Thixoforming : Semi-solid Metal Processing

The investigations show that the rheocasting process is also possible for higher
melting steel alloys. A semi-solid slurry with a fine, globular microstructure can be
successfully created by controlling the casting temperature and holding time in a cup.
In a subsequent inline-forming step, the semi-solid billet produced is forged to a
demonstration part. This final part shows a globulitic homogeneous microstructure
in a large part of the volume but still includes several local failures, which have to be
taken into account for the design of the forming strategy and advanced component
geometries.
10.4
Simulation of the Thixoforming Process
To clarify the boundary conditions and the temperature distributions during the
process, two types of simulation software were applied to analyse the thixoforging
process by numerical calculation. Due to high deformation during the forming
operation, it is necessary to have the possibility of remeshing to retain the functionality
of the mesh. With LARSTRAN/SHAPE, which is based on viscoplastic,
modelling a remeshing is applicable. It is also possible to take the material properties
into account, which consider the high dependence of the flow stress in the solidus
interval (Section 6.1.6.3). The commercial standard software ABAQUS has its main
advantages in the calculation of complex thermal problems. The used solver
ABAQUS-Standard is a general-purpose finite element program that is suitable for
static, thermal, electrical and transient problems. In this work, in the thermal
coupling of component and tools during the cooling operation is of particular
interest. In addition to the advantage of consideration of the tools as elastic or
plastic dies that support detailed information about temperature gradients in the
tools, it is also possible to define the latent heat individually for a given temperature in
contrast to LARSTRAN/SHAPE (Section 6.2.3).
10.4.1
Material Properties and Boundary Conditions
10.4 Simulation of the Thixoforming Processj399
As shown in Chapter 6, several values are necessary as input data for the simulation.
Most of the properties can be found in the literature, but in case of the flow stress,
radiation and heat transfer additional analyses have to be made. The determination of
the flow stress for thixoforging processes with lower liquid-phase fraction has already
been described with the compression test (Section 6.1.6.3). For the determination of
boundary conditions such as radiation and heat transfer, a combination of experimental
and simulative analyses is necessary to ensure precise reproduction of the
temperature development.
10.4.1.1 Radiation
Radiation losses, which greatly influence the process steps during heating and while
closing the dies, are determined by using cylinders (20 mm diameter 20 mm), that