Metal powder for additive manufacturing

Abstract

The present invention relates to a metal powder for additive manufacturing, the composition of which comprises the following elements, expressed in contents by weight:-15% < = Mn < = 35%,-6% < = Al < = 15%,-0.5% < = C < = 1.8%,-0 < = Si < = 0.5%,-P < = 0.013%,-S < = 0.015%,-N < = 0.100%, and the balance being Fe and unavoidable impurities. And optionally comprising Ni < = 8.5% by weight and / or Cr < = 2.5% by weight and / or B < = 0.1% by weight and / or one or more elements selected from the group consisting of Ta, Zr, Nb, V, Ti, Mo and W in a cumulant amount of up to 2.0% by weight, the balance being iron and unavoidable impurities resulting from processing. The invention also relates to a method for producing such a powder and to a method for producing a printed part therefrom.

Description

Technical Field

[0001] The present invention relates to a metal powder for the manufacture of steel parts, and in particular for the additive manufacturing of steel parts. The invention also relates to a method for manufacturing a metal powder. The powder according to the invention is particularly well suited for the manufacture of safety parts or structural parts with low density for vehicles, such as land motor vehicles. It can also be used in particular for the manufacture of parts for defense, naval, or armor applications. Background Art

[0002] Environmental restrictions force car manufacturers to continuously reduce the CO2 emissions of their vehicles. To this end, car manufacturers are looking for various ways to reduce the weight of motor vehicles.

[0003] This can be achieved in particular by reducing the density of the steel used to manufacture the component by alloying it with other metals that are lighter than iron.

[0004] Steels containing high levels of manganese, aluminum and carbon, often referred to as triplex steels, may show density levels below 7.4 g / cm3. Their solidified structures show an austenitic structure that may contain kappa carbides (Fe,Mn)3AlCx and ferrite.

[0005] However, they are difficult to manufacture by conventional casting methods due to their high aluminum and carbon content. They also exhibit some macrosegregation of manganese, carbon and / or aluminum, resulting in banding when they are laminated and possible formation of brittle phases that lead to cracks. Summary of the invention

[0006] The object of the present invention is therefore to remedy the disadvantages of the prior art by providing a new method for obtaining components having a low density without manufacturability problems.

[0007] To this end, a first object of the present invention comprises a metal powder for additive manufacturing, the composition of which, expressed in terms of content by weight, comprises the following elements:

[0008] -15%≤Mn≤35%

[0009] -6%≤Al≤15%

[0010] -0.5% ≤ C ≤ 1.8%

[0011] -0≤Si≤0.5%

[0012] -P≤0.013%

[0013] -S≤0.015%

[0014] -N≤0.100%

[0015] and optionally contains Ni≤8.5 wt.% and / or Cr≤2.5 wt.% and / or B≤0.1 wt.% and / or one or more elements selected from Ta, Zr, Nb, V, Ti, Mo and W in a cumulative amount of up to 2.0 wt.%, the balance being iron and unavoidable impurities resulting from processing.

[0016] The metal powder according to the invention may also have the optional features listed below, considered alone or in combination:

[0017] the powder particles have an austenitic microstructure containing up to 1% by weight of kappa carbides (Fe,Mn)3AlCx and up to 20% by weight of ferrite and up to 1% by weight of AlN,

[0018] -The density of the powder is less than 7.0g / cm 3 ,

[0019] - the average particle size is between 1 μm and 150 μm,

[0020] - the average particle size is between 1 μm and 20 μm,

[0021] - the average particle size is between 20 μm and 63 μm,

[0022] The average particle size is between 60 μm and 150 μm.

[0023] A second object of the present invention includes a method for producing a metal powder for additive manufacturing, the method comprising:

[0024] -a) melting the elements and / or the metal alloy at a temperature of at least 100° C. above the liquidus temperature to obtain a molten composition according to claim 1,

[0025] - b) atomizing the molten composition with a gas pressurized to 10 to 30 bar through a nozzle having a diameter of at most 4 mm.

