effect of dual swirling plasma arc cutting parameters

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ORIGINAL RESEARCH

Effect of dual swirling plasma arc cutting parameters

on kerf characteristics

Jiayou Wang & Zhengyu Zhu & Conghui He & Feng Yang

Received: 29 July 2009 /Accepted: 3 June 2010 /Published online: 19 June 2010# Springer-Verlag France 2010

Abstract A numerical control 3-D processing system was

constituted for dual swirling plasma arc cutting. The effect of cutting energy parameters and operating gases on kerf

characteristics was then investigated experimentally, so as

to provide a reference for appropriately selecting process

parameters to improve cut quality. It is shown that kerf

widths reduce, and the bevel angle and the straightness

increase with an increase of cutting speed and a decrease of

arc current. Moreover, a smaller bevel angle, together with

greater straightness and more dross, exhibits on the low

speed side of the cut. As the oxygen content of the

operating gas decreases, kerf widths decrease and the dross

increases, while the bevel angle varies slightly on the high

speed side of the cut. For the pure oxygen and pure air

processes, the bevel angle on the low speed side and the

straightness of cut surface are the smallest, but the pure

oxygen cut surface is the roughest due to the occurrence of

a saw-like kerf.

Keywords Plasma arc cutting . Swirling arc .

Kerf characteristics . Cut quality . Automatic cutting

Introduction

Plasma arc cutting (PAC) is a thermal cutting process of

high efficiency and high quality, primarily suitable for

metallic materials. This process utilizes an arc plasma at

high temperature and fast speed to melt the base metal andto expel the molten material immediately, thus forming a

cut. According to the cutting operating gas that suits for the

base material to be cut, and the PAC is commonly classified

into air, O2, N2, Ar-H2 and N2-H2 plasma processes [1, 2].

To improve further cutting quality and to lower cutting cost,

some new concept plasma cutting processes have been

developed, such as oxygen-shielded air plasma cutting,

single and dual swirling plasma cutting, and ultrasonic

frequency pulsed plasma cutting [3 – 6]. With the rapid

developments in cutting power supply and process control

techniques, furthermore, automatic and high-power plasma

cutting comes recently true [7 – 9]. Thus, the PAC process

has found growing use in modern metal manufacturing

industries.

Of the new concept PACs, the swirling and dual plasma

processes are particularly attracting in cutting steels. The

swirling process can remarkably improve the inclination

and shoulder of cut surface while decreasing the adherent

dross to the cut [5]. In the dual plasma arc cutting, an outer

gas stream works to protect the nozzle against rapid burn,

and also to shield the cut surface from nitrogen if using

oxygen gas [4, 10]. Therefore, a desirable cut surface of

good shape and reprocessability is available directly on the

low speed side of kerf, but also the life of consumable parts

is obviously increased. In practice, the selection of

appropriate cutting parameters is very important for the

good process results. To understand better the dual swirling

plasma process and to provide a reference for actual cutting

application, the present work constitutes a numerical

control three dimensional (3- D) processing system for dual

swirling plasma arc cutting, and then investigates experi-

mentally the effect of arc current, cutting speed and

operating gas type on kerf characteristics.

J. Wang (*) : Z. Zhu : C. He : F. Yang

Provincial Key Laboratory of Advanced Welding Technology,

School of Materials Science and Engineering,

Jiangsu University of Science and Technology,

2 Mengxi Road,

Zhenjiang, Jiangsu 212003, People’s Republic of China

e-mail: [email protected]

Int J Mater Form (2011) 4:39 – 43

DOI 10.1007/s12289-010-0990-y



Plasma arc cutting system and experimental procedure

The dual swirling plasma arc cutting system is schemati-cally illustrated in Fig. 1. This system primarily consists of

plasma cutting power supply, cutting torch, the manipulator

of three rectangular coordinates, process control unit, and

operating gas and cooling water sources. Cutting power

supply is a digital signal processor (DSP) controlled

inverter of constant current output characteristic, the

positive and negative of which are connected respectively

to the workpiece and the electrode. A hafnium bar is

inserted in the electrode tip to act as the cathode. The water-

cooled torch of dual nozzles is placed perpendicularly

against the workpiece surface; the plasma and shielding

gases flow respectively through the inner and outer nozzles

in clockwise swirling state. A numerical control manipula-

tor of high precision carries the torch to move in the y and/

or z directions, and can drive the workpiece to give out

cutting speed in the x direction. Arc current and operating

gas flowrate are digitally set and displayed in real time. In

cutting, an arc is initiated between the hafnium cathode and

the inner nozzle by a high frequency igniter, and then

transferred to the workpiece. As the main arc steadily

burns, the control unit automatically cuts off the pilot

current of the arc, and simultaneously gives an arcing signal

to start the manipulating motion.

