidification and will promote grain refinement by acting as nucleation sites for the melt to solidify. Solidification by growth of crystals nucleating on the TiN particles will compete with epitaxial growth of crystals from previously solidified material. Crystals nucleating on the TiN refiner in the undercooled melt during the additive manufacturing process will grow as equiaxed grains until they are engulfed by the epitaxial solidification front or until they connect to other solidification structures.

[0077] According to one embodiment, the powder bed fusion manufacturing method is selected from selective laser melting (SLM) or electron beam melting (EBM). In both embodiments, a powder bed is used, the powder is provided as a layer and an energy source will pass over the area of the powder layer to be melted, whereby the powder is exposed to the energy source and thus melted or at least partially melted. After the desired part of the powder layer is melted, a new layer is provided and this continues until the desired object is obtained.

[0078] In SLM, the energy source is one or more laser beams, while in EBM the energy source is an electron beam. SLM is performed in an inert atmosphere, such as an argon or nitrogen atmosphere. Furthermore, the method can use supports if needed, for example for stiffening small angles, which will then be removed. Furthermore, SLM printing is performed directly on loose powder layers.

[0079] In EBM, each powder layer is preheated before it is locally melted by the electron beam. In 1*10 -5 The method is performed under vacuum and high temperature in a bar. Furthermore, in EBM, each new powder layer is first pre-sintered using the electron beam before actual printing of the powder layer starts.

[0080] According to one embodiment, the powder layer thickness is between 10 pm and 250 pm. For example, in SLM, the layer thickness is between 10 pm and 80 pm, for example between 10 pm and 45 pm, while in EBM the layer thickness is between 10 pm and 250 pm.

[0081] According to one embodiment, the additive manufacturing method is direct energy deposition (DED). In this type of method, an energy source is used to build a local melt pool. Metal powder is fed into this melt pool as a filler material. The position of the melt pool is constantly moved so that a three-dimensional body is built by solidified material. The energy source can be a laser beam or a plasma arc. The heat generated from the energy source should be sufficient to melt the surface of the substrate, thereby forming a melt pool. Powder is added to the pool by using a focused powder flow, which means that the powder is propelled in the focused energy source so as to be melted. The DED method is usually performed under an inert protective gas atmosphere that protects the melt pool. The material feed angle can be changed depending on the predetermined shaped object.

[0082] Furthermore, due to the additive manufacturing method and the Fe-Cr-Al powder composition as defined above or below, no post-treatment such as heat treatment or shape machining can be required. Furthermore, a reduction in deposition rate can be avoided, thereby increasing the deposition productivity.

[0083] The additively manufactured object obtained from the Fe-Cr-Al powder as defined above or below will perform well at temperatures up to 1350 °C. In addition, the inventive object will have a significantly high temperature corrosion resistance and high oxidation, sulphidation and carburization resistance. Furthermore, the additively manufactured object will have a significantly high temperature creep strength, shape stability and high electrical resistivity. The additively manufactured object is particularly useful as an electrical heating element or as a component in high temperature applications (applications operating between 400 °C and 1350 °C). The additively manufactured object is also particularly useful as a component in electrical heating applications. The object can also be used to protect other objects from high temperature wear and corrosion. Thus, the inventive object can be used in both electrical heating and high temperature applications.

[0084] The present invention is further described by the following non-limiting examples.

[0085] Example

[0086] Powder composition

[0087] Four Fe-Cr-Al powders were manufactured with different titanium and nitrogen contents (see Table 1 for their composition). Powders 1 and 2 are comparative examples, powders 3* and 4* are inventive powders. The powders were manufactured by induction melting and subsequent gas atomization. The metal melt with the specific composition was poured through a small melt nozzle into an atomization chamber filled with an inert atmosphere. Using a high-speed gas nozzle system, the melt stream was broken up into very fine droplets, which were cooled down and then transformed into solidified particles in flight within fractions of a second. The particles were collected and cooled down to ambient temperature in an inert atmosphere. The powders were sieved to -45 pm.

[0088] Table 1 Composition of Fe-Cr-Al powders (in wt.-%)

[0089] powder 1 2 3* 4* gas Ar Ar [N2] [N2] Fe balance balance balance balance Cr 20.88 21.01 20.93 20.29 Al 5.20 5.18 5.24 5.19 Si 0.24 0.26 0.29 0.27 Ni 0.24 0.18 0.21 0.18 Mn 0.19 0.15 0.17 0.17 Ti 0.50 0.07 0.24 0.49 Zr 0.074 0.075 0.077 0.073 N 0.024 0.044 0.073 0.10 Y 0.06 0.06 0.07 0.06 C 0.024 0.022 0.024 0.024 P 0.008 0.008 0.008 0.007 S 0.0002 0.0004 0.0002 0.0002 O 0.0073 0.01425 0.0060 0.0063

[0090] The grain refinement effect obtained by introducing the TiN modifying agent has been obtained and visually perceived in the solidified microstructure of the powder as atomized. Qualitatively, the degree of single crystallinity versus polycrystallinity can be visually perceived by means of a "grain contrast imaging technique" or "electron channeling contrast imaging technique", briefly outlined as follows. The Fe-Cr-Al powder is mixed with electrically conductive balsa wood powder and formed into a solid cylindrical puck. One of the flat faces of the puck is ground to sufficient depth and then polished to a very high surface finish. Thereby, when analyzed by means of a scanning electron microscope (SEM), the polished part of many powder particles will be visible on the polished puck surface. The depth reached by the incident SEM electrons into the crystalline metallic material under investigation and the number of backscattered electrons reflected back therefrom depend on the crystal orientation of the investigated crystal in the sample. Thus, with respect to the direction of the incident electrons, different crystal orientations of the grains will result in different amounts of backscattered electrons being reflected, resulting in the end in a contrast difference between these investigated grains. This effect is thus best perceived with a backscattered electron detector.

