Pure Mo Powder Research Articles

Spray pyrolysis-based synthesis of pure molybdenum powder with nanoscale primary particles and its sintering characteristics

Molybdenum (Mo) is a refractory metal with exceptional high-temperature stability, resistance to creep, and low coefficient of thermal expansion, it thus has great potential for various high-temperature applications. Nevertheless, conventional methods of synthesizing Mo powder, such as mechanical milling and wet chemical processing, have encountered limitations in producing sintered bodies with high relative density and a fine grain structure while maintaining a low sintering temperature. Therefore, it is crucial to develop an efficient powder synthesis method that enables precise and uniform control over the powder's size and shape while minimizing contamination. Such an approach would contribute to achieving excellent formability and sinterability for the resulting Mo sintered body. In this study, we successfully demonstrated the ultrasonic spray pyrolysis (USP) technique in combination with hydrogen reduction for synthesizing pure Mo powder. The resulting Mo powder exhibited micro-sized secondary particles composed of nanoscale primary particles, leading to improved formability and sinterability, essential for fabricating high-quality sintered bodies. In order to comprehensively understand the properties of the as-prepared molybdenum oxide and hydrogen-reduced Mo powder, we employed various methods to analyze its physicochemical characteristics systematically. Subsequently, the fine Mo particles were subjected to a pressureless sintering process after a conventional uniaxial powder molding. Due to its excellent formability and sinterability, we achieved a Mo sintered body with a grain size of approximately 3 μm while maintaining a relatively high sintering density of 98.3% at a comparatively low sintering temperature of 1400 °C. The resulting microstructure contributed to a relatively high hardness of around 234 Hv. These results indicate the successful synthesis of pure Mo powder with desirable properties and highlight its potential in various applications, particularly those requiring high-quality sintered bodies with enhanced hardness.

(Digital Presentation) Fabrication of AA7075 Containing Mn-Rich Phases By Spark Plasma Sintering and Evaluation of the Pitting Corrosion Resistance

Aluminum alloy 7075 (AA7075) is known as a light-weight and high strength material. AA7075 mainly contains alloying elements such as Zn, Mg, Cu, and Fe to increase strength by intermetallic particles (IMPs). Cu-containing and Fe-containing IMPs in aluminum alloys act as initiation sites of localized corrosion such as pitting in chloride solutions1-3). These IMPs in aluminum alloys show higher potential relative to the Al-matrix and act as cathodic reaction sites4). As for AA7075, it was reported that an increase in pH by the oxygen reduction reaction around Cu-containing IMPs contributes to the dissolution of Al-matrix, followed by pitting3). Therefore, it would appear that the inhibition of the oxygen reduction reaction on the IMPs prevents the pitting corrosion of AA7075.Spark plasma sintering (SPS) is a sintering technique which has been applied to fabricate various materials. When stainless steel powders and pure Mo powders are mixed and sintered by SPS, stainless steel containing Mo-rich phases was fabricated5).In this study, AA7075 containing Mn-rich phases was fabricated by SPS. Gas-atomized AA7075 powders and pure Mn powders were mixed and sintered at 773 K for 15 min. After sintering, hot-forging was conducted at 773 K. After hot-forging, heat-treatment was conducted at 688 K for 2 h, followed by furnace-cooling. After that, the specimens were polished with a diamond paste down to 0.25 µm. AA7075 was fabricated same procedure as AA7075 containing Mn-rich phases. The surface of specimens was observed using an optical microscope and a field emission scanning electron microscope (FE-SEM) equipped with energy dispersive X-ray spectroscopy system (EDS). Electrochemical measurements were performed in 0.1 M NaCl at pH 6.0 adjusted with NaOH. With the exception of the electrode area, the surfaces of specimens were coated with an epoxy resin. The size of the exposed electrode area was ca. 5 mm × 5 mm or 100 µm × 100 µm. Surface treatment of AA7075 was performed in 0.1 M MnSO4. The specimens were polarized in the solution at -0.9 V vs. Ag/AgCl (3.33 M KCl) for 5 hours. A dip-dry corrosion test was conducted to evaluate the corrosion resistance of the specimens. The specimens were dipped in 0.1 M NaCl (pH 6.0) for 20 min and dried at 298 K and 50%RH for 60 min.Al- and Mn-containing films were formed on Cu-containing IMPs by surface treatment in 0.1 M MnSO4. In the 20 cycles of dip-dry corrosion test, MnSO4-treated specimen showed less discoloration than non-treated specimen. It was found that the Al- and Mn-containing films inhibited the oxygen reduction reaction by cathodic polarization measurements. Mn-rich phases were detected in Mn-added AA7075. Mn-rich phases consisted of Mn, Al-Mn, and Al-Mn-Cu intermetallic phases. The size of Mn-rich phases was ca. 20 - 150 µm. In the dip-dry corrosion test, mass-loss decreased with Mn content. After 40 cycles, mass-loss of 5.0 mass% Mn-added AA7075 was twice as lower as that of Mn-free AA7075. After the dip-dry corrosion test, pitting corrosion of the matrix was observed and Mn phases were dissolved. Al- and Mn-containing films were formed on Cu-containing IMPs around the Mn-rich phases after the corrosion test. It was found that the Al- and Mn-containing films inhibited the oxygen reduction reaction by cathodic polarization measurements.

