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Lubrication and delubrication at PowderMet 2012
technical trends
Lubrication and delubrication at PowderMet 2012 Dr James McCall, from NuvoSci Consulting, reviews what he considers to be the strongest collection of presentations concerning lubrication and delubrication delivered at PowderMet 2012 in many years.
T
he presentations at PowderMet included are two dedicated sessions, Lubrication I and II comprising five papers and the Special Interest Programme on Delubrication Science, Problems & Development, which entailed eight presentations. A panel discussion at the end of the SIP discussed the current performance and projected lubrication needs for the industry. There were also other papers related to lubrication in other sessions, and highlights of selected presentations are provided in this report.
Metal-free lubricants The performance of a wide variety of potential admixed lubricants was discussed by D Biro (Lonza) in “Evaluation of lubricant chemistries for powder metal applications”, coauthored by R Skillen (Lonza) and D Heaney (CISP). Metal-free lubricants (17) representing four different chemistries were evaluated at 0.75 wt% in straight iron powder at room temperature for their effect on powder, green, ejection and sintered properties. Acrawax C was used as the reference. All lubricants had been ground to a similar particle size distribution (D50=7-14, D90=12-50 μm) to minimise the influence of this property. Compaction was done at 579 MPa; a significant variation in compressibilities led to variation in green densities,
which ranged from 6.91-7.09 g/cm3. Several of new lubricants had better individual properties than the reference Acrawax C, and research is continuing to optimise performance. V Paris (Rio Tinto) covered other admixed lubricants in “Novel highperformance lubricants for conventional and high-density compaction”, coauthored with Y Thomas (NRCC-IMI) and S St-Laurent (Rio Tinto). The lubrication performance of eight admixed composite proprietary lubricant formulations were compared with Kenolube at a level of 0.7 wt% in an iron-copper-carbon composition. Flow rates and apparent densities were significantly poorer than for Kenolube, but the addition of a flow enhancer improved these properties. Compressibilities at room temperature and at 60°C were generally quite similar, but several of the lubricants had slightly better compressibility than Kenolube at 60°C, giving green densities of 7.20 g/cm3 or higher. The ejection performances of the lubricants varied, but lubricant PR-1 was more lubricious than Kenolube at both room temperature and 60°C. Relative to Kenolube, experimental lubricant PR-1 had equal apparent density and flow (with flow enhancer present), somewhat poorer compressibility, but better ejection performance.
Adding nanographite A Tamashausky (Asbury Graphite Mills) discussed nanographite as a
0026-0657/12 ©2012 Elsevier Ltd. All rights reserved
potential admixed lubricant in “The utility of nanographite/near-graphene materials as solid-phase, integral diewall lubricants in PM”. Typical usage levels of conventional graphite are not sufficient to benefit from the good lubricity of this material; simply not enough of the material is located at the die walls. The objective of the research was to explore the use of nanographite alone, or in conjunction with conventional graphite, to act as an efficient die-wall lubricant. Typical natural flake graphite used in PM has a particle size of about 5 μm, which provides a surface area of about 15 m2/g. This contrasts with the natural and synthetic platelet nanographites reported here, with an apparent mean particle size of about 1 micron and a surface area of about 350 m2/g. This is an “apparent” size because it is actually an agglomerate of much finer primary particles. For this reason, a conventional particle size measurement is not useful; instead, the author developed a Lamella Thickness Index (LTI), a metric based on the surface area, to approximate the number of graphene (fully-delaminated graphite) layers present. This gives a measure of the extent of delamination. FC0208 mixes were prepared at in a lab V-blender with graphite, nanographite and lubricant admixed as required. The ejection behavior of 30 tsi-compacted FC0208 mixes containing no admixed lubricant were
July/August 2012 MPR
19
Figure 1. (Courtesy of S. Luk, R.T. Warzel III, P. Hofecker).
