Thixoforming : Semi-solid Metal Processing

32j 2 Metallurgical Aspects of SSM Processing
systems is the formation of intermetallic phases (IMPs) in the residual liquid, and the
width of the terminal freezing range (TFR), which is the non-equilibrium partial
solidification range near termination of solidification that indicates the alloy s
proneness to hot tearing.
In the following sections, the above-mentioned criteria of temperature sensitivity,
proneness to segregation and hot tearing, and IMP-formation are discussed for
selected alloying systems. Both light metals (aluminium and magnesium) and ironbased
alloys are considered. In addition, some metallurgical aspects are discussed
with regard to the characteristics of slurry formation in the rheo- and thixo-routes,
and the impact of variations in the alloy compositions on the structure and properties
of SSM components.
2.2
Temperature Sensitivity S and Solid–Liquid Fraction
For the thixo-formability of alloys, the correct adjustment of the solid–liquid fraction
is of crucial importance. In general, systems with a wide freezing rage are beneficial.
In a real thixo-process, the temperature will always be subject to some error. The
impact of temperature variations on the present initial solid fraction, f S , can be
expressed by the (negative) slope of the equilibrium solid fraction curve:
S ¼ df S
dT
ð2:1Þ
S depends on the alloy s composition and also on the initial amount of solid or liquid.
It is advisable to consider the average slope in the partial freezing range f S ¼ 0.4–0.6.
The average sensitivity is inversely proportional to this partial freezing range.
Therefore, as for selection of appropriate alloys, the temperature sensitivity S should
be small.
Table 2.1 indicates that for the light metal cast alloys A356 and AZ91 a temperature
error of 5 K results in a variation of solid fraction of only about 0.04, but for the
wrought alloy AA6082 the variation is about 0.135 or roughly 25% of the initial
fraction. As for the Fe-based alloys, the tool steel X210CrW12 proves to be more
suitable for SSM processing than the bearing steel 100Cr6.
Table 2.1 Relevant temperatures and temperature sensitivities of various alloys [2, 3].
Alloy TS ( C) TL ( C) T50 ( C) DT 40–60 ( C) S (K 1 )
AlSi7Mg (A356) 557 614 574 17 0.0083
AlSi1Mg (AA6082) 557 647 637 7 0.027
AlSi9Cu3 (A380) 548 603 566 10 0.039
AZ91 470 600 570 22 0.0087
100Cr6 1348 1461 1427 19 0.010
HS6-5-2 1175 1432 1360 35 0.0055
X210CrW12 1221 1366 1291 50 0.0045