FSSP Enhancement of TA15 Titanium Alloy TIG Welded Joint Microstructure and Microhardness

Literature Overview

This paper, published in the Transactions of Nonferrous Metals Society of China (2018, Vol. 28, No. 1, pp. 55-65), presents a systematic investigation of the effects of friction stir spot processing (FSSP) on the microstructure and microhardness of TIG-welded TA15 titanium alloy joints. The research was conducted at Northwestern Polytechnical University's State Key Laboratory of Solidification Processing and the Shaanxi Key Laboratory of Friction Welding Technologies. The work was supported by the National Natural Science Foundation of China (Project 51405389), Fundamental Research Funds for the Central Universities (3102015ZY024), and the Shanghai Key Laboratory of Digital Manufacture for Thin-walled Structures (2014003).

Core Technical Findings

The study demonstrates that FSSP effectively improves both the microstructure and microhardness of TA15 alloy sheets and TIG-welded joints. The most significant finding is the substantial increase in average microhardness in the weld nugget zone after FSSP treatment, while the heat-affected zone (HAZ) shows only a modest hardness improvement. This differential response reflects the fundamental difference between the plastic deformation and dynamic recrystallization in the stirring zone versus the thermal-only influence in the HAZ.

The stirring zone exhibits two peak hardness values along the width direction, which is attributed to the asymmetric material flow pattern characteristic of friction stir processing. The surface forms a bright white layer composed of fine grains, while surface oxidation remains not evident under the investigated processing parameters.

Microstructural Analysis

The microstructural evolution in the FSSP-treated zones can be understood through the following mechanisms:

• Stirring zone: Dynamic recrystallization produces ultra-fine grains through intense plastic deformation and elevated temperature. The two hardness peaks along the width direction correspond to regions of maximum strain rate and grain refinement, typically associated with the leading and trailing edges of the tool interaction.
• Weld nugget zone: The pre-existing coarse weld microstructure undergoes partial dynamic recrystallization and grain refinement through the severe plastic deformation imposed by the FSSP tool. This results in a significant hardness increase, partially compensating for the softening that typically occurs in TIG weld zones.
• HAZ: Limited plastic deformation in this zone results in only modest microstructural refinement and hardness improvement. The thermal cycle of FSSP may contribute to some precipitation changes, but the effect is secondary to the deformation-induced refinement.

Engineering Practice Implications

For titanium alloy pipe and fitting fabrication, the combination of TIG welding followed by FSSP post-weld treatment offers a promising approach to enhance local mechanical properties without the need for extensive post-weld heat treatment. Several practical considerations emerge from this study:

1. Surface finish improvement: The bright white fine-grain layer on the FSSP-treated surface provides improved surface integrity, which is particularly beneficial for corrosion-sensitive applications such as chemical processing pipes and marine structural components.
2. Localized strengthening: The ability to selectively strengthen specific weld zones through FSSP provides a tool for addressing stress concentration areas in complex pipe geometries such as tees, reducers, and elbows.
3. Oxidation control: The finding that surface oxidation is not evident under the investigated parameters is encouraging for titanium alloy applications, where oxidation resistance is a primary design consideration.

However, several limitations must be acknowledged when considering FSSP for industrial pipe fabrication:

• Geometry constraints: FSSP is limited to relatively flat or slightly curved surfaces, making it less suitable for tight-radius pipe bends or complex three-dimensional geometries.
• Tool wear and cost: The severe deformation conditions imposed on the FSSP tool result in significant tool wear, increasing production costs for high-volume manufacturing.
• Residual stress introduction: While the study focuses on microstructure and hardness, FSSP inevitably introduces residual stresses that may affect fatigue performance and dimensional stability.

Key Reflections and Technical Insights

The asymmetric hardness distribution with two peaks in the stirring zone is a characteristic signature of friction stir processing and reflects the complex material flow dynamics around the rotating tool. From a process control perspective, this asymmetry must be accounted for in applications where uniform mechanical properties are required, such as in pressure vessel welds subject to hydrostatic testing.

The significant hardness improvement in the weld nugget zone is particularly noteworthy because TIG welds in titanium alloys typically exhibit reduced hardness compared to the base metal due to grain coarsening during the welding thermal cycle. The ability of FSSP to partially restore hardness through dynamic recrystallization provides a practical solution to this common problem, potentially reducing the need for costly post-weld heat treatment cycles.

The absence of evident surface oxidation is a critical finding for titanium alloy applications. Unlike many post-weld treatment processes that require elevated temperatures and may introduce oxidation, FSSP operates at temperatures below the melting point and in a relatively inert environment provided by the tool workpiece contact. This makes it particularly suitable for titanium alloys where even thin oxide layers can significantly degrade corrosion resistance.

Reference Value and Outlook

This study provides compelling evidence that FSSP can serve as an effective post-weld treatment for titanium alloy TIG welds, improving both microstructure and mechanical properties. The technique is particularly promising for applications where localized strengthening is required, such as in high-stress regions of pipe fittings and structural components. Future research should investigate the effects of FSSP on fatigue crack propagation resistance, evaluate the technique's applicability to thick-section welds, and develop process windows that minimize residual stress while maximizing microstructural refinement.