Microstructure and Mechanical Properties of TIG Welded TC4 Forged Titanium Alloy Plates

Literature Overview

This study by Zhang Hang and colleagues from CRRC Changchun Railway Vehicles Co., Ltd., in collaboration with Changchun University of Technology and Changchun Institute of Technology, investigates the microstructure, crystallographic orientation, and mechanical properties of TIG welded joints on 14 mm thick forged TC4 (Ti-6Al-4V) titanium alloy plates. The work was published in Iron and Steel Vanadium Titanium (2024, Vol. 45, No. 5) and supported by the Jilin Provincial Science and Technology Development Plan and the China Postdoctoral Science Foundation. The multi-pass butt welding was conducted using non-consumable tungsten electrode argon arc welding, and the results provide valuable data on the weldability limitations of this widely used aerospace and rail transit grade titanium alloy.

Core Technical Findings

The weld bead morphology was described as aesthetically sound, indicating acceptable process control for the multi-pass procedure. However, the microstructural analysis reveals significant phase transformations across the weld zone. The weld nugget zone is characterized by columnar grains and elongated acicular martensite alpha-prime (alpha') phase, which forms due to the rapid cooling rates inherent to TIG welding of titanium alloys. The heat-affected zone (HAZ) consists of a (alpha + beta) + alpha' mixed microstructure, while the base metal retains its original (alpha + beta) duplex structure typical of the forged condition.

The EBSD analysis provides particularly insightful crystallographic orientation data. The weld nugget zone exhibits an interlocking grain distribution with preferred orientations that vary by phase. Notably, the root pass weld shows a grain orientation distribution more aligned along specific crystallographic directions, whereas the base metal grain orientation distribution is relatively non-uniform. This orientation anisotropy is significant because it directly influences the directional mechanical response of the joint under service loading.

Mechanical Performance and Defect Analysis

The tensile strength of the welded joint was measured at 982 MPa with a post-fracture elongation of 6.0%, both of which are below the base metal values. This reduction is consistent with the formation of the brittle martensitic alpha' phase in the weld metal, which has limited plasticity. More critically, all bend test specimens exhibited cracking or fracture, indicating that the joint lacks the ductility required for forming operations. This is a fundamental weldability limitation of TC4 under TIG welding without post-weld heat treatment (PWHT).

The hardness profile from weld center to base metal shows a characteristic pattern: hardness first decreases and then increases. Within the weld nugget zone, the cap pass microhardness is slightly lower than the root pass. This difference is attributed to the thermal history of successive passes — the root pass cools more rapidly due to less thermal mass above it, promoting more alpha' formation, while the cap pass benefits from preheating by underlying passes, allowing slightly more alpha + beta equilibrium phases to form.

Engineering Practice Implications

For rail transit applications where TC4 components are used in high-stress structural elements, this study highlights several critical engineering considerations. The bend test failure rate of 100% suggests that any application requiring post-weld forming or cold work is not feasible without a tempering or solution treatment cycle. The 982 MPa tensile strength, while still high in absolute terms, represents a significant drop from typical forged TC4 plate properties, which can exceed 1100 MPa.

In practice, this finding mandates that TIG welded TC4 joints in safety-critical applications must undergo PWHT — typically a solution treatment at approximately 950-1000 degrees Celsius followed by controlled air cooling or furnace cooling — to convert the martensitic alpha' back to a ductile alpha + beta microstructure. The EBSD orientation data also suggests that residual stress patterns may be anisotropic, which should be considered in fatigue life predictions for railway bogie components.

Study Insights and Reflections

The combination of optical metallography, EBSD, and mechanical testing in this study provides a comprehensive characterization that goes beyond simple strength values. The observation that the root pass has a different crystallographic orientation preference than the cap pass is particularly valuable for engineers designing multi-pass weld procedures, as it suggests that the thermal history of each pass independently influences the resulting microstructure and potentially the fatigue initiation sites. The 6.0% elongation, while low, is not unexpected for an untempered titanium weld joint, but it underscores the necessity of post-weld treatment in any production environment. For rail transit manufacturers, this data should inform weld procedure qualification testing under relevant standards such as ASME Section IX or EN 15614, ensuring that PWHT parameters are included in the WPS to achieve acceptable bend test results.