A mechanical joining process of a bar and nut to a hard hot-stamped steel sheet was developed, because the resistance projection welding generally used for joining of bolts and nuts to hot-stamped sheets has low joinability. In this process, a slightly smaller hole than a bar and nut was made in the sheet with the bar and nut having a small round or chamfer at the bottom corner, and then the bar and nut were inserted into the hole. The punched hole was ironed with the bar and nut during the insertion, and thus the joint strength became high. No pre-punching is required for the sheet, whereas higher strength than that for the sheet is necessary for the bar and nut. For the bar and nut having a radius of 1.0 mm at the bottom corner, the ironed surface was the largest, and thus the maximum joint strength was obtained.

In the hot working of multiphase steels, the thermal history before hot working affects the phase balance. The aim of this study is to investigate the effect of the thermal history just before hot working on the hot workability by performing hot uniaxial tensile tests. Duplex stainless steel was chosen as the test material, as this material has a typical dual-phase structure consisting of δ ferrite and austenite, and its phase balance changes with the working temperature. The phase ratio of δ ferrite is higher at high temperatures, and the ratio of the austenite phase increases as the temperature decreases. To clarify the effect of the cooling rate just before hot working on hot workability, a hot tensile test was carried out with two extremely different cooling rates (0.3 °C/s, 10.0 °C/s) from the heating temperature of 1250 °C. The hot working test temperature was in the range of 650 to 1150 °C. The hot workability under the rapid cooling condition was higher than that under the slow cooling condition in the hot working temperature range of between 850 to 1150 °C, but this tendency was eliminated or reversed under conditions below 750 °C. The reason for this change was clarified by investigating both the phase balance just before hot working, which depends on the cooling rate, and the strength of each single phase. Thus, for accurate measurement of the hot workability of multiphase materials, consideration of the thermal history just before hot working is important.

Magnesium and its alloys are expected to be applied in medical and electric device industries. Magnesium has excellent characteristics such as light weight, high strength, and low toxicity to humans since it is an element essential to our health. The demand for manufacturing small-diameter tubular products has increased leading to small-sized devices. Tube hydroforming (THF) is a plastic processing method that has less restriction on the shape and size of workpieces owing to deformation by a liquid tool. THF has been predicted to enable the manufacture of small-diameter tubular parts made of magnesium alloy. In this work, warm THF was applied to the processing of small-diameter ZM21 magnesium alloy tubes with 2.0 mm outer diameter and 0.15 mm thickness. The effects of temperature, effects of internal pressure, and the amount of axial feed on formability in THF were confirmed. As a result, the influences of temperature on the deformation resistance, bursting pressure, and ductility of the tube were elucidated. Furthermore, the appropriate amount of axial feed and internal pressure were clarified for expanding the small-diameter tubes. Consequently, a bulge with an outstanding height of 1.5 times the outer diameter was successfully fabricated experimentally. The deformation characteristic of tube was also evaluated.

The actual punch load, contact pressure, and frictional stress on the punch surface has been measured in air-stamp hammer forging. An example of stamping where the punch is inserted into the center of a circular cylinder in a circular die with flash land has been chosen for the detection of contact stress on the punch. The punch load was detected by fitting strain gauges on the side wall of the inner hole machined on the bottom surface of the punch. Pressure and friction sensors were installed on the top flat surface of the punch, the corner of the punch, and the flash land surface. In the stamping process where the work material is only pressed by the punch top, the pressure on the punch was nearly equal to the punch load divided by the top area of the punch. When the compressed bulged material came into contact with the outer side surface and the deformed material came into contact with the bottom of the punch, the friction on the corner of the punch increased. The punch load increased rapidly when the material filled the die cavity. The pressure and friction applied on the flash land surface increased as the flash thickness and temperature decreased with the progress of stamping.