Extrusion behavior of metal–ceramic composite pipes in multi-billet extrusion process

Two-layer composite pipes consisting of zirconia and stainless steel were fabricated by a multi-billet extrusion (MBE) technique. The MBE process is characterized by coextruding different kinds of particulate pastes containing a water-based binder, joining them in chambers and finally obtaining a desired extrudate by means of dies. It is shown that sound extrudates can be obtained no matter what mixing ratios of zirconia and stainless steel powders in both inner and outer layers are used. The MBE requires a larger amount of binder compared to conventional rod extrusion. Furthermore, a better flowability is necessary for the outer layer than for the inner one in the extrusion process of a two-layer pipe, as a result of the characteristic structures of the extrusion dies. The amount of binder can be minimized by using mixed powder with a wider particle size distribution and a die with a longer land. It is effective to add a small amount of glycerol or use partially stabilized zirconia powder in restraining the crack formation in extrudates during drying period.