Keywords

Introduction

Melt Atomization is the dominant method used commercially to produce metal and alloy powders from Al, Cu, Fe, Ti, Ni, and other alloys because high production rates favor economies of scale. Atomization, as the name implies, involves the energetic disintegration of a liquid into micrometer-sized droplets. In fact, any material that can form a liquid phase can be atomized. During melt atomization, a molten metal stream is forced through an orifice at moderate pressures, and is subsequently disintegrated into droplets by the impingement of high-energy jets of a particular fluid medium, which may be either gas or liquid, such as inert gas, water or oil. Gas atomization is often preferred over other atomization methods for the production of pre-alloyed powders since a careful degree of control can be exercised on powder chemistry, powder cleanliness, and powder size distribution and morphology. A schematic diagram of a typical gas atomization unit is shown in Fig. 36.1 [1]. The melt is delivered to an atomizer nozzle where it is fragmented into micrometer-sized droplets by means of energetic jets. The ligaments and other irregular shapes that form first during disintegration, and subsequently during interactions with aerodynamic forces, are spheroidized, a process that is driven by the high surface energy that is typical of molten metals. The molten spheres subsequently experience solidification into powder particles during flight. Particles are collected normally in the lower region of the atomization chamber, frequently via a cyclone separator device. Another type of atomization process that is available is centrifugal atomization, which uses the centrifugal forces of a spinning disk/cup or rotating electrode to disperse the liquid stream into droplets. However, melt atomization accounts for more than 95% of available atomization capacity worldwide [2]. Atomization is the most flexible technique, which allows for the production of a broad range of alloy compositions with extensive control over resulting powder characteristics and properties. Atomization is also the most critical step in spray deposition processing [3], in which melt atomization is used to produce the metallic droplets, which subsequently impinge and collect on a deposition surface to form a three-dimensional preform. Unlike conventional spray liquid processing, there is also thermal energy dissipation and phase transformation reactions during melt atomization of metal powders.

Fig. 36.1
figure 1_36

Schematic diagram of gas atomization structure and processing [1]. (From B. Zheng et al. Metall. Mater. Trans. B 40, 2009)

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The principle of melt atomization is ancient and formed the basis for a British patent in 1872 to produce lead powders by drawing-off and spraying molten lead using a stream injector [4]. Since the first large-scale production of atomized iron powder during World War II, melt atomization technology has been steadily implemented and improved partly to the widespread application of prealloyed powders, for example in thermal spray coatings and net shaped components. The latter is generally achieved by compacting fine powders using various powder metallurgy technologies for aerospace, automotive, tools of petroleum, etc.

The melt atomization is generally referred to as two-phase atomization (or twin fluid atomization). The fluid being atomized is typically a molten metal, while a secondary fluid is used as the atomization media to break-up the molten metal into droplets. During atomization, jets of the secondary fluid are formed and accelerated using a stream injector. These jets are then focused onto a stream of molten metal to promote disintegration.

During atomization, the bulk liquid is disintegrated into fine droplets in the micro-sized range, which exhibit a much larger surface-to-volume ratio as compared to the starting materials. The driving force for atomization is generally provided by the kinetic energy of the atomization media. Hence, the atomization of molten metals inherently involves the transfer of energy from the atomization media to the molten metal and the creation of a large amount of surface area. From an energy conservation standpoint, the kinetic energy imparted by the atomization media on the molten metal is partially dissipated in two important processes: to overcome the viscous forces of the molten metal that resist deformation and to overcome the surface energy forces that resist free surface creation. The kinetic energy that retains in the atomization media is eventually dissipated in the environment. The energy that is transferred from the atomization media to the molten metal in the form of surface energy for unit mass of powders produced, ΔE s, may be estimated from a simple formulation [3]:

$$\Delta {E_{\rm{s}}} = {s_1}\left( {\sum {{S_{\rm{d}}}} - \sum {{S_{\rm{b}}}} } \right)$$
(36.1)

where σ 1 is the surface energy of the molten metal; \(\sum {{S_{\rm{d}}}}\) and \(\sum {{S_{\rm{b}}}}\) are the total surface areas of the droplets and that of the bulk material for the unit mass of powder produced.

Classification of Melt Atomization

Melt atomization processes can be classified into various categories, according to the physical properties and flow characteristics of the atomization fluid: water atomization, oil atomization, and gas atomization. Gas atomization can be further classified into subsonic gas atomization, supersonic gas atomization, and ultrasonic gas atomization. The considerations in selecting a particular melt atomization method include economic factors, production scale, the physical and chemical properties of fluid to be atomized and powder to be produced, and the morphology of the powder desired [3, 5].