[0026] A third object of the invention comprises a method for producing a printed part by additive manufacturing, wherein a powder according to the invention is printed by laser powder bed fusion.

[0027] The printing method according to the present invention may also have the optional features listed below, considered individually or in combination:

[0028] - the method comprises a first step of forming a powder layer having a thickness lower than 100 μm, and a second step of forming a shaped layer by melting at least part of the powder layer with a focused laser beam in an atmosphere consisting essentially of an inert gas,

[0029] -This method is set with the following parameters:

[0030] -Limit laser power to a maximum of 500W,

[0031] - Scanning speed is 300mm / sec to 2000mm / sec,

[0032] - Linear energy density of 190 J / m to 500 J / m,

[0033] - Scanning pitch is 50μm to 120μm,

[0034] - Volume energy density of 100 J / mm3 to 330 J / mm3.

[0035] A fourth object of the invention comprises a printed part obtained according to the invention having a cellular solidified structure with an equivalent diameter below 2 μm. DETAILED DESCRIPTION

[0036] The present invention will be better understood by reading the following description, which is provided for illustrative purposes only and is not intended to be limiting in any way.

[0037] Manganese is present in the composition according to the invention in a content of 15% to 35% by weight. Manganese is an essential alloying element to such an extent, mainly due to the fact that alloying with very large amounts of manganese and carbon stabilizes austenite until room temperature in the final component, and austenite can then withstand large amounts of aluminum without losing stability and without converting to excessive ferrite or martensite. In order to enable the alloy to have excellent ductility, the manganese content must be equal to or higher than 15% by weight. However, when the manganese content is higher than 35% by weight, the precipitation of the β-Mn phase will deteriorate the ductility of the alloy. Therefore, the manganese content should be controlled to be equal to or greater than 15% by weight, but less than or equal to 35% by weight. In a preferred embodiment, it is equal to or greater than 15.5% by weight or even equal to or greater than 16.0% by weight. Its amount is more preferably 25% to 31% by weight, or even better 26% to 30% by weight.

[0038] Aluminum is present in the composition according to the invention in a content of 6% to 15% by weight. Adding aluminum to high manganese austenitic steel effectively reduces the density of the alloy. In addition, this significantly increases the stacking fault energy (SFE) of austenite in the final component, thereby causing a change in the strain hardening behavior of the alloy. Aluminum is also one of the main elements of nano-sized κ carbides (Fe, Mn) 3AlCx, so its addition significantly enhances the formation of such carbides. On the one hand, in order to ensure austenite stability and the possible precipitation of κ carbides, and on the other hand, in order to control the formation of ferrite, the aluminum concentration of the alloy of the present invention should be adjusted. In addition, it has been observed that an aluminum amount below 6% by weight results in a material density in the final component higher than 7.0 g / cm 3 Therefore, the aluminum content should be controlled to be equal to or greater than 6 wt %, but less than or equal to 15 wt % to avoid removal of the austenite phase. In a preferred embodiment, the aluminum content is 6 wt % to 12 wt %, or even better 6 wt % to 10 wt %.

[0039] The carbon content is set at 0.5 wt % to 1.8 wt %. Carbon plays an important role in the formation of the microstructure of the final component. Its main role is to stabilize austenite (which is the main phase of the microstructure of the steel component) and provide strengthening. A carbon content lower than 0.5 wt % will reduce the proportion of austenite, which leads to a reduction in both ductility and strength of the alloy. However, since it is the main constituent element of kappa carbide (Fe, Mn) 3 AlC x, a carbon content higher than 1.8 wt % may promote the precipitation of such carbides in a coarse manner on the grain boundaries, which leads to a reduction in the ductility of the alloy.

[0040] Preferably, the carbon content is 0.6 to 1.3 wt%, more preferably 0.8 to 1.2% by weight, in order to obtain sufficient strength.

[0041] Silicon is present in the composition according to the invention in an amount of 0% to 0.5% by weight. It has been observed that the addition of up to 0.5% by weight of silicon suppresses hot cracking that occurs when the final component is produced by additive manufacturing. However, additions above 0.5% by weight lead to cold cracking failures when the final component is produced by additive manufacturing. Preferred ranges are 0.05% to 0.5% by weight, 0% to 0.25% by weight, or even better 0.05% to 0.25% by weight.

