Weld Quality Improvement using GMAW-GTAW Hybrid Welding Process for Aluminum Alloy

doi:10.21203/rs.3.rs-4917879/v1

One of the major challenges in welding aluminum alloys is the formation of porosity in weld metal, which diminishes mechanical properties and usability. Therefore, this research aims to study the feasibility of developing a welding technique using a hybrid welding to improve weld quality by minimizing porosity. The hybrid welding process consists of two techniques: gas metal arc welding and gas tungsten arc welding. High-speed video analysis examined metal transfer, weld pool behavior, and oxide cleaning action. Radiographic testing was employed to evaluate the porosity in weld metal. The results demonstrated a significant decrease in the number of porosities. Additionally, this research proposes guidelines for welding variable selection and equipment setup, offering a comprehensive strategy for achieving improved weld quality.

Gas Tungsten Arc Welding

Gas Metal Arc Welding

Hybrid welding

Oxide cleaning action

Welding of aluminum

Welding porosity

Welding of aluminum is considered to be slightly more difficult than steel due to high thermal and electrical conductivity, high thermal expansion coefficient, refractory aluminum oxide (Al₂O₃) formation tendency, and low stiffness. However, the increase in applications of aluminum alloys in all sectors of industry is a driving force for technologists to develop viable and efficient technologies for joining of aluminum without much adverse effect on their mechanical, chemical, and metallurgical performances desired for longer life. The major problems in welding of aluminum alloy are tending of enormous porosity formation in weld metal and loss of their mechanical properties in heat heat-affected zone (HAZ) due to welding heat.

Gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) are the most extensive gas-shielded arc-welding processes used in joining aluminum and its alloys due to their preferable flexibility and economy. GTAW welding is a high-quality and stable process that has characteristics of less spatter and better weld bead appearance. To remove the aluminum oxide film, an alternative current can be used. During direct current electrode positive (DCEP) polarity of the arc, the cathodic cleaning action of cathode spots occurs [1, 2]. However, the GTAW process is associated with some problems, such as a low deposition rate, a tendency for incomplete penetration, and incomplete fusion between joint members [3].

Nevertheless, GMAW welding provides the advantage of a high-efficiency welding method with a high deposition rate due to high heat input. The excessive heat input can also cause quality problems such as deeper penetration and more serious distortion. Despite many advantages of the GMAW process, some shortcomings still need to be resolved, such as welded distortion [4] and porosity [5]. The porosity is a serious problem that would deteriorate joint strength [6]. It is well known to all that the porosity formation is greatly influenced in aluminum alloys by the low vaporization point element such as Mg, Zn content that tends to entrap occluded gases porosity during welding [7–9].

In recent years, extensive studies have been reported to investigate the effect of welding heat input on the mechanical and metallurgical properties of aluminum alloys welded metal. Liang et al. [10] showed that the GTAW – CMT hybrid welding process could increase weld penetration and produce a preferable weld profile. Additionally, the microstructures of the joint coarsened, and a precipitation-strengthening phase appeared. Ascari et al. [11] reported that GMAW current is significant on porosity formation. Rodriguez-Hernandez et al. [12] also indicated that the GMAW- GTAW hybrid welding process could reduce the degradation in the heat-affected zone.

However, only a few studies of the formation of porosities of the welded metal and the investigations have not covered the welding quality of the GMAW- GTAW hybrid welding process. In order to investigate the possibilities of improving welding quality, a GMAW- GTAW hybrid welding process was employed in this study. Adding a stable GTAW arc can increase the total heat input with high-quality oxide cleaning action on the workpiece and the high deposition rate capacity of the GMAW welding process. This study aimed to investigate the effect of GTAW oxide cleaning action on the weld quality and porosity formation of GMAW- GTAW hybrid welding process.

