As-Deposited Microstructure Characteristics of Arc Additive Manufactured TC4 Titanium Alloy Thin-Walled Parts

Literature Overview

This paper, published in Welding (Issue 11, 2015, pp. 53-56), was authored by researchers from Xinxiang Vocational and Technical College, Huanghe University of Technology, and the State Key Laboratory of Advanced Welding and Joining at Harbin Institute of Technology. The study investigates the as-deposited microstructure of TC4 titanium alloy thin-walled parts manufactured by arc additive manufacturing (AAM). This is a significant contribution to the understanding of microstructural evolution in additively manufactured titanium alloys, which is critical for predicting the mechanical behavior and service performance of AM components.

Core Technical Findings

The study identified two types of striations in the as-deposited thin-walled parts: parallel striations and arc striations. Parallel striations are the result of solid-state phase transformation caused by post-heating effects from subsequent deposition layers. Arc striations are traces of the liquid phase contour during the metal deposition process. The as-deposited parts can be divided into two distinct regions: the top region and the middle-to-bottom region.

Microstructural Zones in As-Deposited Parts

Region	Thermal History	Microstructure	Phase Composition
Top region	Last layer deposition, temperature above beta transformation temperature	Alpha-prime, alpha-m, small amount of basket-weave, beta	Alpha-prime, alpha-m, basket-weave, beta
Middle-bottom region	Post-heating from subsequent layers, varying degrees	Basket-weave, colony, lamellar, small residual alpha-prime, beta	Basket-weave, colony, lamellar, residual alpha-prime, beta

The top region experienced the highest temperature during the final deposition layer, remaining above the beta transformation temperature (approximately 995°C for TC4). This resulted in a microstructure dominated by alpha-prime (martensitic) phase and alpha-m (metastable alpha) phase with small amounts of basket-weave and beta phase. The middle-to-bottom region underwent varying degrees of post-heating from subsequent layers, leading to a more complex microstructure with basket-weave, colony, lamellar, and residual alpha-prime phases.

Engineering Practice Implications

The microstructural heterogeneity observed in arc additive manufactured TC4 parts has profound implications for mechanical property prediction and component design. The top region, with its martensitic alpha-prime phase, is expected to exhibit higher strength but lower ductility compared to the middle-to-bottom region. The basket-weave and lamellar structures in the middle-to-bottom region provide a better balance of strength and toughness.

For engineering applications, this microstructural variation must be accounted for in component design and post-processing. Stress-relief annealing can homogenize the microstructure by transforming alpha-prime to alpha-plus-beta, but the degree of transformation will vary across the part thickness. The study provides essential baseline data for developing appropriate heat treatment schedules for AAM TC4 components.

Comparison with Conventional TC4 Microstructures

Microstructure Type	Strength	Ductility	Fatigue Resistance	Typical Application
Alpha-prime (as-deposited top)	High	Low	Poor	Requires post-heat treatment
Basket-weave (middle-bottom)	Moderate	Good	Good	Structural components
Lamellar (post-heat treated)	High	Moderate	Excellent	Aerospace fasteners
Equiaxed alpha (post-heat treated)	Moderate	High	Good	Pressure vessels

Study Insights and Reflections

The identification of parallel striations and arc striations provides valuable insight into the thermal-metallurgical history of arc additive manufactured parts. The parallel striations, caused by post-heating from subsequent layers, represent a unique feature of additive manufacturing that does not exist in conventional welding. This post-heating effect creates a gradient in microstructure through the part thickness, with the bottom layers experiencing the most post-heating and the top layers experiencing the least.

The study's classification of the as-deposited part into top and middle-to-bottom regions is a practical and useful framework for understanding microstructural evolution. In production settings, this knowledge can be used to predict where mechanical property variation is most likely and to design appropriate post-processing routes. For example, parts with critical fatigue life requirements should be designed to avoid placing high-stress regions in the top zone where alpha-prime martensite is prevalent.

The research also highlights the importance of process parameters in controlling the as-deposited microstructure. Deposition speed, heat input, layer thickness, and interlayer temperature all influence the thermal cycle and, consequently, the microstructure. The study provides a foundation for developing process-microstructure-property relationships that can guide the optimization of arc additive manufacturing parameters for TC4 titanium alloy.

In summary, this study establishes a clear understanding of the microstructural characteristics of as-deposited arc additive manufactured TC4 titanium alloy thin-walled parts, demonstrating that the thermal history creates distinct microstructural zones with different phase compositions, which must be carefully considered in component design, post-processing, and quality assurance for additive manufacturing applications.