Medium manganese powder for additive manufacturing, printing components, and method for manufacturing the same.

Abstract

The present invention relates to manganese powder for the manufacture of steel parts, particularly for additive manufacturing thereof, and to printed parts, having a composition comprising, by weight percent, C: 0.03~0.60%, Mn: 2.5~12.0%, O≦0.100%, P≦0.013%, S≦0.015%, N≦0.200%, and optionally, one or more of the following elements by weight percent: Al≦1.0%, Mo≦0.65%, B≦0.004%, Si≦3%, Ti≦0.2%, Nb≦0.2%, V≦0.3%, Sn≦0.1%, Sb≦0.1%, Ni≦1.0%, Cr≦1.0%, Cu≦1.0%, with the remainder of the composition being iron and unavoidable impurities resulting from processing.

Description

Technical Field

[0001] The present invention relates to medium manganese powder for the production of steel parts, particularly for their additive manufacturing. The present invention also relates to printed parts made from the medium manganese powder. The present invention also relates to a method for manufacturing printed parts.

Background Art

[0002] Ferroalloy powders for additive manufacturing are usually produced by an atomization process in which a molten alloy stream is extruded through a nozzle and impinged with an injection of gas introduced into such a stream immediately before exiting the nozzle, thereby obtaining fine metal droplets. The alloy droplets are cooled while falling within an atomization tower to form powder particles.

[0003] The cooling rate that such powder particles undergo during falling is not uniform among the particles, which can result in the formation of an unstable fine structure with non-uniform proportions of phases and particle sizes.

[0004] When manufacturing ferroalloy parts by additive manufacturing using such powder particles, the powder is exposed to a complex heat path, and the resulting parts exhibit a non-uniform fine structure due to time- and temperature-dependent phase transformations, which may result in non-uniform use characteristics. Furthermore, the laser powder bed fusion (LPBF) method itself may also induce a non-uniform fine structure in printed parts due to the continuous heating and cooling processes of all layers, particularly the final heating process at high temperatures. Also, since the method parameters used in LPBF generate a specific heat path, different fine structures, and thus mechanical properties, may be obtained in parts printed using different parameters.

Summary of the Invention

Problems to be Solved by the Invention

[0005] [[ID=三十年]]Therefore, an object of the present invention is to solve the above-mentioned drawbacks and provide a printed part by additive manufacturing having a standard deviation of nano-hardness value of 0.30 GPa or less.

[0006] Preferably, the component has an average nanohardness of 4.0 GPa or higher.

[0007] Another object of the present invention is to provide a method for manufacturing printed parts that has improved robustness against variations in volumetric energy density (VED) used for printing the parts. In particular, the objective is to have yield strength (YS) variations of 100 MPa or less, where the YS variation is calculated as the difference between the maximum and minimum YS values ​​measured in at least two parts printed with different volumetric energy densities (VED).

[0008] Preferably, the YS fluctuation is 80 MPa or less. [Means for solving the problem]

[0009] An object of the present invention is achieved by providing the metal powder described in claim 1. The metal powder may also include the features described in claim 2. Another object is achieved by providing the method according to claim 3. Another object of the present invention is achieved by providing the printed component described in claim 4. The printed component may also include the features described in claim 5. Another object is achieved by providing the method according to claim 6. Another object is achieved by providing the method according to any one of claims 7 to 9.

[0010] Next, the present invention will be described in detail with reference to the attached drawings and illustrated by examples without limitation. [Brief explanation of the drawing]

[0011] [Figure 1] This figure shows the normalization frequency and Gaussian fit of the nano hardness values ​​of printed components. [Modes for carrying out the invention]

[0012] Next, the composition of the powder according to the present invention will be described, and the content will be expressed in weight percentage.

[0013] The carbon content should be between 0.03% and 0.60%. Carbon plays an important role in the formation of the microstructure of the final component, particularly in the formation of ferrite, bainite, martensite, and retained austenite. Its main role is to impart strength and hardness. Carbon also contributes to hardenability. However, a carbon content exceeding 0.60% leads to a decrease in printability. On the other hand, a carbon content of less than 0.03% does not provide sufficient strength. Preferably, the carbon content is between 0.03% and 0.50%, more preferably between 0.03% and 0.45%, and even more preferably between 0.05% and 0.45%.

