Download DOUBLE-SIDED ARC WELDING INCREASES WELD JOINT

DOUBLE-SIDED ARC WELDING INCREASES
WELD JOINT PENETRATION
Connecting two torches to the two terminals of a single power supply achieves more concentrated
arcs that increase weld joint penetration
By Y. M. Zhang and S. B. Zhang
Joining of components plays a critical role in determining the cost and productivity of manufacturing and the
quality of final products. As the primary method for metal joining, welding is practiced in almost all
industries. Cost reduction and productivity improvement in welding operation can therefore generate
considerable impacts on competitiveness of various manufacturing industries.
Plasma Arc
Welding
Power
Supply
PA Electrode
PA
Torch
Plasma Arc
Work
(a)
Plasma Jet
and Keyhole
Current and
Arc Zone
Current
lines
Work
(b)
Plasma Jet Zone
(no current and arc)
Fig. 1 Conventional plasma arc welding.
(a) System configuration. (b) Current flow.
Welding time and joint preparation are
among the most important factors
dominating the cost and productivity of
welding. If the desired penetration of the
weld pool can be achieved in a single
pass, the welding speed will be the major
factor that determines the welding time.
When the thickness of the part increases,
grooves may have to be machined so that
the desired penetration may still be
obtained in a single pass. In this case, the
welding time remains unchanged at the
expense of groove preparation. If the
thickness of the part further increases,
multiple passes may become mandatory in
addition to the joint preparation. Both the
welding time and the cost for the joint
preparation will significantly increase.
However, if the penetration capability is
increased so that joint preparation and
multiple passes be eliminated, the resultant
impact on cost and productivity will be
quite significant.
Laser beam and electron beam welding
can produce much deeper penetration than arc welding [1]. However, the costs associated with laser
Y.M. Zhang and S.B. Zhang are with the Welding Research and Development Laboratory, Center for Robotics and
Manufacturing Systems, University of Kentucky, Lexington, KY.
1
beam and electron beam welding are often unacceptable for many applications. In addition, their
requirement on joint machining and fixture is often too high for many industries. In short, compared with
the most widely used metal joining method, arc welding, laser beam and electron beam welding achieve
deep penetration at the expense of high cost.
IMPROVING PENETRATION WITHOUT INCREASING COSTS
This work focuses on improving weld penetration without substantial cost increase. Instead of using
another welding process, arc welding process will be performed in a novel method. It is known that
plasma arc welding (PAW) can achieve deeper penetration than other arc welding because of its more
concentrated arc and plasma jet [2]. However, the majority of the welding current in PAW earths through
the top surface of the base metal [3] so that only the plasma jet which has been ionized and heated by the
arc, rather than the arc itself, can directly penetrate into the keyhole (Fig. 1). If the arc can directly
penetrate into the keyhole, the penetration will be significantly increased. Hence, this paper proposes
increasing the penetration by placing a second torch from the opposite side of the base metal plate. The
two torches are directly connected to the two terminals of the power supply. The resultant welding current
loop becomes power supply-torch-work-torch-power supply instead of the conventional power supplytorch-work-power supply loop. As a result, the welding current, thus the arc, is guided into the keyhole
(Fig. 2). The weld penetration is significantly increased. In this paper, this arc welding process is referred
to as double-sided arc welding (DSAW).
It should be pointed out that other
Second Electrode
approaches have been applied to increase
weld penetration during arc welding. One
Second Arc
approach is to use flux to change the fluid
Plasma Arc
flow in the weld pool during gas tungsten
Work
Welding
arc (GTA) welding.
Significant
Power Supply
Plasma Arc
improvement in penetration has been
(Primary Arc)
PA
observed [4].
Compared with the
Torch
proposed
DSAW,
this approach can be
(Primary
Torch) PA Electrode
used without the restriction of back-side
accessibility. However, in order to apply
Fig. 2 Proposed double-sided arc welding. The second
this technology, different fluxes (chemical
torch can be either gas tungsten arc, gas metal arc, or
composition) must be carefully designed
plasma arc, depending on the application.
for different materials. Because of the
sensitivity of the penetration capability to the amount of the added flux, even coating of the flux is a basic
requirement. Another method that has been applied to increase weld penetration involves the use of two
independent sets of torch and power supply for simultaneously welding the work from the opposite two
sides. Two conventional power supply-torch-work-power supply loops parallelly function and
approximately twice penetration is achieved. Although it does not change the mechanism of conventional
arc welding where the power supply-torch-work-power supply loop is essential, this approach has found
successful applications when the both sides of the work are accessible.
