Thixoforming : Semi-solid Metal Processing

326j 9 Rheocasting of Aluminium Alloys and Thixocasting of Steels
Figure 9.16 The bag serves for the restraint of the oxide face of the
slugs. Thus, only oxide-free material from the middle of the billet
reaches the gating, on the right: The peeled oxide outer surface
found in the restraining cavity confirms the functionality of the
system.
is achieved to minimize gas inclusions and other specific casting defects. Already the
ingate system of the first mould sample featured an oxide restraining system
(Figure 9.16). By pressing the reheated billet through a hole in the slug insert, part
of the outer skin and of the oxide skin at the side is removed in a circular cavity around
the biscuit. The oxide-resistant skin from the front of the billet is moved into a bag
behind the slug insert bag. This preferentially clean material reaches the inside of
the gating channel.
The cross-sectional area of the gating channel is slightly decreased with respect to
the area of the ingate of the order of 1.1:1. By pressurizing the gating system, the
laminar metal flow is promoted.
The geometry of such a cavity is of special significance for the process flow. A high
volume of the cavity leads to great metal losses whereas too small a system can take
the outer skin only partly and oxide inclusions can arise in the component.
9.3.3
Model Tests
Different test geometries were developed and used in the characterization of the
process properties of SSMs. Fork and finger samples were the first moulds, followed
by a step die with variable ramps, a meandering die, a sample with a T-shaped
geometry and a plate sample, which were used for model tests. All mould inserts are
designed so that they can be integrated into the framework of a modular die-casting
tool. Three tool frameworks in different measurements were available for the
examinations. The plate dies are integrated into a multiple mould so that with this
form four test parts can be produced in one shot whereas all other dies have only one
cavity.
T Geometry The T-shaped sample is part of the basic flow experiments. The used
sample serves for the characterization of high-melting material with the help of