Improvement of cut quality
in plasma arc cutting with a micro-jet shroud
S. Kim1,2, J. Heberlein1, J. Lindsay2 and J. Peters2
1
Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55414,USA
2
Hypertherm Inc. Hanover, NH 03755, USA
Abstract:
The further development and deployment of thermal plasma technologies has
been limited by the incomplete understanding of fluid dynamic instabilities that
occur when a highly constricted plasma arc interacts with the cold gas
entrainment. Such instability of the cutting process negatively affects the cut
quality and reproducibility. This research is motivated by the need to obtain
fundamental understanding of the fluid dynamics of the plasma flow and its
interaction with cold gas entrainment. This work characterized the arc
instabilities, and focused on plasma torch design changes that would optimize cut
quality. In order to characterize the fluid dynamic instabilities, Schlieren imaging
with a high speed CCD camera was used to visualize the plasma arc column and
ambient gas flow. In order to estimate cut quality, cut angles on each side of cut
samples and surface roughness comparison were performed. Modified torch
designs with micro-jet injection into the shear layer were evaluated and
compared with the original designs with respect to the fluid dynamic instability
and cut quality.
Keywords:
Plasma arc instability, cut quality, ambient gas entrainment and micro-jet
1. Introduction
Plasma arc cutting is commonly used to cut
most metals in many applications, including
heavy-duty equipment manufacturers, steel
service
centers,
automotive
industries,
shipbuilding and art. The plasma gas reaches
high temperatures (typically, from 15,000 to
30,000 K) by the electric heating from an
electric arc, and dissociates and ionizes,
generating a plasma. The plasma gas flows
inside the torch around the cathode, is
constricted by the nozzle inside the torch, and
then reaches the anode. The plasma arc
fluctuations resulting from fluid dynamic
instabilities pose some limits on further
improvements of cut quality in plasma arc
cutting technology. Such instabilities occur
when a highly constricted plasma jet interacts
with the cold gas environment. The goal of this
research has been 1) the characterization of the
arc instabilities by means of plasma and ambient
gas flow visualization, and 2) improvement in
cutting performances such as cut angle and
surface roughness according to different torch
designs.
2. Approach
All experiments reported here were conducted
using a Hypertherm HT 2000 plasma arc cutting
system run at a current of 200 A dc. Oxygen gas
was used for cutting operations.
A. Diagnostic methods
In order to visualize the plasma arc width and
ambient gas entrainment, high speed Schlieren
imaging was used for the characterization of
fluid dynamic instability [3]. Measured arc
width average value shows arc constriction
while standard deviation of the arc width
indicate the degree of plasma arc instability. In
addition, ambient gas entrainment into the
plasma jet was determined from the nitrogen
line emission using a narrow bandpass filter
with a high speed CCD camera (refer to [3] for
detailed information).
B. Torch design modification
The original torch consumables and two
modified consumables with straight and swirl
micro-jet shield gas were used for this study.
The main differences are the employment of
micro-channels for the shield gas (see Fig. 1) in
the nozzle after blocking the space between the
nozzle and shield cap. The reason for employing
the micro-jets from the micro-channels
surrounding the plasma gas jet is that we can
expect the micro-jets around the arc to reduce
the large scale shear layer turbulence and the
associated entrainment of the cold ambient gas
into the hot plasma jet, thus increasing the arc
stability [4]. The modified torch with swirl
micro-jets has 20° swirling component in the
jets.
Figure 1. Schematic of the modified (left) and original
(right) HySpeed torches [3]
C. Cutting performance
As for measures of cutting performances, the cut
edge angle and angle variation, and the cut
surface roughness are measured. The
measurements have been performed on the four
cut sides of 3x3 inch square cut samples, (0.5
inch thickness). Cut angles are measured using
five points along a line at the center of each
side. After fitting the five points into a straight
line, the angle is defined as the angle between a
line perpendicular to the sample surface and the
fitted line as shown in Figure 2.
