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Ultrafast laser helical drilling of three-dimensional shaped holes using synchronized adaption of energy deposition

: He, C.; Buehring, J.; Zibner, J.; Gillner, A.

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Journal of Laser Micro/Nanoengineering. Online journal 13 (2018), No.1, pp.31-35
ISSN: 1880-0688
Journal Article, Electronic Publication
Fraunhofer ILT ()

We present a three-dimensional (3D) drilling strategy for symmetrically shaped microholes using helical drilling optics and ultrashort pulsed laser radiation. In the helical drilling process, the helical movement of laser beam is performed by using a rotating Dove prism, bore diameter and taper are controlled by a motorized wedge prism and a linear table separately. More precisely, the helical diameter of focused laser beam is determined by the tilt angle of wedge prism, which alters the incident angle of the incoming raw laser beam in front of focusing lens, while the linear table regulates a defined lateral offset, which determines the incident angle of focused laser beam on workpiece. A helical path of laser beam in the borehole is primarily determined by the positions of wedge prism and linear table. Distinguished from a classic helical drilling process with a fixed helical path, a 3D helical process is characterized by a dynamic helical path generated by altering the position of wedge prism or linear table and hole taper during helical process. To fabricate a 3D shaped microhole using helical optics, both classic and dynamic helical drilling process are essential. In this work, a micro-funnel with an opening angle of 90 degree and a straight slant as a drilling example is fabricated in a 0.5 mm thick stainless steel with an ultrashort pulsed laser having a 7 ps pulse duration. A cylindrical stem hole can be drilled by a classic helical process, the hole entrance is shaped by the 3D helical drilling. During this process the deposited laser energy in the volume can be dynamically adapted by synchronizing the helical diameter and laser pulse energy. The results demonstrate the feasibility of extending the drilling conicity and high precision of shaped hole. Moreover, neither micro cracks nor recast layer is detected on the hole wall by using ultrashort laser pulses.