Porosity evolution during heating of copper made from powder by friction extrusion

Friction extrusion was used to compact and extrude solid copper rods from feedstock powders. Following extrusion, considerable porosity was observed throughout the extrudate cross section due to the entrained porosity in the feedstock material and the extrusion process. The thermal stability of the extrudate was investigated via a series of heat treatments. Porosity evolution exhibited three distinct stages—an unchanged plateau (0–300 °C) followed by an abrupt increase (400–500 °C) and ultimately a reduction, as the temperature increases (>500 °C). The peak porosity measured was ∼25%. The underlying driving force for pore evolution is described as the competition between the internal pore pressure, material strength, and sintering kinetics, as a function of temperature. The observed porosity evolution and driving force are not expected to be limited to copper. Thus, this manuscript reveals an important consideration regarding the microstructure thermal stability as advanced manufacturing methods involving direct powder extrusion are explored.