Machining fibre metal laminates and Al2024-T3 aluminium alloy

• Main Author: Giasin, Khaled
• Other Authors: Pinna, Christophe ; Ayvar Soberanis, Sabino
• Published: University of Sheffield 2017
• Subjects: 629.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.701781

The present thesis investigates the machining performance of an aerospace structural material commercially known as GLARE fibre metal laminate and its metal constituent aluminium Al2024-T3 aerospace alloy using commercially available solid carbide twist drills. The objective is to quantify the effects of the cutting parameters and two modern coolant technologies on cutting forces and a number of hole quality parameters. The generated drilling cutting forces, quality of machined hole and drilling-induced damage and defects when drilling GLARE fibre metal laminates were experimentally studied. Drilling-induced defects and damage investigated were surface roughness, burr formation at both sides of the workpiece and interlayer burr, hole size and circularity error, chip formation as well as damage described at the macro level (delamination area) using computerised tomography (CT) scan, and at the micro level (fibre matrix debonding, chipping, adhesions, cracks) using scanning electron microscopy (SEM). The experimental results have been statistically analysed using full factorial and response surface methodology statistical techniques to generate multiple regression models which makes it attractive as an indirect tool predicting the machining outputs prior the start of actual tests. Moreover, the analysis of variance (ANOVA) was employed to determine the percentage contribution of drilling parameters on cutting forces and hole quality outputs. The results indicated that the presence of coolant during the drilling process of GLARE could significantly improve hole quality. The use of cryogenic liquid nitrogen was found to eliminate the formation of waste on the borehole surface and burr formation at the hole exit. Using minimum quantity lubrication coolant was found to reduce the workpeice temperature compared to dry drilling at room temperature. Both coolants reduced the surface roughness compared to dry drilling but increased the cutting forces especially when using cryogenic liquid nitrogen. The cutting parameters results indicated that a maximum operating feed rate of 300 mm/min and a maximum spindle speed of 6000 rpm is recommended for superior hole quality results. Moreover, drilling at or below those levels of cutting parameters did not lead to severe delamination or fibre pull outs in the laminate compared to the higher cutting parameters used in the study. In addition, the fibre orientation and workpiece thickness were found to play a significant role on surface roughness and hole size but did not have a considerable impact on cutting forces due to the small thickness of glass fibre layers in the laminate. Adhesion and built up edge was found to be the main wear mechanism when drilling monolithic aluminium alloy, while adhesion and abrasion of the primary and secondary facets of the drill were identified to be the main wear process that occurs in drilling GLARE laminates.