Effect of Weld Reinforcement Removal on Residual Stress Distribution in Aluminum Alloy TIG Welds

Literature Overview

This study, published in Welding (2017, Vol. 5, pp. 10–13) by Shi Kangning, Wang Dianlong, Liang Zhimin, and Han Shanguo from Hebei University of Science and Technology and Guangdong Welding Institute, investigates the effect of weld reinforcement removal on the residual stress distribution in TIG welds of marine 5-series aluminum alloys. The research was supported by the National Natural Science Foundation of China (Grant No. 51205106) and a defense research project (2013DFR70160). The study employed ultrasonic measurement techniques to characterize both longitudinal and transverse residual stresses in butt welds of 3 mm and 5 mm thick aluminum alloy plates before and after weld reinforcement removal.

Background and Significance

Residual stresses in welded structures are a critical factor influencing fatigue life, stress corrosion cracking susceptibility, and dimensional stability. In marine applications, where 5-series aluminum alloys (such as AA5083, AA5456, and AA5086) are widely used for hull structures, the control of residual stresses is particularly important due to:

• The marine environment's high chloride content, which promotes stress corrosion cracking.
• The cyclic loading conditions experienced by marine structures.
• The relatively low fatigue strength of aluminum alloys compared to steels.

Weld reinforcement removal is a common post-weld treatment used to improve surface quality, reduce stress concentrations, and facilitate inspection. However, its effect on the residual stress field is complex and not always beneficial. This study provides valuable quantitative data on this topic.

Experimental Methodology

Test Materials and Geometry

Ultrasonic Residual Stress Measurement

The ultrasonic method (also known as the ultrasonic elastic constant method) is a non-destructive technique for measuring residual stresses based on the dependence of ultrasonic wave velocity on the stress state of the material. The method relies on the acoustoelastic effect, where the velocity of ultrasonic waves changes in response to applied or residual stresses.

Key advantages of the ultrasonic method for this application:

• Non-destructive and non-contact measurement.
• Capable of measuring both longitudinal and transverse stresses.
• Suitable for aluminum alloys where other methods (such as X-ray diffraction) may be less accurate due to the material's low elastic modulus.
• Provides through-thickness stress profiles when combined with stepwise removal techniques.

Measurement Configuration

• Probe frequency: 5 MHz (typical for aluminum alloy residual stress measurement).
• Measurement directions: Longitudinal (parallel to weld axis) and transverse (perpendicular to weld axis).
• Measurement positions: Along the weld centerline and at various distances from the weld centerline (typically 0, ±5, ±10, ±15, ±20, ±30 mm).
• Temperature compensation: Applied to account for thermal effects during measurement.

Results and Analysis

Residual Stress Distribution Before Reinforcement Removal

Longitudinal Residual Stress

The longitudinal residual stress distribution exhibits the following characteristics:

• Maximum tensile stress occurs in the near-weld region (within ±5 mm of the weld centerline).
• The 3 mm plate shows slightly lower longitudinal residual stress in the near-weld region compared to the 5 mm plate. This is attributed to greater plastic deformation in the thinner plate, which releases some stress through geometric deformation.
• Compressive stress regions exist on both sides of the weld, at distances beyond ±15 mm, as required by force equilibrium.
• The stress distribution is symmetric about the weld centerline.

Transverse Residual Stress

The transverse residual stress distribution shows:

• Tensile stress in the weld metal and near-weld HAZ, reaching values of 80–120 MPa.
• Compressive stress at distances beyond ±10 mm from the weld centerline.
• The magnitude of transverse stress is generally lower than the longitudinal stress.

Residual Stress Distribution After Reinforcement Removal

After the weld reinforcement (the portion of the weld above the original plate surface) is removed by grinding or machining, the residual stress distribution changes significantly:

Longitudinal Residual Stress

Key observations after reinforcement removal:

• The longitudinal residual stress curve exhibits a characteristic "camel-back" shape, with lower stress at the weld center and maximum stress in the near-weld region (±5 mm).
• The removal of the weld reinforcement redistributes the stress field, shifting the maximum stress from the weld center to the near-weld HAZ.
• The overall magnitude of longitudinal stress is not significantly reduced by reinforcement removal.
• The compressive stress regions on the far sides of the weld remain largely unchanged.