[0014] Manganese is present in the powder composition according to the present invention at a content of 2.5% to 12.0%. Manganese is an essential alloying element for enhancing the hardenability of steel and suppressing the formation of transformed ferrite and cementite. Manganese also prevents austenitic decomposition during the continuous heating and cooling processes at low temperatures that the part undergoes during printing. Different printed layers result in less microstructural transformation due to the continuous heating and cooling processes in the additive manufacturing method, thereby maintaining a more uniform microstructure. Manganese also contributes to strength and hardness. Preferably, the manganese content is 2.5% to 11.0%, more preferably 2.5% to 10.0%, and even more preferably 2.5% to 9.0%.

[0015] Several elements can be optionally added to the powder composition according to the present invention.

[0016] Aluminum may be present in the composition according to the present invention at a concentration of up to 1.0% to reduce manganese segregation during solidification and improve toughness. Aluminum concentrations exceeding 1.0% may reduce the printability of the parts. Preferably, the aluminum content is 0.05% to 1.0%, or 0.2% to 1.0%, more preferably 0.5% to 1.0%, and even more preferably 0.5% to 0.9%.

[0017] Molybdenum can be added in a content of up to 0.65%. Molybdenum helps to improve the hardenability of steel, reduce the tempering of the molten powder layer during the printing process, and thus maintain a uniform microstructure. Furthermore, molybdenum helps to reduce the steel's susceptibility to temper embrittlement and increase its toughness. Preferably, the molybdenum content can be added up to 0.50%, more preferably up to 0.40%, and even more preferably up to 0.30%. Preferably, the minimum amount of molybdenum can be 0.05%, more preferably 0.10%, and even more preferably 0.15%.

[0018] To improve the hardenability and toughness of the parts, boron can be optionally added up to 0.004%. Preferably, the minimum boron content is 0.0002%.

[0019] Silicon may be optionally present in a content of up to 3% to help stabilize retained austenite by suppressing carbide precipitation. Silicon also increases strength by solid solution strengthening without causing loss of ductility. In preferred embodiments, silicon is limited to 2%, or 1%, or even better, 0.5%.

[0020] Titanium and niobium can be optionally added up to 0.2% or 0.1%, tin and antimony up to 0.1%, vanadium up to 0.3%, and nickel, copper, and chromium up to 1.0% or 0.5% to achieve quenching and strengthening.

[0021] The remainder consists of iron and unavoidable impurities resulting from processing. Phosphorus, sulfur, nitrogen, and oxygen are the main impurities. They are not intentionally added and may be present in the ferroalloy and / or pure elements used as raw materials. Nitrogen may also be introduced during atomization. Their content is preferably controlled to avoid harmful changes in microstructure and / or increased brittleness. Therefore, their content is limited to 0.013%, 0.015%, 0.200%, and 0.100%, respectively.

[0022] The powder is obtained initially by mixing and melting pure elements and / or ferroalloys as raw materials. It can also be obtained by using pre-alloyed ingots of the desired composition.

[0023] Ferroalloys refer to alloys of iron having a high proportion of one or more other elements such as manganese, silicon, aluminum, molybdenum, etc. The alloying elements can alternatively be added as pure elements (usually having a purity of more than 99 wt%). The pure elements can be, in particular, carbon and pure metals such as iron, molybdenum, aluminum, manganese, boron, nickel, zirconium, titanium, tantalum, tungsten, niobium, vanadium, chromium. Those skilled in the art know how to mix different ferroalloys and pure elements to reach the desired composition.

[0024] When a composition is obtained by mixing pure elements and / or ferroalloys or scraps in appropriate proportions, the composition is heated at a temperature Th which is at least 100 °C higher than its liquidus temperature, and maintained at this temperature to melt all the raw materials and homogenize the melt in the atomization chamber.

[0025] Thanks to this superheat, the reduction in the viscosity of the molten composition helps to obtain powders with a high sphericity having an appropriate particle size distribution without satellites. However, since the surface tension increases with temperature, it is preferable not to heat the composition at a temperature 450 °C higher than its liquidus temperature. Preferably, the composition is heated at a temperature at least 200 °C higher than its liquidus temperature to promote the formation of highly spherical particles. More preferably, the composition is heated at a temperature 270 °C higher than its liquidus temperature.

