| Author: | Dan, Xingdong |
| Title: | Developing high-performance titanium alloys through additive manufacturing |
| Advisors: | Chen, Zibin (ISE) Chan, K. C. (ISE) |
| Degree: | Ph.D. |
| Year: | 2026 |
| Department: | Department of Industrial and Systems Engineering |
| Pages: | xxvii, 186 pages : color illustrations |
| Language: | English |
| Abstract: | Titanium and its alloys have long been valued for their high strength-to-weight ratio, corrosion resistance, and biocompatibility, making them crucial in aerospace, biomedical, and energy applications. Despite these advantages, a long-standing challenge persists in achieving an optimal balance between strength and ductility. Conventional alloying and thermomechanical processing strategies often enhance one property at the expense of the other. Recently, heterostructured materials have emerged as a promising design concept to overcome this trade-off by combining soft and hard phases to promote hetero-deformation-induced (HDI) strengthening. Simultaneously, additive manufacturing (AM), particularly laser-directed energy deposition (L-DED), enables spatial control of alloy composition and microstructure in a layer-by-layer manner, providing a new platform for the fabrication of titanium alloys with in-situ alloying and engineered heterostructures. This thesis explores AM-enabled strategies to enhance the mechanical performance of titanium alloys (especially α-titanium alloys) via two synergistic approaches: (i) in-situ aluminum alloying to optimize solid-solution strengthening and microstructural refinement, and (ii) spatially controlled compositional and structural heterogeneity to harness interface-mediated strengthening mechanisms. Chapter 1 introduces the background of titanium and its alloys, including phase stability (α/α+β/β systems), recognized microstructures (equiaxed, lamellar, bimodal), and the strength-ductility dilemma; it also outlines the promise of AM for titanium alloys and key concepts in heterostructured materials (definitions, typical architectures—bimodal, gradient, lamellar—and their strengthening effects in overcoming the trade-off). Chapter 2 describes the experimental methodology, including L-DED (LENS™) processing under controlled atmosphere, parameter selection and scan strategy, and fabrication of both homogeneous and heterogeneous Ti-Al builds alongside as-cast comparators; the comprehensive characterization methods (X-ray diffraction, optical microscopy, scanning electron microscopy with energy-dispersive spectroscopy, electron probe microanalysis, electron backscatter diffraction, transmission/scanning transmission electron microscopy), the mechanical property evaluation testing (vickers microhardness, uniaxial tension with digital image correlation), and thermal property measurements. In Chapter 3, a series of Ti-Al alloys with Al concentrations ranging from 0 to 6 at% were fabricated via LENS™. Compared to conventional as-cast counterparts, the AM-fabricated Ti-6Al samples exhibited ~90% improvement in yield strength and ~100% enhancement in ductility. These superior properties are attributed to the formation of refined basketweave α grains and high dislocation density induced by the rapid solidification conditions of AM. Chapter 4 builds upon this alloying strategy by designing a heterogeneous multi-gradient Ti/Ti-10Al architecture, achieved through dynamic modulation of powder feedstock during printing. The resulting structure consisted of alternating soft and hard regions with gradient composition and microstructure. This heterostructure achieved an impressive combination of ~760 MPa yield strength and ~33% fracture strain — outperforming both homogeneous Ti and Ti-10Al samples. The enhanced mechanical synergy is governed by the gradient stress partitioning, strain delocalization, and ductility compensation across chemically graded interfaces. In Chapter 5, a lamellar Ti-3.5Al/Ti-6Al-4V heterostructure was developed to investigate the role of interfacial stresses in AM. The significant mismatch in thermal expansion coefficients (CTE) between the layers generated residual stress fields and geometrically necessary dislocations (GNDs), leading to strong back-stress hardening. This engineered structure exhibited a yield strength of ~1090 MPa and an elongation of ~10%, simultaneously exceeding the strength and ductility of homogeneous Ti-6Al-4V. Microstructural analyses confirmed that hetero-deformation-induced (HDI) hardening and crack deflection at interfaces played key roles in the observed mechanical enhancements. Chapter 6 consolidates the major findings of this thesis into a unified framework, demonstrating how in-situ alloying, spatially engineered heterostructure, and interface design can be effectively integrated through AM to overcome the conventional strength-ductility trade-off in titanium alloys. Chapter 7 outlines future research directions, including: (i) the application of post-processing treatments such as cold rolling, annealing, and heat treatments to assess their impact on microstructural stability and mechanical synergy in AM-fabricated heterostructures; (ii) the integration of real-time monitoring during L-DED to enable microstructure-by-design strategies; and (iii) systematic comparisons between AM-induced and conventionally fabricated heterostructures to evaluate interfacial characteristics, dislocation-based strengthening mechanisms, and strain hardening behavior. |
| Rights: | All rights reserved |
| Access: | open access |
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