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Dynamic testing for secure powder characterisation
special feature
Dynamic testing for secure powder characterisation Modern powder characterisation techniques now extend well beyond the tradition of shear testing, bringing insight and information to support the needs of metal powder processors.
F
or many, powder characterisation still suggests shear cell analysis; an approach pioneered in the 1960’s. However, in the intervening decades, our need to understand, manipulate and control powder behaviour has grown significantly. Today’s powder metallurgists handle the broadest of product portfolios, and face exacting demands in terms of the powder properties delivered, with flowability being especially important. Those working at the cutting edge of powder engineering increasingly recognise the limitations of shear testing, and other traditional techniques, and the need to apply newer methods to secure more comprehensive and relevant powder characterisation. One such method is dynamic powder testing, which directly measures the flow properties of consolidated, conditioned, aerated and even fluidised powders. With a proven capability of delivering process relevant information, it complements shear, and indeed bulk property measurement, providing insight that is not otherwise accessible. Applied together, these three methodologies deliver an unprecedented level of information about the behaviour of metal powders — information that can be used to engineer a product towards market-defining quality.
Making powders flow The ecological credentials and cost efficiency of powder metallurgy con-
16
MPR January/February 2013
Figure 1: Detect the difference. Shear data characterise these two titanium dioxide samples as identical, while dynamic data highlight a clear difference.
0026-0657/13 ©2013 Elsevier Ltd. All rights reserved
tinue to promote its steady growth, in terms of both new materials and new applications. The first step in any powder metallurgy process is to produce a blend of elemental or alloy powder, as well as any necessary additives that will ultimately deliver a component of the requisite quality. The flowability of the blend constituents, alone and in combination, is crucial since it directly influences the efficiency of this front end of the process, up to and including die filling. Powders with optimal flow characteristics will not only process smoothly, maximising plant throughput, but will also fill the die efficiently, to produce a high-quality, blemish-free finished component. As a result, although additives are frequently incorporated to enhance flow properties, the inherent characteristics of the metal powder are key factors that directly influence its suitability for any specific application and consequently its market value. For a powder to flow, the particles within it must move relative to one another. The ease with which this happens varies considerably and is influenced by a number of factors, including the level of friction between particles, which is a function of properties such as surface texture and chemical composition, and the likelihood of mechanical interlocking. Mechanical interlocking is where particles fit together like pieces of a jigsaw puzzle and exert significant resistance to movement. It is strongly affected by particle shape. The degree of cohesion and adhesion in the system can also be highly influential. Van der Waals forces, electrostatics and the effect of magnetics, amongst others, will all influence cohesivity, the strength with which particles stick to one another. Conversely, adhesive forces arise from interaction between the particles and, for example, process equipment surfaces, as well as from liquid bridging between particles, where powders contain moisture. This complexity makes the prediction of powder flow behaviour a challenge — one that is, on a practical level, beyond current industrial capabilities. The pragmatic alternative is to identify powder testing strategies that quantify flow behaviour in ways that relate to their in-use performance, such that defined specifications give rise to the
metal-powder.net
Figure 2: At any given shoe speed the aluminium powder exhibits better performance than the tungsten, as evidenced by the recorded filling ratios.
in-process behaviour required. This is a task that has exercised powder testing experts for a number of decades.
