Residual Stress Distribution and Post-Weld Heat Treatment Stress Relaxation in TC2 Titanium Alloy Plate Welds

Overview of the Study

This literature presents a comprehensive numerical simulation and experimental investigation of residual stress distribution in TC2 (Ti-6Al-4V equivalent) titanium alloy plate welds and the effectiveness of post-weld heat treatment (PWHT) for stress relaxation. TC2 is widely used in aerospace structures, pressure vessels, and chemical equipment, where residual stresses can significantly affect fatigue life, dimensional stability, and susceptibility to stress corrosion cracking. The study employs finite element analysis (FEA) to model the welding thermal-mechanical cycle and validates the results through experimental measurements.

Residual Stress Formation Mechanisms

Residual stresses in welded joints develop due to the non-uniform thermal expansion and contraction during welding and cooling. In titanium alloys, the situation is particularly challenging due to:

• High thermal sensitivity of the microstructure to cooling rates
• Limited ductility at elevated temperatures compared to steels
• Low thermal conductivity leading to steep thermal gradients
• High thermal expansion coefficient relative to many other alloys
• Susceptibility to stress corrosion cracking under residual tensile stress

Thermal-Mechanical FEA Model

The study employs a coupled thermal-mechanical FEA model that captures:

1. Thermal analysis: Transient heat conduction with moving heat source (double-ellipsoidal model), accounting for temperature-dependent material properties and phase transformations.
2. Mechanical analysis: Elastic-plastic deformation with temperature-dependent yield stress, thermal expansion, and plastic strain accumulation.
3. Remelting effect: Proper treatment of material properties reset at fusion boundary during multi-pass welding.
4. Material model: Anisotropic thermal expansion and temperature-dependent elastic-plastic properties for TC2.

Residual Stress Distribution Results

The simulation reveals a complex residual stress distribution characteristic of titanium alloy welds:

The peak residual stresses in TC2 welds are notably high, reaching 80–95% of the material yield strength (350–380 MPa for solution-treated condition). This is significantly higher than typical residual stresses in carbon steel welds (which are usually limited to 50–70% of yield strength due to greater plastic deformation capacity).

Effect of Welding Parameters on Residual Stresses

The study systematically varies welding parameters to assess their influence on residual stress magnitude:

Post-Weld Heat Treatment Stress Relaxation

The study evaluates the effectiveness of various PWHT regimes for stress relaxation in TC2 welds:

Key Findings on Stress Relaxation

The most effective single-stage stress relief for TC2 welds occurs at 550 °C for 2 hours, achieving 40–55% stress reduction with minimal impact on mechanical properties. However, this leaves residual stresses of 125–210 MPa, which may still be problematic for fatigue-critical applications.

For applications requiring near-complete stress relief, solution treatment at 950 °C achieves 70–85% reduction but fundamentally alters the microstructure, requiring subsequent aging to restore mechanical properties. The solution-plus-aging sequence provides the best combination of stress relief (75–90%) and property restoration, but at significantly higher cost and potential distortion.

Effect of Cooling Rate on Post-PWHT Properties

The cooling rate after PWHT significantly affects the final microstructure and properties:

• Water quenching after 950 °C solution treatment produces fully transformed microstructure (primary α + transformed β) with excellent toughness but lower strength.
• Air cooling after 950 °C produces a mixed microstructure with balanced properties.
• Furnace cooling after 950 °C promotes grain coarsening and reduced strength but improved creep resistance.
• For stress relief at 550 °C, cooling rate has minimal effect on properties since no phase transformation occurs.

Experimental Validation

The FEA results are validated through experimental residual stress measurements using:

• Neutron diffraction: Provides bulk stress measurements at various depths, confirming the predicted stress distribution within ±15% accuracy.
• X-ray diffraction: Surface stress measurements showing peak longitudinal stresses of 280–340 MPa, consistent with simulation.
• Hole-drilling method: Incremental drilling measurements confirming the stress gradient through the weld thickness.
• Strain gauge method: Real-time stress monitoring during welding, validating the thermal-mechanical model.

The experimental validation demonstrates that the FEA model accurately predicts both the magnitude and distribution of residual stresses, with the primary discrepancy occurring at the very surface where the actual stress state may be affected by surface preparation and measurement artifacts.

Engineering Practice Recommendations

For engineers fabricating TC2 titanium alloy pressure vessels and structural components, the following recommendations emerge:

1. For fatigue-critical applications: Implement PWHT at 950 °C solution treatment followed by aging at 540 °C for 6 hours to achieve maximum stress relief (75–90%) while restoring mechanical properties.
2. For dimensional stability requirements: Use multi-pass welding with controlled interpass temperature (below 150 °C) combined with 550 °C stress relief to achieve acceptable distortion control with moderate stress reduction.
3. For cost-sensitive production: Accept higher residual stresses with single-pass welding followed by 550 °C stress relief, provided that fatigue analysis accounts for the remaining stress levels.
4. For thick-section fabrication: Apply multi-stage stress relief (500→600→700 °C) to progressively relax stresses while minimizing distortion in thick components.
5. For hydrogen service applications: Ensure complete stress relief through solution treatment, as residual tensile stresses significantly increase susceptibility to hydrogen embrittlement in TC2 alloys.

FMEA Considerations for Residual Stress-Related Failures

Applying Failure Mode and Effects Analysis (FMEA) to residual stress management:

Key Questions and Technical Reflections

The study highlights an important trade-off that engineers must navigate: the degree of stress relief achieved is inversely related to the preservation of as-welded mechanical properties. Solution treatment provides the best stress relief but requires subsequent aging to restore strength, introducing additional process complexity and potential distortion. For many pressure vessel applications, the 550 °C stress relief represents a practical compromise, but engineers must verify through fatigue testing that the remaining residual stresses do not compromise the design life.

Another critical consideration is the interaction between residual stresses and corrosion environments. The study confirms that TC2 welds with residual tensile stresses exceeding 200 MPa exhibit significantly reduced resistance to chloride-induced stress corrosion cracking. This finding has direct implications for specifying PWHT requirements for pressure vessels operating in corrosive environments, where the stress relief threshold should be set lower than for non-corrosive service.

The multi-stage stress relief approach (500→600→700 °C) deserves further investigation as a potential alternative to full solution treatment for applications requiring moderate stress relief without complete microstructural alteration. This approach may offer a cost-effective middle ground between minimal stress relief and full solution treatment.

Summary and Conclusions

This study provides comprehensive insights into residual stress formation and relaxation in TC2 titanium alloy welds, demonstrating that peak residual stresses reach 80–95% of yield strength without PWHT. The most effective stress relief strategy depends on the specific application requirements: 550 °C stress relief for moderate reduction (40–55%) with minimal property impact, or 950 °C solution treatment followed by aging for maximum relief (75–90%) at higher cost. For pressure vessel engineers, the critical takeaway is that residual stress management must be integrated into the fabrication specification from the design stage, with PWHT requirements determined by the service environment, fatigue loading spectrum, and susceptibility to environmentally assisted cracking. The FEA model validated through experimental measurements provides a reliable predictive tool for optimizing welding sequences and PWHT parameters to achieve the required stress state while maintaining acceptable mechanical properties and dimensional accuracy.