Gas Atomization Process Research Articles

Printability and microstructural evolution of a near-eutectic three-phase V-based alloy

This paper presents the very first study on additive manufacturing (AM) of a high melting point near-eutectic V–9Si–5B alloy via direct energy deposition (DED). Tailored V–9Si–5B powder material was produced by means of a gas atomization (GA) process. A novel setup for the DED experiments was developed and an overview of the production parameters for manufacturing of crack-free specimens is given. The microstructural evolution of the three-phase V–9Si–5B alloy is described by means of SEM, EBSD and STEM analyses during the entire process chain, i.e. the gas atomization of the powder material, the consolidation via DED and the heat treatment of the compacts. First mechanical tests demonstrate the high hardness and the competitive creep resistance of the AM V–9Si–5B material in comparison to other three-phase V-based alloys.

Effects of Si Addition on Microstructure and Mechanical Properties of the Al Alloys with High Cr Content Prepared by Gas Atomization and SPS Processes

Effects of Si Addition on Microstructure and Mechanical Properties of the Al Alloys with High Cr Content Prepared by Gas Atomization and SPS Processes

Study on Microstructure and Mechanical Properties of the Al–25Cr–5Si (at%) Alloy Powder Using Gas-Atomization and SPS Process

In order to investigate the microstructure and mechanical properties on Al–25Cr–5Si (at%) alloy, a mixed powder with pure elements and an alloy powder using a gas atomization process were used. Fine and high purity Al–25Cr–5Si (at%) alloy powder was successfully prepared by gas atomization and densified using a spark plasma sintering (SPS) process. The overall powder size distribution of the mixed Al, Cr, and Si elemental powders was in the range of 10–15 μm. The atomized Al–Cr–Si alloy powder was fine and spherical in morphology and difficult to be formed by intermetallic formation. Densification was clearly confirmed at 1000 °C, with almost isolated pores formed, by clear removal of pores between particles, deformation of particles, an increase in the number of contacts, and a change in size between particles. As a result of XRD analysis of the sintered compacts, single phase was observed using the mixed powder, but the compact using gas atomization remained the alloy phase even at the process temperature. The Vickers hardness of the compacts by mixed powder was observed at 59.70 Hv and the compact using gas atomized powders on the temperature 1000 °C of the Vickers hardness increased to 702.6 Hv. The compressive yield strength of the compact with mixed powder was 195.24 MPa and the compressive strength of the compact with gas atomized powder increased to 802.07 MPa. It is considered not to be decomposed by the AlCrSi, Al13Cr4Si4 and Al8Cr5 phases sintering process, resulting from the improvement of mechanical properties. We have obtained a fine and high purity Al–25Cr–5Si (at%) alloy powder, which has been successfully manufactured by gas atomization. The atomized Al–Cr–Si alloy powder was fine and spherical in morphology and difficult to be formed by intermetallic formation. Alloy powders have been densified using a pulsed current-activated sintering (PCAS) process. The Vickers hardness of the compacts by mixed powder was observed at 59.70 Hv and the compact using gas atomized powders on the temperature 1000 ℃ of the Vickers hardness increased to 702.6 Hv. The compressive yield strength of the compact with mixed powder was 195.24 MPa and the compressive strength of the compact with gas atomized powder increased to 802.07 MPa.

Sliding wear of cold sprayed Ti6Al4V coatings: Effect of porosity and normal load

Ti6Al4V coatings were made from spherical and irregular powders manufactured using plasma gas atomization and Armstrong processes, respectively. The coatings, made from the two different powders, were distinctly different in terms of porosity content and hardness. Sliding wear tests were performed at different normal loads on both Ti6Al4V coatings. Despite low porosity and high hardness, irregular powder (IP) deposited coatings showed higher wear rates compared to spherical powder (SP) deposited coatings. In case of IP coatings, abrasive ploughing by the wear debris led to high wear rates, whereas presence of porosity in SP coatings, entrapped the wear debris generated during sliding and decreased the wear rate. Increase in normal load led to a decrease in wear rate and CoF in both coatings. The decrease in wear rate was due to tribo-oxidation in IP coatings whereas the combined effect of entrapment of debris along with tribo-oxidation resulted in much lower wear rates in SP coatings. Transmission electron microscopy (TEM) analysis showed that the third bodies filled in the pores consisted of highly deformed material with ultrafine grain microstructure and micron sized particles from the counterface while the wear track had comparatively coarse grain microstructure. Results indicated that despite low hardness of SP Ti6Al4V coatings, presence of porosity facilitated for the entrapment of wear debris generated from first bodies which inhibited abrasive ploughing and contributed to low wear rates.

