View
218
Download
0
Category
Preview:
Citation preview
7/30/2019 Presentation EDM 8
1/30
PAULO PECAS & ELSA HENRIQUES
Effect of the powder concentraton and
dielectric flow in the surface morphology inelectric discharge machining with powder-
mixed dielectric (PMD-EDM)
B
Y
7/30/2019 Presentation EDM 8
2/30
INTRODUCTION
Since 1940 a considerable research effort has fostered a deepunderstanding, prediction and control of the electro-discharge
machining process (EDM).
One of the preferential areas is related to the surface qualityanalysis, namely the study of EDM finishing and polishing
steps. Several research approaches like the discharge process
modification, the electrode morphology and the electrodeorbital movement have been undertaken. Also Approachesbased on the conjunction of the EDM process with the
electrochemical machining, with ultrasonic machining and with acontouring robot were also followed.
Another research approach, which is presenting promisingand reliable results, consists of the use of conductive and semi-conductive powder materials suspended in the dielectric.
7/30/2019 Presentation EDM 8
3/30
INTRODUCTION
Promotes gap increasing Stability of the process increases
Polishing time and the influence of the electrode area in
the surface roughness are reduced.
Effect of powder material suspended in the dielectric :
7/30/2019 Presentation EDM 8
4/30
INTRODUCTION
Jeswani: Uses graphite powder suspended in the dielectric and Reported
that the powder concentration has an important influence on the
discharge process, increasing both the gap distance and the discharging
rate.
Powder concentration: 4 g/l.
Mohri et al. and Narumiya et al. : Uses silicon, aluminium and graphite
as powder materials and their results showed that the gap distance
increases with the powder concentration and is larger for the
aluminium powder but there is no direct relation between the surfaceroughness and the gap distance. In fact, the best results concerning the
surface roughness were achieved for low powder concentrations levels
and for silicon and graphite powders.
Powder concentration: 2-40 g/l.
Results of various scientists:
7/30/2019 Presentation EDM 8
5/30
INTRODUCTION
Yan and Chen and Ming and He: The powder particles contribute
to the reduction of surface cracks and to the smoothness and
homogenisation of the white layer.
Powder concentration : 2-5 g/l.
Uno and Okada: Silicon powder particles, besides, generate the
increasing of the gap distance also reduces the discharge
impact force
Powder concentration: 23g/l.
Wong at al: Confirms the results achieved by previous
researchers. Accordingly the best surface finishing was
obtained for silicon and graphite powders with a low
powder concentration (2 g/l).
7/30/2019 Presentation EDM 8
6/30
INTRODUCTION
Klocke et al : Used Silicon powder with avg particle size 10 m
and 10 g/l flushing flow.
It promotes a softer transition from the white layer into the
matrix material the the observed with powder free dielectric. Pecas and Henriques: Increasing of the polishing time induces a
clear improvement of the surface quality if the PMD-EDM
conditions are used and only a fair improvement for the
conventional EDM conditions.
From all above research results, silicon & graphite powder
materials allow a consistent improvement of the surface
roughness.
Powder concentration: 1-20 g/l
Grain size: 1-15 m
To study effect of powder suspended dielectrics in the thermal
influenced flow
7/30/2019 Presentation EDM 8
7/30
EXPERIMENTAL SETUP, MATERIALS & SURFACE
ANALYSIS
To perform expt., EDM programme
was selected having typical
sequence of 13 machining regimes.
Electrode penetration: 0.5 mm
Dielectric: Kerosene-Castrol SE
Fluid 180 and with powder-
mixed dielectric for the same
parameter setting (Fig. 1).
Powder material: 99.5 % pure
silicon.
Particle size: 10 m.
Fig. 1 Flushing nozzle and 1600
mm2machined sample a) with conventional
dielectric conditions and b) with siliconpowder suspended in dielectric
7/30/2019 Presentation EDM 8
8/30
EXPERIMENTAL SETUP, MATERIALS & SURFACE
ANALYSIS
Silicon powder was added only in last 3 regimes.
Polishing time: 100 min.
