Proceedings of the 10th International Conference on Frontiers of Design and Manufacturing
June 10~12, 2012, Chongqing, China
Study on Arc Stability of Underwater Wet Flux-Cored Arc Welding
Yonghua Shi1, Zepei Zheng2 and Jin Huang3
School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640
E-mail: 1yhuashi@scut.edu.cn, 2zhengzepei1987@163.com, 3becartjin@qq.com
Abstract: Stable arc is essential to good arc welding quality.
Due to the effects of water and pressure, the arc stability of wet
flux-cored arc welding (FCAW) is much worse than those of
in-air welding, and the unstable arc results in bad welding quality.
Investigating in influence factors of arc stability of wet FCAW
can provide theoretical guidance for improving of wet FCAW
quality. Standard deviation (SD) and relative standard deviation
(RSD) are used as the index of arc stability. And the arc stability
of wet FCAW in different welding conditions is studied based on
underwater welding experiments. Sensitivity models of arc
stability have been built and sensitivities of welding parameters
on arc stability, such as welding current, voltage, speed and
water depth, are analyzed. The results of this study can provide
theoretical instructions for optimizing of wet FCAW process
parameters.
increasingly applied. This study performs an analysis on
electrical signals of underwater wet FCAW and
investigates the factors that have influences on welding
arc stability, aiming at finding out the key influence
factors of arc stability in underwater wet FCAW. The
results of this study can be used as theoretical instructions
for improving wet FCAW quality.
Keywords: Underwater welding, Arc stability, Sensitivity
analysis
1
Introduction
With the accelerated exploitation of marine resources,
many marine structures have been built during the past
decades. Underwater wet welding has been increasingly
applied in the construction and maintenance of subsea
pipelines, oil platforms, bridges and others for its
convenience and flexibility.
The present studies of underwater wet welding focus
on the welding material, namely changing the components
of electrode covering or flux-cored wire to improve the
underwater welding quality. However, besides the suitable
welding material, the parameters and welding power
source play an important role in keeping the welding
progress stable.
Due to the effect of water and hydraulic pressure,
higher electric field intensity is required to ionize the gas
in underwater welding arc column and maintain a stable
welding arc. Studies show that the range of welding
parameters which keep the welding progress stable
becomes increasingly narrower with the increment in
water depth [1], as shown in Fig. 1.
Stable arc is essential to good welding quality.
Underwater welding parameters and hydraulic pressure
have a great influence on welding arc stability. Scholars
have done researches on arc stability of hyperbaric gas
metal arc welding (GMAW) and gas tungsten arc welding
(GTAW)[2-3]. Pessoa et al. studied arc stability on
underwater wet welding using rutile and oxidizing
commercial electrodes[4]. Mazzaferro and Machado
studied arc stability of wet manual metal arc welding
(MMAW) in shallow water[5]. But for arc stability of wet
flux-cored arc welding (FCAW), there are no related
reports published. As the efficiency of FCAW is much
higher than MMAW, underwater wet FCAW is
Figure 1 Effect of pressure on acceptable hyperbaric welding
parameters: 1bar corresponds to 10m of water depth[1]
2
Equipments and Materials
The experiments are conducted in a hyperbaric cabin. As
shown in Fig. 2, a three-dimensional motion platform is
installed in the cabin, which is used to control the
three-dimensional motion of welding torch and adjust the
welding speed and the contact tip to workpiece distance
(CTWD). The wire feeder is also placed in the cabin while
the welding power source out of the cabin. Before welding,
water is poured into the cabin to make sure that the arc is
underwater during the welding process. The cabin is also
pressurized by compressed gas, which is used to simulate
the different water depths (0.1MPa pressure is equivalent
to 10m of water depth).
Figure 2 Hyperbaric underwater wet FCAW and data
acquisition system of welding electrical signals
The self-shielded flux-cored wire used in the
underwater welding is 2mm in diameter. Bead-on-plate
(a) The original waveform of current
70
9
65
8
60
7
σ I/A
55
6
50
5
45
40
4
Current SD
Inverse of current CV
35
3
30
2
320
(b) The current waveform after wavelet filtering
360
400
Welding current I/A
440
Figure 4 Influence of welding current on current SD and
inverse of current CV
Figure 3 Arc current waveforms of underwater wet FCAW
100
8
90
7
80
6
70
σI /A
4 Indexes of Wet Welding Arc Stability
Generally, the welding arc voltage (U) is proportional to
arc length. When the arc is unstable, it will inevitably lead
to fluctuations of arc voltage and welding current (I).