compared; the admixed carbon (0.9 wt%) was conventional (9 μm) flake graphite, 350 m2/g nanographite (about 7 graphene layers present), or 400 m2/g nanographite (about 6 graphene layers present): average ejection forces were 13,900, 7200 and 7000 lbs-f, respectively, compared to 4680 lbs-f for the mix containing 0.9 wt% conventional flake graphite with 0.75 wt% ethylene bis stearamide (EBS) wax as lubricant. Experiments were also made with reduced EBS lubricant content. TR bars made with 0.9 wt% nanographites (the same 350 and 400 m2/g ones) and reduced (0.1 wt%) EBS content had lower average ejection forces (2740 and 2660 lbs-f, respectively) than if conventional flake graphite had been used with 0.1 wt% EBS (3320 lbs-f). Thus, the admixed nanographites provided significantly improved lubricity than conventional flake graphite, but still not to the level obtained with admixed EBS wax. The reduced-wax compositions containing the nanographites, however, had lower sintered hardnesses and strengths. The intriguing results of this examination of nanographites warrant additional study.
Warm compaction process S Luk presented “Warm-die compaction process to achieve higher green density and green strength”, co-authored with his N.A. Höganäs colleagues
20
MPR July/August 2012
R.T. Warzel III and P. Hofecker. This paper served as an overview of the role of lubricants in an FC0205 mix in the warm-compaction process. Better mechanical properties derive from higher densities, which can be obtained by using, for example, copper infiltration, high-temperature sintering, doublepress-double-sintering or powder forging. Warm (tool+powder) compaction was introduced as another option in the 1990s, and now warm-die compaction is being promoted. In the 1990s, warm compaction required temperatures of about 120°C. Now, a heated die (6080°C) alone can be used. The lubricant must, at least initially, be a solid but be capable of migrating to the die walls upon compaction. The authors compared a low-melting solid, such as stearic acid (m.p. of about 68°C) with higher-melting polyethylene wax (m.p. 104°C) at 0.75 wt% in an FC0205 composition. Although the compressibilities were very similar at 25°C, they were very different at 75°C, with the PE wax being much less compressible. A very high green strength (29 MPa) was obtained with the 550-MPa-compacted mixture containing the PE wax. Above 550 MPa, and with a warm (60°C) die, the PE wax did not provide sufficient lubricity: ejection was noisy and parts were scored. At a given compaction pressure, green expansion was higher with stearic acid than with PE wax, at both temperatures. The performances of Acrawax C EBS wax and Intralube E in the same FC0205 system were compared.
Compaction was done at various pressures and at die temperatures of 25, 60 and 95°C. Green density, green expansion, green strength and lubricity were measured. The evaluated premixes contained 0.75 wt% lubricant (Mix A=EBS, Mix E3=Intralube E), and reduced loadings of Intralube E (0.30 wt%=Mix E1 and 0.50 wt%=Mix E2). Rank-ordering of the compressibilities changed with compaction temperature. At the highest compaction pressure (690 MPa), mixes E1 and E2 were the most compressible and had similar compressibilities at 25 and 60°C; at 95°C mix E2 had much superior compressibility and was able to reach 7.31 g/cm3. Green strengths of about 13 MPa or higher are desirable for safe handling of green parts; this was achieved for all of the mixes containing Intralube E using warm or hot dies. Green expansion (springback) was quite similar for all the mixes under a given compaction pressures at 25°C, but mixes E2 and E3 had higher springbacks at the highest pressures at 60°C and at all pressures at 95°C. There were marked differences in lubricity between the mixes, both as measured stripping and sliding pressures. At 60°C and the highest compaction pressure, the stripping and sliding pressures with 0.75 wt% EBS were about equal to that for 0.50 wt% Intralube E. The mix containing 0.75 wt% Intralube E had much better lubricity, and indeed had the best lubricity at all die temperatures and compaction pressures.