Second
Torch
2
SETTING UP THE EXPERIMENTS
GTA Electrode
GTA Torch
Gas Tungsten Arc
Work
Plasma Arc
(Primary Arc)
Variable Polarity
Plasma Arc
Welding
Power Supply
PA
Torch
PA Electrode
Fig. 3 Experimental set-up for feasibility studies.
An experimental system has been
developed (Fig. 3) to verify the proposed
DSAW approach. This system, with gas
tungsten arc as the second arc,
successfully conducted the proposed
DSAW. This verified the feasibility of the
proposed DSAW approach. To confirm
the effectiveness of the proposed DSAW
in improving the penetration, extensive
experiments, in comparison with regular
variable polarity plasma arc (VPPA)
welding, have been conducted
using the developed DSAW
experimental system.
Comparative experiments are
conducted on 5052 Al at flat
position. Both the shielding gas
and plasma gas are pure argon.
The flow rate of the plasma gas,
(a)
(b)
Fig. 4 Penetration comparative experiments on 6.5 mm thick plates. the flow rate of the shielding gas
for plasma arc, and the diameter
No. filler. Bead-on-plate. (a) DSA weld. (95A, 47V, 4.7 mm/s).
of the orifice are 2.6 sfch, 30
(b) Regular VPPA weld (100A, 30V, 1.1mm/s). Bead-on-plate.
sfch, and 2.57 mm, respectively
for both DSAW and regular
VPPA welding. The flow rate of the shielding gas for GTA in
DSAW is 20 sfch. The duration of positive and negative current in
each pulse are all 15 ms, and the amplitudes in positive and negative
duration are equal. The arc voltage in DSAW is higher than it is in
regular VPPA welding due to the second arc.
Fig. 4 illustrates the penetration status of a DSA weld in comparison
with a regular VPPA weld. The thickness of the plate is 6.5 mm.
For DSAW, the current is 95A and the voltage is 47 V. For
VPPA, the current is 100 A and the arc voltage is 30 V. The
Fig. 5 DSA butt weld on 9.5
mm thick plate (110 A, 49V, 2.7 welding speed for DSAW is 4.7 mm/s, while the welding speed is
1.1 mm/s for VPPAW. It can be seen that the joint has been well
mm/s). No filler.
penetrated by DSAW. However, although the power input during
VPPAW is approximately 3 times as it is during DSAW, the joint is just penetrated approximately 37
percent by regular VPPAW.
3
(a)
(b)
Fig. 6 Behaviors of plasma arc in DSAW. Welding current: 90 A.
(a)-EN period. (b)-EP period.
Fig. 5 shows a DSA weld made
on 9.5 mm thick plate. In this
case, the welding current is 110 A
and the welding speed is 2.7
mm/s. It can be seen that the joint
has been well penetrated.
However, for regular VPPAW,
when the welding speed is 2.5
mm/s, 220 A current is needed in
order to achieve a full penetration
[5].
Extensive comparative experiments have also been done for stainless steel, mild steel, and other Aluminum
alloys. Similar results have always been observed as in Figs. 4 and 5. Experiments confirmed that the
proposed DSAW did dramatically improve the penetration in all the cases.
PRINCIPLES OF THE PROCESS
As it has been stated earlier, by placing the second torch, welding current flows through the keyhole,
instead of earthing through the work surface, in the proposed DSAW. As a result, the plasma arc is guided
into the keyhole and becomes more concentrated. Hence, the penetration capability is dramatically
increased. It is apparent that the welding current must flow through the work during DSAW. Thus, in
order to verify the principle, proposed above, of DSAW in improving penetration, only two questions
should be answered: does the welding current flow through the keyhole, and does the plasma arc really
become more concentrated?
Direct observation of the existence of the welding current inside the keyhole is not easy. More work will
be directed to resolve this issue.
To answer the second
question, the plasma arc
has been monitored during
welding. The images in
Fig. 6 shows the behaviors
of the plasma arc during
electrode-positive
(EP)
and
electrode-negative
(EN) periods. It can be
(a)
(b)
Fig. 7 Behaviors of plasma arc in regular VPPAW. Welding current: 90 seen that during EN
period, the plasma arc is
A. (a)-EN period. (b)-EP period.
much more concentrated
than it is in a regular VPPAW (Fig. 6(a) and Fig. 7(a)). However, during EP period, the concentration of
the plasma arc in DSAW and regular VPPAW is similar (Fig. 6(b) and Fig. 7(b)). It is known that
4
penetration is primarily due to the EN period. Hence, the proposed DSAW is characterized by the very
high concentration of its plasma arc.
