A study of forming loads and metal flow characteristics during the backward extrusion of aluminium

Backward extrusion is a popular industrial technique for the manufacture of tube and cans. This report serves to examine the forming loads, metal flow and extrusion defects arising during the backward extrusion of aluminium cups. The report surveys the literature of extrusion with particular interest in backward extrusion. Extrusion was carried out with the aid of a specially constructed 50mm backward extrusion tool designed and manufactured by the author. Five extrusion ratios were examined, using both the direct and indirect extrusion directions and flat, conical and nosed punch ends were used to examine metal flow and forming loads for various punch profiles. Forming loads were found to be much greater for direct extrusion and the usual relationship was derived for load/extrusion ratio characteristics. The effects of friction and lubrication were examined and these effects on load and flow patterns were noted. Incremental forming of split billets with grids scribed on the mid-plane was carried out for a variety of punch profiles. This revealed the evidence of a dead metal region forming and arc below the end of flat punches. The geometry of dead metal regions was compared with that derived by an upper bound technique and found to agree well. Also, upper bound loads using this model were derived and found to compare favourably. Forming using conical punch profiles showed that the adoption of a smooth profile similar in geometry to the dead metal zone reduced extrusion leads significantly. Extrusion defects were examined and found to fall into three categories, namely cavitation, internal cracking and shearing of the corner. Cavitation was found to occur when the base thickness becomes smaller than the wall thickness, which was also proved theoretically. Also cracking was found to occur only at large extrusion ratios during direct extrusion. Finally, in the light of conclusions drawn from the work, recommendations were made as to suggestions for further work which could usefully continue the present work.