Metal Powders for Additive Manufacturing

Abstract

The present invention relates to a metal powder for additive manufacturing, which comprises the following elements, expressed as a weight content: 15%≦Mn≦35% 6%≦Al≦15% 0.5%≦C≦1.8% 1.6%≦Si≦3.5% P≦0.013% S≦0.015% N≦0.100% and optionally one or more elements selected from Ta, Zr, Nb, V, Ti, Mo and W in a cumulative amount of Ni≦8.5 wt.% and / or Cr≦2.5 wt.% and / or B≦0.1 wt.% and / or up to 2.0 wt.%, the balance being iron and unavoidable impurities resulting from smelting. The present invention also deals with methods for producing such powders and for producing printed parts therefrom.

Description

[Technical Field]

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

[0002] Environmental regulations are forcing automakers to continually reduce vehicle CO2 emissions, which is why they are exploring every possible way to reduce the weight of their vehicles.

[0003] This can be achieved, inter alia, by reducing the density of the steel used to manufacture the parts and by alloying the steel with other metals that are lighter than iron.

[0004] Steels containing high levels of manganese aluminum and carbon, often referred to as triplex steels, can exhibit density levels below 7.4 g / cm. Their solidification structure likely exhibits an austenitic structure with kappa carbides (Fe,Mn)3AlCx and ferrite.

[0005] However, their high aluminum and carbon content makes them difficult to manufacture by conventional casting methods. They also exhibit some macrosegregation of manganese, carbon, and / or aluminum, which can form brittle phases that can cause banding and cracking when stacked. Summary of the Invention [Problem to be solved by the invention]

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to remedy the shortcomings of the prior art by providing a new method for obtaining low density parts without manufacturability problems. [Means for solving the problem]

[0007] To this end, a first object of the present invention is to provide a metal powder for additive manufacturing, which comprises, expressed as a content by weight of the following elements: 15%≦Mn≦35% 6%≦Al≦15% 0.5%≦C≦1.8% 1.6%≦Si≦3.5% P≦0.013% S≦0.015% N≦0.100% and optionally one or more elements selected from Ta, Zr, Nb, V, Ti, Mo and W in a cumulative amount of Ni≦8.5 wt.% and / or Cr≦2.5 wt.% and / or B≦0.1 wt.% and / or maximum 2.0 wt.%, the balance being iron and unavoidable impurities resulting from smelting.

[0008] The metal powder according to the invention may also have any of the characteristics listed below, individually or in combination: The powder particles have an austenite microstructure containing up to 1 wt.% kappa carbide (Fe,Mn)3AlCx and up to 20 wt.% ferrite and up to 1 wt.% AlN. The powder is 7.0 g / cm 3 having a density of less than The average particle size is 1 to 150 μm. The average particle size is 1 to 20 μm. The average particle size is 20 to 63 μm. The average particle size is 60 to 150 μm.

[0009] A second object of the present invention is a method for producing metal powder for additive manufacturing, comprising: a) melting the element and / or metal alloy at a temperature at least 100°C above the liquidus temperature to obtain the molten composition according to claim 1; b) atomizing the molten composition through a nozzle with a diameter of up to 4 mm using a gas pressurized to 10-30 bar; The method comprises:

[0010] A third object of the invention consists of a method for producing printed parts by additive manufacturing, in which a powder according to the invention is printed by laser powder bed fusion.

[0011] The printing method according to the invention may also have any of the features listed below, considered individually or in combination: The method includes a first step of forming a powder layer having a thickness of less than 100 μm, and a second step of forming a shaped layer by using a focused laser beam to melt at least a portion of the powder layer in an atmosphere consisting essentially of an inert gas. The method is configured with the following parameters: - Laser power is limited to a maximum of 500W. - The scanning speed is 300 to 2000 mm / s. - Linear energy density is 190-500J / m, - The hatch spacing is 50 to 120 μm, - Volumetric energy density is 100~330J / mm 3 is.

