Spheroidal particle stability in semisolid processing

Abstract

A model for diffusion-controlled spherical particle growth is presented and solved numerically, showing how, on cooling at sufficient rate from a given fraction solid, growth velocity first increases, and then decreases rapidly when solute fields of adjacent particles overlap. An approximate analytical solution for the spherical particle growth velocity is then developed and shown to be valid until the solute fields begin to overlap. A particle stability model is next presented, building on the above analytic solution. This model permits prediction of the maximum cooling rate at which a semi-solid slurry or reheated semi-solid billet can be cooled while still retaining the spherical growth morphology. The model shows that particle stability is favored by high particle density, high fraction solid and low cooling rate. The predictions of the stability model are found to be in good quantitative agreement with experimental data collected for Al-4.5wt%Cu alloy. Engineering applications of the results obtained are discussed.

Grain Morphology of As-Cast Wrought Aluminium Alloys

“…low Q 0 values. The morphology change with grain refinement has been observed previously in grain refinement processes for the production of semi-solid slurries 5,22,32) and for ultrasonic grain refinement of alloys. [33][34][35] Interestingly Ti additions have no discernable effect on the SDAS, even in alloys with low Q 0 values where they have a very large effect on the grain size.…”

“…A spherical grain structure is obtained where the number of successful nucleation events is large enough that the final grain morphology is non-dendritic. 5,34) Given that an alloy has a characteristic SDAS, particular grain morphologies can be defined. In Fig.…”

“…The morphology of equiaxed grains has often been related to the breakdown of spherical growth. 5,9) However, it is also known that dendrites coarsen during soldification 10) to the point that the grains become spherical. 11) There is also evidence in peritectic systems such as Al-Ti and Mg-Zr that whilst initial growth is dendritic as shown by the morphology of the Ti- 12) or Zr-enriched regions, 13) the final grain morphology can be spherical or globular.…”

“…1. Whilst there are some situations where grains will grow spherically, 5,9) e.g. skip stage 2 and go directly to stage 3 or 4 in the figure, it is likely that the same processes are acting.…”

Solidification microstructures and solid-state parallels: Recent developments, future directions

“…Theoretical and experimental studies of non-steady-state growth transients have also been carried out that provide new insights into transitions between globular and dendritic microstructures in both equiaxed alloy solidification [142] and semi-solid processing [143]. Related progress in understanding transitions between columnar and equiaxed microstructures is reviewed in Section 5.2.…”