Characteristics and mechanisms of weld formation during oscillating arc narrow gap vertical up GMA welding

Abstract

Large metal structures are applied increasingly in manufacturing industry, such as ship building industry, pressure vessel, pipeline engineering, and ocean engineering. Narrow gap vertical up GMA welding has been applied in such large metal structures with the advantage of higher production efficiency. In this paper, the characteristics and mechanisms of weld formation during oscillating arc narrow gap vertical up GMA Welding were investigated by experiment and simulation, and the influences of the oscillating parameters on weld formation were analyzed, such as oscillating angle and oscillating speed. The results show that the weld formation of vertical up GMA welding is apt to appear convex shape, due to the molten liquid metal accumulating at the middle of pool with the action of gravity, and oscillating arc is beneficial to reduce convex degree. The desired weld bead with little convex degree has been achieved by using the optimization parameters.

1 Introduction

Large metal structures are applied increasingly in manufacturing industry, such as ship building industry, pressure vessel, pipeline engineering, and ocean engineering. In such large metal structures, conducting welding at vertical up position is inevitable due to the difficulty for changing welding position [1]. Narrow gap GMA welding (NG-GMAW) is a promising welding process with the advantages of less material consumption, higher productivity, lower thermal stress, and less deformation in welding heavy plates compared with traditional GMA welding. The narrow gap GMA welding process at vertical up position is able to reduce welding time and promote manufacturing efficiency in large metal structures.

However, in vertical up NG-GMAW, it is difficult to obtain desired weld bead partly because the molten pool is apt to flow down under the action of gravity [2]. Many methods have been developed to improve this problem, including applying pulse current power to decrease heat input, oscillating arc, and employing Lorenz force to against the gravity of the molten pool. In pulsed GMA welding, the current rises to a peak for forming the droplet and melting groove, then, in the background current phase, the current is lowered to reduce the overall heat input. Therefore, the overall heat input of pulsed GMA welding can generally become lower than that in constant current welding, due to the possibility of lowering the average current. For example, G. Lothongkum et al. [3] apply pulse current for horizontal, vertical, overhead welding of 304 L austenitic stainless. The magnetic field generators are fitted at the welding torch for generating magnetic field in the molten pool. The current flowing through the molten pool or the eddy current by additional power produces electromagnetic forces under the action of the magnetic field. The direction of electromagnetic force could be set to against the gravity of molten pool by adjusting the direction of the current or the magnetic field [4]. Decreasing heat input and adding electromagnetic force are two main methods to counteract flow of molten pool, while equipment for adding electromagnetic force is complex and not suitable for narrow gap welding. Among them, the oscillating arc is not only applied in vertical up GMA welding for controlling molten pool [5] but also in NG-GMAW for ensuring sufficient penetration of sidewall with increasing heat input of the sidewall [6–8]; however, the study on oscillating arc narrow gap vertical up GMA welding process is not reported. Therefore, the oscillating arc narrow gap vertical up position GMA welding process is proposed in this paper.

Due to the effects of the narrow gap groove, oscillating arc, and gravity of molten, the weld bead formation during oscillating arc vertical up NG-GMAW is characterized with deep penetration and convex surface. In multiple layer narrow gap welding, the serious convex surface shape is defined as a formation defect because it is able to result in interlayer defect of incomplete fusion. Study on the mechanism of weld bead formation and control of formation defect is crucial for oscillating arc vertical up NG-GMAW. Many efforts have been made to analyze formation mechanism and reduce formation defect. P. F. Mendez and T. W. Eagar [9] explained the penetration and defect in the molten pool at high current welding, the arc pressure at high currents resulted in a gouging region which is the direct cause of several weld pool defects, and a simple model is developed to predict the onset of humping defect. In order to illustrate the mechanism of humping in high-speed GMAW, Nguyen et al. [10, 11] researched the relationship between the location of droplet and the humping defects. At high welding speed, the weld pool became elongated, shallow and narrow; the location of droplet was closer to the edge of the weld pool, and the molten metal under the arc flowed to the rear of the welding pool with a very high speed so that only a very thin layer remained, then premature solidification of the wall jet resulted in the humping defect. According to the above references, the humping phenomenon is resulted by the molten metal flowing to the rear of the welding pool with a very high speed. Similarly, convex surface shape in vertical up weld bead is also caused by molten metal flowing to the tail of molten pool under the gravity. Therefore, some analytical methods of the reference can be applied to explain the mechanisms of weld formation during vertical up narrow gap GMA welding.

