By: Khai Pham Xuan, Kazuya Saginawa, Rie Tanabe, Yoshiro Ito
Department of Mechanical Engineering, Nagaoka University of Technology
Recently, high precision molds for small products are being highly required in many industrial applications, especially in information communication technology. Mold materials with high hardness, such like cemented tungsten carbide (WC-Co), are difficult to machine by classical chip-removal techniques and the electrical discharge machining (EDM) is mainly used technology in microfabrication of hard materials. Even though the accuracy of EDM is fairly high K.H. Ho and S.T. Newmann, State of the art electrical discharge machining (EDM), it has several drawbacks such as slow machining rate and the additional time and cost needed for polishing and finishing processes. Recently, laser machining becomes an effective technique for machining of hard materials. Many experimental results show that short pulse laser machining allows removal of very small amount of material with little heat affected zone compared with nanosecond or longer pulse lasers.
In this study, direct fabrication on WC-Co using an ultrashort laser pulses have been carried out. Micro-holes of 100 to 300 mm diameters on plates of WC-Co of 0.5 to 1 mm thick have been successfully fabricated within several minutes by laser trepanning method. An amplified Ti: sapphire laser system which delivered 150 fs pulses at 785 nm with 1 kHz repetition rate was used. To scan focused laser beam on sample surface, we used a galvanometer scan head with an f-θ lens. The machining conditions, such as scan speed, focus position and polarization of the laser beam, were examined to obtain less tapered, more symmetric holes with less, negligible heat affected zone and high machining rate. Our experimental results show that machining speed is high enough to apply the femtosecond laser machining of WC-Co to practical mold fabrication: It took only 16 seconds to drill a through hole of 100 mm diameter on plate of WC-Co of 0.5 mm thick and less than 4 minutes for a through hole of 300 mm diameter on plate of WC-Co of 1 mm thick. Observation by a scanning electron microscope revealed that periodic surface structure with the period of less than 300 nm was formed on the entire side surface of drilled holes.