[0042] Nickel can optionally exist with the content of up to 8.5 wt %.Nickel can be used as a diffusion barrier for hydrogen.Being higher than 8.5 wt % of nickel is not desirable, because it promotes the formation of cementite that damages (Fe, Mn)3AlCx carbide.Nickel can also be used as an effective alloying element, because it stabilizes austenite, and also promotes the formation of ordered compounds such as B2 components in ferrite, thereby causing other strengthening.But, especially for cost reasons, it is 6.0 wt % or less or 4 wt % or less that it is desired that nickel is added to be limited to a maximum content, and preferably 0.1 wt % to 2.0 wt % or 0.1 wt %% to 1.0 wt %.When not adding nickel, the composition may still comprise up to 0.1 wt % of nickel as impurity.

[0043] Chromium may optionally be present in an amount of up to 2.5% by weight to increase the strength of the steel by solid solution hardening. It also enhances the high temperature corrosion resistance of the steel according to the invention. However, since chromium reduces the stacking fault energy and stability of austenite, its content must not exceed 2.5% by weight, and is preferably 0.1% to 2.0% by weight or 0.1% to 1.0% by weight. When no chromium is added, the composition may still contain up to 0.1% by weight of Cr as an impurity.

[0044] Boron may optionally be present in an amount of up to 0.1 wt%. Boron has a very low solid solubility and a strong tendency to segregate at grain boundaries, thereby interacting strongly with lattice defects. Boron can therefore serve to limit the precipitation of intergranular kappa carbides.

[0045] Tantalum, zirconium, niobium, vanadium, titanium, molybdenum and tungsten are elements that can optionally be used to achieve hardening and strengthening, in particular by precipitation of nitrides, carbonitrides or carbides. However, when their cumulative amount is above 2.0% by weight, preferably above 1.0% by weight, or even better above 0.5% by weight or above 0.3% by weight, there is a risk that excessive precipitation may cause a reduction in toughness, which must be avoided.

[0046] The balance is made up of iron and the inevitable impurities produced by processing. Phosphorus, sulfur and nitrogen are the main impurities. They are not intentionally added. They may be significantly present in the ferroalloys and / or pure elements used as raw materials. Nitrogen may also be introduced during atomization. It is preferred to control their content to avoid harmful changes in the microstructure and / or to avoid increasing brittleness. Therefore, their content is limited to 0.013 weight %, 0.015 weight % and 0.1 weight % respectively. In a preferred embodiment, their content is limited to 0.005 weight %, 0.015 weight % and 0.01 weight % respectively.

[0047] The microstructure of the powder is mainly austenite and may optionally contain up to 1 wt% of kappa carbide (Fe, Mn)3AlCx, up to 1 wt% of AlN and up to 20 wt% of ferrite. In a preferred embodiment, the optional ferrite content may be 0.5 wt% to 10 wt% or 0.5 wt% to 5 wt%, or even better 0.5 wt% to 4.0 wt%.

[0048] The powder can be obtained by first mixing and melting the pure elements and / or ferroalloys as raw materials. It can also be obtained by using a pre-alloyed ingot of the desired composition.

[0049] Ferroalloys refer to various alloys of iron with a high proportion of one or more other elements (e.g. manganese, silicon, aluminum, niobium, boron, chromium, molybdenum, ...). The main alloys are FeMn (usually containing 70-80 wt% Mn), FeAl (usually containing 40-60 wt% Al), FeSi (usually containing 15-90 wt% Si), FeNi (usually containing 70-95 wt% Ni), FeB (usually containing 17.5-20 wt% B), FeCr (usually containing 50-70 wt% Cr), FeMo (usually containing 60-75 wt% Mo), FeNb (usually containing 60-70 wt% Nb), FeV (usually containing 35-85 wt% V), FeW (usually containing 70-80 wt% W).