metal-powder.net
Various lubricant packages were discussed by R Warzel III (N A Höganäs) in “Lubrication for demanding applications”, co-authored by H Rodrigues and R Goto (Engineered Sintered Components). He compared the performance (at 0.75 wt%) of Acrawax, Intralube E, and Starmix Boost to an undisclosed reference (at 0.80 wt%) in an FC0208 system. All mixes contained a machinability additive (SM-3) at 0.2 wt%. The same mixes were also evaluated on a production part: a vane rotor having a green density of 6.85 g/cm3. Flow rates and apparent densities equal to or better than the reference blend, except for Acrawax, which gave lower apparent density and slower flow rate. Compressibilities were very similar up to a green density of about 7.10 g/cm3, above which Intralube E was more compressible. Intralube E and Starmix Boost provided the lowest green expansion. The lowest peak ejection pressure was found for the Starmix Boost mix, about 10% lower than that for the reference. The better lubricity found in the laboratory translated to better part-to-part weight stability and better surface finish in the production part. Higher dimensional change was noted for production parts made with the Starmix Boost, not observed in the laboratory tests. New bonded lubricants were discussed by K McQuaig (Hoeganaes Corp) in “Improved die-fill performance through binder treatment”, co-authored with S Patel and P Sokolowski (Hoeganaes Corp) and
S. Shah, G. Schluterman and J. Falleur (Cloyes Gear & Products). The benefits of bonded lubricants include improved apparent density, flow rate, dusting resistance and mix homogeneity. Four kinds of FC0205 mixes were compared: premix, Ancorbond, and the experimental EXP1-bonded and EXP2-bonded lubricants. Lubricant loading was 0.75 wt%. Testing was done at room temperature and 75°C. The experimental mixes had much higher apparent densities. At 75°C, the four systems had quite similar compressibilities but EXP1 and EXP2 had significantly lower stripping pressures than either the regular premix or the Ancorbond mix (see Figure). In a production trial, an ABS wheel sensor was pressed to a green density of 6.8 and 7.0 g/cm3. Both EXP1 and EXP2 mixes provided better surface finish than either the premix or the Ancorbond mix. No differences were seen in sintered mechanical properties. Bonded mixes were also the subject of the presentation by F Hanejko (Hoeganaes Corp) on “Advances in lubrication technology in PM to promote higher sintered densities”. The first Ancorbond system used an organic binder/lubricant system to improve mix homogeneity, improve powder flow and reduce dusting. Improvement in binders and lubricants led to warm compaction applications with Ancordense and Ancordense 450, using both heated die and heated powder, to achieve high green densities and strengths. The newer Ancormax
Figure 2. K. McQuaig, S. Patel, P. Sokolowski, S. Shah, G. Schluterman, J. Falleur.
metal-powder.net
system requires only a heated die. Hoeganaes has a new metal-free binder/ lubricant under development that can be used at 0.25 wt% loading and requires only die heating, at 107°C. Finally, the consequences of poor control of the delubing process were discussed by J. Engquist (BurgessNorton) in “Methods for measuring and quantifying effective delubrication – a practical approach.” Many quality issues related to sintered products can be traced back to poor delubrication. Implementing process control monitoring of the delube zone can greatly help in troubleshooting. Poor delubrication can lead to problems such as carbon buildup on heat exchangers and shortening the life of belts, for example. In a case study, the cause of short belt life was investigated in detail with the aim to predict and improve the life. A fixed length of belt was measured weekly, and the rate-of-change in length was plotted as a function of lifetime. Every time the belts were shortened, the carbon content was measured. They found a negative correlation of carbon content with belt life. Adjusting the furnace gas composition and the time-attemperature in the delube zone greatly reduced the amount of sooting and deposition on the belt, and consequently increased belt life. The systematic collection and analysis of process data provides a better understanding of the entire process, and can allow one to implement predictive maintenance procedures.
Figure 3. (Courtesy of S. Luk, R.T. Warzel III, P. Hofecker).
