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Thixoforming
The potential for forming processes based upon semi-solid metal
alloys was first recognised
in the early 1970s. The microstructure of a suitable alloy
comprises spheroidal particles of
solid surrounded by liquid phase of a lower melting point,
rather than the interlocking tree-
like dendrites of conventionally cast alloy. It is this
microstructure that gives the material itsthixotropic properties,
i.e. when sheared the material flows, but when allowed to stand,
it
thickens. Thixoforming is one member of a family of semi-solid
forming processes and it
possesses characteristics of both casting and forging. The
non-dendritic feedstock necessary
for thixoforming is usually produced using electromagnetic
stirring in a continuous casting
process.
The Thixoforming Process
The thixoforming process incorporates four operations, figure
1.
Firstly, a bar of thixoformable raw material is cut into
appropriate slug lengths. Then the slugs are heated in a controlled
manner, using either an induction coil or amuffle furnace, into a
uniform mushy state.
The heated slug is transferred to the shot sleeve of a suitably
modified die castingmachine and injected into a die.
The component feeder and gating systems are then removed using a
clipping press orband saw.
Figure 1.Schematic of a thixoforming process cell.
Mould Materials
Thixoforming allows the use of softer, cheaper die materials due
to the reduced mechanical
stresses acting on the die during filling and the lower alloy
processing temperature, e.g. for
example, graphite, easily machinable stainless steels and
disposable one shot non-metallic
dies have all been used. This permits the application of
thixoforming to the small production
volumes required by rapid prototyping and mass customised
production.
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Advantages of Thixoforming
The advantageous attributes of thixoforming have been described
as follows:
It is an energy efficient process which is easily automated and
controlled to achieveconsistency
Production rates are similar to pressure die casting or
better
Smooth filling of the die with no air entrapment and low
shrinkage porosity gives partsof high integrity
Lower processing temperatures reduce the thermal shock of the
die, promote die lifeand allow the processing of high melting point
alloys (such as tool steels and stellites) that
are difficult to form by other means
Fine, uniform microstructures give enhanced component
properties
Weight savings can be achieved through optimised component
design.
The high mechanical, geometric and surface quality of components
produced through
thixoforming can justify the removal of additional production
processes such as machining
steps and the need for reinforcing inserts. A simpler, more
flexible production process
requires fewer resources for control and allows a more rapid
response to changing customer
requirements.
The extent to which these benefits are achieved depends upon the
design of the component
and its dies, the optimisation of the processing conditions, the
integration of the technology
into an existing production process and the demands of the
organisation's business
environment. Not only are such benefits difficult to quantify in
an analysis, but they are also
dependent upon managerial decisions.
Limitations of the Thixoforming Process
A number of drawbacks currently limit the commercial viability
of thixoforming:
The high cost of raw material and the low number of its
suppliers
The considerable research effort and expense required to
implement a viablemanufacturing process due to the limited process
knowledge
The higher die development costs than for conventional forming
technologies becauseof the lack of available process experience and
design rules
The higher level of training required for personnel employed to
operate and maintain athixoforming plant compared to traditional
operators.
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Despite the recent development of product quality and customer
service as sources of
manufacturing competitiveness, in many industries cost is still
regarded as the primary
consideration. The relatively high price of suitable feed stock
for the thixoforming process is
therefore perceived to be the major barrier to its
commercialisation. As these prices are
related to volumes of raw material production, they are unlikely
to fall until demand
increases significantly.
Concluding Remarks
The characteristics of the thixoforming process allow savings to
be made in many other
areas (e.g. removing machining operations, increased sales,
reduced material use, surface
treatments, energy savings) and so will often offer economic
benefits overall. The
automotive component illustrated in figure 2 is an example where
the near-net-shape,
mechanical integrity, high strength, optimum ductility and high
volume of production
characteristics of thixoforming have enabled a cost effective
design solution to be achieved.
The issue for potential industrial users is therefore to
evaluate accurately the full extent of
the costs and benefits to be gained through implementing the
technology. The difficulties in
achieving this arise from the reliability of predictions of
future cash flows and evaluation of
what are often considered to' be intangible benefits.
Figure 2.An example of a thixoformed automotive component.
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