Performance evaluation and analysis of the use of CO2 cooling for conventional drilling of carbon fibre reinforced plastics

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Abstract

Machining defects induced by conventional drilling of carbon fibre reinforced plastics (CFRPs), of which the most concern is delamination damage both to the surface of the component and to the machined surface of the hole, usually occur due to the heterogeneity and anisotropic properties of the material. Among previous research work on conventional drilling of CFRPs, attempts to minimise delamination damage have focused on the optimisation of tool material, geometry and cutting parameters. Although the application of cryogenic cooling has been shown to improve performance in metal machining, there has been little research work reported on its application to conventional drilling of CFRPs. Therefore, the objectives of this research were to evaluate the application of cryogenic cooling, for which CO2 was used as the main cutting fluid, in conventional drilling of CFRPs and present a detail explanation of the effect on machining performance and mechanism associated with cryogenic machining of these materials.

Drilling experiments with liquid nitrogen (LN2) pre-cooled tools, with CO2 cooling and when machining dry at room temperature were performed on CFRPs (carbon/epoxy) plaques using TiAlN and diamond coated solid tungsten carbide drills. The performance evaluation was based on measurement of thrust force, tool wear and delamination damage to the entry/exit of the hole and internal damage to machined surface of the hole. Cutting temperature and characteristics of machined surface (fracture behaviour of carbon fibres and epoxy matrix) produced when drilling with cryogenic cooling and dry at room temperature were also investigated.

In this research, it was found that application of cryogenic cooling (LN2 pre-cooling and CO2 cooling) to conventional drilling of CFRPs resulted in an improvement in machining performance with respect to quality of the hole. Less exit delamination damage and internal damage to machined surface of the hole were produced when machining with cryogenic cooling compared to room temperature dry drilling. However, the use of cryogenic cooling in conventional drilling of CFRPs did not improve machining performance with respect to cutting forces and tool wear. In fact, it resulted in higher thrust force and average flank wear compared to machining dry at room temperature. The reduction of delamination and internal damage and the increase of thrust force and rate of tool wear were found to be due to the higher abrasiveness, strength and stiffness of CFRP plaques that were retained during drilling with cryogenic cooling. The cutting temperature was shown to be lower than room temperature dry drilling due to the more effective removal of heat from the cutting zone. It was shown that the cutting temperature was reduced by 14-27% when drilling with cryogenic cooling, for which the use of a CO2 cooling system provided the highest cooling ability, at a cutting speed and feed rate of 100 m/min and 0.06 mm/rev respectively. It was shown that drilling with cryogenic cooling resulted in a more brittle fracture behaviour and less thermal softening of the epoxy matrix in CFRP plaques compared to that produced by room temperature dry drilling. This indicates higher strength and stiffness of the epoxy matrix that were retained during drilling with cryogenic cooling hence resulting in higher abrasiveness, strength and stiffness of the plaque due to more rigid support of the matrix. Since the drilling-induced damage, which was shown in previous research work to degrade the mechanical properties and performance of CFRP components, was reduced, the application of cryogenic cooling can therefore be beneficial when implemented in conventional drilling of CFRPs to improve productivity. However, tool material has to be optimised to compensate with shorter tool life due to increased rate of tool wear. Although no significant difference in thrust force was produced when drilling with CO2 cooling and when drilling with a LN2 pre-cooled tool at the same cutting speed and feed rate, less damage to the machined surface was produced when drilling with CO2 cooling. This was found to be due to higher capability in reducing the cutting temperature than LN2 pre-cooling of the tool. Therefore, the application of cryogenic cooling by continual supply of CO2 (or LN2) is more preferable and more practical to be implemented in the production process in industry than cryogenic pre-cooling of the tool.

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