To investigate the effect of cutting process parameters

on kerf characteristics, a number of cutting experiments

were carried out. Experimental conditions were: 45 – 75 A

for arc current, 5 – 50 mm/s for cutting speed, 6 mm for

testpiece thickness, 2 and 3 mm for torch standoff height,

31 and 12 L/min at 0.81 MPa respectively for the

flowrates of plasma and shielding gases, 1.1 mm for the

diameter of the hafnium core in the electrode, and1.2 mm for the inner nozzle orifice diameter. A mild

steel plate of size 120×40 mm was rigidly fixed on the

+P

Power

Supply

Stage

ProcessControl

M o t i o n C o n t r o l

z y

x

13

4

5 5

2

CurrentDetection

Pilot current

Arcing signal

Torch

Cooling waterGas control

Setting& Display

6

7

8

Fig. 1 Schematic diagram of dual swirling plasma arc cutting system.

1-Electrode, 2-Plasma gas, 3-Inner nozzle, 4-Outer nozzle, 5-Shield-

ing gas, 6 -Hafnium, 7 -Arc, 8-Workpiece

0 10 20 30 40 500

1

2

3

4

K

( m m )

V c (mm/s)

0 10 20 30 40 50

V c (mm/s)

0 10 20 30 40 50

V c (mm/s)

I a (A)

45

75

K t K b

a Kerf widths

I a (A)

45

75

θ L θ R

-10

-5

0

5

10

15

θ ( d

e g . )

b Bevel angle

I a (A)

45

75

StR StL

0.2

0.4

0.6

S t

( m m )

c Straightness

Fig. 3 Effect of cutting energy parameters on kerf shape (h=3 mm)

K t

K b

S t

θ

Left Right

Dross

Fig. 2 Cut shape factors

40 Int J Mater Form (2011) 4:39 – 43



work stage, and finally a cut of 80 mm length was

longitudinally formed in the middle of the plate. After

cutting, an observed section was obtained at 20 mm apart

from the end of the cut to evaluate kerf shape. The shape

factors of the cut usually include [11]: top kerf width( K t ), bottom kerf width ( K b), bevel angle (θ), straightness

or flatness (S t ), and the adherent dross to the cut, as shown

in Fig. 2.

In the experiments, the cross section of kerf was

digitally photographed, and then the geometrical param-

eters of kerf were accurately measured in computer

based on the enlarged picture of the cross section. To

inspect the reliability of experimental results, moreover,

cutting test was actually repeated twice for every set of

process parameters (i.e., a combination of arc current,

cutting speed, and the type of operating gas). However,

a set of experimental results were finally presented

below for a convenient expression, because of the

measured values of the shape parameters are consider-

ably close twice.

Kerf characteristics

Effect of cutting energy parameters

The plasma cutting energy is determined by such parame-

ters as arc current, arc voltage and cutting speed, of which

the arc voltage is influenced by torch standoff height, arc

current, cutting speed and arc constricted degree. So, arc

current ( I a) and cutting speed (V c) are two dominant energy

parameters for given torch parameters and operating gas.

Figure 3 shows the effect of cutting energy parameters on

kerf shape at the torch standoff height (h) of 3 mm. Here,

oxygen and air were used respectively as the plasma and

shielding gases; θ L and θ R denote the bevel angle, and S tLand S tR represent the straightness, respectively on the left

and right sides of the cut.

With an increase in cutting speed and a decrease of arc

current, cutting energy decreases, and the flushing action of

the arc on cut surface becomes weak. Accordingly, kerf

widths ( K ) decrease and the straightness increases, while

the bevel angle goes great due to a rapider decrease of

bottom kerf width than top kerf width. At low cutting

speeds, arc voltage rises as the main area of arc anode spots

submerges, and thus more heat from the anode spots andarc column acts on the bottom of the cut. Finally, several

negative bevel angles occur at 5 mm/s because of K b more

than K t . On the right side of the cut, the bevel angle

obviously becomes smaller as a result of the lower actual

cutting speed induced by the swirling effect of the operating

V c (mm/s) I a (A)

5 3010 50

45

75

3mm

Fig. 4 Photographs of kerf shape at different cutting energy

parameters (h=3 mm)

1

2

3

4

air+airO2+O2 air+O2O2+air



K

( m m )

V c (mm/s)

30

40

K t K ba Kerf widths

0

5

10

15

θ ( d

e g . )

V c (mm/s)

30

40

θ L θ Rb Bevel angle

0.2

0.3

0.4

0.5

S t

( m m )

V c (mm/s)

30

40

StR StLc Straightness

Fig. 5 Effect of cutting operating gas type on kerf shape ( I a=60 A

and h=2 mm)

Int J Mater Form (2011) 4:39 – 43 41



gas, although the straightness is somewhat greater than that

on the left side.

