- Email: [email protected]
UMT promises tight control of particle size
UMT promises tight control of narticle size Many of today’s advanced powder metallurgy (PM] manufacturing processes require powders with we//-defined and reproducible geoSuch properties are diffi= metric properties. cult to achieve with conventional powder production techniques such as gas and water atomization, and mechanical processing. To overcome these shortcomings, start-up company Uniform Metal Technologies LLC (UMT) has developed the Precision Particle Spray which it c/aims offers (PPS) process, unprecedented control of size and shape. A/i Alagheband, President, and Chris Brown, CEO, describe UMI% process and place it in context with the development of established powder production methods.
0
VW the history of the powder metallurgy (PM) industry, many ways of producing metal powder have been developed. At present, powders of various materials can be purchased that have been produced by such diverse processes as gas atomization (a process with many variationsj,
r
Gas atomization
t Water atomization Centrifugal (spinning cup) atomization Vacuum atomization ‘Tandem’ atomization
IL
Close-coupled Free fall High pressure EIGA PIGA Consumable electrode
Melt spinning
’F/-f
y--p15ns Hydrometallurgical
techniques J
FIGURE 1: Major commercially available metal powder production processes.
30 MPR November
1998
0026-0657/98/US$19.00
water atomization, mechanical attrition and alloying, melt spinning, the rotating electrode process (REP), and a wide variety of chemical reactions producing powder precipitates and by-products. Each of these processes has several variations and permutations, designed to alter the material properties or the geometric properties (e.g. mean particle size, particle shape) of the final product. Looking at Figure 1, we see the diversity of powder producing processes. An obvious question is: why are there so many ways to produce what would seem to be a rather simple product? Historically, many of these processes were developed to meet the unique processing needs of various materials (such as steel, tungsten carbide, superalloys, etc.) and those of various parts manufacturing processes (such .as press-and-sinter, highpressure compaction, hot isostatic pressing (HIP), etc.). As an example of this historical process, let us look at the development of some of the current commonly used powder production processes. By the early 1960s water atomization was the main method used to make the irregularly shaped particles of various grades of steel for classic press-and-sinter PM. However, it is not easy to make clean, spherical, oxide-free and impurity-free powders of reactive alloys such as titanium or any of the nickel-based superalloys by this method. The need for these powders in the HIPing of superalloy parts for the aerospace industry led to the development of the rotating electrode process by Nuclear Metals (now Starmet) of Concord, MA, USA, in the late 1960s and early 1970s. In addition to REP, inert gas atomization (IGA) and vacuum atomization were also developed through the mid-1970s as responses to the need for HIPed titanium and superalloy parts for the aerospace industry. At the same time, other groups were working on the parts manufacturing process now known as metal injection moulding (MIM). By the early 198Os, MIM had attracted a great deal of interest for its ability to create intricately detailed stainless steel parts without compaction. However, the process required very fine powder as a feedstock. At the time, interest in very fine powder production was primarily due to interest in rapidly solidified microstructures. As MIM started to grow, powder producers became aware of the market’s potential to consume a great deal of very fine powder and began to alter their processes to create higher yields. Those with inert gas atomization processes met with the
Copyright
0 1998,
Elsevier Science Ltd All rights reserved
most success at adapting their process to produce powders suitable for MIM feedstock. These developments are a good example of how the evolution of powder production processes has been led by the introduction of new materials and new parts manufacturing processes. Most of the processes being used today for powder production were initially developed in the 1960s and 1970s in response to the demand for different and higher quality materials. The geometric properties of the powder, such as particle size, particle size distribution and particle shape, and the lot-to-lot repeatability of these properties were less crucial to the parts manufacturing processes of the day (e.g. press-and-sinter, high pressure compacting and HIP). During the 1980s and 1990s however, the situation has changed somewhat. Several new parts manufacturing processes have emerged in which the geometric properties of the powder, as listed above, are crucial to repeatable and accurate manufacture of high quality parts. MIM, thermal spray (TS) and metal rapid prototyping (MRP) are three major processes in which the geometric properties of the powder are just as important to the success of the process as the material properties. The Precision Particle Spray (PPSTM) process was developed by Uniform Metal Technologies LLC (UMTTM)to overcome the shortcomings of conventional powder production techniques in meeting the needs of certain market segments. The proprietary process is designed to enable metal powders to be made with a precisely tailored particle size and particle size distribution. The process also allows precise duplication of these properties from lot to lot. UMT