Therefore, the arc current and voltage fluctuations also
reflect the arc stability. After being sampled, the welding
voltage and current are saved as a data set in the computer.
Analyzing the dispersion degree of these data can reveal
how large the current and voltage fluctuations are. From a
statistical point of view, sample range, standard deviation
(SD) and coefficient of variation (CV, also known as
relative standard deviation) can be used to reflect the
dispersion of the data. The smaller the sample range, SD
and CV are, the smaller the data fluctuation is. This
indicates that the voltage or current is more stable when
the data represent the welding voltage or current.
Therefore, the welding current range (∆I) and voltage
range (∆U), the current SD (σI) and voltage SD (σU), the
current CV (CI=σI/I) and voltage CV (CU=σU/U) can be
used as the indexes of arc stability[2]. Allowing for the fact
that signal range is susceptible to accidental factors, here
we use SD and CV of welding current and voltage to
measure the arc stability. In this study, inverses of CVs
(iCI=1/CI，iCU=1/CU) are adopted because of the very
iCI
3 Waveforms of Underwater Welding Electrical
Signals
When the welding wire is chosen, the welding current,
voltage, CTWD, welding speed and water depth become
the main influence factors on underwater welding quality.
In general, the CTWD of GMAW can be fixed according
to the diameter of welding wire. Here the CTWD was set
to 20mm. The underwater welding current waveforms
with the welding speed of 10mm/s were sampled. As
shown in Fig. 3, the current of underwater wet FCAW
fluctuates more intensively than that of air welding.
small value of coefficient of variation.
The underwater wet FCAW experiments were
conducted in a hyperbaric cabin and the electrical signals
were sampled. The σU, σI, CU, CI in different welding
conditions were calculated by a self-developed signal
processing software. When the CTWD is 20mm, the
welding speed is 10mm/s, the arc voltage is 34V and the
depth is 0.1m, the influence of welding current on current
SD and the inverse of CV is depicted in Fig. 4. As the
welding current increases, the SD of current shows a
tendency to increase, indicating that the welding arc is
more unstable, but the deterioration of arc stability is not
serious.
When the welding speed is 10mm/s, the welding
current is 320A and the depth is 0.1m, the influence of
welding current on current SD and CV is shown in Fig. 5.
It shows that as the voltage increases, the arc stability gets
better when the arc voltage is less than 32V, while it gets
worse greatly when the arc voltage exceeds 32V. When
the arc voltage is 32V, the arc becomes more stable.
When the welding speed is 10mm/s, the welding
current is 310A and the voltage is 30V, the influence of
water depth on SD and CV of welding voltage is depicted
in Fig. 6. It shows that the arc stability gets worse greatly
as the depth increases.
60
5
50
iCI
welds have been made on Q235B steel.
Welding current and voltage signals were acquired at
10 KHZ and then inputted to the data acquisition card.
Self-developed data acquisition software is used to capture
and analyze the underwater wet FCAW electrical signals.
4
40
Current SD
30
3
Inverse of current CV
20
2
26
28
30
32
Arc voltage U/V
34
36
Figure 5 Influence of welding voltage on current SD and
inverse of current CV
5 Sensitivity Analysis of Arc Stability
5.1 Establishment of the empirical equations
The experimental results show that the arc stability
changes monotonically as the welding parameters (i.e.
current, voltage, etc.) change, and their relationship accord
with the exponential function. So the empirical formulas
can be expressed as follows:
(1)
σ U = a0 I a1U a2 S a3 D a4
iCU = a0' I a1 'U a2 ' S a3 ' D a4 '
(2)
Where S is welding speed; D is water depth; a0 and a0′ are
constants; a1, a2, a3, a4, a1′, a2′, a3′ and a4′ are regression
coefficients.
Taking the natural logarithm of Eqs. (1) and (2) gives
regression analysis forms as follows:
(3)
ln σ U =ln a0 + a1 ln I + a2 ln U + a3 ln S + a4 ln D
'
'
'
'
'
(4)
ln(iCU ) =ln a0 + a1 ln I + a2 ln U + a3 ln S + a4 ln D
Eqs. (3) and (4) can be modeled by the method of multiple
linear regression.