In this paper, the weld bead formation characteristics during oscillating arc narrow gap vertical up position GMA welding were analyzed. A three-dimensional numerical heat transfer and fluid flow model was developed to explain the mechanism of the weld bead formation. Finally, the effects of the oscillating parameters on weld bead were studied and the defect free joint was obtained in oscillating arc narrow gap vertical up GMA welding process.

2 Experimental apparatus and procedure

The schematic diagram of oscillating arc narrow gap GMA welding torch is shown in Fig. 1. The contact tip is screwed into the lower part of conducting rod which is bent to form an angle of 7°. The wire is fed through the inner hole of the conducting rod and stretches out from the contact tip, so an angle of 7° is formed between the wire and the axis of the conducting rod. The realization of the oscillating arc is that the wire and the contact tip rotate back and forth with the conducting rod through a pair of gears driven by a motor. In the present experiment, the wire out from the contact tip orientates toward the welding direction. In order to observe the behavior of the molten pool, the CanRecord 5000 × 2 high-speed camera with 3000 frames per second is employed to view the front of the molten pool.

Fig. 1

Schematic diagram of oscillating arc narrow gap vertical up GMA welding

The base metal is low carbon steel plate of 25 mm in thickness and the filler wire is ER50-6 with 1.2 mm in diameter. The chemical compositions of base metal and filler wire are listed in Table 1. Ar (92%) + CO2 (8%) mixture gas is selected as shielding gas with the flow rate of 15 L min⁻¹. The welding parameters used in this experiment are listed in Table 2.

Table 1 Chemical compositions of base metal and filler wire (wt.%)

Table 2 Welding parameters

The transfer mode near sidewall is projected transfer, while it presents global transfer at the middle of groove with the welding parameters listed in Table 2. Transfer frequency near the side is higher than that at the middle of groove in the oscillating arc narrow gap welding. Detailed analysis of the droplet transfer in oscillating arc narrow gap welding have been shown in our another published article [12].

In order to have a deep understanding of heat and mass transfer process in oscillating arc narrow gap welding molten pool in vertical up welding position, the transient three-dimensional numerical model combined with the feature of oscillating arc narrow gap welding was established with the software of flow-3d. This model takes account into not only the influences of arc pressure, electromagnetic force, and surface tension, but also the effect of droplet, swing trajectory, narrow gap groove, and welding position.

3 Results and discussion

3.1 Formation characteristics

Figures 2 and 3 show the cross sections of weld bead at the flat position and vertical up position by oscillating arc NG-GMAW, respectively. The geometry of the narrow gap groove and welding parameters during flat position welding are the same as vertical up welding, which are shown in Fig. 1 and Table 2. After welding in the flat position, cross sections were removed from flat weld joint, and ground with grinding papers and polished. The saturated ferric chloride solution was used to etch the samples, and the profiles of the beads were traced by the optical microscope. As shown in Fig. 2, the surface of the flat position weld bead is concave shape, and the two sides of the weld bead are higher than the central. As shown in Fig. 3, compared with weld bead in flat position, the surface of weld bead at vertical up position is convex. More specifically, there is a crest at the central and two troughs close to the sidewalls. The sides of weld bead are slightly higher than the through but lower than the crest. The convex surface is apt to result in interlayer incomplete fusion in multi-layer welding process; Fig. 4 shows the interlayer defect. So, the serious convex surface can be defined as a formation defect in narrow gap welding. It is also clear from Figs. 2 and 3 that oscillating arc vertical up NG-GMAW process allows the deeper penetration under the same weld parameters comparing with flat position oscillating arc NG-GMAW.

Fig. 2

Cross section of oscillating arc NG-GMAW weld at flat position

Fig. 3

Cross section of oscillating arc NG-GMAW weld at vertical up position

Fig. 4

Top and longitude views of weld craters at flat and vertical up position. a Flat position. b Vertical up position

Figure 4 displays the top and the longitudinal views of weld craters using the same welding parameters, but in the flat position and the vertical up position, respectively. In the flat position, from Fig. 4, the transition from weld bead top to the base metal is smooth and the final weld profile does not show any sign of surface depression. Meanwhile, from Fig. 4, there is a heavy gouge in the final weld profile in vertical position by the combined actions of arc forces and the molten pool gravity. And the slop from weld bead top to base metal in vertical up position is deeper than it in the flat position.

In summary, the weld bead of oscillating arc NG-GMAW in vertical up position takes the characteristics with convex surface and deeper penetration, and there is a heavy gouge in the final weld bead profile.

