TC4 Titanium Alloy K-TIG Welded Joint Microstructure and Properties Investigation

Literature Overview

This research published in Heat Processing Technology (Volume 55, Issue 10, 2026, pages 61-67) by Jiang Tiantian and colleagues from Luoyang Ship Material Research Institute and the National Key Laboratory of Marine Corrosion and Protection investigates the K-TIG welding of 10 mm thick TC4 titanium alloy plates. TC4 (equivalent to Ti-6Al-4V) is the most widely used titanium alloy in aerospace, marine, and medical applications due to its excellent strength-to-weight ratio, corrosion resistance, and biocompatibility. The study explores the effect of welding current on penetration depth and weld width, establishes single-pass through-welding parameters, and comprehensively characterizes the weld joint microstructure and mechanical properties.

Core Technical Content and Process Analysis

K-TIG welding of titanium alloys presents unique challenges due to the material's high reactivity with atmospheric gases, high melting point, low thermal conductivity, and significant coefficient of thermal expansion. The keyhole effect in K-TIG welding enables deep penetration with single-pass welding, which is particularly advantageous for thick titanium plates where multi-pass welding would result in excessive heat input and adverse microstructural changes. However, achieving stable keyhole formation and maintaining weld quality throughout the process requires careful parameter control.

Welding Current Effects on Penetration and Geometry

The researchers systematically investigated the effect of welding current on weld penetration depth and weld width for 10 mm thick TC4 plates. The key findings were: at 520 A, complete penetration was achieved, indicating the threshold current for keyhole formation through the full plate thickness. At 580 A, single-face welding with double-side formation was achieved, meaning the weld bead formed properly on both the top and bottom surfaces without additional backing or filler material on the root side. This single-face, single-pass capability is a significant productivity advantage for thick plate welding.

Microstructural Characterization

The weld microstructure analysis revealed a complex microstructural evolution characteristic of titanium alloy welding. The weld metal exhibited a lamellar alpha phase, acicular martensitic alpha phase, and residual beta phase. This mixed microstructure is typical of titanium alloy welds cooled at relatively high rates, where the rapid solidification promotes the formation of fine acicular alpha structures within the transformed beta grains. The presence of residual beta phase indicates that the cooling rate was not sufficient to transform all beta phase to alpha during solidification and subsequent cooling.

In the heat-affected zone, two distinct regions were identified. The coarse grain zone contained equiaxed alpha phase and acicular alpha phase, while the fine grain zone contained equiaxed alpha phase and lamellar alpha phase. This microstructural variation reflects the different thermal histories experienced by different regions of the HAZ, with the coarse grain zone experiencing peak temperatures well above the beta transus temperature and the fine grain zone experiencing temperatures in the alpha-beta two-phase field.

Mechanical Property Evaluation

The hardness distribution across the weld joint showed that the upper portion of the weld had lower hardness values compared to the middle and bottom portions. The lower and middle regions of the joint exhibited uniform microhardness values with minimal variation between zones. This hardness distribution pattern is related to the microstructural differences, with the finer acicular structures in the middle and lower weld regions providing higher hardness through solid solution strengthening and precipitation effects.

A notable finding was that the impact toughness and impact strength of both the weld metal and heat-affected zone were higher than those of the base metal. This improvement in toughness is attributed to the grain refinement that occurs during the welding process. The rapid heating and cooling during welding refines the grain structure, particularly in the weld metal where the fine acicular alpha structures provide both strength and toughness. This is somewhat counterintuitive, as welding typically degrades mechanical properties, but in this case, the base metal likely had a coarser prior beta grain structure that was refined during welding.

Engineering Practice Implications

The K-TIG welding of 10 mm thick TC4 titanium alloy plates with single-pass penetration represents a significant advancement in titanium welding productivity. Traditional multi-pass welding of thick titanium plates requires extensive shielding gas coverage, multiple layers of weld metal, and careful interpass temperature control. The single-pass capability demonstrated in this study could substantially reduce welding time, gas consumption, and overall production cost while potentially improving joint quality by minimizing thermal cycling.

For aerospace applications, where titanium components are extensively used in airframes, engines, and landing gear, this technology could enable more efficient manufacturing of thick titanium structural components. In marine applications, where titanium is used for seawater systems, propellers, and underwater structures, the improved toughness of the welded joints is particularly beneficial for resisting impact loading and fatigue cracking.

Key Process Parameters and Results Summary

Study Insights and Reflections

This research provides valuable insights into the K-TIG welding of thick titanium alloy plates, demonstrating that single-pass welding of 10 mm TC4 is achievable with appropriate current levels. The microstructural analysis reveals the complex phase transformations that occur during welding and their implications for mechanical properties. The finding that welded joints exhibit higher toughness than the base metal is particularly encouraging and suggests that K-TIG welding may be preferable to certain heat treatment conditions for achieving optimal mechanical properties in titanium components.

One area that warrants further consideration is the long-term performance of these joints under fatigue loading and elevated temperatures. While the as-welded properties are excellent, titanium alloys can be susceptible to stress corrosion cracking and creep at elevated temperatures. Additionally, the residual stress distribution in single-pass welded thick plates should be characterized, as high residual stresses can negatively impact fatigue life and dimensional stability. For practical implementation, the shielding gas coverage requirements for single-pass welding of 10 mm plates must be carefully managed to prevent contamination, as titanium is extremely sensitive to oxygen and nitrogen pickup. Overall, this study establishes a solid foundation for the application of K-TIG welding in thick titanium plate manufacturing, with the potential to significantly improve productivity and joint quality in aerospace and marine industries.