Acoustic emission signatures of electrical discharge machining

Acoustic emission signatures of
electrical discharge machining
International Academy for Production Engineering
66th General Assembly – Guimaraes, Portugal, Aug. 21-27, 2016
CIRP Annals - Manufacturing Technology Volume 65, Issue 1, 2016, Pages 229 - 232
A. Klink, M. Holsten, S. Schneider & P. Koshy

© WZL/Fraunhofer IPT
Acknowledgements5
Conclusions and outlook4
Results of sequential discharge experiments3
Results of single discharge experiments2
Introduction and experimental setup1
Outline

Acoustic Emission (AE) in the context of EDM
 AE – transient elastic waves in
response to mechanical loading:
– Inherently generated in cutting
processes
– Generated also during EDM, but not
well understood
 Availability of process information
[1,2]
 Monitoring of gap phenomena
using AE is possible
[1] Smith C, Koshy P (2013) Applications of Acoustic Mapping in Electrical Discharge Machining. CIRP Annals – Manufacturing Technology 62:171–174.
[2] Goodlet A, Koshy P (2015) Real-Time Evaluation of Gap Flushing in Electrical Discharge Machining. CIRP Annals – Manufacturing Technology 64:241–244.
What unique information can be extracted from AE acquired during EDM?
AE sensor
workpiece
disk tool

Possible sources of Acoustic Emission in EDM
AE sensor
AE signal
gas
bubble plasma channel
shock
waves
tool
workpiece
Possible sources for AE:
 Plasma channel
 Gas bubble
originating from the
rapid vaporization of
the liquid dielectric in
the vicinity of the
plasma channel
 Shock waves
emanating from the
formation, implosion
and rebound of the
gas bubble
Discharges in dry EDM do not yield significant AE: the plasma channel
can therefore be excluded as a source

Analysis on source of signal generation
0 40 80 120 160
-1.0
-0.5
0.0
0.5
1.0
AE/V
time / µs
sensor 1
sensor 1disk tool
sensor 2
sensor 2
AE during EDM can be attributed to the dynamics of gas bubbles
Experimental trial
with two identical
AE sensors:
 In contrast to
Sensor 1, the
second sensor
that only detects
the shock wave,
registers much
weaker signals
 Swapping of
sensors reveals
same results

Correlations between AE, Force and High-Speed-Images
0 100 200 300 400
-3
0
3
6
9
force/V
time / µs
on-time
1st collapse
2nd collapse
pressure
pulse
3rd collapse
-0.6
-0.3
0.0
0.3
0.6
AE/V
EMI
time delay
pressure pulse
1st collapse
2nd collapse
3rd collapse
a
d
fe
c
g h i
(A)
(B)
 Interpretation of the AE signal
by referring to the discharge
force signal
 Signals span ~200 times the
discharge on-time
Single discharge between wire electrode and force sensor

(f) 175 µs
(g) 200 µs (h) 230 µs (i) 270 µs
(a) 0.5 µs (c) 50 µs
(d) 100 µs (e) 112 µs
0.5 mm
bubble
electrode
dielectric
force
sensor
bubble inception
first collapse first rebound
second reboundsecond collapse third collapse
maximum
bubble
volume
bubble implosion
Stages of bubble expansion from high speed imaging
Time steps:
 inception and rapid
growth of the gas bubble
 maximum expansion
 bubble compression,
leading to its first collapse
 first rebound
 second collapse
 second rebound
 third collapse

Spectrogram of AE burst and time evolution at 330 kHz
600
400
500
300
200
100
0
frequency/kHz
-60
-65
-70
-75
-80
-85
dB
0 100 200 300 400
0.00
0.15
0.30
0.45
0.60
amplitude/V
time / µs
EMI pressure
pulse
2nd collapse
3rd collapse
1st collapse
 Extraction of additional
information through
examination of
spectrogram
 A slice of the spectrogram
at a frequency of 330 kHz
that corresponds to the
maximum amplitude:
Accentuation of features
such as second and third
collapse which were not
as readily apparent in the
AE signal before