July/August 2012 MPR
21
Lubrication and delubrication at PowderMet 2012 Dr James McCall, from NuvoSci Consulting, reviews what he considers to be the strongest collection of presentations concerning lubrication and delubrication delivered at PowderMet 2012 in many years.
T
he presentations at PowderMet included are two dedicated sessions, Lubrication I and II comprising five papers and the Special Interest Programme on Delubrication Science, Problems & Development, which entailed eight presentations. A panel discussion at the end of the SIP discussed the current performance and projected lubrication needs for the industry. There were also other papers related to lubrication in other sessions, and highlights of selected presentations are provided in this report.
Metal-free lubricants The performance of a wide variety of potential admixed lubricants was discussed by D Biro (Lonza) in “Evaluation of lubricant chemistries for powder metal applications”, coauthored by R Skillen (Lonza) and D Heaney (CISP). Metal-free lubricants (17) representing four different chemistries were evaluated at 0.75 wt% in straight iron powder at room temperature for their effect on powder, green, ejection and sintered properties. Acrawax C was used as the reference. All lubricants had been ground to a similar particle size distribution (D50=7-14, D90=12-50 μm) to minimise the influence of this property. Compaction was done at 579 MPa; a significant variation in compressibilities led to variation in green densities,
which ranged from 6.91-7.09 g/cm3. Several of new lubricants had better individual properties than the reference Acrawax C, and research is continuing to optimise performance. V Paris (Rio Tinto) covered other admixed lubricants in “Novel highperformance lubricants for conventional and high-density compaction”, coauthored with Y Thomas (NRCC-IMI) and S St-Laurent (Rio Tinto). The lubrication performance of eight admixed composite proprietary lubricant formulations were compared with Kenolube at a level of 0.7 wt% in an iron-copper-carbon composition. Flow rates and apparent densities were significantly poorer than for Kenolube, but the addition of a flow enhancer improved these properties. Compressibilities at room temperature and at 60°C were generally quite similar, but several of the lubricants had slightly better compressibility than Kenolube at 60°C, giving green densities of 7.20 g/cm3 or higher. The ejection performances of the lubricants varied, but lubricant PR-1 was more lubricious than Kenolube at both room temperature and 60°C. Relative to Kenolube, experimental lubricant PR-1 had equal apparent density and flow (with flow enhancer present), somewhat poorer compressibility, but better ejection performance.
Adding nanographite A Tamashausky (Asbury Graphite Mills) discussed nanographite as a
0026-0657/12 ©2012 Elsevier Ltd. All rights reserved
potential admixed lubricant in “The utility of nanographite/near-graphene materials as solid-phase, integral diewall lubricants in PM”. Typical usage levels of conventional graphite are not sufficient to benefit from the good lubricity of this material; simply not enough of the material is located at the die walls. The objective of the research was to explore the use of nanographite alone, or in conjunction with conventional graphite, to act as an efficient die-wall lubricant. Typical natural flake graphite used in PM has a particle size of about 5 μm, which provides a surface area of about 15 m2/g. This contrasts with the natural and synthetic platelet nanographites reported here, with an apparent mean particle size of about 1 micron and a surface area of about 350 m2/g. This is an “apparent” size because it is actually an agglomerate of much finer primary particles. For this reason, a conventional particle size measurement is not useful; instead, the author developed a Lamella Thickness Index (LTI), a metric based on the surface area, to approximate the number of graphene (fully-delaminated graphite) layers present. This gives a measure of the extent of delamination. FC0208 mixes were prepared at in a lab V-blender with graphite, nanographite and lubricant admixed as required. The ejection behavior of 30 tsi-compacted FC0208 mixes containing no admixed lubricant were
July/August 2012 MPR
19
Figure 1. (Courtesy of S. Luk, R.T. Warzel III, P. Hofecker).
