Synergistic optimization of machining parameters for surface hardness of the machined Ti–6Al–4V alloy before and after residual stress release

The surface hardness of machined Ti–6Al–4V alloy is a critical indicator of surface quality and fatigue performance. While cutting parameters significantly influence this hardness, prevailing optimization strategies focus solely on maximizing the as-machined hardness, neglecting the instability of machining-induced residual stresses. These stresses inevitably release during service, disrupting the surface equilibrium and altering hardness, which can compromise long-term performance and reliability. To address this gap, this study proposes a novel multi-objective optimization framework that synergistically optimizes the surface hardness both before and after residual stress release. A hybrid experimental design was employed, and the Focused Ion Beam-Digital Image Correlation (FIB-DIC) ring-core method was integrated with nanoindentation to quantitatively measure the hardness variation (Δ H , the potential hardness decrease upon stress release) caused by a controlled, complete release of residual stress, simulating in-service stress relaxation. Kriging-based models were then developed to accurately establish the relationships between machining parameters and three objectives: machined surface hardness ( H ), hardness decrease (Δ H ), and Material Removal Rate (MRR). The NSGA-III was employed to resolve the optimization model, yielding Pareto-optimal solutions. The results demonstrate that solely pursuing high initial hardness or high MRR can lead to large softening effects (up to Δ H = 0.721 GPa), whereas the proposed method identifies parameter sets that maintain high initial hardness (7.12 GPa) while minimizing subsequent softening (Δ H = 0.62 GPa), thereby ensuring a superior stabilized hardness in service. This research provides a new methodology for selecting machining parameters that guarantee not only superior initial quality but also long-term performance stability for high-reliability titanium alloy components.