30
3
25
2.5
20
2
15
1.5
σ U = 0.323I 1.0043U −1.2358 S −0.0104 D 0.0780
(5)
= 3.713I −1.0688U 2.3297 S −0.0485 D −0.0737
(6)
iCU
The calculated values of the reciprocal of voltage CV
according to Eqs. (5) and (6) and the actual measured
values are compared and shown in Fig. 7. The comparison
of the calculated values of voltage SD and the actual
measured values is shown in Fig. 8. The results indicate
that the models are accurate.
Inverse of voltage CV
Voltage SD
10
Caculated values of iCU
iCU
σ U/V
35
1
5
0.5
0
20
40
60
Water depth H/m
80
Figure 6 Influence of welding depth on voltage SD and
inverse of voltage CV
According to the voltage SD and the reciprocal of
voltage CV shown in Table 1, multiple linear regression
method can be utilized to determine the regression
coefficients in Eqs. (3) and (4). The empirical formulas
are obtained as follows:
25
20
15
10
10
The underwater wet FCAW experiments were
conducted in the hyperbaric cabin which was filled with
high pressure gas to simulate different depths. Welding
current, voltage, welding speed and depth were selected as
the main influence factors of arc stability. The CTWD
value was set to 20mm. The welding voltage signals were
sampled and the values of voltage SD and the reciprocal
of voltage CV were calculated. The experimental
parameters and results are presented in Table 1.
20
30
Measured values of iCU
Figure 7 Calculated and measured iCu in arc stability
analysis
2.6
Calculated values σU /V
Table 1 Experimental parameters and results of underwater
welding arc stability
Welding
Water
Current Voltage
σU
speed
depth
iCU
(A)
(V)
(V)
(mm/s)
(m)
320
34
10
0.1
1.07
32.31
340
34
10
0.1
1.23
27.63
360
34
10
0.1
1.33
25.53
380
34
10
0.1
1.41
23.97
400
34
10
0.1
1.36
23.06
420
34
10
0.1
1.40
22.23
440
34
10
0.1
1.54
22.13
460
34
10
0.1
1.57
21.72
320
30
10
0.1
1.36
21.98
320
31
10
0.1
1.20
26.06
320
32
10
0.1
1.15
27.80
320
33
10
0.1
1.02
32.21
320
34
10
0.1
1.07
32.31
320
35
10
0.1
1.04
33.93
300
32
6
0.1
1.13
28.37
300
32
8
0.1
1.14
27.92
300
32
10
0.1
1.23
26.13
300
32
12
0.1
1.15
25.23
310
30
10
0.1
1.24
24.25
310
30
10
10
1.61
19.08
310
30
10
20
1.57
17.85
310
30
10
30
2.01
15.38
310
30
10
40
1.87
15.04
310
30
10
50
2.22
13.80
310
30
10
60
2.44
13.50
30
2.1
1.6
1.1
0.6
0.6
1.6
Measured values σU / V
2.6
Figure 8 Calculated and Measured σU in arc stability analysis
5.2 Sensitivity analysis of underwater welding arc
stability
The sensitivity analysis is a mathematical method to find
out the key factors and sort them in accordance with their
importance[6,7]. In this study, it is used to analyze the
influence level of welding parameters on arc stability, and
to find out the most important influence factor of arc
stability.
Respectively, taking the partial derivatives with
respect to I, U, S and D of Eqs. (5) and (6), the sensitivity
formulas are obtained.
The arc voltage SD and CV sensitivities with respect
to welding current I are:
(7)
d (σ U ) / dI = 0.324 I 0.0043U −1.2358 S −0.0104 D 0.0780
d (iCU ) / dI = −3.968 I −2.0688U 2.3297 S −0.0485 D −0.0737 (8)
The sensitivities with respect to arc voltage U are:
d (σ U ) / dU = −0.399 I 1.0043U −2.2358 S −0.0104 D 0.0780 (9)
d (iCU ) / dU = 8.65 I −1.0688U 1.3297 S −0.0485 D −0.0737 (10)
The sensitivities with respect to welding speed S are:
d (σ U ) / dS = −0.0034 I 1.0043U −1.2358 S −1.0104 D 0.0780 (11)
d (iCU ) / dS = −0.18 I −1.0688U 2.3297 S −1.0485 D −0.0737 (12)
And the sensitivities with respect to water depth D
are:
d (σ U ) / dD = 0.0238 I 1.0043U −1.2358 S −0.0104 D −0.922 (13)
weak.