Effect of Pressure on the Gas Atomizer to Fabricate Stainless Steel Metal Powder

Metal powder is used in the Powder Metallurgy (PM) application process. Most of the metals used in the PM are stainless steel made by the gas atomization process. This study uses the free fall gas atomizer. The material was used to produce the metal powder from various forms of stainless steel 304 raw material, which is melted in an electric induction furnace. This method is very practical to be applied in the large-scale metal processing industries. While the gas pressure variation results show that metal powder with a smaller size will be produced more using high gas pressure. The free fall gas atomizer has successfully produced stainless steel 304 metal powder with the size <40 μm and have a spherical shape. The well-rounded sphericity for 8 bar pressure, 10 bar pressure, and 12 bar pressure are 61.1%, 41.7%, and 37.5% respectively. It can be concluded that 12 bar pressure produces the smallest size range of powder about <40 µm with the most quantity about 1.11%wt, followed by 10 bar pressure about 0.41%wt and 8 bar pressure about 0.07%wt.

Study on the continuous phase evolution and physical properties of gas-atomized high-entropy alloy powders

In this study, AlCoCrFeNi high entropy alloy (HEA) powders were fabricated by gas atomization process, and the effects of annealing heat treatment on phase evolution and mechanical properties were investigated. The as-atomized powders have pure BCC phase with a spherical shape and equal composition distribution, and then the FCC and sigma phase sequentially generated after annealing. The mechanical property such as hardness was evidently enhanced, which was caused by precipitation hardening effect. After the raw powders were annealed at 600 °C, the FCC (Al–Ni) phase began to precipitate, the its phase intensity raised with the annealing temperature. Then, the sigma phase (Fe–Cr) formed as the annealing temperature reached 800 °C. Both mechanical properties and lattice constant were influenced by heating effect. According to the results, the lattice became loose with the increasing temperature. In summary, the mechanical properties and phase constitutions of gas-atomized AlCoCrFeNi HEA powders can be adjusted via annealing process, resulting in precipitation hardening effect.

Enhancement of microstructure uniformity and grain refining efficiency of Al-5Ti-1B alloy via powder extrusion

Microstructure uniformity and grain refining efficiency of Al-5Ti-1B alloy rod prepared by powder extruded process and continuous casting and rolling process are investigated in detail by X ray diffractometer, scanning electron microscope and electron probe. It is found that TiB2 and TiAl3 particles are uniformly distributed in the powder extruded Al-5Ti-1B alloy via gas atomization and hot extrusion process. The samples with 0.2% powder extruded Al-5Ti-1B alloy rod holding for 2 min consist of equiaxed grains with an average diameter of 183 µm. Even the holding time extended to 180 min, the average diameter is still stable (229 µm). Al-5Ti-1B alloy rod prepared by the powder extruded process presents better grain refinement and stronger resistance to grain coarsening.

Effect of NiTi Powder Gas Atomization Process on the Selective Laser Melting Moldability and Alloys' Superelastic

Effect of NiTi Powder Gas Atomization Process on the Selective Laser Melting Moldability and Alloys' Superelastic

Hot deformation characteristics of Ti43.5Al5Nb1V1Y alloy by hot press sintering

In this paper, Ti43.5Al5Nb1V1Y pre-alloyed powders densified by hot-pressing sintering, were prepared by gas atomization process. Isothermal hot compression simulation tests for Ti43.5Al5Nb1V1Y alloys were carried out at the conditions of deformation temperature of 1050 °C–1200 °C, strain rate of 0.001 s−1–0.5 s−1 and deformation of 50%. The experimental results show that the flow stress change is inversely proportional to the deformation temperature, but proportional to the strain rate. The initial strain flow curves show obvious work hardening characteristics, and compared to other alloys, the dynamic recrystallization curve features of the alloy were more obvious. The hot deformation activation energy of the vacuum hot-pressing sintering Ti43.5Al5Nb1V1Y alloy is 320.861 kJ mol−1. The experimental values of the constructed peak stress constitutive model agree well with the calculated values. Based on Prasad instability criterion and Dynamic Material Model, the hot processing map of Ti43.5Al5Nb1V1Y alloy with true strain of 0.5 is constructed, and the ideal deformation conditions are deformation temperatures of 1080 °C–1200 °Cand strain rates of 0.0015 s−1–0.04 s−1.

