Comparative Study of EB Welding and TIG Welding Microstructures and Mechanical Properties in TC4 Titanium Alloy

Literature Overview

This study, published in Hot Working Technology (2012, Vol. 41, No. 7, pp. 105-108), was conducted by Zhang Fengying, Deng Juanli, Jiang Chaoping, and Zhang Xuemin from the School of Materials Science and Engineering at Chang'an University. Supported by the State Key Laboratory of Solidification Technology at Northwestern Polytechnical University and the China Postdoctoral Science Foundation, this work provides a systematic comparison between electron beam (EB) welding and tungsten inert gas (TIG) welding of TC4 (Ti-6Al-4V) titanium alloy, focusing on solidification microstructure, micro-phase structure, and room-temperature tensile properties of the weld joints.

Core Technical Findings

The researchers examined the solidification microstructure, micro-phase composition, and static tensile properties at room temperature for both EB-welded and TIG-welded TC4 joints. The findings reveal fundamental differences in thermal cycles and their metallurgical consequences.

Microstructure Comparison

In the TIG weld joint, the heat-affected zone (HAZ) exhibits coarse equiaxed grains, while the weld zone displays coarse columnar crystals. The columnar grain growth direction initially starts perpendicular to the weld-HAZ interface and gradually transitions to vertical upward growth. This directional transition is a direct consequence of the relatively low heat input and slower cooling rate characteristic of TIG welding, which allows grain growth to respond to changing thermal gradients during solidification.

In contrast, the EB weld joint also shows equiaxed grains in the HAZ and columnar crystals in the weld zone, but both grain types are significantly finer than those in the TIG weld. Crucially, the columnar grain growth direction remains consistently perpendicular to the weld-HAZ interface throughout the entire weld zone. This is attributed to the extremely high energy density and rapid solidification rate of EB welding, which creates a steep and stable thermal gradient that directs grain growth uniformly.

Micro-Phase Structure

The micro-phase structure within the original β grains of the TIG weld zone consists of Widmanstätten α plates, acicular martensitic α′, and β matrix. The EB weld zone, however, contains predominantly fine acicular martensitic α′ and β matrix, with significantly reduced Widmanstätten α plate content. This difference arises because the much faster cooling rate in EB welding suppresses the diffusion-controlled Widmanstätten α transformation and promotes the diffusionless martensitic transformation.

Mechanical Properties

The tensile testing results demonstrate that the EB weld joint exhibits slightly higher strength than the TIG weld joint, while the ductility of the EB weld joint is significantly superior to that of the TIG weld joint. This combination of improved strength and substantially enhanced ductility makes EB welding the preferred process for critical TC4 applications where joint reliability is paramount.

Engineering Practice Implications

From a practical engineering standpoint, this comparison carries significant implications for the selection of welding processes in titanium alloy applications, particularly in aerospace structures and high-performance pipe fittings. The superior ductility of EB welds is especially important for components subjected to cyclic loading or impact, where crack initiation and propagation are critical failure modes.

For pipe fitting fabrication, where geometric complexity often precludes EB welding, TIG welding remains the workhorse process. However, the study highlights areas for improvement: controlling TIG welding parameters to minimize HAZ grain coarsening, and considering post-weld heat treatment to refine the microstructure and improve joint properties. The observation that Widmanstätten α plates are detrimental to ductility suggests that thermal cycling procedures aimed at dissolving Widmanstätten α and promoting equiaxed α could enhance TIG weld performance.

Key Reflections

The fundamental lesson from this study is that welding process selection is not merely a matter of capability but of metallurgical consequence. The energy density difference between EB and TIG welding—typically 10³ W/mm² for EB versus 10¹-10² W/mm² for TIG—translates directly into grain refinement, phase control, and mechanical property differences. For engineers working with titanium alloy pipe fittings, this underscores the importance of understanding the thermal cycle's metallurgical impact rather than treating welding as a purely mechanical joining operation.