Metal powder for additive manufacturing

Abstract

The invention relates to a metal powder for additive manufacturing having a composition comprising the following elements, given in terms of weight content:\n0.01% ≤ C ≤ 0.2%, 4.6% ≤ Ti ≤ 10%, (0.45 xTi) – 0.22% ≤ B ≤ (0.45 xTi) + 0.70%, S ≤ 0.03%, P ≤ 0.04%, N ≤ 0.05%, O ≤ 0.05%\nand optionally comprising:\nSi ≤ 1.5%, Mn ≤ 3%, Al ≤ 1.5%, Ni ≤ 1%, Mo ≤ 1%, Cr ≤ 3%, Cu ≤ 1%, Nb ≤ 0.1%, V ≤ 0.5%\nand includes eutectic precipitates of TiB2 and Fe2B, the remainder consisting of iron and process impurities, a volume fraction of TiB2 equal to or greater than 10% and an average bulk density of the powder equal to or less than 7.50 g / cm3. The invention also relates to a method of producing it by atomization.

Description

Metal powder for additive manufacturing The present invention relates to a metal powder for the production of steel parts and in particular for use in additive manufacturing. The present invention also relates to a method for producing metal powder. FeTiB2 steels are of great interest due to their high and excellent E-modulus, low density, and high tensile strength. However, it is difficult to produce these steel sheets with desired yield from conventional routes, which limits their applications. Therefore, the aim of the present invention is to overcome the aforementioned problems by providing FeTiB2 powders that can be efficiently used to produce parts via additive manufacturing methods while maintaining the desired application properties. To this end, the first object of the present invention is to provide a metal powder having a composition consisting of the following elements, expressed on a weight basis: 0.01% ≤ C ≤ 0.2% 4.6% ≤ Ti ≤ 10% (0.45 xTi) – 0.22% ≤ B ≤ (0.45 xTi) + 0.70% S ≤ 0.03% P ≤ 0.04% N ≤ 0.05% O ≤ 0.05% And optionally includes: Si ≤ 1.5% Mn ≤ 3% Al ≤ 1.5% Ni ≤ 1% Mo ≤ 1% Cr ≤ 3% Cu ≤ 1% Nb ≤ 0.1% V ≤ 0.5% and containing precipitates of TiB2 and Fe2B, the remainder consisting of iron and unavoidable impurities resulting from production, with a volume percentage of TiB2 equal to or greater than 10% and an average bulk density of the powder between 7.50 g / cm3 or less. The metal powder according to the invention may also have the optional features listed in each of claims 2 to 4, taken individually or in combination. A second object of the invention includes a method for producing a metal powder for additive manufacturing, comprising the following: - Melting of metal elements and / or alloys at a temperature of at least 50°C above the liquidus temperature in order to obtain a molten composition containing the following by weight: 0.01% ≤ C ≤ 0.2%, 4.6% ≤ Ti ≤ 10%, (0.45 xTi) – 0.22% ≤ B ≤ (0.45 xTi) + 0.70%, S ≤ 0.03%, P ≤ 0.04%, N ≤ 0.05%, O ≤ 0.05% And optionally includes: Si ≤ 1.5%, Mn ≤ 3%, Al ≤ 1.5%, Ni ≤ 1%, Mo ≤ 1%, Cr ≤ 3%, Cu ≤ 1%, Nb ≤ 0.1%, V ≤ 0.5%, The rest consists of iron and impurities from production and - Atomizing the molten mixture through a nozzle with pressurized gas. The method according to the invention may also include the optional features listed in each of claims 6 to 8, taken individually or in combination. The invention will be better understood by reading the following description, which is provided for illustrative purposes only and is in no way limiting. The powder according to the invention has a specific composition that is balanced to achieve desired properties when used to produce parts. Carbon content is a limitation for weldability because cold cracking resistance and toughness in the HAZ (heat affected zone) are reduced if the carbon content is higher than 0.20%. Resistance weldability is significantly improved when the carbon content is equal to or less than 0.050 wt%. Due to the titanium content of the steel, the carbon content is preferably limited to avoid initial precipitation of TiC and / or Ti(C,N) in the liquid metal. The maximum carbon content should preferably be limited to 0.1% and even more desirably to 0.080% so that TiC and / or Ti(C,N) precipitates are produced mainly during solidification or in the solid phase. Silicon is an optional element but when added due to solid solution hardening, it effectively helps to increase tensile strength. However, excessive addition of silicon causes the formation of sticky oxides which are difficult to remove. To maintain desirable surface properties, the silicon content should not exceed 1.5% by weight. The element manganese is optional. However, at levels equal to or greater than 0.06%, manganese increases hardenability and contributes to solution-solid hardening, thereby increasing tensile strength. It combines with any sulfur present and thus reduces the risk of hot cracking. However, at manganese contents above 3% by weight, there is a greater risk of harmful manganese precipitates forming during solidification. The aluminum element is optional. However, at a