[0050] The alloying elements can alternatively be added as pure elements (generally with a purity of more than 99% by weight). The pure elements can in particular be carbon and pure metals such as iron, aluminum, manganese or nickel, zirconium, titanium, tantalum, molybdenum, tungsten, niobium, vanadium, chromium.

[0051] A person skilled in the art knows how to mix different ferroalloys and pure elements to achieve a target composition.

[0052] After the composition has been obtained by mixing the pure elements and / or ferroalloys in appropriate proportions, the composition is heated at a temperature of at least 100°C above its liquidus temperature and maintained at this temperature to melt all the raw materials and homogenize the melt. Due to this overheating, the reduction in the viscosity of the molten composition helps to obtain a powder with high sphericity without satellite particles and with an appropriate particle size distribution. That is, since the surface tension increases with temperature, it is preferred not to heat the composition at a temperature of more than 450°C above its liquidus temperature.

[0053] Preferably, the composition is heated at a temperature of at least 200°C above its liquidus temperature to promote the formation of highly spherical particles. More preferably, the composition is heated at a temperature of 250°C above its liquidus temperature.

[0054] In one embodiment of the invention, the composition is heated from 1650°C to 1800°C, which represents a good compromise between viscosity reduction and surface tension increase.

[0055] The molten composition is then atomized into fine metal droplets by forcing the molten metal stream through the orifice of a nozzle at moderate pressure and by impinging it with a gas jet (gas atomization). Just before the metal stream leaves the nozzle, a gas is introduced into the metal stream for generating turbulence when the entrained gas expands (due to heating) and enters a large collection volume (atomization tower). The atomization tower is filled with gas to promote further turbulence of the molten metal jet. The metal droplets cool during their fall into the atomization tower. Gas atomization is preferred because it is conducive to producing powder particles with a high degree of roundness and a low number of satellite particles.

[0056] The atomizing gas is preferably argon or nitrogen or a mixture thereof. Both of them increase melt viscosity more slowly than other gases such as helium, which promotes the formation of smaller particle sizes. They also control chemical purity and play a role in the good morphology of the powder. Since the molar weight of nitrogen is 14.01 g / mole, the molar weight of argon is 39.95 g / mole by comparison, finer particles can usually be obtained with argon than with nitrogen. On the other hand, the specific heat capacity of nitrogen is 1.04 J / (g K), while the specific heat capacity of argon is 0.52 J / (g K). Therefore, nitrogen increases the cooling rate of the particles. Whenever nitrogen is used as a component of the atomization process, AlN up to 1 wt % can be formed by the combination of aluminum and nitrogen.

[0057] Gas pressure is important because it directly affects the particle size distribution. In particular, the higher the pressure, the higher the cooling rate. Preferably, the gas pressure is set to 10 bar to 30 bar, or even better 24 bar to 30 bar, to promote the formation of particles whose size is most compatible with additive manufacturing techniques.

[0058] The nozzle diameter has an influence on the molten metal flow rate and, therefore, on the particle size distribution and on the cooling rate. The nozzle diameter is preferably limited to 4 mm to limit the increase in average particle size and the decrease in cooling rate.

[0059] The metal powder obtained by atomization can be used as is, or can be sieved to retain particles whose size is better suited to additive manufacturing techniques for later use. For example, in the case of additive manufacturing by powder bed fusion, the range 20 μm to 63 μm (called fraction F2) is preferred, and the range 20 μm to 40 μm is even better. In the case of additive manufacturing by laser metal deposition or direct metal deposition, the range 60 μm to 150 μm (called F3) is preferred, and the range 40 μm to 125 μm is even better. Fraction F1 covering particle sizes below 20 μm or even 10 μm can be used, for example, in binder jetting.

[0060] Components made of metal powder according to the present invention can be obtained by additive manufacturing techniques such as: Powder Bed Fusion (LPBF), Direct Metal Laser Sintering (DMLS), Electron Beam Melting (EBM), Selective Heat Sintering (SHS), Selective Laser Sintering (SLS), Laser Metal Deposition (LMD), Direct Metal Deposition (DMD), Direct Metal Laser Melting (DMLM), Direct Metal Printing (DMP), Laser Cladding (LC), Binder Jetting (BJ), Cold Spray (CS), Thermal Spray (TS), High Velocity Oxygen Fuel (HVOF).

