1 Introduction

Because of the high productivity and low investment costs, arc welding processes are frequently used welding technologies in industry. Particularly for joining materials such as stainless steels or titanium, magnesium, and nickel-based alloys, excellent shielding gas coverage is needed in order to ensure a high joint quality.

Insufficient shielding gas coverage of the process area may lead to process instabilities, spatters, pores, oxide inclusions, or cracks [1]. In addition, enhanced concentrations of oxygen lead to an oxidation of heated areas of the parent material (so called heat-tints) [2, 3] which indicate a dramatic reduction of the corrosion resistance.

In order to prevent corrosion and to increase the optical appearance of the joint, heat-tints either have to be avoided or removed subsequently after the welding process [4, 5]. Because removing of the heat-tints is often associated with high costs or the usage of hazardous substances and dangerous goods, gas trailing shields are frequently used to reduce or fully avoid the formation of heat-tints.

A large number of various constructional concepts of gas trailing shields are used in industrial applications. Simplified concepts are based on the usage of extended gas nozzles using the existing shielding gas flow of the welding torch. Complex concepts with a separate gas supply are used in order to significantly increase the area of the shielding gas coverage at the parent material.

A frequently used concept is the so called “gas box”, which is mainly characterized by a separated gas supply as well as an included welding torch. Gas box concepts widely differ in terms of the gas inlet, the gas distribution as well as the laminarization of the turbulent shielding gas flow.

Already slight changes of the process parameters and/or the component geometry may result in a serious deterioration of the shielding gas coverage. Thus, the transferability of gas trailing shields between different welding tasks is limited. The constructional design often needs to be adjusted by the user according to the welding task.

Since no general design recommendations for gas trailing shields are available in the literature, changes of the constructional design are often performed step by step. This procedure is associated with an increased expenditure of time and costs, even when existing designs of gas trailing shields are readily available.

In order to be able to gain a comprehensive insight into the shielding gas flow field and the related physical processes in arc welding, a variety of different numerical as well as diagnostic methods were presented in the past [6–14]. CFD-simulations enable a spatial and time dependent resolution of all process variables, e.g., flow velocity or temperature. Numerous numerical studies have been carried out to understand complex phenomena such as the effects of shielding gas components and vaporization in tungsten inert gas welding [6, 7], as well as gas metal arc welding [8, 9]. Concerning the shielding gas coverage, numerical models were used to describe the effects of both the process variables and the geometry of torch components on the shielding gas flow field [10].

In addition to numerical models, a high number of diagnostic methods are available in order to characterize the shielding gas coverage in arc welding. For the visualization and characterization of the flow field in arc welding, the usage of the particle image velocimetry (PIV) [11] and Schlieren-Technique [10, 12, 13] is possible. For a quantitative evaluation of the shielding gas coverage, the oxygen concentration can be determined using the lambda sensor principle [14]. These methods have been successfully used for the analysis of the shielding gas flow as well as the development of welding torches in GMA welding [10]. However, neither numerical nor experimental investigations regarding the constructional design of gas trailing shields and the underlying cause and effect chains are described in the literature.

In this paper, two major contamination mechanisms concerning gas boxes are discussed. By the use of numerical as well as experimental methods, the reasons of these contaminations are visualized and their influencing parameters are characterized. Subsequent, general design recommendations are presented in order to gain robust as well as cost efficient constructional concepts for industrial application.

2 Methodical approach

In this paper, two industrially used constructional concepts are described: gas boxes using commercially available diffusers at the gas inlet as well as gas boxes with a flow separation, e.g., perforated or sintered metal plates at the bottom of the box, Fig. 1.

Fig. 1
figure 1

Basic designs of the investigated gas box concepts

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In order to ensure a purposeful development, an enhanced understanding of the flow field inside the gas box, as well as its interactions with the welding process, and the constructional design of the system is essential. Therefore, numerical as well as experimental methods were combined systematically.

A verification of the results gained from the numerical model was carried out by welding tests on the austenitic stainless steel 304.

The computational domain of the numerical model used in the investigations includes the shape of the gas box as well as the torch, the workpiece, and the gas flow region (Fig. 2).

Fig. 2
figure 2

Computational domain of a gas box with two diffusers

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The equation system is based on a magneto-hydrodynamic-model (MHD) containing both the standard equations of computational fluid dynamics (conservation of mass, momentum, and energy) as well as formulations for the consideration of electromagnetic phenomena such as the current density, the magnetic induction, Lorentz force, and resistive heating. In addition to radiative phenomena, the effects of the anodic and cathodic sheath regions as well as turbulence effects were taken into account.