compared; the admixed carbon (0.9 wt%) was conventional (9 μm) flake graphite, 350 m2/g nanographite (about 7 graphene layers present), or 400 m2/g nanographite (about 6 graphene layers present): average ejection forces were 13,900, 7200 and 7000 lbs-f, respectively, compared to 4680 lbs-f for the mix containing 0.9 wt% conventional flake graphite with 0.75 wt% ethylene bis stearamide (EBS) wax as lubricant. Experiments were also made with reduced EBS lubricant content. TR bars made with 0.9 wt% nanographites (the same 350 and 400 m2/g ones) and reduced (0.1 wt%) EBS content had lower average ejection forces (2740 and 2660 lbs-f, respectively) than if conventional flake graphite had been used with 0.1 wt% EBS (3320 lbs-f). Thus, the admixed nanographites provided significantly improved lubricity than conventional flake graphite, but still not to the level obtained with admixed EBS wax. The reduced-wax compositions containing the nanographites, however, had lower sintered hardnesses and strengths. The intriguing results of this examination of nanographites warrant additional study.
Warm compaction process S Luk presented “Warm-die compaction process to achieve higher green density and green strength”, co-authored with his N.A. Höganäs colleagues
20
MPR July/August 2012
R.T. Warzel III and P. Hofecker. This paper served as an overview of the role of lubricants in an FC0205 mix in the warm-compaction process. Better mechanical properties derive from higher densities, which can be obtained by using, for example, copper infiltration, high-temperature sintering, doublepress-double-sintering or powder forging. Warm (tool+powder) compaction was introduced as another option in the 1990s, and now warm-die compaction is being promoted. In the 1990s, warm compaction required temperatures of about 120°C. Now, a heated die (6080°C) alone can be used. The lubricant must, at least initially, be a solid but be capable of migrating to the die walls upon compaction. The authors compared a low-melting solid, such as stearic acid (m.p. of about 68°C) with higher-melting polyethylene wax (m.p. 104°C) at 0.75 wt% in an FC0205 composition. Although the compressibilities were very similar at 25°C, they were very different at 75°C, with the PE wax being much less compressible. A very high green strength (29 MPa) was obtained with the 550-MPa-compacted mixture containing the PE wax. Above 550 MPa, and with a warm (60°C) die, the PE wax did not provide sufficient lubricity: ejection was noisy and parts were scored. At a given compaction pressure, green expansion was higher with stearic acid than with PE wax, at both temperatures. The performances of Acrawax C EBS wax and Intralube E in the same FC0205 system were compared.
Compaction was done at various pressures and at die temperatures of 25, 60 and 95°C. Green density, green expansion, green strength and lubricity were measured. The evaluated premixes contained 0.75 wt% lubricant (Mix A=EBS, Mix E3=Intralube E), and reduced loadings of Intralube E (0.30 wt%=Mix E1 and 0.50 wt%=Mix E2). Rank-ordering of the compressibilities changed with compaction temperature. At the highest compaction pressure (690 MPa), mixes E1 and E2 were the most compressible and had similar compressibilities at 25 and 60°C; at 95°C mix E2 had much superior compressibility and was able to reach 7.31 g/cm3. Green strengths of about 13 MPa or higher are desirable for safe handling of green parts; this was achieved for all of the mixes containing Intralube E using warm or hot dies. Green expansion (springback) was quite similar for all the mixes under a given compaction pressures at 25°C, but mixes E2 and E3 had higher springbacks at the highest pressures at 60°C and at all pressures at 95°C. There were marked differences in lubricity between the mixes, both as measured stripping and sliding pressures. At 60°C and the highest compaction pressure, the stripping and sliding pressures with 0.75 wt% EBS were about equal to that for 0.50 wt% Intralube E. The mix containing 0.75 wt% Intralube E had much better lubricity, and indeed had the best lubricity at all die temperatures and compaction pressures.
