Zhuojin Pipe Fitting Co., LtdNumerical Simulation of Multi-Pass Multi-Layer Surfacing: Residual Stress and Path Optimization
Literature Overview
The paper by Zhang Xue et al., published in Mechanical Science and Technology (Vol. 42, No. 3, 2023, pp. 408–414), presents a finite element simulation study of multi-pass multi-layer surfacing on high-temperature alloy substrates. The authors, from Chongqing University, used Marc finite element software to investigate the effects of surfacing thickness and welding path on residual stress distribution. The research was supported by the National Natural Science Foundation of China (Grant 51775068).
Core Technical Content
Multi-pass multi-layer surfacing is a common technique for building up thick overlay layers on components, particularly in the repair and fabrication of high-temperature alloy components such as turbine blades, exhaust manifolds, and heat exchanger tubes. The residual stress developed during multi-pass surfacing is a critical concern, as it can lead to distortion, cracking, and reduced fatigue life. The study systematically varied the surfacing thickness and welding path (pass-by-pass versus layer-by-layer) and analyzed the resulting residual stress fields.
The key findings are:
- Increasing surfacing thickness leads to higher peak transverse residual stress and changes the local stress direction, subjecting the surfacing layer to multi-axial stress states.
- Stress relaxation occurs between adjacent surfacing layers due to the thermal influence of preceding and following welds.
- Layer-by-layer welding produces a more uniform residual stress distribution with lower peak values compared to pass-by-pass welding.
Residual Stress Analysis
| Parameter | Effect on Residual Stress |
|---|---|
| Surfacing thickness (increasing) | Peak transverse stress increases; stress direction changes |
| Surfacing thickness (excessive) | Multi-axial stress state; higher cracking risk |
| Pass-by-pass welding | Higher peak stress; less uniform distribution |
| Layer-by-layer welding | Lower peak stress; more uniform distribution |
| Inter-layer thermal influence | Stress relaxation between layers |
| Welding sequence | Significantly affects final stress field |
The numerical simulation revealed that as the total surfacing thickness increases, the peak transverse residual stress rises progressively. This is because each additional layer adds thermal strain that is not fully relaxed by subsequent cooling. The stress direction also shifts, transitioning from predominantly transverse to a more complex multi-axial state. This multi-axial stress condition increases the likelihood of crack initiation and propagation, particularly in high-strength high-temperature alloys.
Welding Path Comparison: Pass-by-Pass vs. Layer-by-Layer
The comparison between pass-by-pass and layer-by-layer welding sequences is one of the most practically significant findings of this study.
| Aspect | Pass-by-Pass | Layer-by-Layer |
|---|---|---|
| Stress distribution uniformity | Poor | Good |
| Peak residual stress | Higher | Lower |
| Thermal cycling per pass | More severe | Less severe |
| Distortion risk | Higher | Lower |
| Production efficiency | Potentially higher | Slightly lower |
| Equipment requirement | Standard | May require path planning |
In pass-by-pass welding, the welder completes all passes in one layer before moving to the next layer. This results in a steep thermal gradient between the hot, freshly welded pass and the cool, previously solidified passes. The thermal mismatch generates high residual stresses.
In layer-by-layer welding, the welder completes one pass in each layer before returning to the first layer. This distributes the thermal input more evenly across the entire surfacing volume, reducing thermal gradients and residual stresses. The inter-layer stress relaxation effect is more pronounced in layer-by-layer welding because each pass is deposited onto a substrate that is still partially warm from the previous layer.
5W2H Analysis of Surfacing Process Optimization
Applying the 5W2H framework to the surfacing process optimization:
| Question | Analysis |
|---|---|
| What | Residual stress in multi-pass multi-layer surfacing |
| Why | Thermal gradients and constrained cooling |
| Where | High-temperature alloy components |
| When | During welding and subsequent cooling |
| Who | Welding engineers and process planners |
| How | Finite element simulation and path optimization |
| How much | Peak stress reduction of 20–30% with layer-by-layer |
The simulation provides quantitative data for optimizing the welding path. The recommended approach is to use layer-by-layer welding for thick surfacing layers (exceeding 5 mm) to minimize residual stress and distortion. For thinner layers (less than 3 mm), the difference between pass-by-pass and layer-by-layer is less pronounced, and pass-by-pass may be acceptable for efficiency reasons.
Engineering Practice Integration
In practice, the choice of welding path is often constrained by production schedules and equipment capabilities. However, the simulation results provide a strong basis for advocating layer-by-layer welding in critical applications. For example, in the repair of turbine blade platforms or the fabrication of high-pressure steam pipe elbows, where distortion and cracking are unacceptable, layer-by-layer welding should be the default approach.
A practical implementation strategy is:
- Use finite element simulation to predict residual stress for the planned welding sequence.
- If the predicted peak stress exceeds the allowable limit (typically 70% of the yield strength of the surfacing layer), switch to layer-by-layer welding.
- Apply inter-pass temperature control (typically 150–250 °C) to further reduce residual stress.
- Perform post-weld stress relief heat treatment (PWHT) at 650–750 °C for high-temperature alloys.
- Verify the final residual stress using X-ray diffraction or neutron diffraction.
Key Questions and Reflections
The study raises important questions about the scalability of the simulation results. The finite element model used Marc software with specific material properties and boundary conditions. In practice, the material properties of high-temperature alloys vary significantly between heats and suppliers, and the boundary conditions depend on the clamping arrangement, which is difficult to replicate exactly in simulation.
Another consideration is the effect of welding defects on residual stress. The simulation assumes perfect welds without porosity, lack of fusion, or undercut. In reality, defects act as stress concentrators and can significantly alter the residual stress field. Integrating defect analysis with residual stress simulation would provide a more comprehensive assessment of surfacing quality.
The stress relaxation phenomenon between layers is particularly interesting. It suggests that the thermal history of the entire surfacing sequence is more important than the individual pass parameters. This has implications for welding procedure qualification, where each pass is typically qualified independently. A holistic approach that considers the entire surfacing sequence may be more appropriate for thick overlay applications.
Summary
This study provides valuable quantitative insights into residual stress management in multi-pass multi-layer surfacing of high-temperature alloys. The key recommendations are to limit surfacing thickness to avoid multi-axial stress states and to prefer layer-by-layer welding over pass-by-pass welding for stress reduction. The finite element simulation methodology is a powerful tool for process optimization, but it must be validated with experimental residual stress measurements for specific applications. Engineers working in high-temperature alloy surfacing should incorporate these findings into their welding procedure development and qualification programs.