Buried Arc application in Hybrid Laser-Arc Welding of Thermo-Mechanical Control Process Steels

doi:10.21203/rs.3.rs-3206217/v1

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Buried Arc application in Hybrid Laser-Arc Welding of Thermo-Mechanical Control Process Steels

https://doi.org/10.21203/rs.3.rs-3206217/v1

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published 14 Dec, 2023

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Hybrid laser arc welding (HLAW) has gained significant attention in recent years due to its ability to provide high-quality welds with improved productivity. One of the key challenges in HLAW is the efficient utilization of the welding arc energy. This work aims to analyze the HLAW process with gas metal buried arc welding (GMBAW), and compare the results obtained with typical results of laser welding and HLAW with non buried arc. Tests were conducted at two welding speeds, 1.0 m/min and 1.5 m/min, and the results were compared with autogenous laser bean welding (LBW) and HLAW without a buried arc, on ASTM A709 steel that was manufactured by Thermo-Mechanical Control Process (TMCP). The buried arc HLAW resulted in a more uniform weld bead, with a reduced heat-affected zone. Moreover, the laser power required to achieve full penetration of the weld joint was smaller for the buried arc HLAW variation.

HLAW

GMBAW

laser

penetration

bead geometry

Hybrid laser arc welding (HLAW) has emerged as a versatile joining technique, combining the advantages of laser and arc welding processes. Both heat sources acting simultaneously in the same weld pool, allows the union of high speeds and penetrations with tolerance to misalignments, offering improved weld quality, increased productivity and enhanced process control. Even in situations where full penetration cannot be achieved in just one pass, the application of HLAW for the root pass makes possible not only reduce the number of welding passes, but also changes the bevel size and angle, reducing procedures related to joint preparation and material waste [1]. HLAW is already used in automotive, aerospace, naval and oil and gas industries [2].

Another process that can be used in high penetration applications is the GMBAW, which is applied with a high current density combined with a forcibly short arc. In this case, the arc is established in a depression in the center of the molten pool, increasing the penetration and enabling higher welding speeds. The GMBAW also has high deposition rates, due to the high wire-feed speeds required to keep the short arc length and to the high currents combined with a long stick-out, which assist in wire fusion [3].

These characteristics of GMBAW can also be beneficial when applied in HLAW, since the deep depression formed by the pressure of the arc would reduce the thickness perceived by the laser beam, improving weld penetration. These factors could contribute to increased productivity and cost reduction, making the acquisition of a hybrid welding system more accessible. Despite the potential benefits of using these two processes together, the related literature is scarce. Some results were presented by Gook et al. [4], Pan et al. [5] and Wahba et al. [6], since its successful implementation required careful consideration of process parameters. Optimize and control all the variables in this process, such as laser power, arc current, wire feed rate, shielding gas, laser and arc alligment, among others, is crucial to achieve reliable and consistent results, a stable operation, prevent excessive spattering and minimize the formation of defects. In addition, the effects of the high heat input of the buried arc process in the heat-affected zone (HAZ) adjacent to the weld bead, compared to the traditional process, were not evaluated in depth. This evaluation is important in structural steels manufactured by TMCP, which, despite having good weldability, can suffer a reduction in hardness in the HAZ, especially in operations with high heat input, degrading the original base material properties [7].

Therefore, this work aims to analyze the HLAW process with GMBAW, and compare the results obtained with typical results of laser welding and HLAW with non buried arc.

To evaluate the behavior and characteristics of the HLAW with buried and non buried arc (colled “long arc” in this work) and LBW, welds were performed with welding speeds (S) of 1.0 m/min and 1.5 m/min and current ranging from approximately 260 A (long arc) and 470 A (buried arc), and laser power ranging from 4.5 to 10 kW.

The weld beads were performed in SENAI Innovation Institute for Laser Processing, using a PRECO Inc. SL8600 hybrid process machine, that includes a disk laser source from Trumpf (TruDisk 10002) provided with a continuous-wave (CW) near-infrared (NIR, λlaser = 1030 nm) laser beam, with spot size of 0.6 mm, maximum average power of 10 kW. The welding head (LaserMech) has focal distance of 450 mm. A digiplus A7 PMDAC-1000 multiprocess welding source was used for gas metal arc welding (GMAW). Voltage and current data were collected during all tests using a data aquisition system (SAP V4) from the welding source. The GMAW torch and laser arrangement is shown in Fig. 1a. A high-speed camera (PHANTOM MIRO M310) with a 320 x 240 pixels resolution, 20000 fps image acquisition rate, exposure time of 5 µs and Lens aperture of f/16.0 was used to monitor the process. A schematic representation of the experimental arrangement used is shown in Fig. 1b. The high-speed camera was positioned laterally to the XY table, in order to capture the torch's inclination.