[0091] The results of this qualitative analysis on the powder particles in the size range of 1 pm to 45 pm in the four powders are shown in Figure 1a - d). The result of this analysis is that the powder with the combination of high titanium content and high nitrogen content (powder 4) resulted in the highest degree of polycrystallinity and the smallest average grain size. The powder particles of powder 4 also showed the highest amount of cubic TiN precipitates. The powder with the moderate levels of titanium and nitrogen content (powder 3) showed the second highest polycrystallinity. The powder with both low titanium content and low nitrogen content (powder 2) showed the lowest degree of polycrystallinity. Powder 1 showed no or only limited grain refinement. It is thus concluded that in order to obtain the grain refinement imparted by the modifying agent, both the titanium level and the nitrogen level should be increased to obtain a TiN modifying agent that promotes ferrite grain nucleation.

[0092] Printing

[0093] Many builds were printed with each of these powders using the same printing parameter settings. The four different powders with the compositions described above were provided to the SLM machine by adding them to the powder delivery system. During the printing process, the powder was provided by the powder delivery system in the machine and a blade spread a layer of powder over the build plate. Then, according to the 3D map provided for a 20x20x20 mm 3 cube, the laser was passed through the powder layer, whereby the powder layer was exposed to the laser beam and thus melted. After the powder layer was melted, a new layer was provided until the desired sample was formed according to the 3D map.

[0094] The thickness of the powder layer was 20 pm. Printing was performed in an inert atmosphere using argon. The scanning speed was 500 mm / s. The power of the energy source was 95 W.

[0095] The sample was allowed to cool down to room temperature in an inert atmosphere. Then, the as-built cubes were cut from the build plate without prior heat treatment.

[0096] Evaluation

[0097] The four printed cubes were visually inspected. In contrast to the cubes containing powder 1 or 2, no cracks were observed for the cubes containing powder 3 or 4. Figure 2a The printed cube (object 2) containing powder 2 was disclosed, wherein cracks were visible (see arrows in Figure 2a The printed cube (object 4) containing powder 4 was disclosed. Figure 2b

[0098] The microstructure was analyzed in as-built condition. The grain mapping identified by electron backscatter diffraction (EBSD) analysis (accelerating voltage 20 kV, sampling size 1 pm, minimum 10 pixels per grain) of a polished vertical section parallel to the build direction of the cubes revealed that the cube containing powder 4 (object 4 in Figure 3b ) showed significantly smaller grain sizes than the other cubes (in terms of the cube containing powder 2 (object 2 in Figure 3a ). This difference is related to the increased levels of titanium and nitrogen. According to the ECD; the 20 largest grains of object 4 were 106 ± 21, in contrast to 164 ± 52 for object 2. Accordingly, the grains / mm2 2 > 100 pm of object 4 were 1428, in contrast to 397 for object 2.

[0099] The SEM + EBSD analysis performed on object 4 revealed that, although solidification still mainly occurred epitaxially in case of finer grains, the grain ensemble presented a more equiaxed morphology. However, the SEM analysis could not confirm what the mechanism was for the grain refinement of powder 4 compared to powder 2. This could be a combination of grain pinning and inoculation, both of which could be related to the TiN inoculant in the melt pool.

[0100] The SEM + EBSD analysis performed on object 2 showed that the columnar grains were mostly aligned parallel to the build direction. This microstructure was characterized by coarse columnar grains, up to mm in length. The epitaxial columnar growth of the grains over multiple layers indicated that there was no active inoculant and that it was not possible to form fine grains within the melt pool by heterogeneous nucleation. It was found that this material was extremely prone to cracking, with most of the cracks observed in the transverse direction.

[0101] ​Tensile testing of the printed objects showed that object 4 had higher ductility than object 2, which can be due to finer grains.

[0102] Oxidation testing showed similar results for both objects; thus, the TiN modifier had no negative effect on the oxidation resistance of object 4.

[0103] Thus, it is clear that the TiN modifier in the Fe-Cr-Al powder has a very positive effect on the grain refinement and crack tolerance of the as-built part.

Claims

1. A Fe-Cr-Al powder, characterized in that... The powder has the following composition in weight percent: The remaining iron and unavoidable impurities; Where Ti / N ≥ 3.3, TiN exists as a modifier that promotes grain refinement.

2. The Fe-Cr-Al powder according to claim 1, wherein the Cr content is from 18.0% to 24.0% by weight.

3. The Fe-Cr-Al powder according to claim 1 or 2, wherein the Al content is from 4.0% to 6.0% by weight.

4. The Fe-Cr-Al powder according to claim 1 or 2, wherein the Ti content is from 0.30% to 1.00% by weight.

5. The Fe-Cr-Al powder according to claim 1 or 2, wherein the N content is from 0.09% to 0.20% by weight.

6. The Fe-Cr-Al powder according to claim 1 or 2, wherein the Zr content is from 0.07% to 0.10% by weight.

7. The Fe-Cr-Al powder according to claim 1 or 2, wherein the powder size is less than 120 μm.

8. A method for manufacturing a three-dimensional object, using an additive manufacturing method and Fe-Cr-Al powder according to any one of claims 1 to 7.

9. The method of claim 8, wherein the additive manufacturing method is selected from powder bed fusion or direct energy deposition (DED) methods.

10. The method of claim 9, wherein the powder bed fusion method is SLM or EBM.

11. An additively manufactured object comprising the powder according to any one of claims 1 to 7 or manufactured by the method according to any one of claims 8 to 10.

12. The additively manufactured object according to claim 11, wherein the object is a high-temperature resistant heating element or a high-temperature resistant component.

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