Investigation of minimum quantity lubrication effects in drilling CFRP/Ti6Al4V stacks

Minimum quantity lubrication (MQL) has been extensively applied to the machining processes of various metallic materials; however, its impact on the drilling characteristics of metallic-composite stacks has not been thoroughly investigated. The present paper is thus aimed at identifying the influences of the MQL environments on the drilling process of one type of composite-titanium compound stacks constituted by T700/FRD-YZR-03 carbon/epoxy laminates and the Ti6Al4V sheets. A series of drilling trials were performed using tungsten carbide twist drills to investigate the feasibility of the MQL for the multilayer stack machining. A quantitative analysis was conducted on various drilling outputs including the drilling forces, surface morphologies of cut composite holes, hole defect extents, hole geometrical accuracy and tool wear signatures. A comparative investigation between the MQL and dry cutting conditions on the stack machinability was also carried out in order to quantify the potentials gained by the MQL machining process. The results indicate that the MQL drilling can yield several benefits in improving the surface morphologies of cut composite holes and in alleviating the drill wear severity, but it fails to reduce the drilling thrust forces, decrease the delamination extents and minimize the hole cylindricity errors when compared with the conventional dry cutting conditions.