and the PPS
UMT was founded in 1997 by two Massachusetts Institute of Technology (MIT) graduates: Ali Alagheband (president and director of marketing) and Chris Brown (CEO and director of technology). They obtained their Masters degrees in Mechanical Engineering in 1996 from MIT, where they both focused their studies on manufacturing and product development. UMT’s pilot plant is situated in Worcester, Massachusetts. The production facility will be fully operational by mid-October, and has the capability of producing ferrous and non-ferrous spherical metal powders down to 15 pm in size. In order to finance UMT’s plant, funds have been raised from individual investors. MIT is also a shareholder of the company. The aim of the company is to produce high quality and customized spherical metal powders for use in industries such as filtration, metal injection moulding, rapid prototyping and thermal spray coatings. In UMT’s PPS process, a laminar jet of liquid metal is broken into uniform droplets by a mechanically applied vibration in the melt. This technique is very similar to that used in ink jet printers (the continuous jet type, not
Pressure, II
Ill1
P
1 Ii
1 II
Molten Metal
Jet Velocity, vj
Wavelength,
h L
FIGURE 2: Schematic of the break-up of a laminar jet due fo capillary instability. the drop-on-demand type). Obviously, the apparatus has to be substantially modified to create a jet of liquid metal! As a liquid jet issues from an orifice, droplets are formed by various processes depending on the jet velocity, v. For a very slow jet, the break-up is dominated by gravity (drop-on-demand). In an intermediate flow regime (moderate jet speeds), break-up is dominated by the surface tension (capillary force) of the jet itself For an extremely fast jet, break-up is dominated by aerodynamic surface friction with the surrounding medium. In a sense, conventional gas atomization creates powder through the third mechanism. In that process, a turbulent jet of liquid metal is created by pouring metal through a nozzle. A high-pressure gas jet breaks up this stream into randomly sized droplets. High relative velocities between the metal jet and the ‘ambient’ gas are created by this high-pressure gas jet. The PPS process, however, operates in the second regime, where the metal stream has a moderate speed relative to the ambient medium. In this regime, the break-up of the jet is dominated by capillary instability. A liquid jet will always have a tendency to break up into droplets because the surface energy of a sphere of liquid is lower than that of a cylinder of the same volume. The phenomenon of jet break-up due to capillary instability has been a significant research topic for over a century. English physicist Lord Rayleigh was the first to study and mathematically model the phenomenon in 1878. He theorized that, if a laminar jet is perturbed by an infinitesimal amount at the exit of the nozzle, the disturbance will grow
MPR November 1998 31
FIGURE 3: Tin powder produced by the PPS process with size distribution controlled to 25%.
particle is within about 3% of the mean diameter. A feedback control system implemented in the laboratory has brought this variance down to about 1.5%. Smaller orifices are required to make finer powders. Hence, the pressure required to create a liquid jet increases as the desired particle size decreases. Current pressure capabilities therefore limit the lowest particle size to about 15 pm. The primary advantage of the PPS over conventional techniques is its ability to produce particular size cuts of powder on demand. Conventional gas atomization, by contrast, produces a large range of particle sizes, only some of which might be in demand at a given time. This forces powder producers to plan carefully, since their supply curve of particle size versus quantity is a normal distribution around a mean which is only somewhat controllable, whereas their demand curve may look very much different to a lognormal distribution. To date, the PPS process has been used to make tin, aluminium and copper powders (Figures 3 and 4). The maximum particle size has been 750 pm and the minimum particle size has been 30 pm. UMT’s pilot production machine, however, will be able to produce particles down to 15 pm in diameter of materials with melting temperatures as high as 1600°C.