3.2 Molten pool forming process

In order to understand the formation of convex weld bead in vertical up position, the sequence of events taking place in the formation of convex molten pool was recorded by high-speed camera. The weld pool cannot be horizontally observed using high-speed camera because the sidewalls of narrow gap groove block the view. Figure 5 shows the establishment of the convex molten pool. As shown in Fig. 5a, the arc was at the left side of the narrow gap groove, and the surface of molten pool was concave shape. Then, as shown in Fig. 5b, the arc swung to the right side of the narrow gap groove, and the height of molten pool increased but the surface was still concave shape. As the moving of the filled metal wire at the welding direction, the weld molten pool was elongated, and the molten metal flow to the rear of molten pool by the action of gravity and arc pressure, so the molten pool height increased with the process going on until the balance between the amount of melting and solidification. Figure 5c, d shows that the molten height increased and the molten pool surface changed to flat shape from concave shape. From Fig. 5e, f, the middle of weld pool was higher than the side of weld pool with the molten metal flowing to rear of weld pool, that is, the surface of weld pool had become convex. Then the convex weld pool moved with the welding speed and solidified to convex weld bead.

Fig. 5

The establishment of the W-shape weld pool. a Arc dwelling at the left side. b Arc dwelling at the right side. c Height of pool increasing. d Arc swing to the right side. e Arc swing to the left side. f W-shaped weld pool

The numerical simulation with flow-3d software was applied to study the behavior of molten pool. The mesh density as 0.25 mm/mesh was used in this simulation, and the total number of mesh is 136,000. The governing equations computational fluid dynamics (CFD) simulations of a weld pool are the continuity equation, Navier–Stokes equation, and the energy equation. The material properties and variables are summarized in Table 3. The initial temperature of the simulation model is set as room temperature T ₀. Thermal radiation and thermal convection exist on the surface of workpiece. So the boundary condition of the surface is expressed as the following equations:

$$ \lambda \frac{\partial T}{\partial \overrightarrow{n}}=-{Q}_{\mathrm{conv}}-{Q}_{\mathrm{rad}} $$

(1)

$$ {Q}_{\mathrm{conv}}={h}_{\mathrm{conv}}\left(T-{T}_0\right) $$

(2)

$$ {Q}_{\mathrm{rad}}=\varepsilon {k}_b\left({T}^4-{T}_0^4\right) $$

(3)

Table 3 Main prosperities used in simulation

where $\overrightarrow{n}$ means the surface normal, T is the temperature, and Q _conv is the thermal convection, and Q _rad means the thermal radiation, h _conv is the convective heat transfer coefficient, and ε is the thermal radiation of surface, and k _b is the Boltzmann’s constant.

The energy on the free surface of sidewall and groove bottom is balanced among the arc heat flux (Q _a ), heat dissipation by convection and radiation, and heat loss due to evaporation (Q _evap). The boundary condition of this surface is expressed as Eq. 4.

$$ \lambda \frac{\partial T}{\partial \overrightarrow{n}}={Q}_a-{Q}_{\mathrm{conv}}-{Q}_{\mathrm{rad}}-{Q}_{\mathrm{evap}} $$

(4)

where λ is the thermal conductivity.

Ellipsoid heat source was employed in this simulation model; the heat flux distribution (Q _a (r)) is calculated as Eq. 5.

$$ {Q}_a(r)=\frac{\sqrt{2}\eta UI}{2\pi \sqrt{\pi }{\sigma}_x{\sigma}_y{\sigma}_z} \exp \left(-\frac{{\left(x-{x}_1\right)}^2}{2{\sigma}_x^2}-\frac{{\left(y-{y}_1\right)}^2}{2{\sigma}_y^2}-\frac{{\left(z-{z}_1\right)}^2}{2{\sigma}_z^2}\right) $$

(5)

where η means effective coefficient of the GMA welding; U is the welding voltage; I is welding current; x, y, and z are the coordinates of the calculation point in the simulation; σ _x , σ _y , and σ _z are the distribution coefficient of the heat source.

The effects of droplet, narrow groove, and oscillating arc on the behavior of molten pool has been considered in this simulation, and it can simulate molten pool forming process of the oscillating arc narrow gap welding. While the arc heat source distribution in narrow gap welding is different from conditional welding, so it needs to be modified in the succeeding research.

The flow-3d software was applied to study the behavior of molten pool during oscillating arc narrow gap welding, and the 3D simulation results indicate that the molten pool is with convex shape, as shown in Fig. 6.

Fig. 6

3D simulation figure of oscillating arc narrow gap vertical up welding molten pool

Temperature and flow field of longitudinal section of vertical up welding molten pool was shown in Fig. 7. The molten liquid metal flows to the molten pool rear from the front with the action of gravity, which results more liquid metal accumulating at the molten pool rear. But the solidified metal and surface tension at the rear hampers the further flowing, then even little liquid metal returns to the front of molten pool with the action of surface tension and electromagnetic force. In the rear of the molten pool, the liquid metal layer is thick as most liquid metal flows to the rear, while the gouge appears at the front of molten pool due to insufficient molten pool metal. At vertical up welding position, because the front of molten pool is very thin as the molten liquid metal flow to the rear, deep penetration can be obtained with thin rapid flow region due to arc can melt the base metal more easily.