In the case of water and oil atomization, the atomization fluid is accelerated via a fluid injector and generally released as discrete jets. The basic mechanism in water atomization is based on momentum transfer, where the molten metal stream is broken up under the impact, rather than shear, from water droplets [6]. Separation and desiccation are normally necessary to collect the metallic powders from the slurry of atomization fluid and powders. Water atomized powders generally are quite irregular in morphology as compared to those generated using gas atomization since the cooling rate that is present during water atomization is approximately one to two orders of magnitude larger than that for gas atomization. Oil atomized powders have intermediate densities that fall between those corresponding to water and gas atomized powder as the quench rate is slower and oxidation not as pronounced. Water and oil atomization methods are generally used for high tonnage production of metallic powders when irregular particle shape and a certain degree of contamination can be tolerated.

Gas atomization is the process where the molten metal is disrupted by a high velocity gas. Subsonic, supersonic, and ultrasonic gas atomization utilize different gas velocity jets, as implied by the names used to describe the techniques. During gas atomization, a high pressure atomization gas, such as air, nitrogen, argon, or helium, is discharged from the pressuring reservoir of the atomizer into a chamber, which is typically maintained at a low environmental pressure. The compressible atomization gas is accelerated to a high velocity as it expands from the high reservoir pressure into the low pressure chamber. The energetic impingement of high velocity gas jet causes the molten metal stream to deform and disintegrate into fine droplets. The gas-to-metal ratio is an important factor that governs particle size for gas atomization, rather than being dominated by pressure of the medium like in the case of water atomization.

Atomizer and Processing Parameter

The gas atomizer can be classified into two basic types, open-type (free-fall design) and close-type (confined design) atomizers as shown in Fig. 36.2, according to the relative position between the atomization gas jets and molten metal stream [5, 8]. In free-fall designs, the metal is allowed to fall under the action of gravitational forces for a certain distance (2–20 cm) prior to interacting with the atomization gas jets. Due to the rapid velocity decay as the gas moves away from the jet, it is very difficult to bring the mean diameter of powder below 60 μm on Fe-based alloys with free fall atomizers [2]. In confined designs, the metal travels a very short distance or is prefilmed before being impinged upon by the high energy gas jets. The fluid medium is introduced into the metal stream just before it leaves the nozzle, serving to create turbulence as the entrained fluid expands due to heating and exits into a large collection volume exterior to the orifice. Confined atomizers enhance the yield of fine powder particles by maximizing gas velocity and density on contact with metal. The advantages of confined atomizer designs include their higher atomization efficiency and more stable spray relative to that associated with free-fall atomizers. However, confined atomizers are extremely sensitive to metal freeze-up, a condition that occurs when the liquid metal at the end of the delivery tube solidifies as a result of the combined effects of prefilming and rapid heat extraction that occur at the orifice of the delivery tube [3]. The interaction of the gas stream with the nozzle tip can also generate either negative or positive pressure, causing increased metal flow rate or alternatively, completely blocking the flow of metal from the crucible. Thus, great care is needed in setting up confined nozzles. Supersonic atomizers are primarily close-type atomizers. As the gas jets exit the nozzle orifices, their velocity decays rapidly, resulting in the initially supersonic flow degrading into subsonic flow only a short distance from the orifices. Ultrasonic gas atomization has been widely used for fine powder (~10 μm) atomization [3]. Different from other gas atomization methods, the gas jets formed by an ultrasonic gas atomizer carry shock waves with frequencies beyond the audible range (i.e., 20–100 kHz) [9–11]. The shock waves are meant to increase the efficiency of the molten metal breakup process.

Fig. 36.2
figure 2_36

Two-fluid atomizer designs: (a) free fall design (gas or water), and (b) confined design (gas only), according to the relative position between the atomization gas jets and molten metal stream [7]. (Courtesy of Syracuse University Press, Syracuse, New York, 1972)

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In confined annular atomizer designs, the rapid flow of atomization gas passing the nozzle edge creates a recirculating region and results in a negative pressure region under the nozzle exit orifice (called aspiration effect) during gas atomization. The aspiration effect, as shown schematically in Fig. 36.3 [3], leads to shearing of the metal stream by the flowing gas, which then forms an envelope of liquid sheet right above the focal point of the gas jets. The liquid sheet is then disrupted into fragments and droplets.

Fig. 36.3
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Gas and melt flow patterns in a confined atomizer with the aspiration effect

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The pressures used in conventional gas atomization are typically in the range of 100–600 psi (0.6–4 MPa) [8], and gas velocities in the nozzles range from Mach 1 to 3 [3]. The typical metal flow rates through single orifice nozzle range from about 1 to 90 kg/min, and typical gas flow rate ranges from 1 to 50 m3/min. The effective gas velocities range from 20 m/s to supersonic velocities, depending on nozzle design. The superheat of the molten metal, the differential temperature between the metal melting point and the temperature for atomizing molten metal, is generally about 75–150°C [2].

