Drilling of micro-scale high aspect ratio holes with ultra-short lasers

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Nasrollahi, Vahid ORCID: https://orcid.org/0000-0002-5697-8062 (2020). Drilling of micro-scale high aspect ratio holes with ultra-short lasers. University of Birmingham. Ph.D.

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Abstract

Laser micro drilling is a very attractive option for a growing number of industrial applications due to its intrinsic characteristics, such as high flexibility and process control and also capabilities for non-contact processing of a wide range of materials. However, there are some constrains that limit the applications of this technology, i.e. taper angles on side walls, edge quality, geometrical accuracy and achievable aspect ratios of produced structures. To address these process limitations and control the morphology and quality of the holes that are important factors in broadening the application areas of this technology, three different approaches have been proposed and developed in this research:

(i) A novel method for two-side laser processing is proposed. The capabilities of this method are investigated with a special focus on its key enabling technologies for achieving high accuracy and repeatability in two-side laser drilling. The pilot implementation of the proposed processing configuration and technologies is discussed together with an in-situ, on-machine inspection procedure to verify the achievable positional and geometrical accuracy. It is demonstrated that acceptable alignment accuracy is achievable using this pilot two-side laser processing platform. In addition, the morphology of holes with circular and square cross-sections produced with one-side laser drilling and the proposed method was compared in regard to achievable aspect ratios and holes’ dimensional and geometrical accuracy and thus to make conclusions about its capabilities.
(ii) Effects of a wide fluence spectrum associated with the use of femto-second lasers on achievable aspect ratios were investigated by employing lenses with different focal distances. It was demonstrated that the achievable aspect ratio can be increased substantially just by varying the lenses’ focal distances. In addition, the quality of produced holes in terms of taper angle and cylindricity was investigated and the results showed that the quality would be improved by increasing the fluence and/or decreasing the focal distance. At the same time, the limitations in drilling holes with low focal distance lenses were discussed, i.e. sensitivity to defocusing, risks of recast formations inside the holes and bending effects, that should be considered in designing processes for high aspect ratio percussion drilling.
(iii) A beam shaping solution for laser micro drilling has been designed and implemented to achieve a top-hat spatial profile. The morphology of the high aspect ratio holes in terms of cylindricity, circularity, tapering angle, heat affected zone (HAZ) and penetration depth was investigated. The capabilities and limitations of the proposed beam shaping solution for producing micro-scale high aspect ratio holes are discussed, i.e. its sensitivity to defocusing, and the resulting hole morphology compared with that achievable with a Gaussian beam spatial distribution. It’s been shown that the penetration depth and aspect ratios achievable with a Gaussian beam are higher, but the use of a top-hat beam improves the holes geometrical accuracy, especially the deviations of the holes from cylindricity are less and also the holes are with a lower tapering angle. The top-hat spatial distribution minimises not only HAZ but also fluence at the beam spot area can be tailored accurately in respect to the ablation threshold.

To assess the holes’ morphology, depth and quality in all these approaches, a non-destructive method i.e. a high-resolution X-ray tomography (XCT) with a minimised measurement uncertainty was employed.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):