[0061] In particular, the present invention can utilize the LPBF method, which is a layer-upon-layer additive manufacturing technique. A thin layer of metal powder is evenly distributed onto a substrate platform, typically metal, using a coating mechanism, which is fixed to an indexing table that moves along a vertical axis. This occurs inside a chamber containing a strictly controlled atmosphere. After the individual layers have been distributed, the 2D slices of the component geometry are fused by selectively melting the powder. This is achieved with a high-power laser beam, typically an ytterbium fiber laser. The laser energy is strong enough to allow the particles to be completely melted (welded) in the form of tracks or strips. Basically, after completing one track, the process is repeated with the next track, which is separated from the first track by a scan pitch. The process is repeated layer by layer until the part is completed. Overhanging geometries are supported by unmelted powder from the previous layer. The main process parameters used in LPBF are typically layer thickness, scan pitch, scan speed and laser power. After completing the process, the remaining powder is screened for reuse.

[0062] The method for producing an additively manufactured component by LPBF comprises a first step of forming a powder layer with a powder according to the invention. Preferably, the powder layer is less than 100 μm. Above 100 μm, the laser may not be able to melt the powder in all layer thicknesses, which may lead to pores in the component. Preferably, the layer thickness is kept between 20 μm and 60 μm to optimize the melting of the powder.

[0063] In a second step, the focused laser beam forms a shaped layer by melting at least part of the powder layer under process conditions detailed below.

[0064] In the case of LPBF, the individual layers of the printed component are at least partially melted in an atmosphere consisting essentially of an inert gas.

[0065] The laser power is preferably limited to a maximum of 500 W. Preferably, the laser power is set above 80 W to facilitate melting in all layer thicknesses. In a preferred embodiment, the laser power is between 175 W and 300 W.

[0066] The scanning speed is preferably 300 mm / sec to 2000 mm / sec, and more preferably 300 mm / sec to 700 mm / sec. Below 300 mm / sec, the excess energy provided by the laser may result in keyhole voids and / or spatters that, if not properly dragged outside the powder bed, are deposited on the powder layer, which creates voids in the printed part. Above 2000 mm / sec, the energy provided by the laser to the powder may be insufficient to melt the powder in all layer thicknesses.

[0067] The Linear Energy Density (LED) is preferably between 190 J / m and 500 J / m. LED is defined as the ratio between the laser power and the scan speed expressed in m / sec. Below 190 J / m, the LED may not be sufficient to properly print the part (due to the keyholing effect). Above 400 J / m, the excess energy provided by the laser may result in spatter that, if not properly dragged outside the powder bed, will deposit on the powder layer. Such deposits create voids in the printed part.

[0068] The scanning pitch is preferably 50 μm to 120 μm. Below 50 μm, individual points of the printed part may re-melt multiple times, which may cause overheating. Above 120 μm, unmelted powder may be trapped between two tracks. More preferably, the scanning pitch is 70 μm to 110 μm.

[0069] The volumetric energy density (VED) is preferably 100 J / mm 3 Up to 330J / mm 3 VED is defined as P / (v·h·l t ), where P is the laser power, v is the scanning speed, h is the scanning distance, and l t is the powder layer thickness.

[0070] The present invention may alternatively utilize an LMD method which is a layer-by-layer additive manufacturing technique. The LMD laser beam forms a melt pool on a metal substrate into which powder is fed by a carrier gas. The powder melts under the protection of the protective gas to form a deposit that is fused to the substrate. The desired geometry is built layer by layer in this way by a gantry system or a robotic arm that controls both the laser and the powder delivery nozzle.

[0071] The laser power is preferably 600 W to 1000 W. Preferably, the laser power is set to 600 W to 800 W.

[0072] The scanning speed is preferably 5 mm / sec to 40 mm / sec, and more preferably 15 mm / sec to 30 mm / sec.

