Hot Extrusion of Thin-Wall Multichannel Copper Profiles

This paper presents the development of a unique, net shape, hot-extrusion process to produce precision, thin-wall, multichannel copper profiles for high efficiency heat-exchangers. This process is a departure from conventional copper extrusion, which is a nonisothermal process used primarily to produce simple semifinished products and hollow profiles requiring cold drawing after hot extrusion. A lab-scale apparatus was developed to simultaneously extrude multiple heated billets through a porthole type hollow die to form the multi-channel profiles. The process is performed at 700–750 °C, essentially at isothermal extrusion conditions. Temperature and tooling strength considerations necessitated the use of superalloys for the apparatus (which included dies, container, ram stems, and support tooling). A 250 kN computer controlled servo-hydraulic MTS^® machine was used to provide the extrusion ram force. Two part designs were extruded to demonstrate process feasibility and versatility. A two-channel design with 0.2 mm wall thicknesses and an 11-channel design with wall-thicknesses of 0.3 mm were extruded. The extrusion ratios for these profiles are 67 and 25, respectively. Experimental data and an approach to analytically model the process are presented. Because solid-state welds in the tube walls are necessitated by the use of hollow extrusion dies, the microstructure in these regions is also presented.