Mo-Si-B Alloy Formed by Optional Laser Melting Process.

In this research article, the molybdenum alloy was prepared by solid-solid doping, selecting pure Mo powder, amorphous Si powder, and B powder as the experimental raw materials for SLM molding. The density and mechanical properties of Mo-Si-B alloys prepared by SLM technology under different processes and compositions were explored, and at the same time, the microstructure of the obtained alloy was observed. The result shows that, with a laser power of 250 W, a scanning speed of 500 mm/s, and a scanning distance of 60 μm, the Mo4.5-Si2-B (at.%) alloy has the highest forming rate under the 120°parameters of the rotating scanning strategy, and the highest density is 94.22%.

Evaluating the Corrosion Resistance of AA7075 Containing Mn-Rich Phases Fabricated By Spark Plasma Sintering

Aluminum alloy 7075 (AA7075) are known to have light-weight and high strength. AA7075 mainly contains alloying elements such as Zn, Mg, Cu, and Fe to increase strength by intermetallic particles (IMPs). However, Cu-containing and Fe-containing IMPs show higher potential relative to the Al-matrix and act as initiation sites of localized corrosion such as pitting in chloride solutions1-2). In particular, Cu-containing IMPs in AA7075 become cathode and strongly affect the initiation and growth of localized corrosion2). As for AA7075, it was reported that breakdown of Al-oxide films occurs around Cu-containing IMPs due to an increase in pH by the oxygen reduction reaction, followed by pitting. Therefore, it would appear that the inhibition of the breakdown of passive films around the IMPs AA7075. Spark plasma sintering (SPS) is a sintering technique which has been applied to fabricate various materials. When stainless steel powders and pure Mo powders are mixed and sintered by SPS, stainless steel containing Mo-rich phases was fabricated3).In this study, AA7075 containing Mn-rich phases was fabricated by SPS. Gas-atomized AA7075 powders and pure Mn powders were mixed and sintered at 773 K for 15 min. After sintering, hot-forging was conducted at 773 K. After hot-forging, heat-treatment was conducted at 688 K for 2 h, followed by furnace-cooling. After that, the specimens were polished with a diamond paste down to 2.5 µm. The surface of specimens was observed using an optical microscope and a field emission scanning electron microscope (FE-SEM) equipped with energy dispersive X-ray spectroscopy system (EDS). The open circuit potential (OCP) of the specimen was measured for 72 h in 0.1 M NaCl at pH 6.0. With the exception of the electrode area, the surfaces of specimens were coated with an epoxy resin. The size of the exposed electrode area was ca. 5 mm × 5 mm. A dip-dry corrosion test was conducted to evaluate the corrosion resistance of specimens. The specimens were dipped in 0.1 M NaCl (pH 6.0) for 20 min and dried at 298 K and 50%RH for 60 min.Mn-rich phases were detected in Mn-added AA7075. Mn-rich phases consisted of pure Mn and Al-Mn intermetallic phases. The size of Mn-rich phases was ca. 20 - 150 µm. In the OCP measurements, OCP of Mn-added AA7075 decreased immediately after the immersion in 0.1 M NaCl, and the minimum value of the OCP of the Mn-added AA7075 was found to be lower than that of Mn-free AA7075. It was observed that the OCP of 5.0 mass% Mn-added AA7075 was lower than that of Mn-free AA7075. After the OCP measurements, the corrosion of Mn-rich phases was confirmed, and the color of these phases changed to dark. In the cyclic dip-dry corrosion test, mass-loss decreased with Mn content. After 40 cycles, mass-loss of 5.0 mass% Mn-added AA7075 was twice as lower as that of Mn-free AA7075. After the cyclic dip-dry corrosion test, pitting corrosion of the matrix was observed and Mn phases were dissolved.

