Gas-atomized Powder Research Articles

Processability of a Hot Work Tool Steel Powder Mixture in Laser-Based Powder Bed Fusion.

Powder bed fusion of metals using a laser beam system (PBF-LB/M) of highly complex and filigree parts made of tool steels is becoming more important for many industrial applications and scientific investigations. To achieve high density and sufficient chemical homogeneity, pre-alloyed gas-atomized spherical powder feedstock is used. For high-performance materials such as tool steels, the number of commercially available starting powders is limited due to the susceptibility to crack formation in carbon-bearing steels. Furthermore, scientific alloy development in combination with gas-atomization is a cost-intensive process which requires high experimental effort. To overcome these drawbacks, this investigation describes the adaption of a hot work tool steel for crack-free PBF-LB/M-fabrication without any preheating as well as an alternative alloying strategy which implies the individual admixing of low-cost aspherical elemental powders and ferroalloy particles with gas-atomized pure iron powder. It is shown that the PBF-LB/M-fabrication of this powder mixture is technically feasible, even though the partly irregular-shaped powder particles reduce the flowability and the laser reflectance compared to a gas-atomized reference powder. Moreover, some high-melting alloying ingredients of the admixed powder remain unmolten within the microstructure. To analyze the laser energy input in detail, the second part of the investigation focuses on the characterization of the individual laser light reflectance of the admixed alloy, the gas-atomized reference powder and the individual alloying elements and ferroalloys.

Phase evolution within multiphase stainless steels during simulated hot isostatic pressing cycles

Stainless steel hardfacing alloys are being developed for wear and corrosion resistant applications in pressurised water reactor environments. Two examples of this, the austenitic Tristelle 5183 and triplex RR2450 were produced by gas-atomisation before undergoing consolidation using hot isostatic pressing. The phase evolution of these alloys during simulated hot isostatic pressing cycles was observed in-situ, using synchrotron X-ray diffraction. During these cycles, the metastability of the gas-atomised powders is revealed, which influences the rate of high-temperature γ→δ transformation within the RR2450 alloy. Additionally, a high-strength silicide phase, named π-ferrosilicide, forms within these alloys. It decomposes by a eutectoid π→δ+M7C3 transformation, demonstrating a high carbon solubility within this phase. The observations of this study demonstrate the need to carefully consider the process parameters during hot isostatic pressing for such complex alloys, since alloy phase transformation rates are heavily influenced by the starting condition of the gas-atomised powder.

Influence of particle size on surface oxide of 316L stainless steel powders for hot isostatic pressing

The powder oxide state is essential for resilient powder metallurgy components using Hot Isostatic Pressing (HIP) process. In fact, it can affect the behaviour of the powder during the different additive manufacturing processes. However, the oxide layer properties as a function of particle size are not well known yet. This work summaries a detailed surface characterization of different 316L particles’ size of a gas atomization powder to permit a better understanding and control of additive manufacturing processes. X-ray photo-electron spectroscopy (XPS), Raman spectroscopy, Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM) were combined to characterize the surface condition of the powders. The results show an evolution of the oxide thickness in function of the particle’s size. This oxide appears like an amorphous micro-structure. An evolution of its composition can be observed through the oxide. The main composition is iron oxide (FeO and Fe3O4 types), chromium oxide (Cr2O3 type) and manganese oxide (Mn2O3 type). In addition, nanometric nodules of manganese oxide (MnO type) were observed on the particles’ surface by TEM and Scanning Electron Microscope (SEM). These variations can explain behaviour difference between small and large particles.

