Study of Effect of Electrode Arrangement on Layer Geometry and Fusion Zone Morphology under Twin-Arc Surfacing

Introduction. Increasing the productivity of single-wire surfacing through raising the wire feed rate causes defects — undercuts and poor fusion between layers, which reduces the quality of the deposited coating and increases the reject rate. To solve this problem, multiwire surfacing techniques are being developed in a shielded gas environment which increase productivity without compromising quality. The literature shows that the relative position of electrodes in multiwire systems affects significantly the thermal and electrophysical characteristics of the arc, and therefore, the geometry of the reinforcement and the shape of the fusion penetration. However, the available studies are fragmentary: there is insufficient data on the morphology of the fusion zone, and the relationship of its parameters and specific electrode arrangement schemes under twin-arc surfacing in a shielded environment, which leaves a scientific gap. The objective of this research is to evaluate the change in the geometric parameters of the reinforcement of the deposited layer and the morphology of the fusion zone with different relative positions of the electrodes under twin-arc surfacing in a shielded gas environment.

Materials and Methods. The experiment was conducted on a 6-axis Fanuc 120iD robot with an EWM Titan XQ500 power source and an experimental surfacing head consisting of two welding torches. The layers were deposited on steel substrates of grade St3 using the GMAW Pulse method with Sv 08G2S wire with a diameter of 1.2 mm in an Ar/CO₂ environment (98%/2%) under a fixed surfacing mode (WFS = 6.5 m/min for each torch, TS = 4 mm/s, MW = 150°C). The following parameters were adopted as the studied factors: distances between electrodes (z = 15, 18, 21 mm), their angle of inclination (α = 5°, 10°). On the cross-sections of the deposited layers prepared by grinding and etching, the geometric parameters of reinforcement (height h, width S, wetting angle γ) and fusion penetration (depths a, a₁, width b) were measured. Quantitative analysis of the weld geometry was performed using Digimizer software to assess the effect of the relative position of the electrodes on the formation of the layer.

Results. It has been found that the distance between the electrodes (z) affects significantly the reinforcement geometry: a growth of z causes an increase in the layer width (S) and the wetting angle (γ), but a decrease in its height (h). The axial fusion depth (a) demonstrated a nonlinear dependence on z, reaching a maximum (~2.2 mm) at z = 18 mm. The inclination angle (α) had a minor effect (<5%)on the reinforcement parameters, but affected significantly the shape of the main fusion zone (a₁): an increase in α decreased a₁ and made the penetration more gently sloping. At z = 21 mm, the impact of α on the penetration disappeared. The relationships between the relative positions of the electrodes under twin-arc surfacing, the geometric parameters of the reinforcement, and the depth of the fusion zone were specified.

Discussion. The explanation of the established dependences is based on the change in the thermal and electrophysical properties of the electric arc depending on the mutual arrangement of the electrodes. The axial depth of fusion penetration depends not only on the distance between the electrodes, but also on the volume of the weld pool. With an excessive volume of the weld pool for a specific surfacing mode, a damping effect of heat flows from the electric arc to the base metal occurs — the volume of the weld pool absorbs part of the heat, which causes a decrease in the depth of penetration. The change in the arc pressure vector with an increase in the angle between the electrodes explains the decrease in the depth of the main fusion zone.

Conclusion. The regularities of the effect of the mutual arrangement of electrodes on the geometry of the deposited layer and the shape of the fusion zone under twin-arc surfacing in a shielded gas have been experimentally established. It is shown that an increase in the distance between the electrodes results in an increase in the width of the bead, a decrease in its height, and an increase in the wetting angle. It has been noted that the penetration depth depends on the volume of the weld pool. It is determined that the angle of inclination of the electrodes in the studied modes has an insignificant effect — less than 5% — on the geometry of the deposited metal, although hypothetically, it can be enhanced at smaller interelectrode distances. The data obtained extract clear trends and form the basis for further in-depth study of the thermal and electrophysical aspects of the process of twin-arc surfacing in a shielded gas environment.

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