Assessing different powder characterisation techniques Many traditional powder testing methods, such as flow through an orifice, angle of repose and tapped density, suffer from poor reproducibility. Equipment set-ups and procedures are not standardised, and manual sample preparation and analysis can give rise to operator variability. Equally importantly, however, these tests attempt to classify flow behaviour with just a single figure and experience suggests that this is insufficient and oversimplistic. The net result is that such tests not only exhibit poor sensitivity but also fail to provide descriptors of powder behaviour that are directly relevant for process studies. Powders are most precisely thought of as three component bulk assemblies containing: complex and variable solid particles; a usually unquantified amount of air; and a certain degree of moisture. Powder behaviour is defined by an intricate network of interactions between these components, via the mechanisms previously outlined, and is influenced by a vast array of
parameters. This makes both measurement and processing an exacting challenge, and it means that while single number techniques may provide some insight, they fail to supply the understanding needed by modern processors. The development of shear cells marked an important step forward in testing sophistication and application of the resulting data. Shear testing was developed in combination with a hopper design methodology that remains the benchmark today, and as a result it is still a valuable test method. However, a lack of suitable alternatives has resulted in the application of shear testing well beyond the initial intent. Over the years, the reproducibility of shear testing has undoubtedly improved through the steady refinement of instrument design and test methodologies, but that does not mean it has evolved into an ideal tool for process-related powder characterisation. Shear testing determines how a consolidated powder will transition from a static state into a dynamic regime, and this lends itself well to hopper behaviour with which it is now synonymous. However, it is less relevant when considering how powders behave in a loosely packed state or once in that dynamic regime. Furthermore, shear testing can still fail to detect minor differences that influence processability, especially between relatively free-flowing powders. Dynamic powder testing is arguably the most recent powder testing
January/February 2013 MPR
17
breakthrough with proven industrial merit. It involves generating values of flow energy from measurements of the axial and rotational forces acting on a blade as it passes through a powder sample along a defined path. Rotating the blade downwards pushes the powder against the base of the test vessel, applying a compacting force, while an upward traverse produces a low-stress, lifting action. The flow energies that are measured, Basic Flowability Energy (BFE) and Specific Energy (SE), respectively, directly quantify how easily the powder flows under different conditions and have been shown to reliably correlate with process performance.
Dynamic testing has the intrinsic appeal of evaluating the powder in motion. Furthermore, for manufacturers it offers the capability to apply test conditions that simulate the different stresses that exist within the process environment to directly assess how the powder will respond in-plant. Powders can be tested in an aerated or even fluidised state to directly quantify the impact of air on the powder: how flow changes as the powder picks up air and how rapidly the impact of entrained air dissipates on standing. Furthermore, dynamic testing is sufficiently sensitive to provide effective differentiation when techniques such as shear testing reach their limits.
Choosing between shear and dynamic testing Figure 1 shows shear and dynamic test data for two samples of titanium dioxide. The shear test data suggest that the two samples are identical, but flow energy data detects clear differences. These data suggest that in certain processing environments — for example, when subject to moderately high stress in a hopper, these two samples are likely to behave in a similar way. However, in other circumstances, such as die filling, mixing or gravitational flow, their performance will differ. This variation would not be detected by shear testing alone and could ultimately lead to in-process problems or variation in product quality. This simple comparison highlights the benefits of applying a multi-faceted powder testing strategy to generate a selection of properties that capture the full behavioural profile of a powder. Once this is implemented it becomes possible to identify those variables that define performance in a specific unit operation so that analysis can be focused in the most productive way. The following case study demonstrates this approach in more detail.
Case Study: Applying a multi-faceted powder testing strategy to investigate die filling performance
Figure 3: Aeration and compaction both impact the performance of aluminium powder. The tungsten in contrast performs poorly when compacted but is relatively unaffected by aeration.
