Ultrasonic Impact Treatment Effects on TC4ELI Titanium Alloy TIG Welded Joints

Literature Overview

Published in Ordnance Materials and Science (Vol. 49, No. 1, 2026, pp. 62–67), this study by Ma Xiaojun et al. from Jiangsu University and the China Ship Scientific Research Center addresses a critical challenge in naval engineering: the high post-weld residual stresses in TC4ELI (Ti-6Al-4V ELI) titanium alloy joints. The research is supported by the National Key Laboratory of Ship Structural Safety open fund and the China Postdoctoral Science Foundation (389885), underscoring its relevance to marine and naval applications where fatigue and corrosion resistance are paramount.

Residual Stress Reduction Through Ultrasonic Impact Treatment

The primary objective of ultrasonic impact treatment (UIT) is to introduce beneficial compressive residual stresses that counteract the tensile residual stresses generated during welding. The results are remarkable: longitudinal residual stress at the weld center was reduced to −944 MPa, and at the weld toe to −584 MPa. Transverse residual stress reached −881 MPa at the weld center and −432 MPa at the weld toe. These values represent a complete reversal from the typical tensile residual stresses (often +300 to +500 MPa) found in untreated titanium alloy welds.

Residual Stress Distribution

Location	Longitudinal Stress (MPa)	Transverse Stress (MPa)
Weld Center	−944	−881
Weld Toe	−584	−432

The magnitude of these compressive stresses approaches the yield strength of TC4ELI (approximately 900 MPa), indicating that the UIT process has plastically deformed the surface layers to a depth of typically 0.5–1.5 mm. This deep compressive stress layer is highly effective in inhibiting fatigue crack initiation at the weld toe, which is the most common fatigue failure location in welded structures.

Mechanical Property Enhancement

The UIT process produced measurable improvements in both hardness and impact toughness:

Microhardness increase: approximately 5% improvement, attributed to surface work hardening and grain refinement in the impacted zone.
Impact toughness increase: approximately 11% improvement, which is particularly significant for titanium alloys where the HAZ is typically the weakest link in terms of ductility and toughness.

These improvements are attributed to the combined effects of work hardening, grain refinement, and the introduction of dislocation structures that impede crack propagation. The enhancement in impact toughness is especially valuable for naval applications where impact loading and low-temperature service are concerns.

Corrosion Performance Improvement

A frequently overlooked benefit of UIT is its positive effect on corrosion resistance. The study confirms that UIT-treated TC4ELI welded joints exhibit superior corrosion resistance compared to untreated joints. This improvement is attributed to several mechanisms:

Closure of micro-porosity: The plastic deformation introduced by UIT compresses and closes micro-voids and micro-pores near the surface, reducing the number of corrosion initiation sites.
Work hardening layer: The hardened surface layer has a more uniform microstructure with fewer grain boundary discontinuities, reducing susceptibility to localized corrosion.
Compressive stress field: Compressive residual stresses inhibit the opening of micro-cracks that would otherwise serve as pathways for corrosive media penetration.

For naval applications where joints are exposed to seawater for extended periods, this corrosion resistance improvement translates directly into extended service life and reduced maintenance requirements.

Engineering Practice and Process Parameters

The UIT process parameters that influence the final residual stress state include:

Parameter	Typical Range	Effect
Impact frequency	20–25 kHz	Determines stress wave characteristics
Amplitude	0.1–0.3 mm	Controls plastic deformation depth
Overlap ratio	50–70%	Ensures uniform coverage
Impact angle	45–60°	Optimizes surface deformation
Treatment depth	0.5–1.5 mm	Defines compressive layer thickness

For naval engineers, the integration of UIT into the welding production sequence is straightforward: it is applied after welding and any required post-weld heat treatment (PWHT), before final machining or coating. The process is non-destructive, does not require consumables, and can be automated for production lines.

Study Insights and Implications

This research provides quantitative evidence that UIT is an effective post-weld treatment for TC4ELI titanium alloy welded joints, simultaneously addressing the three major durability concerns in naval engineering: fatigue, impact resistance, and corrosion. The compressive residual stresses achieved (exceeding 900 MPa at the weld center) are among the highest reported in the literature for titanium alloys, suggesting that the process parameters were well-optimized. For shipbuilders and naval architects, this study supports the inclusion of UIT in the welding procedure specification for critical fatigue-critical joints, such as those in hull structures, submarine pressure hulls, and propulsion systems. The combination of residual stress relief, mechanical property enhancement, and improved corrosion resistance makes UIT a highly attractive value-added treatment that can significantly extend the service life of titanium alloy naval components.