[0026] Preferably, the composition is heated to 1500 - 1950 °C, which represents a good compromise between viscosity reduction and surface tension increase.

[0027] Next, the molten composition is atomized into fine metal droplets (gas atomization) by pushing the molten metal flow through an orifice and nozzle at moderate pressure and impacting it with a gas jet. The gas is introduced into the metal flow just before it exits the nozzle, and the entrained gas expands (due to heating), helping to create turbulence as it exits into an atomization tower with a large collection volume. The atomization tower is filled with gas to promote further turbulence in the molten metal jet. The metal droplets are cooled as they fall within the atomization tower. Gas atomization is preferred over water atomization because it is advantageous for producing powder particles with high roundness and low number of adhering particles.

[0028] The atomizing gas is preferably argon, nitrogen, or a mixture thereof. Both increase the melt viscosity more slowly than other gases, such as helium, and promote the formation of smaller particle sizes. They also control the purity of the chemical properties and play a role in the good form of the powder. Since the molar weight of nitrogen is 14.01 g / mol compared to 39.95 g / mol of argon, finer particles can usually be obtained using argon rather than nitrogen. On the other hand, the specific heat capacity of nitrogen is 1.04 J / (gK) compared to 0.52 J / (gK) of argon. Therefore, nitrogen increases the cooling rate of the particles. Preferably, argon is used to avoid contamination of the composition by nitrogen and the formation of AlN.

[0029] Gas pressure is important because it directly affects the particle size distribution. In particular, higher pressure results in a faster cooling rate. Preferably, the gas pressure is set to 10-30 bar, or even better, 20-30 bar, to promote the formation of particles whose size is best suited to additive manufacturing techniques.

[0030] The nozzle diameter affects the molten metal flow rate, and therefore the particle size distribution and 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.

[0031] Optionally, the metal powder obtained by atomization may be dried at 100°C, preferably in a vacuum chamber, to further improve its fluidity.

[0032] The metal powder obtained by atomization can be used as is, or it can be sieved to maintain a particle size that is better suited to subsequent additive manufacturing techniques.

[0033] For example, in addition manufacturing by laser powder bed fusion, a range of 20 to 63 μm (called fraction F2) is preferred, and a range of 20 to 45 μm is even better.

[0034] In binder spraying, particles with a fraction F1 having a size in the range of less than 20 μm, and even less than 10 μm, are used.

[0035] Fraction F3 covers particle sizes of 63–150 μm, or even 45–150 μm.

[0036] The components according to the present invention, made from metal powder, can be obtained by additive manufacturing techniques such as laser 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 energy deposition (DED), direct metal laser melting (DMLM), direct metal printing (DMP), laser cladding (LC), binder jetting (BJ), cold spray (CS), thermal spray (TS), and high-velocity oxygen fuel (HVOF).

[0037] Preferably, the present invention can utilize the LPBF method, which is a layer-upon-layer additive manufacturing technique. Thin layers of metal powder are uniformly distributed onto a substrate platform, usually metal, fixed to an indexing table that moves along a vertical axis, using a coating mechanism. This is done in a chamber containing a strictly controlled atmosphere. Once each layer is distributed, each 2D slice of the part shape is fused together by selectively melting the powder. This is achieved using a high-power laser beam, usually a ytterbium fiber laser. The laser energy is strong enough to allow for complete melting (welding) of the particles in the form of tracks or strips. Essentially, once a track is complete, the method is repeated with the next track, separated from the first track by a hatch interval. This method is repeated layer by layer until the part is complete.

[0038] The overhang shape is supported by unmelted powder from the previous layer. The main method parameters used in LPBF are typically layer thickness, hatch spacing, scanning speed, and laser power.

[0039] After the process is complete, any remaining powder is sieved and reused.

[0040] A method for manufacturing additive-formed parts using LPBF includes a first step of forming a powder layer using the powder according to the present invention. Preferably, the powder layer is less than 100 μm thick. If it is greater than 100 μm, the laser may not melt the powder at all layer thicknesses, which may result in porosity in the part. Preferably, the layer thickness is maintained at 20 to 60 μm to optimize powder melting.

[0041] In the second step, the focused laser beam forms a molded layer by melting at least a portion of the powder layer under the method conditions detailed below. In the case of LPBF, each layer of the printed component is at least partially melted in an atmosphere consisting substantially of an inert gas.