[0012] A fourth object of the invention consists of a printed part obtained according to the invention, having a cellular solidification structure with an equivalent diameter of less than 2 μm. DETAILED DESCRIPTION OF THE INVENTION

[0013] The invention will be better understood from reading the following description, which is provided purely for illustrative purposes and is not intended to be limiting in any way.

[0014] Manganese is present in the composition according to the invention at a content of 15 to 35 wt. %. Manganese is an essential alloying element for such grades, primarily due to the fact that alloying with such large amounts of manganese and carbon stabilizes the austenite in the final component to room temperature, allowing it to withstand large amounts of aluminum without subsequently being destabilized and transformed into excessively large amounts of ferrite or martensite. To allow the alloy to have excellent ductility, the manganese content must be 15 wt. % or more. However, if the manganese content exceeds 35 wt. %, the precipitation of the β-Mn phase reduces the ductility of the alloy. Therefore, the manganese content needs to be controlled to be 15 wt. % or more but not more than 35 wt. In a preferred embodiment, it is 15.5 wt. % or more, or even less than 16.0 wt. The amount is more preferably 25 to 31 wt. %, or even better, 26 to 30 wt. %.

[0015] Aluminum is present in the composition according to the present invention at a content of 6 to 15 wt. The addition of aluminum to high-manganese austenitic steel effectively reduces the density of the alloy. Furthermore, it significantly increases the stacking fault energy (SFE) of the austenite in the final component, which then leads to changes in the strain-hardening behavior of the alloy. Aluminum is also one of the main elements in the nano-sized kappa carbide (Fe,Mn)3AlCx, and therefore, its addition significantly promotes the formation of such carbides. The aluminum concentration in the present alloy should be adjusted to ensure austenite stability and possible precipitation of kappa carbides, on the one hand, and to control the formation of ferrite, on the other hand. Furthermore, aluminum amounts less than 6 wt. % can reduce the density of the final component to 7.0 g / cm. 3 It has been observed that this results in a material density exceeding 1000 MPa. Therefore, the aluminum content needs to be controlled to be greater than or equal to 6% by weight but less than or equal to 15% by weight to avoid the removal of the austenite phase. In a preferred embodiment, the aluminum content is between 6 and 12% by weight, or even better between 6 and 10% by weight.

[0016] The carbon content should be between 0.5 and 1.8 wt.%. Carbon plays an important role in shaping the microstructure of the final component. Its primary role is to stabilize austenite, the predominant phase in the microstructure of the steel component, and to provide strengthening. A carbon content below 0.5 wt.% reduces the proportion of austenite, resulting in a decrease in both the ductility and strength of the alloy. However, because carbon is the main component of the kappa carbide (Fe,Mn)3AlCx, a carbon content above 1.8 wt.% promotes the coarse precipitation of such carbides at grain boundaries, thereby reducing the ductility of the alloy.

[0017] The carbon content is preferably 0.6 to 1.3% by weight, and more preferably 0.8 to 1.2% by weight in order to obtain sufficient strength.

[0018] Silicon is present in the composition according to the invention in a content of 1.6-3.5 wt%. It has been observed that the addition of at least 1.6 wt% silicon suppresses hot cracking during the production of final parts by additive manufacturing. However, additions of more than 3.5 wt% silicon result in cold cracking defects during the production of final parts by additive manufacturing. Preferred ranges are 1.7-3.5 wt%, 1.8-3.5 wt%, 1.7-2.5 wt%, or even better 1.8-2.2 wt%.

