Investigation on the cutting responses of CFRP/Ti stacks: With special emphasis on the effects of drilling sequences

The compound CFRP/Ti stacks, as an advanced engineering structure, have been commonly applied in the manufacture of large aircraft structural components. However, their inhomogeneous characteristics and poor machinability have given rise to the great difficulty for the drilling process in one-shot time. Meanwhile, owing to their different mechanical and thermal performances of the involved CFRP and Ti phases, the cutting responses in the bi-material (i.e. CFRP-Ti interface) region are of great complexity. In comparison to the previous investigations, this paper was designed to specially address the effects of drilling sequences (i.e. CFRP -> Ti and Ti -> CFRP) on the machining outputs of CFRP/Ti stacks with more comprehensive aspects. The obtained results show that the drilling sequence does play an important role in determining the involved machining responses of CFRP/Ti stacks. When drilling the stacks from the CFRP layer, the maximum magnitudes of drilling forces and cutting temperatures show higher values than those when drilling from the Ti6Al4V layer. Meanwhile, the difference between the entry diameter of the CFRP layer and that of Ti6Al4V layer increases with the elevation of feed rate during drilling from CFRP layer while the diameter of Ti6Al4V is always larger than that of the CFRP layer by 0.02-0.03 mm and presents disparity to the drill diameter by 0.02 mm when drilling from Ti6Al4V layer. From the viewpoint of drilled hole accuracy, it is preferable to drill CFRP/Ti stacks from the Ti6Al4V layer while the drilling sequence of CFRP -> Ti is mostly preferred in terms of reducing the delamination damages in practical engineering production.