Mechanism of Breakup and Powder Morphology

The atomization of a bulk liquid into droplets relies on the mechanical disturbances which an atomizer imposes on a liquid. As a result of the various atomizer designs and the large number of processing variables involved, the actual breakup process of liquid into droplets may differ from case to case, although the overarching driving force is provided by surface energy. The breakup of a liquid is intimately coupled to the interactions that occur between the liquid and the surrounding environment or atomization media. Review of various mechanisms that have been proposed to explain “atomization” shows that most of the mechanisms of droplet formation involve three basic stages during the gas atomization process for the disintegration of an instable liquid sheet into droplets as shown in Fig. 36.4 [12]. These are: formation and growth of disturbance waves; disruption of liquid sheet into fragments; and formation of droplets by further breakup of fragments. Atomization of molten metals may also be divided into three important fundamental processes: primary atomization, secondary atomization, and solidification. The solidification events that are associated with metals may affect the breakup processes and the resultant size distribution of the droplets. If the droplet size produced in the primary atomization is sufficiently small, droplets may already be solid or partially solid prior to secondary atomization. In this case, there may not be sufficient time for secondary atomization to occur.

Fig. 36.4
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Mechanism of disintegration of a liquid sheet into droplets with three basic stages during the gas atomization process [12]. (From N. Dombrowski, W. R. Johns: Chem. Eng. Sci. 18, 1963)

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The particle morphology is determined largely by the rate of solidification and varies from spherical, if a low heat capacity gas is employed, to highly irregular if water is used. The irregular shape of water atomized powder is attributed to the relative higher solidification rate as compared to the spheroidizing rate of liquid droplets. If the spheroidization time (or residence time), t sph, of liquid droplet is shorter than its solidification time, t sol, particle shape tends to be spherical. Particles tend to be irregular if spheroidization time is longer. A more irregular morphology is sometimes observed when there are impurities present which lower surface tension and hence increase spheroidization time [2]. The morphology of gas atomized powder particles is generally spherical. “Satellite” powders are sometimes formed when finer powder is gas atomized, which is believed to be caused by the circulation of gas within the atomizing chamber. The fine particles are blown back into the spray plume, where they collide with larger and still partly molten particles. Spherical particles have ideal flow characteristic and are desirable for feeding thermal spray and laser powder cladding or net shaped deposition processes.

Particle Size Distribution

The relative velocity between the liquid and the gas is considered to be one of the most important factors that affect the liquid breakup process during gas atomization. For a given gas nozzle design, particle size is controlled by the atomizing media pressure and melt flow rate. The droplet size distribution for various gas-atomized alloys has been reported generally to follow a lognormal distribution [13–17]. Two numbers: d 50, median mass diameter, and σ g, geometric standard deviation, are usually used to describe the entire size distribution. The mass probability density function, p(d), of the droplet-size distribution can be expressed by [18–20]:

$$p(d) = \frac{1}{{\sqrt {2{\rm{\pi }}} \ln \,{\sigma _{\rm{g}}}}}\exp \left[ { - \frac{{{{(\ln \,d - \ln \,{d_{50}})}^2}}}{{2{{(\ln \,{\sigma _{\rm{g}}})}^2}}}} \right]$$
(36.2)

where d is the droplet size. Generally, the powder size distribution is represented in terms of a cumulative frequency, f(d i), which is defined as the fraction of powders that fall in the size range that is smaller than d i. The mass median diameter d 50 is defined as the droplet size that corresponds to the 50% cumulative frequency. d 50 can be well predicted for gas atomizing Al alloys powder by using a correlation developed by Lubanska [15]:

$${d_{50}} = {K_{\rm{d}}}{D_{\rm{n}}}{\left[ {\frac{{{\eta _{\rm{m}}}}}{{{\eta _{\rm{g}}}W}}\left( {1 + \frac{{\dot M}}{{\dot G}}} \right)} \right]^{\frac{1}{2}}}$$
(36.3)

where K d is a constant, D n is the melt stream diameter (i.e., the nozzle diameter), η m (m2/s) and η g (m2/s) are the kinematic viscosity of the melt and gas, respectively, \(\dot M\) (kg/s) and \(\dot G\) (kg/s) are the melt and gas flow rates, respectively, W is the Weber number, \(W = v_{\rm{i}}^2{\rho _{\rm{m}}}{D_{\rm{n}}}/{\gamma _{\rm{m}}}\), where ρ m (kg/m3) and γ m (J/m2) are the density and surface tension of the melt, respectively. σ g is the geometric standard deviation characterizing the spread of the droplet size distribution centered around d 50, and can be estimated by the following empirical equation [16, 21]:

$${\sigma _{\rm{g}}} = qd_{50}^j$$
(36.4)

where q and j are constants, and the unit of d 50 is micrometer herein.

