Modelling the semisolid processing of metallic alloys

Semi-solid metal alloys, as used in industrial thixoforming, have a special microstructure of globular grains suspended in a liquid metal matrix. The complex rheological properties are strongly influenced by the local solid fraction, particle shape, particle size and state of agglomeration. It was analysed how the microstructure develops in dependence of the shear rate and cooling rate during the solidification and it was observed that the average particle size increased with increasing shear rate and decreasing cooling rate. In order to account for those phenomena, the rate of crystal growth and the relationship between average particle diameter and viscosity was modelled by applying the Sherwood two-film model for the mass transport. The dependence of the viscosity from the particle size were modelled with a modified Krieger–Dougherty model. Based on the rheological and microstructural observations an evaluation method was elaborated that allows for the construction of objective master curves that are independent of the particle growth during the experimentation. The isothermal experiments for the characterisation of the rheological behaviour consisted of step-change of shear-rate and yield-stress experiments. From the experimental data the steady-state flow curves could be determined as well as the time-dependent relaxation of the shear stress after a change of shear rate. The steady-state rheological behaviour was found to be shear thinning. Nevertheless, immediately after a shear-rate change an overshoot was observed that resulted from a short-time shear-thickening behaviour. The yield stress was found to strongly depend on the microstructure and the degree of agglomeration of the solid phase. With increasing rest time the yield stress was increasing strongly, because of the agglomeration of the solid particles. Based on the step-change of shear-rate experiments a single-phase flow has been developed that consists of a modified Herschel–Bulkley approach and accounts for the thixotropic as well as for the yield-stress behaviour of the alloys.

Flow behaviour of semi-solid metal alloys

Semisolid metal processing (SSM) or thixoforming is a new technology that offers several advantages over liquid processing and solid processing. This process utilizes semisolid behavior as well as reduces macrosegregation, porosity, and forming forces during shaping process. A lot of research work has been carried out by various researchers in order to exploit the potential of this process to produce different products especially for automotive industry. This paper will summarise the rheological behavior of aluminium alloys in semisolid slurries, thixoformability of modified aluminium alloys, the effect of feedstock production method on mechanical properties, and the importance of developing low-cost raw materials for semisolid processing.

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An Overview of Semisolid Processing of Aluminium Alloys

Semi-solid metal alloys, as used in thixoforming, have a special microstructure of globular grains suspended in a liquid metal matrix. The complex rheological properties are strongly influenced by the local solid fraction, particle shape, particle size and state of agglomeration. There is a high demand for models and software tools allowing the simulation of semi-solid casting processes. The material under investigation is a tin-lead alloy (Sn-15%Pb) which exhibits a similar microstructure to aluminium alloys. The experiments were performed with a concentric cylinder rheometer of the Searle type. Initially, the liquid alloy is cooled down to the semi-solid range under constant shearing and then kept under isothermal conditions for further experimentation. Based on the experimental data, a single-phase model has been derived where the semi-solid alloy is regarded as a homogeneous material with thixotropic properties and the microstructure is characterised by a structural parameter. The model consists of two parts: the equation of state, including a finite yield stress, and a rate equation for the structural parameter. The model equations are employed in numerical software and used for the simulation of characteristic filling cases and the comparison with the conventional filling.

Time-dependent rheological properties of semi-solid metal alloys

In order to properly model and control the semi-solid processing of metallic alloys, their thixotropic behaviour requires proper characterisation. In particular, the effects of shear rate, shear time, temperature and rest time on the rheology of such slurries needs to be understood and quantified. A purpose-built high temperature Searle rheometer was used to determine the rheological behaviour of aluminium alloy slurries at shear rates from 3.1 to 124.8 s −1 , periods of shear of up to 60 min for each shear rate, and periods of rest (no stirring) of up to 60 min for Al–4wt.%Si and A356 alloys. Continuous cooling rheometry was used to determine the coherency point of the alloys. Isothermal fractions solid of 0.36 for Al–4%Si and 0.33 for A356 were investigated. Isothermal tests were used to follow the temporal evolution of viscosity, which was found to be significantly different for both alloys, particularly at low shear rates. Steady state viscosity values that were determined over a range of shear rates indicated severe pseudoplastic behaviour, as measured by a viscosity–shear rate power law index less than −1. This work confirms that this finding is an actual rheological feature and not an artefact of a particular measurement device. A study using shear rate jumps determined the isostructural behaviour of the alloys by discounting equipment inertial effects. It is shown that peak stress or apparent viscosity is a better indicator of slurry thixotropy than the hysteresis loop from shear ramping experiments, and the work also shows that the effect of agglomeration on fluidity is an important parameter to measure as it has consequences for thixoforming.

Experimental investigation of the transient and steady state rheological behaviour of Al–Si alloys in the mushy state

A rheological investigation has been conducted using a concentric cylinder rheometer on a semi-solid sn-15% Pb alloy at solid fractions between 0.2 and 0.5. The results show anomalous flow behaviour in that, at low shear rates, the shear stress falls with increasing shear rate up to 250 s−1 under steady-state conditions. Possible explanations of this anomalous behaviour are discussed and it is concluded that it is a genuine feature of the bulk slurry and not an experimental artefact. It is further suggested to be a consequence of the rapid breakdown of the internal structure of solid particle clusters within the slurry which occurs at low shear rates.

N.G. Henderson

Papers

Anomalous rheological behaviour of semi-solid alloy slurries at low shear rates