[0073] The powder feed rate is preferably 5 g / min to 40 g / min, and more preferably 10 g / min to 25 g / min.

[0074] The layer thickness is preferably 0.2 mm to 1.5 mm and more preferably 0.4 mm to 1 mm.

[0075] The laser spot diameter is preferably 0.5 mm to 4 mm, and more preferably 1 mm to 3 mm. The laser beam shape may be Gaussian or flat top.

[0076] The carrier gas may be selected from nitrogen, helium, or argon, or a mixture thereof, and its flow rate is preferably 2 l / min to 8 l / min, and more preferably 4 l / min to 6 l / min.

[0077] The shielding gas may be selected from nitrogen, helium, or argon, or a mixture thereof, and its flow rate is preferably 8 l / min to 20 l / min, and more preferably 10 l / min to 15 l / min.

[0078] Depending on the additive manufacturing method used, the microstructure of the component can be different, but in each case it is a solidified microstructure. Such a solidified microstructure is determined by the temperature gradient (G) and the growth rate (R) because they involve high cooling rates typical of additive manufacturing methods [Kou, S. (2020). Welding metallurgy. John Wiley & Sons].

[0079] As detailed by Agarwal, G. (2019) and shown in Figure 2.12 of "Study of Solidification Cracking during Laser Welding in Advanced High Strength Steels. A Combined Experimental and Numerical Approach. Delft University of Technology", the values ​​of G and R and the cooling rate define different regions of the presence of solidified structures that can be cellular, cellular-dendritic, or columnar-dendritic.

[0080] In the present invention, when the LPBF method is used, solidification cellular unit cells with an equivalent diameter below 2 μm are observed.

[0081] In contrast, when the LMD method was used, a combination of solidification cells was observed: cellular-dendrite cells with an equivalent diameter below 10 μm and columnar-dendrite cells with an equivalent diameter between 10 μm and 20 μm.

[0082] Example

[0083] The following examples and tests presented hereinafter are non-limiting in nature and must be considered for illustrative purposes only. They will illustrate the advantageous features of the present invention, the importance of the parameters selected by the inventors after extensive experimentation, and further determine the properties that can be achieved by the metal powder according to the present invention.

[0084] Different metal compositions as described in Table 1 were first obtained by mixing and melting ferroalloys and pure elements.

[0085] Table 1

[0086] powder Mn (wt%) Al (wt%) C (weight %) Si (wt%) Fe <![CDATA[1 ＊ ]]> 28 7 0.8 0.05 margin <![CDATA[2 ＊ ]]> 28 7 0.8 0.32 margin 3 28 7 0.8 <![CDATA[ 1.0 ]]> margin 4 28 10 0.8 <![CDATA[ 1.0 ]]> margin 5 26 7 0.8 <![CDATA[ 1.4 ]]> margin

[0087] *According to the invention

[0088] P, S and N were kept below 0.013 wt%, 0.015 wt% and 0.1 wt%, respectively.

[0089] These metal compositions were heated up to 1800°C (i.e. 200°C to 350°C above the liquidus temperature) and then gas atomized with nitrogen under the following process conditions:

[0090] - Gas pressure: 20 bar

[0091] -Nozzle diameter: 2.5mm to 3mm.

[0092] The powders were then sieved and classified into fractions F1 to F3. They were evaluated for flowability, sphericity and roundness and were found to be satisfactory for additive manufacturing purposes. The density of the powders was about 6.9 g / cm 3 .

[0093] Fraction F2

[0094] For powders 1, 2 and 3, the microstructure of the F2 fraction was determined by XRD and is summarized in Table 2.

[0095] Table 2

[0096]

[0097] *According to the invention

[0098] Fraction F2 of such powder was then used to print a series of 22 1 cm 3 The cube:

[0099] -Laser power 150W to 200W,

[0100] - Scanning speed 300mm / sec to 1100mm / sec,

[0101] - Scanning pitch 70μm to 110μm,

[0102] - layer thickness 20 μm to 40 μm,

[0103] -Linear energy density (LED) 180J / m to 500J / m,

[0104] -Volume Energy Density (VED) 100J / mm3 Up to 330J / mm 3 .

