Machinability Investigation of Ti-6Al-4V Titanium Alloy during CNC Turning Under Dry, Wet, and Minimum Quantity Lubrication Conditions: A Taguchi L16 and ANOVA-Based Multi-Response Optimisation Study

Abstract

Ti-6Al-4V (Grade 5) titanium alloy, constituting over 50 percent of all titanium alloy production, is extensively used in aerospace, biomedical, and automotive applications demanding its unique combination of high specific strength, corrosion resistance, and biocompatibility. However, its low thermal conductivity, high chemical reactivity, and tendency to spring back make it one of the most challenging materials to machine, characterised by high cutting temperatures, rapid tool wear, and poor surface integrity. This study presents a systematic experimental investigation of CNC turning machinability parameters — surface roughness (Ra), tool flank wear (VB), and cutting temperature — for Ti-6Al-4V under three cooling conditions (dry, wet, and Minimum Quantity Lubrication — MQL) using a Taguchi L16 orthogonal array with four factors: cutting speed (60–150 m/min), feed rate (0.10–0.20 mm/rev), depth of cut (0.3–0.8 mm), and cooling method. Uncoated carbide (WC-Co, K20 grade) inserts were used throughout to isolate material and cooling effects from coating contributions. ANOVA identifies cutting speed as the dominant factor (42.8% contribution) followed by feed rate (26.4%) and cooling method (16.9%). MQL outperforms both dry and wet machining for Ra and VB across all conditions. The optimal parameter combination (Vc = 120 m/min, f = 0.10 mm/rev, ap = 0.3 mm, MQL) achieves Ra = 1.21 µm and VB = 0.092 mm, validated by confirmation experiments within 3.4% of Taguchi predictions. Regression models for Ra and VB are presented with R² values of 0.942 and 0.961 respectively. SEM analysis of worn tool surfaces confirms that adhesive wear and built-up edge formation are the dominant wear mechanisms under all conditions, with MQL significantly reducing BUE formation