Thin-walled copper tubes of various shapes are required to reduce the environmental impact of products by improving the performance of their heat exchangers. Tubes of various outer diameters along the longitudinal direction are conventionally manufactured by rotary swaging. However, the shape of the die is just transferred and thin-walled copper tubes are prone to torsion due to friction. To solve these problems, we applied the flexible ball-spin forming (FBS) method, in which ball-spin tools are pressed against a rotating circular tube at an arbitrary axial position. The FBS method was suitable for the flexible diameter reduction of thick tubes, whereas it caused polygonal or torsional defects in the case of thin tubes. Therefore, in this paper, we propose a method of spin forming using a torus-shaped tool for thin tubes. This method can hold tubes in the circumferential direction and maintain a near-circular shape. As a result, the tube diameter was effectively reduced and a reduction ratio of up to 57.1 % was achieved.

Solid-phase forge welding (SPFW) is an effective method for bonding dissimilar metals in bulk. However, the bonding parameters for achieving strong and stable bonding by SPFW are not clarified. To investigate the parameters, a new test method to evaluate SPFW was developed. Using this method, we prepared test pieces bonded with SUS304/A1070 with various surface enlarging ratios and surface pressures on the interface. Tension tests were performed to measure bonding strength. It was found that strong bonding was obtained in specimens exceeding the surface enlarging ratio = 3.0, the surface pressure = 1740 MPa. After the tension tests, SUS304 fracture surfaces were observed by electron microscope. As a result, it was found that the SUS304 fracture surface was divided into the oxide film area and the exposed virgin area. In addition, oxide films were cracked after the surface enlargement, and A1070 adhesion was observed only on the virgin area, which was exposed in the gap of oxide films.