Microstructural coarsening of semi-solid aluminium alloys

H.V Atkinson,D Liu

doi:10.1016/j.msea.2008.06.013

Abstract

Semi-solid metal processing involves processing metallic alloys between the solidus and the liquidus. The microstructure must be non-dendritic and consist of spheroids of solid in a liquid matrix. Two potential routes to suitable starting material are recrystallisation and partial melting (RAP) and cooling slope (CS). Here the microstructural coarsening of such materials in the semi-solid state is compared with rates in the literature. A slightly lower coarsening rate was observed for RAP route 2014 alloy with 37% fraction liquid than for CS route 2014 alloy with 17% fraction liquid despite the higher fraction liquid for the former. For the CS route, an increase in fraction liquid gave a higher coarsening rate. A relatively high coarsening rate was observed in a modified 2014 alloy with the Fe, Mn and Zn stripped out of the composition and a relatively low Si content. The coarsening rate was also relatively high for alloy 201 prepared by the CS route compared with alloy 2014 either by RAP or CS. It is likely that relatively low coarsening rates are associated with the presence of particles which are inhibiting the migration of liquid film grain boundaries, either through a pinning mechanism or through impeding diffusion through the liquid film at the boundary.

Highlights

Semisolid metal processing involves processing metallic alloys between the solidus and the liquidus
The coarsening rate was relatively high for alloy 201 prepared by the cooling slope (CS) route compared with alloy 2014 either by recrystallisation and partial melting (RAP) or CS
The RAP route is allied to the Strain Induced Melt Activated (SIMA) route [12] but in the SIMA route working takes place ‘hot’, i.e. above the recrystallisation temperature, and is followed by a period of cold work prior to reheating

Summary

Introduction

Semisolid metal processing involves processing metallic alloys between the solidus and the liquidus. The process depends on the material behaving in a thixotropic way in the semisolid state i.e. flowing when sheared but thickening again when allowed to stand This behaviour requires the microstructure to consist of spheroids of solid surrounded by a liquid matrix. Two routes were used to produce thixotropic material - recrystallisation and partial melting (RAP) [6] and cooling slope casting (CS) [7] The former has already been used commercially for the production of components, the latter is part of the process called New Rheocasting (NRC) [8,9] and has wider potential applications [e.g. 10,11]. The spheroids tend to be very well rounded in comparison with those produced by liquid state routes to thixoformable microstructures (for example, magnetohydrodynamic stirring) and give advantageous flow properties [13]. The main disadvantage is that wider diameter billets are difficult to work and there may be inhomogeneity in the degree of work introduced across the component, leading to inhomogeneous spheroidal microstructure across the cross-section

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Discussion

Conclusion