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Title: Conventional and ultrasonic assisted drilling of carbon fibre reinforced polymer/Titanium alloy stacks
Author: Dahnel, Aishah Najiah
ISNI:       0000 0004 6496 7579
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2017
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Drilling through Carbon Fibre Reinforced Polymer (CFRP)/Titanium (Ti) stacks is important for mechanical assembly of aircraft, however, there are concerns over rapid tool failure and part damage, which lead to reduced productivity and hole quality. Limited research has shown that Ultrasonic Assisted Drilling (UAD) has potential to improve hole quality when drilling CFRP and Ti individually. This has attracted the attention of aircraft manufacturers to evaluate the performance of UAD for CFRP/Ti stacks applications. This thesis presents three main studies of experimental work, which investigate tool wear mechanisms that govern tool life and hole quality when drilling CFRP/Ti stacks in a single continuous operation (one-shot) using carbide (WC-Co) drills. Study 1 involved conventional drilling of CFRP/Ti stacks as opposed to drilling CFRP and Ti individually using constant cutting parameters (cutting speed = 50 m/min; feed rate = 0.05 mm/rev). Study 2 and 3 investigate the effect of cutting parameters (cutting speeds = 25, 50, and 75 m/min; feed rates = 0.025, 0.05, and 0.075 mm/rev) during conventional drilling and UAD of CFRP/Ti stacks, and the performance of UAD was also evaluated. The hole quality was assessed in terms of hole diameter, CFRP damage (delamination and pull-out) and Ti surface integrity (burr formation, roughness, and hardness). The rapid tool failure when drilling CFRP/Ti stacks was found to be due to complex tool wear mechanisms, i.e. strong Ti adhesion on the abraded cutting edges and hence cutting edge fragmentation as the adhered Ti detached. Holes with inconsistent diameters, 22% – 62% more CFRP entrance delamination, 170% – 530% more CFRP pull-out and 720% higher Ti burr (after 80 holes) were observed when one-shot drilling of CFRP/Ti stacks compared to the case of drilling the materials individually (Study 1). It was shown (in Study 2 and 3) that using a low cutting speed and a high feed rate is important for better tool life during both conventional and UAD of CFRP/Ti stacks, although this does not completely resolve hole quality issues. Ti adhesion reduced, hence cutting edges wore uniformly and gradually, which resulted in a longer tool life, consistent hole diameter and reduced Ti burr when drilling CFRP/Ti stacks with reduced cutting speed and increased feed rate due to lower heat generation and contact time. However, CFRP damage when drilling CFRP/Ti stacks was not significantly affected by cutting parameters and tool wear, rather it was found to be substantially influenced by Ti chips evacuating through the CFRP. The application of UAD on CFRP/Ti stacks did not provide any significant advantage in tool wear / life compared to conventional drilling within the range of cutting parameters and cutting tool used, although there was slight improvement in hole quality. The cutting force profiles and Ti chip morphology indicated that UAD exhibited continuous cutting, i.e. the tool did not disengage from the workpiece during drilling. Even though the use of UAD was beneficial to produce more consistent hole diameter (between CFRP and Ti of the stacks), 33% lower machined Ti surface roughness and a marginal increase in Ti hardness compared to conventional drilling, there was no significant improvement in CFRP damage and Ti burr height. When drilling CFRP/Ti stacks, tool life and productivity could not be improved by the application of ultrasonic assistance using the range of parameters investigated, and finishing operations would still be needed to improve the hole quality. UAD could be more advantageous for drilling titanium individually than CFRP/Ti stacks.
Supervisor: Not available Sponsor: Kementerian Pengajian Tinggi ; Malaysia ; International Islamic University Malaysia
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TA Engineering (General). Civil engineering (General)