DEVELOPMENT OF A MARAGING STEEL POWDER FOR ADDITIVE MANUFACTURING

Simon Hoeges GKN Sinter Metals Engineering GmbH

Radevormwald, Germany

Christopher T. Schade and Robert Causton Hoeganaes Corporation, Cinnaminson, NJ

ABSTRACT

Maragıng steels are high strength- high hardness steels that are not hardened by carbon. These steels rely on the precipitation of intermetallics compounds and generally contain high levels of cobalt, molybdenum and nickel. Elements such as titanium, vanadium, aluminum and niobium have been added to enhance the precipitation process thus increasing the strength. The steels are generally hardened by aging at approximately 500 oC. The requirements for powders for Additive Manufacturing (AM) are not well understood. Water atomization as an alternative to the widely used gas atomization is presented and the powder characteristics as well as the behaviour of the powders during AM is compared. The processing of this material by Additive Manufacturing (AM) needs to be evaluated as well. In this paper the mechanical properties and microstructure of a 1.2709 Maraging Steel are characterised after being processed by Selective Laser Melting.

INTRODUCTION Additive Manufacturing describes a range of emerging production processes which have a high potential to enhance the capabilities of powder metallurgy production. The metal based processes can be divided into material deposition techniques, e.g. Laser metal deposition (LMD) or powder bed techniques, e.g. Electron Beam Melting (EBM), 3D-Printing or Selective Laser Melting (SLM). While material deposition (Figure 1 (a)) is widely used for repair and overhaul applications as well as generation of large structures with lower complexity, powder bed is used to produce complex near net shape parts. In powder bed techniques like SLM a thin layer of metal powder is deposited on a substrate, see Figure 1 (b). The powder layer is typically in the range of 30-150 µm. In SLM and EBM a high intensity energy beam fully melts the powder in the designated areas as well as parts of the previous powder layer to achieve high density and fusing between succeeding layers. Using 3D-Printing a binder is printed into the powder layer to glue particles together producing a green part. The platform is then lowered, a new powder layer applied until

the parts are produced. 3D-Printing results in a green part which has to be debindered and sintered after Additive Manufacturing according to conventional powder metallurgical routines. In SLM and EBM parts with a high density (> 99.5 % relative density) can be achieved as produced.

(a) (b) Figure 1. (a) Schematic of material deposition process (b) Schematic of powder bed process Depending on the used material heat treatment needs to be performed to achieve similar mechanical and metallurgical properties as forged materials. Therefore SLM or EBM can be used for applications where high mechanical strength and durability are required. To achieve serial material properties the materials need to fulfill chemical specifications as well as requirements of the AM process on the powder properties. In material deposition the process depends on the amount of material fed into the melt pool, in powder bed the powder needs to be spread as a thin reproducible layer without voids. Therefore depending on the AM process the particles need to fulfill a certain particle size distribution, density, flowability, sphericity and tap density. Nowadays a variety and constantly growing number of applications is under development or in implementation for Additive Manufacturing. The advantage of geometrical freedom is used for

• dental restorations, where SLM is used to produce individualized parts in series, • prototypes of parts intended for large scale production to shorten development cycles • medical individualized implants for bone substitution • spare parts on demand • aerospace applications with lightweight design • mold injection tool inserts to achieve conformal cooling • arts and jewelry with freeform design requirements.

The success of further applications depends more and more on the cost of the produced parts. Figure 2 shows the dependency of part cost on part design complexity for conventional manufacturing. Using AM the part cost is independent of the complexity of the part so that the high cost is reasonable for a small number of applications as mentioned above.

Figure 2. Dependency of part cost on part design complexity

To extend the possible range of applications to e.g. small series production the cost of the technology needs to be reduced by increasing the productivity of the AM technology [1]. A further limitation is the availability and cost of materials for Additive Manufacturing. Focus of Research so far has been on the use of gas atomized high alloy powders. Generally gas atomized powders are preferred for additive manufacturing because of the spherical nature of the powder. Water atomization is the most common and economical technique to produce metal powders. Generally water atomizing, due to the rapid cooling rate, produces powders that are irregular in shape. In addition, the high water pressures impact more energy into the molten metal stream leading to the rough shape of the powder particles. This irregular shape is less desirable because it increases the flow time and possibly reduces the packing density. However, if a low water to metal ratio is used in water atomizing along with a high pressure, a spherical powder with a particle size distribution optimized for additive manufacturing can be produced. The growth of additive manufacturing is tied to the materials available and the cost of those materials. Alloys such as stainless steel, nickel alloys, superalloys, tool steels and cobalt alloys are being used in a range of applications from medical to aerospace. To ensure uniform and consistent part builds all of the powders must have consistent flow and high packing density. One alloy that has seen considerable interest is a maraging steel (Table I) [2-3].

