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Strength in depth counts if the going gets tough
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Strength in depth counts if the going gets tough Work carried out by researchers from leading non-ferrous powder manufacturer Ecka suggests that dispersion-strengthened copper alloys can set the pace in hot and high-speed applications... opper is widely used in industry when high electrical and thermal conductivity is needed. A wide variety of copper alloys are commercially available that offer higher strength levels, but usually at the cost of reduced conductivity values. Precipitation hardened alloys can be a good compromise if both high levels of conductivity and strength are required. However, if the material is subjected to prolonged heating at temperatures above the initial precipitation heat treatment either or both properties may suffer. The only way to overcome this to use dispersion-strengthened copper alloys, where ultrafine ceramic particles such as oxides or carbides in homogenous distribution generate the high mechanical strength needed. And that's where powder metallurgy comes into its own. The particle distribution required to manufacture dispersionstrengthened alloys cannot be achieved by melting technologies because of the tenden-
C
The rare but vital combination of strength and conductivity is needed in engineering applications cy of the ceramic particles to segregate. They normally form the slag of a melt. The starting powders are mechanically alloyed by ball-milling. During milling, two processes are being carried out at the same time: breaking the powder down through cold work during the milling, and particle growth due to cold welding of the powder particles. In the first stage of milling both processes are roughly in balance leading to an intimate and homogeneous mixture of the different components. If reactive ingredients are used, reactions between the different components or
the alloy additions and the milling atmosphere are enhanced because, as the powder particles are broken down, continuously fresh and reactive surfaces are created. Both mechanical alloying and reaction milling (RMMA) offer the possibility of producing a large variety of alloy compositions with specific designed properties. In principle attritors as well as ball mills are convenient milling devices, but ball-milling offers possible economic production in large batch sizes. The low contamination achieved by milling minimises the effect of impurities on electrical conductivity. Copper materials are used when high electrical and/or thermal conductivity is needed, but for engineering applications strength is often important too. This means that the combination of conductivity and strength is the determining factor in deciding suitability for any particular application. Unfortunately, optimisation of conductivity always works against strength, and the other way round. As can
Table 1: examples of dispersion strengthened PM-copper alloys and properties Alloy Hardness [HV 30] El. Conductivity [% IACS] A Cu – Al – C – O 170 65
Ductility A10 [%] < 10
B
Cu – Al – B – O
160
80
< 15
C
Cu – Al – Ti – C – O
180
65
< 10
D
Cu – Al – Ti – C – O
220
48
<7
E
Cu – Al – B – O
120
90
< 20
16
MPR April 2005
0026-0657/05 ©2005 Elsevier Ltd. All rights reserved.
Figure 1. Japan’s Bullet Trains (Shinkansen) use the properties of advanced materials such as dispersionstrengthened copper. This ensures efficient transfer of power to each of the train’s axles, maximising acceleration and maintaining its 260kph top speed. Courtesy: Meha Kulyck/Science Photo Library.
be seen in Table 1, the RMMA-process offers the advantage of combining the high-temperature performance of dispersion- strengthened copper with a wide range of combinations of strength and conductivity. The potential of this process to design materials for specific applications can be shown by considering examples: Electrodes for MIG/MAG welding and electro discharge machining (EDM) With alloy "A" the aim was to get high values for electrical conductivity as well as mechanical strength. The addition of carbon led to lower wear under frictional applications and reduced burn-out due to electro-erosion. This material shows excellent properties when used for contact tips for MIG/MAG-welding. The high wear resistance to thermal and electric impact
metal-powder.net
leads to between three and four times longer lifetime of the contact tips compared to standard electrodes made from conventional CuCrZr alloy. With improvements to the extrusion process, it has become possible to offer this material as bimetal tubes. Concentrated at point of need This delivery form has become the standard raw-material for production of contact tips. It is a perfect example of material design and optimisation of materials properties and processing being adapted to the application. The expensive, high-performance material is concentrated at the point where it is needed and even the cheap pure copper used for the outer skin improves the properties of the electrode as it helps to dissipate the heat from the
contact tip due to its higher thermal conductivity, an additional contribution to extend lifetime. The cross section is shown in Figure 2. Another advantage of this Cu-Al-C-Oalloy is its good machinability, which opens other fields of application. As the dispersed particles act as chip breakers, this alloy shows short cuts and low burr formation under machining. This, together with its stiffness and resistance against electro-erosion makes it interesting for electrodes for electrical discharge machining (EDM). EDM electrodes often have to reproduce filigree structures. If pure copper is used as electrode material, the manufacturing of such complicated electrodes is very cost intensive, because of the timeconsuming removal of the burrs, which has to be done extremely carefully because of
April 2005 MPR
17
Figure 2. Dispersion strengthened copper has made dramatic differences in performance In advanced materials for MIG/MAG welding tips and their production.
