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Single-pass full-penetration welding for thick stainless steel using high-current GMAW

ABSTRACT This paper describes a high-current Gas Metal Arc Welding (GMAW) process using a constant voltage power source developed for high-efficiency welding of thick stainless steel plates. A short arc length as a welding condition is preferable to concentrate the arc and obtain a deep penetration for thick plate welding. However, it is easily found that an unstable arc derived from a short circuit happens irregularly by using a general purpose GMAW with constant voltage characteristics. A ‘low-frequency modulated voltage control’ was applied for stabilizing a high-current arc that can maintain a short arc length without short circuiting. The welded joint using the developed welding process showed the similar corrosion resistance as that using the submerged arc welding; however, porosity was observed in the welded joints using the developed welding process in the current region of 400 A or above. Consequently, a new current waveform control that changes the welding current dynamically and periodically has been developed, which realized reducing porosity and single-pass full-penetration welding of 9-mm-thickness type 304 stainless steel plates with square groove.


Introduction
Welding processes with a range of characteristics, including submerged arc welding, plasma welding, laser welding, and GMA welding are used to weld thick stainless steel plates. Submerged arc welding is a proven, widely used thick plate welding process and a range of welding procedures including single-pass full-penetration welding of 20 mm thick plates have been reported [1][2][3][4][5]. Plasma welding makes it possible to carry out full-penetration welding using keyhole welding with no backing and single-pass full-penetration welding of plates up to 9.5 mm thick has been reported [6,7]. Welding of thicker plates has been made possible by laser welding and laserarc hybrid welding, and single-pass fullpenetration welding of stainless steel with a plate thickness of 12 mm to 50 mm has been reported [8][9][10][11][12][13]. In recent years, 100 kW class high-power laser oscillators have also been developed [14] and are expected to find wider applications for thicker plates. However, this kind of high-efficiency welding process has a range of limitations, including the type of joints with which it can be used, the welding position, the required accuracy for the groove and gap and the specificity of the jig system and thus it cannot necessarily be used with every joint.
Although the welding efficiency is lower than with the above processes, GMA welding has excellent flexibility and universality and can be used for a range of joints. Accordingly, GMA welding is often used as a versatile welding process for complex products and structures with a wide range of joints and for large item -small-scale products. Solid wire or flux-cored wire [15][16][17][18] may be used as the welding wire for GMA welding of stainless steel and, characteristically, penetration is deep with the former and bead appearance is good with the latter. In this study, we focussed on using GMA welding with solid wire with deep penetration in order to develop a single-pass fullpenetration welding method for thick plates.
A range of low to medium welding current, no higher than 350A, is used in most cases of GMA welding of stainless steel with solid wire. A higher current is preferable to perform single-pass fullpenetration welding of thick plates however the welding performance at high currents of 400A and above remains largely unknown. Despite the concern that in high-current GMA welding of stainless steel the corrosion resistance of the joint will decrease due to the welding heat input [19], the corrosion resistance of stainless steel welded joints made using high-current GMA welding at 400A or above is yet to be clarified.
It is also known that multiple porosities occur as the current rises during GMA welding of stainless steel using solid wire [20]. Since this is evidently one reason that high-current conditions are not being widely used, the present authors believe that the establishment of a method of reducing porosities is essential for the successful application of GMA welding with high currents of 400A and above. Methods of reducing porosities during GMA welding include the low-frequency pulsed MIG welding used with aluminium alloy reported by Yamamoto et al., in which the molten pool is vibrated by two periodically alternated different pulse conditions, promoting the expulsion of gas present in the molten pool to the exterior and reducing porosities [21][22][23][24][25][26]. Izutani et al. have also reported that porosities can be reduced by the use of low-frequency superimposed pulse waveforms with pulsed MAG welding of galvanized steel sheet [27]. However, there have been no reported findings for GMA welding of stainless steel in the high current range of 400A and above and there is no established method of reducing porosities.
Accordingly, in the present study, the aim of which was to develop a high-quality, high-efficiency GMA welding method for thick stainless steel plates, 'lowfrequency modulated voltage control' [28,29], a method developed for stabilizing the high-current GMA welding of general ferrous materials, was investigated for its arc stabilization effect when applied to stainless steel. The corrosion resistance of joints welded by high-current GMA welding and a method of reducing porosity using welding current/arc voltage waveforms control were also investigated. Here, we report on the development of single-pass fullpenetration welding method of thick stainless steel plate developed on the basis of these results.