Controlled geometries
FIGURE 4: Uniform 78 pm diameter particles of Sn-37Pb produced by the PPS process using a 50 pm orifice. exponentially with time and sinusoidally with space. The growth factor of the disturbance depends on the density, viscosity and surface tension of the liquid, as well as the diameter of the jet. A schematic of this break-up is shown in Figure 2. Any laminar jet will break up into randomly sized particles due to disturbances naturally present in the system (surface irregularities or burrs in the nozzle, fluctuations in the ambient medium, etc.). If, however, a disturbance is imposed on the jet that is much larger than any of the random disturbances present, the break-up of the jet can be controlled. This is the operating principle of the PPS process. By varying the diameter and velocity of the liquid metal jet, as well as the frequency of the imposed vibratory disturbance, droplets of any size can be produced. Uniform powders, where all the particles are the same size, can be produced by holding all the process parameters constant. Powders with specific particle size distributions can be produced by altering the process parameters, and by blending uniform powders of different sizes. Experimentally, the PPS process can produce powders in which the diameter of each
32 MPR November 1998
As stated earlier, many of the recent advanced PM manufacturing processes require powders with controlled and repeatable geometric characteristics. Three of these are thermal spray, rapid prototyping and metal injection moulding. Thermal spray: In a powder based thermal spray process, powder is fed into a plasma or high velocity oxy fuel (HVOF) jet and sprayed onto a substrate to impart improved properties such as hardness, erosion and corrosion resistance, good lubricity, or improved electrical properties. Such processes are used, for example, to put wear-resistant coatings of tungsten carbide cobalt alloy on turbine blades for aircraft engines. The typical powder size desired ranges from 50-100 pm depending on the plasma ‘gun’ used and manufacturer preferences. To obtain a proper coating, each particle of powder should reach the substrate in the same thermal state: a ‘mushy’ state in which each particle is part solid, part liquid. Particle-to-particle variations in thermal state are due primarily to non-uniformities in the powder. Particles that are too large, for instance, are too solid upon reaching the substrate and can cause gaps and other errors in the coating. Particles that are too small, on the other hand, are vaporized before even reaching the substrate. In addition to both of these phenomena, the variation in particle size distribution from lot-to-lot changes the flow characteristics of the powder, forcing users to calibrate their sprayers for each new lot of powder.
Rapid prototyping: There are over ten companies and institutions working on commercially viable MRP processes. A few, such as DTM Corp (Austin, TX, USA), already have working models on the market. Each of these processes makes different demands on the powder used. Some are optimized using uniform powders for accurate section definition and to improve accuracy and surface finish, and some prefer a tight bimodal size distribution to optimize packing efficiency and therefore achieve high final densities. Still others depend on accurate and repeatable powder flow characteristics, something that can only be achieved by controlling the particle size, shape and size distribution from lot to lot. Metal injection moulding: One of the primary problems confronting MIM is the availability of cost-effective fine powders. Like MRP, each MIM manufacturer has its own notions of the optimal powder particle size distribution for best results depending on their machinery and mould designs. Additionally, repeatable powder characteristics, though not the only factor affecting MIM part yields, would assist in making MIM a more reliable and repeatable process. For example, keeping powder/binder mixture flow properties, such as viscosity, constant from one lot to the next is crucial tq maintaining quality parts and high part yields. It is known that changes in powder size and size distribution change the viscosity markedly. Changes in size distribution from lot to lot, even given a constant mean particle diameter, will alter the packing density and surface activity of the lot. This will in turn affect binder usage (through powder loading), debinding characteristics, shrinkage, sintering, final density and surface finish. If these properties are to be controlled, regulating the particle size distribution is a necessary first step. UMT is currently scheduling experiments with various companies to test the suitability of its powders in each of these processes. These trials will begin in November. As Robert W Cardy, CEO and chairman of Carpenter Technology (Reading, PA, USA), said in his keynote address to this year’s PM2TEC meeting in Las Vegas: “Wouldn’t it be wonderful to be able to produce the exact particle size desired? Think of the cost savings.. Maybe these things are possible in the next century.” UMT’s precision particle spray process looks set to fulfill this dream before the century turns. Contact: Ali Alagheband and Chris Bmwn Uniform Metal Technologies LLC 60 Prescott Street Worcester MA 01605, USA. Tel: +l-508-831-7020, ext. 611; Fax: +l-508-831-7215; E-mail: alialagh@,europa.com; brownurn@ mindspringcom ?? URL: www.umtpowders.com
WE HAVE A COMMON TARGET
Quality coupled with efficiency is everyone’s target. Pometon and its Customers have achieved this by following sophisticated statistical controls and investing in the latest technology. These combined efforts guarantee success. Both your success and ours.
A wide range of iron, copper (electrolytic and atomised), bronze (fully alloyed and pre-mixed), brass, tin, zinc, lead powders is produced under the above concept.