[0019] Nickel may optionally be present in a content of up to 8.5 wt. %. Nickel can be used as a diffusion barrier against hydrogen. Amounts of nickel greater than 8.5 wt. % are undesirable because they promote the formation of cementite, which impairs the (Fe,Mn)3AlCx carbides. Nickel can also be used as an effective alloying element because it stabilizes austenite and promotes the formation of ordered compounds in ferrite, such as the B2 component, providing additional strengthening. However, for cost reasons, among others, it is desirable to limit nickel additions to a maximum content of 6.0 wt. % or 4 wt. % or less, preferably 0.1-2.0 wt. % or 0.1-1.0 wt. However, if nickel is not added, the composition may contain up to 0.1 wt. % nickel as an impurity.

[0020] Chromium may optionally be present in a content of up to 2.5 wt.% to increase the strength of the steel by solid solution hardening. Chromium also improves the high-temperature corrosion resistance of the steel according to the invention. However, since chromium reduces the stacking fault energy and the stability of austenite, its content should not exceed 2.5 wt.%, preferably 0.1% to 2.0 wt.% or 0.1 to 1.0 wt.%. However, if no chromium is added, the composition may contain up to 0.1 wt.% Cr as an impurity.

[0021] Boron may optionally be present at a content of up to 0.1 wt. %. Boron has very low solid solubility and a strong tendency to segregate to grain boundaries, where it interacts strongly with lattice imperfections. Boron can therefore be used to limit the precipitation of intergranular kappa carbides.

[0022] Tantalum, zirconium, niobium, vanadium, titanium, molybdenum and tungsten are elements that may optionally be used to achieve hardening and strengthening, especially by precipitation of nitrides, carbonitrides or carbides, but if their cumulative amount is more than 2.0 wt.%, preferably more than 1.0 wt.%, or even better more than 0.5 wt.% or 0.3 wt.%, there is a risk that excessive precipitation will cause a decrease in toughness, which must be avoided.

[0023] The remainder consists of iron and unavoidable impurities resulting from refining. Phosphorus, sulfur, and nitrogen are the main impurities. They are not intentionally added. They may be present, in particular, in the ferroalloys and / or pure elements used as raw materials. Nitrogen can also be introduced during atomization. Their content is preferably controlled so as not to adversely change the microstructure and / or increase embrittlement. Therefore, their contents are limited to 0.013%, 0.015%, and 0.1% by weight, respectively. In a preferred embodiment, their contents are limited to 0.005%, 0.015%, and 0.01% by weight, respectively.

[0024] The powder microstructure is predominantly austenite and may optionally contain up to 1 wt.% kappa carbide (Fe,Mn)AlC, up to 1 wt.% AlN, and up to 20 wt.% ferrite. In a preferred embodiment, the optional ferrite content may be between 4 and 10 wt.%.

[0025] The powder can be obtained by first mixing and melting pure elements and / or ferroalloys as raw materials. The powder can also be obtained by using pre-alloy ingots of the required composition.

[0026] Ferroalloys refer to various alloys of iron with a high proportion of one or more other elements, such as manganese, silicon, aluminum, niobium, boron, chromium, molybdenum, etc. The main alloys are FeMn (usually containing 70-80% Mn by weight), FeAl (usually containing 40-60% Al by weight), FeSi (usually containing 15-90% Si by weight), FeNi (usually containing 70-95% Ni by weight), FeB (usually containing 17.5-20% B by weight), FeCr (usually containing 50-70% Cr by weight), FeMo (usually containing 60-75% Mo by weight), FeNb (usually containing 60-70% Nb by weight), FeV (usually containing 35-85% V by weight), and FeW (usually containing 70-80% W by weight).

[0027] The alloying elements may alternatively be added as pure elements (usually having a purity of more than 99% by weight), which may be, inter alia, carbon and iron, aluminum, manganese or pure metals such as nickel, zirconium, titanium, tantalum, molybdenum, tungsten, niobium, vanadium, chromium, etc.

[0028] Those skilled in the art know how to mix different ferroalloys and pure elements to arrive at a target composition.

