Performance Enhancement of TC4ELI Titanium Alloy TIG Welds by Ultrasonic Impact Treatment

Literature Overview

The research published in Ordnance Materials and Science and Engineering (2026, Vol. 49, No. 1, pp. 62–67) by Ma Xiaojun, Wang Jinjie, Zhang Deyang, Wang Jun, Zhao Yong, Wang Feiyun, and Fu Juan investigates the application of ultrasonic impact treatment (UIT) to TIG welds of TC4ELI titanium alloy for marine applications. The study is supported by the National Key Laboratory of Ship Structure Safety open fund and the China Postdoctoral Science Foundation (389885), underscoring the significance of this research for naval engineering. The primary objective is to address the elevated residual stress levels in TC4ELI welds through UIT and to evaluate the resulting improvements in residual stress, mechanical properties, and corrosion resistance.

Core Findings and Residual Stress Reduction

The UIT treatment achieved significant reductions in residual stress at both the weld center and weld toe regions. The longitudinal residual stress decreased to minus 944 MPa at the weld center and minus 584 MPa at the weld toe, while the transverse residual stress decreased to minus 881 MPa and minus 432 MPa respectively. These compressive residual stress values are substantially beneficial for fatigue life improvement and resistance to stress corrosion cracking.

The magnitude of compressive residual stress achieved at the weld center approaches the yield strength of TC4ELI titanium alloy, which typically ranges from 830 to 900 MPa in the annealed condition. This level of compressive stress is sufficient to significantly improve fatigue performance by counteracting applied tensile stresses during cyclic loading.

The UIT treatment also improved the hardness and impact toughness of the weld joint by approximately 5% and 11% respectively. The hardness improvement is attributed to work hardening induced by the plastic deformation of the surface layer during the impact process. The impact toughness improvement suggests that the compressive residual stress state and surface refinement contribute to enhanced crack initiation resistance.

Metallurgical Mechanisms of UIT

Ultrasonic impact treatment operates by repeatedly impacting the weld surface with high-frequency ultrasonic waves, typically in the range of 20 kHz, using a hardened needle or pin. This process induces plastic deformation of the near-surface material, which produces several beneficial metallurgical effects. The primary mechanism is the introduction of compressive residual stress through plastic strain accumulation, which counteracts the tensile residual stresses generated during welding.

Beyond residual stress modification, UIT promotes grain refinement in the surface layer through dynamic recrystallization and mechanical twinning. In titanium alloys, the alpha phase can undergo mechanical twinning under compressive loading, which contributes to both work hardening and improved ductility. The formation of fine deformation bands and dislocation structures near the weld toe also contributes to the observed hardness improvement.

The improved corrosion resistance after UIT treatment can be attributed to multiple factors. The compressive residual stress state reduces the driving force for crack initiation under corrosive environments. The refined surface microstructure provides a more uniform and less susceptible surface for corrosion attack. Additionally, the work hardening effect may improve the passive film stability on the titanium alloy surface, enhancing resistance to localized corrosion in marine environments.

Engineering Application for Marine Structures

TC4ELI titanium alloy is widely used in marine applications due to its excellent combination of strength, corrosion resistance, and low density. However, welding titanium alloys introduces significant residual stresses that can compromise the fatigue and corrosion performance of welded joints, particularly in the harsh marine environment where cyclic loading and chloride-induced stress corrosion cracking are major concerns.

The application of UIT to TC4ELI welds represents a practical post-weld treatment that can be applied selectively to critical weld regions without requiring extensive equipment or process modifications. The treatment is non-destructive, can be applied to as-welded joints, and does not require heat treatment that could affect the overall mechanical properties of the component. This makes UIT particularly suitable for in-situ treatment of large marine structures where post-weld heat treatment is impractical.

For marine applications, the improvement in both fatigue resistance and stress corrosion cracking resistance is of paramount importance. The compressive residual stress introduced by UIT effectively raises the threshold for fatigue crack initiation, while the improved surface microstructure and compressive stress state reduce susceptibility to chloride-induced stress corrosion cracking. These combined benefits can significantly extend the service life of titanium alloy welded structures in marine environments.

Study Insights and Reflections

This study demonstrates the effectiveness of ultrasonic impact treatment as a practical post-weld improvement technique for TC4ELI titanium alloy welded joints. The substantial reduction in residual stress, coupled with improvements in hardness, impact toughness, and corrosion resistance, validates UIT as a valuable tool for enhancing the performance of titanium alloy welds in demanding marine applications. The achieved compressive residual stress levels are particularly impressive, approaching the yield strength of the base material and providing excellent fatigue life enhancement potential.

One consideration for practical implementation is the uniformity of UIT treatment across large weld lengths. The study focuses on localized treatment effects, but in engineering practice, consistent treatment quality over extended weld lengths requires careful control of impact parameters, travel speed, and overlap between impact zones. Future research should also investigate the long-term stability of the induced compressive residual stress under thermal cycling and prolonged exposure to marine environments, as well as the interaction between UIT and other post-weld treatments such as shot peening or laser shock peening for synergistic performance enhancement.