APPLICATION OF HOT WORKABILITY STUDIES TO EXTRUSION PROCESSING .3. PHYSICAL AND MECHANICAL METALLURGY OF AL-MG-SI AND AL-ZN-MG ALLOYS

The conditions for homogenisation and cooling to improve the extrusion velocity and surface quality are derived from analysis of the influen ce of solidification segregation and constitutive particles in lowerin g the minimum melting temperature. Over-aged particles of Mg2Si do not reduce surface quality as do large equilibrium ones and do not raise the strength as much as Mg and Si in solution. A Si level above the Mg 2Si ratio augments the hot strength less than excess Mg and strengthen s the extrudate while diminishing toughness. During homogenisation coo ling and pre-heating, the precipitates in both Al-Mg-Si and Al-Zn-Mg a lloys must be limited in size to permit solution as they pass through the extrusion zone. Both alloys have low quench sensitivity suiting th em for rapid cooling upon exit from the press so that the extrudate is ready for age hardening. The influence of additional elements such as Cu, Mn, Zr and Cr on both extrusion process and product are reviewed. (C) 1997 Published by Elsevier Science Ltd.