Welding system and specimen materials
The configuration of the welding system used in the present study is shown in Figure 1. The welding power source comprises two digital inverter-controlled power sources operated in parallel linked by highspeed serial communication cable, with current changes based on circuits equivalent to a welding power source with constant-voltage characteristic by a welding dedicated LSI (ASIC) which achieves optimal power supply characteristics for each current range by high-speed digital calculation [30]. The rated output current was 650A and the rated usage rate was 100%. The water-cooled welding torch, which had a cooling structure optimized using copper alloy 3D modelling technology, could be used under the rated output conditions described above.
The materials used in the experiments are shown in Table 1. SUS304, a widely used general-use stainless steel was used as the base metal, YS308 was used as the welding wire and 98%Ar-2%O 2 was used as the shielding gas.

Arc stabilization by low-frequency modulated voltage control
For the welding of thick plates by GMA welding, it is preferable for the arc to be short to achieve a deep penetration due to a concentrated arc but, on the other hand, when the arc is short, there is an increased likelihood of a short circuit between the welding wire and the molten pool. Since the molten pool oscillates irregularly when an unintended short circuit occurs, so the arc voltage, which indicates the arc length, also changes irregularly which becomes a disturbing factor for arc length control. Accordingly, in the present study, we focussed on low-frequency modulated voltage control [28,29], a welding current/arc voltage waveforms control which stabilizes the arc during high-current GMA welding, and investigated the effectiveness of this with welding currents of 400A   Table 2. Bead-on-plate welding was performed under both of these welding current conditions at the same set voltage, with and without low-frequency modulated voltage control.
The welding currents and arc voltage waveforms for welding currents of 400A and 600A are shown in Figures 2 and 3. The welding currents and arc voltage waveforms are shown with and without low-frequency modulated voltage control, and the waveforms over    for the welding current, since numerical calculation was performed to reproduce the current changing rate with a DC reactor for high-current welding [31], there is no large change in the welding current associated with changes in the arc voltage. The arc voltage fluctuates within ranges of approximately −9 V ~ +5 V for an average voltage of 28.5 V under a welding current of 400A and approximately −5 V ~ +2 V for an average voltage of 34.5 V under a welding current of 600A. Fluctuations of the arc voltage with a mainly constant welding current reflect changes in the arc length; in particular, an arc voltage waveform where the voltage is reduced instantaneously by several volts from the average voltage is the result of a short circuit. On the other hand, Figures 2(b) and 3(b) show the welding current and arc voltage waveforms when lowfrequency modulated voltage control was used at, respectively, 400A and 600A welding currents. Under both welding current conditions, welding current and arc voltage change periodically due to low-frequency modulated voltage control but no instantaneous decrease in arc voltage of several volts or more, which would indicate a short circuit, was noted.
As for the standard deviations of arc voltage shown in Figures 4 and 5, the standard deviation of arc voltage was smaller, and the irregular fluctuations in arc voltage were more inhibited, when lowfrequency modulated voltage control was applied than when there was no low-frequency modulated voltage control. This is thought to be because short circuiting, and the fluctuations in the arc length it causes, were inhibited and so the arc was stabilized by low-frequency modulated voltage control. When low-frequency modulated voltage control is used, the arc pressure is raised with the welding current increase during the high voltage period, and the molten pool is strongly pressed down, thus being prevented from approaching the welding wire tip, with the result that short circuiting is prevented.
The optimal low-frequency modulated voltage control conditions (a combination of voltage amplitude and voltage frequency) were verified based on the above findings. Bead-on-plate welding was performed for the five conditions of welding current ranges of 200A, 300A, 400A, 500A and 600A with low-frequency modulated voltage control conditions changed for each condition, making a total of 18 combinations of six voltage amplitudes (±1 V, ±2 V, ±3 V, ±4 V, ±5 V, ±6 V) and three voltage frequencies (50 Hz, 100 Hz, and 150 Hz) for each of these welding currents and the standard deviations of  arc voltage were compared. The low-frequency modulated voltage control conditions when the standard deviation for arc voltage was smallest under each of the welding currents are shown in Table 3. Since, in the range of welding currents of 300A and under, the droplet transfer mode becomes short circuit transfer and no clear effects of low-frequency modulated voltage control could be observed, the voltage amplitude in the range of welding currents of 300A and below is shown as ±0 V.
The standard deviation of the arc voltage does not reduce as the voltage amplitude increases and standard deviation is minimum at a voltage amplitude of ±4 V when the welding current is 400A and ±5 V when the welding current is in the region of 500A and above. If the voltage amplitude increases, the molten pool is pressed down by the strong arc pressure during the high-voltage period and this effectively prevents short circuit but, on the other hand, that if the voltage amplitude is too large, the molten pool may be pressed down excessively and destabilized. It is at the higher of the welding currents of 400A and above that the optimal voltage amplitude becomes larger. The reason that the optimal voltage amplitude becomes larger at 500A is that instantaneous short circuit is more likely to occur due to the liquid column extending at the wire tip becoming longer [32] at high welding currents so that a momentary short circuit is likely to occur, and the molten pool surface must be pressed down more strongly to prevent this. The above findings suggest the existence of a voltage amplitude that contributes to arc stabilization with a value appropriate to the conditions of the welding current.
Of the voltage frequencies of 50 Hz, 100 Hz and 150 Hz, 100 Hz is optimal, irrespective of the conditions of the welding current. If the voltage frequency is too small, the low voltage period per each cycle of voltage oscillation cycle becomes long, and over this long period the molten pool is not strongly pushed down and short circuiting cannot be controlled. If the voltage frequency is too large, on the other hand, the high voltage period per voltage oscillation cycle becomes shorter and the molten pool cannot be adequately supported, which suggests that there is an optimal range for voltage frequencies that contribute to arc stabilization.