Fig. 7

Temperature and flow field of longitudinal section of vertical up welding molten pool

Figure 8 shows the molten pool forming process with simulation method. At the beginning of welding, the molten pool was concave. When t = 2.351 s to 3.063 s, the oscillating arc began to melt some base metal in the bottom and sidewalls of narrow gap groove, and the melted base metal formed molten pool with transferred droplet together. Molten pool metal spread to a certain height along the fusion area, then the molten pool metal at the highest points of fusion area solidified firstly to two fixed point at the sidewalls when oscillating arc was away. When t = 3.42 s to 3.766 s, the molten liquid metal which flows from the front of molten pool with the action of gravity only accumulates at the middle of pool, as the shape of the two sides of molten pool has been fixed due to the fixed points of sidewalls, and the middle of the molten pool is higher but concave shape still. The height of the middle of molten pool increases and the molten pool change to convex shape from concave shape as the filler metal is supplemented constantly, when t ≥ 4.485 s the formation is convex shape.

Fig. 8

Evolution of vertical up welding molten pool (welding current 168A, welding voltage 25 V, welding speed 150 mm min⁻¹, oscillating angle 55°, oscillating speed 343°, dwell time 300 ms)

3.3 Effects of parameters on weld bead formation

Figure 9 shows the simulation results of oscillating and non-oscillating arc narrow gap vertical up welding, respectively. The simulation results indicate that the oscillating arc can achieve stable molten pool and favorable weld formation with sufficient sidewall penetration, while non-oscillating arc can result molten pool flowing down and poor weld formation. One of the reasons is that the interface tension between molten pool and sidewalls of narrow gap groove can restrain the action of gravity due to oscillating arc promotes more molten pool metal to sidewalls. Moreover, it plays a key role in control of molten pool that oscillating arc is benefit to reduce the high temperature retention time. Figure 10 shows welding thermal circles of test points in the centre of the narrow gap groove with oscillating arc and non-oscillating arc.

Fig. 9

3D figures of oscillating and non-oscillating arc narrow gap vertical up welding molten pool. a Oscillating arc. b Non-oscillating arc

Fig. 10

Welding thermal circles with oscillating and non-oscillating arc

Figure 11 shows the simulation results with different oscillating speeds. It indicates that the convex degree of weld bead reduces as the oscillating speed increasing. In the oscillating arc narrow gap welding, heat input declines as the oscillating speed increases, which is benefit to decrease the high temperature retention time. Therefore, in order to achieve favorable weld formation with lower convex degree, even with concave shape, it should apply high oscillating speed.

Fig. 11

Simulation results with different oscillating speed

As shown in Fig. 12, the convex degree reduces as the oscillating angle increases. The molten pool flows down as the direction of gravity when the oscillating angle is less than 30°, and the weld formation is with defect of infusion in sidewalls. The width of the molten pool increases as the oscillating angle increases, with the corresponding result is that the heat dissipation area increases. Moreover, more liquid metal is promoted to the area close to sidewall as the oscillating angle increases, which results in the decline of molten pool height. Therefore, increasing oscillating angle is benefit to control convex surface in narrow gap vertical up GMA welding.

Fig. 12

Simulation results with different oscillating angle

Based on the above analysis result, the favorable weld formation without defect of serious convex surface was achieved with the appropriate welding parameters, as shown in Fig. 13.

Fig. 13

Cross sections of weld bead with optimization welding parameter

4 Conclusion

In this paper, the weld bead formation characteristics and mechanisms during oscillating arc narrow gap vertical up GMA welding have been studied with experiment and simulation. The following conclusions are from the present research.

• 1. The oscillating arc narrow gap vertical up GMA welding is characterized by deep weld penetration and convex surface. The convex surface is apt to result in interlayer incomplete fusion in multi-layer welding process. So the serious convex surface can be defined as a formation defect in narrow gap welding.

• 2. The molten liquid metal which flows from the front of molten pool with the action of gravity, only accumulates at the middle of pool, because the shape of the two sides of molten pool has been fixed. Therefore, the molten pool changes to convex shape from concave shape as the mount of filler metal increases.

• 3. Oscillating arc can achieve stable molten pool and favorable weld formation with sufficient sidewall penetration. Increasing oscillating angle and speed is benefit to achieve convex surface in narrow gap vertical up GMA welding.