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International Academy for Production Engineering
68th CIRP General Assembly | Tokyo | August 19-25, 2018
M. Holsten, P. Koshy, A. Klink, A. Schwedt
Anomalous influence of polarity
in sink EDM of titanium alloys
Anomalous influence of polarity in sink EDM of titanium alloys
M. Holsten, P. Koshy, A. Klink, A. Schwedt
68th CIRP General Assembly
Tokyo, August 21, 2018
2/13
Machinability as a function of polarity
A positive tool polarity is conventionally used in sink EDM for roughing operations
Application of this polarity for Ti and Ti6Al4V alloys results in virtually no material removal
This is however not the case for the titanium alloy Ɣ-TiAl
This research explored the mechanisms behind this intriguing phenomenon
Anomalous influence of polarity in sink EDM of titanium alloys
M. Holsten, P. Koshy, A. Klink, A. Schwedt
68th CIRP General Assembly
Tokyo, August 21, 2018
3/13
Polarity-dependent removal at different discharge durations
The anomalous polarity effect was observed at all discharge durations
Anomalous influence of polarity in sink EDM of titanium alloys
M. Holsten, P. Koshy, A. Klink, A. Schwedt
68th CIRP General Assembly
Tokyo, August 21, 2018
4/13
Effect of different pulse shapes for the Ti6Al4V alloy
A significant increase in pulse energy due to a change in pulse shape from triangular to
rectangular did not quite increase the removal rate for the tool positive polarity
Anomalous influence of polarity in sink EDM of titanium alloys
M. Holsten, P. Koshy, A. Klink, A. Schwedt
68th CIRP General Assembly
Tokyo, August 21, 2018
5/13
Surface topography as a function of pulse shape and polarity
For both pulse shapes, the topography corresponding to the tool positive polarity
indicated no characteristic craters, but exhibited several microcracks
This pointed to the likelihood of a hard/brittle material species on the work surface for
the tool positive polarity
Anomalous influence of polarity in sink EDM of titanium alloys
M. Holsten, P. Koshy, A. Klink, A. Schwedt
68th CIRP General Assembly
Tokyo, August 21, 2018
6/13
Surface hardness as a function of polarity
The surface/subsurface corresponding to the tool positive polarity did
indeed refer to a significant increase in hardness
Anomalous influence of polarity in sink EDM of titanium alloys
M. Holsten, P. Koshy, A. Klink, A. Schwedt
68th CIRP General Assembly
Tokyo, August 21, 2018
7/13
Material accretion on the machined surface
For the tool positive polarity,
examination of the machined
surface indicated the
deposition of a material layer
during the course of machining
The growth of this layer
corresponded to a decrease in
both the surface roughness
and the machining speed
(measured in terms of the tool
position versus time)
Anomalous influence of polarity in sink EDM of titanium alloys
M. Holsten, P. Koshy, A. Klink, A. Schwedt
68th CIRP General Assembly
Tokyo, August 21, 2018
8/13
Effect of polarity on the cross-section of machined surfaces
For the tool positive
polarity, the machined
surface indicated a thick
and continuous layer, which
was identified to be TiC
For the tool negative
polarity such a layer was
relatively thin and
intermittent
The TiC layer, by virtue of
being a thermal barrier,
inhibits material removal for
the tool positive polarity
Anomalous influence of polarity in sink EDM of titanium alloys
M. Holsten, P. Koshy, A. Klink, A. Schwedt
68th CIRP General Assembly
Tokyo, August 21, 2018
9/13
Effect of polarity on energy density
Single discharge experiments
indicated a smaller crater diameter,
but a higher workpiece material
removal volume, for the tool
negative polarity
This indicates the TiC formation on
the work surface to be less
pronounced for the tool negative
polarity due to the higher energy
partition and the higher energy
density being responsible for the
removal of incipient TiC layers
Anomalous influence of polarity in sink EDM of titanium alloys
M. Holsten, P. Koshy, A. Klink, A. Schwedt
68th CIRP General Assembly
Tokyo, August 21, 2018
10/13
Effect of polarity on acoustic emission
Acoustic emission (AE) in EDM relates to the energy apportioned to the gas
bubbles [ Klink et al, CIRP Annals 2016]
The formation of the TiC thermal barrier under the tool positive polarity was
seen to correspond to an increase in AE due to the reduction in energy
transmitted through the workpiece
Anomalous influence of polarity in sink EDM of titanium alloys
M. Holsten, P. Koshy, A. Klink, A. Schwedt
68th CIRP General Assembly
Tokyo, August 21, 2018
11/13
Effect of working fluid
A comparison of the material removal rate in hydrocarbon oil and water
indicated the source of carbon for TiC formation to be the oil rather than
the graphite electrode
Anomalous influence of polarity in sink EDM of titanium alloys
M. Holsten, P. Koshy, A. Klink, A. Schwedt
68th CIRP General Assembly
Tokyo, August 21, 2018
12/13
Effect of discharge duration on TiC formation for Ɣ-TiAl
In the case of Ɣ-TiAl, the
formation of TiC was
significant only at
discharge durations as
high as 300 µs
A higher energy input was
required for TiC formation
due to the thermal
conductivity of Ɣ-TiAl being
1.5 to 3.0 times that of
Ti6Al4V at elevated
temperatures
36µs300µs
Anomalous influence of polarity in sink EDM of titanium alloys
M. Holsten, P. Koshy, A. Klink, A. Schwedt
68th CIRP General Assembly
Tokyo, August 21, 2018
13/13
Acknowledgments
More information can be found in:
M. Holsten, P. Koshy, A. Klink, A. Schwedt, Anomalous
influence of polarity in sink EDM of titanium alloys,
CIRP Annals 67 (2018) 221-224.