Magnetic
force
The concentration of the plasma arc can be explained by the
welding current direction and its induced magnetic field. In
DSAW, the welding current flows approximately normally
through the work. Welding current in such a direction generates
a magnetic filed (Fig. 8). Under the effect of this magnetic field,
the current lines (welding current) are driven towards the axis
(Fig. 8). The welding current is concentrated both in the arcs
and in the work. Because of the spatial correlation between the
welding current and the arc distribution, the arcs are therefore
converged.
Magnetic
force
Work
Magnetic
line
Current
lines
Fig. 8 Current and induced magnetic
field.
DEPTH-TO-WIDTH RATIO
Penetration, width (mm)
9
8
Width (VPPAW)
7
Width (DSAW)
6
5
Penetration (DSAW)
4
Penetration (VPPAW)
3
2
90
92
94
96
98
100
102
104
106
Current (A)
Fig. 9 Comparative experiments in penetration depth and
width. Material: 5052 Al. Thickness: 6.5 mm. Welding
speed for DSAW: 4.7 mm/s, for VPPA: 1.1 mm/s.
The width of the weld pool is the
primary factor determining the width of
the heat-affected-zone (HAZ), thus the
material degradation and mechanical
properties of the resultant welds. For a
particular application, the required depth
of the weld pool is given. Thus, the
depth-to-width ratio plays the most
important role in determining the width of
the HAZ.
Extensive experiments have been
conducted to investigate the depth-towidth distinctions of DSAW
in comparison with regular
VPPAW. The results are
documented in Fig. 9. It
can be seen that for regular
plasma arc welding, when
the
welding
current
increases, the depth and
width of the weld pool
increase
simultaneously.
With Filler
No Filler
The depth-to-width ratio
Fig. 10 DSA welds under different conditions. No undercuts are
does
not
significantly
observed.
change. Consequently, an
increase in the penetration
5
can only be achieved at the expense of increasing the width of the weld pool. Such a property is not
preferred. For DSAW, when the welding current increases, the penetration depth increases rapidly, while
the resultant increase in the width of the weld pool is insignificant. This implies that the increase in the depth
of penetration in
DSAW is primarily
caused
by
an
increased depth-towidth ratio. Hence,
increasing
penetration will not
subject a substantial
increase in the width
of the weld pool,
(a)
(b)
thus the width of the
Fig. 11 Microstructure of DSA weld (2024 Al).
HAZ.
(a) Weld-HAZ interface. (b) HAZ.
DISTORTION
Thermal distortion is caused by uneven heat input during welding. It is undesired and should be minimized.
It is evident that the DSA heats the work from both sides of the work and tends to reduce the thermal
distortion.
DISCONTINUATION
Undercut and porosity are the two major weld discontinuations associated with PAW. Although porosity
may successfully be eliminated by using high pure gas [6], the prevention of the undercut has not been an
easy issue [6, 7]. In fact, in regular plasma arc welding, both the plasma pressure and the gravity serve as
detaching forces which tend to separate the liquid pool from the solid material. The retaining force, the
surface tension between the solid material and the melted metal, tends to prevent the liquid metal from
being separated from the solid material. If the retaining force is not sufficient, undercut occurs. In the
proposed DSAW, the plasma arc welds underneath. The gravity serves as a retaining force, instead of a
detaching force. In addition, the pressure of the second arc adds another retaining force. Hence, the
undercut can be prevented.
Extensive experiments
have been conducted in
the feasibility study and
preliminary investigation
phase of this project.
No undercut has been
produced despite the
arc-blow and electrode
erosion
(Fig. 10).
(a)
(b)
Fig. 12 Microstructure of VPPA weld (2024 Al).
(a) Weld-HAZ interface. (b) HAZ.
6
However, when a regular plasma arc welding is performed, a great care has to be taken in order to prevent
the undercut.
TENSILE STRENGTH AND DUCTILITY
To test the mechanical properties of DSA welds, extensive experiments have been done. The table below
summaries the test results for 5454 Al butt welds.
Weldment
(#)
1
2
3
4
5
Average
Yield Strength
Ksi
MPa
17.2
118.2
18.1
124.4
18.3
126.1
18.9
130.2
18.5
127.6
18.2
125.3
Ultimate Strength
Ksi
Mpa
36.3
250
35.9
247.7
34.6
238.1
37
255
34
233.8
35.6
244.9
Elongation Percentage
for 2 in (50mm)
21.3
18.9
16
17.4
16.8
18.1
Failure
Location
Base Metal
Base Metal
HAZ
HAZ
HAZ
It can be seen that there is no significant distinction in the ultimate strength between DSA welds and
annealed base metal. (The ultimate strength is 36 Ksi for the annealed 5454 Al [8]). The average ultimate
strength is slightly better than the ultimate strength in gas-shielding arc welding which is 34 Ksi [9]. The
final failure occurs either in base metal or HAZ. The yield strength is also higher than it is in the annealed
base metal (17ksi) [8]. It is clear that the tensile strength of DSA welds can be regarded as the same as
the annealed base metal and is slightly better than regular arc welds. Although the ductility test reading
(18.1%) is slightly lower than the similar reading in annealed base metal, it is higher than the ductility of
regular arc welds (17%) [9]. Hence, the mechanical properties of DSA welds are quite satisfactory.
