Three-Dimensional Residual Stress Analysis of Magnetically Controlled Narrow Gap TIG Welded TC4 Titanium Alloy Plates

Literature Overview

This study, published in the Journal of Mechanical Engineering (2019, Vol. 55, Issue 6), investigates the three-dimensional residual stress distribution in thick TC4 titanium alloy test plates welded using magnetically controlled narrow gap TIG (MNT-TIG) welding. The work was conducted by researchers from the Guangdong Provincial Institute of Welding Technology and the Institute of Metal Research, Chinese Academy of Sciences. The significance of this research lies in the fact that TC4 titanium alloy is widely used in aerospace, petrochemical, and power generation industries where thick-section components are common, yet the through-thickness residual stress field in such joints remains poorly characterized compared to surface-level measurements.

Core Technical Findings

The study employed two complementary measurement techniques: the indentation strain method for surface residual stress mapping and the full release strain method for through-thickness three-dimensional residual stress evaluation. Test plates of 100 mm and 31 mm thickness were welded under identical magnetically controlled narrow gap TIG conditions to isolate the effect of thickness on residual stress development.

The key findings reveal a pronounced thickness-dependent residual stress pattern:

Parameter	100 mm Plate	31 mm Plate
Surface longitudinal peak stress	600–700 MPa	~250–320 MPa
Surface transverse peak stress	600–700 MPa	~250–320 MPa
Stress as % of yield strength	70–80%	40–50%
Primary stress development mechanism	Cumulative transverse shrinkage	Limited cumulative shrinkage

The critical insight is that the substantially higher residual stresses in the 100 mm plate are not merely a consequence of greater total heat input, but rather stem from the cumulative effect of transverse contraction across multiple weld passes. In narrow gap welding, the constrained geometry limits lateral deformation, forcing the accumulated contraction strains to manifest as elevated longitudinal and transverse tensile stresses in the weld and heat-affected zone.

Post-Weld Heat Treatment Effects

Post-weld vacuum heat treatment at 650 °C was applied to both plate thicknesses, and the results demonstrate significant stress relief:

Metric	Before PWHT	After PWHT at 650 °C
Maximum stress reduction	—	>50%
Residual peak stress (surface)	600–700 MPa (100 mm)	<200 MPa
Through-thickness stress at weld center	High gradient	Near zero
Surface stress distribution	Peak near weld center	Redistributed

The through-thickness stress field at the weld centerline after heat treatment approaches zero in all three directions, which is particularly beneficial for fatigue performance and stress corrosion cracking resistance. The redistribution of surface residual stresses after PWHT suggests that the heat treatment does not merely reduce stress magnitude but fundamentally alters the stress field topology, likely due to non-uniform relaxation rates across the weld cross-section.

Engineering Practice Implications

For thick-section TC4 titanium alloy fabrication in aerospace and petrochemical applications, several practical conclusions can be drawn. First, the 650 °C vacuum heat treatment is highly effective and should be considered mandatory for components exceeding 30 mm thickness. Second, the near-zero through-thickness stress state after PWHT significantly improves resistance to hydrogen-assisted cracking and stress corrosion cracking, which are critical failure modes in titanium alloys. Third, the cumulative transverse shrinkage mechanism identified in this study provides a physical basis for optimizing welding sequence and preheat strategies to minimize residual stresses before heat treatment.

The magnetically controlled narrow gap TIG process itself offers distinct advantages for titanium alloy welding: the magnetic field stabilizes the arc and promotes deeper penetration with narrower weld profiles, reducing the number of passes and total heat input. However, the constrained gap geometry paradoxically amplifies residual stresses in thick sections, underscoring the importance of post-weld stress relief.

Key Questions and Reflections

A noteworthy observation is the relatively low residual stress in the 31 mm plate despite similar welding parameters per pass. This suggests that thickness effects on residual stress are non-linear and that a threshold thickness exists beyond which cumulative shrinkage becomes the dominant factor. Engineers should be cautious about extrapolating residual stress data from thin-section test results to thick production components. Additionally, the study does not address the effect of PWHT on mechanical properties, which is important since 650 °C is close to the β-transus temperature of TC4 and may influence grain size and phase distribution.

Summary

This study provides valuable quantitative data on three-dimensional residual stress in thick TC4 titanium alloy MNT-TIG welds and demonstrates the effectiveness of 650 °C vacuum heat treatment in reducing peak stresses by over 50%. The identification of cumulative transverse shrinkage as the primary mechanism driving thickness-dependent stress elevation offers a clear physical understanding for process optimization. For engineering practice, this work reinforces that thick-section titanium alloy components require comprehensive post-weld heat treatment protocols, and that residual stress management must be an integral part of the welding procedure specification from the design stage onward.