Transverse Residual Stress

• The transverse residual stress is moderately reduced by reinforcement removal.
• The reduction is more pronounced in the weld center region than in the near-weld region.
• The overall stress distribution pattern remains similar, with tensile stress near the weld and compressive stress at distance.

Mechanistic Interpretation

Stress Redistribution During Reinforcement Removal

The removal of weld reinforcement involves the sequential removal of material from the weld surface. As material is removed, the residual stresses in the removed layer are released, and the remaining material undergoes elastic redistribution to maintain equilibrium. This process can be understood through the following steps:

1. Initial state: The weld reinforcement carries a portion of the residual stress field, with high tensile stress at the weld center.
2. Removal of reinforcement: The tensile stress in the reinforcement is released, causing the underlying material to expand slightly in the longitudinal direction.
3. Stress redistribution: The expansion of the underlying material induces compressive stress at the weld center and tensile stress in the near-weld region.
4. Final state: The stress field reaches a new equilibrium with the characteristic "camel-back" distribution.

Role of Plate Thickness

The difference in stress response between 3 mm and 5 mm plates can be attributed to:

• Geometric constraints: Thicker plates provide greater constraint against deformation, resulting in higher residual stresses.
• Heat input distribution: In thicker plates, the heat input is more concentrated, leading to steeper thermal gradients and higher residual stresses.
• Plastic deformation: Thinner plates undergo more plastic deformation during welding, which partially releases residual stresses through geometric deformation.

Effect on Transverse Stress

The reduction in transverse residual stress upon reinforcement removal is less pronounced than the redistribution of longitudinal stress. This is because:

• The transverse stress field is less concentrated at the weld surface.
• The removal of reinforcement primarily affects the longitudinal stress equilibrium.
• The transverse constraint from the base metal is maintained regardless of reinforcement removal.

Engineering Practice Implications

Design Considerations

1. Fatigue life assessment: The "camel-back" stress distribution after reinforcement removal may be less favorable for fatigue performance than the original distribution, as the maximum stress is now located in the near-weld HAZ rather than at the weld surface. Engineers should consider this when evaluating fatigue life.
2. Stress corrosion cracking: The redistribution of residual stresses may affect susceptibility to stress corrosion cracking. The near-weld HAZ, which may contain sensitized microstructure, is now subjected to higher tensile stress, potentially increasing SCC risk.
3. Dimensional stability: Reinforcement removal can introduce additional distortion due to stress redistribution. This should be considered in the manufacturing sequence and fixturing design.

Process Recommendations

Inspection and Quality Control

• Residual stress measurement should be performed before and after reinforcement removal to verify the stress state.
• Ultrasonic testing should be conducted after reinforcement removal to ensure no cracks have been introduced during grinding or machining.
• Surface finish after reinforcement removal should meet the specified requirements (typically Ra ≤ 6.3 μm for marine applications).

Key Reflections

This study provides valuable quantitative data on the residual stress redistribution caused by weld reinforcement removal in aluminum alloy TIG welds. The key finding is that reinforcement removal does not significantly reduce the overall longitudinal residual stress but rather redistributes it, creating a "camel-back" distribution with maximum stress in the near-weld HAZ. This has important implications for fatigue life assessment and stress corrosion cracking susceptibility.

Several questions remain for future investigation:

• How does the depth of reinforcement removal affect the residual stress redistribution? Is there an optimal removal depth that minimizes the maximum stress?
• What is the combined effect of reinforcement removal and post-weld treatments such as peening or thermal stress relief on the final residual stress state?
• How do the residual stress distributions measured in this study correlate with fatigue life and stress corrosion cracking behavior?

Study Insights and Implications

The study demonstrates that weld reinforcement removal, while beneficial for surface quality and inspection accessibility, does not inherently improve the residual stress state in aluminum alloy TIG welds. Engineers should not assume that reinforcement removal reduces residual stresses and should evaluate its impact case by case. For marine applications where fatigue and stress corrosion cracking are critical concerns, alternative or complementary treatments such as peening or thermal stress relief may be more effective. The ultrasonic measurement technique employed in this study is a valuable tool for non-destructive residual stress assessment and should be incorporated into quality assurance procedures for critical welded structures.