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 secondderivative-of-displacement data with respect to time and permits calculations of viscosities at shear rates in excess of 1000 s Ϫ1 . Alternatively, the DFV can be operated like a conventional parallel-plate viscometer, attaining shear rates as low as 10 Ϫ5 s Ϫ1 . 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 Paиs as the shear rate increased from approximately 200 to 1300 s Ϫ1 in less than 4 ms. The viscosity later increased to about 10 Paиs as the shear rate decreased from 1300 to 30 s Ϫ1 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 straininduced, 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 ЊC to 611 ЊC). 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.

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Aluminum-copper alloys offer both high strength and excellent ductility suitable for a number of automotive applications to reduce vehicle weight; however, the alloys are difficult to cast because of their tendency for hot tearing. In this work, semi-solid gravity casting of an aluminum-copper alloy, B206, was conducted in constrained rod casting molds to study the feasibility of using the process to reduce or eliminate hot tearing. To demonstrate the feasibility of gravity casting of the metal slurries, a fluidity test was also conducted. Results show that the hot tearing susceptibility of the aluminum-copper B206 alloy cast in semi-solid state is lower than those cast in liquid state with high superheat temperatures. The grain size of the semi-solid cast Al-Cu samples appears to be finer than those cast in liquid state with high superheat temperatures. In addition, the metal slurries had sufficient fluidity to fill the molds even with low gravity pressures. The results suggest that semi-solid gravity casting is a feasible process to help reduce hot tearing.

Measuring fracture toughness of metallic glasses (MGs) is challenging, and a large scatter has been observed which has been at least partially attributed to varying processing conditions. Here, we investigated the influence of main processing conditions, cooling rate and processing environment, on the fracture toughness of MGs. Through a thermoplastic forming based toughness evaluation approach, we found that processing effects can be dramatic and typically very specific to the alloy. Hence, general predictions of processing effects are challenging and MG specific characterization are required; however we offer some insight how the cooling rate effect can be explained with fictive temperatures.

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