Multi-objective optimisation of sustainable lubrication in end milling of AISI D2 aerospace die steel using hybrid GWO-CSA and NSGA-II

Purpose This study aims to investigate the end milling performance of AISI D2 die steel under different lubrication conditions to improve productivity, surface finish and tool life. A comparison between vegetable-based neem oil and ServoCut S mineral oil was conducted to identify a sustainable cutting fluid for machining hardened die steels used in aerospace tooling and precision manufacturing. Design/methodology/approach Experiments were designed using the Taguchi L16 orthogonal array to analyse the effects of spindle speed, feed rate and depth of cut on material removal rate (MRR), surface roughness (Ra) and tool wear (VB). Signal-to-noise ratio analysis identified significant parameters. A hybrid optimisation framework integrating grey wolf optimiser (GWO), crow search algorithm (CSA) and RBF-assisted NSGA-II was applied for multi-objective optimisation. SEM and ImageJ software were used to examine surface morphology and wear mechanisms. Findings Depth of cut showed the greatest influence on MRR, while spindle speed mainly affected Ra and VB. Neem oil outperformed dry and mineral-oil machining, increasing MRR by 7%–8%, reducing Ra by 20% and lowering VB by 27%. Optimised neem oil machining achieved MRR = 65.54 mm3/s, Ra = 1.79 µm and VB = 0.204 mm. SEM analysis confirmed smoother surfaces and lower wear under neem oil lubrication. Originality/value The study presents a hybrid optimisation framework combining Taguchi design, GWO-CSA and RBF-assisted NSGA-II. It demonstrates neem oil as a sustainable and effective alternative to conventional mineral cutting fluids for milling hardened die steels.