Viscosity Of Semi-solid Alloy Research Articles

Experimental Investigation on the Rheological Behavior of Hypereutectic Al-Si Alloys by a Precise Rotational Viscometer

Determining the viscosity of low solid volume fraction semisolid alloys is important for predicting the gradient of primary particles within functionally graded materials (FGMs), produced by in-situ casting processes. In this study, a new precise rotational viscometer was developed and used to measure the viscosity of Al-22 pct Si and Al-30 pct Si semisolid alloys up to solid volume fractions of 9 and 20 pct, respectively. Three kinds of typical curves, viscosity (η) vs solid volume fraction (fs), shear time (t), and shear rate (\( \dot{\gamma } \)), were derived from the results of viscosity measurements for both Al-Si alloys. In the semisolid Al-Si alloys, thixotropic behavior was not detected at low solid volume fractions, but this behavior was obviously observed with increasing solid volume fraction and could be described by an analytical model. Finally, the results showed that the equilibrium viscosity of the semisolid alloys with thixotropic properties decreased by increasing shear rate according to the Ostwald–De Waele power law. A special test, developed in this research, was used to show the effect of agglomeration on the viscosity of the semisolid alloys.

Viscosity Estimation of Semi-Solid Alloys by Drop Weight Backward Extrusion Test under High Speed Deformation

A simple method for estimating the apparent viscosity of semi-solid alloys was investigated. A drop weight of 50Kg and backward extrusion equipments were used to test the deformation. Backward extrusion was employed in a closed die system due to the very little separation of liquid from solid phases compared with other methods at very high deformation rate. The half of the calculated impact velocity, 700mm/s was considered as an average velocity for deformation during the test, since the slop of the velocity changes against time is constant in the middle stage of the deformation. The initial height of the sample and the thickness of the residual compressed solid at the bottom of the cup were used to calculate the amount of apparent viscosity of A356 aluminum alloy in this work.

Study on Apparent Viscosity of Semi-Solid Alloys during Rheocasting

An analytical model of apparent viscosity was built up based on analysis of energy dissipation during rheocasting. By study the evaluation law of microstructure analyzed with a quantitative metallographic analysis system for semi-solid alloys, the variation law of apparent viscosity with time has been obtained. The model was verified in the experiment of A356 alloys with a coaxial double-bucket rheometer, and the results of simulated experiment and theoretical calculation are in a good agreement.

Deformation behavior of semi-solid A356 Al–Si alloy at low shear rates: The effect of sample size

The rheological behavior of semi-solid metal slurries at low shear rates can be characterized by parallel plate compression tests of cylindrical discs where the height ( h) is much smaller than the diameter ( d) and the axial flow is assumed negligible or zero against the radial flow. In order to confirm the validity of required aspect ratio, two sets of cylindrical test samples with ( h/ d) of 0.4, d = 24 mm small discs, and 1.8, d = 75 mm billets, were prepared with similar microstructures. The small specimens were tested in a parallel plate squeezing flow rheometer to determine the viscosities which were compared to the viscosity values of samples (billets) with larger aspect ratios using a similar large scale apparatus. For the billets, rheocast samples, it was shown that the dendritic primary α-Al structure has the highest viscosity. The viscosity decreases when the structure comprises a rosette or globular morphology. Similar results were obtained for the discs, thixocast samples, where the specimens have similar initial microstructure. The results confirmed that for low shear rates, less than 10 −2 s −1, the viscosity of semi-solid alloy is dependent mainly on the morphology of the primary particles and not the sample size.