Fully dense and crack free molybdenum manufactured by Selective Laser Melting through alloying with carbon
Release time:
2025-09-09
1. State of the Art
Molybdenum (Mo) and its alloys are metals with a unique combination of properties. Their high melting point, high strength at elevated temperature, low thermal expansion, high thermal and electrical conductivity and high corrosion resistance to many molten metals and glasses make them to an ideal material for a wide range of applications [1]. This includes electronic and electrical devices, medical equipment, high temperature furnaces, aerospace components and lighting. [1].Molybdenum and its alloys are usually produced using powder metallurgy (PM) [1], with all its limitations regarding the possibility to manufacture parts with complex geometries and small batches. Selective Laser Melting (SLM) on the other hand is a manufacturing method perfectly suited to produce highly complex parts in small or even individualized batches [2]. However, the production of fully dense and crack free molybdenum parts with adequate mechanical properties via SLM has not been reported yet. Faidel et al. [3] achieved a optically measured relative density of up to 82.5% in the perpendicular cross section using a 200 W laser. They concluded that higher densities require higher laser powers. Wang et al. [4] could increase the relative density to 99.1% by using a 400 W laser, plasma–spheroidized powder,a scan strategy with 67° layer rotation and a support structure for preserving heat and thereby reducing temperature gradients.
The major challenge when selectively melting molybdenum is the formation of cracks. The key factors triggering the initiation of hot and cold cracks in molybdenum are described in Braun et al. [5]. This paper of the same research groups as the present one titled “Molybdenum and tungsten manufactured by SLM – Analysis of defect structure and olidification mechanisms” is published in this special issue of the International Journal of Refractory Metals and Hard Materials. Braun et al.conclude that oxygen impurities cause hot cracks, weakened grain boundaries and elevated DBTT and must therefore either be eliminated or controlled, e.g. by alloying [5].
A material with comparable difficulties when processing it by SLM,which has received more scientific attention compared to molybdenum is tungsten. It can also be processed to nearly full density [5–7], but cracking could not be overcome so far in a reliable manner [5,8]. Similar to pure molybdenum [5,9], SLM of pure tungsten suffers from a coarse columnar grain structure due to planar epitaxial solidification [5,8,10] in combination with weak grain boundaries due to impurity segregation, in particular oxygen [5,8]. In addition to grain boundary weakening, oxygen further negatively influences the evolution of the molten track due to its influence on the wetting behavior of the melt [11,12]. Furthermore, it is believed that oxides act as nucleation sites for pores, reduce density, and trigger mechanisms to form open-porosity and spatter formation [12]. Alloying is a promising approach to produce defect free refractory metal parts via SLM. In literature, the effects of the addition of 6 wt% tantalum [10] and 0.5 wt% ZrC nanoparticles [8] to tungsten are discussed. Tantalum leads to the formation of a submicron intragranular cellular structure, which captures nanopores from being pushed to grain boundaries acting there as crack nucleation sites [10]. However, cracking was reduced but not eliminated. When adding 0.5 wt% ZrC the grain size can be significantly reduced thereby increasing the crack resistance and lowering the crack density by 88.7% [8]. A finer microstructure also means larger grain boundary areas, less specific grain boundary segregation [13] and therefore stronger grain boundaries.
The aim of the present study is to obtain crack free, fully dense molybdenum components with mechanical properties at least comparable with that of pressed / sintered / deformed and recrystallized molybdenum by applying a threefold strategy:
i. Minimizing the oxygen content of the powder and the process atmosphere
ii. Working at elevated temperatures to reduce thermal gradients and stresses
iii. Alloying molybdenum with carbon for refining the microstructure and preventing the weakening of the grain boundaries by the formation of molybdenum oxides as described by the same research groups in the International Patent Application WO 2019/068117(A1) [14]
2. Experimental procedure
In the present work pure molybdenum and pre–alloyed Mo - C powder were used. The powders were fractioned by sieving with mesh sizes of 10 μm and 63 μm. Table 1 summarizes the particle sizes measured by using a laser diffraction particle size analyzer (Malvern Mastersizer 2000, Malvern, UK), the carbon content measured by applying the combustion method (Leco CS-230, Leco, USA), the oxygen content measured by carrier gas hot extraction (Leco TC–500) and the tap– /bulk density according to ASTM B527 for both powders. The scanning electron microscopy (SEM) images in Fig. 1 a) and b) show the spherically shaped pure Mo and Mo – C powder particles with small satellites. Both powders have comparable oxygen contents.