Morphology and microstructure of Fe–Cr–W–B alloy powders prepared by argon gas atomization

Gas atomization was used to prepare the Fe–W–B and Fe–Cr–W–B alloy powders. The morphology, size distribution, microstructure and fluidity of the alloy powders were examined. Discussion of the effect of Cr on the morphology and microstructure of powders was carried out. The media diameter, flowability and apparent density of atomized Fe–Cr–W–B powders were 81.4 μm, 15.75 s/50g and 5.28 g cm−3, respectively. The gas-atomized powders consisted of α-(Fe,Cr) solid solution, where W and B atoms were dissolved as solute atoms. After heat treatment, (Fe,Cr)W2B2, a stable ternary boride, formed with sufficient atomic diffusion time and energy in the alloy powders. In the Fe–Cr–W–B alloy powders, the Cr elements dissolved into the ternary boride, causing severe lattice distortion and inducing the structure of boride to change from orthorhombic into tetragonal. Moreover, due to the high nucleation rate and low critical nucleation supercooling, the borides in Fe–Cr–W–B powders were more dispersive and tinier (even up to the submicron scale) than Fe–W–B alloy powders. This method can prepare the excellent powder for Fe–Cr–W–B alloy, which can be used as a potential wear-resistant material.

Production of powders for metal additive manufacturing applications using surface grinding

Metal powders for additive manufacturing (AM) applications are commonly produced using atomization techniques, which are known to be expensive, capital intensive and highly inflexible. In this work, we explore an alternative route for producing metal powders by the use of surface grinding. We demonstrate the production of nearly monodisperse AISI 52100 steel powders for AM applications. The formation of these powder particles is analyzed using material characterization techniques. We then evaluate the usability of the produced powders for metal AM using a custom-built directed energy deposition setup. Post-process analysis of laser-assisted deposition tracks shows that our powders perform just as well as conventional gas-atomized powders, resulting in void and inclusion-free metallurgical bonding.

Plasma spheroidization of gas-atomized 304L stainless steel powder for laser powder bed fusion process

Particles of AISI 304L stainless steel powder were spheroidized by the induction plasma spheroidization process (TekSphero-15 spheroidization system) to assess the effects of the spheroidization process on powder and part properties. The morphology of both as-received and spheroidized powders was characterized by measuring particle size and shape distribution. The chemistry of powders was studied using inductively coupled plasma optical emission spectroscopy for evaluation of composing elements, and the powder’s microstructure was assessed by X-ray diffraction for phase identification and by electron backscattered diffraction patterns for crystallography characterization. The Revolution Powder Analyzer was used to quantify powder flowability. The mechanical properties of parts fabricated with as-received and spheroidized powders using laser powder bed fusion process were measured and compared. Our experimental results showed that the fabricated parts with plasma spheroidized powder have lower tensile strength but higher ductility. Considerable changes in powder chemistry and microstructure were observed due to the change in solidification mode after the spheroidization process. The spheroidized powder solidified in the austenite-toferrite solidification mode due to the loss of carbon, nitrogen, and oxygen. In contrast, the as-received powder solidified in the ferrite-to-austenite solidification mode. This change in solidification mode impacted the components made with spheroidized powder to have lower tensile strength but higher ductility.

Microstructure and mechanical properties of AlCoCrFeNi2.1 eutectic high-entropy alloy synthesized by spark plasma sintering of gas-atomized powder

Eutectic high-entropy alloys (EHEAs) are usually prepared by melting method. Here, bulk eutectic high-entropy alloys, AlCoCrFeNi2.1, with a phase constitution of FCC and BCC phases, was successfully prepared by spark plasma sintering using gas-atomized powders as starting materials. With the increase of sintering temperature, a gradual microstructure evolution from lamellar eutectic to massive eutectic was observed, which should be caused by the decrease of surface energy. When sintering temperature was 1200 °C, the lamellar eutectic microstructure was completely transformed into a massive eutectic microstructure. The bulk EHEA shows the best mechanical performance when the sintering temperature is 1000 °C. The compressive yield strength, fracture strength, and ductility reached 758.60 MPa, 2626.29 MPa, and 45.45%, respectively. Furthermore, the tensile yield strength, fracture strength and ductility reached 538.13 MPa, 963.55 MPa, and 13.31%, respectively. The improved properties can be attributed to phase interfaces between FCC and BCC, and lamellar microstructures in EHEA, strengthening the alloys by inhibiting dislocation motion. The hardness of the alloy sintered at 1000 °C also reached a maximum value of 294.3 HV at 1000 °C, which has the best wear resistance, and the wear mechanism was adhesive wear. The phase interface orientation relationship of AlCoCrFeNi2.1 EHEA satisfies the N-M relationship, which is < 110>FCC//<100>BCC,{111}FCC//{110}BCC.