MICROSTRUCTURE
Fig. 13 Microstructure of 2024 Al base
metal.
The mechanical properties are primarily determined by the
microstructure. Figs. 11 and 12 give the microstructures at
the weld-HAZ interface and in the HAZ for the DSA weld
in Fig. 4(a) and for the regular VPPA weld in Fig. 4(b),
respectively. The microstructure in the base metal is shown
in Fig. 13. Although the grains have increased after welding
(Figs. 11 and 13), finer grains, in both the HAZ and the
weld zone, have been observed than in regular VPPA weld.
Such finer grains are mainly due to the narrowed weld and
HAZ, thus shorter heat-treatment period. Hence, the
microstructure explained the satisfactory mechanical
properties of DSAW.
7
APPLICATION ISSUES
There are a number of issues related to applying the DSAW to production.
The first issue is the accessibility of both sides of the work. Although many applications do not meet the
requirement of accessibility, the cases where both sides of the work are accessible are still not limited due
to the diversity of applications. For example, the external tank of the space shuttle and many pipes can be
accessed from both sides during welding.
Because the arcs exist on the both sides, the arc voltage is increased in comparison with regular arc
welding. As a result, the nominal maximum current of the used power supply may not be achieved. Our
experiments were conducted using a 200 A variable polarity plasma arc welding power supply. However,
because of the higher arc voltage, the actually achievable welding current is only approximately 110 A
during DASW. The authors plan obtaining a more powerful power supply to weld thicker materials.
Other application related issue is the alignment of the two torches during welding. To ensure an adequate
joining, the joint must be throughoutly fused by the weld pool. For thin material joining where the size of
the weld is small, the two torches must be well aligned. However, for thick material, a larger error in the
alignment of the two torches may be permitted. Basically, the required accuracy for the alignment is close
to the accuracy required on seam tracking.
CONCLUSIONS
•
•
•
•
•
An experimental set-up has been established to verify the proposed DSAW approach. It is shown that
the proposed DSAW configuration works.
Experiments demonstrated the effectiveness of the proposed DSAW in improving depth-to-width ratio
and penetration.
Preliminary studies found that the plasma arc during DSAW becomes much more concentrated.
Although more studies should be conducted to better understand the principle of DSAW in improving
penetration, this finding has provided a convincing support to explain the experimental results of
DSAW in improving penetration.
Preliminary studies have also demonstrated the merits of DSAW in decreasing HAZ, eliminating
undercuts, reducing thermal distortion, and achieving satisfactory microstructure and mechanical
properties.
A significant restriction of DSAW is the mandatory requirement on the double side accessibility of the
work. Because of the wide application of arc welding in manufacturing and application diversity, the
cases where both sides of the work are accessible are still not limited.
REFERENCES
1. Laser Welding Handbook. 8th edition, Vol. 2: Welding Processes, AWS, 1991.
2. E. Craig, 1988. “The plasma arc welding-a review,” Welding Journal, 67(2): 19-25.
8
3. J. Dowden, P. Kapadia, and B. Fenn, 1993. “Space charge in plasma arc welding and cutting,”
Journal of Physics (D): Applied Physics, Vol. 26: 1215-1223.
4. T. Paskell, C. Lundin, and H. Castner, 1997. “GTAW flux increases weld joint penetration,” Welding
Journal, 76(4): 57-62.
5. M. Tomsic and Barhorst, 1984. “Keyhole plasma arc welding of aluminum with variable polarity
power,” Welding Journal, 63(2): 25-32.
6. M. R. Torres, J. C. McClure, A. C. Nunes, and A. C. Gurevitch, 1992. “Gas contamination effects in
variable polarity plasma arc welded aluminum,” Welding Journal, 71(4): 123s-131s.
7. R. Hou, D. M. Evans, J. C. McClure, A. C. Nunes, and G. Garcia, 1996. “Shielding gas and heat
transfer efficiency in plasma arc welding,” Welding Journal, 75(10): 305s-1310s.
8. B. Irving, 1994. “Welding the four most popular aluminum alloys,” Welding Journal, 73(2): 51-55.
9. J. R. Davis (Editor), 1994. ASM Specialty Handbook: Aluminum and Aluminum Alloys, ASM
International, 376-415.
9