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DEFECTS IN HIGH PRESSURE CASTING PROCESSES
High Fraction of Solid
Spheroidal Microstructure
Low Fraction of Solid
Dendritic Microstructure
Prone to Shrinkage Porosity Core Pins
Figure 11.3 Graphical illustration showing the effects of phase separation in
semi-solid metalworking.
semi-solid processing is partially solid and partially liquid. This two-phase mixture does not necessarily remain homogeneous. The
liquid phase flows easily and in some cases will leave its solid counterpart behind, resulting in phase separation.
Cases of phase separation have been observed when the metal fill front must travel a significant distance within the die cavity
while flowing around multiple cores. Figure 11.3 is a graphical illustration of this phenomenon. The cores choke the passage of
the solid phase. The resulting product has nonuniform material properties as the microstructure of the metal near the gate is highly
spheroidal while the microstructure of the metal at the last location to fill is dendritic. Solidification shrinkage also becomes a prob-
lem in the dendritic region.
In many cases, the phase separation does not affect the func- tionality of the products produced using semi-solid metalworking.
The dendritic structure may go unnoticed. However, secondary processes such as machining or trimming can open a pathway to
11.5 PREDICTING DEFECTS
167
solidification shrinkage associated with a dendritic region. In pres- sure vessels, this phenomenon can cause significant problems.
Variation in the mechanical properties caused by phase sepa- ration can be a serious issue in structural members. The dendritic
region of the component has inferior mechanical properties in comparison to the bulk material. Moreover, porosity caused by
solidification shrinkage can act as a stress concentrator, increasing the chances of failure.
Actions can be taken related to die design to minimize phase separation during semi-solid metalworking. Gating can signifi-
cantly affect phase separation induced during fill. The distance the metal front must flow to fill the runner system and die cavities
must be minimized. Each time the metal front must change direc- tion, the potential for phase separation increases. Choke points
such as cores should be minimized. Necessary choke points should be fed with metal on both sides, such as with the use of a
fan gate.
Phase separation may be unavoidable for some applications. Overflows can be utilized to capture the dendritic region of the
component. Unfortunately, the use of overflows reduces process- ing yield. The location of the dendritic region, however, can be
manipulated by varying the location of the gate. In some cases, the dendritic region can be selectively located in a noncritical
location within the product.
11.5 PREDICTING DEFECTS
Both product designers and component producers must be aware of the potential defects that may occur in components manufac-
tured using high integrity die casting processes. Many potential defects are well understood. Other defect types are unique to
squeeze casting and semi-solid metalworking. With a clear un- derstanding of the mechanisms that induce defects, efforts may be
made prior to product launch to minimize their occurrence.
Flow modeling software has developed significantly over the past decade, and many software packages include data for ana-
lyzing vacuum die casting, squeeze casting, and semi-solid met- alworking processes. These analytical tools should be utilized to
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DEFECTS IN HIGH PRESSURE CASTING PROCESSES
predict the formation of contaminant veins, ideally while the de- signs of both the product and the die can be modified. The pre-
diction of phase separation using computer flow modeling is not yet possible. The currently available computational fluid dynamic
software defines the semi-solid metal as a high viscosity fluid rather than as a true two-phase mixture. A method of modeling
two-phase flow was proposed in 1997 and efforts are currently underway to develop the proposal into a viable computer model.
2
REFERENCES
1. Keeney, M., J. Courtois, R. Evans, G. Farrior, C. Kyonka, A. Koch, K. Young, ‘‘Semisolid Metal Casting and Forging,’’ in Stefanescu, D. editor, Metals
Handbook, 9th ed., vol. 15, Casting, ASM International, Materials Park, OH,
1988, p. 327. 2. Alexandrou, A., G. Burgos, and V. Entov, ‘‘Semisolid Metal Processing: A
New Paradigm in Automotive Part Design,’’ SAE Paper Number 2000-01- 0676, Society of Automotive Engineers, Warrendale, PA, 2000.
