DOI

10.17077/etd.por88e8m

Document Type

Thesis

Date of Degree

Fall 2017

Degree Name

MS (Master of Science)

Degree In

Mechanical Engineering

First Advisor

Ding, Hongtao

First Committee Member

Sugiyama, Hiroyuki

Second Committee Member

Xiao, Shaoping

Abstract

Carbon fiber-reinforced plastics (CFRPs) have gained widespread popularity as a lightweight, high-strength alternative to traditional materials. The unique anisotropic properties of CFRP make processing difficult, especially using conventional methods. This study investigates laser cutting by ablation as an alternative by comparing two near-infrared laser systems to a typical mechanical machining process. This research has potential applications in the automotive and aerospace industries, where CFRPs are particularly desirable for weight savings and fuel efficiency.

First, a CNC mill was used to study the effects of process parameters and tool design on machining quality. Despite high productivity and flexible tooling, mechanical drilling suffers from machining defects that could compromise structural performance of a CFRP component. Rotational feed rate was shown to be the primary factor in determining the axial thrust force, which correlated with the extent of delamination and peeling. Experimental results concluded that machining quality could be improved using a non-contact laser-based material removal mechanism.

Laser machining was investigated first with a Yb:YAG fiber laser system, operated in either continuous wave or pulse-modulated mode, for both cross-ply and woven CFRP. For the first time, energy density was used as a control variable to account for changes in process parameters, predicting a logarithmic relationship with machining results attributable to plasma shielding effects. Relevant process parameters included operation mode, laser power, pulse overlap, and cross-ply surface fiber orientation, all of which showed a significant impact on single-pass machining quality. High pulse frequency was required to successfully ablate woven CFRP at the weave boundaries, possibly due to matrix absorption dynamics. Overall, the Yb:YAG fiber laser system showed improved performance over mechanical machining. However, microsecond pulses cause extensive thermal damage and low ablation rates due to long laser-material interaction time and low power intensity.

Next, laser machining was investigated using a high-energy nanosecond-pulsed Nd:YAG NIR laser operating in either Q-Switch or Long Pulse mode. This research demonstrates for the first time that keyhole-mode cutting can be achieved for CFRP materials using a high-energy nanosecond laser with long-duration pulsing. It is also shown that short-duration Q-Switch mode results in an ineffective cutting performance for CFRP, likely due to laser-induced optical breakdown. At sufficiently high power intensity, it is hypothesized that the resulting plasma absorbs a significant portion of the incoming laser energy by the inverse Bremsstrahlung mechanism. In Long Pulse mode, multi-pass line and contour cutting experiments are further performed to investigate the effect of laser processing parameters on thermal damage and machined surface integrity. A logarithmic trend was observed for machining results, attributable to plasma shielding similar to microsecond fiber laser results. Cutting depth data was used to estimate the ablation threshold of Hexcel IM7 and AS4 fiber types. Drilling results show that a 2.2 mm thick cross-ply CFRP panel can be cut through using about 6 laser passes, and a high-quality machined surface can be produced with a limited heat-affected zone and little fiber pull-out using inert assist gas. In general, high-energy Long Pulse laser machining achieved superior performance due to shorter pulse duration and higher power intensity, resulting in significantly higher ablation rates. The successful outcomes from this work provide the key to enable an efficient high-quality laser machining process for CFRP materials.

Keywords

Carbon fiber reinforced polymer, Fiber laser, Keyhole, Laser cutting, Long duration, Nanosecond pulse laser

Pages

xv, 98 pages

Bibliography

Includes bibliographical references (pages 91-98).

Copyright

Copyright © 2017 Timothy Donald Heiderscheit

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