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Anomalous influence of polarity in sink EDM of titanium alloys

  • 1. International Academy for Production Engineering 68th CIRP General Assembly | Tokyo | August 19-25, 2018 M. Holsten, P. Koshy, A. Klink, A. Schwedt Anomalous influence of polarity in sink EDM of titanium alloys
  • 2. Anomalous influence of polarity in sink EDM of titanium alloys M. Holsten, P. Koshy, A. Klink, A. Schwedt 68th CIRP General Assembly Tokyo, August 21, 2018 2/13 Machinability as a function of polarity A positive tool polarity is conventionally used in sink EDM for roughing operations Application of this polarity for Ti and Ti6Al4V alloys results in virtually no material removal This is however not the case for the titanium alloy Ɣ-TiAl This research explored the mechanisms behind this intriguing phenomenon
  • 3. Anomalous influence of polarity in sink EDM of titanium alloys M. Holsten, P. Koshy, A. Klink, A. Schwedt 68th CIRP General Assembly Tokyo, August 21, 2018 3/13 Polarity-dependent removal at different discharge durations The anomalous polarity effect was observed at all discharge durations
  • 4. Anomalous influence of polarity in sink EDM of titanium alloys M. Holsten, P. Koshy, A. Klink, A. Schwedt 68th CIRP General Assembly Tokyo, August 21, 2018 4/13 Effect of different pulse shapes for the Ti6Al4V alloy A significant increase in pulse energy due to a change in pulse shape from triangular to rectangular did not quite increase the removal rate for the tool positive polarity
  • 5. Anomalous influence of polarity in sink EDM of titanium alloys M. Holsten, P. Koshy, A. Klink, A. Schwedt 68th CIRP General Assembly Tokyo, August 21, 2018 5/13 Surface topography as a function of pulse shape and polarity For both pulse shapes, the topography corresponding to the tool positive polarity indicated no characteristic craters, but exhibited several microcracks This pointed to the likelihood of a hard/brittle material species on the work surface for the tool positive polarity
  • 6. Anomalous influence of polarity in sink EDM of titanium alloys M. Holsten, P. Koshy, A. Klink, A. Schwedt 68th CIRP General Assembly Tokyo, August 21, 2018 6/13 Surface hardness as a function of polarity The surface/subsurface corresponding to the tool positive polarity did indeed refer to a significant increase in hardness
  • 7. Anomalous influence of polarity in sink EDM of titanium alloys M. Holsten, P. Koshy, A. Klink, A. Schwedt 68th CIRP General Assembly Tokyo, August 21, 2018 7/13 Material accretion on the machined surface For the tool positive polarity, examination of the machined surface indicated the deposition of a material layer during the course of machining The growth of this layer corresponded to a decrease in both the surface roughness and the machining speed (measured in terms of the tool position versus time)
  • 8. Anomalous influence of polarity in sink EDM of titanium alloys M. Holsten, P. Koshy, A. Klink, A. Schwedt 68th CIRP General Assembly Tokyo, August 21, 2018 8/13 Effect of polarity on the cross-section of machined surfaces For the tool positive polarity, the machined surface indicated a thick and continuous layer, which was identified to be TiC For the tool negative polarity such a layer was relatively thin and intermittent The TiC layer, by virtue of being a thermal barrier, inhibits material removal for the tool positive polarity
  • 9. Anomalous influence of polarity in sink EDM of titanium alloys M. Holsten, P. Koshy, A. Klink, A. Schwedt 68th CIRP General Assembly Tokyo, August 21, 2018 9/13 Effect of polarity on energy density Single discharge experiments indicated a smaller crater diameter, but a higher workpiece material removal volume, for the tool negative polarity This indicates the TiC formation on the work surface to be less pronounced for the tool negative polarity due to the higher energy partition and the higher energy density being responsible for the removal of incipient TiC layers
  • 10. Anomalous influence of polarity in sink EDM of titanium alloys M. Holsten, P. Koshy, A. Klink, A. Schwedt 68th CIRP General Assembly Tokyo, August 21, 2018 10/13 Effect of polarity on acoustic emission Acoustic emission (AE) in EDM relates to the energy apportioned to the gas bubbles [ Klink et al, CIRP Annals 2016] The formation of the TiC thermal barrier under the tool positive polarity was seen to correspond to an increase in AE due to the reduction in energy transmitted through the workpiece
  • 11. Anomalous influence of polarity in sink EDM of titanium alloys M. Holsten, P. Koshy, A. Klink, A. Schwedt 68th CIRP General Assembly Tokyo, August 21, 2018 11/13 Effect of working fluid A comparison of the material removal rate in hydrocarbon oil and water indicated the source of carbon for TiC formation to be the oil rather than the graphite electrode
  • 12. Anomalous influence of polarity in sink EDM of titanium alloys M. Holsten, P. Koshy, A. Klink, A. Schwedt 68th CIRP General Assembly Tokyo, August 21, 2018 12/13 Effect of discharge duration on TiC formation for Ɣ-TiAl In the case of Ɣ-TiAl, the formation of TiC was significant only at discharge durations as high as 300 µs A higher energy input was required for TiC formation due to the thermal conductivity of Ɣ-TiAl being 1.5 to 3.0 times that of Ti6Al4V at elevated temperatures 36µs300µs
  • 13. Anomalous influence of polarity in sink EDM of titanium alloys M. Holsten, P. Koshy, A. Klink, A. Schwedt 68th CIRP General Assembly Tokyo, August 21, 2018 13/13 Acknowledgments More information can be found in: M. Holsten, P. Koshy, A. Klink, A. Schwedt, Anomalous influence of polarity in sink EDM of titanium alloys, CIRP Annals 67 (2018) 221-224.