Modelling the Semisolid Processing of Metallic Alloys

Abstract

Semi-solid processing of metallic alloys and composites utilises the thixotropic behaviour of materials with non-dendritic microstructure in the semi-solid state. The family of innovative manufacturing methods based on this behaviour has been developing over the last twenty years or so and originates from scientific work at MIT in the early 1970s. Here, a summary is given of:-routes to spheroidal microstructures; types of semi-solid processing; and advantages and disadvantages of these routes.Background rheology and mathematical theories of thixotropy are then covered as precursors to the main focus of the review on transient behaviour of semi-solid alloy slurries and computational modelling. Computational Fluid Dynamics (CFD) can be used to predict die filling. However, some of the reported work has been based on rheological data obtained in steady state experiments, where the semi-solid material has been maintained at a particular shear rate for some time. In reality, in thixoforming, the slurry undergoes a sudden increase in shear rate from rest to 100s -1 or more as it enters the die. This change takes place in less than a second. Hence, measuring the transient rheological response under rapid changes in shear rate is critical to the development of modelling of die filling and successful die design for industrial processing. The modelling can be categorised as one phase or two phase and as finite difference or finite element. Recent work by Alexandrou and co-workers and, separately Modigell and co-workers, has led to the production of maps which, respectively summarise regions of stable/unstable flow and regions of laminar/transient/turbulent fill. These maps are of great potential use for the prediction of appropriate process parameters and avoidance of defects. A novel approach to modelling by Rouff and coworkers involves micro-modelling of the 'active zone' around spheroidal particles.There is little quantitative data on the discrepancies or otherwise between die fill simulations and experimental results (usually obtained through interrupted filling).There are no direct comparisons of the capabilities of various software packages to model the filling of particular geometries accurately. In addition, the modelling depends on rheological data and this is sparse, particularly for the increasingly complex two-phase models. Direct flow visualisation can provide useful insight and avoid the effects of inertia in interrupted filling experiments.

Rheological behavior of semi-solid AZ91D magnesium alloy at steady state

“…This is consistent with the experimental observations reported in earlier works [4][5][6] . The relationship can be described by the following equation [5] :…”

“…Furthermore, some models for the rheology of semi-solid metal slurry in steady state have been presented [4][5][6][7][8][9] . However, these works are mainly concentrated at Al alloys and Sn-Pb alloys.…”

Transient semi-solid behaviour: modelling step-change up and step-change down in shear rate using a micromechanical approach

“…Many experiments were achieved in the last past two decades to characterize the semisolid rheology. However, many disparities appear between literature results [1][2][3][4][5][6][7]. As far as the steady-state is concerned, the presence of strong shear thinning behaviour is now well-admitted.…”

“…As far as the steady-state is concerned, the presence of strong shear thinning behaviour is now well-admitted. However, the value of the strain rate sensitivity parameter (power-law index when the behaviour is mathematically represented using a power-law relation) ranges from -1 to 1 depending on the solid fraction and the shear rate range of interest [1][2][3][4][5][6][7]. As far as the transient behaviour is concerned, literature results show contradicting tendency for the shear-rate dependence.…”

“…More promising modelling are those introducing structural parameters to represent the microstructural changes under shearing [1]. Recently, Favier et al [8] propose a constitutive equation accounting for the mechanical role of liquid and solid and of their spatial distribution within the material on the overall behaviour of semisolids.…”