Rheology and microstructure of semi-solid aluminum alloys compressed in the drop-forge viscometer

Abstract

The rheological behavior and microstructure of semi-solid aluminum alloys were studied using a novel apparatus, the drop-forge viscometer (DFV). The viscometer determines force from the second-derivative-of-displacement data with respect to time and permits calculations of viscosities at shear rates in excess of 1000 s⁻¹. Alternatively, the DFV can be operated like a conventional parallel-plate viscometer, attaining shear rates as low as 10⁻⁵ s⁻¹. Rapid compression experiments (in the DFV) result in first rapidly increasing, then decreasing, shear rates. In a typical experiment, the viscosity decreased from about 100 to 1 Pa·s as the shear rate increased from approximately 200 to 1300 s⁻¹ in less than 4 ms. The viscosity later increased to about 10 Pa·s as the shear rate decreased from 1300 to 30 s⁻¹ over 2 ms. The minimum viscosity obtained depended on the maximum shear rate, not the duration of shear. The dual observed phenomena of (1) a very rapid drop of viscosity with increasing shear rate followed by (2) a relatively slow increase of viscosity with decreasing shear rate thereafter have potential significance for future machine and process design. For example, it should be possible to form higher fraction solid slurries than is now feasible by applying vigorous shear to semi-solid slurries just before the metal is introduced to the die entrance. The DFV was used to calculate viscosity as a function of shear rate for samples produced by the commercial strain-induced, melt-activated (SIMA) and magnetohydrodynamic (MHD) methods, as well as the recently developed Massachusetts Institute of Technology (MIT) method. Isothermal experiments were conducted between fraction solid of 0.44 and 0.67 for the various alloys (corresponding to a temperature range of 579 °C to 611 °C). The viscosity of the commercial semi-solid Al-Si alloys A357 and A356 produced by the various methods was similar. Separation of liquid and solid phases was not observed in rapid compression experiments shorter than 10 ms, either visually or with energy-dispersive spectroscopy (EDS) characterization. At low compression velocities, segregation was observed and increased with increasing amounts of strain. The maximum fraction solid compressed at high and low shear rates were 0.67 and 0.69, respectively.