This section outlines the experimental procedure and analysis techniques employed in the research. The experiment is divided into two sections, covering the observation of cathode spots during oxide cleaning action and the development of the GTAW – GMAW hybrid welding process. Initially, bead-on-plate welding will be conducted on aluminum alloy AA5083 plates with dimensions of 150 × 40 × 5 mm using the GTAW process. Various welding parameters and welding current waveforms will be employed. Following this, the porosity formation of GMAW weld metal will be assessed using radiographic testing. Subsequently, the experimental set-up for the GTAW – GMAW hybrid welding process on an aluminum plate will be prepared to observe arc behaviors, molten droplet transfer, and cleaning action on the surface of the aluminum plate. The GMAW welding filler metal used in the experiment is AWS A5.10 ER5356 with a diameter of 1.2 mm.

3.1 The effect of the balanced waveform control of AC-GTAW on cleaning action quality.

GTAW welding was conducted using an alternating current power source and a standard GTAW torch. The shielding gas employed was 100% Ar, with a gas flow rate of 15 L/min and a 5 mm distance between the electrode tip and the base metal. The welding was carried out in the alternating current mode with a welding current of 100 A and an arc voltage of 15 V. Balanced waveform control was applied under three different DCEN-DCEP ratio conditions: 50%-50%, 40%-60%, and 30%-70%, respectively. The welding was performed as spot welding, with a 5-second arc time.

After welding, macrostructure analysis of the weld metal surface, oxide cleaning action surface, and unmelted surface was conducted. The surface of the oxide cleaning action was analyzed and processed using Image-J computer programming. This technique can evaluate the area fraction of the cleaned and uncleared areas of the cleaning zone. Meanwhile, the oxide cleaning behavior during the arc was observed using a high-speed camera (Photon, FASTCAM SA1.1). The high-speed video camera was placed at an angle of approximately 45° with the base metal. The 16-bit grayscale images were captured with a frame rate of 5,000 frames per second (fps) and an exposure time of 20 µs. These camera settings allowed the recording of the arc plasma column, development of cathode spots, and oxide cleaning action during the arc at high resolution.

3.2 The study of the porosity distribution during GMAW welding.

Bead-on-plate welds were performed using GMAW with a direct current electrode positive polarity. The shielding gas was 100% Ar, with a gas flow rate of 20 L/min. The contact tip to work distance (CTWD) was maintained at 15 mm. The welding was carried out at a current of 200 A and an arc voltage of 30 V. Three different travel speeds were employed: 45, 55, and 65 cm/min. Weld metal soundness was assessed using computed radiography (CR). Simultaneously, the location of porosity in the weld metal was observed using macrostructure image analysis. A high-speed video camera was utilized to observe droplet formation and the metal transfer mode during welding. The welding conditions employed in this study followed the manufacturer's suggestions and the recommended procedure [3, 13–15].

3.3 Study of the welding quality of weld metal using GMAW – GTAW hybrid arc.

GMAW and GTAW welding were combined in a hybrid welding process. The GMAW torch was positioned in front of the GTAW torch with the same alignment. The travel angle of the GMAW torch was set perpendicular to the base metal. Subsequently, the GTAW torch was tilted at a 20° travel angle. The welding parameters were selected based on the experiment as described above. Figure 1 and Fig. 2 present the schematic illustration of the experimental set-up and the GMAW – GTAW hybrid welding process torch position, respectively.

The amount of porosity was also evaluated using computed radiography compared to that of the conventional GMAW. The GMAW and GTAW arc shape and metal droplet behaviors were observed and analyzed using a high-speed camera. The high-speed video camera allowed recording plasma column, distribution of cathode spot, oxide cleaning action, and metal droplet transfer during GMAW – GTAW hybrid welding.

4.1 The oxide cleaning action effect of AC-GTAW

The macrostructure and area fraction (%) of the oxide cleaning action surface were analyzed in three areas within the weld pool interface. The area fraction (%) of the cleaned oxide surface near the fusion line of GTAW weld metal is presented in Table 1. The red areas indicate the remaining oxide surface after welding. The average area fraction of the cleaned oxide surface for DCEN 50% - DCEP 50%, DCEN 40% - DCEP 60%, and DCEN 30% - DCEP 70% was 75.81%, 80.58%, and 84.18%, respectively. The area fraction (%) of the cleaned oxide surface reached its highest value with the balanced waveform of DCEN 30% - DCEP 70%. This suggests an increased oxide cleaning efficiency with an elevated DCEP ratio. It is evident that the welding waveform ratio of DCEN – DCEP is correlated with the oxide cleaning action in the cleaning zone.