level equal to or higher than 0.005%, aluminum is a very effective element for deoxidizing steel. However, above a content of 1.5% by weight, excessive initial precipitation of aluminum occurs, causing processing difficulties. At levels above 0.030%, sulfur usually precipitates in excessively large amounts in the form of manganese sulfides, which are harmful. Phosphorus is a known grain boundary segregating element. Its content should not exceed 0.040% to maintain sufficient hot malleability and thus avoid cracking. Optionally, nickel, copper or molybdenum can also be added, these elements increase the tensile strength of the steel. Due to economic considerations, these additions are limited to 1% by weight. Optionally, chromium can be added to increase tensile strength. This element allows large amounts of carbides to precipitate. However, its content is limited to 3% by weight to produce a more economical steel. A chromium content of 0.080% or less is best. This is because too much chromium leads to the precipitation of more carbides. Also, niobium and vanadium can be added in amounts equal to or less than 0.1% and equal to or less than 0.5%, respectively, to provide additional hardness in the form of fine precipitated carbonitrides. Titanium and boron play an important role in the powder according to the invention. Titanium is present in an amount between 4.6% and 10%. When the titanium content is less than 4.6% by weight, TiB2 precipitation does not occur sufficiently because the volume fraction of TiB2 precipitate is less than 10%, thus preventing a significant change in the elastic modulus, which may remain below 240 GPa. When the titanium content is more than 10% by weight, coarse initial precipitation of TiB2 occurs in the liquid metal, causing problems in the products. In addition, the liquefaction temperature increases and a superheat of at least 50 °C cannot be achieved with the standard atomization process. The eutectic precipitate of FeTiB2 occurs following solidification. The eutectic nature of the precipitate gives the formed microstructure a particular homogeneity and fineness that is beneficial for the mechanical properties. When the amount of eutectic precipitates of FeTiB2 exceeds 10% by volume of TiB2 precipitates, the modulus may exceed about 240 GPa, thus allowing the design of much lighter structures. This value can be increased by up to 15% by volume in the case of steels containing alloying elements such as chromium or molybdenum, to exceed about 250 GPa. This is because when these elements are present, the maximum amount of TiB2 that can be obtained in the eutectic precipitate increases. As previously explained, titanium must be present in sufficient quantities to cause endogenous TiB2 formation. In the context of the present invention, "free Ti" refers to the content of unbound titanium in the form of precipitates. The free titanium content can be evaluated according to the following formula, where B represents the boron content in the powder: Free Ti = Ti - 2.215 x B According to the invention, the content of titanium and boron is in such a way as to satisfy the following relationship: −0.22 ≤ B − (0.45×Ti) ≤ 0.70 In the above range, the free titanium content is less than 0.5%. It is preferable to determine the free titanium at an index between 0.30 and 0.40%. The precipitation occurs in the form of two successive eutectics: first, FeTiB2 and then Fe2B, this second endogenous precipitation of Fe2B occurring to a greater or lesser extent depending on the boron content of the alloy. The amount of precipitation in the form of Fe2B can be up to 8% by volume. This second precipitation also occurs according to a eutectic scheme which allows a uniform fine distribution to be achieved, thus ensuring the desired uniformity of the mechanical properties. The Fe2B precipitate complements the TiB2 precipitate, the maximum amount of which is associated with the eutectic. Fe2B plays a similar role to TiB2, increasing the elastic modulus and reducing the density. This allows for precise adjustment of the mechanical properties by varying the complement of the Fe2B precipitate to the TiB2 precipitate. This property can be used in particular to achieve an elastic modulus greater than 250 GPa in steel. When the steel contains an Fe2B content equal to or greater than 4% by volume, the elastic modulus increases by more than 5 GPa. When the Fe2B content is greater than 7.5% by weight, the elastic modulus increases by more than 10 GPa. The bulk density of the metal powder according to the invention is unexpectedly favorable. In fact, the bulk density of the metal powder according to the invention is equivalent to a maximum index of 7.50 g / cm3. Thanks to this low powder density, a part made from such a metal powder, manufactured by additive manufacturing, exhibits a reduced density together with an improved modulus of elasticity. The