Pometon MFTAL POWDERS AND GRANULES POMETON Via Cirtonvollozione,
5p.A. - I-30030 Moerne (Venice) Italy Tel. t39.41.2903611 Faxt39.41 h41624
62
MPR
November 1998 33
0
VW the history of the powder metallurgy (PM) industry, many ways of producing metal powder have been developed. At present, powders of various materials can be purchased that have been produced by such diverse processes as gas atomization (a process with many variationsj,
r
Gas atomization
t Water atomization Centrifugal (spinning cup) atomization Vacuum atomization ‘Tandem’ atomization
IL
Close-coupled Free fall High pressure EIGA PIGA Consumable electrode
Melt spinning
’F/-f
y--p15ns Hydrometallurgical
techniques J
FIGURE 1: Major commercially available metal powder production processes.
30 MPR November
1998
0026-0657/98/US$19.00
water atomization, mechanical attrition and alloying, melt spinning, the rotating electrode process (REP), and a wide variety of chemical reactions producing powder precipitates and by-products. Each of these processes has several variations and permutations, designed to alter the material properties or the geometric properties (e.g. mean particle size, particle shape) of the final product. Looking at Figure 1, we see the diversity of powder producing processes. An obvious question is: why are there so many ways to produce what would seem to be a rather simple product? Historically, many of these processes were developed to meet the unique processing needs of various materials (such as steel, tungsten carbide, superalloys, etc.) and those of various parts manufacturing processes (such .as press-and-sinter, highpressure compaction, hot isostatic pressing (HIP), etc.). As an example of this historical process, let us look at the development of some of the current commonly used powder production processes. By the early 1960s water atomization was the main method used to make the irregularly shaped particles of various grades of steel for classic press-and-sinter PM. However, it is not easy to make clean, spherical, oxide-free and impurity-free powders of reactive alloys such as titanium or any of the nickel-based superalloys by this method. The need for these powders in the HIPing of superalloy parts for the aerospace industry led to the development of the rotating electrode process by Nuclear Metals (now Starmet) of Concord, MA, USA, in the late 1960s and early 1970s. In addition to REP, inert gas atomization (IGA) and vacuum atomization were also developed through the mid-1970s as responses to the need for HIPed titanium and superalloy parts for the aerospace industry. At the same time, other groups were working on the parts manufacturing process now known as metal injection moulding (MIM). By the early 198Os, MIM had attracted a great deal of interest for its ability to create intricately detailed stainless steel parts without compaction. However, the process required very fine powder as a feedstock. At the time, interest in very fine powder production was primarily due to interest in rapidly solidified microstructures. As MIM started to grow, powder producers became aware of the market’s potential to consume a great deal of very fine powder and began to alter their processes to create higher yields. Those with inert gas atomization processes met with the
Copyright
0 1998,
Elsevier Science Ltd All rights reserved
most success at adapting their process to produce powders suitable for MIM feedstock. These developments are a good example of how the evolution of powder production processes has been led by the introduction of new materials and new parts manufacturing processes. Most of the processes being used today for powder production were initially developed in the 1960s and 1970s in response to the demand for different and higher quality materials. The geometric properties of the powder, such as particle size, particle size distribution and particle shape, and the lot-to-lot repeatability of these properties were less crucial to the parts manufacturing processes of the day (e.g. press-and-sinter, high pressure compacting and HIP). During the 1980s and 1990s however, the situation has changed somewhat. Several new parts manufacturing processes have emerged in which the geometric properties of the powder, as listed above, are crucial to repeatable and accurate manufacture of high quality parts. MIM, thermal spray (TS) and metal rapid prototyping (MRP) are three major processes in which the geometric properties of the powder are just as important to the success of the process as the material properties. The Precision Particle Spray (PPSTM) process was developed by Uniform Metal Technologies LLC (UMTTM)to overcome the shortcomings of conventional powder production techniques in meeting the needs of certain market segments. The proprietary process is designed to enable metal powders to be made with a precisely tailored particle size and particle size distribution. The process also allows precise duplication of these properties from lot to lot. UMT
