With the advancement of high-performance machine tools and ultrahard cutting tool materials, High-Speed Dry Milling (HSDM) has emerged as a promising finishing technique for machining difficult-to-cut materials such as titanium and its alloys. However, this process creates intense thermomechanical loads at the tool-chip and tool-part interfaces, which induce significant metallurgical alterations in the subsurface of the machined surface layer. These alterations include Dynamic Recrystallization (DRX), phase transformation and textural orientation changes, which modify the mechanical properties of the subsurface and consequently influence the fatigue strength and service life of manufactured components.
Despite extensive research into the integrity of machined surfaces of titanium and its alloy, most studies have focused on the turning process, with limited theoretical analysis of the physical mechanisms governing the modification of near-surface mechanical and metallurgical properties during high-speed machining. Furthermore, the emission of fine and ultra-fine metal particles during machining represents a critical risk to the safety and health of operators, which has led to the adoption of increasingly stringent regulations to limit exposure. These emissions also influence cutting performance and tool life. While previous studies have investigated the emission of particles from various materials, research on the Ti-6Al-4V alloy remains limited. The distinct mechanisms of chip formation and particle release in Ti-6Al-4V alloy requires an in-depth study of this material’s behavior during machining, in order to capitalize on its potential while ensuring process efficiency, environmental sustainability and operational safety.
In this study, the microstructural evolution mechanisms, along with other surface integrity features (plastic strain, burr formation, surface roughness, hardness, etc.) and particle emissions induced by thermo-mechanical loads during HSDM of Ti-6Al-4V, are investigated using multiscale modeling and experimental approaches. This manuscripts-based research is structured as follows:
First, a temperature-displacement coupled Finite Element (FE) model, replicating the actual milling process, was established for the HSM of Ti-6Al-4V alloy. In the proposed model, Abaqus/Explicit solver was utilized to couple the material damage and its fracture energy. The time-varying chip geometry was digitized into small elements with distinct cutter rotation angles and discrete time intervals. The model was validated against cutting forces, chip morphology and temperature; Strong correlations between numerical and experimental results were obtained. This FE model provided critical state field variables, including temperature, stress, and strain, essential for subsequent microstructural evolution predictions. Following that, the Zenner-Holloman (Z-H) and Hall-Petch (H-P) equations were combined in a user-defined subroutine (FORTRAN) to model dynamic recrystallization (DRX) and estimate grain size and microhardness. The results indicated that at higher cutting speeds and feed rates, grain refinement occurred more prominently in the adiabatic shear band (ASB) than on the cutting surfaces. In contrast, microhardness exhibited an inverse correlation with grain size, increasing with cutting speed and feed rate. The influence of plastic strain and temperature on grain size during chip segmentation was also examined. Experimental validation confirmed a strong correlation between the developed model and observed results regarding grain size and parts microhardness. Secondly, using a user-defined subroutine, thermo-metallo-mechanical phase transformation theoretical models were integrated into Ti–6Al–4V cutting simulation using Abaqus/Explicit software. The temperature and stress-strain field distributions were retrieved from a validated simulated model during the rapid heating of the alloy. These field states were then used to model the cooling process, determining the volume fraction of the a’-phase. The results revealed that during the rapid heating, the phase transformation (i.e., a- to B)-phase starts within the region where the temperature exceeds the initial phase transitions of Ti-6Al-4V alloy, and almost all of the a-phase has been transformed into B-phase when the cutting temperature crossed the transus temperature. Upon cooling, the quenching effect induced a reverse transformation from the B-phase to the a’-phase, further refining the grain structure.
Thirdly, a simplified milling model was developed, where the undeformed chip thickness is approximated as a sinusoidal function for residual stress prediction within the milled part. The periodic fluctuations of plastic strain and residual stress during the milling process were unveiled.
Finally, the simulated residual stress profiles for the machined surface and subsurface were compared with experimental measurements, and the influence of thermo-mechanical loads on residual stress formation was thoroughly investigated. Finally, an experimental approach was employed to investigate the correlation between machining parameters, chip formation, surface integrity, and particle emission. The results demonstrated that increasing cutting speed initially led to greater chip serration, deformed layer thickness, particle emissions, and surface roughness, followed by a subsequent decrease. In contrast, increasing feed rate and depth of cut consistently intensified these effects. A direct relationship was observed between particle emissions and machined surface integrity parameters such as plastic deformation depth, burr formation, surface roughness, and chip morphology. This research enhances theoretical understanding and practical applications in the field of manufacturing, offering valuable insights for industries, researchers, and academic institutions. The findings contribute to the development of optimized machining strategies that improve surface integrity and mitigate the environmental and health impacts of fine particle emissions in HSDM of Ti-6Al-4V alloy.
| Date | 26 Jun 2025 |
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| Original language | American English |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Victor Songmene (Supervisor) & Esther T. Akinlabi (Co-supervisor) |
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