[0029] Once the composition is obtained by mixing the pure elements and / or ferroalloys in the appropriate proportions, it is heated to a temperature at least 100°C above its liquidus temperature and maintained at this temperature to melt all the ingredients and homogenize the melt. This reduced viscosity of the molten composition, thanks to the heating, helps to obtain powders with high sphericity and a suitable particle size distribution without satellites. However, since surface tension increases with temperature, it is preferable not to heat the composition to a temperature more than 450°C above its liquidus temperature.

[0030] Preferably, the composition is heated to at least 200°C above its liquidus temperature to promote the formation of highly spherical particles. More preferably, the composition is heated to 250°C above its liquidus temperature.

[0031] In one embodiment of the present invention, the composition is heated to 1650-1800°C, which represents a good compromise between viscosity reduction and surface tension increase.

[0032] The molten composition is then atomized into fine metal droplets (gas atomization) by forcing the molten metal stream through an orifice, a nozzle, at moderate pressure and colliding it with a jet of gas. The gas is introduced into the metal stream just before it exits the nozzle and serves to create turbulence as the entrained gas expands (due to heating) and exits into a large collection volume, the atomization tower. The latter is filled with gas to promote further turbulence in the molten metal jet. The metal droplets cool as they fall into the atomization tower. Gas atomization is preferred because it produces powder particles with high roundness and a low number of satellites.

[0033] The atomizing gas is preferably argon or nitrogen or a mixture thereof. Both gases increase the melt viscosity more slowly than other gases, such as helium, thereby promoting the formation of smaller particle sizes. They also control the purity of the chemicals and play a role in the morphology of the powder. Since nitrogen has a molar weight of 14.01 g / mol compared to argon's 39.95 g / mol, argon typically produces finer particles than nitrogen. Meanwhile, nitrogen's specific heat capacity is 1.04 J / (gK) compared to argon's 0.52. Therefore, nitrogen increases the cooling rate of the particles. Whenever nitrogen is used as a component in the atomizing process, the combination of aluminum and nitrogen can form up to 1 wt. % AlN.

[0034] The gas pressure is important as it directly affects the particle size distribution. In particular, the higher the pressure, the higher the cooling rate. Preferably, the gas pressure is set between 10 and 30 bar, or even better between 24 and 30 bar, to promote the formation of particles whose size is most compatible with additive manufacturing techniques.

[0035] The nozzle diameter influences the molten metal flow rate and therefore the grain size distribution and the cooling rate. To limit the increase in the average grain size and the decrease in the cooling rate, the nozzle diameter is preferably limited to 4 mm.

[0036] The metal powder obtained by atomization can be used as is or can be sieved to retain particles of a size better suited to the additive manufacturing technique used subsequently. For example, for additive manufacturing by powder bed fusion, the range of 20 to 63 μm (called fraction F2) is preferred, and even better, the range of 20 to 40 μm. For additive manufacturing by laser metal deposition or direct metal deposition, the range of 60 to 150 μm (called F3) is preferred, and even better, the range of 40 to 125 μm. Fraction F1, covering particle sizes below 20 μm or even 10 μm, can be used, for example, for binder jetting.

[0037] Parts made with the metal powder according to the invention can be obtained by additive manufacturing techniques such as powder bed fusion bonding (LPBF), direct metal laser sintering (DMLS), electron beam melting (EBM), selective thermal 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 spraying (TS), high velocity oxygen fuel spraying (HVOF).

[0038] Specifically, the present invention utilizes the additive manufacturing (LPBF) process. Thin layers of metal powder are uniformly dispensed using a coating mechanism onto a substrate platform, typically metal, fixed to a moving index table on a vertical axis. This occurs in a chamber containing a tightly controlled atmosphere. As each layer is dispensed, each 2D slice of the part shape is fused together by selectively melting the powder. This is achieved using a high-power laser beam, typically an ytterbium fiber laser. The laser energy is intense enough to allow complete melting (welding) of particles in the form of tracks or strips. Essentially, once a track is completed, it is repeated with the next track, which is separated from the first by a hatch spacing. This process is repeated layer by layer until the part is complete. The overhanging shape is supported by unmelted powder from the previous layer. The main process parameters used in LPBF are typically layer thickness, hatch spacing, scanning speed, and laser power. After the process is complete, the remaining powder is sieved and reused.