Particle shape and size analysis for metal powders used for additive manufacturing: Technique description and application to two gas-atomized and plasma-atomized Ti64 powders

The particle size and shape distributions of metal powders used in additive manufacturing powder bed fusion processes are of technological importance for the final built product. Current three-dimensional (3D) measurements of these distributions assume a spherical shape, while techniques that measure both size and shape are always two-dimensional (2D) measurements of particle projections. This paper describes a set of techniques using X-ray computed tomography, combined with various mathematical algorithms, to measure the 3D size, shape, and internal porosity of individual particles. Calibrated by a limited amount of visual examination of 3D images of individual particles, these techniques can classify powder particles as single near-spherical (SnS) particles, and non-spherical (NS) particles, which consist of either single highly non-spherical particles or multi-particles, where two or more smaller particles have been joined together. From this 3D data, other algorithms can generate 2D particle size and shape information to compare with the results of 2D measurement techniques. These techniques are applied to two metal powders composed of a specific alloy of titanium with aluminum and vanadium, denoted as Ti64, which is in common use as a powder for selective laser or electron beam melting powder bed additive manufacturing. One powder was made with a gas-atomization process, the other with a plasma-atomization process, both have been recycled, and both pass the specifications for additive manufacturing use. The powders differ in the fraction of NS particles and porous particles, in their size and shape distributions, and in average shape and size statistics. The SnS/NS classification enables one to show how these classes contribute to the overall particle size distributions, even for a single powder type, and is useful for comparing different sources of powder as well as studying how the size/shape distributions of a powder might change over multiple recycling events.

Phase field simulation of dendrite growth in gas atomized binary Al–Ni droplets

Catalytic powders with fine microstructures can be produced from a rapidly solidified gas atomized Al–Ni alloy. The solidification microstructure of the droplets is closely linked with heat flow conditions. Thus, the heat transfer conditions between the gas and droplet are essential to microstructural evolution. In this study, a phase field model for simulating single and multiple dendrite growth in a binary Al–Ni alloy was constructed and the microstructural evolutions occurring during a gas atomization process were evaluated. Temporal variations in the heat transfer coefficient and the boundary heat flux were taken into account. The results revealed that the heat transfer coefficient of the atomized droplet in flight is correlated with the droplet size and the relative velocity between the droplet and the atomizing gas. During the simulation, the competition between boundary heat flux extraction and latent heat release from phase transition causes a recalescence process in thermal history, thereby affecting the gradient temperature distribution and, consequently, the dendrite morphology. Dendrite growth under the effects of the heat transfer coefficient is restrained continuously because of the decreasing amount of extraction. The computational results confirmed the fine homogeneous microstructure and low microsegregation levels of gas atomized powders.

Design and Fabrication of High Activity Retention Al-Based Composite Powders for Mild Hydrogen Generation.

Al–Bi–Sn–Cu composite powders for hydrogen generation were designed from the calculated phase diagram and prepared by the gas atomization process. The morphologies and structures of the composite powders were investigated using X-ray diffraction (XRD) and a scanning electron microscope (SEM) equipped with energy-dispersive X-ray (EDX) spectroscopy, and the results indicate that the Cu additive enhanced the phase separation between the Al-rich phase and the (Bi, Sn)-rich phase. The hydrogen generation performances were investigated by reacting the materials with distilled water. The Al–Bi–Sn–Cu powders reveal a stable hydrogen generation rate, and the Al–10Bi–7Sn–3Cu (wt%) powder exhibits the best hydrogen generation performance in 50 °C distilled water which reaches 856 mL/g in 800 min. In addition, the antioxidation properties of the powders were also studied. The Al–10Bi–7Sn–3Cu (wt%) powder has a good resistance to oxidation and moisture, which shows great potential for being the hydrogen source for fuel cell applications.

