Investigation of Surface Topology Effects on Metal Flow of Aluminum and Steel During Hot Compression

An investigation was conducted to study the effect of die surface roughness and lay on metal flow and the friction factor under lubricated hot compression. 6061-T6 aluminum and AISI 1018 steel rings and rectangular bar stock workpieces were compressed on H-13 tool steel platens of varying roughnesses ranging from Ra 0.25 – 6.1 µm (10 – 240 µin). Each platen was machined in such a way to ensure that the surface roughness would be unidirectional. Platen temperatures of 149 °C and 204 °C (300 °F and 400 °F) were used for all aluminum ring samples whereas platen temperatures ranging from 149 – 204 °C (300 – 400 °F) were used for all steel ring samples. Platen temperatures ranging from 121 – 204 °C (250 – 400 °F) were used for all aluminum and steel rectangular bar stock samples. All workpieces were compressed on a 22.6-metric ton (25 ton) hydraulic press composed of an experimental die set as well as appropriate measurement devices. Aluminum workpieces were compressed using high temperature vegetable oil and boron nitride as a lubricant. Steel workpieces were compressed using an aqueous graphite lubricant. The ring compression test was used under varying surface roughnesses to quantify metal flow in terms of interface surface friction seen between the platen and workpiece. It was found that friction factor increased with platen surface roughness as well as with workpiece orientation relative to the platen lay. The side compression test, or cigar test, was used to quantify metal flow in terms of true strains. Three workpiece orientations relative to the platen lay were used (0°, 45°, and 90°) to determine how platen lay affected metal flow. It was found that true strain generally increased with platen surface roughness as well as with workpiece orientation. It was also determined that in all cases, an increased platen temperature did not have much of an influence on metal flow