The SLM experiments were performed using a customized, lab–scale SLM machine equipped with a fiber laser and substrate plate heating.The oxygen level in the building chamber was kept below 20 ppmv for all experiments. Argon was used as shielding gas. Samples with dimensions 15 mm × 15 mm × 10 mm and for mechanical testing 35 mm × 6 mm × 6 mm were fabricated on molybdenum substrate plates with optimized process parameters. The process parameters are specified by the line energy LE = P/v with laser power P and scanning speed v. During the building process the substrate plate was kept at a temperature of 500 °C and 800 °C, respectively. The scanning strategy included zig–zag pattern, 67° rotation between adjacent layers and a layer shift of 0.5 mm. The building process was followed by a controlled cooldown with a rate of 100 K per hour. All samples were separated from the substrate plate using wire–cut Electrical Discharge Machining (EDM). Table 2 summarizes the process– and scanning parameters used.
The density of all samples was measured using Archimedes' method.A reliable density measurement was ensured by covering the samples with a thin paraffin layer before measurements. In this way, open porosity does not falsely contribute to the measured density. Specimens of sample A, B and D were cut in half in (referred to as side view) and perpendicular (referred to as top view) to the building direction. The ground, polished and etched (Murakami acid) surfaces were investigated using the optical microscope (OM) Axio Imager A2m (Zeiss,Germany). Transmission Electron Microscope (TEM) for sample B and Electron Backscatter Diffraction (EBSD) images for samples A and B were conducted by FELMI ZFE Graz. For TEM (FEI Tecnai F20, Thermo Fisher, USA) the illustration methods Bright Field (BF), Energy Filtered (EF), High–Angle Annular Dark–Field (HAADF) and High Resolution (HR) where used. For EBSD measurements a Zeiss Ultra 55 (Zeiss,Germany) with a Forward Scatter Detector (FSD) was applied. Inverse Pole Figures (IPF) were created. The fracture surfaces of samples A and B and the powder morphology were investigated using the Scanning Electron Microscope (SEM) JSM–7610F (Jeol, Japan). The ambient temperature 3–point bending test was carried out according to DIN EN ISO 3327 with four samples for each parameter set. The load was applied in building direction. The hardness of all samples was characterized according to Vickers with a load of 98.0665 N using the Emco Test m1c 010 (Emco–Test Prüfmaschinen GmbH, Austria). For bending strength and hardness mean values and standard deviations were calculated. Statistical significance was assessed within a 95% confidence interval using the unpaired t–test.
3. Results in processing pure molybdenum and Mo – 0.45 wt% C by SLM
3.1. Microstructure, cracking and fracture surface
Fig. 2 shows polished and etched top– and side view cross sections of pure molybdenum (sample A) and Mo – 0.45 wt% C (samples B and D) processed by SLM. Sample A and B were both manufactured with 0.66 J/mm line energy at a substrate plate temperature of 800 °C,sample D with 0.58 J/mm line energy and a substrate plate temperature of 500 °C. The OM images of sample A (Fig. 2 a) and d) show cracks and porosity in both views. In top view the cracks located at the grain boundaries form a network that reflects the scanning path of the laser. The side view reveals epitaxially grown columnar grains, intergranular cracks and pores (Fig. 2 d). The addition of 0.45 wt% carbon to molybdenum results in a refined grain structure without cracks and pores. In the top view the microstructure of Mo – 0.45 wt% C also reflects the laser path but with finer interlocking grains than in pure molybdenum. The side view image shows that epitaxial grain growth along the building direction is effectively suppressed. No preferential direction is recognizable in the microstructure of the Mo – 0.45 wt% C sample. The top view image of Sample D, produced with 500 °C substrate plate temperature, shows one long transgranular crack, which can be clearly identified as a stress crack.