Fabrication of Mo-Dispersed Type 304L Stainless Steel By Spark Plasma Sintering and Its Corrosion Resistance in 0.1 M NaCl

Stainless steels are widely used in industries because of their high corrosion resistance. The Cr-rich oxide film on the surface protects the balk metal. For type 316 stainless steel, 2~3 mass% Mo was added to improve corrosion resistance in chloride solutions. The pitting corrosion resistance of type 316 stainless steel is known to be higher than that of type 304 stainless steel1). The role of Mo has been investigated for long time. It was proposed that Mo forms Mo-oxides and inhibits corrosion attack2). In addition, it is suggested that Mo changes the structure of the Cr oxide layers on stainless steels 3).Spark Plasma Sintering (SPS) has been applied to fabricate various types of composite materials. The advantage of the SPS technique is rapid heating and cooling 4). When two metal powders were mixed and sintered, dual-phase materials are expected to be formed because sintering is conducted below the melting points of the metals.In this study, we fabricated Mo-dispersed type 304L stainless steel by spark plasma sintering, and the corrosion resistance was assessed in 0.1 M NaCl.Mo-dispersed type 304L stainless steel was fabricated by SPS. As the starting materials, pure Mo powders and type 304L stainless steel powders were used. Table 1 shows the chemical composition of type 304L stainless powders. Sintering was performed at 1373 K for 20 min. After sintering, the specimen was heat-treated at 1573 K for 5 h in vacuum and quenched in water. After that, the solution treatment was conducted at 1373 K for 30 min and water quenched. The specimen surface was polished down to 1 μm using a diamond paste. Sintered type 304L and type 316L were used as reference materials. The same SPS procedure and heart-treatments were applied to prepare those.The surface of Mo-dispersed type 304L was inspected using an optical microscope. Scanning electron microscope (SEM) was also used. Electron Probe Micro Analyzer (EPMA) analysis of surface was conducted.Potentiodynamic polarization curves were measured in 0.1 M NaCl solution (pH 6.0) at 298 K. Specimens were covered by epoxy resin, with the exception of the electrode area (5 mm × 5 mm).On the basis of the surface observations and X-ray diffraction, it was suggested that the steel matrix of Mo-dispersed type 304L is austenite, but many voids were generated. Mo-rich phase were observed on the entire surface. The size of Mo-rich phase was 100 ~ 200 μm. From EPMA analysis, the average concentration of Mo in Mo-rich phase was determined to be 4.3 mass%. In the steel matrix, the average Mo concentration was 2.8 mass%.In polarization measurements in 0.1 M NaCl, the passive current density of Mo-dispersed type 304L and type 316L was unstable, probably because of the presence of voids. However, the pitting potentials of Mo-dispersed type 304L were found to be higher than those of type 316L. The selective dissolution of Mo-rich phases was thought to prevent the initiation of pitting effectively in chloride environments.References; Nishimoto, I. Muto, Y. Sugawara, N. Hara, J.Electrochem.Soc., 166 (11), C3081-C3089 Hashimoto, K. Asami, K. Teramoto, Corros. Sci., 19 (1979), 3-14. F. Montemor, A. M. P. Simões, M. G. S. Ferreira, M.D.C. Belo, Corros. Sci., 41 (1999), 17-34.Omori, Mater. Sci. Eng., A287 (2000), 183-188. Figure 1

Additive Manufacturing of Pure Mo and Mo + TiC MMC Alloy by Electron Beam Powder Bed Fusion

A metal matrix composite powder of molybdenum (Mo) + TiC was produced by mechanical alloying (MA) and used in additive manufacturing by electron beam powder bed fusion along with pure Mo powder to form sandwich structures. The Mo + TiC solid layers formed mixed structures of Mo with discrete TiC particles, eutectic Mo + TiC, and Mo dendrites. Thermodynamic modeling showed that the system contained an invariant eutectic reaction in the composition range used and indicated that the system was highly sensitive to changes in composition and temperature.