Droplet Flow Dynamics

During gas atomization, the droplets are accelerated or decelerated due to the drag force resulting from the velocity difference with the local atomization gas. The motion of an individual droplet along the spray-axis is governed by the following equation [22, 23]:

$${\rho _{\rm{d}}}{V_{\rm{d}}}\frac{{{\rm{d}}{v_{\rm{d}}}}}{{{\rm{d}}t}} = {V_{\rm{d}}}\left( {{\rho _{\rm{d}}} - {\rho _{\rm{g}}}} \right)g - \frac{1}{2}{\rho _{\rm{g}}}{A_{\rm{s}}}{C_{\rm{d}}}\left| {{v_{\rm{d}}} - {v_{\rm{g}}}} \right|\left( {{v_{\rm{d}}} - {v_{\rm{g}}}} \right)$$
(36.5)

where v d (m/s), ρ d (kg/m3), V d (m3), and A s (m2) are velocity, density, volume and cross-sectional area of a droplet, respectively; ρ g (kg/m3) and v g (m/s) are the density and velocity of the GA gas, and g (m/s2) is the gravitational acceleration. Since the gas-atomized droplets are treated as spherical in shape, \({V_{\rm{d}}} = ({\rm{\pi }}/6){d^3}\) and \({A_{\rm{s}}} = ({\rm{\pi }}/4){d^2}\), where d (m) is the effective droplet diameter. For a spherical droplet during GA, the drag coefficient, C d, can be estimated by [24]:

$${C_{\rm{d}}} = 0.28 + \frac{{6\sqrt {Re} + 21}}{{Re}}$$
(36.6)

where Re is Reynolds number determined by:

$$Re = \frac{{{\rho _{\rm{g}}}d\left| {{v_{\rm{g}}} - {v_{\rm{d}}}} \right|}}{{{\mu _{\rm{g}}}}}$$
(36.7)

where μ g (Ns/m2) is the gas dynamic viscosity.

The gas velocity reaches a maximum at the exit of the atomizer nozzle, and subsequently decreases with an approximately exponential decay as the flight distance increases. Because of the velocity difference between the droplets and the impinging gas stream during gas atomization, the droplets are subjected to an accelerating drag force. The velocity of atomized droplets increases with increasing gas pressure. Figure 36.5 shows a typical Al alloy droplet velocity as a function of droplet size and flight distance for a gas pressure of 2.76 MPa [25]. In this case, the droplets are initially accelerated to a maximum value (the gas velocity at that point) due to the gas drag force. Once the gas velocity has been exceeded, the velocity decreases monotonically due to the retarding drag force from the gas. Small diameter droplets (e.g., 5–20 μm) are readily accelerated, whereas larger droplets have a larger inertia and hence resist the acceleration force.

Fig. 36.5
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Velocity variation of atomization gas and droplets with flight distance and droplet size during gas atomization of Al alloys for a gas pressure of 2.76 MPa [25]. (From B. Zheng et al. Metall. Mater. Trans. B 40, 2009)

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Cooling Rates and Microstructure

The powder particles’ size resulting from atomization allow cooling rates many orders of magnitude above those in casting processes, ranging from 102 to 107 K/s, which is known as rapid solidification, a non-equilibrium process. Gas atomization can produce high cooling rates also due to the initial high relative velocity to the droplets and the fast moving cold gas stream. The cooling rate of the droplets depends on several process parameters, such as gas composition, gas pressure, superheat, gas/melt mass flow ratio, and atomizer design, etc. The cooling rates depend on the heat exchange between the atomized particles and the surrounding medium via two mechanisms: radiation towards the atomizer chamber, and convection into the cooling gas. The latter is the predominant mechanism, given the temperature gradient and flow conditions that are typical of melt atomization. As the cooling rate experienced by the atomized particles depend on their size, amorphous, supersaturated and well-developed microstructures can be found in a gas atomization batch with an appropriate composition [1].

In summary, melt atomization is a primary and widely used powder metallurgy process, in which an alloy melt jet is energetically disintegrated into micrometer-sized powders under a controlled environment and non-equilibrium thermal and solidification conditions. It is widely used to produce a variety of powders from almost any metal.