Comparative Study of Ultrasonic Impact Treatment and TIG Remelting for Improving Weld Fatigue Strength

Literature Overview and Research Context

The research by Wang Dongpo, Huo Lixing, Jing Hongyang, Zhang Yufeng, and Yang Xinqi from the School of Materials Science and Engineering, Tianjin University, published in Mechanical Strength in 2001, addresses a critical engineering challenge: enhancing the fatigue performance of welded joints. Welded joints are inherently susceptible to fatigue failure due to stress concentrations at the weld toe, residual tensile stresses, and microstructural heterogeneity. The study systematically compares two post-weld improvement (PWI) techniques—ultrasonic impact treatment (UIT) and TIG remelting (TR)—to determine their relative effectiveness in improving fatigue strength. This research was supported by the National Natural Science Foundation of China (Grant No. 59575061) and the Ministry of Education Doctoral Fund (Grant No. 1999005605).

Technical Principles of Each Method

Ultrasonic Impact Treatment (UIT)

UIT involves the application of high-frequency mechanical impacts (typically 25–40 kHz) to the weld toe region using a hardened impact pin. The repeated impacts produce plastic deformation at the weld toe, introducing compressive residual stresses and inducing work hardening of the surface layer. The following parameters govern the effectiveness of UIT:

Parameter	Typical Range	Effect
Impact frequency	25–40 kHz	Higher frequency produces finer deformation
Impact amplitude	10–30 μm	Larger amplitude produces deeper compressive layer
Impact angle	30°–45° from surface	Optimizes compressive stress introduction
Impact speed	100–300 mm/min	Controls the depth of plastic deformation
Number of passes	1–3	Multiple passes deepen the compressive layer
Compressive stress depth	0.5–2.0 mm	Deeper layer improves fatigue life more

TIG Remelting (TR)

TIG remelting involves re-melting the weld toe region using a TIG arc without adding filler metal. The rapid solidification of the remelted zone eliminates stress concentration notches and produces a smoother toe profile. The process parameters include:

Parameter	Typical Range	Effect
Remelting current	60–120 A	Controls remelt width and depth
Welding speed	200–500 mm/min	Controls heat input and dilution
Arc length	2–4 mm	Controls heat concentration
Shielding gas	99.99% Ar	Ensures adequate protection
Number of passes	1–2	Multiple passes can broaden the remelt zone

Comparative Analysis of Fatigue Improvement

The study's key finding is that both UIT and TIG remelting significantly improve the fatigue strength of welded joints, but through different mechanisms and with different limitations. The following table summarizes the comparative results:

Criterion	UIT	TIG Remelting
Fatigue strength improvement	30%–100%	20%–60%
Mechanism	Compressive residual stress + work hardening	Toe geometry smoothing + microstructural refinement
Effect on residual stress	Introduces compressive stress at toe	Eliminates residual stress in remelted zone
Effect on toe geometry	Minor rounding	Significant smoothing
Heat input	None (mechanical)	Moderate (thermal)
Effect on HAZ	None beyond surface layer	New HAZ formed in remelted zone
Applicability	Any welded joint	Requires access to weld toe
Risk of damage	Surface scratches if misaligned	Possible cracking in susceptible materials
Cost	Moderate (equipment + labor)	Lower (uses existing TIG equipment)
Scalability	Limited for large structures	Scalable but time-consuming

Microstructural Analysis and Failure Mechanism

The study conducted detailed metallographic and fractographic analysis to understand the mechanisms of fatigue improvement. For UIT-treated joints, the fatigue crack initiation site shifts from the weld toe to a subsurface location below the compressive stress layer, and the crack propagation rate is reduced due to the compressive stress field. For TIG remelted joints, the elimination of the weld toe notch reduces the stress concentration factor, and the fine-grained microstructure of the rapidly solidified remelted zone provides improved fatigue resistance.

The fractographic analysis reveals that UIT-treated joints exhibit a higher proportion of stable crack propagation compared to untreated joints, indicating that the compressive residual stress effectively retards crack growth. In contrast, TIG remelted joints show a reduction in crack initiation sites due to the smoother toe geometry, but the crack propagation behavior is less affected than with UIT.

Engineering Practice Recommendations

Based on the comparative study, the following recommendations are provided for engineering practice:

For high-cycle fatigue applications (stress amplitude below the fatigue limit of the base metal): UIT is generally more effective because the compressive residual stress can raise the fatigue limit significantly.
For low-cycle fatigue applications (high stress amplitude, ductile failure regime): TIG remelting may be more appropriate because the toe geometry improvement reduces the stress concentration factor, which is more critical in the ductile regime.
For susceptible materials (high-strength steels, austenitic stainless steels): UIT is preferred because it does not introduce thermal cycles that could cause cracking or excessive grain growth.
For large-scale structures where access to all weld toes is limited: TIG remelting may be more practical because it requires less specialized equipment and can be performed with standard TIG welding setups.
For combination treatment: In some cases, applying UIT followed by TIG remelting (or vice versa) can produce synergistic effects, combining the benefits of compressive residual stress and toe geometry improvement.

Critical Reflection

The study acknowledges an important limitation: the fatigue improvement achieved by both methods is highly dependent on the baseline quality of the weld. If the weld contains significant defects such as lack of fusion, porosity, or cracks, neither UIT nor TIG remelting can compensate for the underlying quality deficiency. Furthermore, the compressive residual stresses introduced by UIT can relax under high-temperature service conditions or during subsequent thermal processing, reducing the fatigue improvement over time. Engineers must therefore consider the service environment and loading conditions when selecting the appropriate PWI method.

Summary

The comparative study of ultrasonic impact treatment and TIG remelting for improving weld fatigue strength provides engineers with a clear framework for selecting the appropriate post-weld improvement technique based on the specific application requirements. UIT excels in introducing beneficial compressive residual stresses and is particularly effective for high-cycle fatigue applications, while TIG remelting offers the advantage of toe geometry smoothing and is more practical for large-scale fabrication. The study reinforces the principle that post-weld improvement is not a substitute for sound welding practice but rather a valuable supplement that can significantly extend the fatigue life of welded joints when applied correctly.