For the modeling of diffusion effects, the combined diffusion coefficients of argon air mixtures calculated by Murphy [15] were used. Based on the assumption of a local thermodynamic equilibrium (LTE), all thermophysical and transport properties were included as functions of temperature and pressure. A description of the equation system for modeling welding arcs can be found in [9, 16].

The flow characteristics of the diffusers as well as the perforated or sintered metal plates were taken into account by including additional source terms in the momentum conservation equation. The material specific parameters of the momentum source were gained from pressure loss measurements and a non-linear least squares method [16].

The fundamental contamination mechanisms are described on the basis of a gas box with characteristic dimension (length: 150 mm; width: 50 mm; height: 30 mm). However, the findings are transferable to other gas box dimensions.

In order to ensure transferability between the different design concepts, all investigations were done using a tungsten electrode (WLa 15) with a diameter of 3.2 mm and a tip angle of 40°. The electrode tip to workpiece distance was 3 mm. We used argon 4.6 for all investigations with a shielding gas volume flow of the torch of 10 l/min. The perforated plate used had a material thickness of 1 mm and the diameter of the holes was 1 mm. The material thickness of the sintered metal plate was 4 mm, the grain size varied from 0.2–0.315 mm.

3 Contamination mechanisms

An essential parameter of gas boxes, which determines its operational capability, refers to the distance between the bottom of the gas box and the workpiece surface (working distance). Higher distances lead to a better accessibility but also to a significantly increased sensitivity to process parameters and interferences on the shielding gas coverage.

In order to ensure stable and reproducible gas coverage in industrial usage, the working distance often has to be reduced down to 1–2 mm. To increase the working distance, main contamination mechanisms have to be identified and eliminated with respect to economic aspects.

Numerical calculations have shown that contamination of gas boxes with atmospheric gases are based on two main mechanisms—eddy formation inside the gas box and intake of atmospheric gases at the edges close to the arc process due to the highly accelerated arc-induced flow. In Figs. 3 and 5, the resulting transport of atmospheric gases for these two mechanisms is shown. Green streamlines indicates the flow field of the shielding gas starting at the gas inlet of the gas box as well as of the welding torch, red lines point out the flow of atmospheric gases starting from outside the box. Hereinafter, these two contamination mechanisms are described in detail.

Fig. 3
figure 3

Streamlines of a gas box with two diffusers current 150 A, working distance 5 mm, shielding gas volume flow gas box 20 l/min

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3.1 Intake of atmospheric gases by eddy formations

This contamination mechanism is related to gas boxes with diffusers and is based on an eddy formation, which leads to an intake of atmospheric gases into the center of the vortex (Fig. 3).

The vortex is mainly driven by the arc-induced flow as well as the opposing gas flow of the diffusers. The higher the eddy intensity is, the bigger is the transport of atmospheric gases into the gas box. Consequently, the quality of the gas coverage is determined by all parameters, which influence the flow of the arc as well as the flow of the diffusers, and thus the eddy intensity.

The arc induced flow is mainly determined by the welding current, the used shielding gas composition and flow rate as well as the included angle of the electrode tip and the shape of the electrode tip itself. The flow of the diffusers is driven by the shielding gas volume flow used as well as the characteristic of the material and the geometry of the diffusers.

With an increasing number of diffusers, flow velocities in areas close to the diffusers are reduced. As a result, the shear stresses, and thus, the eddy intensity reduce as well. The possible working distance depends strongly on the diffuser used and its geometry as well as material properties. This is the major drawback of this approach. In addition, an increased number of diffusers lead to increased constructional effort as well as to an enhanced number of wear parts.

In order to realize enhanced working distances, the formation of eddies inside the gas box needs to be prevented. The vortex formation is mainly driven by the interaction between the arc induced flow and the gas flow coming from the diffusers, thus separation of both flows is required. This can be done by using an additional insert in the gas box bottom (e.g., a perforated plate or a sintered metal plate) to separate the arc and the diffuser flow fields, see Fig. 4. The separation of the flow fields results furthermore in a significant reduction of the number of factors influencing the shielding gas coverage.

Fig. 4
figure 4

Velocity field of a gas box using two diffusers without (top) and with an additional perforated plat (bottom)—current 150 A, working distance 5 mm, shielding gas volume flow gas box: 20 l/min

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3.2 Intake of atmospheric gases by the highly accelerated arc-induced shielding gas flow

The second main source for contamination inside gas boxes is related to the intake of atmospheric gases at the lower edges of the gas box close to the arc process due to the highly accelerated shielding gas flow of the arc, Fig. 5. Compared to the contamination of gas boxes resulting from eddy formations, this contamination mechanism occur only at high working distances when using the same shielding gas volume flow.