Fig. 3 shows images of the Mo – 0.45 wt% C sample processed at 800 °C (sample B) in side view with a higher magnification. The OM image in Fig. 3 a) reveals different solidification microstructures. The microstructure clearly reflects melt pools as highlighted in b). These melt pools have half cylindrical shape with different widths due to the 67°–layer rotation with respect to the cutting plane. The microstructure is composed of elongated cells with different size and orientation,visible in Fig. 3 a)–d). The cells on the bottom of the melt pools are fine sized while those in upper regions–, especially adjacent to the border are coarser. The cells appear to be arranged in colonies with similar orientation. Fig. 3 c) and d) show cell colonies in HAADF images in longitudinal and transverse direction, respectively. The cell diameter is in the range of 1 μm. The cells are surrounded by a closed network of a segregated phase, visible in Fig. 3 c) and d). The diffraction pattern of the cell and segregated phase were calculated by Fourier transforming the corresponding regions in the HRTEM image in Fig. 3 e). Measuring the crystal lattice spacings (not shown) proves that alpha–Mo phase is surrounded by a network of Mo2C. Fig. 3 e) also shows the Mo2C – Mo interface in detail.
The EBSD images in Fig. 4 a) and b) confirm the OM images.Alloying molybdenum with 0.45 wt% C leads to a smaller grain size and epitaxial grain growth is suppressed. Adjacent cell colonies are separated by high angle grain boundaries and may therefore be referred to as grains (Fig. 4 c) and d). The misorientation angle between grains is higher than 10°, see misorientation profiles 1 and 2 in Fig. 4 e) and f).The misorientation angles in the point–to–point profile in Fig. 4 f) reveals a high angle grain boundary at a distance of 1.6 μm and misorientation angles of < 0.5° within the grains.
Representative fracture surfaces of pure molybdenum (sample A)and Mo – 0.45 wt% C (sample B) in different magnifications are given in Fig. 5. For pure molybdenum the fracture mode is almost entirely intergranular, see Fig. 5 a) and c). For sample B the fracture surface is mainly transgranular with only small shares of intergranular failure.The transgranular fracture follows crystallographic planes and is indicated by river patterns which are clearly visible in Fig. 5 d).
In Table 3 the carbon and oxygen contents of Mo – 0.45 wt% C(sample B, 0.66 J/mm, 800 °C) are compared to those of the Mo –0.45 wt% C powder. 522 μg/g carbon and 93 μg/g oxygen were lost during the building process.
In Fig. 6 a) the densities of pure molybdenum and the Mo – 0.45 wt% C samples are shown. The pure molybdenum sample A (0.66 J/mm,800 °C) has the lowest density of 9.97 ± 0.02 g/cm3. The Mo – 0.45 wt% C samples B (0.66 J/mm, 800 °C) and C (0.58 J/mm, 800 °C) have nearly the same density of 10.07 ± 0.02 g/cm3 and 10.08 ± 0.02 g/cm3.
, respectively, while the density of sample D (0.58 J/mm, 500 °C)amounts to 10.02 ± 0.02 g/cm3.
In Fig. 6 b) the mean bending strength and hardness with standard deviations for all pure molybdenum and Mo – 0.45 wt% C samples are given. All bending samples show brittle fracture without any ductility.
