Synergistic effects of hBN and ZnO nanofluids coupled with vortex-tube cooling during milling of selective laser melted Ti-6Al-4V alloy: A thermo-tribological assessment

Selective laser melted (SLM) Ti-6Al-4V components are widely used in aerospace, biomedical and defense applications, but their as-built parts inherently exhibit poor surface quality, high residual stresses and a heterogeneous, martensitic α′-dominated microstructure that complicates secondary machining. The present study investigates a hybrid eco-friendly cooling/lubrication strategy in which hexagonal boron nitride (hBN) and zinc oxide (ZnO) nanofluids prepared at 0.6 vol.% in a biodegradable vegetable-based synthetic oil are coupled with Ranque–Hilsch vortex-tube (VT) cooling during the milling of SLM Ti-6Al-4V. Eight machining environments were compared: dry, MQL, VT, MQL+VT, hBN-MQL, ZnO-MQL, hBN-MQL+VT and ZnO-MQL+VT, at constant cutting parameters ( vc = 90 m/min, f = 0.1 mm/rev, a p = 0.8 mm, a e = 12 mm ). The nanofluids were characterized in terms of pH, dynamic viscosity and thermal conductivity, and machining performance was assessed through cutting temperature (Tc), average surface roughness (Ra), flank wear progression (VB), power consumption (P), coefficient of friction (CoF) and specific wear rate. Tool wear mechanisms were further elucidated through SEM imaging and EDX point analysis. Compared with the dry baseline (Tc ≈ 474 °C, Ra ≈ 0.326 μm, VB ≈ 542 μm, P ≈ 490 W, CoF ≈ 0.539), the ZnO-MQL+VT hybrid produced the largest improvements: Tc, Ra, VB, P and CoF were reduced by 24.7%, 44.5%, 53.1%, 39.4% and 52.3%, respectively, while the specific wear rate decreased by 31.0%. SEM/EDX analysis confirmed that adhesion, diffusion and built-up edge (BUE)/built-up layer (BUL) formation, accompanied by coating delamination, were the dominant wear modes in dry and partially cooled conditions, whereas these mechanisms were strongly suppressed under hybrid nanofluid+VT environments. The findings provide a coherent thermo-tribological framework for designing sustainable hybrid cooling/lubrication strategies tailored to the specific challenges of additively manufactured titanium alloys.