Fabrication of functionally graded rods with three cylindrical layers made by spark plasma extrusion (SPE) process and characteristics of each layer

Abstract

Functionally graded (FG) rods with three layers in the radial direction were fabricated by a spark plasma extrusion (SPE) process. The FG rods consisted of a pure Ti compact in the center, a 15TiB/Ti composite layer in the middle, and a 25TiC/Ti composite layer on the outside. Extrusion was carried out at a temperature of 1000 °C, at a pressure of 50 MPa with extrusion ratios (cross-sectional area before extrusion/cross-sectional area after extrusion) of 2.3 (from d_i = 15 mm to d_o = 10 mm) and 3.5 (from d_i = 15 mm to d_o = 8 mm). An FG rod with a length of over 40 mm and a high relative density could be obtained by the SPE process. The cross-sectional area ratio of the pure Ti layer after extrusion significantly decreased compared to the 15TiB/Ti and 25TiC/Ti composite layers due to the higher ductility of the pure Ti. On the contrary, the cross-sectional area ratio of the 25TiC/Ti composite layer increased after the extrusion, because the 25TiC/Ti composite layer had quite low ductility. Due to the sufficient material flow during the extrusion, the TiC particles in the 25TiC/Ti composite layer of the FG rod were uniformly distributed along the extrusion direction. Furthermore, during the SPE process, the TiB clusters in the TiB/Ti composite layer were transformed into TiB whiskers, and the TiB whiskers were mainly oriented parallel to the extrusion direction. The tensile strength of the TiB/Ti composite layer fabricated by the SPE process improved due to the transformation of the TiB whiskers and their orientation along the extrusion direction.