Millisecond fibre laser drilling of alumina-alumina CMC

Alumina-Alumina (OX-Ox)CMCs are a technical ceramic composed of alumina fibres in an alumina matrix, with a primary usage case in the aerospace industry. Ox-Ox CMCs offer the traditional benefits of high toughness and bending strength, along with resistance to oxidation and corrosion associated with monolithic alumina, coupled with low brittleness and crack arresting behaviour of fibre in matrix composites. Whilst these properties are highly desirable, CMCs present significant difficulties for conventional mechanical machining, with laser machining offering potential solutions providing the impetus to the work presented in this paper. This work presents a parametric study of the effects of frequency and pulse energy for 10mm deep laser percussion drilled holes and a response surface study for these two variables, using an industrially representative 20kW peak power laser. Results show that for an 800µm diameter hole, a taper angle of <1⁰ and average recast layer thickness of <40µm are achievable, while the response surface study highlights the importance of considering input variables as a complex system, particularly for recast layer thickness.