TIG Arc Additive Manufacturing of Typical TC4 Frame Structures

Literature Overview

This paper, published in Hot Working Technology (2018, Vol. 47, Issue 1), investigates the application of TIG arc additive manufacturing (AM) technology for fabricating typical TC4 (Ti-6Al-4V) titanium alloy frame structures. The authors from Beijing Institute of Technology and the Ordnance Engineering Academy of the People's Liberation Army Army Ordnance Engineering College studied the effect of process parameters on single-track morphology, developed a 10° rotation strategy for achieving isotropic formation in square frame structures, and proposed a path planning approach for a "田" character (grid) frame structure with arc-shaped lap joints at cross intersections.

Process Parameters and Single-Track Formation

TIG arc additive manufacturing combines the benefits of arc welding (high deposition rate, low cost) with the design freedom of additive manufacturing. For TC4 titanium alloy, the key process parameters include welding current, arc voltage, travel speed, wire feed speed, and shielding gas flow rate. The study employed a controlled variable method to determine the optimal parameter range for stable single-track formation.

Parameter	Typical Range	Effect on Track Morphology
Welding Current	150–250 A	Higher current increases deposition width and height
Arc Voltage	18–25 V	Higher voltage increases arc length and heat input
Travel Speed	200–500 mm/min	Higher speed decreases deposition rate and track height
Wire Feed Speed	300–600 mm/min	Higher feed increases deposition volume
Shielding Gas Flow	15–25 L/min	Insufficient flow causes oxidation; excessive flow causes turbulence

The optimal parameter window for stable single-track formation on TC4 typically involves a heat input of 4–8 kJ/mm with a wire feed speed matched to the travel speed to maintain a consistent deposition rate. The heat input must be carefully controlled because TC4 has a relatively low thermal conductivity (approximately 7 W/m·K) and a high melting point (1668 °C), making it susceptible to both insufficient fusion at low heat input and excessive grain coarsening at high heat input.

Isotropic Formation Strategy for Frame Structures

A critical challenge in arc additive manufacturing of frame structures is the anisotropy introduced by the directional solidification of each deposited layer. As the arc moves along the build direction, the grain structure and mechanical properties develop directional characteristics that can lead to non-uniform behavior in multi-directional loading scenarios.

The study proposes rotating the welding torch and wire feed rod assembly by 10° relative to the build direction. This rotation strategy achieves isotropic stable formation by distributing the directional solidification effects across multiple orientations. The 10° angle was determined through experimental optimization to balance the competing effects of arc stability, wire feeding consistency, and grain orientation control.

For square frame structures, this rotation strategy ensures that the deposited material properties are relatively uniform regardless of the loading direction, which is essential for structural applications where frames may experience multi-axial stresses. The rotation also helps to mitigate the stair-step effect that can occur at corners and intersections when depositing material in a strictly linear path.

Path Planning for Grid Frame Structures

The "田" character frame structure, which consists of a grid pattern with internal cross members, presents additional challenges in path planning due to the need to deposit material at intersections where previously deposited tracks meet. The study proposes using arc-shaped lap joints at cross intersections to ensure smooth transitions and minimize defects.

The arc-shaped lap joint strategy involves curving the deposition path at intersections so that the new track gradually overlaps with the existing track rather than making an abrupt perpendicular transition. This approach has several advantages:

Reduced thermal stress: The gradual overlap allows heat to dissipate more evenly, reducing the risk of cracking at high-stress concentration points.
Improved fusion: The arc-shaped transition ensures complete fusion between adjacent tracks, minimizing porosity and lack of fusion defects.
Surface quality: The smooth transition produces a better surface finish at intersections, reducing the need for post-processing.
Mechanical integrity: The gradual overlap distributes mechanical loads more evenly across the intersection, improving fatigue resistance.

Engineering Practice Implications

For aerospace and defense applications where TC4 frame structures are used (such as satellite brackets, missile components, and aircraft structural elements), this study provides a practical framework for TIG arc additive manufacturing process development. The key engineering considerations include:

Layer thickness control: Each deposited layer should be 1–2 mm thick to balance deposition rate with heat input management and distortion control.
Interlayer temperature: Maintaining interlayer temperature between 100–200 °C is recommended to prevent excessive grain growth while ensuring adequate fusion with the previous layer.
Shielding gas management: Due to the high reactivity of titanium at elevated temperatures, the shielding gas envelope must be carefully designed to prevent oxidation of the deposited material and the heat-affected zone.
Post-build heat treatment: A stress-relief annealing at 540–620 °C followed by solution treatment and aging at 950 °C/2 h + 540 °C/6 h is recommended to achieve the desired mechanical properties and relieve residual stresses.

The TIG arc additive manufacturing approach offers significant cost and weight advantages over conventional machining of TC4 components, as titanium alloy machining generates substantial waste material and requires expensive cutting tools. The additive approach can achieve material utilization rates of 80–90% compared to 20–40% for machining, making it particularly attractive for complex frame geometries.

Study Insights and Reflections

This research demonstrates the practical viability of TIG arc additive manufacturing for TC4 titanium alloy structural components. The 10° rotation strategy for achieving isotropic formation is a simple yet effective solution to the anisotropy problem that has limited the structural applications of arc AM. The arc-shaped lap joint path planning approach for grid structures addresses a critical practical challenge that is often overlooked in academic studies but is essential for production implementation. The study's emphasis on path planning and process optimization rather than purely on material characterization reflects a mature understanding of the requirements for industrial adoption of additive manufacturing. For future development, the integration of real-time process monitoring (such as arc voltage and current monitoring, thermal imaging, and acoustic emission) with adaptive path planning will be essential for achieving the dimensional accuracy and mechanical property consistency required for critical aerospace applications. The TIG arc AM process, with its relatively low equipment cost and high deposition rate, represents a promising bridge between traditional manufacturing and advanced additive technologies for titanium alloy structural components.