Effect of Mo addition mode on the microstructure and mechanical properties of TiC–high Mn steel cermets

In TiC- and Ti(C,N)-based cermets, the wettability of the ceramic phase with the metallic binder is commonly increased through supplementation with Mo in the form of pure Mo powder or Mo2C. Herein, TiC–high Mn steel cermets were fabricated by conventional powder metallurgy techniques using Fe–Mo pre-alloyed powders as binders to guarantee uniform Mo distribution, and the cermet preparation process was optimized and investigated in detail. The microstructures of the thus obtained cermets were observed by scanning electron microscopy and compared to those of a Mo-free cermet and a cermet prepared using pure Mo powder. The grain size of Fe–Mo powder cermets exceeded that of the Mo-free cermet but was much smaller and more homogeneous than that of the Mo powder cermet. For Fe–Mo powder cermets, angular and tetragonal TiC particles were observed at Mo contents of <1.2 wt%, while round shapes became dominant at higher Mo contents. The hardness of Fe–Mo powder cermets increased with increasing Mo content, as did transverse rupture strength, which was maximal (2264 MPa) at a Mo content of 2.4 wt%, while impact toughness was maximal (11.2 J/cm2) at a Mo content of 1.2 wt%. The above values exceeded those reported for similar conventional cermets, and the use of Fe–Mo pre-alloyed powder as a metallic binder was therefore concluded to be an attractive strategy of increasing the strength and toughness of TiC–high Mn steel cermets.

Preparation and Performance of Sintered Fe-2Cu-2Mo-0.8C Materials Containing Different Forms of Molybdenum Powder.

Fe-2Cu-2Mo-0.8C powder mixtures were prepared by mixing Fe, Cu and C elemental powders with different forms of Mo-containing powder (pure Mo powder, prealloyed Mo-Fe powder and mechanically alloyed Mo-Fe powder, respectively). The powder mixtures were warm pressed under different pressures and temperatures. Properties of the green compacts and the sintered parts were tested to investigate the effects of the different ways of introducing molybdenum. The test results show that a green density of 7.32 g/cm3 was obtained for Fe-2Cu-2Mo-0.8C powder mixtures containing mechanically alloyed Mo-Fe powders, under a warm compaction pressure of 800 MPa and warm pressing temperature of 120 °C, respectively. The sintered Fe-2Cu-2Mo-0.8C specimens added with mechanically alloyed Mo-Fe powders had a density of 7.31 g/cm3, a hardness of 95 HRB and a tensile strength of 618 MPa, respectively. Compared with the sintered samples, added Mo in the forms of pure Mo and prealloyed Mo-Fe powder, the sintered parts added with mechanically alloyed Mo-Fe powders had more uniform microstructure, better mechanical and wear-resistant properties.

Investigations on microstructure and wear resistance of Fe-Mo alloy coating fabricated by plasma transferred arc cladding

A wear-resistant Fe-Mo alloy coating was fabricated on the AISI 1045 steel substrate by plasma transferred arc (PTA) cladding, using pure Mo powders as the precursor material. The microstructure, microhardness and wear resistance of the coating were investigated by scanning electron microscope (SEM) with energy dispersive spectrum (EDS), X-ray diffraction (XRD), Vickers hardness and pin-on-disk wear testers, respectively. It was found that the coating presented rapidly solidified microstructure mainly consisting of cellular R-Fe63Mo37 dendrites and a small amount of lamellar R-Fe63Mo37/α-Fess dendrites uniformly distributed in the α-Fess matrix. Based on the eutectic growth and undercooling theories, the possibility of the R/α peritectic coupled growth in the rapidly solidified peritectic Fe-Mo alloy was discussed. The extensive growth of columnar crystallites into the substrate provided good metallurgical bonding at the interface between the coating and the AISI 1045 steel substrate. The results showed that the microhardness and wear resistance of the coating were much higher than that of the AISI 1045 steel substrate. The transferred oxides layer containing MoO3 formed on the worn surface had a good friction-reducing property and also contributed to the wear resistance of the coating.

Preparation and characterization of Mo/Al2O3 composites

In order to improve the performance of molybdenum, the Mo/Al2O3 composites were prepared by using a hydrothermal method for the synthesis of the precursor powders and subsequent powder metallurgical processing. The morphologies of the composite powders and the microstructures and properties of the composites were investigated. Compared with the pure Mo powder, the grains of composite powders are smaller because of the existence of the fine Al2O3 particles. The results from the sintered composites show that the fine Al2O3 particles are evenly distributed in the Mo matrix and well bonded with the Mo matrix. With increasing Al2O3 content, all the values of the micro-hardness, compressive strength and flow stress at 0.08 strain are increased. The strengthening effect is more remarkable at elevated temperatures. At room temperature, the compressive strength and the flow stress at 0.08 strain of the composite with 40vol.% Al2O3 are 1.67 and 2.01 times greater than those of pure molybdenum, respectively, while the values are up to 2.02 and 2.52 at 1100°C.