Workpiece: Hardened mould steel AISI H13 (54 HRc).
The powder concentration, the flushing flow and
the electrode area were varied during the tests in order to assess
their influence over the process technological performance,
measured through the craters' average diameter and depth, the
white-layer thickness, the final surface roughness and the type
of discharges occurred.
7/30/2019 Presentation EDM 8
9/30
EXPERIMENTAL SETUP, MATERIALS & SURFACE
ANALYSIS
Monitoring system (Fig.2) identifies
discharges as:
Normal discharges- 30 % of reference
Voltage.
Electric-arc discharge- Above 70 % of
Ionisation voltage.
Short circuit discharge- Below 7 V.
Open circuit discharge type is pointed
for long ionization time meaning that
the dielectric condition
(ex.: contamination) inhibits discharge
ignition.
Type of discharges:
7/30/2019 Presentation EDM 8
10/30
RESULTS AND DISCUSSION
The influence of the silicon
powder concentration:
The powder concentrationwas varied from 0 g/l(conventional conditions) to
20 g/l. A copper electrode with 1600
mm2 of square surface areawas used with a constantdielectric flow rate of 1
l/min20 g/l.
Fig 3 presents images ofsamples performed withdiffeent silicon powder
concentrations.
7/30/2019 Presentation EDM 8
11/30
RESULTS AND DISCUSSION
7/30/2019 Presentation EDM 8
12/30
RESULTS AND DISCUSSION
The occurred abnormal discharge are solely of short-circuit type what can be
explained by an excess of powder particles in the gap promoting the formation
of particle bridges between the electrode and the workpiece.
7/30/2019 Presentation EDM 8
13/30
RESULTS AND DISCUSSION
Surface craters formed explains the surface roughness distribution.
The presence of a high concentration of silicon powder contributes to
the formation of valleys on a matrix of smooth discharges (Fig. 6)
7/30/2019 Presentation EDM 8
14/30
RESULTS AND DISCUSSION
The presence of excessive particles of silicon powder causes the occurrence of
short-circuiting that can be pointed out as one of the main reasons for the
formation of the referred deep valleys.
The use of silicon powder concentration in the range of 1 to 3 g/l promotes theformation of slightly smoother craters resulting in the reduction of the surface
roughness. Even when observed with the scanning electron microscope it is
difficult to identify the craters' borders in the sample with the lowest surface
roughness obtained for a silicon powder concentration of 2 g/l (Fig. 7c)
7/30/2019 Presentation EDM 8
15/30
RESULTS AND DISCUSSION
But the surface roughnessobtained for the highest
powder concentration is very
superior compared to powder
free dielectric in which only
small percentage of short
circuiting is observed.
The use of silicon powder
concentration in the range of
1 ot 3 g/l
promotes the formation of
slighty smoother craters
resulting into reduction of the
surface roughness.
7/30/2019 Presentation EDM 8
16/30
RESULTS AND DISCUSSION
Due to presence of powder particles several discharging paths
within one single input pulse is generated,so a single input pulsedue to presence of powder generate several discharging spots.
This phenomena leads to reduction of the matrix craters diametereven for a higher concentration range of silicon powder.
This behaviour is shown in fig.8
7/30/2019 Presentation EDM 8
17/30
RESULTS AND DISCUSSION
The expected behavior due to the phenomena referred for the craterdiameter, the fig. 9
The presence of powder particles causes the reduction of the averagecrater depth for the low concentration range and the increase of it forthe high concentration range.
7/30/2019 Presentation EDM 8
18/30
RESULTS AND DISCUSSION
The increase of the powder concentration tends to increase the gapdistance. And the larger gap reduce the discharge channel impulsive
force contributing to the formation of smoother craters, thus
generating a more homogeneous and thinner white-layer(fig 10).
The distribution of the white layer thickness with the powder
concentration is shown in fig.11
7/30/2019 Presentation EDM 8
19/30
RESULTS AND DISCUSSION
7/30/2019 Presentation EDM 8
20/30
RESULTS AND DISCUSSION
To study the influence of the dielectric flow in the surface
morphology the powder concentration was set up to 2 g/l for the
tests with silicon suspended dielectric.
