Mechanistic study of chip-induced secondary machining on hole-wall morphology in CFRP/Ti stack drilling

Abstract

CFRP/Ti heterogeneous stacks are extensively applied in critical aircraft structures due to their superior mechanical and physical properties. Stack drilling is an effective approach to improve machining efficiency and hole-making accuracy; however, the pronounced disparity in physical properties between CFRP and Ti alloys poses significant challenges in achieving consistent hole quality during drilling. In this study, the secondary machining effect of Ti alloy chips on the CFRP hole wall during CFRP/Ti stack drilling is systematically investigated. An analytical strategy combining incomplete drilling experiments with energy-dispersive X-ray spectroscopy (EDS) is employed to elucidate the interaction mechanisms between Ti alloy chips and the CFRP hole wall under dry drilling and cryogenic drilling conditions. The results indicate that Ti alloy chips primarily interact with the smeared matrix on the machined CFRP surface. Under dry drilling conditions, matrix failure is dominated by plastic deformation, and the action of Ti alloy chips promotes the formation of a new smeared matrix layer on the hole wall. In contrast, under cryogenic drilling conditions, matrix failure is governed by brittle behavior, whereby the interaction with Ti alloy chips leads to further removal of the original smeared matrix, resulting in a combined morphology characterized by block-like matrix debris and exposed fibers.