In general, Ti-6Al-4V alloy sheets are formed by press forming in the temperature range of 600 and 900 ℃, which improves ductility. Because of the low ductility of Ti-6Al-4V alloy at temperatures between room temperature and 600 ℃, fracture easily occurs during press forming. Hence, to achieve press forming at 300 ℃, a method of press forming of Ti-6Al-4V alloy sheets at warm forming was developed. In this method, the punch motion and blank holding force were applied separately and incrementally to prevent fracture at the punch radius. In addition, the effects of forming conditions such as lubrication condition and punch motion on warm incremental press forming were investigated to achieve warm press forming without lubrication. As a result, by controlling the punch motion and punch speed in warm incremental press forming, we were able to obtain a formed cup without lubrication, the same as by forming with lubrication. By comparing the results at 300 and 500 ℃, we clarified that the formability was not affected by the forming temperature in warm incremental press forming.

Ultrasonic waves are effective in examining the contact state between the die and the material during sheet metal forming. In this study, ultrasonic measurements were performed on wrinkles and contaminant in press forming. A model specimen simulating wrinkles was used in the measurement of wrinkles. The contaminant was placed on a flat specimen and sandwiched between dies in the ultrasonic measurement of contaminant. The reflection intensity of 2.25 MHz ultrasonic waves irradiated to the convex part of the wrinkle decreased as the number of wrinkles with a wrinkle wavelength of 4 mm increased. However, the ultrasonic reflection intensity at the convex part of the wrinkles was not related to the number of wrinkles in 5 MHz ultrasonic waves. The contaminant changed the reflection characteristics of ultrasonic waves. The frequency characteristics of the reflected wave changed markedly at a specific frequency because of wrinkling and contamination. 2.25 MHz and 5 MHz ultrasonic waves were able to detect contaminants of the size occurring at the production site and wrinkles with a wrinkle height of 0.01 mm not only from the ultrasonic reflection intensity but also from the frequency characteristics.