Mechanistic modeling of strain heterogeneity amplification in machining ductile refractory alloys

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This study investigates the microstructural origins of poor machinability in tantalum-tungsten (Ta-10W) alloys through integrated orthogonal cutting experiments, electron backscatter diffraction (EBSD) analyses, and microstructure-dependent finite element method (FEM) simulations. Experimental results identified a critical transition at approximately 90 m/min where cutting forces decreased by 58.3%, chip thickness reduced by 89.8%, and tool crater wear decreased by 29.5%. The transition is governed by strain rate-dependent crystallographic heterogeneity: at low cutting speeds, prolonged tool-chip contact time and thermal-mechanical coupling amplify the mechanical contrast between α-fiber and γ-fiber grains, intensifying strain heterogeneity and thickening the secondary shear zone (SSZ) to 52.15 μm, ultimately triggering macro-scale flow instability characterized by chip buildup and progressive shear angle reduction. These predictions were experimentally validated through cross-sectional SSZ thickness measurements and chip EBSD analysis, which confirmed progressive SSZ thickening and spatially heterogeneous strain accumulation at low cutting speeds. This instability mechanism is explained through the Molinari-Moufki chip stability framework, which predicts decreasing shear angles under developing free-surface perturbations. In contrast, high cutting speeds raise strain rates and reduce contact time, suppressing heterogeneity amplification and confining deformation to a thin, stable SSZ of 13.54 μm. The study establishes that Ta-10W’s poor machinability fundamentally stems from its intrinsic crystallographic heterogeneity being amplified under low strain rate conditions, providing a mechanistic framework for optimized machining strategies and microstructural control of refractory metals.

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