Figure 4 gives several examples of cut cross-section

photograph. At the lower arc current and the slower

cutting speeds, the more dross attaches to the bottom of

the cut, particularly on the right side (i.e. low-speed side)

of the cut. It is also demonstrated that the difference

between the left and right bevel angles obviously becomes

great with increasing cutting speed at 75 A. As a result, the

dross-free and nearly perpendicular cut surfaces of

acceptable flatness exhibit on the low-speed side of the

cut.

Effect of operating gases

The operating gas is a combination of the plasma and

shielding gases for the dual plasma arc cutting. If the gas

contains a content of oxygen, the heat produced from the

oxidation reaction of oxygen with metal, along with the

electric heat of the arc, will contribute to the formation of

kerf. Figure 5 shows the effect of the common operating

gases on kerf shape at h=2 mm and I a =60 A, where the

horizontal coordinate indicates the combination type of the

“ plasma + shielding” gas.

When the operating gas varies from “O2+O2”, “O2+air ”,

“air+O2” to “air+air ”, the oxidation reaction heat decreases

during cutting owing to a reduction of the oxygen content in the gas. Accordingly, kerf widths narrow to a varying

extent, where the greatest widths imply the highest cutting

efficiency of the “O2+O2” process. On the other hand, the

bevel angle changes more remarkably on the right side than

on the left one of the cut. For the pure oxygen and pure air

processes, the right bevel angle and the straightness of cut

surface are the smallest. Regardless of the operating gas and

cutting speed, the right bevel angle is much smaller than the

left one, whereas the right straightness is obviously greater

than the left one, which also further verifies the results

mentioned in Fig. 3b and c.

Figure 6 indicates the kerf shape for different operatinggases at 30 and 40 mm/s. It is clear that the more dross

attaches to the bottom of the cut in the usage of the

operating gas containing air. For the “O2+O2” process, an

indentation and plenty of oxide dross readily occur on the

internal sides of the cut. Figure 7 gives the photographs of

Plasma

+

Shield

O2+O2

O2+air

air+O2

air+air

5 30

Low-speed side 3mm

V c (mm/s)

Fig. 7 Photographs of cut surface at different operating gases ( I a=

60 A and h=2 mm)

V c (mm/s)Plasma

+

Shield 30 40

O2+O2

O2+air

air+O2

air+air

3mm

Fig. 6 Photographs of kerf shape at different operating gases ( I a=

60 A and h=2 mm)

42 Int J Mater Form (2011) 4:39 – 43



the right cut surface at 5 and 30 mm/s, where the arrow

indicates the direction of cutting. Some thick cutting lines

are visible on the “O2+O2” cut surface regardless of

cutting speed, in which a saw-like kerf was actually

viewed from the front side of the testpiece. It is also

demonstrated that the cut surface is smoother at 30 mm/s

for the “O2+air ” and “air+O2” processes, although the

indentation phenomenon occurs also at 5 mm/s. On the pure air cut surface, there is much residue of oxide and

nitride primarily along the drag lines, thus affecting its

surface smoothness.

The above experimental results interpret that the

“O2+O2” plasma process is undesirable in the high

tolerance plasma cutting. Moreover, there is no great

difference in kerf shape for the “O2+air ” and “air+O2”

processes, whereas the “air+air ” process can produce a cut

of better geometrical shape. Thus, the latter three processes

may be practically selected according to the different

demands for the cutting efficiency, electrode cost and cut

quality.

Conclusions

The present work constitutes a numerical control 3-D

processing system for the dual swirling plasma arc cutting,

and then investigates experimentally the effect of process

parameters on kerf characteristics. It is shown that:

(1) With a decrease of arc current and an increase of

cutting speed, kerf widths decrease, and the bevel

angle and the straightness increase. On the low speedside of the kerf, there always is a smaller bevel angle

although greater straightness and more dross readily

exhibit, and finally a dross-free and nearly perpendic-

ular cut surface of acceptable flatness can be obtained

for succeeded processing.

(2) As the oxygen content of the operating gas decreases,

kerf widths decrease and the dross increases, while the

bevel angle varies slightly on the high speed side of

the cut. For the pure oxygen and pure air processes,

the bevel angle on the low speed side and the

straightness of cut surface are the smallest, but the

pure oxygen cut surface is the roughest due to

the occurrence of a saw-like kerf.

Acknowledgements Thanks are due to the financial supports from

the National Natural Science Foundation of China (Grant No.

50675091) and the Natural Science Research Plan of Jiangsu

Provincial Universities (Grant No. 06KJA430006).

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effect of dual swirling plasma arc cutting parameters

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