Thixoforming : Semi-solid Metal Processing

11
Thixoextrusion
Frederik Knauf, Rene Baadjou, Gerhard Hirt, Reiner Kopp,
Simon M€unstermann, and Rainer Telle
11.1
Introduction
Extrusion is a forming process where a billet is inserted in a container first and
thereupon extruded through a forming die by a punch. The conventional extrusion
process is industrially mainly applied for light metal- and copper-based alloys.
Relatively low flow stresses compared with the tool material used, moderate temperatures
and the possibility of solid-state welding during the process open up the
possibility of producing light metal profiles with complex cross-section geometry by
bridge dies. These profiles require little or no further surface treatment and achieve
excellent mechanical properties by appropriate heat treatment.
Compared with light metal alloys, the conventional steel extrusion process is
characterized by high thermal and mechanical stresses. This results in limitation of
cross-section complexity and alloy selection, very short tool life and limited industrial
use. Steel profiles are therefore typically produced by profile rolling, which, however,
requires many rolling passes even though the cross-sections achievable are restricted
to relatively simple geometries.
Hence semi-solid extrusion of steels might open up new possibilities for the
production of steel profiles. The reduced flow stresses of semi-solid processes lead to
lower mechanical tool stress and accordingly the extrusion of conventionally nonextrudable
steel alloys or an increasing profile complexity might become possible.
Also, the specific globular microstructure (see Chapter 3) which is obtained in
thixoforming could be of interest for some materials and applications.
However, identifying decisive operating parameters and maintaining the process
window defined by those parameters in a continuous extrusion process is a
challenging task for process development. Although experience and results obtained
in sequenced thixoforming methods such as thixoforging and thixocasting may be
transferred to the realization of steel thixoextrusion, continuous thixoextrusion
differs considerably in terms of material handling, tool design and process control.
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