Correlation between Peak Force and Peak AE signals
0.0 1.5 3.0 4.5 6.0 7.5 9.0
0.00
0.75
1.50
2.25
3.00
peakAE/V
peak force / V
 Remarkable linear
correlation between
peak discharge
force and peak AE
amplitude
 Range of
experiments
including variation
of
– Discharge current
– Open circuit voltage
– On-time
Peak force may be
estimated from the AE
signal

Dynamics of bubble collapse in EDM gap space
expansion compression split impingement
bubble
(a) 150 µs (b) 200 µs
0.4 mm
bubble
debris
Jet impingement may change removal mechanism for on-times exceeding ~100 µs

Correlations between process parameters and RMS AE I/II
100 150 200 250
0.04
0.05
0.06
0.07
0.08
0.09
AERMS/V
open circuit voltage / V dielectric
oil water
0.00
0.02
0.04
0.06
At a higher gap width due to increased open circuit voltage, the bubble does less work against the
surrounding fluid as it initially expands and it assumes a larger volume: higher bubble pressure and a
larger contact area yields higher AE. This hypothesis is verified by higher AE RMS corresponding to
lower kinematic viscosity of the dielectric fluid (water: 1 mm²/s vs. oil: 3.8 mm²/s).
AE correlates with the energy available for bubble dynamics
 RMS AE increases
with the open circuit
voltage although
discharge energy and
the resulting MRR are
constant
 RMS AE depends
strongly on gap width
and dielectric viscosity

Correlations between process parameters and RMS AE II/II
10 100
0.03
0.04
0.05
0.06
0.07
0.08
0.09
AERMS/V
pulse on-time / µs
workpiece cathode
workpiece anode
2 5 20 50 100 200 300 400
0.027
0.028
0.029
0.030
0.031
λ / (W/mK)
brass
aluminum
copper
500
Crossover in oil can be explained by carbon formation on anode at large pulse on-times > ~20 µs that
apportions more energy into the gas bubble. Thermal conductivity of electrode material further
determines energy in the gas bubble. The greater the thermal conductivity of anode, less energy is
available in the bubble and lower the AE
 Polarity effect for
MRR in oil:
– Long pulse:
workpiece cathode
– Short pulse:
workpiece anode
 More energy
partition to the
anode
 For water dielectric,
red curve stays
above the blue one
with no crossover

Conclusions and outlook
AE sensor
AE signal
gas bubble
plasma channel
shock waves
tool
workpiece
(a) 150 µs
bubble
debris
 Conclusions
– The AE signal was shown to essentially
reflect the energy available within the
gas bubble
– It comprises features pertaining to the
pressure pulse from the rapid expansion
of the gas bubble, followed by several
collapse and rebound cycles
– Jet impingement was identified to
contribute to removal for discharges
with long pulse on-times (> ~100 µs)
– Sequential discharge results are in line
with new theories regarding pyrolytic
carbon deposition, which depends on
the polarity and dielectric fluid used and
the pulse on-time
 Outlook
– There is a high potential for AE enabling
further unique insights into EDM

Acknowledgements
 German Research Foundation (DFG), Collaborative
Research Center SFB/TRR 136 “Process Signatures”
(Bremen, Aachen, Oklahoma) sub project F02.
 Alexander von Humboldt Foundation – Alumni Program,
Support for: Fundamental Investigations Into Acoustic
Emission From Electrical Discharge Machining,
Professor Philip Koshy, McMaster University, Canada.
 Industrial Research Circle EAK (Arbeitskreis
Elektroerosive Bearbeitung): Support for pre-competitive
fundamental and applied research in the area of EDM
and vast support in re-building research capabilities and
infrastructure after the big fire at WZL in February 2016.
EAK

Acoustic emission signatures of electrical discharge machining

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