Microstructure and Mechanical Properties of TC4 Titanium Alloy K-TIG Welded Joints

Literature Overview

The research by Cui Shu-wan, Shi Yong-hua, and Zhang Cheng-shi, published in the Transactions of Nonferrous Metals Society of China in 2021, examines the weld metal zone (WMZ) microstructure, grain boundary characteristics, and mechanical properties of 12 mm-thick Ti-6Al-4V (TC4) titanium alloy plates welded by keyhole TIG (K-TIG) welding at varying heat inputs ranging from 2.30 to 2.62 kJ/mm. The study integrates optical microscopy, electron backscatter diffraction (EBSD), tensile testing, and Charpy impact testing to establish comprehensive heat input–structure–property relationships.

Core Technical Findings

Microstructure Evolution

The K-TIG welds exhibited good formation with no obvious defects across the entire heat input range studied. The microstructural evolution in the WMZ follows a clear trend:

As heat input increases, α-laths grow longer due to the extended time at elevated temperatures allowing for greater α-phase growth. Simultaneously, the α′ martensitic phase and residual β phase both decrease, indicating more complete β-to-α transformation during cooling. The EBSD results confirm that the proportion of high-angle grain boundaries in the WMZ increases with heat input, suggesting that higher thermal cycles promote grain boundary migration and recrystallization.

Mechanical Properties

The tensile strength of the WMZ gradually decreases with increasing heat input, while elongation and impact toughness both increase. This classic strength–ductility trade-off is driven by the coarsening of α-laths and the reduction of α′ martensite, which are harder but more brittle phases.

Process Analysis and Standards Context

K-TIG welding represents a significant advancement over conventional TIG for thick-section titanium alloy welding. The keyhole mechanism enables deeper penetration and higher deposition rates, making it suitable for pipe and fitting applications where wall thicknesses of 8–20 mm are common. The heat input range of 2.30–2.62 kJ/mm studied here is relatively moderate for K-TIG, indicating that even within a narrow window, significant microstructural and property variations can occur.

For titanium alloy pipe applications governed by standards such as ASTM B381, AMS 4911, or GB/T 36210, the weld metal mechanical properties must meet or exceed specified minimum values. The study demonstrates that heat input control is critical: excessive heat input reduces tensile strength, potentially falling below the base metal specification, while insufficient heat input results in brittle α′-rich microstructures with poor impact toughness.

Engineering Practice Implications

1. Heat input windowing for K-TIG welding of TC4 titanium alloy should target the 2.30–2.62 kJ/mm range for 12 mm plate thickness. Process windows narrower than this may be required depending on the specific property requirements of the application.
2. Post-weld heat treatment may be necessary to homogenize microstructure and relieve residual stresses, particularly when welding thick-walled pipes or complex fittings where heat input variation between passes is unavoidable.
3. EBSD-based grain boundary analysis should be considered for qualification testing of critical titanium alloy welds, as high-angle GB proportion provides a quantitative indicator of microstructural quality that correlates with mechanical performance.
4. Multi-pass welding strategies must account for the cumulative heat input effect on previously deposited layers, as each subsequent pass acts as a thermal cycle that further modifies the microstructure of underlying weld metal.

Key Questions and Reflections

A notable observation is that no obvious defects were found across the entire heat input range, which suggests that K-TIG offers robust defect avoidance capability for titanium alloy welding. However, the study does not address hot cracking susceptibility, which is a well-known concern for titanium alloys with wide solidification ranges. The absence of cracking in this study may be attributed to the relatively low heat inputs used or to the specific plate composition.

The increase in high-angle GB proportion with heat input is mechanistically interesting. Higher thermal cycles promote grain boundary migration, but the driving force and kinetics of this process in rapidly solidifying weld metal warrant further investigation through thermodynamic modeling and simulation.

Summary

This study establishes clear heat input–microstructure–property relationships for K-TIG welding of TC4 titanium alloy, providing valuable guidance for process optimization in titanium alloy pipe and fitting fabrication. The finding that moderate heat inputs within a narrow window can achieve good weld formation without defects, combined with the demonstrated microstructural evolution trends, offers a practical framework for qualifying K-TIG processes for aerospace and energy industry applications where titanium alloy integrity is paramount.