**2. Principle of rotary-die equal channel angular pressing method**

The rotary-die equal channel angular pressing (RD-ECAP) method was developed for structuring fine grains in light metal, such as aluminum alloys, magnesium alloys, titanium and so on. In general, using equal channel angular pressing (ECAP), a large strain can be introduced into a billet by simple shear deformation without changes in the cross-sectional area; the billet develops fine grains after several passes of ECAP. However, in conventional ECAP method, the billet must be removed from the die and reinserted back for the next pressing, making the process inefficient. Using the RD-ECAP method, up to 4 passes of ECAP-style severe plastic deformation is possible without billet removal.

 Schematic diagrams of the RD-ECAP method are shown in Fig. 4. It consists of four cylindrical channels meeting at the center of the rotary die and four punches in the corresponding channels. The sample is set into the center of the hole. Then, the four punches are placed into the holes from the four directions and the die is set on a die holder. The die is heated to about 500–700 K and a plunger presses the punch at the top. The sample is extruded to the left direction because the right punch and the bottom punch are locked in place due to contact with the die holder. The remaining two channels are used for the conventional ECAP extrusion process. The punch at the top is pushed completely into the die to complete one extrusion or RD-ECAP process. After this extrusion, the die is rotated clockwise 90o to the initial configuration with the exposed punch at the top, and a second pressing is performed. The process continues until the die returns to its former position, after 4 passes. Then, because the sample is not reduced and the die is enough big, temperature of the sample is able to control with control of temperature of the die. The informal name for RD-ECAP is the Japanese term "Mochitsuki", which is the common process of making rice cake by pressing steamed rice again and again.

In the present work, the samples can be processed under conditions of 573–773K at an approximately 0.9–2.4 mm/s punch speed at 300MPa or lower. By the RD-ECAP method, ECAP processing could be repeatedly done without sample removal. In addition, the temperature of the sample could be easily controlled. In our study, samples processed over 30 cycles (one cycle=one extrusion and 90° die rotation) were obtained. One RD-ECAP cycle could be processed in 30 s. Therefore, RD-ECAP has the advantage of being energy efficient.

Fig. 4. Schematic diagram of rotary-die equal channel angular pressing. (a) initial state, (b) after one pass, and (c) after 90° die rotation

Compared with the conventional ECAP die consisting of two channels intersecting at an angle, the RD-ECAP die is easy to make because the channels in the RD-ECAP die are formed with two straight holes. Though there are many channels in the RD-ECAP die, the sample is always pressed from the same direction and general press equipment can be used.