The pure molybdenum sample reached with 267 ± 51 MPa the lowest mean bending strength of all samples. Alloying with 0.45 wt% carbon leads to a 340% increase of the bending strength to 1180 ± 310 MPa for the same processing parameters 0.66 J/mm and 800 °C (see sample B). The Vickers hardness of sample B amounts to 343 ± 5 HV10 which is by 65% higher than the 208 ± 4 HV10 of the pure molybdenum sample. Reducing the line energy from 0.66 J/mm in Mo – 0.45 wt% C by 12% to 0.58 J/mm leads to an insignificant reduction of the mean bending strength by 12% to 1036 ± 169 MPa, while the hardness significantly increases to 359 ± 3 HV10. Lowering the substrate plate temperature to 500 °C while keeping the line energy at 0.58 J/mm leads to a reduction of the bending strength to 659 ± 53 MPa and significantly increases hardness by 12% to 402 ± 6 HV10.
4. Discussion of alloying concept
4.1. Microstructure, cracking and fracture surface
Pure molybdenum processed via SLM suffers from coarse columnar grains, cracks and pores [3,4] and generally shows a fully intergranular fracture surface. Processing molybdenum powder with a low oxygen content of 161 μg/g at a temperature of 800 °C, which is well above the DBTT of conventionally processed molybdenum in a highly inertized atmosphere does not suppress these defects entirely and does not lead to a changed fracture mode (see Fig. 5 a) and c)). This result is in agreement with our findings published in Braun et al. [5]. Alloying molybdenum with 0.45 wt% carbon, on the other hand, leads to a refined microstructure without cracks, a significantly reduced porosity and a predominantly transgranular fracture behavior.
The refined microstructure, see OM and EBSD images in Figs. 3–4,can be explained by constitutional supercooling. The addition of an alloying element with limited solubility in the solid, e.g. carbon to molybdenum, leads to an increased solute concentration of the alloying element in the melt ahead of the solidification front. The melt is cooled below its freezing point leading to the breakup of the planar solidification front into cells. This effect is called constitutional supercooling. This is the first time that a cellular microstructure has been reported for molybdenum processed by SLM. For other alloys, such as for example AlSi10Mg [15], CoCrMo [16] and 316 L [17] produced via SLM, such structures have already been observed. The EBSD measurements in Fig. 4 d) and f) show that multiple adjacent cells have similar crystallographic orientation with a misorientation angle < 0.5° proving that they belong to the same cell colony. Alpha–Mo phase is surrounded by a closed network of molybdenum carbide, which is Mo2C, as revealed by the measured lattice parameters. The cell growth direction and the size depend on the local temperature gradient in the highly dynamic SLM melt pool. Fig. 3 a) and b) show different cell sizes depending on their location in the solidified melt pool. This implies that the temperature gradient and the cooling rate are higher on the bottom of the melt pool than on the top. Adjacent cell colonies are separated by misorientation angles of > 10°, thus representing high angle grain boundaries. Furthermore, the high angle grain boundaries, see Fig. 4 c) and d), are highly interlocked.
The crack–free microstructure and the transgranular fracture mode in Mo – 0.45 wt% C can be attributed to two main reasons:
i. The addition of carbon leads to a grain refinement and the change from a planar to a cellular solidification front with enhanced grain boundary interlocking. Thereby, the grain boundary area is increased, and the specific content of segregated oxygen is reduced. In fact, oxygen segregated at grain boundaries could no longer be detected in Mo – 0.45 wt% C by TEM investigations. In addition, a portion of 92 μg/g oxygen was outgassed possibly via the formation of CO, further reducing the oxygen content within the sample. The loss of 522 μg/g carbon during the building process might also be due to the formation of CO but mainly caused by the reaction with the residual oxygen content in the building chamber
ii. The elevated substrate plate temperature reduces the thermal gradient and thermally induced stresses. It was found that a substrate plate temperature of 800 °C is sufficient to entirely suppress cracking for Mo – 0.45 wt% C while a few cracks can still be observed at 500 °C.
