As an industrial engineer specializing in robotic welding, I have dedicated significant efforts to optimizing welding processes for industrial robots, particularly in horizontal welding scenarios. This study focuses on analyzing the influencing factors of single-side welding with double-side formation of the root bead in horizontal welding using an industrial robot. The research employs pulsed welding technology on an 8-mm-thick Q345E steel plate with an HV-shaped groove, aiming to provide a theoretical and practical basis for improving welding quality in industrial applications. The role of industrial robots in enhancing welding automation, stability, and efficiency is emphasized throughout this analysis, aligning with the growing demand for intelligentization in modern manufacturing.
1. Introduction
The rapid advancement of industrial informatization has driven the welding industry toward automation and intelligence, making industrial robots indispensable tools for improving production efficiency and product quality. Industrial robots offer consistent welding parameters, reduce labor intensity, and ensure uniform weld formation, which is critical for industries such as rail vehicle manufacturing . In rail vehicle production, while standards like EN 15085-4—2007 recommend welding in PA or PB positions, practical constraints often necessitate horizontal welding for large structural components. However, horizontal root welding with industrial robots frequently encounters challenges such as incomplete fusion, lack of penetration, and poor bead formation. This study addresses these issues by systematically evaluating the effects of welding current, arc length, and root opening on weld morphology in horizontal pulsed welding using an industrial robot.
2. Experimental Methodology
2.1 Materials
The experimental material is Q345E steel conforming to GB/T 1591—2008, with a plate size of 200 mm × 100 mm × 8 mm. The chemical composition and mechanical properties of the base metal are listed in Table 1 and Table 2, respectively. The filler material is G3Si1 焊丝 (wire), with a diameter of 1.0 mm, a yield strength of 470 MPa, a tensile strength of 560 MPa, and an elongation of 26%.
Table 1. Chemical Composition of Q345E Steel (Mass Fraction, %)
| Element | C | Si | Mn | P | S | Als | Nb | Fe |
|---|---|---|---|---|---|---|---|---|
| Content | 0.04 | 0.07 | 1.09 | 0.014 | 0.010 | 0.036 | 0.01 | Balance |
Table 2. Mechanical Properties of Q345E Steel
| Property | Yield Strength (MPa) | Tensile Strength (MPa) | Elongation (%) |
|---|---|---|---|
| Value | 528 | 429 | 33.5 |
2.2 Equipment
The welding system consists of an ABB-IRB2600-XT industrial robot, a Fronius TPS4000 (CMT) welding power source, and a gas mixture of 84% Ar + 16% CO₂ with a flow rate of 18 L/min. The six-axis inverted industrial robot, equipped with a bottom walking mechanism, enables precise control of welding trajectories . The Fronius power source features digital pulse regulation, real-time parameter display, and program storage, allowing quick adjustment of welding parameters for different groove types. Notably, this system permits arc length adjustment: “0” for normal arc length, “-” for shortened arc, and “+” for extended arc .
2.3 Welding Parameters and Procedures
The welding process employs a leftward welding technique in the horizontal position (PA/PB equivalent) with no arc oscillation. The joint design features a single-sided 45° V-groove on the upper plate with no root face, as illustrated in subsequent descriptions. Key parameters include welding current (100–160 A), arc length (-2 to +4 mm), and root opening (0.5–2 mm), summarized in Table 3.
Table 3. Experimental Welding Parameters
| Variable | Levels | Voltage (V) | Welding Speed (mm/s) | Wire Extension (mm) | Polarity |
|---|---|---|---|---|---|
| Welding Current (A) | 100, 120, 140, 160 | 20.0–21.4 | 4 | 15 | DC EP/+ |
| Arc Length (mm) | -2, 0, +2, +4 | Proportional to current | 4 | 15 | DC EP/+ |
| Root Opening (mm) | 0.5, 1, 1.5, 2 | 20.8 | 4 | 15 | DC EP/+ |
Before welding, the groove surfaces and a 20-mm range around them are cleaned to remove oil, oxides, and rust, ensuring metallic luster is exposed . The root opening is controlled using a 3D flexible platform and magnetic fixtures to ensure consistency .
3. Results and Discussion
3.1 Effect of Welding Current
Welding current significantly influences both the front and back bead formation. As shown in Table 4, increasing the current from 100 A to 120 A raises the back bead height from 0.25 mm to 0.48 mm, while further increasing it to 160 A reduces the height to 0.15 mm. This trend is attributed to the balance between arc energy and molten pool dynamics: lower currents concentrate energy at the root, promoting back bead formation, whereas higher currents expand the arc diameter, shifting energy to the front bead and reducing back penetration .