[0042] The printing parameters are layer thickness, laser power, scanning speed, and hatch spacing. The combination of printing parameters is expressed as a single parameter using volumetric energy density (VED). VED is preferably 60 to 380 J / mm3. VED is defined as P / (v·h·lt), where P is the laser power, v is the scanning speed, h is the hatch spacing, and lt is the powder layer thickness.

[0043] The laser power is preferably limited to a maximum of 550 W. Preferably, the laser power is set to over 80 W to facilitate melting at all layer thicknesses. In a preferred embodiment, the laser power is 150 to 300 W. The scanning speed is preferably 200 to 2000 mm / second, more preferably 200 to 700 mm / second. Below 200 mm / second, the excess energy provided by the laser may result in keyhole porosity and / or sputtering, which, if not properly drawn out to the outside of the powder bed, will deposit on the powder layer and create voids in the printed part. Above 2000 mm / second, the energy provided to the powder by the laser may not be sufficient to melt the powder at all layer thicknesses.

[0044] The hatch spacing is preferably 0.05 to 0.12 mm. If it is less than 0.05 mm, each point of the printed part may be remelted multiple times, which can lead to overheating.

[0045] At intervals greater than 0.120 mm, unmelted powder may become trapped between the two tracks. Preferably, the hatch spacing is 0.07 to 0.12 mm.

[0046] Next, the microstructure of the printed component according to the present invention will be described.

[0047] The printed component has a microstructure consisting of 5% to 50% retained austenite and the remainder being martensite and / or transformation zone ferrite and / or bainite. Preferably, the microstructure contains 5% to 30% retained austenite. The martensite can be fresh martensite, tempered martensite, or both, preferably fresh martensite. In the LPBF method, each layer undergoes heating and cooling processes during the melting of the layer with the previous layer, meaning different thermal paths in each layer. Between subsequent printed layers, the microstructure may be heated and rapidly cooled in or above the transformation zone region (greater than Ac1 to less than Ac3).

[0048] Transformation zone ferrite is produced from this heating process at temperatures between Ac1 and Ac3. Transformation zone ferrite is different from ferrite (hereinafter referred to as "transformed ferrite") that can be formed by the transformation of austenite into ferrite and carbides during the cooling process.

[0049] If the target temperature exceeds Ac3 or falls within the transformation zone, austenite is also formed during this heating process.

[0050] During cooling, a portion of the austenite is kept at ambient temperature, and, due to the alloy design according to the present invention, particularly the manganese which enhances the hardenability of the steel and suppresses the transformation of austenite into ferrite and cementite, a portion of it transforms into martensite or bainite.

[0051] The printed component according to the present invention has a standard deviation of nanohardness values ​​of 0.30 GPa or less.

[0052] Preferably, the component has an average nanohardness of 4.0 GPa or higher, depending on its carbon content.

[0053] Nanohardness is measured according to the ISO 14557 standard.

[0054] Preferably, the average nanohardness is measured using at least 500 nanoindentations. More preferably, at least 800 nanoindentations are used, and even more preferably, at least 1000 nanoindentations are used.

[0055] The printed parts according to the present invention are manufactured in a robust manner with respect to printing parameters. In particular, they have a YS variation of 100 MPa or less, and this YS variation is calculated as the difference between the maximum and minimum YS values ​​measured in at least two parts printed with different VEDs. [Examples]

[0056] The following examples and tests presented below are not limiting in nature and should be considered for illustrative purposes only. They demonstrate the 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 by the metal powder according to the present invention.

[0057] Table 1 - Composition The tested metal compositions are summarized in the table below, with elemental content expressed in weight percentage, and were obtained by first mixing and melting ferroalloy and pure elements.

[0058] [Table 1]

[0059] P, S, and N were maintained at less than 0.013% by weight, 0.015% by weight, and 0.200% by weight, respectively.

[0060] Table 2 - Atomization Parameters The metal composition was heated to a temperature Th corresponding to superheating exceeding the liquidus temperature ΔTo, and then atomized with argon using a nozzle diameter of 2.5 mm under the following conditions.