Fine spherical powder production during gas atomization of pressurized melts through melt nozzles with a small inner diameter

A pressure-gas atomizer was developed, in which the melts were pressurized through melt nozzles with a small inner diameter, aiming for a small mass median diameter (MMD, d50,3) and high productivity of fine spherical powders. The maximum melt flow resistance in a melt nozzle, defined as the sum of the capillary resistance and viscous pressure drop, was analyzed by varying the inner diameter of the melt nozzle (D0). The calculation results indicate that the maximum melt flow resistance increases quickly with the decrease of D0, and varies in an order of 100–102 kPa for different metal melts when D0 reduces from 4.0 mm to 0.5 mm. Atomization runs with three kinds of aluminium (Al) alloys were accomplished using melt nozzles with different inner diameters in a pilot plant whereby an over-pressure in a range of ∆pl = 30–45 kPa can be maintained on the melts to enhance the melt flowing in the melt nozzle. The experimental results indicate that the atomization efficiency can be well improved by reducing the inner diameter of the melt nozzle, which resulted in a small MMD, narrow particle size distribution and high fine powder yield. For Al-I alloy powders, when the inner diameter of the melt nozzle reduces from D0 = 3 mm to D0 = 1 mm, the particle MMD reduces from d50,3 = 86.13 μm to d50,3 = 40.42 μm, and the powder yield <53 μm increases from 27.60% to 62.57%. For Al-III alloy powders, when the inner diameter of the melt nozzle reduces from D0 = 4 mm to D0 = 2 mm, the particle MMD reduces from d50,3 = 120.10 μm to d50,3 = 54.82 μm and the powder yield <53 μm increases from 20.70% to 48.20%. Moreover, the satellite particles and lamellae sticking on the particle surface were reduced when a melt nozzle with a small inner diameter was employed in a gas atomization process.

Effect of heat treatment on the phase evolution and mechanical properties of atomized AlCoCrFeNi high-entropy alloy powders

In this study, gas-atomized AlCoCrFeNi high-entropy alloy (HEA) powders were prepared by gas atomization process instead of ball milling method and the variations of annealing temperature on the crystal structure, microstructure, phase evolution and mechanical properties were investigated. This equatomic HEAs powder possesses an initial phase of BCC while the FCC and minor sigma phases could be adjusted via the annealing treatment. After the annealing process at 900 °C over 3 h, the phase constitution of gas-atomized AlCoCrFeNi gradually reached its stable state, composed of FCC + BCC phases. The BCC phase was identified as one kind of FeCo compound and served as the base of AlCoCrFeNi HEAs. During the annealing process, Cr element was separated and (Al, Ni)-rich phases were distributed in the uniform FeCo base. Based on such a microstructure, we found that the precipitated FCC and sigma phases can lead to a significant increase of the mechanical performance of AlCoCrFeNi HEA. In summary, our results suggest that the precipitation hardening is an important mechanism for obtaining a high strength AlCoCrFeNi HEA alloy.

Experimental determination of the nuclei number in the deeply undercooled and rapidly solidified powder particles of high-alloyed steel

In metallurgical practice, solidification of metallic materials usually starts by heterogeneous nucleation. Homogeneous nucleation can occur in deeply undercooled melts. Gas atomization represents one of the methods which can be used to achieve deep melt undercooling. In the gas atomization process, a certain number of the melt droplets can even reach hypercooling, which may provide suitable conditions for homogeneous nucleation in such particles.The paper presents an original approach to the quantitative analysis of the number of crystallization nuclei originated in deeply undercooled melt by the process of homogeneous nucleation using the particles of rapidly solidified (RS) powder of high alloyed Cr-Mo-V tool steel of a ledeburite type prepared by nitrogen gas atomization. For microstructural analysis, RS powder particles were sieved into six standard-sized granulometric fractions ranging from 50 μm to 125 μm. The maximum volumetric density of homogeneously formed crystallization nuclei was identified using the methods quantitative stereology in the granulometric fraction of RS powder from 56 μm to 63 μm with the value approximately 1.5 × 106 mm−3.