the detailed structure. If graphite electrodes are used, special machinery is required to ensure that graphite dust is collected during machining. Graphite electrodes have a limited life. The use of the Cu-Al-C-O-alloy for EDM electrodes solves both problems. Overall, manufacturing of the electrodes becomes cheaper because of lower burr formation and lower wear under working EDM conditions. Dispersion-strengthened alloys have shown their value in electrodes for resistance spot welding. Alloy "B" was developed especially for electrode caps for resistance spot welding (Figure 3). Dispersionstrengthened copper alloys are superior to conventional copper alloys such as CuCr, CuZr or CuCrZr because of their hightemperature stability. Plastic deformation The key issue during resistance spot welding is to control current density. Due to high operating temperatures, the top of the electrode caps show step increases in plastic deformation (mushrooming). This leads to increased electrode contact, reduced current density and heat generation. If the values fall below a critical limit, the electrode pair has to be changed or the outer shape has to be recreated by tip dressing. Zinc alloying onto the electrode top when welding coated zinc sheet promotes mushrooming. These effects lead to unwanted downtime, reducing efficiency of automated production lines with robotic systems. The use of dispersion-strengthened alloys guarantees extended electrode life as the dispersed particles impart hightemperature strength. There is less mushrooming and the diffusion of zinc atoms into the electrode is inhibited. Special electrodes are needed for resistance welding
18
MPR April 2005
Figure 3. The specially developed dispersion alloys for resistance spot welding showed themselves to be superior in performance to conventional alloys because of their high-temperature stability.
Process flexibility allows development of alloys with the potential to replace high-strength bronzes with new and outstanding PM materials and submerged welding For speciality resistance welding such as micro-welding or welding chain links the electrodes must be harder. Compared to the alloy "B" electrodes where the material needs a certain ductility to be suitable for the production of electrode caps by cold heading these electrodes are mostly produced by machining. So ductility, sometimes even electrical conductivity, is of less importance; hardness of the material is all that counts. High-temperature performance The RMMA process allows hardness optimisation simply by changing the alloy composition and using a higher proportion of dispersed particles. Variations can be made in the choice of alloying elements, composition and properties of the raw powders, eg particle size distribution and shape. Process parameters like milling time, milling atmosphere, ball powder ratio can be adjusted as well. This flexibility enables development of alloys with the potential to substitute high-strength bronzes like CuCoBe or CuNi2Si with new outstanding PM materials. These have comparable hardness properties at room-temperature, but otherwise
Figure 4. Working underwater is especially stressful for weld tips. Dispersion-strengthened copper electrodes last about 2.5 times conventional units.
unreachable high-temperature performance due to dispersion hardening. In submerged welding the lifetime of electrodes (Figure 4) out of Alloy D exceeds those made of CuNi2Si by 2.5 times! Current-gathering strips in electric train pantographs. High electrical conductivity to minimise energy losses is required in current gathering elements of pantographs. The current collection device of an electric train, affects the efficiency of the power supply network. But mechanical properties are important too, especially since railway transport speeds have gone up. Faster trains are more attractive for would-be passengers. But operational specifications are keeping pace with new
The author This feature was abstracted from Ecka Discup® -new high-performance PM copper materials - process properties and applications by Markus Hofmockel and Hans-Claus Neubing, who both work for Ecka in Germany. Ecka Discup® is a family of dispersion-strengthened copper alloys used in this research. These PM materials are produced by a combination of mechanical alloying and reaction milling, where oxides and/or carbides are incorporated into the copper matrix with a very homogeneous distribution. Materials with high-temperature stability are created, but each powder particle exhibits both high strength and high conductivity. After milling the consolidation of the powder to full density is carried out using common techniques such as cold isostatic pressing and hot extrusion.