[0061] [Table 2]

[0062] Next, the powder was sieved and classified into fractions F1 to F3. Its fluidity, sphericity, and roundness were evaluated, and it was found to be satisfactory for additive manufacturing applications. The tap density of the powder, measured according to standard B527-15, was approximately 4.5 ± 0.1 g / cm3.

[0063] Table 3 - Printing Parameters Next, using powders A and E, a series of 1 cm³ cubes were printed by LPBF with the following parameters.

[0064] [Table 3]

[0065] Next, the cube is evaluated, and the corresponding microstructure determined by the surface fraction is summarized in Table 4.

[0066] Table 4 - Microstructure of a Printed Cube

[0067] [Table 4]

[0068] The surface fraction of the phases in the microstructure was determined by examining samples cut from printed parts and polished.

[0069] The volume fraction of retained austenite was determined by X-ray diffraction (XRD).

[0070] The identification of the remaining phases in tests according to the present invention and tests not according to the present invention, namely the identification of transformation interval ferrite, transformed ferrite, bainite, fresh martensite, tempered martensite, and cementite, is carried out by a combination of different methods.

[0071] Unlike "transformed ferrites" that can be produced after annealing, transformed ferrites are particularly rich in carbon and manganese; that is, their carbon and manganese content is higher than that of transformed ferrites. Therefore, transformed ferrites and transformed ferrites can be distinguished by observing micrographs taken with a FEG-SEM microscope after etching with Nital or Picral / Nital reagent. In such micrographs, transformed ferrites appear medium gray, while transformed ferrites appear dark gray due to their higher carbon and manganese content. Fresh martensite appears light gray under FEG-SEM observation after etching with Nital or Picral / Nital reagent. Cementite can sometimes be identified as bright spots, but their fineness makes their identification difficult with such techniques.

[0072] The presence of cementite was confirmed by transmission electron microscopy (TEM). Indeed, using such techniques, crystallographic identification of the phase is possible without any doubt. Furthermore, the presence of tempered martensite and bainite was also evaluated by TEM observation.

[0073] Finally, EBSD systems combined with SEM / FEG and TEM were complementary techniques used to identify different phases, particularly transformation-interval ferrites and bainites, as they exhibit different misorientation angle distributions.

[0074] In the tests according to the present invention, austenite and transitional ferrite are formed during the transitional annealing process between the continuous heating and cooling steps of the printed layer, and the austenite does not transform into transitional ferrite and cementite during cooling due to the manganese content in cube 1. A portion of the austenite transforms into martensite at a temperature below Ms at the end of the cooling process. Therefore, the microstructure is uniform and contains only retained austenite, transitional ferrite, and martensite.

[0075] In cube 2 printed with a powder having a manganese content lower than that required for the present invention, austenite and fresh martensite can more easily decompose into transformed ferrite and cementite, forming tempered martensite. The cementite content is less than 1% by surface fraction.

[0076] Table 5 - Nanohardness values ​​of cubes For cube 1, the dimensions are 100 × 100 μm. 2 The two squares and cube 2 are 100 × 100 μm. 2 In the three squares, the nanohardness values ​​of the cubes were measured at the center of the cubes using the nanoindentation method according to the ISO 14557 standard. The applied load was 3.50 mN, the loading rate was 0.2 mN / sec, and the unloading rate was 0.5 m / sec. The residence time at maximum load was 10 seconds. The spacing between indentations was 4 μm. This corresponds to 1084 nanoindentations in cube 1 and 1711 nanoindentations in cube 2. Figure 1 shows the normalization frequency and Gaussian fit of the nanohardness values ​​of the printed parts.

[0077] Next, the average nanohardness is calculated, the standard deviation of the nanohardness values ​​is evaluated, and the results are summarized in Table 5 below.

[0078] [Table 5]

[0079] As shown in Figure 1, the nanohardness distribution is narrower for cube 1, and this is numerically reflected in Table 5 by both the standard deviation and the coefficient of variation (CV) (ratio of the standard deviation in GPa units to the average nanohardness in GPa units). The manganese content contributes to austenite stabilization and therefore maintains a uniform microstructure.

[0080] Next, using the powder having the composition shown in Table 1, and using the parameters summarized in Table 6, 15 rectangular sub-sized tensile test specimens (according to standard ASTM E8 / E8M) were printed using the LPBF series with a layer thickness of 20 μm.

