STUDY ON REDUCTION OF POLISHING TIME OF METAL MOLD USING LASER IRRADIATION

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At present, the cold working die steel, SKD11 is commonly being used as the metal molds due to its superior wear and corrosion resistance. In electrical discharge machining(EDM) of die steel, a hard white layer, that is different from the base metal, is formed on the metal mold surface causing rapid heating and cooling. Cracks and micro-craters appear on the surface of white layer depending machining conditions used. Such heat-affected zones(HAZ) can greatly shorten the life of metal molds. Therefore, the removal of this layer after EDM is necessary. Under the present situation, HAZ is removed by hand finishing. However, this process extends the machining time and raises the price of metal mold manufacturing.Therefore, a laser surface modification with melting has been proposed as a new surface modification method instead of the mechanical removal method. This method releases defects such as cracks and micro-craters and improves surface roughness using melting and solidifying HAZ by us … More ing a laser beam as the heat source. In this study, the laser beam was irradiated in three-dimensional metal mold with a area of 50 x 50mm^2. The surface modification was conducted using a 1kW CW carbon-dioxide laser. Argon gas was used on the metal mold coaxial with the laser beam in order to avoid oxidation of the surface. The laser beam was irradiated onto the electrical discharge machined surface with a laser output power of 800W, a feed speed of200 mm/min, a defocused condition of 5mm and step width of 1mm. As the result, the following conclusions are obtained.(1)Surface defects such as cracks and micro-craters generated by EDM could be released due to melting of surface layer. Subsequently, a melting layer could be obtained which is distributed uniformly after irradiation. It could be therefore shown that the new surface modification method improved the electrical discharge machined surface.(2)The surface roughness of about 25μmRy in the electrical discharge machined surface of metal mold could be decreased to 2〜5μmRy due to the overlapped laser treatment. The increase in surface roughness could be observed in the case of overlapped laser treatment surfaces. Less