Ball-milling treatment of gas-atomized Ti 48Al 2Cr 2Nb powder and its effect on preventing smoking during electron beam powder bed fusion building process

Smoking is the explosive powder-scattering by Coulomb interaction, and it is a difficult issue to avoid in electron beam powder bed fusion (EB-PBF) process. In this study, we investigated the effectiveness of mechanical ball milling to prevent powder-bed smoking in EB-PBF using gas-atomized Ti 48Al 2Cr 2Nb powder. The surface of the gas-atomized powder was confirmed to be composed of outer porous TiO 2 + Al 2 O 3 and inner dense Al 2 O 3 rich bilayer oxide film that gives rise to a high electrical resistance. The electrical resistivity after ball milling with 300 rpm (BM300) was significantly decreased due to the formation of the degenerated TiO 2 and Ti n O 2n–1 -type suboxide. The electrical equivalent circuit of the powder bed of BM300 was essentially a single resistor-capacitor (RC) circuit, owing to the mechanically deformed outer oxide film. Besides, through ball milling at 300 rpm for 30 min, the relaxation time of the gas-atomized powder was significantly decreased 1/16 times, and its electrostatic force decreased approximately 1/57 times. Finally, the suppressing effect of smoking on the EB-PBF building process was experimentally examined using ball-milled powders. It was clearly demonstrated that mechanical ball milling is an effective method to restrict smoking during the EB-PBF building process without any chemical treatment and composition change.

Chemical recovery of spent copper powder in laser powder bed fusion

In laser powder bed fusion (LPBF), recovered unfused powder from the powder bed often degrades upon sequential processing through mechanisms like thermal oxidation and particle satelliting from ejected weld spatters and particle-laser interactions. Given the sensitivity of LPBF performance and build quality to powder properties, spent powder is generally discarded after a few build cycles, especially for materials that are sensitive towards surface oxidation. This increases feedstock material costs, as well as costs associated with machine downtime during powder replacement. Here, a new method to chemically reprocess spent LPBF metal powder is demonstrated under ambient conditions, using a heavily oxidised Cu powder feedstock recovered from prior LPBF processing as a model material. This is compared to an equivalent virgin Cu powder. The near-surface powder chemistry has been analysed, and it is shown that surface oxide layers present on spent Cu powder can be effectively reset after rapid reprocessing (from 5 to 20 min). Diffuse reflectance changes on etching, reducing for gas-atomised virgin Cu powder due to the formation of anisotropic etch facets, and increasing for heavily oxidised spent Cu as the highly absorptive oxide layers are removed. The mechanism of powder degradation for moisture sensitive materials like Cu has been correlated to the degradation of LPBF deposits, which manifests as widespread and extensive porosity. This extensive porosity is largely eliminated after reprocessing the spent Cu powder. Chemically etched spent powder is therefore demonstrated as a practical feedstock in LPBF in which track density produced is comparable to virgin powder.

Additive manufacturing of aluminum metal matrix composites: Mechanical alloying of composite powders and single track consolidation with laser powder bed fusion

The thermomechanical properties of particle-reinforced metal matrix composites (MMCs) are attractive for many structural applications, but fabrication of MMC parts with conventional methods is difficult, costly, and typically limited to simple geometries. Additively manufacturing particulate MMCs with laser powder bed fusion (LPBF) would be an ideal method, but the laser consolidation of these materials has been largely unsuccessful in matching the properties of conventionally produced MMCs. The challenges include spreading the heterogeneous powder, distributing small ceramic particles, and forming a strong bond between the metal and the ceramic. Here, by mechanically alloying AlSi10Mg and TiB 2 , we manufacture highly-reinforced aluminum composite powders with morphology and composition tailored for LPBF process conditions. We show that this powder flows and packs similarly to gas atomized aluminum powder and can be consolidated into smooth, continuous tracks. Moreover, the method of feedstock powder production is scalable and commercially viable. Our results demonstrate conditions under which MMCs with high ceramic loading can be produced by additive manufacturing, potentially opening up many new applications for these high-performance materials.

