Optimisation of CNC Milling Process Parameters for Ti-6Al-4V Titanium Alloy Under Minimum Quantity Lubrication using Taguchi-Grey Relational Analysis: A Multi-Objective Approach

Abstract

Due to its low thermal conductivity, high chemical reactivity with cutting tools, and tendency to work- harden during machining, Ti-6Al-4V titanium alloy presents significant machinability challenges. This study presents a systematic multi- objective optimisation of CNC end- milling process parameters for Ti-6Al-4V under Minimum Quantity Lubrication (MQL) using a Taguchi L16 orthogonal array combined with Grey Relational Analysis (GRA). Four process parameters, spindle speed (60 to 120 m/min), feed per tooth (0.02 to 0.08 mm/tooth), axial depth of cut (0.4 to 1.2 mm), and MQL flow rate (80 to 150 mL/h), were investigated at four levels each for their influence on surface roughness (Ra), cutting tool life, and material removal rate (MRR). GRA composite optimisation identified the optimal parameter combination as spindle speed 80 m/min, feed per tooth 0.04 mm/tooth, axial depth of cut 0.8 mm, and MQL flow rate 120 mL/h. At these optimal conditions, Ra was measured at 0.68 micrometres, tool life at 24.6 min, and MRR at 6.84 cm³/min, representing a Grey Relational Grade improvement of 34.2% over the initial parameter setting.