Deep Penetration Welding Mechanism of Medium-Thick Titanium Alloy Using Dual-Pulse TIG Process

Literature Overview

This paper, authored by Yang Qingfu, Luo Zhiwei, Zeng Caichou, Jiang Zihao, Cong Baoqiang, and Qi Baojin from Beihang University and Capital Aero-Mechanical Company, addresses a critical challenge in aerospace and pressure vessel manufacturing: the deep penetration welding of medium-thick titanium alloy plates using a dual-pulse TIG process. Published in Rare Metal Materials and Engineering in 2025, this work is supported by National Natural Science Foundation grants (U20B2031, 52075024). The research investigates the fundamental welding mechanism that enables deeper and more stable penetration compared to conventional single-pulse TIG welding.

Core Technical Content and Mechanism Analysis

The dual-pulse TIG process operates by superimposing two distinct current pulses within each welding cycle — a high-amplitude penetration pulse and a low-amplitude filling pulse. This dual-pulse architecture fundamentally alters the arc plasma dynamics, molten pool geometry, and solidification behavior compared to conventional pulse TIG.

The key mechanism lies in the temporal decoupling of penetration and filling functions. During the high-current penetration pulse phase, the arc concentrates energy at the pool center, driving deep penetration through enhanced arc pressure and electromagnetic stirring. During the low-current filling pulse, the arc transitions to a wider, more distributed heat input pattern that promotes lateral spreading and surface filling without excessive dilution.

The authors demonstrate that the dual-pulse configuration produces a narrower, deeper weld bead with reduced dilution of the base metal. This is particularly significant for titanium alloys, where excessive dilution with filler metal can compromise the metallurgical compatibility and corrosion resistance of the weld zone.

Microstructure and Mechanical Properties

The microstructural evolution in the dual-pulse TIG weld zone of titanium alloy reveals distinct characteristics. The fusion zone exhibits a refined acicular alpha-beta structure, with the alpha lamellae spacing reduced by approximately 30–40% compared to single-pulse TIG. This refinement is attributed to the enhanced thermal cycling effect and the controlled cooling rate achieved through the dual-pulse heat input profile.

The mechanical properties show notable improvements: tensile strength of the weld metal approaches the base metal values (within 5–8%), while impact toughness at room temperature is significantly enhanced. The grain boundary character of the heat-affected zone (HAZ) demonstrates reduced Widmanstätten alpha formation, indicating that the dual-pulse thermal cycle is gentler on the base metal microstructure than continuous or single-pulse processes.

Engineering Practice Implications

For bimetal pressure vessel fabrication involving titanium cladding or titanium alloy components, this dual-pulse TIG technology offers several practical advantages. The reduced dilution ratio means that when welding titanium-clad carbon steel vessels, the risk of iron contamination in the titanium overlay layer is minimized. The deeper penetration capability reduces the number of passes required for thick-section titanium components, improving productivity and reducing total heat input.

In the context of hydrogenation reactor fabrication, where titanium alloy weldments must resist hydrogen embrittlement and high-pressure hydrogen environments, the refined microstructure achieved through dual-pulse TIG welding provides superior resistance to intergranular cracking. The narrow weld geometry also facilitates more effective post-weld heat treatment (PWHT) to achieve the required stress relief without excessive grain growth.

Key Questions and Reflections

A critical question arising from this research is the scalability of dual-pulse TIG to production welding of large-diameter titanium pressure vessels. The mechanism described is well-established for laboratory-scale plates, but maintaining consistent dual-pulse arc stability over long welding lengths on curved surfaces presents significant engineering challenges. The arc length control, gas shielding coverage, and electrode wear characteristics all become more demanding as weld length increases.

Another important consideration is the interaction between dual-pulse TIG welding and subsequent cladding operations. If a titanium alloy base is to be clad with a nickel-based alloy (such as Inconel 625) for enhanced corrosion resistance, the microstructural state of the base weld must be carefully considered. The refined alpha-beta structure produced by dual-pulse TIG may require different cladding parameter windows than a conventionally welded base.

Study Insights and Summary

This research represents a significant advancement in understanding the fundamental physics of deep penetration TIG welding for titanium alloys. The dual-pulse approach elegantly solves the inherent contradiction between achieving deep penetration and maintaining low dilution — a challenge that has limited the application of TIG welding to thick-section titanium components. For practitioners in bimetal pressure vessel manufacturing, the key takeaway is that dual-pulse TIG provides a viable single-pass welding solution for titanium sections up to approximately 8–10 mm, which can substantially simplify fabrication sequences and improve joint quality. The mechanism understanding gained here also provides a foundation for developing hybrid processes that combine dual-pulse TIG with laser or plasma assistance for even thicker sections.