Therefore, the GTAW welding condition used in GMAW – GTAW hybrid welding process will be DCEN 30% - DCEP 70% unbalanced AC waveform. Because of the higher oxide cleaning efficiency of GTAW can remove the oxide that originated after solidifying of GMAW weld metal. Moreover, the GTAW welding current used in the GMAW – GTAW hybrid welding process will be higher enough for remelting the GMAW weld metal surface.

4.2 Distribution of porosity in GMAW weld metal

Table 2 displays the results of the amount of porosity, and the size distribution of GMAW and GMAW – GTAW hybrid welding process of aluminum weld metal. The amount of porosity in GMAW weld metal significantly increased with an increase in welding travel speed. Meanwhile, the size distributions of porosity decreased with the rising welding travel speed. These observations suggest that a higher welding travel speed leads to an increased solidification rate of the weld metal. Similarly, oxide films were formed immediately after solidification of the weld pool surface at the tailing. Therefore, there is a high probability that porosity could be entrapped in the weld pool, having insufficient time to move away from the weld pool [19, 20].

On the contrary, the findings revealed a significant reduction in the amount of porosity in every welding compared to the standard GMAW process. Specifically, the GMAW – GTAW hybrid welding process with a travel speed of 45 and 55 cm/min exhibited zero porosity and defects. Similarly, the porosity size distributions for a welding travel speed of 65 cm/min were also significantly reduced. This demonstrates that the GMAW – GTAW hybrid welding process can remarkably decrease porosity in the weld metal of aluminum alloy AA5083.

Meanwhile, high-speed images revealed that the metal transfer mode of GMAW was short circuit transfer with a transfer rate of 10–13 drops per second. It was observed that the weld pool convection in the case of short circuit transfer was disturbed. The stirring effect, caused by this disturbance in the weld pool, led to drawing gas into the solidifying aluminum weld pool, resulting in porosities in the completed weld [20].

Table 2

Amounts of porosity in GMAW weld metal and GMAW – GTAW hybrid welding weld metal with different welding travel speeds..
Porosity sizes in GMAW weld metal (mm)	GMAW process			GMAW–GTAW hybrid process
	Welding travel speed (cm/min)			Welding travel speed (cm/min)
	45	55	65	45	55	65
0.01–0.30	5	5	15	-	-	2
0.31–0.60	4	9	11	-	-	2
0.61–0.90	-	1	5	-	-	-
0.91–1.20	-	-	3	-	-	-
Accumulate amount of porosity	9	15	34	0	0	4

The macrostructure of the weld metal cross-section revealed that porosity was mainly distributed near the surface of the weld metal, indicating that the porosity tended to sweep and move up to the weld pool surface during solidification. Accordingly, gas porosity was entrapped near the weld metal surface.

4.3 The welding quality of GMAW – GTAW hybrid welding process

The GMAW – GTAW hybrid welding process was performed using the welding conditions as described above. GMAW torch was placed leading GTAW torch to combine the oxide cleaning action at the weld metal surface. Figure 3 to 5 showed the arc appearance of GMAW – GTAW hybrid welding process with the travel speed of 45, 55, and 65 cm/min, respectively.

It was observed that the metal droplet transfer behavior in the GMAW – GTAW hybrid welding process was similar to conventional GMAW. However, the behavior of anode spots at the molten tip of the wire electrode was slightly unstable. A significant challenge associated with the GMAW – GTAW hybrid welding process was the disturbance in the welding current pathway of GMAW caused by the alternating current of GTAW. This was clearly visualized in Fig. 3d) to 3e) where the arc plasma of the GMAW – GTAW hybrid welding process moved away from the arc axis. The dominant effect of the movement of arc plasma can be explained by the current pathway of the arc plasma [22]. During the DCEN cycle of GTAW, the current pathway of GMAW passes from the arc plasma to the arc plasma of GTAW. However, the effect of the arc disturbance due to the current pathway is irrelevant to the weld bead appearance.

