Semi-solid processing (SSP) is now an established technology, there being many companies who are increasing the use of thixo-forming techniques. The principal reason forth-is interesting position is that SSP enables the combination of the high mechanical properties resulting from plastic deformation with the economic advantages of near-net-shaping. This work analyses the formability of Al-4.5% Cu alloys in the semi-solid state by forging and by extrusion (thixo-forging and thixo-extrusion). Forging and direct extrusion tests were carried out in order to make feasible the production of simple-shaped geometry. Forming forces were monitored during the processes in order to enable the analysis of the deformation behavior. The products obtained showed good quality in terms of dimensional accuracy and surface finish, comparable to those obtained by conventional forming, for both processes. For the particular geometry studied, the maximum force required for thixo-forging was 35% of that required to forge from. the solid state. For thixo-extrusion, the maximum force required was 20% of that required for conventional extrusion. These values can be reduced by increasing the liquid fraction in the feed stock (which, however, must be kept low to prevent material decohesion during deformation) or decreasing the degree of deformation. Macrostructures of the products showed highly homogeneous material, with fine equi-axial grains, free from residual stresses. Some separation of liquid to the outer regions could be observed, which, however, can be avoided by increasing the deformation rate. The microstructures showed higher Cu content In solution than in conventional forged and extruded products, which can lead to more convenient conditions for further homogenization treatments. (C) 1999 Elsevier Science S.A. All rights reserved.934249