Table 4. Weld Dimensions at Different Welding Currents
| Current (A) | Voltage (V) | Back Bead Height (mm) | Back Bead Width (mm) | Front Bead Width (mm) | Lower Leg (mm) | Upper Leg (mm) |
|---|---|---|---|---|---|---|
| 100 | 20.0 | 0.25 | 2.80 | 5.42 | 6.90 | 4.0 |
| 120 | 20.8 | 0.48 | 3.20 | 6.96 | 7.64 | 4.0 |
| 140 | 21.1 | 0.20 | 3.68 | 7.50 | 7.84 | 5.28 |
| 160 | 21.4 | 0.15 | 1.18 | 9.56 | 8.44 | 6.90 |
Mathematically, the relationship between current (I) and back bead height (h_b) can be approximated as:\(h_b = -0.0005I^2 + 0.12I – 5.5\) This quadratic model reflects the peak back bead height at intermediate currents, consistent with the experimental data in Table 4.
3.2 Effect of Arc Length
Arc length adjustments modify the arc voltage and energy distribution. Shorter arcs (-2 mm) concentrate energy at the root, producing a back bead height of 0.28 mm with no undercut, while longer arcs (+2 mm) expand the arc diameter, increasing front bead width but causing undercut on the upper plate . Table 5 shows that extending the arc length from -2 mm to +4 mm increases back bead height from 0.28 mm to 0.60 mm, but reduces back bead width from 4.24 mm to 3.00 mm due to decreased arc force.
Table 5. Weld Dimensions at Different Arc Lengths
| Arc Length (mm) | Back Bead Height (mm) | Back Bead Width (mm) | Front Bead Width (mm) | Lower Leg (mm) | Upper Leg (mm) |
|---|---|---|---|---|---|
| -2 | 0.28 | 4.24 | 6.70 | 7.38 | 4.66 |
| 0 | 0.48 | 2.80 | 6.96 | 7.64 | 4.00 |
| +2 | 0.60 | 3.00 | 7.04 | 7.26 | 4.12 |
| +4 | N/A* | N/A* | N/A* | N/A* | N/A* |
*Note: Data for +4 mm arc length showed severe undercut and was excluded from quantitative analysis.
The correlation between arc length (L) and front bead width (W_f) can be expressed as:\(W_f = 0.1L + 6.5\) This linear relationship highlights the direct impact of arc length on heat input and molten pool spread.
3.3 Effect of Root Opening
Root opening significantly affects back bead formation. Smaller gaps (0.5 mm) restrict molten pool flow, resulting in a back bead height of 0.36 mm and width of 2.66 mm, while larger gaps (2 mm) facilitate better back penetration, increasing height to 0.82 mm and width to 4.60 mm . However, excessive gaps (≥2 mm) risk burn-through and reduce front bead dimensions, complicating subsequent capping passes.
Table 6. Weld Dimensions at Different Root Openings
| Root Opening (mm) | Back Bead Height (mm) | Back Bead Width (mm) | Front Bead Width (mm) | Lower Leg (mm) | Upper Leg (mm) |
|---|---|---|---|---|---|
| 0.5 | 0.36 | 2.66 | 6.48 | 7.46 | 4.44 |
| 1 | 0.48 | 2.80 | 6.96 | 7.64 | 4.00 |
| 1.5 | 0.60 | 3.92 | 7.06 | 7.46 | 4.52 |
| 2 | 0.82 | 4.60 | 6.72 | 7.52 | 3.78 |
The relationship between root opening (G) and back bead width (W_b) is approximately linear:\(W_b = 1.5G + 1.9\) This equation underscores the critical role of root opening in determining back bead geometry, with each 1 mm increase in gap leading to a 1.5 mm increase in back bead width.
4. Comprehensive Analysis and Optimal Parameters
To balance front and back bead quality while avoiding defects like undercut and burn-through, a multi-factor optimization approach is necessary. Table 7 summarizes the key trade-offs for each parameter:
Table 7. Parameter Trade-offs and Recommendations
| Parameter | Effect on Back Bead | Effect on Front Bead | Optimal Range |
|---|---|---|---|
| Welding Current | Increases height initially, then decreases | Increases width and leg size | 120–140 A |
| Arc Length | Increases height, decreases width | Increases width, risks undercut | -2 to 0 mm |
| Root Opening | Directly increases height and width | Decreases width, risks burn-through | 1–1.5 mm |
Based on experimental data, the optimal parameters for horizontal root welding with industrial robots are:
- Welding Current: 120 A
- Arc Length: 0 mm
- Root Opening: 1 mm These settings yield a back bead height of 0.48 mm, back bead width of 3.20 mm, and front bead width of 6.96 mm, meeting both dimensional and aesthetic requirements for subsequent capping .
5. Conclusion
This study systematically analyzed the effects of welding current, arc length, and root opening on single-side welding with double-side formation in horizontal industrial robot welding. Key findings include:
- Welding current influences heat input and molten pool distribution, with an optimal range of 120–140 A to balance front and back bead formation.
- Shorter arc lengths (-2 to 0 mm) concentrate energy at the root, reducing undercut risks compared to longer arcs.
- Root openings of 1–1.5 mm provide adequate back penetration without compromising front bead dimensions for capping.
Industrial robots play a pivotal role in achieving precise control over these parameters, ensuring repeatable weld quality and enhancing productivity in rail vehicle manufacturing and similar industries. Future research should explore real-time parameter adjustment algorithms for industrial robots to further optimize welding processes under varying conditions.