18
MPR January/February 2013
An experimental study was carried out to compare the die-filling performance of tungsten (Dv 50 4μm, angular shape) and aluminium (Dv50 134μm, irregular shape) powders. The small scale rig used mimics a commercial operation and consists of a stationary die (volume 10ml, diameter 25 mm) and a motordriven powder-filled shoe that moves at a controlled velocity of between 50 and 300 mm/s. Filling ratios, the actual mass of powder in the die following a single pass of the shoe, compared to a mass calculated from the die volume and bulk density of the powder, were determined for both materials as a
metal-powder.net
function of shoe velocity. The results show that the aluminium powder processes better at all shoe speeds than the tungsten, which can be processed successfully, but only at a low shoe speed, which equates to a productivity constraint. In further studies, the impact of the initial packing state of the powder was also investigated through the use of aerated, conditioned and compacted feeds. The compacted feed was produced by tapping the sample 20 times, while a lightly aerated state was reached by passing air through the material at a velocity of 20 mm/s (aluminium) and 10 mm/s (tungsten), immediately before loading the shoe. Aeration was found to improve the filling ratio of aluminium relative to the conditioned sample, while compaction caused a reduction in process efficiency. With tungsten, the effect of compaction is more dramatic, but aeration has a negligible impact (see Figure 3). Both powders were fully characterized using an FT4 Powder Rheometer™ (Table 1) to determine a rationale for all the observed behaviours (see reference 1 for method details). The flow energy parameters, BFE and SE, indicate that tungsten is less free-flowing than aluminium. The measured shear stress values provide little differentiation between the two powders. It can be argued that the calculated values of cohesion and unconfined yield strength indicate that shear analysis can, indirectly, detect differences between the samples. However, these derived values are often subject to greater variability, and significant discrepancies can be introduced by the analysis technique itself. Furthermore, what is clear is that shear analysis is unable to provide any insight into the impact of process variables such as the initial packing state. Developing an understanding of the impact of the initial packing state of the feed focuses attention specifically on dynamic and bulk powder properties. The Aeration Ratio (AR), the ratio of aerated to conditioned flow energy, is far higher for aluminium than for tungsten, while the Consolidation Index is lower. This suggests that the low flow energy of aluminium will be reduced dramatically by aeration and that the
metal-powder.net
Table 1: Contrasting the properties of aluminium and tungsten powders Measurements:
Aluminium
Tungsten
Conditioned Bulk Density, CBD (g/ml)
1.24
4.17
Basic Flowability Energy, BFE (mJ)
3300
5964
Specific Energy, SE (mJ/g)
4.4
6.7
Aeration Ratio, AR
172
26
Pressure Drop across the powder bed at 2 mm/s air velocity, PD15 (mbar)
1.4
15.3
Consolidation Index, CI20Taps - factor by which flow energy increases after tapping, relative to BFE
1.43
2.32
Bulk Density after consolidation with 20 taps, (g/ml)
1.34
4.97
Compressibility - volume change after consolidation with 15kPa direct pressure (%)
3.5
11.1
Shear Stress, τ2 (kPa)
1.57
1.64
Unconfined Yield Strength (kPa)
0.45
1.09
Cohesion, Co (kPa)
0.120
0.297
11.6
5.5
Flow Function, FF – from 3kPa shear test already high flow energy of tungsten will be increased considerably higher by compaction. Both effects directly mirror the experimental observations made in terms of the effect of initial packing state. Permeability data help to elucidate an understanding of the impact of aeration. The fine, relatively cohesive tungsten powder is highly resistant to the passage of air relative to the coarser aluminium which exhibits much weaker interparticulate bonding. With the tungsten the air channels through the sample, rather than lubricating each particle, diminishing its impact on flow energy. The aluminium particles, in contrast, can be individually separated by the air due to the lower strength of the cohesive bonds, which ultimately fluidises the bed, causing a dramatic reduction in flow energy that translates to improved die-filling performance. In addition, the greater permeability of aluminium results in relatively rapid air release once the powder is in the die, and this further enhances the efficiency of the die-filling process.
Conclusion Although there are many powder testing techniques in use today, relatively few provide the information needed to
comprehensively understand powder behaviour across a range of different processes and applications. For metal powder producers, this is essential since engineering products that process reliably and go on to deliver components of the necessary quality is crucial for success. Dynamic powder testing is a proven technique for process-related powder characterisation, and when used in combination with shear and bulk property testing provides a secure basis for highly informative powder testing strategies. The case study presented here illustrates the application of this multifaceted approach and demonstrates the insight it can provide for those developing and manufacturing metal powders that deliver exemplary performance.
References [1] R Freeman, “Measuring the flow properties of consolidated, conditioned and aerated powders — A comparative study using a powder rheometer and a rotational shear cell,” Powder Technology, 2007, vol. 174, pp. 25-33. [2] R. Freeman and X Fu “Understanding die filling performance and its importance on final components” Powder Metallurgy 2008, Vol 51 pp 196-201.
