Dual-Pulse TIG Deep Penetration Welding Mechanism for Medium-Thickness Titanium Alloy

Literature Overview

This paper by Yang Qingfu, Luo Zhiwei, Zeng Caiyou, Jiang Zihao, Cong Baoqiang, and Qi Bojin from Beihang University and Capital Aerospace Machinery Co., Ltd., published in Rare Metal Materials and Engineering (2025, Vol. 54, Issue 3, pp. 688-696), investigates the deep penetration welding mechanism of medium-thickness titanium alloy using dual-pulse TIG technology. The research is supported by the National Natural Science Foundation of China (Grants U20B2031 and 52075024). The study addresses a fundamental challenge in titanium alloy welding: the limited penetration depth and low welding efficiency of conventional TIG processes for medium-thick sections.

Process Configuration and Experimental Setup

The study uses 6 mm thick TC4 (Ti-6Al-4V) titanium alloy as the test material. Three arc modes are compared: DC (constant current), low-frequency pulse, and dual-pulse (low-frequency + ultrasonic frequency superposition). The dual-pulse mode superimposes a high-frequency (ultrasonic) pulse onto a low-frequency base pulse, creating a unique current waveform that modulates the arc behavior in a way that neither pure DC nor simple low-frequency pulsing can achieve.

Arc Mode	Current Waveform	Penetration Depth	Weld Width	Weld Efficiency
DC (Constant Current)	Steady current	Shallow	Wide	Low
Low-Frequency Pulse	Sinusoidal pulse	Moderate	Moderate	Moderate
Dual-Pulse (LF + HF)	Superimposed dual frequency	Deep	Narrow	High

Finite element simulation was employed to study the dynamic behavior of the weld pool temperature field and flow field under dual-pulse conditions. The simulation reveals that the dual-pulse mode increases the melt flow velocity within the weld pool, effectively exciting a deep penetration keyhole at the pool center and promoting downward heat source transfer.

Deep Penetration Mechanism

The dual-pulse TIG welding mechanism can be understood through the interaction of two frequency components. The low-frequency pulse establishes the overall energy input and melting rate, while the ultrasonic frequency component creates rapid current fluctuations that induce electromagnetic stirring and acoustic radiation pressure on the molten pool surface. This combination produces several beneficial effects:

Enhanced electromagnetic stirring - The high-frequency component generates strong Lorentz forces that intensify melt convection, transporting heat deeper into the workpiece.
Keyhole excitation - The rapid current pulses create transient high current density peaks that can momentarily exceed the threshold for keyhole formation, even at average current levels below the conventional keyhole threshold.
Heat source downward shift - The combined electromagnetic and acoustic effects push the effective heat source deeper, increasing penetration while reducing surface width.
Stable keyhole maintenance - Unlike single-pulse keyhole TIG, the dual-pulse mode maintains keyhole stability through the continuous modulation of current, reducing the risk of keyhole collapse.

The resulting weld joint achieves a tensile strength of 964 MPa with a joint strength ratio of 98% and an elongation of 3.7%, demonstrating near-equal-strength joint performance. This is remarkable for a TIG-welded titanium alloy joint, as conventional TIG welding of TC4 typically yields joint strength ratios of 80-90%.

Welding Microstructure and Properties

The deep penetration achieved by dual-pulse TIG creates a unique thermal cycle in the weld zone. The rapid heating and cooling associated with keyhole welding promotes fine grain formation in the weld metal. The microstructure of the weld zone is expected to consist of fine acicular alpha phase within a transformed beta matrix, similar to but finer than that produced by conventional K-TIG welding. The HAZ experiences peak temperatures in the alpha-beta field, producing a mixture of blocky and acicular alpha phases.

The near-equal-strength performance (98% joint strength ratio) indicates that the dual-pulse process achieves excellent metallurgical compatibility between weld metal and base metal. The elongation of 3.7%, while modest by titanium alloy standards, is acceptable for structural applications where strength is the primary concern.

Engineering Practice Implications

For medium-thickness TC4 titanium alloy fabrication, dual-pulse TIG welding offers several advantages over conventional processes:

Reduced number of passes - Deep penetration reduces the number of weld passes, minimizing multi-pass reheating effects and improving overall joint quality.
Higher productivity - The combination of deep penetration and high welding speed increases deposition rate significantly.
Reduced distortion - Lower total heat input compared to multi-pass conventional TIG reduces residual stress and distortion.
Better mechanical properties - Near-equal-strength joints eliminate the need for post-weld heat treatment in many applications.

However, dual-pulse TIG requires specialized power sources capable of generating the precise dual-frequency current waveform. The equipment cost is higher than conventional TIG, and the process requires careful parameter optimization for each material thickness and composition.

Key Questions and Reflections

The study demonstrates the technical feasibility of dual-pulse TIG for medium-thickness titanium alloy, but several practical questions remain. Can the process be scaled to thicker sections (10-20 mm)? What is the effect of welding position on dual-pulse stability? How does the process perform with different titanium alloys (TA5, TA15, Ti-6242)? The finite element model used in this study should be validated against experimental thermocouple measurements to ensure the predicted thermal cycles are accurate. Additionally, the long-term performance of dual-pulse TIG welds under cyclic loading and corrosion environments deserves investigation, as the fine microstructure may be susceptible to stress corrosion cracking in aggressive environments.

Study Insights and Implications

This paper represents a significant advancement in titanium alloy welding technology. The dual-pulse TIG process effectively overcomes the fundamental limitations of conventional TIG welding for medium-thickness sections by leveraging the synergistic interaction of low-frequency and ultrasonic current modulation. The near-equal-strength joint performance achieved (98% strength ratio) is particularly noteworthy and opens up new possibilities for titanium alloy structural fabrication. For engineering practice, the adoption of dual-pulse TIG requires investment in specialized equipment and process development, but the benefits in terms of productivity, joint quality, and reduced post-weld treatment make it a compelling option for high-value titanium alloy applications in aerospace and medical industries.