The dielectric flow rate wa varied from 0.5 to 2.5 l/min for the
several electrode areas between 100 to 6400 mm^2.
In fig 12. one can observe that there is a dielectric flow rate at
which surface roughness is minimum and beyond this value of
dielectric flow rate the is no decrease in roughness.
The flow rate level that minimises the surface roughness is
independent of dielectric condition.
7/30/2019 Presentation EDM 8
21/30
RESULTS AND DISCUSSION
7/30/2019 Presentation EDM 8
22/30
RESULTS AND DISCUSSION
For flow rates lower than the one that minimises the surface
roughness, one can observe that an extreme percentage of
abnormal discharges from 1 to 10 regimes.
This phenomenon occurs as the dielectric removal power for the a
small flow rate is not enough to clean eroded particles efficientlyfrom the gap zone.
So an excessive presence of eroded particles contributes to the
machined surface deterioration.
This phenomenon is clearly detected in the abnormal dischargesdirtribution with the dielectric flow rate.(fig 13).
7/30/2019 Presentation EDM 8
23/30
RESULTS AND DISCUSSION
For higher flow rates, two types of situation can occur:
1) An excessive dielectric pressure can create instability in the
discharge zone contributing to the surface roughness deterioration.
As mentioned by chow et al.[27]
2)The dielectric flow rate increase beyond certain limits does not
produce additional effect in the surface roughness since its power
is wasted in workpiece and electrode walls.These two situations
can be observed in fig.12
7/30/2019 Presentation EDM 8
24/30
RESULTS AND DISCUSSION
7/30/2019 Presentation EDM 8
25/30
RESULTS AND DISCUSSION
With regards to the last three polishing regimes for the
conventional dielectric condition, the gap distance is so small thateven with high dielectric flow rates the gap cleaning is noteffective,leading to occurance of abnormal discharge.(fig 14)
At lower dielectric rate even if we use silicon powder, it ispossible to observe occurance of abnormal discharge.
Actually the powder particles increases the gap distance thecleaning action of dielectric is fruitful if the flow rate issufficiently strong to remove the eroded particles.
For the particular case of 1600 mm^2 the use of dielectric flowrate higher than 1.0 l/min avoids the occurance of abnormaldischarge promoting the reduction of the surface roughness.
7/30/2019 Presentation EDM 8
26/30
RESULTS AND DISCUSSION
The sample topography
analysi allows one to validate
the behaviors till mentioned.
As exhibit by image a) and
d) of the fig.15
7/30/2019 Presentation EDM 8
27/30
RESULTS AND DISCUSSION
The diameter of the craters in not significantly affected by the
dielectric flow rate variation for the both dielectric conditions
tested (fig 16).
Deep craters are identified for lower dielectric flow rate and crater
depth tends to become constant with its increasing (fig.17).
The dielectric condition exhibits a significant influence in the
crater dimensions since with the use of silicon powder the crater
width is reduced about 5 um and the crater depth is reduced about
1 um.So, the use of the PMD-EDM conditions contributes to the
formation of smaller and less deep craters, but does not change the
craters dimension behavior with the dielectric flow rate variation.
7/30/2019 Presentation EDM 8
28/30
RESULTS AND DISCUSSION
7/30/2019 Presentation EDM 8
29/30
CONCLUSIONS
Two parameters of the process were studied over several electrode
areas: the powder concentration and the flushing flow rate.Crater diameters, crater depth and the white-layer thickness are
reduced by the use of silicon powder particles suspended in the
dielectric. This reduction is evident even for a small level of
powder concentration.Short-circuiting occurs for higher concentration range that causes
formation of deep valleys. Therefore, powder concentration that
enables a better surface morphology is in the range of 2 to 3 g/l.
The performance of the polishing regimes, measured by thepercentage of abnormal discharges, is affected by the dielectric
flow rate when the silicon powder is used.
With regards to the surface quality, there is a dielectric flow rate
that minimizes the surface roughness for each electrode area
7/30/2019 Presentation EDM 8
30/30
Thank
you
Recommended