The influences of La doping method on the microstructure and mechanical properties of Mo alloys

In this paper, the effects of the La doping method on the microstructure and mechanical properties of Mo alloys were investigated. The powders were fabricated by the industrial produced pure Mo powder, solid–solid doping method (S–S), solid–liquid doping method (S–L) and liquid–liquid doping method (L–L), respectively. In addition, the powders were processed into sintered Mo, rotary swaged Mo and Mo wire. The results indicated that the powders became finer and the grain boundaries of the Mo prepared by S–L(S–L Mo) and L–L (L–L Mo) had the ability to migrate and wrap the La2O3 particles into the grain interior during sintering with the addition of La. The tensile strength and elongation of the rotary swaged L–L Mo were the highest and the tensile elongation was raised to 42% at room temperature. Moreover, the L–L Mo wire had the highest tensile strength after the treatment at all the annealing temperatures and the S–L and L–L Mo wire possessed higher recrystallization temperature than the Mo prepared by S–S (S–S Mo) and pure Mo wire obtained.

Structural evaluation of Mo–Si–B multiphase alloy during mechanical alloying and heat treatment

In this study, an attempt has been made to synthesis Mo–Si–B multiphase alloy powder using a combination of mechanical alloying (MA) and various heat treatments. For this purpose, Mo–14Si–10B (at.-%) elemental powders and pure Mo powder were separately milled using an attritor mill. Mechanically alloyed (MAed) powders were annealed in an atmosphere controlled furnace under constant temperature with different holding times. Metallurgical characteristics of MAed and/or annealed powders were evaluated by SEM, optical microscopy, X-ray diffraction and atomic absorption spectroscopy. The lattice parameter of Mo and Mo alloy were estimated using Nelson-Riley function. The results did not show any solid solution of α-Mo or formation of any related intermetallics after MA. However, α-MoSi2 and small amounts of Mo5Si3 were identified when the MA powders were subjected to high temperature annealing over 10 h. Also, no intermetallic compounds could be identified when powders without MA, were subjected to annealing treatment.

X-ray Study on the Formation of Nano-crystallite and Amorphous Phase of Mo-Fe by Ball Milling.

Pure Mo powders are mechanically attrited with a hardened steel vial and balls in an Ar atmosphere to produce nanostructural and amorphous Mo-Fe alloys. A time evolution of the solid state amorphization caused by the attrition is classified into three stages. In the initial stage the crystallite size of Mo decreases down to 10 nm together with the increase in the lattice strain up to 1.8%. Plastic deformation is a dominating cause of the crystallite size reduction. In the next stage, the crystallite size decreases down to a minimum dimension of 5nm. The reduciton of the grain size is accompanied by a decrease in the lattice strain and the solid solutioning of the Fe impurities which are introduced from the vail and balls in the prior stage. The grain size reduction in this stage is interpreted in terms of recovery process of the stored strain. In the final stage where the Fe content in the Mo lattice saturates about 18at%, a transition from the nano-crystallites to an amorphous phase takes place.

Refinement of Ti and Mo using electron beam melting

The main problem during regeneration of Ti scrap is to reduce the oxygen content in the final ingot since the refinement degree of dopants such as Al, Mo, Ni and Cr is of minor importance. Investigations were carried out of the dependance of overheating temperature, duration of the electron beam influence and surfaces at different thermal conditions, on the degree of refinement for different technological parameters, namely the velocity of movement of the starting material and the beam-power density. The kinetic factors limiting the refinement of Ti from hot metal dopants and oxygen were examined and optimum conditions have been established. Results related to refinement of Mo during its purification from Al and Fe contaminations are presented too. The starting alloy was prepared either using the alumo-thermal method or by mixing pure Mo powders with other species. Mo ingots containing 0.2 and 0.3% Fe and variable amounts of Al, namely 2.9 and 3.34% of alternatively tablets made of mixed metal powders containing 2–4% Al and 0.5% Fe were melted by an electron beam. The purity of the refined Mo was above 99.9%.