metal-powder.net
Various lubricant packages were discussed by R Warzel III (N A Höganäs) in “Lubrication for demanding applications”, co-authored by H Rodrigues and R Goto (Engineered Sintered Components). He compared the performance (at 0.75 wt%) of Acrawax, Intralube E, and Starmix Boost to an undisclosed reference (at 0.80 wt%) in an FC0208 system. All mixes contained a machinability additive (SM-3) at 0.2 wt%. The same mixes were also evaluated on a production part: a vane rotor having a green density of 6.85 g/cm3. Flow rates and apparent densities equal to or better than the reference blend, except for Acrawax, which gave lower apparent density and slower flow rate. Compressibilities were very similar up to a green density of about 7.10 g/cm3, above which Intralube E was more compressible. Intralube E and Starmix Boost provided the lowest green expansion. The lowest peak ejection pressure was found for the Starmix Boost mix, about 10% lower than that for the reference. The better lubricity found in the laboratory translated to better part-to-part weight stability and better surface finish in the production part. Higher dimensional change was noted for production parts made with the Starmix Boost, not observed in the laboratory tests. New bonded lubricants were discussed by K McQuaig (Hoeganaes Corp) in “Improved die-fill performance through binder treatment”, co-authored with S Patel and P Sokolowski (Hoeganaes Corp) and
S. Shah, G. Schluterman and J. Falleur (Cloyes Gear & Products). The benefits of bonded lubricants include improved apparent density, flow rate, dusting resistance and mix homogeneity. Four kinds of FC0205 mixes were compared: premix, Ancorbond, and the experimental EXP1-bonded and EXP2-bonded lubricants. Lubricant loading was 0.75 wt%. Testing was done at room temperature and 75°C. The experimental mixes had much higher apparent densities. At 75°C, the four systems had quite similar compressibilities but EXP1 and EXP2 had significantly lower stripping pressures than either the regular premix or the Ancorbond mix (see Figure). In a production trial, an ABS wheel sensor was pressed to a green density of 6.8 and 7.0 g/cm3. Both EXP1 and EXP2 mixes provided better surface finish than either the premix or the Ancorbond mix. No differences were seen in sintered mechanical properties. Bonded mixes were also the subject of the presentation by F Hanejko (Hoeganaes Corp) on “Advances in lubrication technology in PM to promote higher sintered densities”. The first Ancorbond system used an organic binder/lubricant system to improve mix homogeneity, improve powder flow and reduce dusting. Improvement in binders and lubricants led to warm compaction applications with Ancordense and Ancordense 450, using both heated die and heated powder, to achieve high green densities and strengths. The newer Ancormax
Figure 2. K. McQuaig, S. Patel, P. Sokolowski, S. Shah, G. Schluterman, J. Falleur.
metal-powder.net
system requires only a heated die. Hoeganaes has a new metal-free binder/ lubricant under development that can be used at 0.25 wt% loading and requires only die heating, at 107°C. Finally, the consequences of poor control of the delubing process were discussed by J. Engquist (BurgessNorton) in “Methods for measuring and quantifying effective delubrication – a practical approach.” Many quality issues related to sintered products can be traced back to poor delubrication. Implementing process control monitoring of the delube zone can greatly help in troubleshooting. Poor delubrication can lead to problems such as carbon buildup on heat exchangers and shortening the life of belts, for example. In a case study, the cause of short belt life was investigated in detail with the aim to predict and improve the life. A fixed length of belt was measured weekly, and the rate-of-change in length was plotted as a function of lifetime. Every time the belts were shortened, the carbon content was measured. They found a negative correlation of carbon content with belt life. Adjusting the furnace gas composition and the time-attemperature in the delube zone greatly reduced the amount of sooting and deposition on the belt, and consequently increased belt life. The systematic collection and analysis of process data provides a better understanding of the entire process, and can allow one to implement predictive maintenance procedures.
Figure 3. (Courtesy of S. Luk, R.T. Warzel III, P. Hofecker).
July/August 2012 MPR
21