metal-powder.net
At high speed the transfer of electric energy from network to electric motor can be disturbed by ice or water performance levels, particularly the need to provide high resistance against burn-out due to electro-erosion and friction during sliding of the contact pair under new operational conditions. At high speed (>200km/h) the transfer of electric energy from the network to the motor of the electric train becomes a very technical problem. Disturbing factors such as ice or water on the wire cause instabilities leading to interrupted movement followed by sparking. Arc discharge increases wear of the contact pair by material evaporation - promoting further arc formation.
metal-powder.net
As roughness of the contact materials increase, the destructive cycle of increased mechanical friction, increasing power demand, and increased contact temperature accelerates. The materials commonly used in these high-attrition areas, such as CuCrZr or copper-based sintered powder compositions, have no more potential for further development. Reduced friction Only dispersed-strengthened copper alloys can provide the high electrical conductivity required to minimise energy losses and heat generation as well as mechanical strength at elevated temperatures to reduce friction under high speed conditions. A new PM-alloy with high electrical conductivity (See Table 1, Alloy "E") has been formulated by shifting the alloy composition towards a lower dispersed particle content. A compromise between mechanical strength and ductility has been made to fulfil the application's requirements.
References [1] AV Nadkarni, J E Synk, ASM Handbook, vol. 7: Powder Metallurgy, p. 711 [2] AV Nadkarni, E Klar, W M Shafer, Metals Engineering Quartely10, August 1976, p. 10-14 [3] A K Lee, L E Sanchez-Caldera, J -H Chun, N P Suh, Mat Res Soc Symp Proc, Materials Research Society 1989 [4] B Kieback, G Lotze, C H Sauer, H Kubsch, Patentschrift DE 44 18 600 C2, 1997 [5] J S Benjamin: Dispersion strengthened Superalloys Mechanical Alloying, Metallurgical Transactions , 1 (1970), 2943-2951 [6] J S Benjamin, T.E. Violin: The Mechanism of Mechanical Alloying, Metallurgical Transactions A, 5 (1974), 1929-1934
April 2005 MPR
19
Strength in depth counts if the going gets tough Work carried out by researchers from leading non-ferrous powder manufacturer Ecka suggests that dispersion-strengthened copper alloys can set the pace in hot and high-speed applications... opper is widely used in industry when high electrical and thermal conductivity is needed. A wide variety of copper alloys are commercially available that offer higher strength levels, but usually at the cost of reduced conductivity values. Precipitation hardened alloys can be a good compromise if both high levels of conductivity and strength are required. However, if the material is subjected to prolonged heating at temperatures above the initial precipitation heat treatment either or both properties may suffer. The only way to overcome this to use dispersion-strengthened copper alloys, where ultrafine ceramic particles such as oxides or carbides in homogenous distribution generate the high mechanical strength needed. And that's where powder metallurgy comes into its own. The particle distribution required to manufacture dispersionstrengthened alloys cannot be achieved by melting technologies because of the tenden-
C
The rare but vital combination of strength and conductivity is needed in engineering applications cy of the ceramic particles to segregate. They normally form the slag of a melt. The starting powders are mechanically alloyed by ball-milling. During milling, two processes are being carried out at the same time: breaking the powder down through cold work during the milling, and particle growth due to cold welding of the powder particles. In the first stage of milling both processes are roughly in balance leading to an intimate and homogeneous mixture of the different components. If reactive ingredients are used, reactions between the different components or
the alloy additions and the milling atmosphere are enhanced because, as the powder particles are broken down, continuously fresh and reactive surfaces are created. Both mechanical alloying and reaction milling (RMMA) offer the possibility of producing a large variety of alloy compositions with specific designed properties. In principle attritors as well as ball mills are convenient milling devices, but ball-milling offers possible economic production in large batch sizes. The low contamination achieved by milling minimises the effect of impurities on electrical conductivity. Copper materials are used when high electrical and/or thermal conductivity is needed, but for engineering applications strength is often important too. This means that the combination of conductivity and strength is the determining factor in deciding suitability for any particular application. Unfortunately, optimisation of conductivity always works against strength, and the other way round. As can
Table 1: examples of dispersion strengthened PM-copper alloys and properties Alloy Hardness [HV 30] El. Conductivity [% IACS] A Cu – Al – C – O 170 65
Ductility A10 [%] < 10
B
Cu – Al – B – O
160
80
< 15
C
Cu – Al – Ti – C – O
180
65
< 10
D
Cu – Al – Ti – C – O
220
48
<7
E
Cu – Al – B – O
120
90
< 20
16
MPR April 2005
0026-0657/05 ©2005 Elsevier Ltd. All rights reserved.