An 9.5 mm thickness ASTM A709 HPS 70W, produceded by controlled rolling followed by accelerated cooling, was used as the base material (supplied by Usiminas). This steel has a yield strength greater than 485 MPa, a tensile strength limit between 586 and 758 MPa and an elongation greater than 19%. The filler wire used was AWS 70S-6 with a diameter of 1.2 mm. The chemical composition specifications fo the base metal and filler wire are shown in Table 1.

Table 1

Chemical composition (wt.%) of base material and filler wire.
Material	C	Si	Mn	P	S
A709	≤ 0.11	0.30–0.50	1.10–1.35	≤ 0.020	≤ 0.006
ER70S-6	0.06–0.15	0.15–1.15	1.4–1.85	< 0.020	< 0.035
	Cr	Mo	Ni	Cu	V
A709	0.45–0.70	0.02–0.08	≤ 0.40	0.25–0.40	0.04–0.08
ER70S-6	> 0.15	> 0.15	> 0.15	-	> 0.03

The tests were carried out with two different welding speeds, 1.0 m/min and 1.5 m/min. The process used was arc guiding (GMAW pulling) as it results in increased penetrations. The weld parameters are shown in Table 2. The non buried GMAW process parameters were selected according to the works of Kutsuna and Chen [8], Cao et al. [9] and Liu et al. [10].

The experiments were carried out on 350 mm x 150 mm pieces with 9.5 mm thickness, as shown in Fig. 2. In laser welding, the sheet was machined to ensure a proper joint finish and correct alignment. A backing with the same composition as the base metal was positioned in the weld root region to support the weld pool.

Table 2

Variable welding parameters of the experimental runs.
Process	Set parameters				Measured Parameters		Arc energy (J/mm)	Laser energy (J/mm)
Process	S (m/min)	P (kW)	F (m/min)	U (V)	Im (A)	Um (V)	Arc energy (J/mm)	Laser energy (J/mm)
HLAW buried arc	1.0	4.5	18	35	429	35.9	770	270
HLAW buried arc	1.5	4.8	18	35	438	35.8	520	192
HLAW long arc	1.0	6.2	6	25	282	24.5	330	372
HLAW long arc	1.5	7.7	6	25	260	24.5	220	308
LBW	1.0	9.5	-	-	-	-	-	570
LBW	1.5	10	-	-	-	-	-	400
Observation: S – welding speed; P – laser power; F – wire feed speed; U – voltage; Im – mean current; Um – mean voltage

Initially, a visual inspection was carried out on the beads, then, hardness measurements were performed along the cross-section of the beads, with a distance between impressions of 0.35 mm on the x and y axes, with a load of 1 kgf (HV1).

To evaluate the profile of the molten zone, three samples were taken from the cross section, in regions representing the beginning, middle and end of the bead. The samples were prepared metallographically and etched in Nital 7% for 8 seconds and evaluated in a stereoscope. Measurements were performed according to Fig. 3, using the IMAGEJ software.

The laser power required to achieve 9.5 mm penetration decreased with the introduction of the other heat source (GMAW), Fig. 4. Under the conditions studied to achieve full penetration, the application of buried arc allowed a reduction of 30% of the laser power compared to the HLAW process with long arc, and up to 52% when compared to the LBW, at 1.0 m/min. According to Kutsuna and Chen [8], the deeper the laser incidence region in the molten pool, the greater penetration expected for the HLAW process. As in the GMAW process with buried arc the weld pool has a deeper profile [3], the thickness of material to be penetrated by the laser is smaller, allowing the application of a lower laser power to achieve full penetration. This behavior is illustrated in Fig. 5. When the LBW is used, the laser beam strikes the surface and penetrates the total thickness (X1) of the base metal. When GMAW is added, the laser strikes at an already molten region of the base metal, corresponding to the weld pool. The depression on the surface of the pool reduces the total thickness that must be penetrated by the laser (X2), which is even smaller in the process with the buried arc (X3), whose depression is more accentuated.

