Investigation on residual stresses in milling of Ti-6Al-4V for both rake and flank application of different MWF strategies

Abstract

Abstract This study investigates the effects of both rake and flank applications of different Metal Working Fluid (MWF) strategies on residual stresses in the machining of Ti-6Al-4V alloy. Cryogenic (Liquid CO2), Minimum Quantity Lubrication (MQL) and emulsion strategies were studied with modified CoroMill600 milling cutter via internal channels delivering media to insert rake face and flank face. The cutting force, minimum chip thickness and chip morphology were analyzed to understand more about this novel approach of rake and flank delivery of different MWFs in milling. The results reveal the formation of compressive residual stresses until 55-60µm beneath the machined surface irrespective of the type of MWF strategies. The highest value of compressive residual stress was observed at the machined surface of liquid CO2. The magnitude of the traced compressive residual stress profile shows a trend of positive slope gradient beneath the surface for both parallel and perpendicular to feed directional residual stress components. In contrast, residuals stresses in emulsion and MQL strategies were observed with a different trend in generation of compressive residual stress components, where the parallel to feed directional component shows an inflection point with a an initial negative slope gradient followed by a positive one to beneath the machined surface. An increase in the cutting forces and minimum chip thickness values were also observed for liquid CO2, due to the high shear resistance of Ti-6Al-4V alloy at machining zone, which was confirmed from the chip morphology analysis. Overall results show that cryogenic CO2 leads to higher compressive residual stresses at the surface and positive slope gradient beneath the material. The higher cutting forces in Z-axis and minimum chip thickness value in liquid CO2 are also attribute to the higher compressive stresses in Ti-6Al-4V workpiece at cryogenic CO2 environment.