[0105] The cubes were then evaluated and the corresponding results are summarized in Table 3 below.

[0106] Table 3

[0107]

[0108] *According to the invention

[0109] The printed cubes of Tests 3 to 5 exhibited thermal cracking. Such cracks were present in the solidification front of all cubes.

[0110] Printing cubes according to Trials 1 and 2 of the present invention produced cubes without internal cracks.

[0111] For powders 1, 2 and 3, the microstructure of the printed cubes was determined by XRD and is summarized in Table 4.

[0112] Table 4

[0113]

[0114] *According to the invention

[0115] The microstructure of the printed cubes using Powder 1 was evaluated and showed solidification cellular unit cells with an equivalent diameter below 2 μm. Such unit cell size was determined by the line intercept method of the ASTM E112-10 standard using lateral SEM micrographs.

[0116] The printed cubes corresponding to powder 1 were then subjected to different heating treatments and finally water quenched to evaluate the evolution of their hardness, measured according to standard ASTM E92-17 for heating temperatures and times, as shown in Table 5.

[0117] The evolution of the mechanical properties after evaluation using ASTM standard E8 / E8M-16a for small dimensions is also summarized in Table 5.

[0118] Table 5

[0119] As printed 400℃ 4 hours 500℃ 4 hours 400℃20 hours 500℃20 hours Hardness (HV) 298 297 303 307 306 YS(MPa) 579 559 562 ne ne UTS(MPa) 745 731 741 ne ne Total elongation (%) 40 40 41 ne ne

[0120] *According to the present invention, ne: not evaluated

[0121] It can be observed that, regardless of the heat treatment applied to the printed cube, its hardness is very stable over time. This is also the case for the tensile properties, which are very stable over temperature and time when subjected to temperatures up to 500°C for 4 hours.

[0122] Similar results showing good stability were obtained for the toughness measured by the Charpy test between 0° C. and −180° C. following the ASTM E23-07a standard for the printed cubes corresponding to Powder 1 as summarized in Table 6:

[0123] Table 6

[0124]

[0125] Fraction F3

[0126] For Powder 1, the microstructure of the F3 fraction was determined by XRD and is summarized in Table 7.

[0127] Table 7

[0128]

[0129] *According to the invention

[0130] Fraction F3 of such powder 1 was then used to print a series of 1.5 cm 3 The prism:

[0131] -Laser power 600W to 800W,

[0132] - Scanning speed 15mm / sec to 30mm / sec,

[0133] - powder feed rate 10 g / min to 25 g / min,

[0134] - layer thickness 0.4mm to 1mm,

[0135] -Laser spot: 1mm to 3mm,

[0136] -Laser beam shape: Gaussian or flat-top,

[0137] -Carrier gas: N2, He or Ar,

[0138] -Carrier gas flow rate: 4L / min to 6L / min,

[0139] -Protective gas: N2, He or Ar,

[0140] - Shielding gas flow rate: 10L / min to 15L / min.

[0141] The microstructure of the printed cubes was determined by XRD and is summarized in Table 8.

[0142] Table 8

[0143]

[0144] *According to the invention

[0145] The microstructure of the printed cubes using powder 1 was evaluated and showed a combination of solidification unit cells: cellular-dendrite unit cells with an equivalent diameter below 10 μm and columnar dendrite unit cells with an equivalent diameter of 10 to 20 μm. Such unit cell sizes were determined by the line intercept method of the ASTM E112-10 standard using lateral SEM micrographs.

[0146] The printed cubes corresponding to powder 1 were then subjected to different isothermal heat treatments and finally water quenched to evaluate the evolution of their hardness, measured according to standard ASTM E92-17 for temperature and time, as shown in Table 9. The evolution of their mechanical properties, as evaluated using ASTM standard E8 / E8M-16a small dimensions, is also summarized in Table 9:

[0147] Table 9

[0148] As printed 500℃ 4 hours 700℃ 4 hours 500℃16 hours 700℃16 hours Hardness (HV) 250 269 238 269 232 YS(MPa) 697 794 ne 817 ne UTS(MPa) 977 1037 ne 1056 ne Total elongation (%) 33 42 ne 38 ne

[0149] ne: not evaluated

[0150] There is some improvement in the mechanical properties with temperature, which are very stable up to 500°C for various times.