Table I. Chemistry Specification for Maraging Steel (DIN 1.2709).

This steel, which uses nickel as the primary strengthening element rather than carbon, is known for its’ superior strength and toughness. Despite its high strength the material can be easily machined or formed and after these treatments it can undergo an aging (heat treatment) step that forms intermetallic precipitates involving cobalt, molybdenum and titanium which aid in increasing the tensile strength. Due to the high nickel content the alloy has high hardenability and has wear resistance that is suitable for many tooling applications. The material can be heat treated in air at low temperatures and because of the low thermal coefficient of expansion, has excellent dimensional stability. The low carbon content also helps when used in SLM since the material is not susceptible to thermal stress cracks during cooling [4]. The chemistry for the maraging steel shown in Table I was developed for wrought alloys. A troublesome feature of this chemistry is the use of titanium. Titanium has a high affinity for oxygen and forms stable solid oxides at the melt temperature. A feature of the atomization process is that the metal must be poured through a nozzle with a fairly small orifice (4-8 mm). The oxides tend to adhere to the pouring nozzle and stop the metal flow. This pour flow can have an impact on the shape and particle size of the powder. This is not such an issue in conventional casting of this alloy as normally large ingots or castings are made that have larger flow paths. In order to optimize the flow-ability of this alloy an experimental alloy was made to replace the titanium in the alloy with niobium. Niobium has a lower affinity for oxygen, and in many PM alloys such as 17-4PH and 434Cb is a standard replacement for titanium. Because of its lower affinity for oxygen the oxides to do not form on the pour tube and atomization occurs without slow down or interruption. This lower affinity for oxygen may also be lead to lower oxide inclusions during the SLM process. EXPERIMENTAL PROCEDURE Powder production High-pressure water atomization has proven to be a viable, low-cost process to achieve fine particle size distributions for iron, stainless and low-alloy metal powders. Previous shortcomings relative to powder characteristics, i.e. irregular particle shape, lower tap densities, oxidized surfaces, have been refined to

1.2709 C Si Mn P Cr Mo Ni Ti CoSpecification Max. .03% Max. .10% Max. .15% Max. .010% Max. .025% 4.50-5.20% 17-19% .80-1.20% 8.50-10.0%

more closely replicate gas atomized powder properties. For this study a 1.2709 maraging steel was atomized utilizing water atomizing with pressures around 45 MPa and with niobium replacing titanium. The materials were then sieved to closely match the particle size distribution of the gas atomized powder. This material was then compared to a commercially available gas atomized 1.2709 with titanium. Powder characterization Powders employed in powder bed AM processes require uniform stable flow to maintain the stability of the production process. Since powders used in AM are finer than conventional PM powders Hall or Carney flow may not give results that predict behavior in the AM process. Powders measured with no Hall flow can still be used in the AM process. Therefore new methods for standardization of powder flow need to be evaluated. In this paper the Revolution Powder Analyser from Mercury Scientific is used. Figure 3 (a) shows a schematic of the equipment.

(a) (b) Figure 3. Schematic of the (a) Revolution Powder Analyser © Mercury Scientific Inc (b) powder deposition process during AM The Technology gives a broad spectrum of parameters. Here the avalanche angle (Figure 3 (a)) has been chosen for further evaluation. The avalanche angle is a characteristic parameter when compared with the process of powder deposition (Figure 3 (b)). The quality of a powder layer depends among others on the behavior of the powder in front of the deposition device. Since this is a dynamic system a dynamic characterization using the characterization of moving powders in a moving environment is recommended. If the powder shows a small avalanche angle it will spread smoothly during the deposition of a thin powder layer. Additive Manufacturing Additive Manufacturing was done using the Selective Laser Melting technology in a Renishaw AM 250. The process parameters of: laser power, point to point distance, exposure time and scan strategy have been varied to achieve high density of the produced samples. The density of parts has been measured using gravimetrical analysis based on Archimedes principle. Mechanical Testing Tensile specimens and impact specimens were machined from both the as built and aged specimens according to MPIF standards. Five tensile specimens and impact specimens were evaluated for each condition. The densities of the steels were determined in accordance with MPIF Standard 42. Tensile testing followed MPIF Standard 10 and apparent hardness measurements were made on the tensile and impact specimens, in accordance with MPIF Standard 43.

