Residual Stress Measurement in Titanium Alloy Thin Sheet Laser and TIG Welding

Literature Overview

This paper by He Xiaodong, Zhang Jianxun, Gong Shuili, and Li Jihong from Xi'an Jiaotong University and Beijing Aviation Technology Institute, published in Journal of Materials Engineering (Vol. 31, No. 12, 2003, pp. 7-10), investigates the welding residual stress distribution in titanium alloy thin sheets welded by laser welding and TIG welding. The study employs the small hole release method for residual stress measurement and analyzes the effects of welding method, welding line energy, and post-weld heat treatment on residual stress distribution.

Core Technical Findings

The experimental results reveal significant differences in residual stress distribution between laser welding and TIG welding:

Parameter	Laser Welding	TIG Welding	Comparison
Stress distribution region	Narrow	Wide	Laser has more localized stress
HAZ residual tensile stress	Lower	Higher	Laser stress ~100 MPa lower in HAZ
Weld and fusion line stress	Higher	Lower	Laser stress higher at weld center
Effect of line energy	Higher energy → wider weld → higher HAZ stress	Similar trend	Consistent behavior
Post-weld vacuum heat treatment	Reduces stress by ~90%	Reduces stress by ~90%	Effective for both methods

The most significant finding is that laser welding produces a narrower residual stress distribution region compared to TIG welding, with approximately 100 MPa lower tensile stress in the HAZ. However, the residual stress at the weld center and near the fusion line is higher for laser welding than for TIG welding.

Residual Stress Distribution Analysis

The residual stress distribution in welded joints is governed by the thermal cycle experienced during welding. Laser welding, with its highly concentrated energy input, produces a narrow heat-affected zone with rapid heating and cooling. This results in a narrow stress distribution region with high stress gradients near the weld center.

TIG welding, with its more distributed energy input, produces a wider HAZ with more gradual thermal gradients. This results in a wider stress distribution region with lower peak stresses but more extensive stress-affected areas.

The following table summarizes the residual stress characteristics:

Stress Zone	Laser Welding	TIG Welding	Engineering Implication
Weld center	High tensile stress	Moderate tensile stress	Higher risk of crack initiation in laser welds
Fusion line	High tensile stress	Moderate tensile stress	Similar risk at fusion line
HAZ	Lower tensile stress (~100 MPa less)	Higher tensile stress	Better fatigue performance in laser weld HAZ
Base metal (far field)	Near-zero stress	Near-zero stress	No significant difference

Post-Weld Heat Treatment Effects

The study demonstrates that vacuum heat treatment can reduce residual stress by approximately 90% for both laser and TIG welds. This finding is significant because residual stress is a major contributor to fatigue crack initiation, stress corrosion cracking, and distortion in welded structures.

The effectiveness of vacuum heat treatment is attributed to the following mechanisms:

Stress relaxation through creep at elevated temperatures
Redistribution of plastic strain through thermal expansion
Reduction of dislocation density through recovery and recrystallization

The vacuum environment is critical to prevent oxidation of the titanium alloy surface during heat treatment, as titanium is highly reactive with oxygen at elevated temperatures.

Engineering Practice Implications

For titanium alloy thin sheet fabrication in aerospace and marine applications, residual stress control is critical for ensuring structural integrity and dimensional accuracy. The following table outlines the engineering implications of the study findings:

Application	Recommended Process	Rationale
Thin sheet structural components	Laser welding + vacuum heat treatment	Narrow stress zone, reduced HAZ stress
Thick section fabrication	TIG welding + vacuum heat treatment	Better process stability for thick sections
Fatigue-critical components	Both + vacuum heat treatment	90% stress reduction improves fatigue life
Precision dimensional components	Laser welding + vacuum heat treatment	Less distortion due to narrower heat input

The study findings suggest that laser welding offers advantages for thin sheet titanium alloy fabrication due to its narrower heat-affected zone and lower HAZ residual stress. However, the higher peak stress at the weld center may increase the risk of crack initiation under cyclic loading, necessitating careful consideration of the trade-offs.

Key Questions and Reflections

The study does not address the effect of residual stress on the fatigue performance of laser and TIG welds. While residual stress reduction is generally beneficial for fatigue life, the interaction between residual stress distribution and fatigue crack initiation is complex and may differ between laser and TIG welds.

Additionally, the study does not evaluate the distortion behavior of laser and TIG welded titanium alloy thin sheets. Distortion is a major concern in thin sheet fabrication, and the different heat input characteristics of laser and TIG welding may produce different distortion patterns.

The study also does not address the effect of welding sequence and fixture design on residual stress distribution, which are critical process parameters for controlling distortion and stress in production welding.

Study Insights and Implications

This research provides valuable insights into the residual stress behavior of laser and TIG welded titanium alloy thin sheets. The finding that laser welding produces a narrower stress distribution with lower HAZ stress but higher weld center stress highlights the trade-offs between welding processes for thin sheet fabrication.

The demonstration that vacuum heat treatment can reduce residual stress by 90% for both processes is highly significant for engineering practice, as it provides a reliable method for stress relief in critical titanium alloy components. The work underscores the importance of post-weld heat treatment in achieving acceptable residual stress levels for aerospace and marine applications.

Summary of All Five Studies

These five studies collectively demonstrate the versatility and effectiveness of active agent-enhanced TIG welding (AT-TIG) across different metal systems, including aluminum alloys, low-alloy high-strength steels, and titanium alloys. The common theme is that carefully selected active agents can significantly enhance weld penetration, improve microstructure, and reduce porosity without compromising welding process stability or joint properties.

The SiO2 active agent emerges as the most effective single-component surfactant across multiple metal systems, producing penetration enhancements of up to 3.5× in aluminum alloys and 1.9× in titanium alloys. The SiO2 + Cr2O3 mixed agent offers additional benefits in low-alloy steel welding through acicular ferrite promotion. The CaF2 + MgF2 mixture provides significant porosity reduction in titanium alloy welding.

For residual stress management, the study on laser and TIG welding of titanium alloy thin sheets demonstrates that vacuum heat treatment is an effective post-weld stress relief method, capable of reducing residual stress by approximately 90%. This finding is applicable across all welding processes and metal systems.

The collective insights from these studies suggest that AT-TIG welding with carefully selected active agents represents a promising process development direction for improving welding productivity and quality across multiple industrial sectors. However, comprehensive process qualification including mechanical testing, corrosion testing, fatigue testing, and long-term aging studies is essential before production implementation in critical applications.