Microstructure and Mechanical Properties of TA15 Titanium Alloy Laser Rapid Forming and Rolled Plate Argon Arc Weld Joints

Literature Overview

The paper by Du Borui, Tian Xiangjun, and Wang Huaming from the School of Materials Science and Engineering at Beihang University, published in the Welding Journal in 2013 (Vol. 34, No. 11, pp. 65-68), investigates the microstructure and mechanical properties of argon arc weld joints between laser rapid forming (LRF) TA15 titanium alloy and conventionally rolled TA15 titanium alloy thin plate. The research was funded by the National Basic Research Program of China (Grant No. 2011CB606305). The study addresses a practical engineering challenge: joining additively manufactured titanium alloy components to traditionally manufactured parts in aerospace and biomedical applications.

Laser Rapid Forming Process and Material Characteristics

TA15 titanium alloy is a near-alpha titanium alloy with excellent creep resistance and high-temperature strength, making it suitable for aerospace structural applications. The laser rapid forming process, also known as laser additive manufacturing or selective laser melting, produces components with a unique microstructure characterized by fine grains and columnar crystals oriented along the build direction. The rapid solidification rates and thermal cycling inherent in the LRF process create microstructural features that differ significantly from those of conventionally rolled or forged materials.

The rolled TA15 thin plate, by contrast, exhibits a fully recrystallized equiaxed grain structure resulting from the thermomechanical processing history of rolling and subsequent heat treatment. The microstructural contrast between the two materials creates a heterogeneous weld joint where the two sides of the weld respond differently to the welding thermal cycle.

Weld Microstructure Analysis

The weld metal microstructure of the LRF-to-rolled plate joint exhibits coarse plate-like alpha plus beta phases with epitaxial directional columnar crystal growth. This microstructure is a direct consequence of the heat input during GTAW welding combined with the pre-existing columnar grain structure of the LRF material. The epitaxial growth indicates that the weld metal solidification was influenced by the existing grain orientation of the LRF base metal, leading to preferential crystal growth along the build direction of the additive component.

The rolled plate side of the weld joint shows significant grain coarsening in the heat-affected zone (HAZ), which is a typical response of titanium alloys to high welding heat input. In contrast, the LRF material adjacent to the weld exhibits a transformation from columnar to equiaxed grains in the near-weld HAZ, while grains further from the weld retain their columnar morphology. This differential HAZ response is attributed to the different thermal histories and microstructural starting conditions of the two base materials.

Mechanical Properties and Joint Performance

The microhardness distribution across the weld joint reveals that the LRF material HAZ exhibits the highest hardness values, while the weld metal and the rolled plate HAZ show the lowest hardness. This distribution is consistent with the microstructural observations: the fine grain structure and retained columnar features in the LRF HAZ contribute to higher hardness, while the coarsened grains in the rolled plate HAZ and the weld metal reduce hardness.

The tensile strength of the weld joint is lower than that of the parent materials, which is a common phenomenon in titanium alloy welding due to the formation of brittle phases and grain coarsening in the HAZ. The ductility of the joint is comparable to that of the rolled plate material, and fracture occurred in the rolled plate HAZ, indicating that the rolled plate HAZ is the weakest link in the joint.

Engineering Practice Implications

The findings of this study have direct relevance to the emerging practice of hybrid manufacturing, where additively manufactured components are joined to conventionally manufactured parts. The heterogeneous nature of the weld joint requires careful process parameter selection to minimize the strength mismatch and to prevent premature failure in the rolled plate HAZ. Process modifications such as reduced heat input, preheating of the rolled plate, or post-weld heat treatment may be employed to improve joint performance.

For aerospace applications involving titanium alloy structures, the understanding of LRF-to-rolled plate weld joint behavior is essential for design qualification and airworthiness certification. The study provides the fundamental metallurgical data needed to develop welding procedure specifications for hybrid manufacturing scenarios.

Study Insights and Reflections

This research highlights the metallurgical challenges and opportunities presented by the integration of additive manufacturing with traditional fabrication methods. The differential thermal response of the two base materials creates a complex weld joint microstructure that must be understood and controlled to ensure structural integrity. The identification of the rolled plate HAZ as the fracture location underscores the importance of base material selection and pre-weld preparation in hybrid joint design. As additive manufacturing continues to gain acceptance in critical applications, the development of robust welding procedures for joining dissimilarly processed components will remain a key research and engineering challenge.