[0081] Table 6: Printing parameters of tensile test specimens

[0082] [Table 6] TIFF2026516404000007.tif129169

[0083] For powders of the same composition, print parts with different printing parameters, particularly different power, speed, and hatch distance leading to different VEDs. Print three tensile test specimens and test each parameter set, measuring YS according to ASTM E8 / E8M. Then calculate the average YS from the three obtained YS values.

[0084] Table 7 summarizes the variation in YS calculated from the difference between the measured maximum average YS and minimum average YS for the same chemical composition.

[0085] Table 7 - Mechanical properties of printed parts

[0086] [Table 7] TIFF2026516404000009.tif123169

[0087] Samples printed with the powder according to the present invention (1-5, 6-10, 11-15, and 26-39) exhibit small fluctuations in the obtained YS values ​​and low sensitivity to VED fluctuations, as evidenced by the emphasis on YS fluctuations. As a result, the microstructure of these printed parts is more robust. Manganese contributes to reducing the sensitivity of the microstructure to energy input (i.e., VED) and heating and cooling processes, thereby improving the robustness of the printing method.

[0088] Samples 16-20 and 21-25, printed with low-manganese powder, exhibit a much higher YS value range than those according to the present invention. These samples are more sensitive to printing parameters and VED due to their lower manganese content. The resulting printed parts have more heterogeneous properties.

Claims

1. Metal powder, expressed by weight content, comprising the following elements C: 0.03-0.60% Mn: 2.5-12.0% O ≤ 0.100% P ≤ 0.013% S ≤ 0.015% N ≤ 0.200% Includes, Selectively, one or more of the following elements in weight percentage. Al ≤ 1.0% Mo ≤ 0.65% B: ≤0.004% Si ≤ 3% Ti ≤ 0.2% Nb ≤ 0.2% V ≤ 0.3% Sn ≤ 0.1% Sb ≤ 0.1% Ni ≤ 1.0% Cr ≤ 1.0% Cu ≤ 1.0% Having a composition including, A metal powder in which the remainder of the aforementioned composition consists of iron and unavoidable impurities resulting from processing.

2. The metal powder according to claim 1, wherein the average particle size is 1 to 150 μm.

3. A method for producing metal powder for additive manufacturing according to claim 1, a) Melting elements and / or metal alloys at a temperature Th of at least 100°C above the liquidus temperature to obtain a molten composition having the composition described in claim 1, b) Atomizing the molten composition through a nozzle with a gas pressurized to 10 to 30 bar, Methods that include...

4. By weight percentage C: 0.03-0.60% Mn: 2.5-12.0% O ≤ 0.100% P ≤ 0.013% S ≤ 0.015% N ≤ 0.200% Includes, Selectively, one or more of the following elements in weight percentage. Al ≤ 1.0% Mo ≤ 0.65% B ≤ 0.004% Si ≤ 3% Ti ≤ 0.2% Nb ≤ 0.2% V ≤ 0.3% Sn ≤ 0.1% Sb ≤ 0.1% Ni ≤ 1.0% Cr ≤ 1.0% Cu ≤ 1.0% Having a composition including, The remaining portion of the aforementioned composition is a printed component made from iron and metal powder, which is an unavoidable impurity resulting from processing. The aforementioned printed component has a surface fraction 5-50% residual austenite The remainder is martensite and / or the transformation segment ferrite and / or bainite. A printed component having a microstructure consisting of the following.

5. The printed component according to claim 4, wherein the printed component has a standard deviation of an average nanohardness value of 0.30 GPa or less.

6. The following steps A step of providing a metal powder according to any one of claims 1 to 2, or a metal powder obtained according to claim 3, - A process of printing by laser powder bed fusion, A method for manufacturing a printed component according to claim 4, including the method described in claim 4.

7. The method according to claim 6, comprising a first step of forming a powder layer having a thickness of less than 100 μm, and a second step of forming a molded layer by melting at least a portion of the powder layer in an atmosphere composed of substantially inert gas with a focused laser beam.

8. The method according to any one of claims 6 or 7, wherein the VED is 60 to 380 J / mm3.

9. The laser output is limited to a maximum of 550W. The scanning speed is 200 to 2000 mm / second. The hatch spacing is 0.05 to 0.12 mm. The method according to claim 8.

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