Welding Characteristics of TC4 Titanium Alloy T-Joint Using Double-Sided Double-Arc MIG Welding

Literature Overview

This 2024 study by Gao Fuyang and colleagues from the 725th Research Institute of China Shipbuilding Group Corporation investigates the welding characteristics of a TC4 (Ti-6Al-4V) titanium alloy T-joint configuration using a double-sided double-arc MIG (Metal Inert Gas) welding technique. The T-joint consists of a 30 mm thick TC4 panel (flange) and a 15 mm thick TC4 web plate, representing a thick-section structural configuration typical of marine and aerospace applications. The research employs high-speed photography for droplet transition observation, numerical simulation for temperature field analysis, and comprehensive microstructural and mechanical characterization of the resulting weld joints.

Core Technical Findings

The double-sided double-arc MIG welding technique demonstrated remarkable effectiveness for this thick-section T-joint configuration. Key findings include:

• The droplet transition mode was observed to be continuous globular transition, characterized by stable and uniform droplet detachment.
• The temperature field distribution was symmetric between the two welding sides when both arcs operated simultaneously.
• The weld metal microstructure consisted of coarse columnar grains with internal long needle-like alpha-prime (α'), lamellar alpha (α), and acicular alpha (α) phases.
• The panel-side heat-affected zone (HAZ) exhibited residual primary alpha, lamellar alpha, and residual beta phases.
• The web-side HAZ showed needle-like alpha, lamellar alpha, and residual beta phases.
• The tensile strength of the weld joint exceeded 969 MPa, with fracture occurring in the base metal rather than the weld or HAZ.

Welding Process Analysis

The double-sided double-arc MIG approach represents a sophisticated solution to the challenge of welding thick titanium alloy sections. The technique offers several distinct advantages over conventional single-sided welding:

1. Reduced heat input per side: By distributing the total energy input across two simultaneous arcs, the peak temperature on each side is reduced, minimizing the extent of the coarse grain HAZ.
2. Symmetric thermal cycling: The balanced temperature distribution promotes uniform microstructural evolution on both sides, reducing the risk of asymmetric residual stress development.
3. Improved weld geometry: The simultaneous filling from both sides produces a more uniform weld bead profile with reduced convexity or concavity.
4. Enhanced productivity: The technique effectively doubles the deposition rate compared to single-sided welding, which is particularly valuable for thick-section fabrication.

The high-speed photography observations of the droplet transition revealed that the continuous globular mode is maintained throughout the welding process, indicating stable arc conditions. This is consistent with the use of appropriate shielding gas composition (typically 99.99% pure argon for titanium welding) and optimized wire feed parameters.

Metallurgical Analysis

The microstructural evolution in TC4 titanium alloy welds is governed by the beta transus temperature (approximately 995°C for Ti-6Al-4V) and the cooling rate through this transformation range. The observed microstructures can be interpreted as follows:

• Weld metal: The coarse columnar grain structure is typical of solidification from a high-temperature beta phase. The internal phases (α', lamellar α, acicular α) reflect the cooling rate through the alpha-beta transformation range. The presence of alpha-prime indicates relatively rapid cooling in certain regions.
• Panel-side HAZ: The residual primary alpha phase indicates that this region experienced peak temperatures below the beta transus but above the alpha transformation range, allowing partial dissolution of the original alpha phase.
• Web-side HAZ: The needle-like alpha morphology suggests slightly different thermal conditions, possibly related to the thinner web plate geometry allowing faster cooling.

The hardness distribution, with HAZ hardness exceeding base metal values, is characteristic of titanium alloy welds and results from the precipitation of fine alpha phases during cooling. This is generally acceptable provided the hardness does not exceed limits that would compromise fatigue performance or stress corrosion resistance.

Engineering Practice Integration

The double-sided double-arc MIG technique has significant implications for titanium alloy fabrication in several industrial sectors:

• Marine engineering: Thick-section titanium alloy components for submarine hulls, propeller shafts, and pressure vessels benefit from this technique's ability to produce high-quality welds in thick sections.
• Aerospace structures: Titanium alloy T-joints are common in aircraft fuselage and wing structures, where the combination of high strength and low weight is essential.
• Medical implants: While typically involving thinner sections, the principles of controlled microstructural development demonstrated here are applicable to thicker implant components.

For compliance with relevant standards such as AWS D6.1 (Structural Welding Code - Titanium), ASME BPVC Section IX, or EN ISO 15614-1, the welding procedure must demonstrate that the mechanical properties meet the applicable requirements. The achieved tensile strength exceeding 969 MPa, with fracture in the base metal, indicates that the weld joint meets or exceeds the base metal requirements—a critical acceptance criterion.

Key Questions and Reflections

The research raises several important considerations for practical application. First, the coarse columnar grain structure in the weld metal, while producing acceptable mechanical properties, may be detrimental to fatigue performance and stress corrosion cracking resistance. Post-weld heat treatment (PWHT) at 550–650°C is typically required to refine the weld microstructure and reduce residual stresses, but this was not addressed in the current study.

Second, the symmetric temperature distribution achieved with double-sided welding assumes precise synchronization of the two welding processes. In practice, maintaining identical parameters on both sides simultaneously requires sophisticated equipment and process control. Any asymmetry in the setup could lead to differential cooling rates and potentially asymmetric microstructures.

Third, the study does not address the effects of welding sequence, interpass temperature control, or the influence of joint preparation (groove geometry, fit-up) on the final weld quality. These factors are critical in production welding of thick titanium sections.

Study Insights and Implications

This research demonstrates that the double-sided double-arc MIG technique is a viable and effective approach for welding thick TC4 titanium alloy T-joints, producing welds with mechanical properties that meet or exceed base metal requirements. The technique's potential for improving productivity while maintaining weld quality makes it particularly attractive for industrial applications involving thick titanium sections. For engineers developing welding procedures for titanium alloy fabrication, this study provides evidence that innovative process configurations can overcome the traditional limitations of single-sided welding for thick sections. However, the importance of post-weld heat treatment for microstructural refinement and residual stress relief cannot be overstated, and any production implementation should incorporate this essential step.