Zhuojin Pipe Fitting Co., LtdNumerical Simulation of Multi-Pass Multi-Layer Surfacing and Residual Stress Analysis
Literature Overview
This research by Zhang Xue, Xia Yufeng, Teng Haihao, Peng Mengxia, and Jin Li from Chongqing University, published in Mechanical Science and Technology (Vol. 42, No. 3, pp. 408–414, 2023), employs finite element analysis (FEA) using Marc software to investigate the residual stress evolution during multi-pass, multi-layer surfacing of high-temperature alloys. Supported by the National Natural Science Foundation of China (grant 51775068), the study systematically examines the influence of surfacing thickness and deposition path strategy on post-weld residual stress distribution.
Residual stress is a critical quality parameter in surfacing operations because it directly affects the overlay's fatigue life, dimensional stability, and susceptibility to cracking. For high-temperature alloy overlays used in aerospace and power generation applications, residual stress management is essential for ensuring component longevity.
Numerical Simulation Framework
The Marc finite element model incorporates several key physical phenomena:
- Thermal analysis: A sequential solidification model simulates the heat input from each welding pass, accounting for the moving heat source, convection, and radiation boundary conditions.
- Phase transformation: The model includes solid-state phase transformations specific to high-temperature alloys (e.g., austenite to martensite or ordered phase precipitation).
- Plastic deformation: An elasto-plastic constitutive model captures the thermal plastic strain accumulation during heating and cooling cycles.
- Residual stress calculation: The thermally induced plastic strain that cannot be recovered during cooling is captured as residual stress.
The simulation accounts for the sequential nature of multi-pass welding, where each subsequent pass partially reheats and relaxes the stresses from previous passes.
Effect of Surfacing Thickness on Residual Stress
| Surfacing Thickness (mm) | Transverse Residual Stress Peak (MPa) | Stress Direction Characteristics |
|---|---|---|
| 2.0 | Moderate | Primarily tensile in transverse direction |
| 4.0 | High | Tensile with emerging longitudinal component |
| 6.0 | Very high | Multi-directional stress state |
| 8.0 | Maximum | Complex multi-axial stress field |
The key finding is that increasing surfacing thickness leads to:
- A progressive increase in the transverse residual stress peak value
- A shift in local stress direction, resulting in a multi-axial stress state rather than a simple uniaxial tensile condition
- Increased stress gradients within the overlay thickness
This multi-axial stress state is particularly detrimental because it reduces the material's effective yield strength through the von Mises criterion and increases the likelihood of overlay cracking, particularly in high-strength high-temperature alloys with limited ductility.
Deposition Path Strategy Comparison
The study compares two distinct deposition path strategies:
| Strategy | Description | Residual Stress Peak (MPa) | Stress Distribution Uniformity |
|---|---|---|---|
| Pass-by-pass (逐道堆焊) | Complete all passes in one layer before moving to the next layer | Higher peak | Less uniform |
| Layer-by-layer (逐层堆焊) | Deposit each layer completely (all passes) before starting the next layer | Lower peak | More uniform |
The layer-by-layer strategy produces more uniform residual stress distribution with lower peak values. The mechanism is that in the layer-by-layer approach, each layer's passes provide mutual stress relief through sequential thermal cycling. The subsequent layer's heat input partially anneals the stresses in the previous layer, creating a more balanced stress field throughout the overlay.
In contrast, the pass-by-pass strategy concentrates thermal cycling effects within a single layer, leading to higher localized stress concentrations and less effective stress relief.
Inter-Pass Stress Relaxation Phenomenon
A notable observation from the simulation is the stress release phenomenon between adjacent layers. This occurs because:
- The thermal cycle from a subsequent layer partially reheats the previous layer to a temperature where thermal stress relaxation occurs
- Plastic flow during the subsequent layer's heat input allows stress redistribution in the underlying material
- The cumulative effect of multiple thermal cycles creates a self-relaxing stress field in the interior layers
This finding has direct practical implications: in thick overlay applications, the interior layers experience lower residual stresses than the outermost layers, which have the least subsequent thermal relief.
Engineering Practice Guidelines
Based on the simulation results, the following engineering recommendations emerge:
- Thickness optimization: Surfacings should be designed with the minimum thickness that satisfies functional requirements. Excessive thickness not only increases material cost but also creates unfavorable multi-axial stress states that compromise overlay integrity.
- Path strategy selection: For thick multi-layer overlays, the layer-by-layer strategy should be preferred over the pass-by-pass strategy to achieve lower and more uniform residual stresses.
- Post-weld treatment: For applications where residual stress is critical (e.g., fatigue-critical components), stress relief annealing after surfacing is recommended, particularly for the outermost layers that receive the least natural stress relief.
- Monitoring: Residual stress measurement (e.g., X-ray diffraction or hole-drilling methods) should be incorporated into quality control for critical overlay applications to verify that predicted stress levels are achieved.
Study Insights
The numerical simulation approach provides a powerful tool for optimizing surfacing parameters before physical trials, reducing development time and cost. However, the model's accuracy depends on the quality of the input material properties and the fidelity of the thermal-mechanical coupling assumptions. For high-temperature alloys with complex phase transformation behavior, experimental validation of the simulation predictions is essential.
The finding that deposition path strategy significantly affects residual stress distribution is particularly important for automated surfacing systems where path planning is programmable. Optimizing the path strategy in conjunction with thickness selection can substantially improve overlay quality without changes to consumable or equipment.