Figure 1. Japan’s Bullet Trains (Shinkansen) use the properties of advanced materials such as dispersionstrengthened copper. This ensures efficient transfer of power to each of the train’s axles, maximising acceleration and maintaining its 260kph top speed. Courtesy: Meha Kulyck/Science Photo Library.
be seen in Table 1, the RMMA-process offers the advantage of combining the high-temperature performance of dispersion- strengthened copper with a wide range of combinations of strength and conductivity. The potential of this process to design materials for specific applications can be shown by considering examples: Electrodes for MIG/MAG welding and electro discharge machining (EDM) With alloy "A" the aim was to get high values for electrical conductivity as well as mechanical strength. The addition of carbon led to lower wear under frictional applications and reduced burn-out due to electro-erosion. This material shows excellent properties when used for contact tips for MIG/MAG-welding. The high wear resistance to thermal and electric impact
metal-powder.net
leads to between three and four times longer lifetime of the contact tips compared to standard electrodes made from conventional CuCrZr alloy. With improvements to the extrusion process, it has become possible to offer this material as bimetal tubes. Concentrated at point of need This delivery form has become the standard raw-material for production of contact tips. It is a perfect example of material design and optimisation of materials properties and processing being adapted to the application. The expensive, high-performance material is concentrated at the point where it is needed and even the cheap pure copper used for the outer skin improves the properties of the electrode as it helps to dissipate the heat from the
contact tip due to its higher thermal conductivity, an additional contribution to extend lifetime. The cross section is shown in Figure 2. Another advantage of this Cu-Al-C-Oalloy is its good machinability, which opens other fields of application. As the dispersed particles act as chip breakers, this alloy shows short cuts and low burr formation under machining. This, together with its stiffness and resistance against electro-erosion makes it interesting for electrodes for electrical discharge machining (EDM). EDM electrodes often have to reproduce filigree structures. If pure copper is used as electrode material, the manufacturing of such complicated electrodes is very cost intensive, because of the timeconsuming removal of the burrs, which has to be done extremely carefully because of
April 2005 MPR
17
Figure 2. Dispersion strengthened copper has made dramatic differences in performance In advanced materials for MIG/MAG welding tips and their production.
the detailed structure. If graphite electrodes are used, special machinery is required to ensure that graphite dust is collected during machining. Graphite electrodes have a limited life. The use of the Cu-Al-C-O-alloy for EDM electrodes solves both problems. Overall, manufacturing of the electrodes becomes cheaper because of lower burr formation and lower wear under working EDM conditions. Dispersion-strengthened alloys have shown their value in electrodes for resistance spot welding. Alloy "B" was developed especially for electrode caps for resistance spot welding (Figure 3). Dispersionstrengthened copper alloys are superior to conventional copper alloys such as CuCr, CuZr or CuCrZr because of their hightemperature stability. Plastic deformation The key issue during resistance spot welding is to control current density. Due to high operating temperatures, the top of the electrode caps show step increases in plastic deformation (mushrooming). This leads to increased electrode contact, reduced current density and heat generation. If the values fall below a critical limit, the electrode pair has to be changed or the outer shape has to be recreated by tip dressing. Zinc alloying onto the electrode top when welding coated zinc sheet promotes mushrooming. These effects lead to unwanted downtime, reducing efficiency of automated production lines with robotic systems. The use of dispersion-strengthened alloys guarantees extended electrode life as the dispersed particles impart hightemperature strength. There is less mushrooming and the diffusion of zinc atoms into the electrode is inhibited. Special electrodes are needed for resistance welding
18
MPR April 2005
Figure 3. The specially developed dispersion alloys for resistance spot welding showed themselves to be superior in performance to conventional alloys because of their high-temperature stability.