2.00
d(iCU)/dD
d (iCU ) / dD = −0.274 I −1.0688U 2.3297 S −0.0485 D −1.0737 (14)
When S is 10mm/s, U is 32V and D is 0.1m, the
sensitivity data are presented in Fig. 9.
0.10
U=32V, S=10mm/s, D=0.1m
Sensitivity
0.05
diCu/dI
diCu/dS
0.00
320
370
U=32V, S=10mm/s, I=370A
0.00
0.3
10
30
80
-4.00
420
-0.05
-6.00
Water depth D/m
-0.10
Figure 11 Sensitivity of water depth on arc stability
Welding current I/A
Figure 9 Sensitivity analysis results of welding current and
welding speed
The negative current sensitivity and speed sensitivity
shown in Fig. 9 indicate that the arc stability gets worse as
the welding current and welding speed increase. The
reason is that the underwater welding arc burns in the
bubbles, and there is more resistance to the bubbles with
the increase in welding speed, so that the welding arc is
disturbed and its stability gets worse. However, when the
arc voltage is 32V, S is 10mm/s and D is 0.1m, the
welding current and welding speed have a smaller
influence on arc stability as the current increase.
The positive sensitivities of welding voltage, which
are shown in Fig. 10, indicate that the reciprocal of CV
increases when the welding voltage increases, and it
means that the arc stability gets better. While the arc
stability gets worse as the water depth increases. When the
welding current is 370A, the speed is 10mm/s and the
depth is 0.1m, the influence of welding voltage and
welding depth on arc stability gets greater as the voltage
increases.
10
5
0
-5
-10
-15
-20
-25
I=370A, S=10mm/s, D=0.1m
28
32
diCu/dU
When water depth is 30m, arc voltage is 32V, and
welding speed is 10mm/s, the relationship between arc
stability index and welding parameters is shown in Fig. 12.
The sensitivity of welding voltage on arc stability is
positive sense while the sensitivities of welding current
and water depth on arc stability are negative sense. The
absolute value of voltage sensitivity is large, which
indicates that arc stability is more sensitive to welding
voltage than welding current and water depth. It means
that arc stability can be improved by increasing the
welding voltage within a suitable range.
1.40
1.20
1.00
Sensitivity
-0.15
Sensitivity
0.8
-2.00
0.80
Voltage
0.60
Water depth
Current
0.40
0.20
U=32V, S=10mm/s, D=30m
0.00
280
-0.20
330
380
430
Welding current I/A
Figure 12 Sensitivity analysis of the inverse of welding
voltage CV
36
diCu/dD
Arc voltage U/V
Figure 10 Sensitivity analysis results of welding voltage and
water depth
The negative sensitivities, which are shown in Figs. 9,
10 and 11, indicate that the reciprocal of CV decreases
when the welding current, welding speed and water depth
increases. It means that the arc stability gets worse. The
larger the absolute value of sensitivity is, the greater the
welding parameter’s influence on the reciprocal of CV is.
Therefore, the water depth has a greater influence on arc
stability than welding current and speed.
Fig. 11 reveals that the welding depth sensitivity of
iCU is negative sense and the arc stability gets worse as
the depth increases, especially in shallow water. But in
deep water, the influence of water depth on arc stability is
6
Conclusions
(1) The welding voltage has a great influence on arc
stability in underwater welding. Increasing the welding
voltage appropriately will improve the arc stability. But if
the voltage exceeds its optimum value, it will lead to the
rapidly deteriorating of arc stability.
(2) The sensitivity of welding depth on arc stability is
negative sense, indicating that the arc stability gets worse
as water depth increases, especially in shallow water.
However, in deep water, the influence degree of welding
depth on arc stability gets smaller as the depth increases.
(3) The sensitivities of welding current and speed on arc
stability are negative sense. Although the arc stability gets
worse as the welding current and welding speed increase,
the influence of current is much smaller.
(4) The results of this study can provide theoretical
instructions for optimizing of wet FCAW process
parameters.
Acknowledgements
This work is supported by the National Natural Science
Foundation of China (No. 51175185, 50705030) and
Natural Science Foundation of Guangdong Province (No.
9151064101000065).
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