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Pure titanium foils were bent by laser forming and the effect of α-β transformation and history of heat treatment of specimen on bending angle was investigated. The thickness of specimens was changed from 40 to 100μm, the length of them was 20mm and the width of them was 10mm. The specimens were annealed at 600-1100C for 30 minutes in argon atmosphere. A 20W YVO4 laser was employed and laser power was changed from 2 to 16W. From the experimental results, when laser power was increased, bending angle also increased and it was dramatically changed at the laser powers occurring α-β transformation and melting. Bending angle increased as grain size increased and it jumped up when grain size exceeded the foil thickness and then became constant. Bending angle decreased by annealing after forming and degree of decrease was greater when the annealing temperature before forming was lower. Introduction Recently, reduction of mechanical and electrical parts size is advancing and high precision is required for forming micro parts. In forming metal foils for micro parts, bending with high precision angle becomes more difficult as foil becomes thinner because of the effect of spring back. Laser forming process is one of forming methods for sheets and foils. In this process, distributions of thermal stress and reduction of yield stress due to local rapid heating by laser irradiation is used for bending of sheets. Thus, the effect of spring back is very small and this process is suitable for bending sheets and foils. By the way, general metals appear volume change due to phase transformation when they are heated to high temperature. Although laser forming utilize distribution of volume change, it is not taken into consideration for deformation mechanisms and equations of estimating bending angle, clearly [1-2]. In order to estimate bending angle more precisely, volume change by phase transformation should be investigated. In forming of metal parts, heat treatment is often carried out before and after forming. When heat treatment is performed before forming, microstructure such as grain size and shape, and phase change. When heat treatment is carried out after forming, phase transformation is happened, microstructure changes and residual stress disappears. Those changes by heat treatment affect to formability and shape change after heat treatment. In this study, pure titanium foils were employed for specimen and effect of volume change by phase transformation during forming was studied. By varying laser power, bending angle was measured in the cases occurring no transformation, transformation and surface melting. Next, effect of heat treatment on formability and shape change after heat treatment was investigated. Experimental Method Material. In this study, pure titanium foils with a length of 20mm, width of 10mm and thickness of 40, 50 and 100μm were employed for specimen. They were annealed at 600C for 30 minutes in argon atmosphere. Pure titanium has two crystal structures in solid state called α titanium having hcp structure and β titanium having bcc structure. The α-β transformation temperature is 882.5C [3]. When pure titanium is lower than this temperature, hcp structure appears and if the temperature is over the temperature, α-β transformation to bcc structure is occurred and the volume increases 5.5% [4]. Melting temperature is 1668C [5]. Experimental Apparatus. An experimental apparatus for laser forming and shape measurement is shown in Fig. 1. To prevent oxidizing and nitriding during forming, the specimen was put in a glass chamber with argon atmosphere. A 20W YVO4 laser which wave length is 1064nm was used for laser forming. The glass chamber was fixed on a NC table. For forming, laser beam was irradiated on the specimen and scanned by galvano mirrors to perpendicular to the longitudinal direction. After forming, the glass chamber was moved to shape measuring place and bending angle was measured by a laser displacement sensor. Experimental Procedure. First, effect of volume change by phase transformation on bending angle was investigated. Working conditions are shown in Table 1. Laser power was changed and bending angle was measured. Since the thickness of foils are very small, it was often difficult to distinguish if the transformation occurred or not from observation by optical microscope, micro vickers hardness test was also carried out. Laser Forming Measurement of Bending Angle Laser Displacement Specimen

Effect of Heat Treatment and Transformation on Bending Angle in Laser Forming of Titanium Foils