Thixoforming of Otherwise Wrought Aluminium Alloys: Thermodynamic Prediction of Amenable Compositions

Thixoforming-or semi-solid processing- is the shaping of metal components in the semi-solid state. For this to be possible, the alloy must have an appreciable melting range and, before forming, the microstructure must consist of solid metal spheroids in a liquid matrix. Thixoforming is in commercial use but only with the casting alloys A356 and A357. One of the major challenges for thixoforming is to broaden the range of alloys which can be successfully thixoformed and to develop alloys which are specifically designed to be amenable to the process. Here the critical scientific issues for alloy development for thixoforming will be summarized and recent work on the thermodynamic prediction of amenable compositions discussed

Experimental Determination of the Parameters for Modelling Semi-Solid Processing

The numerical modeling of semi-solid processing requires data on the rheological properties of materials. This data is often obtained by rheometry but there are difficulties with characterizing high solid fractions, where the torque which can be exerted with the rheometer is insufficient. A number of other methods for measuring the flow parameters, including compression between platens, have been utilized. The various methods will be reviewed in this paper. Computational fluid dynamics modelers have often used data from steady state experiments but it is the behaviour during rapid transients which is more relevant to the actual semi-solid processing route

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Modelling the Semi-Solid Processing of Metallic Alloys

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 co-workers 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

Semi-Solid Processing of Metallic Materials

Semi-solid processing involves forming metallic alloys between the solidus and the liquidus. For the process to operate, the microstructure must consist of solid spheroids in the liquid matrix, rather than dendrites. The material then flows when it is sheared but thickens again when allowed to stand, i.e. it behaves thixotropically. This type of behaviour was first discovered by Flemings and co-workers in the 1970s and is utilized in a family of processes, some now applied commercially. Here the current status of semi-solid processing, both technologically and from a research point of view, will be reviewed

A Review of the Modelling of Semi-Solid Processing from an Experimentalist’s Point of View

Semi-solid processing of metallic alloys and composites utilises the thixotropic behaviour of materials with non-dendritic microstructure in the semi-solid state. Modelling of die fill leads to more effective die design to avoid defects. Here approaches to modelling are summarised. They can be categorised as one phase or two phase and as finite difference or finite element. Although many workers have qualitatively compared their predictions with experimental results (usually obtained through interrupted filling), there is little quantitative data. There are no direct comparisons of the capabilities of various software packages to model die fill accurately. In addition, the modelling depends on rheological data. 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

Analysing the semi-solid response under rapid compression tests using multi-scale modelling and experiments

Simulating semi-solid metal forming requires modelling semi-solid behaviour. However, such modelling is difficult because semi-solid behavior is thixotropic and depends on the liquid-solid spatial distribution within the material. In order to better understand and model relationships between microstructure and behavior, this paper presents a model based on micromechanical approaches and homogenisation techniques. This model is an extension of a previous model established in a pure viscoplastic framework to account for elasticity. Indeed, experimental load-displacement signals revealed the presence of an elastic-type response in the earlier stages of deformation when semi-solids are loaded under rapid compression. This elastic feature of the behaviour is attributed to the response of the porous solid skeleton saturated by incompressible liquid. A good quantitative agreement is found between the elastic-viscoplastic predicted response and the experimental data. More precisely, the strong initial rising part of the load-displacement curve, the peak load and the subsequent fall in load are well captured. The effect of solid fraction on mechanical response is in qualitative agreement with experiments

Coarsening rate of microstructure in semi-solid aluminium alloys

In semi-solid metal processing, the temperature is between the solidus and the liquidus. To behave thixotropically, the microstructure must be non-dendritic and consist of spheroids of solid in a liquid matrix. Recrystallisation and Partial melting (RAP) and the cooling slope (CS) are two potential routes to suitable non-dendritic starting material. Here the rate of microstructural coarsening of such materials in the semi-solid state is compared with rates found in the literature. The rate of coarsening depends on the liquid fraction but RAP route 2014 alloy with 37% liquid coarsens slightly more slowly than the CS route 2014 alloy with a lower liquid fraction of 17%, contrary to expectations. For the CS route, an increase in liquid fraction resulted in faster coarsening. A modified 2014 alloy with the Fe, Mn and Zn stripped out of the composition gave a relatively high coarsening rate. The coarsening rate was also relatively high for CS 201 alloy in comparison with either RAP 2014 or CS 2014. Low coarsening rates are thought to be associated with the presence of particles which are inhibiting the migration of liquid film grain boundaries. This could be the result of pinning or of the liquid film impeding diffusion at the boundary

Peak Load in Rapid Compression Tests: Experiments and Micromechanical Modelling

Rapid compression tests are useful to characterize the semisolid behaviour in thixoforming conditions. The load-displacement curves typically display a peak load. The origin, the maximum value and the height of this peak are discussed comparing calculated results obtained using a micro-macro model and experimental data obtained in earlier work. As suggested by experiments, the calculated peak load originates from a 3D continuous solid skeleton, which breaks down under load. For the same initial semisolid structure, the model predicts that decreasing the ram speed decreases the maximum of the peak because of a strain rate effect. In addition, the peak load is found to be quite sensitive to initial internal structure, namely the volume fraction of entrapped liquid, which may change for various soaking times. Finally, the time for the solid skeleton breakdown is found to be in good agreement with experimental results

Flow Visualisation for Semi-Solid Alloys

A recent review of modelling of semi-solid processing has highlighted the need for appropriate methods of validation of models. Experimental details are given here of a flow visualisation method for tin/lead and aluminium alloys