powder can be obtained, for example, by mixing and melting pure elements and / or ferrous alloys as raw materials. Alternatively, the powder can be obtained by melting pre-alloyed compounds. Pure elements are usually preferred to avoid excessive impurities from ferrous alloys as said impurities may facilitate crystallization. However, in the case of the present invention, it has been found that impurities from ferrous alloys are not detrimental to the achievement of the invention. A person skilled in the art is knowledgeable about mixing various iron alloys and pure elements to achieve the target composition. After the composition has been obtained by mixing pure elements and / or alloys of metals in appropriate proportions, the composition is heated to a temperature of at least 50°C above the liquidus temperature and maintained at this temperature until all the raw materials are melted and the melt is homogeneous. Thanks to this superheating, the viscosity of the molten composition is reduced, which helps to obtain a powder with the desired properties. In other words, with the increase in surface tension with temperature, it is preferable not to heat the composition to a temperature higher than 450°C above its liquidus temperature. Preferably, the composition is heated to a temperature of at least 100° C. above its liquefaction temperature. More preferably, the composition is heated to a temperature of 300 to 400° C. above its liquefaction temperature. The molten mixture is then atomized into metal droplets by forcing a stream of molten metal through an orifice, a nozzle, at moderate pressure and impinging on it with a gas jet (gas atomization) or water jet (water atomization). In the case of gas atomization, the gas is introduced into the stream just before the metal stream exits the nozzle and, with the expansion of the trapped gas (due to heating), it creates turbulence and is discharged into a large volume assembly, the atomization tower. This tower is filled with gas to further turbulentize the liquid metal jet. The metal droplets are cooled as they fall through the atomization tower. Gas atomization is preferred because it produces powder particles with a high degree of roundness and a low level of secondary particles. The atomizing gas is argon or nitrogen. Both of these gases increase the viscosity of the melt more slowly than other gases, such as helium, resulting in smaller particle sizes. They also control the purity of the chemical composition, preventing undesirable impurities, and contributing to the desired powder morphology. Compared to nitrogen, finer particles can be achieved with argon because the molar mass of nitrogen is 14.01 g / mole compared to 39.95 g / mole for argon. On the other hand, the specific heat capacity of nitrogen is 1.04 J / (g K) compared to 0.52 for argon. Thus, nitrogen increases the cooling rate of the particles. Gas pressure is important because it has a direct impact on the particle size distribution and microstructure of the metal powder. In particular, the higher the pressure, the higher the cooling rate. As a result, the gas pressure is considered to be between 10 and 30 bar to optimize the particle size distribution and provide favorable conditions for the formation of micro / nanocrystalline phases. Preferably, the gas pressure is considered to be between 14 and 18 bar to allow the formation of particles whose size is most compatible with additive manufacturing methods. The nozzle diameter has a direct effect on the flow rate of the molten metal and thus on the particle size distribution and cooling rate. The maximum nozzle diameter is usually limited to 4 mm to limit the increase in the average particle size and reduce the cooling rate. The nozzle diameter is preferably between 2 and 3 mm to more accurately control the particle size distribution and provide favorable conditions for the formation of a specific microstructure. The gas to metal ratio, expressed as the ratio between the gas flow rate (in kg / h) and the metal flow rate (in kg / h), is preferably maintained between 1.5 and 7, more preferably between 3 and 4. This helps to regulate the cooling rate and thus the formation of a more specific microstructure. According to one embodiment of the invention, if the humidity is increased, the resulting metal powder is dried by atomization to improve its flowability. The drying is preferably carried out at a temperature of 100 degrees Celsius in a vacuum chamber. The metal powder resulting from atomization can either be used as is or it can be dissolved to retain particles whose size is more optimally compatible with the additive manufacturing method and be used later. For example, in the case of additive manufacturing via powder bed compounding, the range is 20-63µm. In the case of additive manufacturing via metal laser deposition or direct metal deposition, the range is 45-150µm. Parts