[0039] A method for producing an additively manufactured part by LPBF includes a first step of forming a powder layer using a powder according to the present invention. Preferably, the powder layer is less than 100 μm thick. If the powder layer is thicker than 100 μm, the laser may not melt the powder for the entire layer thickness, which may result in porosity in the part. Preferably, the layer thickness is kept between 20 and 60 μm to optimize powder melting.

[0040] In a second step, the focused laser beam forms a shaped layer by melting at least a portion of the powder layer under process conditions detailed below.

[0041] With LPBF, each layer of the printed part is at least partially melted in an atmosphere consisting essentially of an inert gas.

[0042] The laser power is preferably limited to a maximum of 500 W. Preferably, the laser power is set above 80 W to facilitate melting at all layer thicknesses. In a preferred embodiment, the laser power is 175-300 W.

[0043] The scanning speed is preferably 300-2000 mm / s, more preferably 300-700 mm / s. Below 300 mm / s, excessive energy provided by the laser can result in keyhole porosity and / or spatter, which, if not properly retracted outside the powder bed, can accumulate in the powder layer and create voids in the printed part. Above 2000 mm / s, the energy provided by the laser to the powder may not be sufficient to melt the powder at all layer thicknesses.

[0044] The linear energy density (LED) is preferably between 190 and 500 J / m. LED is defined as the ratio of the laser power to the scanning speed expressed in m / s. Below 190 J / m, the LED may not be sufficient to properly print the part (due to keyholes). Above 400 J / m, the excessive energy provided by the laser can result in spatter, which deposits in the powder layer if not properly drawn outside the powder bed. Such deposits form voids in the printed part.

[0045] The hatch spacing is preferably 50 to 120 μm. If it is less than 50 μm, each point of the printed part may be remelted multiple times, which may lead to overheating. If it is more than 120 μm, unmelted powder may be trapped between two tracks. More preferably, the hatch spacing is 70 to 110 μm.

[0046] The volumetric energy density (VED) is preferably 100 to 330 J / mm 3 VED is P / (v·h·l t ) where P is the laser power, v is the scanning speed, h is the hatch spacing, and l t is the powder layer thickness.

[0047] Depending on the additive manufacturing method used, the microstructure of the part may vary, but in all cases it is the solidification microstructure, which is measured by the temperature gradient (G) and growth rate (R) associated with the high cooling rates typical of additive manufacturing [Kou, S. (2020). Welding metallurgy. John Wiley & Sons].

[0048] As detailed by Agarwal, G. (2019) and shown in Figure 2.12 in "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 existence of solidification structures which can be cellular, cellular-dendritic, or columnar-dendritic.

[0049] In the present invention, solidification-like cells with an equivalent diameter of less than 2 μm were observed when the LPBF method was used. [Example]

[0050] The following examples and tests presented below are non-limiting in nature and should be considered for illustrative purposes only. They demonstrate advantageous features of the present invention, the importance of parameters selected by the inventors after extensive experimentation, and further establish the properties that can be achieved with metal powders according to the present invention.

[0051] The different metal compositions as listed in Table 1 were first obtained by mixing and melting the ferroalloys and pure elements.

[0052] [Table 1]

[0053] P, S and N were maintained below 0.013 wt%, 0.015 wt% and 0.1 wt%, respectively.

[0054] These metal compositions were heated to 1800° C., ie, 200-350° C. above the liquidus temperature, and then gas atomized with nitrogen under the following process conditions:

[0055] Gas pressure: 20 bar Nozzle diameter: 2.5~3mm The powders were then sieved and classified into fractions F1 to F3. Their flowability, sphericity, and roundness were evaluated and found to be satisfactory for additive manufacturing applications. The density of the powders was approximately 6.9 g / cm. 3 It was.