The apparent density and flow rate of the base powder was tested in accordance with MPIF Standards 3 and 4, and the sieve analysis in accordance with MPIF Standard 5. RESULTS AND DISCUSSION Chemical properties of powder The chemistry of the water atomized powder versus the gas atomized powder is shown in Table II. In the water atomized powder the titanium was replaced with niobium.

Table II. Chemistry of water atomized versus gas atomized 1.2709 tool steel.

Powder particle characterization The physical characteristics of the water and gas atomized powders are shown in Figure 4 and Table III.

(a) (c)

(b) (d)

Figure 4. SEM image of (a) gas atomized maraging steel; (b) water atomized maraging steel and corresponding particle size distributions: (c) gas atomized maraging steel; (d) water atomized maraging steel.

C Ni Co Mo Nb Ti Si CrType (%) (%) (%) (%) (%) (%) (%) (%)

Water Atomized 0.010 17.74 9.66 5.00 0.35 --- 0.02 0.04Gas Atomized 0.010 16.69 10.21 4.90 --- 0.96 0.02 0.12

x10 = 18.38 µm x50 = 31.84 µm x90 = 45.48 µm SMD = 26.54 µm VMD = 31.78 µm x16 = 21.62 µm x84 = 42.07 µm x99 = 57.38 µm SV = 0.23 m²/cm³ Sm = 804.49 cm²/g

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x10 = 13.80 µm x50 = 28.13 µm x90 = 46.00 µm SMD = 22.98 µm VMD = 29.58 µm x16 = 16.50 µm x84 = 41.55 µm x99 = 70.82 µm SV = 0.26 m²/cm³ Sm = 929.14 cm²/g

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The water atomized powder has a slightly finer particle size and higher apparent density (Table III) than the gas atomized powders but a higher oxygen content. Development work is ongoing to lower the oxygen content.

Table III. Chemical and Physical Properties of maraging steel, Gas versus Water Atomized.

As shown in Table III the flow of the particles could not be measured using conventional Hall Flow. Since the gas atomized maraging steel is widely used in Additive Manufacturing a different methodology needs to be used for the characterization of the particle flow. Different powders have been analyzed with the Powder Revolution Analyzer using the system settings Rotation Rate = 0.3 U/min, Numbers of Avalanche = 150, Drum size = 50. There have always been two samples per batch analyzed. Figure 5 shows the result for three measured powders. Water atomized pure iron powder (WA-Fe-Powder) was measured since the flowability of the powder has been sufficient to be processed in the AM machine without changes to the powder deposition device. This can be seen as a minimum requirement for the processability in SLM. Gas atomized maraging steel has been measured as this is the powder most widely used for Additive Manufacturing and is documented as suitable for AM [2-3].

Figure 5. Average avalanche angle of analyzed powders The avalanche angle differs around 1° between water atomized and gas atomized maraging steel powder. The difference in batch to batch is of that number as shown for water atomized iron powder. The results of measuring the avalanche angle have then been cross linked to the processability of the powder. Powder behavior in Selective Laser Melting Powder deposition for layer preparation is crucial to the AM process. If a defect or void appears in the powder layer, this can result in defect in the molten layer and finally resulting in reduced density of the final part. By using different materials and geometries of the powder deposition device, the quality of the powder layer can be adjusted to the used powder. Materials of the deposition device can be abrasive (e.g. ceramic or metal) or flexible (e.g. silicone wiper or carbon fiber brush). Depending on the chosen material uneven surface layers can be compensated or leveled. The geometry of the device can lead to cutting (ceramic or metal knife) grinding (rotating grind wheel) or compaction (cylinder or rotating roller) to

Sieve Size (mesh)Samples C S O N AD Flow +270 +325 +400 -400

Gas Atomized 0.007 .002 .04 .003 3.77 NF 0.0 0.5 10.6 88.9

Water Atomized 0.010 .002 .36 .001 3.84 NF 0.0 0.1 2.4 97.5

increase the apparent density of the powder layer. To compensate poor flowability of powders a vibration unit can be added to the deposition device which can increase the quality and reproducibility of the powder layer. For the deposition of the water atomized tool steel powder a flexible silicone device is used. Figure 6 shows photographs of the powder layer on the first layer (a) and after the finished build process (b). In the first powder layer a horizontal structure of the powder can be observed. This is due to the build platform below the first layer, the effect dissolved after 3-5 layers and did not influence the build process.

