Zhuojin Pipe Fitting Co., LtdMicrostructure Characteristics of As-Deposited TC4 Titanium Alloy Thin-Walled Parts by Arc Additive Manufacturing
Literature Overview
The paper by Zhang Ping, Liu Debo, and colleagues from Harbin Institute of Technology's State Key Laboratory of Advanced Welding and Joining, along with Xinxiang Vocational and Technical College and Huanghe Science and Technology University, was published in Welding (2015, No. 11, pp. 53–56). The study investigates the microstructure characteristics of as-deposited TC4 titanium alloy thin-walled parts fabricated by arc additive manufacturing (AM). The research reveals two distinct types of striations and identifies two microstructural zones within the deposited parts, providing valuable insight into the thermal history and phase evolution during the additive process.
Core Technical Content and Microstructural Zoning
The arc additive manufacturing process deposits metal layer by layer using a welding arc as the heat source. Each newly deposited layer is subjected to the thermal influence of subsequent layers, creating a complex thermal history that varies significantly with position within the final part. The study identifies two distinct regions: the top region and the middle-to-lower region, each with fundamentally different microstructural characteristics.
| Region | Thermal History | Dominant Phases | Microstructural Features |
|---|---|---|---|
| Top region | Temperature above β-transus during final deposition | α′ martensite, α_m Widmanstätten, basket-weave, β | Residual high-temperature structure |
| Middle-to-lower region | Post-deposition heat treatment from subsequent layers | Basket-weave, colony, lamellar, residual α′, β | Thermally evolved structure |
Striation Analysis
Two types of striations are identified in the as-deposited thin-walled parts: parallel striations and arc-shaped striations. The parallel striations are attributed to solid-state phase transformations caused by the post-heating effect of subsequent layers, where the previously deposited metal is reheated above its transformation temperatures. The arc-shaped striations represent traces of the liquid phase contour during each deposition pass, capturing the instantaneous weld pool boundary at the moment of solidification. This dual-striation pattern provides a direct visualization of the thermal cycling history experienced by each layer during the additive build process.
The parallel striations are particularly significant because they indicate that even in as-deposited material, substantial solid-state phase evolution has occurred. The post-heating from subsequent layers drives the initially formed α′ martensite toward equilibrium phases through α′ decomposition and Widmanstätten α formation. This means that the microstructure observed in the as-deposited state is not a simple record of the solidification process but rather a complex product of both solidification and subsequent thermal evolution.
Phase Evolution Mechanisms
In the top region, where the final deposition pass maintained temperatures above the β-transus temperature, the microstructure reflects the direct solidification and cooling behavior of the last layer. The α′ martensite forms during rapid cooling from the β field, while α_m Widmanstätten plates develop from β grain boundaries. The presence of a small amount of basket-weave structure indicates localized regions where cooling rates permitted the formation of this characteristic morphology. The residual β phase corresponds to the prior β grain boundaries that did not fully transform during cooling.
In the middle-to-lower region, the progressive post-heating from each subsequent layer creates a gradient of thermal treatment severity. Layers closer to the top experienced more intense and prolonged post-heating, promoting more complete α′ decomposition and the development of basket-weave, colony, and lamellar structures. The basket-weave morphology, characteristic of TC4 at intermediate cooling rates, dominates in regions that received moderate post-heating. The lamellar structures indicate regions where the thermal cycling was sufficient to drive phase equilibrium toward the α+β equilibrium state.
Engineering Practice Implications
For engineers designing arc additive manufactured titanium alloy components, the microstructural zoning revealed in this study has direct implications for mechanical property prediction and post-processing requirements. The top region, with its higher proportion of α′ martensite, will exhibit higher hardness but lower ductility and potentially lower fatigue resistance compared to the middle-to-lower region. This property gradient must be considered in component design, particularly for load-bearing applications where uniform mechanical properties are required. Post-deposition heat treatment, such as stress relief or solution treatment and aging, becomes essential to homogenize the microstructure and achieve target mechanical properties throughout the part.
Key Reflections and Study Insights
The most important insight from this study is the recognition that arc additive manufacturing produces inherently heterogeneous microstructures that reflect the cumulative thermal history of the entire build process. Unlike conventional welding, where the thermal cycle is relatively well-defined for a single pass, additive manufacturing creates a multi-pass thermal history where each layer experiences the thermal influence of all subsequent layers. This means that the as-deposited microstructure is not merely a solidification product but a complex thermal evolution record. Engineers must approach additive manufactured parts with the understanding that microstructural and mechanical property gradients are intrinsic to the process, and that post-processing strategies must be designed to address these gradients systematically. The identification of striations as thermal history markers also provides a practical tool for process monitoring and quality assessment, as the striation pattern can be used to infer the thermal cycling conditions experienced during fabrication.