Zhuojin Pipe Fitting Co., LtdThree-Dimensional Residual Stress Analysis of TC4 Titanium Alloy Magnetically Controlled Narrow Gap TIG Welded Plates Before and After Post-Weld Heat Treatment
Literature Overview
This study, published in the Journal of Mechanical Engineering (2019, Vol. 55, No. 6, pp. 61-66) by Yu Chen and colleagues from the Guangdong Welding Technology Research Institute and the Institute of Metal Research, Chinese Academy of Sciences, addresses a critical engineering challenge in thick-section titanium alloy fabrication: the quantification and mitigation of welding residual stresses in TC4 (Ti-6Al-4V) plates welded by magnetically controlled narrow gap TIG (MAG-TIG) process. The work examines two plate thicknesses — 100 mm and 31 mm — and employs two complementary measurement techniques: the indentation strain method for surface residual stress mapping and the full-release strain method for through-thickness three-dimensional stress profiling. The study is supported by multiple funding sources including Guangdong Provincial Science and Technology Programs, Guangzhou Municipal Programs, and Pangang Group research grants, underscoring the industrial relevance of thick-section titanium welding for aerospace and petrochemical applications.
Core Technical Findings
Residual Stress Magnitude and Distribution
The most striking finding is the pronounced thickness dependence of residual stress levels. For the 100 mm thick plate, both longitudinal and transverse surface residual stress peaks reach 600–700 MPa, corresponding to 70–80% of the material yield strength. In contrast, the 31 mm plate exhibits significantly lower peak stresses at only 40–50% of yield strength. This disparity is attributed to the cumulative effect of transverse contraction in thicker sections, where each successive weld pass adds contraction strain to the previously deposited layers without adequate relaxation.
| Parameter | 100 mm Plate | 31 mm Plate |
|---|---|---|
| Surface longitudinal stress peak | 600–700 MPa | ~40–50% of yield strength |
| Surface transverse stress peak | 600–700 MPa | ~40–50% of yield strength |
| Percentage of yield strength | 70–80% | 40–50% |
| Primary cause of difference | Cumulative transverse contraction | — |
Effect of 650°C Vacuum Post-Weld Heat Treatment
The post-weld vacuum heat treatment at 650°C proved highly effective in stress relief. Both thicknesses showed a stress reduction exceeding 50%, with remaining peak residual stresses capped at 200 MPa. Notably, through-thickness stresses at the weld centerline were reduced to near-zero values, indicating near-complete stress relaxation in the critical weld zone. However, surface residual stresses exhibited a redistribution pattern post-treatment, suggesting that while peak stresses were relieved, the equilibrium state shifted to a new distribution governed by the softened material properties.
Measurement Methodology Assessment
The indentation strain method provides excellent surface stress mapping capability with spatial resolution suitable for identifying stress gradients near the weld toe and fusion line. The full-release strain method, while destructive, delivers irreplaceable through-thickness data that reveals the complex three-dimensional stress state invisible to surface-only techniques. The combination of both methods offers a comprehensive picture that neither technique could provide alone.
Engineering Practice Implications
Process Selection Guidance
For thick-section TC4 titanium alloy fabrication exceeding 30 mm, the study strongly recommends incorporating a 650°C vacuum stress relief treatment into the manufacturing sequence. The near-complete elimination of through-thickness stresses at the weld centerline is particularly significant for fatigue-critical applications where crack initiation is often governed by the multiaxial stress state.
Residual Stress Control Strategies
The findings suggest a layered approach to residual stress management:
- Preventive measures: Optimize MAG-TIG parameters to minimize peak temperature gradients and reduce thermal contraction forces at the source.
- Process-based mitigation: Utilize interpass temperature control and symmetric welding sequences to partially relieve accumulated stresses during fabrication.
- Post-weld treatment: Apply 650°C vacuum heat treatment as the definitive stress relief step, ensuring that residual stresses remain below 200 MPa throughout the section.
Standards and Specification Considerations
The 200 MPa residual stress threshold achieved post-treatment aligns well with typical acceptance criteria in aerospace specifications such as AMS 2750 and ASTM F2924, which generally require stress relief for titanium components to minimize distortion and improve fatigue performance. For pipeline and pressure vessel applications governed by ASME Section VIII or NB/T standards, the demonstrated stress reduction provides a robust basis for qualifying MAG-TIG as a production welding process for thick titanium sections.
Key Questions and Reflections
The study raises several important questions for further investigation. First, the redistribution of surface stresses post-heat treatment implies that the final stress state is not simply a scaled-down version of the as-welded state but rather a re-equilibrated configuration. Understanding the mechanics of this redistribution is essential for predicting dimensional stability and distortion after stress relief. Second, the study does not address the effect of heat treatment on microstructure evolution — specifically, whether the 650°C treatment induces any α-to-β phase transformation in TC4, which could compromise the mechanical properties of the weld and HAZ. Third, the transition thickness at which residual stress levels begin to increase significantly remains undefined; practical fabrication may require thickness-specific welding procedure specifications.
Study Insights and Implications
This research provides a rigorous, measurement-based foundation for residual stress management in thick titanium alloy welding. The clear correlation between plate thickness and residual stress magnitude offers engineers a predictive framework for estimating the severity of stress accumulation in future projects. The demonstrated effectiveness of 650°C vacuum heat treatment in reducing stresses to below 200 MPa provides a reliable, standards-compliant solution for ensuring component fitness-for-service. The combination of indentation strain and full-release strain measurement techniques sets a benchmark for residual stress characterization in critical weldments, and the methodology can be readily adapted to other alloy systems and welding processes. For practitioners in the titanium alloy fabrication industry, this study confirms that MAG-TIG welding of thick sections, when combined with appropriate post-weld heat treatment, can produce weldments with residual stress levels comparable to those achievable in thinner sections — a significant advancement for applications demanding both high strength and fatigue resistance in massive titanium structures.