The behavior of cathode spots is a crucial characteristic that indicates the welding quality of the weld metal. It was evident that the oxide cleaning action of GTAW during the DCEP cycle within the weld pool and oxide cleaning zone significantly reduced. This suggests that the DCEP arc of GMAW has sufficient efficiency for oxide cleaning action. Furthermore, the trailing edge of the GMAW weld pool is covered by GTAW shielding gas. As a result, the oxidation reaction at the trailing edge of the weld pool surface does not occur, and the cathode spots of the GTAW arc are reduced. Figure 5c) and 5d) show the arc appearance of the GMAW – GTAW hybrid welding process with the oxide cleaning action.

The visualization of the GMAW – GTAW hybrid welding process arc plasma revealed that the metal transfer behaviors were similar to the conventional GMAW process. The results indicated that the metal transfer mode of GMAW in the hybrid welding process, for every welding travel speed, was short circuit transfer with a transfer rate of 15–20 drops per second. The metal transfer rate was slightly higher than the conventional GMAW arc. It can be suggested that the unstable current pathway and the behavior of anode spots in the GMAW – GTAW hybrid welding process affected the arc plasma temperature and surface tension of molten droplets at the tip of a wire electrode, resulting in an increase in the wire melting rate.

The GMAW – GTAW hybrid welding process demonstrates a significant improvement in the welding quality of the weld metal. It can be suggested that the porosity within GMAW weld metal is entrapped during solidification due to disturbed weld pool convection, a high solidification rate of weld metal, and the formation of an oxide film on the weld surface. However, the GMAW – GTAW hybrid welding arc plasma showed that the combination of oxide cleaning action can remove the oxide surface and increase the possibility that porosity can be ejected from the weld surface. The heat from the AC – GTAW arc, which has a high temperature reaching 13,000–15,000 K [22, 23], can remelt the GMAW weld metal surface. Therefore, it can be suggested that the remelt mechanism allows porosities to be rejected from the weld metal surface. Furthermore, the weld bead appearance of the GMAW – GTAW hybrid welding process was acceptable, featuring a cleaned, spatter-free, and smooth ripple surface. In contrast, the weld bead appearance of GMAW exhibited a high level of weld spatter, as shown in Fig. 6.

In this study, the visualization of arc plasma and the evaluation of welding quality successfully observed the GMAW – GTAW hybrid welding. The results demonstrated a significant improvement in weld metal quality, with porosities in the weld metal almost entirely eliminated. Consequently, the weld bead appearance of the GMAW – GTAW hybrid welding process exhibited no spatter and a smooth surface. The dual cleaning action of GMAW and GTAW strongly influenced the oxide cleaning behavior. Simultaneously, the heat from the GTAW arc plasma could remelt the GMAW weld metal surface, allowing porosities to move out of the weld surface. Therefore, the GMAW – GTAW hybrid welding process could be applied to the industry related to the welding of aluminum alloy according to the high deposition rate, and high welding quality.

Funding statement:

This research is supported by the Ministry of Higher Education, Science, Research, and Innovation of Thailand (MHESI) (Grant no. RGNS 65–082).

Acknowledgement

This research is supported by the Ministry of Higher Education, Science, Research, and Innovation of Thailand (MHESI) (Grant no. RGNS 65–082).

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Table 1 is available in the Supplementary Files section.

Table1.docx

Weld Quality Improvement using GMAW-GTAW Hybrid Welding Process for Aluminum Alloy

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental set-up

3. Research methodology

3.1 The effect of the balanced waveform control of AC-GTAW on cleaning action quality.

3.2 The study of the porosity distribution during GMAW welding.

3.3 Study of the welding quality of weld metal using GMAW – GTAW hybrid arc.

4. Results and discussion

4.1 The oxide cleaning action effect of AC-GTAW

4.2 Distribution of porosity in GMAW weld metal

4.3 The welding quality of GMAW – GTAW hybrid welding process

5. Conclusions

Declarations

Funding statement:

Acknowledgement

References

Table

Supplementary Files

Status:

Version 1