Zhuojin Pipe Fitting Co., LtdThermal Stress Analysis of TIG Welded Wall Structures
Literature Overview
The paper published in Journal of Shanghai Jiao Tong University (2008, Vol. 42, S1) by Zhu Zhengqiang, Ma Guohong, and Zhang Hua from Nanchang University presents a three-dimensional thermo-mechanical coupled analysis of wall structures fabricated by TIG welding rapid manufacturing. The study employs element birth-and-death techniques to simulate metal deposition and compares two different deposition paths: Z-pattern welding and single-direction welding. Funded by the National Natural Science Foundation and Jiangxi Provincial Natural Science Foundation, this work addresses critical residual stress and distortion issues in additive manufacturing by welding.
Core Technical Findings
The analysis reveals that different deposition paths produce distinctly different temperature field and residual stress distributions. Single-direction welding generates lower overall temperature fields and residual stresses compared to Z-pattern welding. However, at the bottom end of the wall termination in single-direction welding, the Z-direction stress (σz) is significantly higher, which can cause separation between the wall and the base plate. In Z-pattern welding, the termination end exhibits fluctuating high Z-direction tensile stresses that can lead to delamination.
| Deposition Path | Temperature Field | Overall Residual Stress | Termination Zone Issue | Recommended Countermeasure |
|---|---|---|---|---|
| Single-direction | Lower | Lower | High σz at bottom, wall-base separation risk | Preheat base plate |
| Z-pattern | Higher | Higher | Fluctuating Z-direction tensile stress, delamination risk | Post-weld heat treatment |
The study recommends preheating the base plate before welding and performing post-weld heat treatment on the entire structure to reduce residual stresses and improve service performance.
Technical Interpretation
The element birth-and-death technique used in this simulation is a well-established method for modeling sequential deposition in welding-based additive manufacturing. As each weld bead is completed, the corresponding finite element mesh is activated (born), and the temperature and stress fields are updated accordingly. This approach captures the cumulative thermal and mechanical effects of multi-pass deposition, which is essential for predicting the final residual stress state.
The difference in residual stress between Z-pattern and single-direction welding can be explained by the heat accumulation effect. In Z-pattern welding, the alternating direction creates a more uniform heat distribution but results in higher overall heat input due to overlapping thermal cycles. In single-direction welding, heat accumulates progressively along the deposition direction, leading to a more severe thermal gradient at the termination end. The high σz at the bottom in single-direction welding is attributed to the constraint effect of the base plate on the contracting weld metal during cooling.
Engineering Practice Integration
For practical implementation of welding-based additive manufacturing, the following recommendations emerge from this study:
- Deposition path optimization is critical for minimizing residual stresses and distortion. The choice between Z-pattern and single-direction welding should be based on the specific geometry and loading conditions of the final component.
- Preheating the base plate reduces the thermal gradient between the deposited material and the substrate, lowering the risk of cracking and delamination.
- Post-weld stress relief heat treatment is essential for achieving acceptable residual stress levels, particularly for components subjected to cyclic loading or high-temperature service.
- For thick wall structures, intermediate stress relief treatments between layers may be beneficial to prevent cumulative stress buildup.
Study Insights and Reflections
This research highlights the importance of numerical simulation in optimizing welding-based additive manufacturing processes. The findings demonstrate that residual stress management requires a holistic approach considering deposition path, thermal history, and post-processing. The observation that single-direction welding produces lower overall stresses but higher localized stresses at the termination end suggests that hybrid strategies combining both approaches may offer the best compromise. Future work should explore multi-directional deposition patterns, variable welding speed strategies, and in-situ stress monitoring to further improve the quality of welded additive structures. The study also underscores the need for standardized qualification procedures for welding-based additive manufacturing, particularly regarding residual stress acceptance criteria.