Achieving the same penetration depth with different laser beam power values indicates the possibility of an increase in welding speed or the possibility of welding even thicker joints without the need for higher power lasers. Resulting in higher productivity and lower implementation costs of the process, as it allows the use of less powerful and cheaper lasers.

3.1 Process evaluation

The presence of the buried arc was verified through the images of the high-speed camera, which show an arc with a height lower than that observed for the long-arc process. This characteristic can be observed in Fig. 6A and B where only a small area corresponding to the electric arc is visible, indicating that the desired welding condition was obtained. Images of the hybrid process with long arc, Fig. 6C and D shows the difference in arc height between both conditions. Additional high-speed videos were taken in order to verify the metal transfer and the pool depression for the buried HLAW process at 1.5 m/min, Fig. 6. It can be seen in the extracted frame that the arc is present inside the pool depression and the transfer mode has streaming spray caracteristics.

During the tests, presence of welding fume and spatter was lower for the buried arc, in comparison with the long arc. The reduction of spatter in GMAW with buried arc has already been verified by Dompablo [11] and Budig [12], and similar results were obtained by Pan et al. [5], Gook et al. [4] and Wahba et al. [6] working with buried arc HLAW, this occurs since any spatter generated tends to be trapped inside the arc cavity.

Regarding the surface aspect of the beads, for buried arc HLAW process, no defects, such as porosities or undercuts, were observed at any of the evaluated speeds, Fig. 7. The absence of undercuts in welds performed with buried arc has already been reported by Dompablo [11], Budig [12] and Wahba et al. [13]. In the long arc HLAW process, undercuts (being more significant at the speed of 1.5 m/min) and spatter were observed. In the LBW, a depression on the weld bead, especially at 1.5 m/min, and spatter were observed, these main defects are better evidenced in the cross section presented in Fig. 7.

3.2 Weld bead profile evaluation

The increase in welding speed leads to a reduction in the heat input in the GMAW process, resulting in a smaller bead area and less penetration, as can be seen in Fig. 7. The buried arc HLAW samples (Fig. 7A and B) have a smaller HAZ, due to the lower heat input and higher cooling rate [14]. The cross section of the bead deposited by the long arc HLAWprocess (Fig. 7C and D) has the form described in the literature as a wine glass [15]. As the region penetrated by the arc is smaller, there is a greater thickness of material that must be penetrated by the laser, justifying the higher power used in these tests. In the hybrid process with the buried arc, the penetration of the arc zone was greater than 5 mm, requiring a lower power of the laser beam to achieve full penetration. In the buried arc HLAW, at 1.0 m/min (Fig. 7A), for example, the laser penetration depth is 0.8 mm, while it is 5.7 mm for the equivalent long arc HLAW, justifying the change in laser power to achieve full penetration in each case. In those hybrid welding samples, the joint geometry was defined according to the work of Cao et al. [9], who studied different joint configurations for a similar hybrid process.

As for the LBW samples (Fig. 7E and F), under-fill was present at both welding speeds, making the surface of the bead concave. This defect is more complicated to solve in autogenous laser welding than in the hybrid process, where a slight adjustment of the wire feed speed may be enough to solve the problem.

In the high penetration laser welding process, it is common to observe a solidification crack, resulting from the high ratio between penetration depth and bead width combined with high cooling rates [17, 20]. Barbetta [21] showed that the occurrence of this defect is related to the formation of a bulging region in the molten pool in the laser zone, with the presence of cracks being proportional to the increase in the width of this region. The addition of GMAW in the hybrid process reduces the process cooling rate and the high penetration of the arc zone reduces the depth/width ratio and overlaps the bulge region in the laser zone, reducing the tendency to form cracks in the laser region [17, 22, 23]. Based on this information, taking into account the high heat input and the greater penetration of the arc zone in the buried arc process, it can be expected that the use of the buried arc has an even more significant influence on the reduction of this defect, compared with the GMAW with long arc.