Assessment of joint corrosion resistance
When stainless steel is welded by high-current GMA welding, there is the concern that the joint corrosion resistance may be reduced due to welding heat input [19]. In particular, since it is known that intergranular corrosion occurs due to sensitization in the heataffected zone [19,33], resistance to intergranular corrosion of the welded joints was assessed in the welded joints of SUS304 used in the present study. The comparative index was a joint welded by submerged arc welding, a high heat input welding process widely used for stainless steel. The welding materials and welding conditions are shown in Table 4. The welding conditions for submerged arc welding, which are based on the manufacturer's recommended values, were a welding current of 500A, a welding voltage of 33 V and a welding speed of 40 cm/min and the heat input, which was the comparison criterion, was 24,750 J/cm. The welding conditions for high-current GMA welding were a welding current of 400A, a welding voltage of 29 V and a welding speed of 30 cm/min, which resulted in a heat input of the same order as submerged arc welding (23,200 J/cm) and the lowfrequency modulated voltage control conditions, based on Table 3, were a voltage amplitude of ±4 V and a voltage frequency of 100 Hz. In order to inhibit the formation of excess weld reinforcement and ensure that sufficient arc heat input reached the base metal, welding was performed with a Y-groove, as shown in Figure 6, to fabricate the joint. The penetration shapes of a high-current GMA welded joint and submerged arc welded joint are shown in Figure 7. The penetration shape of the high-current GMA welded joint has a finger-shaped penetration at the centre, deeper than the penetration shape of the submerged arc welded joint. It was also noted that porosities had occurred near the deepest part of the penetration in the high-current GMA welded joint.
Sulphuric acid and ferric sulphate corrosion tests were performed in accordance with JIS G 0572: 2006 on the welded joints thus fabricated and intergranular corrosion resistance was evaluated. As shown in Figure 8, a corrosion resistance specimen was cut so that it was 5 mm in the welding direction, 55 mm in the bead width direction and 15 mm from the widest position of the weld metal in the thickness direction and the entire surface of this was polished with wet emery paper to #600. The test solution was prepared by adding 20.83 g of ferric sulphate to 176 mL of a mixture of distilled water 324 mL and sulphuric acid 176 mL. The mass reduction due to corrosion was measured at 24-h intervals during immersion and the corrosion rate was calculated by dividing the mass reduction by the surface area before corrosion and the immersion time. The maximum cumulative immersion time was 120 h and the corrosion rates obtained are shown in Figure 9. The corrosion rate of the high-current GMA welded joint showed a slightly lower value than the corrosion rate of the submerged arc welded joint. Although it cannot be concluded from these results alone that the corrosion resistance of high-current GMA welded joints is superior, it is suggested that, at least from the viewpoint of corrosion rate, the same heat input control guidelines as those used for submerged arc welding may be adopted.

Development of a porosity reduction method
In section 2.2 it was shown that it is possible to stabilize high-current GMA welding of stainless steel by using low-frequency modulated voltage control. However, even with stabilized high-current GMA welding, porosities may occur in the welded joints as shown in Figure 7(a). Accordingly, the tendencies for porosity occurrence in high-current GMA welding of stainless steel were clarified and a welding current and arc voltage waveforms control was developed for their inhibition.