made of metal powder according to the invention can be produced by additive manufacturing methods such as liquid-powder bed compounding (LPBF), direct metal sintering (DMLS), electron beam melting (EBM), selective metal sintering (SHS), selective laser sintering (SLS), laser metal deposition (LMD), direct metal deposition (DMD), direct metal laser melting (DMLM), direct metal printing (DMP), laser coating (LC), binder jetting (BJ). Coatings made of metal powder according to the invention can also be obtained by manufacturing methods such as cold spray, thermal spray and high-velocity oxy-fuel thermal spraying. Examples The following examples and experiments are non-limiting in nature and should be considered for illustrative purposes only. They illustrate the useful features of the invention, the importance of the components selected by the inventors after extensive testing, and further demonstrate the properties achievable by the metal powder according to the invention. The metallic compositions according to Table 1 were first obtained either by mixing and melting ferrous alloys and pure elements in appropriate proportions or by melting pre-alloyed compositions. The compositions of the added elements, in terms of weight percent, can be seen in Table 1. Table 1 – Melting composition Sample C Ti B Mn Al Si SPV Ni Cr Cu C76 0.053 5.70 2.20 <0.001 0.316 0.571 0.007 0.002 0.213 <0.001 <0.001 <0.001 C75 0.052 5.69 2.19 <0.001 <0.001 <0.001 <0.001 <0.001 0.213 <0.001 <0.001 <0.001 C27 0.019 4.81 1.99 0.189 0.046 0.068 0.001 0.0090 0 0.045 0.033 0.05 C28 0.019 4.81 1.99 0.189 0.046 0.068 0.001 0.0090 0 0.045 0.033 0.05 Nitrogen and oxygen levels were less than 0.001% for all samples. These metal compounds were heated and then gas atomized with argon or nitrogen under the process conditions shown in Table 2. Table 2 – Atomization components For all tests, the common input components for the BluePower AU3000 atomizer are as follows: Start Δ P 60 mbar End Δ P 140 mbar Δ P time 1.5 min Atomizing gas pressure 24 bar Gas start delay time 1-2 s Ball / rod material Al 2 O 3 / Al 2 O 3 Rod outlet diameter 3.0 mm Boron Nitride Boron Nitride Volatile group Heat T (°C) Hold t (min) Atom T (ºC) Gas atom Gas T (ºC) Atom t, mm:ss F1, % F2, % F3, % C76 250 45 1544 Ar 200 0:59 15.6 36.2 33.6 C75 250 45 1546 N 2 200 1:20 18.2 30.7 28.7 C27 260 45 1554 N 2 200 1:05 11.9 19.3 33.6 C28 100 44 1396 N 2 200 1:03 10.5 19.7 32.1 The resulting metal powders were then dried at 100°C under vacuum for 0.5 to 1 day and sieved to separate into three fractions F1 to F3 according to their size. Fraction F1 corresponded to sizes between 1 and 19 µm. Fraction F2 corresponded to sizes between 20 and 63 µm and fraction F3 corresponded to sizes above 63 µm. The elemental composition of the powders was analyzed, in terms of weight percent, and the major elements are listed in Table 3. All other element contents were within the range of the invention. Table 3 – Powder composition Sample Ti B Free Ti TiB 2 vol.%) ) Fe 2 B C76 3.22 1.52 0 7.8 Yes C75 3.63 1.70 0 8.8 Yes C27 4.76 1.99 0.35 10.6 Yes C28 4.87 2.03 0.37 10.8 Yes The bulk density of the powders was determined and can be seen in Table 4. Table 4 – Bulk density F2 TiB 2 fraction (volume percent) Sample Δ T(°C) Atm Bulk density (g / cm 3 ) C76 250 Ar 7.64 7.8 C75 250 N 2 7.63 8.8 C27* 260 N 2 7.50 10.6 C28* 100 N 2 7.47 10.8 *: Examples in accordance with the invention, underlined indicators are outside the invention. Bulk density was measured using a commercial AccuPyc II 1340 densitometer. The device operates on the basis of gas densitometry using argon atoms. This method is more accurate than Archimedes' principle and the use of liquid systems for powder density, which is subject to wettability problems. The samples were first dried to remove moisture. Helium was used because of its small atomic diameter to penetrate into the tiny pores. The measurement method is based on injecting He at a given pressure into a reference chamber and then releasing the gas into a second chamber containing the powder. The pressure in this second chamber is measured. Marriott's law is then used to calculate the powder volume VÉ. V1 is the volume of the first reference chamber, V0 is the volume of the second chamber containing the powder sample. VÉ Powder volume P1 Gas pressure in the first reference chamber P2 is the gas pressure in the second chamber containing the powder sample. The sample weight was measured with a calibrated balance and the corresponding density was then calculated. Based on the examples, it is clear that the powder according to the invention shows a reduced density at a level of 7.50 g / cm3 or less compared to the reference samples in which the density is significantly higher. This result is unexpected because the indicators related to the volume percentages of TiB2 do not correspond to such a gap in density.