and the PPS
UMT was founded in 1997 by two Massachusetts Institute of Technology (MIT) graduates: Ali Alagheband (president and director of marketing) and Chris Brown (CEO and director of technology). They obtained their Masters degrees in Mechanical Engineering in 1996 from MIT, where they both focused their studies on manufacturing and product development. UMT’s pilot plant is situated in Worcester, Massachusetts. The production facility will be fully operational by mid-October, and has the capability of producing ferrous and non-ferrous spherical metal powders down to 15 pm in size. In order to finance UMT’s plant, funds have been raised from individual investors. MIT is also a shareholder of the company. The aim of the company is to produce high quality and customized spherical metal powders for use in industries such as filtration, metal injection moulding, rapid prototyping and thermal spray coatings. In UMT’s PPS process, a laminar jet of liquid metal is broken into uniform droplets by a mechanically applied vibration in the melt. This technique is very similar to that used in ink jet printers (the continuous jet type, not
Pressure, II
Ill1
P
1 Ii
1 II
Molten Metal
Jet Velocity, vj
Wavelength,
h L
FIGURE 2: Schematic of the break-up of a laminar jet due fo capillary instability. the drop-on-demand type). Obviously, the apparatus has to be substantially modified to create a jet of liquid metal! As a liquid jet issues from an orifice, droplets are formed by various processes depending on the jet velocity, v. For a very slow jet, the break-up is dominated by gravity (drop-on-demand). In an intermediate flow regime (moderate jet speeds), break-up is dominated by the surface tension (capillary force) of the jet itself For an extremely fast jet, break-up is dominated by aerodynamic surface friction with the surrounding medium. In a sense, conventional gas atomization creates powder through the third mechanism. In that process, a turbulent jet of liquid metal is created by pouring metal through a nozzle. A high-pressure gas jet breaks up this stream into randomly sized droplets. High relative velocities between the metal jet and the ‘ambient’ gas are created by this high-pressure gas jet. The PPS process, however, operates in the second regime, where the metal stream has a moderate speed relative to the ambient medium. In this regime, the break-up of the jet is dominated by capillary instability. A liquid jet will always have a tendency to break up into droplets because the surface energy of a sphere of liquid is lower than that of a cylinder of the same volume. The phenomenon of jet break-up due to capillary instability has been a significant research topic for over a century. English physicist Lord Rayleigh was the first to study and mathematically model the phenomenon in 1878. He theorized that, if a laminar jet is perturbed by an infinitesimal amount at the exit of the nozzle, the disturbance will grow
MPR November 1998 31
FIGURE 3: Tin powder produced by the PPS process with size distribution controlled to 25%.
particle is within about 3% of the mean diameter. A feedback control system implemented in the laboratory has brought this variance down to about 1.5%. Smaller orifices are required to make finer powders. Hence, the pressure required to create a liquid jet increases as the desired particle size decreases. Current pressure capabilities therefore limit the lowest particle size to about 15 pm. The primary advantage of the PPS over conventional techniques is its ability to produce particular size cuts of powder on demand. Conventional gas atomization, by contrast, produces a large range of particle sizes, only some of which might be in demand at a given time. This forces powder producers to plan carefully, since their supply curve of particle size versus quantity is a normal distribution around a mean which is only somewhat controllable, whereas their demand curve may look very much different to a lognormal distribution. To date, the PPS process has been used to make tin, aluminium and copper powders (Figures 3 and 4). The maximum particle size has been 750 pm and the minimum particle size has been 30 pm. UMT’s pilot production machine, however, will be able to produce particles down to 15 pm in diameter of materials with melting temperatures as high as 1600°C.
