Evolution of microstructure and rheological behavior of alloys in the semisolid state

Semisolid processing is a net-shape metal forming method that combines the advantages of forging and casting. In this work, emphasis is placed on a comparative evaluation of the major methods that produce near-equiaxed microstructures suitable for subsequent semisolid processing: magnetohydrodynamic (MHD) casting, strain-induced melt activation (SIMA), and spray casting. The microstructural, material and process parameters that affect the behavior of semisolid materials are identified in each method. The microstructural evolution of the solid phase within time intervals characteristic of semisolid processing is examined in wrought and cast aluminum alloys. This work shows the presence of abnormal grain growth in some spray cast materials as a result of the initial microstructure. A new model is proposed to describe the spherodization of dendrites in the semisolid state. Unconstrained compression and parallel plate 'viscometry' experiments were carried out in order to examine the rheological behavior of semisolid materials. The presence of strain localization and inhomogeneous deformation, especially at lower volume fractions of solid is observed. Deformation at low liquid contents creates significant internal damage imposing a low liquid content limit, of the order of 40v/o, for the applicability of the process. Strain rate 'jump' tests exhibit the reduction of the apparent strain rate sensitivity with the increase of the liquid content. Modulus measurements indicate that cohesion in semisolid materials is limited. The presence of liquid phase segregation during deformation is observed. Segregation is reduced by increasing triaxiality and strain rate and minimizing the total strain and grain size. A phenomenological model for a two-phase material is proposed, accompanied by a mathematical modeling framework, to describe the resistance to flow of semisolid materials. Semisolid materials exhibit non-uniform, pressure dependent, strain rate independent, granular flow during deformation at medium liquid contents and in the range of strain rates examined. Finally, a simple criterion for processability in the semisolid state is introduced, based on the sensitivity of the liquid content to temperature fluctuations. It is also proposed that processability is enhanced by using microsegregated raw materials, which decrease both the sensitivity of liquid content to temperature and the processing temperature.