Process flexibility allows development of alloys with the potential to replace high-strength bronzes with new and outstanding PM materials and submerged welding For speciality resistance welding such as micro-welding or welding chain links the electrodes must be harder. Compared to the alloy "B" electrodes where the material needs a certain ductility to be suitable for the production of electrode caps by cold heading these electrodes are mostly produced by machining. So ductility, sometimes even electrical conductivity, is of less importance; hardness of the material is all that counts. High-temperature performance The RMMA process allows hardness optimisation simply by changing the alloy composition and using a higher proportion of dispersed particles. Variations can be made in the choice of alloying elements, composition and properties of the raw powders, eg particle size distribution and shape. Process parameters like milling time, milling atmosphere, ball powder ratio can be adjusted as well. This flexibility enables development of alloys with the potential to substitute high-strength bronzes like CuCoBe or CuNi2Si with new outstanding PM materials. These have comparable hardness properties at room-temperature, but otherwise
Figure 4. Working underwater is especially stressful for weld tips. Dispersion-strengthened copper electrodes last about 2.5 times conventional units.
unreachable high-temperature performance due to dispersion hardening. In submerged welding the lifetime of electrodes (Figure 4) out of Alloy D exceeds those made of CuNi2Si by 2.5 times! Current-gathering strips in electric train pantographs. High electrical conductivity to minimise energy losses is required in current gathering elements of pantographs. The current collection device of an electric train, affects the efficiency of the power supply network. But mechanical properties are important too, especially since railway transport speeds have gone up. Faster trains are more attractive for would-be passengers. But operational specifications are keeping pace with new
The author This feature was abstracted from Ecka Discup® -new high-performance PM copper materials - process properties and applications by Markus Hofmockel and Hans-Claus Neubing, who both work for Ecka in Germany. Ecka Discup® is a family of dispersion-strengthened copper alloys used in this research. These PM materials are produced by a combination of mechanical alloying and reaction milling, where oxides and/or carbides are incorporated into the copper matrix with a very homogeneous distribution. Materials with high-temperature stability are created, but each powder particle exhibits both high strength and high conductivity. After milling the consolidation of the powder to full density is carried out using common techniques such as cold isostatic pressing and hot extrusion.
metal-powder.net
At high speed the transfer of electric energy from network to electric motor can be disturbed by ice or water performance levels, particularly the need to provide high resistance against burn-out due to electro-erosion and friction during sliding of the contact pair under new operational conditions. At high speed (>200km/h) the transfer of electric energy from the network to the motor of the electric train becomes a very technical problem. Disturbing factors such as ice or water on the wire cause instabilities leading to interrupted movement followed by sparking. Arc discharge increases wear of the contact pair by material evaporation - promoting further arc formation.
metal-powder.net
As roughness of the contact materials increase, the destructive cycle of increased mechanical friction, increasing power demand, and increased contact temperature accelerates. The materials commonly used in these high-attrition areas, such as CuCrZr or copper-based sintered powder compositions, have no more potential for further development. Reduced friction Only dispersed-strengthened copper alloys can provide the high electrical conductivity required to minimise energy losses and heat generation as well as mechanical strength at elevated temperatures to reduce friction under high speed conditions. A new PM-alloy with high electrical conductivity (See Table 1, Alloy "E") has been formulated by shifting the alloy composition towards a lower dispersed particle content. A compromise between mechanical strength and ductility has been made to fulfil the application's requirements.
References [1] AV Nadkarni, J E Synk, ASM Handbook, vol. 7: Powder Metallurgy, p. 711 [2] AV Nadkarni, E Klar, W M Shafer, Metals Engineering Quartely10, August 1976, p. 10-14 [3] A K Lee, L E Sanchez-Caldera, J -H Chun, N P Suh, Mat Res Soc Symp Proc, Materials Research Society 1989 [4] B Kieback, G Lotze, C H Sauer, H Kubsch, Patentschrift DE 44 18 600 C2, 1997 [5] J S Benjamin: Dispersion strengthened Superalloys Mechanical Alloying, Metallurgical Transactions , 1 (1970), 2943-2951 [6] J S Benjamin, T.E. Violin: The Mechanism of Mechanical Alloying, Metallurgical Transactions A, 5 (1974), 1929-1934
April 2005 MPR
19