Yield stress phenomena in semi-solid alloys

Abstract

Shear stress ramp experiments were performed in order to analyze the yield stress of the aluminium alloys AlSi5 and AlSi5Cu03Mg05Ag06 in the semi-solid state with regard to its dependency on time, solid fraction and particle size in the slurry. The experimental results were not influenced by the different chemical compositions of the alloys. The minimum solid fraction which is required for the occurrence of yield stress is called coherency solid fraction. The yield stress occurring immediately after shearing is called isostructural yield stress. At rest an internal structure is built up and the yield stress increases until the static yield stress is reached. During the transition from isostructural to static, the yield stress is called dynamic yield stress. The coherency solid fraction was estimated as a function of particle size by means of an extrapolation of the static yield stress to zero. The static yield stress is predominantly influenced by the distance between the particles which can be calculated from the particle size and the solid fraction. On the basis of dynamic yield stress measurements, the isostructural yield stress was estimated to be smaller than 10 Pa and an empirical model for the development of the dynamic yield stress was set up. Together, the experimental results allow for a complete description of yield stress as a function of particle distance and time.

Rheological Analysis of Semi-Solid A380.0 Aluminium Alloy / Analiza Właściwości Reologicznych Stopu Aluminium A380.0 W Stanie Stało-Ciekłym

“…The results of this work prove that the yield point occurs in the semi-solid metal alloys. Similar results were also obtained by other researchers, mainly form RwTH Aachen [13]. …”

“…These results mean that longer resting times cause microstructure transformation allowing to rebuild the solid skeleton. Such procedure was used in RwTH Aachen to investigate the thixotropy in the semi-solid metal alloys [13]. In this work the values of the yield stress were measured for 575 (Figures 6, 7) and 570°C (Figures 8, 9).…”

Rheological Characterization of Semi-Solid Metals: A Review

“…Experimental measurements show that an increase in solid fraction results in an increase of viscosity. In addition, also the yield stress increases with higher solid fraction, as demonstrated in scientific literature [15][16][17][18], together with the presence of the thixotropic effects. Other factors, such as particle diameter, or diameter distribution, and the shape of the particles are of minor importance.…”

“…Experiments [6,15] show that the yield stress depends on the solid fraction, the particle size and shape and on the resting time. With increasing resting time, the yield stress increases nearly exponentially, which leads to the concept of three different values of yield stress [15,16]:…”

Identification of the steel viscosity and dynamic yield stress for the numerical modelling of casting simulations in the semi-solid state

“…The value of shear stress recorded at this moment corresponds to the yield stress. A similar procedure was used in RWTH Aachen to investigate thixotropy in semisolid aluminium alloys [18]. In Figure 9 the flow stress curves for the 1.2080 steel for different resting times at 1340°C were shown.…”