Fundamental investigation of the drilling of multimaterial aerospace stacks to aid adaptive drilling

Abstract

Drilling multi-material aerospace stacks is a challenging task due to the different properties of each stack layer material. Carrying out adaptive drilling, i.e. automatically making adjustments to the drilling process in response to information extracted from process signals, would allow for the utilisation of the optimal parameters throughout the drilling cycle, thereby resulting in improved borehole quality and tool life. To aid the development of adaptive drilling, this thesis deals with gaining a fundamental understanding of the drilling of multi-material aerospace stacks in view of, firstly, the effect of different parameters on the cutting process and, secondly, the relationships between process signals and process incidences. In the first part of the thesis, the impact of the interlayer gap width, tool point angle and parameter changeover on the resulting borehole quality is investigated. Adapting the cutting parameters and cooling strategy based on the material being machined was found to result in a stable cutting process and generate boreholes exhibiting small interface damage. Introducing an interlayer gap to a stack comprising aluminium layers resulted in an increase in interlayer burr formation, as opposed to drilling a stack with no interlayer gap. However, the presence of an interlayer gap can ultimately limit the final burr size, due to the sliding action of the upwards-travelling chips over the borehole edges. When drilling stacks comprising a composite layer above a titanium one, the damage on the composite interlayer surface was caused by the drilling of the metal layer below, as a result of the upwards-travelling metal chips and heat accumulation in the tool and stack interface. The wider the interlayer gap, the easier it becomes for metal chips to penetrate the interface and damage the composite surface. The second part of the thesis deals with establishing relationships between signals recorded from different sensors and process incidences, using both a machining centre and examples of the latest generation of aerospace portable drilling units. Thrust force and torque, motor current, acoustic emission and acceleration were found to be suitable process signals for the detection of tool engagement, material transition and tool disengagement, with the thrust force being the most responsive signal to the occurrence of these incidences. Based on the gained knowledge, a decision-making strategy was designed. Its performance was tested and compared to the strategies used in the drilling units available on the market. The proposed strategy, based on gradient monitoring, yields substantial improvements in terms of both reliability and responsiveness when compared to the currently used magnitude-based approaches.

Date of Award	15 Oct 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Heinemann (Main Supervisor) & Kali-Babu Katnam (Co Supervisor)