Fig. 5
figure 5

Streamlines of a gas box with a sintered metal plate—current 150 A. distance gas box—workpiece 5 mm, shielding gas volume flow gas box 20 l/min

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In order to reduce the intake of atmospheric gases into the gas box, a continuous shielding gas outflow over the entire surface between the lower edges of the gas box and the workpiece surface is required (peripheral surface, Fig. 6). At increased working distances, this cannot be realized especially in areas nearby the welding process. In Fig. 6, this effect is illustrated by showing the flow velocity profile between the edge of the gas box and the workpiece surface. High work distances of about 7 mm and more result in a missing outward flow directly below the edge of the gas box. In this region, atmospheric gases can contaminate the inside of the gas box, see Fig. 6.

Fig. 6
figure 6

Standardized velocity profile (black) and position of contamination (red) at areas close to the arc welding process for three different heights of the gas box—construction principle gas box with a sintered metal plate, current 150 A, working distance 5 mm;,shielding gas volume flow gas box 20 l/min

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Because of the Lorentz force, gas is accelerated especially in regions close to the arc process. This transport has to be met with a higher shielding gas flow inlet in this region. Otherwise, atmospheric gases are transported from the outside into the gas box as a consequence of the resulting pressure-equalization. Consequently, the shielding gas flow necessary needs to vary over the gas box bottom. In areas close to the welding process, an increased shielding gas flow compared to areas in the rear part of the gas box has to be applied.

This contamination mechanism is more pronounced at gas boxes with inserts (e.g., perforated or sintered metal plates), because of the resulting unified velocity field at the bottom of the gas box. For the same shielding gas flow rate, the local flow velocity coming from the inserts is lower in process regions and greater in the rear part of the gas box.

The usage of perforated plates correlates with low pressure losses, the usage of sintered metal plates with higher pressure losses. The bigger the pressure loss of the used insert is, the better is the unification of the flow field. Figure 7 shows the velocity field at the bottom of a gas box using no flow separation in comparison to gas boxes using a perforated or a sintered metal plate.

Fig. 7
figure 7

Calculated velocity fields for a gas box with diffusers without an additional insert (top), using a perforated plate (middle) and a sintered metal plate (bottom)—height of the section above the workpiece 3 mm, current, 150 A, working distance 5 mm, shielding gas volume flow gas box, 20 l/min

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By increasing the shielding gas flow of the gas box, the intake of atmospheric gases can indeed be prohibited. Because of the massive increase of the shielding gas volume flow needed, the usage of this approach is not suitable especially in industrial applications with high duty cycles.

In order to ensure an economic operation of the gas box, a defined increase of the gas flow especially near the arc region is necessary. One approach is based on a sintered metal plate with a variable material thickness. By lowering the material thickness in areas close to the process, the pressure loss is reduced and consequently, the local convective mass flux of the shielding gas can be increased significantly.

This results in a shielding gas coverage with low concentration of atmospheric gases even at enhanced working distances. Figure 8 shows the resulting velocity field of a gas box using a sintered metal plate with a constant and a variable material thickness. The influence on the oxygen concentration is shown on the basis of the 20 ppm isoline. In the case of a sintered metal plate with a constant material thickness, more than 20 ppm of oxygen is present in the process area which indicates the formation of heat-tints. By using a sintered metal plate with a variable material thickness, the area of the box coverage can be increased significantly.

Fig. 8
figure 8

Calculated velocity fields and isolines for 20 ppm oxygen for a gas box with a sintered metal plate with constant (top) or variable (bottom) material thickness (bottom)—height of the section above the workpiece 5 mm; working distance 5 mm, shielding gas volume flow gas box 20 l/min

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4 Discussion of different gas box concepts

The operational behavior of gas boxes, especially at higher working distances, is mainly determined by the shielding gas flow inside the gas box. In order to prevent the intake of atmospheric gases into the gas box, a continuous outflow over the entire peripheral surface (Fig. 6) is required.

The necessary distribution of the gas flow within the box can be realized by both the gas inlet system at the top of the box and the flow laminarization at the bottom of the box. Basically, the better the gas distribution system, the fewer requirements to the laminarization system exist, see Fig. 9.

Fig. 9
figure 9

Principal concepts of gas distributions in gas boxes

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A common concept in industrial application is based on a simplified gas distribution system, e.g., a direct inlet, in the upper section of the gas box. Even at low working distances, atmospheric gases can be transported into the gas box easily due to the missing distribution over the entire peripheral surface (Fig. 9a). By the use of a subsequent laminarization system, e.g., metal inserts, in the lower section of the gas box, the transport of atmospheric gases can be reduced or even fully avoided even at higher working distances. However, by the use of metal inserts with a low pressure loss, e.g., perforated plates, the gas laminarization is not sufficient to fully avoid the intake of atmospheric gases. Instead, inserts with a high pressure loss, e.g., sintered metal plates, are needed in order to obtain a sufficient shielding gas distribution (Fig. 9e).