January/February 2013 MPR
19
Dynamic testing for secure powder characterisation Modern powder characterisation techniques now extend well beyond the tradition of shear testing, bringing insight and information to support the needs of metal powder processors.
F
or many, powder characterisation still suggests shear cell analysis; an approach pioneered in the 1960’s. However, in the intervening decades, our need to understand, manipulate and control powder behaviour has grown significantly. Today’s powder metallurgists handle the broadest of product portfolios, and face exacting demands in terms of the powder properties delivered, with flowability being especially important. Those working at the cutting edge of powder engineering increasingly recognise the limitations of shear testing, and other traditional techniques, and the need to apply newer methods to secure more comprehensive and relevant powder characterisation. One such method is dynamic powder testing, which directly measures the flow properties of consolidated, conditioned, aerated and even fluidised powders. With a proven capability of delivering process relevant information, it complements shear, and indeed bulk property measurement, providing insight that is not otherwise accessible. Applied together, these three methodologies deliver an unprecedented level of information about the behaviour of metal powders — information that can be used to engineer a product towards market-defining quality.
Making powders flow The ecological credentials and cost efficiency of powder metallurgy con-
16
MPR January/February 2013
Figure 1: Detect the difference. Shear data characterise these two titanium dioxide samples as identical, while dynamic data highlight a clear difference.
0026-0657/13 ©2013 Elsevier Ltd. All rights reserved
tinue to promote its steady growth, in terms of both new materials and new applications. The first step in any powder metallurgy process is to produce a blend of elemental or alloy powder, as well as any necessary additives that will ultimately deliver a component of the requisite quality. The flowability of the blend constituents, alone and in combination, is crucial since it directly influences the efficiency of this front end of the process, up to and including die filling. Powders with optimal flow characteristics will not only process smoothly, maximising plant throughput, but will also fill the die efficiently, to produce a high-quality, blemish-free finished component. As a result, although additives are frequently incorporated to enhance flow properties, the inherent characteristics of the metal powder are key factors that directly influence its suitability for any specific application and consequently its market value. For a powder to flow, the particles within it must move relative to one another. The ease with which this happens varies considerably and is influenced by a number of factors, including the level of friction between particles, which is a function of properties such as surface texture and chemical composition, and the likelihood of mechanical interlocking. Mechanical interlocking is where particles fit together like pieces of a jigsaw puzzle and exert significant resistance to movement. It is strongly affected by particle shape. The degree of cohesion and adhesion in the system can also be highly influential. Van der Waals forces, electrostatics and the effect of magnetics, amongst others, will all influence cohesivity, the strength with which particles stick to one another. Conversely, adhesive forces arise from interaction between the particles and, for example, process equipment surfaces, as well as from liquid bridging between particles, where powders contain moisture. This complexity makes the prediction of powder flow behaviour a challenge — one that is, on a practical level, beyond current industrial capabilities. The pragmatic alternative is to identify powder testing strategies that quantify flow behaviour in ways that relate to their in-use performance, such that defined specifications give rise to the
metal-powder.net
Figure 2: At any given shoe speed the aluminium powder exhibits better performance than the tungsten, as evidenced by the recorded filling ratios.
in-process behaviour required. This is a task that has exercised powder testing experts for a number of decades.
