In our previous studies, the experimental and numerical analysis results for flat and V-shaped stamping dies show that the contact state can be monitored using ultrasonic waves. However, machine parts made by stamping consist of not only flat surfaces but also inclined or curved surfaces. When an ultrasonic wave is incident on an inclined or curved surface, the modal conversion of the ultrasonic wave occurs. Thus, acoustic fields near the boundary surface become complicated. Therefore, examining the acoustic fields near the boundary only by experiment is difficult. In this study, the propagation behavior of ultrasonic waves on the boundary surface of a workpiece and stamping dies was investigated numerically by using the finite difference time domain (FDTD) method. The acoustic fields were examined using velocity potentials. We applied this numerical analysis method to the case of circular and flat dies with a small groove. The following numerical results were obtained. 1) The reflection of ultrasonic waves by the small groove was successfully visualized by this simulation. 2) When a thin metal plate was firmly attached to the stamping die, the transmitted waves were not greatly affected by the shape of the die.
In the processing of austenitic stainless steel SUS304, strain-induced martensitic deformation occurrs. In micropart processing, the effect on grain size becomes greater than that in the macropart processing. We reported that the grain size affects the stability of the sheared surface length in micropierced parts. In parthicular, the stability of the fractured surface length is affected by the grain size. The grain size of 3.0 µm yields the most stable fractured surface length. However, stabilization mechanism of start position of the fractured surface has not been clarified. In this study the grain deformation and transformation in micropiercing with coarse and ultrafine-grained SUS304 were investigated by EBSD (electron backscatter diffraction) analysis. Furthermore, the effects on the grain phase and misorientation angle in process-affected zones are reported. It was found that strain-induced martensitic deformation affected the stability of the sheared surface. The selection of an appropriate grain size is required to enable micropiercing.