Effect of Heat Treatment on the Al-Cu Feedstock Powders for Cold Spray Deposition

This paper examines the effects of heat treatment on the Al-Cu feedstock powders used for cold spray deposition. In particular, we relate improvement of the powder to changes in the spray characteristics and microstructure of cold-sprayed coatings. The inert-gas-atomized Al-Cu powders were solutionized at 535 °C for 2 hours using a rotary furnace, followed by a water quench. Heat treatment of the powders not only eliminated most of the cellular solidification structure generated by the gas atomization process but also resulted in significant grain growth in the powder particles. After solution treatment, the copper content in the α-aluminum matrix increased as expected. After heat treatment, the powders were immediately cold sprayed onto AA2024 substrates. Heat treatment of Al-Cu feedstock powders significantly increased the deposition efficiency for all compositions tested. For heat-treated powders, the deposition efficiency systematically decreased with the increasing copper content, while the coating hardness increased. The formation of GP zones or θ″/θ′ strengthening precipitates through natural aging was not generally observed in the powder particles, but θ″/θ′ precipitates were observed in some cases for the cold-sprayed material with higher copper content.

Powder Production Technologies

Production of metal powder has been done for decades. Apart from some methods using crushers for powder production, the atomization of a molten liquid melt is the most common way to produce metal powders. Therefore different melting technologies (open furnace, vacuum furnace, induction melting or plasma melting, and more) as well as atomization techniques (water, gas) can be used. Depending on the melting and atomization technology used, the produced powder can differ in specific size and shape. This article provides an overview of the most used technologies in melting and atomization with a special focus on the production of powder for the use in additive manufacturing. A more detailed insight in the gas atomization process using closed couple atomization system is also part of this article.

A Comprehensive Study of Steel Powders (316L, H13, P20 and 18Ni300) for Their Selective Laser Melting Additive Manufacturing

The determination of microstructural details for powder materials is vital for facilitating their selective laser melting (SLM) process. Four widely used steels (316L, H13, P20 and 18Ni300) have been investigated to detail their powders’ microstructures as well as laser absorptivity to understand their SLM processing from raw material perspective. Phase components of these four steel powders were characterized by X-ray diffraction (XRD), synchrotron radiation X-ray diffraction (SR-XRD) and scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) were utilized to reveal the surface structure of these four steel powders. It is found that phase components of H13, P20 and 18Ni300 are mainly composed of martensite and a small amount of austenite due to the high cooling rate during gas atomization processing, while 316L is characterized by austenite. XPS results show that the four steel powders all possess a layered surface structure, consisting of a thin iron oxide layer at the outmost surface and metal matrix at the inner surface. It is found that the presence of such oxide layer can improve the absorptivity of steel powders and is beneficial for their SLM process.

Size-Dependent Phase Transformation during Gas Atomization Process of Cu⁻Sn Alloy Powders.

For binary element atomization, it is essential to investigate the phase transformation from liquid to solid as a functions of the droplet sizes, as well as the reaction competitiveness, during gas atomizing solidification of their nuclei. In the present work, a series of phase transformations of undercooled Cu (60.9 wt.%)/Sn droplets were analyzed when atomized by pressure gas. The results indicated that the microstructures of the obtained powders and their morphologies were highly relevant to the droplet size. According to the phase characteristics analyzed by the microstructural observations in combination with the transient nucleation theory, powders with sizes from 10 to 100 μm were divided into three categories, exhibiting lotus-leaf, island, and stripe morphologies. The competitive formation of Cu6Sn5 or Cu3Sn was also controlled by the droplet sizes, and a diameter of approximately 45 μm was identified as the threshold size. After heat treatment at 300 °C for 4 h, the powders consisted of a single η’ Cu6Sn5 phase. The obtained Cu6Sn5 phase powders can be used in the field of high-temperature applications as intermetallic balls for integrated chip interconnects.

Inert Gas Atomization of High‐Nitrogen TRIP‐Steels

High nitrogen steels with transformation induced plasticity (TRIP) effect have been recently developed within the framework of the project CRC799. These steels are utilized in a form of powder for the needs of this project. Therefore, the current study focuses on the powder production of high nitrogen steels via inert gas atomization process. Specifically, the role of nitrogen content in determination of the basic powder properties and its behavior on the atomization process are investigated. Additionally, the effects of several other process parameters, such as temperature of the melt and gas to metal ratio are discussed. Thereby, the effect of nitrogen content on the median particle size is not confirmed. Meanwhile, an increase in the spraying temperature and a rise in gas to metal ratio lead to a slight decrease in the median particle size. Nitrogen content in the powder is technologically controlled by the atmosphere above the melt before the spraying process. At last, nitrogen shows a significant impact on the phase composition of powder, by stabilizing the austenite phase. The results thus obtained are of high interest for the future application of the designed steels in the powder form.