(a) (b) Figure 6. Photograph of the powder layer (a) before the build and (b) after the build. Powder layers generated of water atomized powder showed qualitative similar characteristics as gas atomized powder during powder deposition. A vibration device was not necessary to achieve a reproducible dense powder layer without defects or disturbance. Part generation using SLM For the production of parts out of gas atomized maraging steel a set of process parameters has been developed to achieve high density (99.5% rel. density). The process parameters exposure time and hatch distance need to be varied to achieve the same results with water atomized powders. For each variation of chemical composition in the different lots the parameters have been varied in a full factorized 2³ parameter variation by producing cubical geometries with 10 mm edge length. Parameters have been chosen for further production of test parts which result in highest density (8.07 g/cm³). The test geometries for tensile test and Charpy V-notch test have then been produced in one production batch each producing 30 parts. The part distribution on the base plate is shown in Figure 7.

(a) (b) Figure 7. Overview of build jobs for 30 specimen for (a) tensile test and (b) Charpy V-notch test.

Direction of powder deposition Base plate

Mechanical properties The mechanical properties of the water and gas atomized powders are shown in Table V. The test specimens in this table have been aged at 490 °C for 6 hours. The specimens built from water atomized powder had lower ultimate tensile strength and hardness than the specimens built from gas atomized powders. The ductility as measured by the impact and elongation values was also lower for the water atomized specimens. However, the niobium level in the 1.2709 water atomized powder was much lower than the titanium in the gas atomized powders (0.35 versus 0.96 w/o) which may have limited the number and size of the precipitates that form. It has also been found that in water atomized powders utilizing precipitation hardening that because of the finer grain size of water atomized powders the aging time to reach peak strength and hardness is generally lower than that utilized for coarser grain materials such as the gas atomized powder. Currently aging time studies and TEM (transmission electron microscopy) are being performed to better understand the differences in mechanical properties. Additional water atomized powders with refined chemistry will be made after this analysis is complete. Table V: Mechanical Properties of Gas and Water Atomized 1.2709 in the aged condition (490

oC for 6 hrs).

Cross sections have been made of the samples to analyze the microstructure. The comparison for different magnitudes can be seen in Figure 8. The microstructure shows near full density for both parts, etching has been performed with waterless Kallings Reagent.

(a) (d)

Apparent Hardness

Total Elongation Density

Material (J) (HRC) (MPa) (MPa) (%) (g/cm3)Water Atomized 5 43 1793 1784 1.4 8.04

Gas Atomized 7 52 2006 1793 3.1 8.00

Impact Energy

UTS 0.20% Offset

(b) (e)

(c) (f) Figure 8. Microscopy in different magnitudes of the cross sections of test parts out of (a)-(c) gas atomized and (d)-(f) water atomized powder CONCLUSIONS Although the parameters for SLM of water atomized powders had to be modified, it was shown that water atomized powders can be successfully utilized. The powder size, morphology and flowability showed little difference between water and gas atomized powders. The Revolution Powder Analyser for quantifying the flowability of AM powders has been successfully tested to build the foundation for possible standardization. The behavior of the powder during AM laser powder bed processing showed no difference between gas or water atomized powders. Minor changes to the process parameters were necessary to achieve the same high density. In addition, the replacement of titanium with niobium seems to be a viable approach to make the powder more conducive to both atomizing and the SLM process. Further refinement of the chemistry, processing parameters and heat treatment are underway to improve the mechanical properties of the water atomized (niobium containing) 1.2709 tool steel powder.

ACKNOWLEDGMENT Thanks to Fraunhofer ILT, Aachen, Germany for performing the measurement of powders on the Revolution Powder Analyzer. REFERENCES 1. Bremen, S., Meiners, W. and Diatlov, A. (2012), Selective Laser Melting. LTJ, 9:33–38.

doi:10.1002/latj.201290018 2. K. Kempen, E. Yasa, L. Thijs, J.-P. Kruth, J. Van Humbeeck, Microstructure and mechanical

properties of Selective Laser Melted 18Ni-300 steel, Physics Procedia, Volume 12, Part A, 2011, Pages 255-263, ISSN 1875-3892, http://dx.doi.org/10.1016/j.phpro.2011.03.033.

3. Contuzzi, N.; Campanelli, S.L.; Casavola, C.; Lamberti, L. Manufacturing and Characterization of 18Ni Marage 300 Lattice Components by Selective Laser Melting. Materials 2013, 6, 3451-3468.

4. F. Feuerhahn, A. Schulz, T. Seefeld, F. Vollertsen, Microstructure and Properties of Selective Laser Melted High Hardness Tool Steel, Physics Procedia, Volume 41, 2013, Pages 843-848, ISSN 1875-3892, http://dx.doi.org/10.1016/j.phpro.2013.03.157.