Figure 2. Cut quality measurements
Relative distance from shield cap to kerf plate [%]
Relative distance from shield cap to kerf plate [%]
The cut angle for each side is determined, as
shown in Figure 3 [2], as the angle between a
line perpendicular to the sample surface and the
fitted line. The positive angle is defined for the
top edge shifted towards the center of the
sample. For an ideal cut, each surface side has
to have the same cut angle and the angle should
be zero degrees. However, often a cut has
definite angles, and the angles vary from side to
side.
0
20
40
60
80
100
1.5
Modified torches with micro-jet shield gas
surrounding the arc jet have better arc
constriction and stabilization, and less ambient
gas entrainment than the original torch. Figure 4
compares the average and standard deviation of
arc width for different torches. The torches with
swirl and straight micro-jets have a more
constricted and stable arc jet. In addition, the
torches with swirl and straight micro-jets have
less entrainment of ambient gas into the plasma
jet (see Fig. 5).
2.5
3
3.5
4
Mean of arc width [mm]
4.5
5
Original HySpeed
Straight micro-jet
Swirl micro-jet
20
40
60
80
100
0
0.05
0.1
0.15
0.2
0.25
Standard deviation of arc width [mm]
0.3
Figure 4. Comparison of arc width (top) and standard deviation
(bottom) of arc width fluctuations for original and straight and
swirl micro-jet torch designs
Relative distance from shield cap to kerf plate [%]
3. Results
2
0
Figure 3. Cut angle determination in cut sample [2]
The surface roughness obtained with the
original and the modified consumables has been
measured by a surface profilometer with 1 Å
resolution.
Original HySpeed
Straight micro-jet
Swirl micro-jet
0
Original HySpeed
Straight micro-jet
Swirl micro-jet
20
40
60
80
100
1
1.5
2
2.5
3
3.5
RNEW
Figure 5. Comparison of entrainment for original and straight
and swirl micro-jet torch designs
The shield cap tip of the modified torches was
shortened as shown in Figure 1. The distance
between the torch tip and the workpiece was
increased for the modified torches with microjets from 4 to 6 mm in order to compensate for
the shortened cap. The modified torches show
lower average values and variation in cut angles
and better surface smoothness than the normal
torches. Table 1 compares the cut angle and
deviation with different torches. Cut flow for
swirl micro-jet was set to 70 scfh for cut angle
optimization. Swirl micro-jet shows the lowest
angle deviation.
The horizontal direction refers to the direction
of the torch movement.
Figure 7. Surface roughness comparison with different cut
speeds
4. Conclusions
Table 1. Cut angle comparison with different cut flows
The original torch showed streaks of ridges on
cut surfaces while the modified torches showed
smooth surfaces for the cuts, as shown in Figure
6. This may be due to increased drag on the
molten material by the swirling component in
the micro-jet.
It can be concluded that micro-jets in the shield
cap can improve not only arc stability by
breaking down the large scale entrainment of
cold ambient gas into the plasma, but also cut
performance by offering improved shear layer
control.
References
[1] J. Peters, J. Heberlein and J. Lindsay,
Spectroscopic diagnostics in a highly
constricted oxygen arc, J. Phys. D: Appl. Phys.,
v 40, pp. 3960-3971, 2007
Figure 6. Comparison of cut surface appearance at different
cutting speeds (showing streaks of ridges for the normal torch
cuts and smooth surfaces for the cuts obtained with the modified
torch)
Figure 7 shows a comparison of the surface
roughness obtained with the original and the
modified torch with swirl micro-jets in
horizontal direction, for the different cut speeds.
[2] J. Peters, B. Bartlett, J. Lindsay, J. Heberlein,
Relating Spectroscopic Measurements in a
Plasma Cutting Torch to Cutting Performance,
Plasma Chem. Plasma Process., v 28, pp. 331352, 2008
[3] S. Kim, J. Heberlein, J. Lindsay and J.
Peters, “Methods to evaluate arc stability in
plasma arc cutting torches,” J. Phys. D: Appl.
Phys, v. 43, December 2010
[4] C. Shih, F. S. Alvi, H. Lou, G. Grag, and A.
Krothapalli, Adaptive flow control of supersonic
impinging jets, AIAA paper 2001-3027, 2001