Modelling the semisolid processing of metallic alloys

7000 series aluminium alloys are currently machined from the wrought state with much waste. There is therefore a motivation to identify effective near net shaping routes. Semi-solid processing is one such potential route. It relies on the thixotropic behaviour of alloys with non-dendritic spheroidal microstructures. In the semisolid state the material thins when sheared and will flow to fill the die. When worked material is reheated, it recrystallises and moving into the semi-solid state, the required spheroidal microstructure develops. Here we examine the early stages of spheroid formation in 7075 aluminium alloy reheated from the as-supplied T6 condition. This alloy is very resistant to recrystallisation in the solid state due to the presence of dispersoid particles pinning grain boundaries. There is a sudden increase in the appearance of spheroidal grains. There is a close association with the position of the first liquid to form. On reheating as-supplied material to around 580°C (fraction of liquid ∼5%), a fully spheroidal microstructure can be obtained.

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Recrystallisation in the semi-solid state in 7075 aluminium alloy

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.

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Microstructural Coarsening of Semi-Solid Aluminium Alloys

Semisolid metal (SSM) processing or thixoforming is widely known as a technology that involves the formation of metal alloys between solidus and liquidus temperatures. For the procedure to operate successfully, the microstructure of the starting material must consist of solid near-globular grains surrounded by a liquid matrix and a wide solidus-to-liquidus transition area. Currently, this process is industrially successful, generating a variety of products with high quality parts in various industrial sectors. Throughout the years since its inception, a number of technologies to produce the appropriate globular microstructure have been developed and applied worldwide. The main aim of this paper is to classify the presently available SSM technologies and present a comprehensive review of the potential mechanisms that lead to microstructural alterations during the preparation of feedstock materials for SSM processing.

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Semisolid Metal Processing Techniques for Nondendritic Feedstock Production

An in situ Mg 2 Si/Al–Si–Cu composite with semisolid structure was fabricated by cooling slope cast and partial remelting process. The as-cast microstructure, and effect of isothermal holding time on the morphology, size and shape factor of the grains were investigated. The results show that the morphology of primary Mg 2 Si and α-Al grains in the composite are globular and/or elliptic after partial remelting process. The size and shape factor of α-Al grains increase with the isothermal holding time.

Semisolid microstructure of Mg2Si/Al composite by cooling slope cast and its evolution during partial remelting process

A fine grained metal composition suitable for forming in a partially solid, partially liquid condition. The composition is prepared by producing a solid metal composition having an essentially directional grain structure and heating the directional grain composition to a temperature above the solidus and below the liquidus to produce a partially solid, partially liquid mixture containing at least 0.05 volume fraction liquid. The composition, prior to heating, has a strain level introduced such that upon heating, the mixture comprises uniform discrete spheroidal particles contained within a lower melting matrix. The heated alloy is then solidified while in a partially solid, partially liquid condition, the solidified composition having a uniform, fine grained microstructure.

James Alan Courtois

Papers

Fine grained metal composition