Thermomechanical properties of cold-sprayed copper coatings from differently fabricated powders

Cold spraying is a material coating deposition process that accelerates powders to high velocities, thereby causing them to undergo intensive plastic deformation upon impact in solid states at temperatures far below their melting point. The critical powder velocity is the most influential factor determining the bonding strength of cold spray coatings and strongly depends on the thermomechanical interactions between the sprayed powder and the substrate material. In this study, three commercially available Cu feedstocks fabricated through electrolysis (EP), gas-assisted water atomization (WA), and inert gas atomization (GA) were characterized and used to produce cold spray coatings. The kernel average misorientation (KAM) data from electron backscatter diffraction (EBSD) analysis were used to determine the plastic strain distribution of the Cu powders and coatings through local crystallographic misorientation. The percentages of the local misorientation within grains with angles higher than 1.5° in the EP (3.0%) and GA (2.8%) powders were similar, whereas that in the WA powder (0.6%) was the smallest. According to the EBSD data, more dynamic recrystallization and, in turn, more plastic strain release occurred in the as-sprayed EP and GA coatings than in the WA coating. The tendency of recrystallization of the twin grains in the as-sprayed GA coating and its influence on the ductility of the coating are discussed in relation to the GA powder's bonding mechanisms. The static recrystallization in the postannealing treatment (at 190 °C for 1 h) of the EP and GA coatings being more prominent than in that of the WA coating supports differences in dynamic recrystallization among the cold-sprayed coatings determined through EBSD analysis. The thermal conductivity of cold-sprayed Cu coatings is influenced by recrystallization and strain release.

Influence of powder characteristics on microstructure and mechanical properties of Inconel 718 superalloy manufactured by direct energy deposition

This work reports a study of the differences between properties of gas atomized (GA) and plasma rotating electrode processed (PREP) Inconel 718 powder and their printed parts. Powder morphology, particle size distribution, chemical composition, flowability and density have been comprehensively analyzed. From that, it can be found the properties of PREP powder were better than GA powder. The densification, microstructure, mechanical properties, and oxidation resistance of GA and PREP samples prepared by direct energy deposition(DED) were also studied. The results showed that the perfect morphology of powder could contribute to a finer microstruture and a more obvious preferential orientation of the printed part. Moreover, the density, mechanical properties and oxidation resistance of the printed parts were all better. This work highlights that properties of powder are crucial to the properties of the printed parts, and emphasises the importance of powder morphology on energy absorption, microstructure and preferential orientation in DED process.

Spreading behavior of Ti 48Al 2Cr 2Nb powders in powder bed fusion additive manufacturing process: Experimental and discrete element method study

The quality of powder beds is a key factor governing the density of bulk parts produced by the powder bed fusion–additive manufacturing (PBF-AM) process. Conventional assessments of powder flowability cannot be used to evaluate spreadability of the spreading process owing to different dynamic conditions used. Therefore, factors that have dominant effects on the powder bed properties must be investigated. In this study, two types of Ti 48Al 2Cr 2Nb powders, produced by the plasma rotating electrode process (PREP) and gas atomization (GA), were used to compare spreading behaviors and powder bed properties. The static and dynamic flowabilities of the PREP powder were better than those of the GA powder, while the packing density and surface roughness of the powder bed were worse. During the spreading process, the recorded recoating angle gradually increased for the GA powder, while it remained constant for the PREP powder. The increase in the recoating angle for the GA powder was due to the preferential segregation of fine particles during the spreading process. The discrete element method (DEM) model was calibrated by the fitting static and dynamic angle of repose. Based on the DEM model, three particle-flow regimes were identified: the vortex region, free flowing region, and shear stress region. In the shear stress region, the contact force between the GA powder particles was lower than that of the PREP powder, resulting in high packing density. Artificial manipulation of the particle size and shape in the DEM simulation revealed that the predominant factor governing the powder bed properties between the GA and PREP powders was the particle size distribution. We believe that our study provides an improved understanding of the powder-spreading mechanism and optimization of the powder bed properties. • Spreading behaviors and powder bed properties were investigated using experimental and discrete element method. • The static and dynamic flowabilities of the PREP powder were better than those of the GA powder, while the packing density and surface roughness of the powder bed were worse. • The predominant factor governing the powder bed properties was the particle size distribution rather than the particle sphericity distribution.