Porosity occurrence tendencies
Bead-on-plate welding was performed with the welding current varied between 400A, 500A and 600A and the porosity occurrence tendency was checked by radiographic testing. The low-frequency modulated voltage control conditions were the values shown in Table 3 along with the welding current. The welding speed was 30 cm/min and the wire extension length was 20 mm, both constant. The radiographic test images of the weld bead are shown in Figure 10. The results of measurements of the maximum number of porosities observed in a 10 mm×10 mm region of a weld in 300 mm long are shown in Figure 11. It is clear from Figures 10  and 11 that a particularly large number of porosities occur when the welding current is 400A. There is also a tendency for the amount of porosities to decrease as the welding current increases. One reason for this may be the solidification rate of the weld metal. Since, at a fixed welding speed, the solidification of the weld metal due to increase of heat input decreases as the welding current increases, the bubbles entrained in the molten pool tend to be expelled to the exterior before the molten pools solidifies, making it difficult for them to remain in the weld metal.

Porosity reduction by dynamic current-oscillation control
As stated above, Yamamoto et al. have reported that when low-frequency pulsed welding of aluminium alloy is performed porosities can be reduced by periodic switching between two pulse conditions [21][22][23][24][25][26]. This is thought to be the result of the gas in the molten metal being floated upward and then vigorously expelled from the molten pool due to the oscillations caused by the changes in height of the bulging of the molten pool above the base metal surface [22,25] with the changes of the arc force associated with the variations of the effective value of the welding current under each of the pulse conditions [23,26]. Accordingly, the present authors have investigated 'dynamic current-oscillation control', a method of controlling the welding current and arc voltage waveforms, with the aim of reducing the porosities by causing oscillations in the molten pool by similarly varying the effective value of the welding current. As shown in the schematic diagram in Figure 12, dynamic current-oscillation control acts by varying the current set at fixed intervals. The current amplitude used was ±50A,±150A and the current frequency was in the range 1 Hz,3 Hz. To distinguish it from the current settings during the high-current period and the low- Figure 10. Radiographic test images of weld bead using high-current GMAW.
current period, the current setting with respect to the average current is hereinafter referred to as the centre setting current. The low-frequency modulated voltage control described in Section 2.2.1 is applied in both the high and low current periods. The periodic changes in set voltage shown in Figure 12 are due to lowfrequency modulated voltage control. Thus, dynamic current-oscillation control is control with the aim of reducing porosities and low-frequency modulated voltage control is control with the aim of arc stabilization for each condition of the current. The low-frequency modulated voltage control conditions are based on the optimal conditions for arc stabilization for the welding currents shown in Table 3 and the conditions shown in Figure 13, where the voltage amplitude and voltage frequency at the welding current are linearly interpolated.
As an example, the welding current and arc voltage waveforms when dynamic current-oscillation control was used with the centre setting current of 400A, a current amplitude of ±100 A and a current frequency of 2 Hz, are shown in Figure 14. In Figure 14, the range 300 ms ~ 550 ms was the high-current period (500A) and the range 550 ms ~ 800 ms was the low-current period (300A). Under the low-frequency modulated voltage control conditions shown in Figure 13, the welding current and arc voltage were changed by a voltage frequency of 100 Hz, using low-frequency modulated voltage control in the high-current period (500A). By comparison, since the voltage amplitude is ±0 V in the low-current period (300A), no change in the welding current or arc voltage was brought about by low-frequency modulated voltage control. The control was repeated at a current frequency of 2 Hz (period 500 ms).
Observation results for the molten pool in both high and low welding current periods are shown in Figure 15. The observation results on the left of   Figure 15 are results of observation of the molten pool from above at an angle of 45° in relation to the bead. The indentation of the molten pool immediately below the arc changes greatly between the lowcurrent period (300A) and high-current period (500A). This is thought to be due to the difference in the arc pressure dependent on the welding current. A greater depression is formed in the highcurrent period where the arc pressure is larger. The observation results on the right of Figure 15 are from observations of the molten pool in the horizontal direction. The height of the top edge of the molten pool changes greatly in the low-current period and high-current period, with the height in the high-current period being greater. This is thought to be because in the high-current period the weld metal that has been pushed away flows to the rear of the molten pool at the same time as the indentation is formed directly below the arc by strong arc pressure, as described above. This suggests that the molten pool fluctuates in height when dynamic current-oscillation control is used, which is similar to the molten pool oscillation behaviour mentioned in the reports of Yamamoto et al. [22,25] and, similarly, the floating up and expulsion of the bubbles in the molten pool is promoted. In order to confirm the bubble expulsion phenomenon, the molten pool phenomena when dynamic current-oscillation control was applied were observed using a high-speed camera. It is clear from the observation results in Figure 16 that bubbles are actually expelled from the molten pool. Changes in the specific convection currents and stirring phenomena in the molten pool associated with the fluctuation of the molten pool have not been elucidated in this study but, since the bubble expulsion phenomenon described above was not observed when dynamic currentoscillation control was not used, oscillation of the molten pool due to dynamic current-oscillation control is thought to have contributed to the promotion of bubble expulsion from the molten pool.
Next, bead-on-plate welding was performed with the centre setting current of 400A, and the current amplitude and current frequency varied. The radiographic test images of the weld bead are shown in Figure 17 and the bead appearance is shown in Figure 18.
As shown in Figure 17, at a current frequency of 1 Hz, porosities occurred at current amplitudes of ±50A and ±100A but porosities did not occur at a current amplitude of ±150A. When the current amplitude was ±100A porosities occurred at current frequency of 1 Hz, but no porosities occurred at current frequencies of 2 Hz and 3 Hz. Thus, porosities show a tendency to decrease as the current amplitude increases and as the current frequency increases.
When the current amplitude is high, the oscillation of the molten pool increases when the difference between the arc pressure during the low-current period and high-current period increases. When the current frequency is high, the number of molten pool  oscillations per unit time increases. As a result, it is thought that, within the scope of these experiments, expulsion of bubbles is promoted by oscillation of the molten pool, and porosities decreased with larger current amplitude and current frequency.
On the other hand, as shown in the Figure 18, the exterior of the bead appears flat when dynamic current-oscillation control was not used. However, as the current amplitude or frequency increases, creases appear on the bead surface and there is a tendency for the appearance to deteriorate. This is thought to be the result of the above-noted enhancement in the fluctuation of the molten pool.
The effects of the dynamic current-oscillation control conditions on the quantity of porosities occurring and bead appearance are shown in Table 5. Viewed in terms of porosity prevention, the effect is greater as the current amplitude and current frequency increase. Viewed in terms of appearance, on the other hand, a flatter bead is obtained as the  current amplitude becomes smaller and current frequency lower. This finding indicates that the porosity control effect of dynamic current-oscillation control is dependent on the permissible range of the bead appearance.

