Surface morphology of inclined CFRP holes when machined under cryogenic environment

Abstract

ABSTRACT Manufacture of modern aircraft components involve production of high value near net-shape carbon fiber reinforced polymer (CFRP) parts, which requires drilling holes for rivets and bolts at acute inclinations. The study explores the surface morphology of the inclined holes (30 ͦ, 60 ͦ, and 90 ͦ) drilled in CFRP under cryogenic cooling using coated carbide tools. Inclined hole drilling performance was measured using machining forces, surface finish, and bore integrity. It has been shown that cryogenic machining of acutely inclined small diameter holes generated at least 40% higher thrust forces compared to dry cutting. Surface quality under cryogenic was enhanced than dry cutting with reduced matrix transverse cracking that helped suppress the fiber debonding and pull out. This mechanism also aids to minimize stress concentration regions along matrix/fiber interfaces due to promotion of fracture mode II in-plane sliding shear of carbon fibers and reduction of fracture mode III out of plane peel-up and push-down interlaminar delamination. Effect of tool coating on bore surface integrity showed that double coat TiN tool with a point angle of 140 ͦ generated at least 40% lower thrust forces than Physical Vapor Deposition (PVD)-coated multilayer TiAlN flat bottom drill during cryogenic-assisted drilling.