Effect of thermal induced porosity on high-cycle fatigue and very high-cycle fatigue behaviors of hot-isostatic-pressed Ti-6Al-4V powder components

• The regrowth of the residual pores in the as-HIPed Ti-6Al-4V powder component occurs after solution heat treatment. • Effect of thermal induced porosity on the very-high cycle fatigue behavior was firstly studied. • The competition of different failure types in the very-high cycle fatigue regime is related to the area density of pores (pore size >40 μm). The present work reports the effect of thermal induced porosity (TIP) on the high-cycle fatigue (HCF) and very high-cycle fatigue (VHCF) behaviors of hot-isostatic-pressed (HIPed) Ti-6Al-4V alloy from gas-atomized powder. The results show that the residual pores in the as-HIPed powder compacts present no obvious effect on the HCF life. The regrowth of the residual pores can be observed after solution heat treatment. The pore location ranks the most harmful for the fatigue life compared with the other initiating defects. The maximum stress intensity factors were calculated. The plastic zone size of fine granular area (FGA) is much less than the characteristic size of the microstructure, and the crucial size of the internal pores in this study is about 40 μm. The failure types of fatigue specimens in the VHCF regime were classified, and the competition of different failure types was described based on the modified Poisson distribution.

Characterization of AlCoCrFeNi High Entropy Alloy Gas Atomized Powder

AlCoCrFeNi alloys are widely studied due to the combination of high strength in a wide temperature range, good corrosion and wear resistance, but they typically present low ductility. Due to this limited ductility, traditional bulk fabrication processes that involve forming are difficult, and casting processes are also non-ideal due to pronounced segregation and coarse microstructure. Therefore, both the selective laser melting and the powder metallurgy are more promising routes for this alloy, both of these use powder as starting material. This work aimed to produce powder of Al20Co20Cr20Fe20Ni20 alloy by gas atomization and to perform its detailed characterization.The results show that the powder is mainly spherical, which resulted in good flowability. Most of the powder (62%) has a size smaller than 75µm, and microscopy analyses confirmed that a B2 dendritic microstructure was formed, with A2 cuboidal precipitates.

Synthesis and magnetic properties of Sb added MnBi magnets for applications at sub-zero temperatures

MnBi-LTP bulk magnets containing substitutional Sb element were fabricated using gas-atomizing process, surfactant-assisted wet magnetic compaction process, and thermal compaction process. Magnetic properties and microstructures were investigated according to the processing parameters. It was found that substitutional Sb element effectively refined the grain size of gas-atomized powders. The conventional ball milling process was able to pulverize these powders into fine particles with a single crystalline structure and minimize their phase decomposition, resulting in a (BH)max of 11.0 MGOe. By analyzing the effects of the compaction pressure on the density and magnetic alignment during the surfactant-assisted wet compaction process, Mn54Bi45.9Sb0.1 bulk magnet with Hc of 10.0 kOe at room temperature and temperature coefficient Hc of 0.68%/K was successfully fabricated. Furthermore, effects of Sb element on the temperature dependency of Hc in the bulk magnet were discussed.

