Electrical discharge-mechanical hybrid drilling of micro-holes in carbon fibre-reinforced polymers

Abstract

The machining of deep micro-holes in carbon fibre-reinforced polymers (CFRP) components exhibits a significantly increased demand in the industry. However, it is difficult to machine CFRP deep micro-holes using conventional mechanical drilling and non-conventional processes individually because of the anisotropic and inhomogeneous characteristics of CFRP. To address this problem, an electrical discharge-mechanical hybrid drilling method was proposed in this study. In this method, a specialized servo control strategy was employed to effectively utilize the electrical discharge machining and mechanical drilling, based on the distinct difference in electrical conductivity between the carbon fibre and the resin in CFRP. This effectively resolved the challenges posed by the high hardness of the carbon fibre for mechanical drilling and the non-conductivity of the resin for EDM, taking advantage of both EDM and mechanical drilling. High-speed photography, processing debris analysis, discharge state monitoring, and finite element simulation were performed to investigate the machining process and material removal mechanism of electrical discharge-mechanical hybrid drilling. The results showed that most of the carbon fibre and resin were individually removed by EDM and mechanical drilling, respectively. However, in the interfacial region between the carbon fibre and resin, both mechanical drilling and EDM occur simultaneously. The heat generated during the EDM of carbon fibre also leads to the thermal decomposition and vaporization of the resin in proximity to the carbon fibre. Furthermore, deep micro-holes machining with a diameter of 330 μm and a depth-to-diameter ratio of 15.1 was performed on CFRP component to validate the advantages of the proposed hybrid drilling method. Compared with EDM, the proposed hybrid drilling method exhibited a 29.1 % increase in efficiency, 56.25 % reduction in taper, and 54.32 % reduction in the heat-affected zone. These outcomes demonstrate that the electrical discharge-mechanical hybrid drilling holds great potential for machining high-quality micro-holes on advanced multilayer composites with anisotropic and inhomogeneous properties.