High-efficiency welding of thick plates by high-current GMA welding
Single-pass full-penetration welded joints of 9 mm thick plates were fabricated using low-frequency modulated voltage control to stabilize the arc and  dynamic current-oscillation control to reduce porosities. The groove shape was a square groove shown in Figure 19 and the root gap was 4 mm. Details welding conditions are shown in Table 6. The current amplitude conditions were ±75A and 1.5 Hz and the lowfrequency modulated voltage control conditions were the conditions shown in Figure 13, corresponding to the set current. The bead appearance, penetration shape and radiographic testing images are shown in Figure 20. It was possible to perform full-penetration welding with no porosities and thus to carry out highquality and high-efficiency welding of stainless steel.

Conclusion
In the present study, methods of stabilizing the arc and reducing porosities were established by welding current and arc voltage waveforms control during high-current GMA welding of stainless steel and a high-efficiency welding method for thick stainless steel plate was developed. The conclusions of this study are summarized as follows.
(1) By applying low-frequency modulated voltage control, in which the set voltage is varied periodically using a welding power source with constant voltage characteristics, to highcurrent GMA welding of stainless steel, it was possible to stabilize the arc in the high-current region of a welding current of 400A and above. (2) The difference in the corrosion rate between high-current GMA welded joints and submerged arc welded joints under the same heat input conditions is small, suggesting the  Figure 19. Groove shape for single-pass full-penetration welding.  Figure 20. Results of single-pass full-penetration welding of 9 mm-thickness stainless plates.
possibility that almost the same heat input control guidelines used with submerged arc welding can be used for high-current GMA welding. (3) Multiple porosities occurred with high-current GMA welding with a welding current of 400A and above, but it was possible to reduce porosities by dynamic current-oscillation control in which welding current was varied periodically. This is thought to be the result that the molten pool was oscillated by the changes in arc pressure associated with changes in the welding current, promoting the expulsion of gas bubbles to the exterior of the molten pool. (4) It was possible to perform non-porosity and stable single-pass full-penetration welding of 9 mm thick plates with square groove by lowfrequency modulated voltage control and dynamic current-oscillation control and thus to achieve high-efficiency and high-quality thick plate welding.