Overview
Français
ABSTRACT
The forming of alloys from an already partially solidified state is highly preferable to that of forming in the liquid state, as it avoids the disadvantages of a rise in temperature. Over the last few years, it has reached the stage of industrial production due to, amongst other factors, the development of methods based on the controlled solidification of the alloy. Every forming technique that has been experimented is derived from the techniques which are used at the fully liquid or fully solid state. Essentially, the characteristics of these processes are high compactness and a high three-dimensional precision of parts, excellent mechanical properties and a surface finish comparable to that of foundry.
Read this article from a comprehensive knowledge base, updated and supplemented with articles reviewed by scientific committees.
Read the articleAUTHOR
-
Michel SUERY: Doctor of Science - Director of Research at the Centre National de la Recherche Scientifique (French National Center for Scientific Research)
INTRODUCTION
The shaping of alloys from the liquid state can be carried out using a variety of processes, all of which have in common the fact that the alloy must be cast in liquid form at a sufficiently high temperature to enable the mold to be filled correctly. This often results in acute problems of segregation, porosity, cracking and shrinkage. Shaping at a lower temperature, from an already partially solidified state, seems, a priori, very interesting; however, it runs up against the dendritic nature of solidification, which means that the alloy cannot be cast successfully in a mold as soon as the solidified volume fraction exceeds around 20 to 30%. Nor can this state undergo significant deformation without the appearance of cracks and segregation of the liquid phase. Work carried out at MIT (Massachusetts Institute of Technology) in the 1970s showed that intense mechanical stirring of the solidifying liquid could prevent the formation of this dendritic structure, resulting in a relatively low-viscosity, semi-solid jelly suitable for shaping in this state. The initial work was soon put to good use in applying the technique to ferrous alloys as part of programs supported by the US government. The feasibility of semi-solid forming was thus demonstrated, not only by injection molding the jelly produced by stirring, but also by molding semi-solid billets obtained by partial remelting of bars or ingots previously solidified from the jelly.
Since then, significant progress has been made. Initially, electromagnetic stirring was widely used to obtain the jelly. Then, new processes, based on controlled solidification of the alloy, were proposed to generate the globular structure more economically. At the same time, other semi-solid shaping processes (forging, rolling, extrusion) were developed.
Rheoforming refers to all shaping processes applied to alloys in the semi-solid state obtained during solidification.
Thixoforming refers to all shaping processes applied to alloys in the semi-solid state obtained during partial remelting.
Unless otherwise specified, the percentages used in the composition of alloys are percentages by weight.
The Ultimate Scientific and Technical Reference
This article is included in
Metal forming and foundry
This offer includes:
Knowledge Base
Updated and enriched with articles validated by our scientific committees
Services
A set of exclusive tools to complement the resources
Practical Path
Operational and didactic, to guarantee the acquisition of transversal skills
Doc & Quiz
Interactive articles with quizzes, for constructive reading
Semi-solid shaping
Semi-solid state
Bibliography
The Ultimate Scientific and Technical Reference