Microstructural features and mechanical properties of a novel Ti- and Zr-modified Al-Mn alloy processed by laser powder bed fusion

The expansion of the material library for the laser powder bed fusion (LPBF) process is essential for the further establishment of the process in areas such as the aerospace and automotive industries. In this study, we designed a high-strength and low-cost Al-Mn-Ti-Zr alloy specifically tailored to the unique conditions of the LPBF process. Gas-atomized pre-alloyed powder was prepared and used as feedstock to fabricate LPBF specimens for microstructural examination and mechanical testing. By taking advantage of the high solidification rate, unconventionally large amounts of Mn (3.7 ± 0.5 wt%) are metastably frozen within the α-Al matrix, contributing significantly to solid solution hardening (~104 MPa ≙ 37% share of yield strength). The as-built specimens exhibit a yield strength of 284 ± 3 MPa, an ultimate tensile strength of 320 ± 1 MPa, and an elongation at fracture of 16.9 ± 0.2%. This new alloy exhibits a bimodal microstructure consisting of alternately distributed fine equiaxed and coarse columnar grain regions. Further microstructural analyses reveal a high number of primary L12 cubic Al3(Tix Zr1 - x) nanoparticles within the equiaxed α-Al grains near the bottom of the melt pool. A highly coherent interface with the α-Al matrix confirms high efficiency for heterogeneous nucleation during solidification. In addition to the Al3(Tix Zr1 - x) nanoparticles, an AlxMn(Fe, Si) phase with a quasi-crystalline structure is observed along the grain boundaries and interdendritic areas.

Influence of the particle size distribution of monomodal 316L powder on its flowability and processability in powder bed fusion

Powder bed fusion (PBF) is the most commonly adopted additive manufacturing process for fabricating complex metal parts via the layer-wise melting of a powder bed using a laser beam. However, the qualification of PBF-manufactured parts remains challenging and expensive, thereby limiting the broader industrialization of the technology. Powder characteristics significantly influence part properties, and understanding the influencing factors contributes to effective quality standards for PBF. In this study, the influence of the particle size distribution (PSD) median and width on powder flowability and part properties is investigated. Seven gas-atomized SS316L powders with monomodal PSDs, a median particle size ranging from 10 μm to 60 μm, and a distribution width of 15 μm and 30 μm were analyzed and subsequently processed. The PBF-manufactured parts were analyzed in terms of density and melt pool dimensions. Although powder flowability was inversely related to the median particle size, it was unrelated to the distribution width. An inverse relationship between the median particle size and the part density was observed; however, no link was found to the distribution width. Likely, the melt pool depth and width fluctuation significantly influence the part density. The melt pool depth decreases and the width fluctuation increases with an increasing median particle size.

Investigation of Mechanical Properties of Parts Fabricated with Gas- and Water-Atomized 304L Stainless Steel Powder in the Laser Powder Bed Fusion Process

The use of gas-atomized powder as the feedstock material for the laser powder bed fusion (LPBF) process is common in the additive manufacturing (AM) community. Although gas-atomization produces powder with high sphericity, its relatively expensive production cost is a downside for application in AM processes. Water atomization of powder may overcome this limitation due to its low cost relative to the gas-atomization process. In this work, gas- and water-atomized 304L stainless steel powders were morphologically characterized through scanning electron microscopy (SEM). The water-atomized powder had a wider particle size distribution and exhibited less sphericity. Measuring powder flowability using the Revolution Powder Analyzer (RPA) indicated that the water-atomized powder had less flowability than the gas-atomized powder. Through examining the mechanical properties of LPBF fabricated parts using tensile tests, the gas-atomized powder had significantly higher yield tensile strength and elongation than the water-atomized powder; however, their ultimate tensile strengths were not significantly different.

Comparison of nickle release and cytocompatibility between porous and dense NiTi alloy

The porous NiTi (pNiTi) samples were produced by sintering evaporation using Ti−50.8Ni (at.%) gas-atomized powders. The samples were analyzed by metallographic microscope and X-ray dispersive spectroscopy (XRD). A comparison of nickel (Ni) release and cytocompatibility between pNiTi and dense NiTi (dNiTi) was made. The results showed that the pNiTi has good mechanical properties. Ni releases from pNiTiin vitro and in vivoaremore serious than those formdNiTi. The proliferation and differentiation of cells cultured with the pNiTi extracting liquidare significantly worse, and the rate of early apoptosis is higher. In conclusion, pNiTi is mechanically similar to bone, but pNiTi releases more Ni and interferes with cell proliferation and differentiation. A significantly cautious approach should be adopted when using it as a medical implant.