Assessing different powder characterisation techniques Many traditional powder testing methods, such as flow through an orifice, angle of repose and tapped density, suffer from poor reproducibility. Equipment set-ups and procedures are not standardised, and manual sample preparation and analysis can give rise to operator variability. Equally importantly, however, these tests attempt to classify flow behaviour with just a single figure and experience suggests that this is insufficient and oversimplistic. The net result is that such tests not only exhibit poor sensitivity but also fail to provide descriptors of powder behaviour that are directly relevant for process studies. Powders are most precisely thought of as three component bulk assemblies containing: complex and variable solid particles; a usually unquantified amount of air; and a certain degree of moisture. Powder behaviour is defined by an intricate network of interactions between these components, via the mechanisms previously outlined, and is influenced by a vast array of
parameters. This makes both measurement and processing an exacting challenge, and it means that while single number techniques may provide some insight, they fail to supply the understanding needed by modern processors. The development of shear cells marked an important step forward in testing sophistication and application of the resulting data. Shear testing was developed in combination with a hopper design methodology that remains the benchmark today, and as a result it is still a valuable test method. However, a lack of suitable alternatives has resulted in the application of shear testing well beyond the initial intent. Over the years, the reproducibility of shear testing has undoubtedly improved through the steady refinement of instrument design and test methodologies, but that does not mean it has evolved into an ideal tool for process-related powder characterisation. Shear testing determines how a consolidated powder will transition from a static state into a dynamic regime, and this lends itself well to hopper behaviour with which it is now synonymous. However, it is less relevant when considering how powders behave in a loosely packed state or once in that dynamic regime. Furthermore, shear testing can still fail to detect minor differences that influence processability, especially between relatively free-flowing powders. Dynamic powder testing is arguably the most recent powder testing
January/February 2013 MPR
17
breakthrough with proven industrial merit. It involves generating values of flow energy from measurements of the axial and rotational forces acting on a blade as it passes through a powder sample along a defined path. Rotating the blade downwards pushes the powder against the base of the test vessel, applying a compacting force, while an upward traverse produces a low-stress, lifting action. The flow energies that are measured, Basic Flowability Energy (BFE) and Specific Energy (SE), respectively, directly quantify how easily the powder flows under different conditions and have been shown to reliably correlate with process performance.
Dynamic testing has the intrinsic appeal of evaluating the powder in motion. Furthermore, for manufacturers it offers the capability to apply test conditions that simulate the different stresses that exist within the process environment to directly assess how the powder will respond in-plant. Powders can be tested in an aerated or even fluidised state to directly quantify the impact of air on the powder: how flow changes as the powder picks up air and how rapidly the impact of entrained air dissipates on standing. Furthermore, dynamic testing is sufficiently sensitive to provide effective differentiation when techniques such as shear testing reach their limits.
Choosing between shear and dynamic testing Figure 1 shows shear and dynamic test data for two samples of titanium dioxide. The shear test data suggest that the two samples are identical, but flow energy data detects clear differences. These data suggest that in certain processing environments — for example, when subject to moderately high stress in a hopper, these two samples are likely to behave in a similar way. However, in other circumstances, such as die filling, mixing or gravitational flow, their performance will differ. This variation would not be detected by shear testing alone and could ultimately lead to in-process problems or variation in product quality. This simple comparison highlights the benefits of applying a multi-faceted powder testing strategy to generate a selection of properties that capture the full behavioural profile of a powder. Once this is implemented it becomes possible to identify those variables that define performance in a specific unit operation so that analysis can be focused in the most productive way. The following case study demonstrates this approach in more detail.
Case Study: Applying a multi-faceted powder testing strategy to investigate die filling performance
Figure 3: Aeration and compaction both impact the performance of aluminium powder. The tungsten in contrast performs poorly when compacted but is relatively unaffected by aeration.