Claims

Claims 1. A metal powder with a composition comprising the following elements expressed by weight content: 0.01% ≤ C ≤ 0.2% 4.6 % ≤ Ti ≤ 10% (0.45 xTi) – 0.22% ≤ B ≤ (0.45 xTi) + 0.70% S ≤ 0.03% P ≤ 0.04% N ≤ 0.05% O ≤ 0.05% and optionally comprising: Si ≤ 1.5% Mn ≤ 3% Al ≤ 1.5% Ni ≤ 1% Mo ≤ 1% Cr ≤ 3% Cu ≤ 1% Nb ≤ 0.1% V ≤ 0.5% and containing precipitates of TiB2 and Fe2B, the remainder consisting of iron and unavoidable impurities resulting from production, wherein the volume fraction of TiB2 is equal to or greater than 10% and the average bulk density of the powder is between 7.50 g / cm3 or less.

2. The metal powder according to claim 1, wherein the volume fraction of Fe2B is at least 4%.

3. A metal powder according to any one of claims 1 or 2, wherein the free titanium content of the powder is between 0.30 and 0.40% by weight.

4. A method for producing a metal powder for additive manufacturing comprising the steps of: - melting the metal elements and / or alloys at a temperature of at least 50°C above the liquidus temperature to obtain a molten composition comprising the following by weight content: 0.01% ≤ C ≤ 0.2%, 4.6% ≤ Ti ≤ 10%, (0.45 xTi) – 0.22% ≤ B ≤ (0.45 xTi) + 0.70%, S ≤ 0.03%, P ≤ 0.04%, N ≤ 0.05%, O ≤ 0.05% and optionally including: Si ≤ 1.5%, Mn ≤ 3%, Al ≤ 1.5%, Ni ≤ 1%, Mo ≤ 1%, Cr ≤ 3%, Cu ≤ 1%, Nb ≤ 0.1%, V ≤ 0.5%, the remainder consisting of iron and impurities resulting from the production and - atomization of the molten mixture through a nozzle with pressurized gas.

5. A method according to claim 4, wherein the melting takes place at a temperature at least 100°C above the liquefaction temperature.

6. A method according to claim 4 or 5, wherein the melting is carried out at a maximum temperature of 400°C above the liquefaction temperature.

7. A method according to any one of claims 4 to 6, wherein the gas is pressurized between 10 and 30 bar.

8. A steel part produced by an additive manufacturing process using a metal powder according to any one of claims 1 to 3 or obtained by the method according to claims 4 to 7.