Claims

1. A metal powder for additive manufacturing, wherein the composition of the metal powder for additive manufacturing comprises the following elements, expressed in terms of content by weight: 15%≤Mn≤35% 6％≤Al≤15％ 0.5％≤C≤1.8％ 0≤Si≤0.5% P≤0.013％ S≤0.015％ N≤0.100％ and optionally comprising: - Ni≤8.5 wt.-% and / or Cr≤2.5 wt.-% and / or B≤0.1 wt.-% and / or one or more elements selected from Ta, Zr, Nb, V, Ti, Mo and W in a cumulative amount of up to 2.0 wt.-%, The balance is iron and unavoidable impurities resulting from processing, and the average particle size of the powder is between 1 μm and 150 μm.

2. The metal powder of claim 1 , wherein the powder particles have an austenitic microstructure comprising, optionally, up to 1 wt. % kappa carbide (Fe,Mn)3AlCx, up to 1 wt. % AlN and up to 20 wt. % ferrite.

3. The metal powder according to any one of claims 1 or 2, wherein the density of the metal powder is lower than 7.0 g / cm 3 .

4. The metal powder according to any one of claims 1 to 3, wherein the average particle size is between 1 μm and 20 μm.

5. The metal powder according to any one of claims 1 to 3, wherein the average particle size is between 20 μm and 63 μm.

6. The metal powder according to any one of claims 1 to 3, wherein the average particle size is between 60 μm and 150 μm.

7. A method for producing a metal powder for additive manufacturing, comprising: -a) melting the elements and / or the metal alloy at a temperature of at least 100° C. above the liquidus temperature to obtain a molten composition according to claim 1, - b) atomizing the molten composition through a nozzle with a gas pressurized to 10 to 30 bar.

8. A method for producing a printed component by additive manufacturing, wherein the powder according to any one of claims 1 to 6 or the powder obtained according to claim 7 is printed by laser powder bed fusion.

9. The method according to claim 8, comprising a first step of forming a powder layer with a thickness lower than 100 μm, and a second step of forming a shaped layer by melting at least part of the powder layer with a focused laser beam in an atmosphere consisting essentially of an inert gas.

10. The method according to any one of claims 8 or 9, wherein: -Limit laser power to a maximum of 500W, - Scanning speed is 300mm / sec to 2000mm / sec, - Linear energy density of 190 J / m to 500 J / m, - Scanning pitch is 50μm to 120μm, -Volume energy density is 100J / mm 3 Up to 330J / mm 3 .

11. A printed component obtained by the method according to any one of claims 8 to 10, having a cellular solidified structure with an equivalent diameter below 2 μm.

12. A method for producing a printed component by additive manufacturing, wherein a powder according to any one of claims 1 to 7 or a powder obtained according to claim 8 is printed by laser metal deposition.

13. The method according to claim 12, comprising a first step of forming a powder layer with a thickness lower than 100 μm, and a second step of forming a shaped layer by melting at least part of the powder layer with a focused laser beam in an atmosphere consisting essentially of an inert gas.

14. The method according to any one of claims 12 or 13, wherein: -Laser power is 600W to 1000W, - Scanning speed is 5mm / sec to 40mm / sec, - powder feed rate of 5 g / min to 40 g / min, - layer thickness from 0.2mm to 1.5mm, - Laser spot diameter is 0.5mm to 4mm, - The laser beam shape is Gaussian or flat-top, -Carrier gas flow rate is 2L / min to 8L / min, - The protective gas flow rate is 8L / min to 20L / min.

15. A printed part obtained by the method according to any one of claims 12 to 14, having a structure consisting of a cellular-dendritic solidification structure with an equivalent diameter below 10 μm and a columnar-dendritic solidification structure with an equivalent diameter of 10 μm to 20 μm.

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