In the case of metal inserts with a high pressure loss, sealing between the metal inserts and the gas box is mandatory. Insufficient sealing of the inserts may lead to a highly increased gas flux at the leakage, which does affect the profile of the gas flow significantly. In combination with an increased turbulent diffusion, the quality of the shielding gas coverage decreases substantially. Regarding applications with a high duty cycle, a temperature resistance sealing is needed. In addition to the sensitivity of the gas coverage on the quality of the sealing, the usage of sintered metal plates correlates with further significant drawbacks such as a missing exchangeability of the inserts which often limits the application of those gas box concepts to clean and spatter-free welding process.

An alternative concept is to ensure a sufficiently good gas distribution by using an appropriate gas inlet system in the first place. This results in significantly lowered requirements concerning the laminarization system. For small working distances of about 3 mm, the laminarization system is not even necessary, see Fig. 10. In order to increase the working distance, metal inserts with low pressure losses are sufficient. The usage of sintered metal plates is not necessary. Compared to the application of sintered metal plates, the usage of a perforated plate correlates with a number of significant advantages, e.g., no sealing is needed, an improved exchangeability of the metal insert, a simple constructional design as well as lower shielding gas flow rates.

Fig. 10
figure 10

Formation of heat-tints on welds using a gas box with a diffuser gas inlet system without (top) a perforated plate in comparison to a gas box including a perforated plate, material 304, material thickness 2 mm; current 200 A, welding speed 60 cm/min, shielding gas volume flow gas box 30 l/min

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The authors therefore recommend gas boxes using diffusers at the gas inlet as well as a perforated plate at the bottom of the gas box. By the use of this constructional concept, heat-tints can be avoided up to a working distance of 11 mm, see Fig. 10.

The very slight traces of stain on the side of the weld are well known in industrial applications and are observable for all investigated construction principles. A commonly used way in industry to fully avoid heat-tints is using a shielding gas with admixtures of hydrogen for the gas box instead of pure argon (e.g., argon-hydrogen or nitrogen-hydrogen mixtures).

Based on the significantly improved accessibility, a given gas box may be used for different applications (e.g., joining flat workpieces as well as pipes). In addition, the improvements in the shielding gas flow lead to a massive drop in the required shielding gas volume flow rate of up to 60%.

5 Additional requirements on gas boxes

The required length of the gas box is determined by the length of the heated area of the workpiece and therefore depends on the energy input of the welding process as well as on the cooling of the base material via heat conduction and radiation. Consequently, different gas boxes are used for different welding tasks in order to limit the length of the gas box and therefore the shielding gas volume flow needed.

In addition, the inclination of the welding torch (push/pull technique) is defined by the constructional design of the gas box. An adjustment of the torch inclination is beneficial especially for welding processes with a high welding speed to be able to ensure a sufficient fusion penetration or to avoid undercuts.

In addition to the need of a high quality shielding gas coverage, both mentioned requirements do reduce the transferability of gas boxes between different applications significantly. Thus, a variety of different gas boxes is used in industrial applications according to the requirements of the welding task.

Both requirements can be met by using a gas box based on a modular structure for length scaling, including a module to adjust the inclination of the torch. By the usage of this approach, a gas box can be used for a variety of different welding tasks. Figure 11 shows such a modular design. The concept is based on a process module, an arbitrary number of extension modules as well as an ending module

Fig. 11
figure 11

Prototype of a modular gas box with a pivoted joint

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The shielding gas as well as the cooling water are supplied at the process module and are transferred to the subsequent modules by force-closure interfaces. An additional supply of gas as well as cooling water is not necessary. The ending module is used to close the cooling circuit. Between the process and the ending module, the implementation of straight and curved extension modules is possible for scaling the gas box. An adjustment of the torch inclination in the range of ±10° is possible due to an additional pivoted joint (inclination module). A contamination of the gas box with atmospheric gases at the pivoted joint is avoided through the use of cylindrical elements

6 Conclusion

In this paper, different constructional concepts of gas trailing shields and the main contamination mechanism with atmospheric gases were presented.

By the use of gas boxes with diffusers as the inlet system and an additional insert with a low pressure loss (e.g., perforated plate) as the laminarization system, a high quality of the shielding gas coverage can be combined with a simple constructional design of the gas box. This concept allows working distances up to 11 mm at low shielding gas flow rates of about 30 l/min.

For industrial applications, this concept could be transferred to a modular design concept which allows an individual length scaling of the gas box depending on the welding task.