The HLAW process with buried arc showed greater penetration of the arc zone in relation to the long arc, Fig. 8, behavior similar to that found by Kah [2] and Katayama [16], who showed that an increase in welding current directly influences the penetration of the arc zone. According to Gook et al. [4], the area and penetration of the arc zone depend on the characteristics of the arc used, and the deeper this region, the better the distribution of filler metal along the bead. Therefore, the results obtained in this study indicate a greater dilution and better distribution of filler metal in the buried arc process. The heterogeneous distribution of filler metal in hybrid welding has already been reported by Kah [2] and Katayama [16]. This is problematic when alloying elements present in the filler metal are used to obtain certain microstructures and mechanical properties in the weld bead [17, 18]. According to the study carried out by Atabaki et al. [19], a molten pool of greater volume and a lower cooling rate help the escape of gases, reducing porosity. Therefore, the hybrid process using the buried arc, whose heat input is higher than that used by the long arc process, should contribute more significantly to the reduction of porosities, in view of the greater volume of the weld pool.

In general, an increase in the volume of the weld pool is observed as the heat input increases. The evaluation of the bead profile showed this behavior, evidencing the influence of the arc mode and the heat input on the weld pool geometry, Fig. 9. Considering the GMAW process alone, the increase in heat input also indicates a higher deposition rate, resulting in a greater volume of deposited material and reinforcement. The difference observed is a result of the buried arc’s superior heat input. Taking into account only the arc zone, in buried arc hybrid welding operations there is an area around 40 mm² for the process at 1.0 m/min and 30 mm² at 1.5 m/min. In the long arc process, the area was smaller, 16 mm² for 1.0 m/min, and 13 mm² for 1.5 m/min. Additionally, the higher wire feed speeds required for buried arc processes result in greater material deposition, which also influences the bead volume and reinforcement height.

3.2.1 Hardness evaluation

The comparison of hardness values obtained in the molten zone and in the soft region for each process is shown in Fig. 10. It should be noted that in no case was the hardness obtained higher than 350 HV1. Above this value, there is a greater susceptibility to the occurrence of cold cracks [25]. The values indicated in the graph at the total heat input of the process, which is the sum of the heat input of the arc process and the laser process. It is observed that the samples welded with similar heat inputs show the same behavior in relation to hardness changes in the molten zone and in the soft region, regardless of the applied welding process, as is the case for the buried arc HLAW at 1.5 m/min and the long arc HLAW at 1.0 m/min. The greatest hardness variations, in relation to the base metal, occurred in the buried arc HLAW at 1 m/min soft region, and in the LBW-1.5 m/min molten zone. Both samples represent, among the processes used, extreme conditions of higher and lower heat input, respectively. According to Borba [24], a higher cooling rate, that occurs in lower heat inputs, results in higher hardness values due to the formation of microstructural constituents that form at lower temperatures.

Based on the results presented, it is concluded that the buried arc GMAW appears as a good option for application in hybrid welding, allowing the reduction of the laser power necessary to obtain full penetration of the joint. The buried arc HLAW resulted in a more uniform weld bead, with a reduced heat affected zone. In addition, the possibility of joining joints in a single pass and increasing the welding speed is interesting for applications in materials that suffer hardness reduction in the thermally affected zone, as is the case with TMCP steels. Further research and developments are still needed to optimize process parameters, expand the range of weldable materials and achieve the full potential of buried arc GMAW in industrial applications.

Ethical Approval

This study followed the guidelines established by the authors institutions (SENAI Innovation Institute and Usiminas).

Competing interests

I declare that the authors have no competing interests that might be perceived to influence the results and/or discussion reported in this paper.

Authors’ contribuitions

All authors contributed to the study conception and design. Usiminas donated the material used in this study. Welding tests were conducted by Natalia Dreveck, who also did the material preparation, data collection and preliminary analysis. Tadeu Borda conducted advanced samples characterizations. Results were discussed between all the authors. The first draft of the manuscript was written by Natalia Dreveck and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Funding

No funding has used by or given to any of the authors in relation to this study.

Availability of data and materials

The relevant data are shown in the paper, any other information needed can by sent by the corresponding author.

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No competing interests reported.

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You are reading this latest preprint version

Buried Arc application in Hybrid Laser-Arc Welding of Thermo-Mechanical Control Process Steels

Status:

Journal Publication

Version 1

Abstract

Figures

1 Introduction

2 Materials and Methods

3 Results and Discussion

3.1 Process evaluation

3.2 Weld bead profile evaluation

3.2.1 Hardness evaluation

4 Conclusions

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1