Influence of Thermal Cycling on Interfacial Stability of Additively Manufactured Titanium Alloys
Introduction to Thermal Cycling in Additive Manufacturing
Additive manufacturing (AM) of titanium alloys involves a unique thermal history characterized by rapid solidification and repeated thermal cycling during successive layer deposition. Unlike conventional wrought processing, each deposited layer undergoes multiple reheating and cooling cycles as subsequent layers are built upon it, creating complex thermal gyrations that profoundly affect microstructural evolution and interfacial stability.
Formation of Interfacial Microstructures
In Ti-6Al-4V produced by wire arc additive manufacturing (WAAM), the as-built microstructure typically consists of coarse prior β grains filled with aligned α-lath colonies, formed during the β→α transformation upon cooling. The repeated thermal cycling during deposition produces a high fraction of high-angle grain boundaries (HAGBs, >15°) and creates nanoscale β films along α-lath boundaries. These β films, enriched with vanadium (a β-stabilizing element), form coherent α/β interfaces that serve as effective barriers to dislocation motion and contribute significantly to the alloy's high strength.
Effects of Thermal Cycling on Interface Stability
1. Interface Movement and Solute Redistribution
During thermo-mechanical cycling between 400°C and 700°C, the α/β interface exhibits dynamic movement driven by solute redistribution. Synchrotron radiation studies have revealed that repeated thermal fluctuations cause:
An increase in lattice strain of the β(110) peak and expansion of the lattice parameter to a=3.22 Å
An increase in β phase fraction to approximately 3.5% ± 0.01%
Dynamic changes in vanadium concentration profiles across the α/β interface
Atom probe tomography confirms that vanadium concentration in the β phase center region reaches 22.4 ± 0.19 at.%, with the V concentration profile changing dynamically as the interface moves back and forth to maintain phase stability. Diffusion-based kinetic modeling (DICTRA) demonstrates that the α/β interface movement becomes significantly more pronounced when stored energy differences of 400–500 J/mole are introduced to the HCP α phase, supporting the experimental observation of dynamic interface behavior during thermal cycling.
2. Temperature-Dependent Interface Degradation
The stability of α/β interfaces in AM Ti-6Al-4V is strongly temperature-dependent:
At 500°C and below: The α/β interfaces remain relatively sharp and stable. The nano-film β layers retain their interfacial coherency, continuing to act as effective slip barriers. The microstructure is primarily governed by thermally activated recovery, with kinking as the dominant deformation mechanism.
Above 700°C: Extensive interfacial degradation occurs, characterized by:
α-lamella fragmentation and severe bending
β-phase penetration along newly formed α/α boundaries, breaking up originally continuous β interlayers
Loss of interfacial coherency due to boundary migration and recovery processes
Accelerated dynamic recrystallization (both discontinuous DDRX and continuous CDRX) nucleating at kink-affected regions
This temperature-dependent destabilization of nano-film β layers facilitates enhanced slip transfer and localized strain accommodation, leading to rapid flow softening and significant reduction in mechanical performance.
3. Martensite Dissolution and Phase Transformations
Thermal cycling also affects the stability of non-equilibrium phases formed during rapid solidification. Martensite (αm), which forms during fast cooling in AM processes, begins to dissolve at temperatures as low as 350–400°C. Upon reheating during subsequent thermal cycles, αm transforms into more stable α+β structures. This dissolution is a slow, diffusion-controlled process that further alters the local interface chemistry and microstructural stability.
Microstructural Evolution Mechanisms
The high fraction of HAGBs in AM Ti-6Al-4V (approximately 80.8% of total boundaries) plays a critical role in interface stability under thermal cycling:
HAGBs as dislocation sources and sinks: The abundant HAGBs promote boundary bulging and migration, lowering the nucleation barrier for discontinuous dynamic recrystallization (DDRX)
Enhanced boundary mobility: In kink-affected regions, localized instability facilitates DDRX nucleation, accelerating the breakdown of the original lamellar structure
Contrast with wrought alloys: Wrought Ti-6Al-4V contains a much larger proportion of low-angle grain boundaries (LAGBs), which restrict boundary mobility and favor gradual subgrain rotation (CDRX) rather than rapid interface destabilization
At 700°C, thermally activated boundary migration and dislocation climb further reduce the nucleation barrier for DDRX in HAGB-rich AM microstructures, whereas CDRX remains the primary recrystallization pathway in wrought alloys with their LAGB-governed structure.
Implications for Service Performance
The thermal cycling-induced interface instability has significant implications for the reliable application of AM titanium alloys in high-temperature environments:
Strength retention: While AM Ti-6Al-4V exhibits superior compressive yield strength at room and intermediate temperatures (300–500°C) due to its fine α-lath structure and stable α/β interfaces, its thermal stability decreases significantly above 700°C due to rapid interface degradation and softening.
Fatigue performance: The breakdown of coherent α/β interfaces and the formation of recrystallized grains can create sites for crack initiation and propagation, potentially compromising fatigue life.
Creep resistance: The high HAGB fraction and localized dislocation accumulation at α-lath boundaries, initially beneficial for creep resistance, become destabilized as interfaces lose coherency under thermal cycling.
Mitigation Strategies
To enhance interfacial stability under thermal cycling conditions, several approaches are being investigated:
Post-build heat treatment: Controlled thermal treatments can stabilize the microstructure by homogenizing solute distribution and reducing residual stresses from thermal cycling
Process parameter optimization: Adjusting deposition strategies (e.g., dwell time, path planning) to achieve more uniform thermal histories and suppress excessive reheating, resulting in finer, more stable α-lath structures
Thermomechanical processing: Combining AM with in-situ forging or interlayer deformation to refine grain structure and improve interface stability
Conclusion
Thermal cycling in additive manufacturing of titanium alloys creates a unique microstructural state with high fractions of high-angle grain boundaries and nano-film β layers at α/β interfaces. While these features provide excellent room-temperature strength, they exhibit limited thermal stability above 700°C, where interface coherency degrades through β-phase penetration, boundary migration, and dynamic recrystallization. Understanding these temperature-dependent interface evolution mechanisms is essential for optimizing AM process design and ensuring reliable performance of Ti-6Al-4V components in demanding service environments.
