Machining Fibre Metal Laminates and Al2024-T3 aluminium alloy

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.