18
MPR January/February 2013
An experimental study was carried out to compare the die-filling performance of tungsten (Dv 50 4μm, angular shape) and aluminium (Dv50 134μm, irregular shape) powders. The small scale rig used mimics a commercial operation and consists of a stationary die (volume 10ml, diameter 25 mm) and a motordriven powder-filled shoe that moves at a controlled velocity of between 50 and 300 mm/s. Filling ratios, the actual mass of powder in the die following a single pass of the shoe, compared to a mass calculated from the die volume and bulk density of the powder, were determined for both materials as a
metal-powder.net
function of shoe velocity. The results show that the aluminium powder processes better at all shoe speeds than the tungsten, which can be processed successfully, but only at a low shoe speed, which equates to a productivity constraint. In further studies, the impact of the initial packing state of the powder was also investigated through the use of aerated, conditioned and compacted feeds. The compacted feed was produced by tapping the sample 20 times, while a lightly aerated state was reached by passing air through the material at a velocity of 20 mm/s (aluminium) and 10 mm/s (tungsten), immediately before loading the shoe. Aeration was found to improve the filling ratio of aluminium relative to the conditioned sample, while compaction caused a reduction in process efficiency. With tungsten, the effect of compaction is more dramatic, but aeration has a negligible impact (see Figure 3). Both powders were fully characterized using an FT4 Powder Rheometer™ (Table 1) to determine a rationale for all the observed behaviours (see reference 1 for method details). The flow energy parameters, BFE and SE, indicate that tungsten is less free-flowing than aluminium. The measured shear stress values provide little differentiation between the two powders. It can be argued that the calculated values of cohesion and unconfined yield strength indicate that shear analysis can, indirectly, detect differences between the samples. However, these derived values are often subject to greater variability, and significant discrepancies can be introduced by the analysis technique itself. Furthermore, what is clear is that shear analysis is unable to provide any insight into the impact of process variables such as the initial packing state. Developing an understanding of the impact of the initial packing state of the feed focuses attention specifically on dynamic and bulk powder properties. The Aeration Ratio (AR), the ratio of aerated to conditioned flow energy, is far higher for aluminium than for tungsten, while the Consolidation Index is lower. This suggests that the low flow energy of aluminium will be reduced dramatically by aeration and that the
metal-powder.net
Table 1: Contrasting the properties of aluminium and tungsten powders Measurements:
Aluminium
Tungsten
Conditioned Bulk Density, CBD (g/ml)
1.24
4.17
Basic Flowability Energy, BFE (mJ)
3300
5964
Specific Energy, SE (mJ/g)
4.4
6.7
Aeration Ratio, AR
172
26
Pressure Drop across the powder bed at 2 mm/s air velocity, PD15 (mbar)
1.4
15.3
Consolidation Index, CI20Taps - factor by which flow energy increases after tapping, relative to BFE
1.43
2.32
Bulk Density after consolidation with 20 taps, (g/ml)
1.34
4.97
Compressibility - volume change after consolidation with 15kPa direct pressure (%)
3.5
11.1
Shear Stress, τ2 (kPa)
1.57
1.64
Unconfined Yield Strength (kPa)
0.45
1.09
Cohesion, Co (kPa)
0.120
0.297
11.6
5.5
Flow Function, FF – from 3kPa shear test already high flow energy of tungsten will be increased considerably higher by compaction. Both effects directly mirror the experimental observations made in terms of the effect of initial packing state. Permeability data help to elucidate an understanding of the impact of aeration. The fine, relatively cohesive tungsten powder is highly resistant to the passage of air relative to the coarser aluminium which exhibits much weaker interparticulate bonding. With the tungsten the air channels through the sample, rather than lubricating each particle, diminishing its impact on flow energy. The aluminium particles, in contrast, can be individually separated by the air due to the lower strength of the cohesive bonds, which ultimately fluidises the bed, causing a dramatic reduction in flow energy that translates to improved die-filling performance. In addition, the greater permeability of aluminium results in relatively rapid air release once the powder is in the die, and this further enhances the efficiency of the die-filling process.
Conclusion Although there are many powder testing techniques in use today, relatively few provide the information needed to
comprehensively understand powder behaviour across a range of different processes and applications. For metal powder producers, this is essential since engineering products that process reliably and go on to deliver components of the necessary quality is crucial for success. Dynamic powder testing is a proven technique for process-related powder characterisation, and when used in combination with shear and bulk property testing provides a secure basis for highly informative powder testing strategies. The case study presented here illustrates the application of this multifaceted approach and demonstrates the insight it can provide for those developing and manufacturing metal powders that deliver exemplary performance.
References [1] R Freeman, “Measuring the flow properties of consolidated, conditioned and aerated powders — A comparative study using a powder rheometer and a rotational shear cell,” Powder Technology, 2007, vol. 174, pp. 25-33. [2] R. Freeman and X Fu “Understanding die filling performance and its importance on final components” Powder Metallurgy 2008, Vol 51 pp 196-201.
January/February 2013 MPR
19