Controlled geometries
FIGURE 4: Uniform 78 pm diameter particles of Sn-37Pb produced by the PPS process using a 50 pm orifice. exponentially with time and sinusoidally with space. The growth factor of the disturbance depends on the density, viscosity and surface tension of the liquid, as well as the diameter of the jet. A schematic of this break-up is shown in Figure 2. Any laminar jet will break up into randomly sized particles due to disturbances naturally present in the system (surface irregularities or burrs in the nozzle, fluctuations in the ambient medium, etc.). If, however, a disturbance is imposed on the jet that is much larger than any of the random disturbances present, the break-up of the jet can be controlled. This is the operating principle of the PPS process. By varying the diameter and velocity of the liquid metal jet, as well as the frequency of the imposed vibratory disturbance, droplets of any size can be produced. Uniform powders, where all the particles are the same size, can be produced by holding all the process parameters constant. Powders with specific particle size distributions can be produced by altering the process parameters, and by blending uniform powders of different sizes. Experimentally, the PPS process can produce powders in which the diameter of each
32 MPR November 1998
As stated earlier, many of the recent advanced PM manufacturing processes require powders with controlled and repeatable geometric characteristics. Three of these are thermal spray, rapid prototyping and metal injection moulding. Thermal spray: In a powder based thermal spray process, powder is fed into a plasma or high velocity oxy fuel (HVOF) jet and sprayed onto a substrate to impart improved properties such as hardness, erosion and corrosion resistance, good lubricity, or improved electrical properties. Such processes are used, for example, to put wear-resistant coatings of tungsten carbide cobalt alloy on turbine blades for aircraft engines. The typical powder size desired ranges from 50-100 pm depending on the plasma ‘gun’ used and manufacturer preferences. To obtain a proper coating, each particle of powder should reach the substrate in the same thermal state: a ‘mushy’ state in which each particle is part solid, part liquid. Particle-to-particle variations in thermal state are due primarily to non-uniformities in the powder. Particles that are too large, for instance, are too solid upon reaching the substrate and can cause gaps and other errors in the coating. Particles that are too small, on the other hand, are vaporized before even reaching the substrate. In addition to both of these phenomena, the variation in particle size distribution from lot-to-lot changes the flow characteristics of the powder, forcing users to calibrate their sprayers for each new lot of powder.
Rapid prototyping: There are over ten companies and institutions working on commercially viable MRP processes. A few, such as DTM Corp (Austin, TX, USA), already have working models on the market. Each of these processes makes different demands on the powder used. Some are optimized using uniform powders for accurate section definition and to improve accuracy and surface finish, and some prefer a tight bimodal size distribution to optimize packing efficiency and therefore achieve high final densities. Still others depend on accurate and repeatable powder flow characteristics, something that can only be achieved by controlling the particle size, shape and size distribution from lot to lot. Metal injection moulding: One of the primary problems confronting MIM is the availability of cost-effective fine powders. Like MRP, each MIM manufacturer has its own notions of the optimal powder particle size distribution for best results depending on their machinery and mould designs. Additionally, repeatable powder characteristics, though not the only factor affecting MIM part yields, would assist in making MIM a more reliable and repeatable process. For example, keeping powder/binder mixture flow properties, such as viscosity, constant from one lot to the next is crucial tq maintaining quality parts and high part yields. It is known that changes in powder size and size distribution change the viscosity markedly. Changes in size distribution from lot to lot, even given a constant mean particle diameter, will alter the packing density and surface activity of the lot. This will in turn affect binder usage (through powder loading), debinding characteristics, shrinkage, sintering, final density and surface finish. If these properties are to be controlled, regulating the particle size distribution is a necessary first step. UMT is currently scheduling experiments with various companies to test the suitability of its powders in each of these processes. These trials will begin in November. As Robert W Cardy, CEO and chairman of Carpenter Technology (Reading, PA, USA), said in his keynote address to this year’s PM2TEC meeting in Las Vegas: “Wouldn’t it be wonderful to be able to produce the exact particle size desired? Think of the cost savings.. Maybe these things are possible in the next century.” UMT’s precision particle spray process looks set to fulfill this dream before the century turns. Contact: Ali Alagheband and Chris Bmwn Uniform Metal Technologies LLC 60 Prescott Street Worcester MA 01605, USA. Tel: +l-508-831-7020, ext. 611; Fax: +l-508-831-7215; E-mail: alialagh@,europa.com; brownurn@ mindspringcom ?? URL: www.umtpowders.com
WE HAVE A COMMON TARGET
Quality coupled with efficiency is everyone’s target. Pometon and its Customers have achieved this by following sophisticated statistical controls and investing in the latest technology. These combined efforts guarantee success. Both your success and ours.
A wide range of iron, copper (electrolytic and atomised), bronze (fully alloyed and pre-mixed), brass, tin, zinc, lead powders is produced under the above concept.
Pometon MFTAL POWDERS AND GRANULES POMETON Via Cirtonvollozione,
5p.A. - I-30030 Moerne (Venice) Italy Tel. t39.41.2903611 Faxt39.41 h41624
62
MPR
November 1998 33