CLADDING TECHNOLOGY SHANXI CO., LTDInterpass Overlap Model for GMAW Overlay Forming
Literature Overview
This paper by Meng Fanjun, Zhu Sheng, Baderma, and Du Wenbo, published in the Journal of Welding in 2011, presents a mathematical model for the interpass overlap (lap amount) in GMAW overlay forming. The research was conducted at the Department of Equipment Remanufacturing Engineering, Academy of Armored Force Engineering, and was supported by the National Natural Science Foundation of China. The work addresses a critical practical challenge in overlay welding: determining the optimal overlap between adjacent weld passes to achieve desired surface geometry, dimensional accuracy, and metallurgical quality.
Core Technical Content and Model Development
The interpass overlap, often referred to as the lap amount or overlap ratio, is defined as the ratio of the overlap width to the weld bead width. In multi-pass overlay welding, this parameter directly influences the final surface profile, layer thickness uniformity, dilution rate, and residual stress distribution. The authors developed a mathematical model that relates the interpass overlap to process parameters including welding current, voltage, travel speed, wire feed speed, and torch oscillation parameters.
Model Parameters and Relationships
| Parameter | Symbol | Typical Range | Influence on Overlap |
|---|---|---|---|
| Welding current | I (A) | 150-400 | Higher current increases bead width |
| Arc voltage | U (V) | 20-35 | Higher voltage increases bead width |
| Travel speed | v (mm/min) | 200-800 | Higher speed decreases bead width |
| Wire feed speed | v_w (mm/min) | 300-1200 | Higher speed increases deposition rate |
| Overlap ratio | k | 0.3-0.7 | Primary control parameter |
| Bead width | b (mm) | 8-25 | Dependent on I, U, v |
The model establishes that the interpass overlap ratio k can be expressed as a function of the welding parameters and the desired surface geometry. For a single-pass overlay bead, the bead width b is approximately proportional to I^0.5 / v^0.3, while the bead height h is approximately proportional to v_w^0.4 / v^0.6. The overlap ratio k = (b - d) / b, where d is the effective spacing between pass centers.
Surface Geometry Prediction
The model further predicts the final surface profile after multiple passes by superimposing individual bead profiles. The key finding is that an optimal overlap ratio of approximately 0.5-0.6 produces the flattest surface with the most uniform layer thickness. Overlaps below 0.3 result in a wavy surface with valleys between beads, while overlaps above 0.7 lead to excessive material deposition and potential undercutting at the bead edges.
Engineering Practice and Application
For overlay welding engineers, this model provides a quantitative tool for planning multi-pass overlay sequences. The practical application involves several steps:
- Initial parameter selection: Based on the required overlay thickness and base material, select welding current, voltage, and travel speed to achieve the target bead width.
- Overlap calculation: Use the model to determine the optimal overlap ratio for the desired surface finish.
- Layer planning: Calculate the number of passes per layer and the total number of layers required to achieve the target overlay thickness.
- Verification and adjustment: After welding a test coupon, measure the actual bead geometry and adjust parameters based on deviations from predicted values.
The model has been validated through experimental overlay welding trials using various filler materials including stainless steel 308L, 316L, and nickel-based alloys on carbon steel substrates. The predicted overlap ratios agreed with measured values within ±5% for typical GMAW overlay conditions.
Process Windows for Optimal Overlay
| Filler Material | Current (A) | Voltage (V) | Speed (mm/min) | Overlap Ratio | Surface Flatness |
|---|---|---|---|---|---|
| ER308L | 200-300 | 22-28 | 300-500 | 0.5-0.6 | ±0.5 mm |
| ER316L | 200-300 | 22-28 | 300-500 | 0.5-0.6 | ±0.5 mm |
| ERNiCrMo-3 | 180-280 | 20-26 | 250-450 | 0.4-0.55 | ±0.7 mm |
| ER347 | 200-300 | 22-28 | 300-500 | 0.5-0.6 | ±0.5 mm |
Critical Reflections and Study Insights
The development of a mathematical model for interpass overlap represents a significant advancement from purely empirical overlay welding practice. However, several important limitations must be acknowledged. The model assumes idealized bead profiles with smooth, symmetric geometry, whereas actual weld beads exhibit irregularities due to oscillation, spatter, and edge effects. Additionally, the model does not account for the effects of heat input on the previously deposited layers, which can cause resolidification of the overlap region and affect the final surface profile.
A particularly important practical consideration is the effect of overlap on dilution. Higher overlap ratios increase the interaction between adjacent beads, which can lead to localized regions of higher dilution and potentially reduced overlay layer composition. For applications requiring strict compositional control, such as corrosion-resistant overlay on carbon steel, the overlap ratio must be carefully balanced against compositional requirements. Engineers should always perform metallographic examination of the overlay layer cross-section to verify dilution levels and microstructure uniformity.
The model also highlights the importance of torch oscillation in achieving uniform overlap. In practice, many GMAW overlay operations employ torch oscillation to widen the effective bead width, and the oscillation parameters must be integrated into the overlap calculation. This paper provides a valuable foundation for further research into overlay forming models that incorporate real-time process monitoring and adaptive parameter control.