4.2. Density, bending strength and hardness
The pure molybdenum sample A reached a density of 9.97 ± 0.02 g/cm3 giving a relative density of 97.7 ± 0.2% compared to 10.2 g/cm3 for pure, dense molybdenum [18]. To be able to specify a relative density of the Mo – 0.45 wt% C samples the theoretical density needs to be known. Under the assumption that in equilibrium the total amount of carbon occurs as Mo2C, this results in a volume fraction of 8.4% Mo2C in the molybdenum matrix. The calculated theoretical density of Mo – 0.45 wt% C amounts to 10.11 g/cm3 (density of Mo2C:9.18 g/cm3 [19]). This gives a relative density of samples B, C and D of99.6 ± 0.2%, 99.7 ± 0.2% and 99.1 ± 0.2%.
The reason for the higher relative density of all Mo – C samples compared to pure molybdenum might be that Mo – C solidifies in a temperature interval and no longer at a solidification point. This gives the melt time to fill arising voids during solidification. Also, the wetting behavior and viscosity of the intercellular liquid, since it is a Mo – Ceutectic, might be beneficial. Another effect could be that superficial oxygen, which is known to alter Marangoni convection and worsen liquid wetting [12], derived either from the powder or the residual oxygen content of the inert gas atmosphere, can be removed by formation of CO.
The results of the 3–point bending test reveal that Mo – 0.45 wt% C processed by SLM reaches a 340% increased mean bending strength of up to 1180 ± 310 MPa compared to pure molybdenum. The results are in the range between the values for recrystallized (around 950 MPa) and stress–relieved (around 1400 MPa) powder metallurgically produced molybdenum (values from internal Plansee database). The differences in the mean bending strength of sample B and C (line energy 0.66 J/mm, 800 °C vs. 0.58 J/mm, 800 °C) are not significant. The Vickers hardness is markedly higher for sample C owing to the lower line energy, which results in higher residual stresses. The lowered bending strength of sample D (0.58 J/mm, 500 °C) is due to inner cracks. Also, the hardness of sample D is higher due to more incomplete recovery based on the lower substrate temperature.
Pure molybdenum produced by SLM shows a hardness value of 208 ± 4 HV10. For comparison, the hardness of powder metallurgically produced pure molybdenum in the deformed and recrystallized state lies between 160 and 180 HV10, and of stress–relieved pure molybdenum typically between 240 and 260 HV10 (values from internal Plansee database). SLM processed Mo – 0.45 wt% C reaches 343 ± 5 HV10. The reason for this high value is the large volume fraction of about 8% Mo2C in a closed network structure.
5. Conclusion
Alloying molybdenum with 0.45 wt% carbon enables for the first time the fabrication of SLM molybdenum parts, entirely free of cracks, with a mechanical strength comparable with that of conventionally produced molybdenum and a fracture mode changed from intergranular to transgranular. Furthermore, for the first time a fine–grained cellular microstructure without epitaxially grown columnar grains could been found for molybdenum. The following conclusions can be made:
• Alloying molybdenum with 0.45 wt% carbon changes the solidification mode from planar to cellular due to constitutional supercooling. The cells are organized in colonies with crystallographic misorientation angles between 0 and 0.5°. Alpha–Mo phase is surrounded by a closed network of Mo2C. Adjacent cell colonies are separated by high angle grain boundaries.
• The typically intergranular fracture mode in pure molybdenum was altered to mainly transgranular.
• 50% of the oxygen and 12% of the carbon contained in the powder was outgassed during SLM process at 800 °C. The oxygen content in the sample was 76 ± 5 μg/g.
• A substrate plate temperature of 800 °C during the SLM process was necessary to completely suppress cracks and to increase the relative density up to 99.7% (absolute density 10.08 g/cm3).
• Mo – 0.45 wt% C reaches a 340% higher mean bending strength in the 3–point bending test compared to pure molybdenum processed by SLM. The value of 1180 ± 310 MPa is in the range of powder metallurgically produced pure molybdenum.
Paper Citation information
International Journal of Refractory Metals & Hard Materials 84 (2019) 105000
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