Microstructure and Tensile Properties of TB8 Titanium Alloy TIG Weld Joints

Literature Overview

This study, published in the Journal of Chinese Nonferrous Metals in 2019 and funded by the Shaanxi Provincial Natural Science Foundation (Grant No. 2018JM5142), investigates the weld microstructure evolution and tensile mechanical properties of TB8 titanium alloy plates joined by gas tungsten arc welding (GTAW / TIG). The authors, Ma Quan and Cao Di from Baoji University of Arts and Sciences, address a critical gap in the welding technology of beta-phase titanium alloys, which are increasingly deployed in aerospace structural components due to their superior formability and high-temperature strength characteristics.

Core Technical Content

TB8 titanium alloy corresponds to the Ti-3Al-8V composition system, classified as a near-beta titanium alloy. The microalloying of vanadium as a strong beta stabilizer shifts the alpha-plus-beta transus temperature (beta transus, T_beta) to approximately 950 degrees Celsius, enabling full solution treatment in the beta field. This fundamental metallurgical feature dictates the weld solidification and subsequent cooling microstructure, which is the central focus of the investigation.

The study examines how varying TIG welding parameters — including welding current, travel speed, arc voltage, and shielding gas flow rate — influence the weld bead geometry, heat-affected zone (HAZ) width, and the resulting microstructural constituents. Typical welding parameters investigated for TB8 plates of 6 mm to 12 mm thickness are summarized below.

The microstructural analysis reveals that the weld metal exhibits a fully acicular martensitic beta (alpha-prime) structure due to the rapid solidification cooling rates inherent to the TIG process. In the HAZ, a gradient of microstructures develops: the coarse-grained HAZ (CGHAZ) shows Widmanstatten alpha plates growing from the prior beta grain boundaries, while the fine-grained HAZ (FGHAZ) retains a finer alpha-plus-beta equiaxed morphology. The base metal microstructure, typically a beta matrix with small amounts of primary alpha, remains largely unchanged beyond the HAZ boundary.

Microstructural Evolution Analysis

The key metallurgical insight from this work is the quantitative relationship between cooling rate and the resulting microstructural features in the TB8 weld zone. The authors employ optical microscopy (OM) and scanning electron microscopy (SEM) to characterize the weld cross-sections, supplemented by X-ray diffraction (XRD) for phase identification and electron backscatter diffraction (EBSD) for crystallographic texture analysis.

In the weld center, cooling rates exceeding 100 degrees Celsius per second produce a fully martensitic transformation, yielding needle-like alpha-prime lamellae with an average thickness of 0.5 to 2 micrometers. As the cooling rate decreases toward the fusion line, the microstructure transitions to a Widmanstatten pattern of alpha plates within a beta matrix. The prior beta grain size in the CGHAZ can reach 100 to 200 micrometers, which is significantly coarser than the base metal grain size of approximately 50 micrometers. This grain coarsening in the CGHAZ is a well-documented phenomenon in titanium alloy welding and represents a potential site for crack initiation under cyclic loading.

The authors also discuss the role of the alpha-prime phase on mechanical properties. While the martensitic beta structure provides high strength, it inherently possesses limited ductility and toughness. The tensile strength of the weld joint typically reaches 950 to 1100 MPa, which is comparable to or slightly exceeds the base metal strength of approximately 930 MPa (for the as-received condition). However, the elongation of the weld metal is reduced to 10 to 14 percent compared to the base metal elongation of 15 to 18 percent, reflecting the embrittling effect of the fully martensitic microstructure.

Engineering Practice Implications

From a practical fabrication standpoint, the findings of this study carry several important implications for engineers involved in titanium alloy component manufacturing. First, the fully martensitic weld microstructure, while strong, may require post-weld heat treatment (PWHT) to restore ductility and toughness. A typical solution treatment at 980 to 1020 degrees Celsius followed by air cooling or aging at 540 degrees Celsius for 2 hours can transform the alpha-prime martensite into a more ductile alpha-plus-beta equiaxed microstructure.

Second, the coarse prior beta grains in the CGHAZ represent a metallurgical weakness that must be addressed through proper welding procedure specification. Reducing the heat input by using higher travel speeds or pulsed TIG welding can limit the CGHAZ width and mitigate grain coarsening. Third, the shielding gas strategy is critical for titanium alloys due to their extreme susceptibility to interstitial pickup of oxygen, nitrogen, and hydrogen. The study confirms that argon shielding with a minimum flow rate of 12 L/min, combined with a trailing gas shield on the back of the weld, is essential to prevent surface oxidation and intergranular embrittlement.

Key Questions and Reflections

Several questions emerge from this study that merit further investigation. The transition from fully martensitic weld metal to Widmanstatten HAZ microstructure creates a significant mechanical property gradient across the weld zone. Under service conditions involving cyclic or impact loading, this gradient could promote crack initiation and propagation at the fusion line or within the CGHAZ. Fatigue and fracture toughness studies on TB8 TIG weld joints, particularly with respect to the CGHAZ, would be valuable complements to the present tensile property data.

Furthermore, the study does not extensively address the effect of interpass temperature control on multi-pass welds, which is a common scenario in thick-section titanium alloy fabrication. For multi-pass TIG welds on TB8 plates thicker than 15 mm, maintaining an interpass temperature below 150 degrees Celsius is recommended to avoid excessive grain growth and to preserve the beneficial precipitation strengthening response during subsequent heat treatment.

Study Insights and Implications for Bimetal Fabrication

The metallurgical principles elucidated in this study are directly transferable to bimetal pressure vessel fabrication involving titanium alloy cladding layers. When titanium clad plates are welded to carbon steel or stainless steel backing layers, the thermal cycles impose on the titanium side are governed by the same fundamental solidification and transformation kinetics. The beta-phase transformation behavior of TB8, and by extension other beta and near-beta titanium alloys such as Ti-10V-2Fe-3Al (VT10), is a critical consideration in the design of welding procedures for titanium-clad pressure vessels.

In practice, the welding of titanium clad plates requires careful thermal management to avoid excessive dilution of the titanium cladding layer by the base steel substrate. The weld overlay procedures must be designed to limit the dilution ratio to below 5 percent for titanium cladding on carbon steel, which is significantly more restrictive than the dilution limits applicable to stainless steel or nickel alloy cladding. The microstructural sensitivity of titanium alloys to interstitial contamination also demands that the welding environment be meticulously controlled, with back-purge gas systems and sealed chambers being standard practice for titanium clad vessel fabrication.

The tensile property data presented in this study provides a baseline for qualification testing of titanium alloy weld procedures under standards such as NB/T 47014 and ASME Section IX. Engineers involved in the design and fabrication of titanium-clad pressure vessels should use the weld joint strength ratios and elongation values reported here as reference points when evaluating procedure qualification results and determining acceptable welding parameter ranges for production welding.