[0056] The microstructure of the F2 fraction was measured by XRD for powders 1, 2 and 3 and is summarized in Table 2.

[0057] [Table 2]

[0058] Then, using fraction F2 of such powder, 1 cm was measured by LPBF using the following parameters: 3A series of 22 cubes was printed.

[0059] Laser output 150~200W, Scanning speed: 300 to 1100 mm / s hatch spacing 70~110μm, Layer thickness 20~40μm, Linear energy density (LED) 180~500J / m Volumetric energy density (VED) 100~330J / mm 3 The cubes were then evaluated and the corresponding results are summarized in Table 3 below.

[0060] [Table 3]

[0061] The printed cubes of Tests 1, 4, 6, 8, and 10 exhibited hot cracks, which were present along the solidification front in all cubes.

[0062] The printed cubes from Test 3 showed large cracks due to internal stresses that led to cold cracking. Such cracks were primarily due to the increased hardness caused by the silicon addition, along with a severe increase in brittleness.

[0063] The printed cubes of Tests 2, 5, 7 and 9 were produced according to the present invention and resulted in cubes without internal cracks.

[0064] The microstructure of the printed cubes was measured by XRD for powders 1, 2 and 3 and is summarized in Table 4.

[0065] [Table 4]

[0066] The microstructure of cubes printed using Powder 2 was evaluated and showed cellular solidification cells with equivalent diameters of less than 2 μm, as measured using cross-sectional SEM micrographs according to the ASTM E112-10 line segment method.

[0067] The microstructure of sample 3 contains several ordered phases corresponding to (Fe,Mn)4(N,C) and (Fe,Mn)3(Si).

Claims

1. Metal powders for additive manufacturing, containing the following elements expressed as weight content: 15%≦Mn≦35% 6%≦Al≦15% 0.5%≦C≦1.8% 1.6%≦Si≦3.5% P≦0.013% S≦0.015% N≦0.100% and optionally, % 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 refining. A metal powder having a composition.

2. The powder particles may optionally contain up to 1 wt. % kappa carbide (Fe, Mn) ３ 10. The metal powder of claim 1 having an austenitic microstructure containing AlCx, up to 1 wt.% AlN, and up to 20 wt.% ferrite.

3. The density of the metal powder is 7.0 g / cm ３ The metal powder according to claim 1 or 2, wherein the metal powder has a molecular weight of less than 10 ...

4. The metal powder according to any one of claims 1 to 3, having an average particle size of 1 to 150 µm.

5. The metal powder according to claim 4, wherein the average particle size is 1 to 20 μm.

6. The metal powder according to claim 4, wherein the average particle size is 20 to 63 μm.

7. The metal powder according to claim 4, wherein the average particle size is 60 to 150 μm.

8. 1. A method of producing metal powder for additive manufacturing, comprising: a) melting an element and / or metal alloy at a temperature 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 using a gas pressurized to 10-30 bar; A method comprising:

9. 10. A method for producing printed parts by additive manufacturing, wherein a powder according to any one of claims 1 to 7 or obtainable according to claim 8 is printed by laser powder bed fusion.

10. 10. The method of claim 9, comprising: a first step of forming a powder layer having a thickness of less than 100 μm; and a second step of forming a shaped layer by using a focused laser beam to melt at least a portion of the powder layer in an atmosphere consisting essentially of an inert gas.

11. Laser power is limited to a maximum of 500W. The scanning speed is 300 to 2000 mm / s, The linear energy density is 190 to 500 J / m; The hatch spacing is 50 to 120 μm, Volume energy density: 100 to 330 J / mm ３ That is, 11. The method of claim 9 or 10.

12. A printed part obtainable by the method according to any one of claims 9 to 11, having a cellular solidification structure with an equivalent diameter of less than 2 μm.

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