80
SEMI-SOLID METALWORKING
Scrap Conveyor Grinder
Trim Press or
Saw Finished
Parts Bin
Die Screw
Feed Bin Thixomolding Machine
Blo w T
ube
Feedstock
Virgin or Regrind
TogglesPiston
Figure 5.12 Graphical representation of a typical manufacturing cell.
above the molding machine. The feedstock is metered into the screw, heated, sheared, and injected into the die cavity. The solid-
ified component is ejected and extracted from the die using au- tomation that then loads the component into a saw or trim press
for removal of the runner system and flash. Scrap is conveyed to a grinder, which chips the off-fall into usable feedstock.
In comparison to conventional die casting, dies used in Thix- omolding威 components have one major difference. After metal
injection is complete, the end of the screw freezes shut. The plug that forms Figure 5.13 keeps the semi-solid mixture from leak-
ing out of the screw. The die must be designed to capture this plug during metal injection, as shown in Figure 5.14. At the start
of metal injection, the plug is shot into a ‘‘catch’’ that captures the plug. The cone-shaped feature around the catch feeds the semi-
solid metal into the die cavity.
The Thixomolding威 process is very flexible. Since the semi- solid metal is completely contained within the screw, the liquid–
solid fraction can be varied without creating complications in handling. As such, multiple microstructures may be obtained by
varying the percent solid, as shown in Figure 5.15. In extreme cases, the metal may be heated to a completely liquid state. This,
in essence, turns the Thixomolding威 machine into a conventional die casting machine.
5.5 SEMI-SOLID METALWORKING EQUIPMENT
81
Semi-solid Metal Screw Housing
Screw
Frozen Plug
Figure 5.13 Schematic of the metal injection screw used in Thixomolding威.
Frozen Plug
Catch Cone-
Shaped Runner
Figure 5.14 Die design for capturing the frozen screw plug.
82
SEMI-SOLID METALWORKING
Figure 5.15 Multiple microstructures may be obtained by varying the percent
solid during metal injection when using the Thixomolding威 process. Courtesy of Thixomat.
5.6 APPLYING SEMI-SOLID METALWORKING
Semi-solid metalworking has many advantages over conventional die casting and squeeze casting. A qualitative comparison of these
processes is shown in Figure 5.16.
8
Semi-solid metalworking cy- cle times are equivalent to those in conventional die casting. This
is due to the minimal time required for solidification. Most com- ponents produced using semi-solid metalworking can be heat
treated without blistering defects to further enhance mechanical properties.
Both conventional die casting and squeeze casting are lower cost in the areas of capital equipment. Unlike vacuum die casting
and squeeze casting, which are used to improve the integrity of conventionally die cast components, most products manufactured
using semi-solid metalworking are conversions from high cost processes, including forging and investment casting.
Although semi-solid metalworking initially was used exclu- sively by the aerospace industry, the automotive industry has em-
braced the technology. Several structural and pressure vessel applications are now in production utilizing aluminum compo-
nents manufactured from semi-solid processes. Next, example alu-
REFERENCES
83 Feature CDC Squeeze SSM
Metal temperature Cycle time
Number of cavities Alloy flexibility
Shrink porosity Oxide entrapment
Equipment cost Automation
Metal cost Recycling
Mechanical properties Heat treatable
Metal heating Metal loss
+ = indicates favorable rating -
+ +
+ -
- ++
++ +
+ -
- +
+ -
- +
+ +
+ +
+ +
+ +
+ +
+ +
+ ++
- ++
+ -
- -
- for billet processes ++
++ -
- for billet processes
Figure 5.16 Comparisons between conventional die casting, semi-solid met-
alworking, and squeeze casting.
minum case studies are presented. The electronics industry has also found semi-solid metalworking ideal in the manufacture of
numerous magnesium components. Case studies for magnesium components are presented as well.
REFERENCES
1. Flemings, M., ‘‘Behavior of Metal Alloys in the Semisolid State,’’ Metallur- gical Transactions,
Vol. 22B, June 1991, p. 269. 2. 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.
3. Alexandrou, A., and G. Burgos. ‘‘Semisolid Metal Processing’’ in M. Tiry- akioglu and J. Campbell editors, Materials Solutions 1998: Advances in
Aluminum Casting Technology, ASM International, Materials Park, OH, 1998,
p. 23. 4. Jorstad, J., and W. Rasmussen, Aluminum Casting Technologies, 2nd ed.,
American Foundry Society, Des Plaines, IL, 1993. 5. Young, K, ‘‘Semi-solid Metal Cast Automotive Components: New Markets
for Die Casting,’’ Transactions Number T93-131, North American Die Cast- ing Association, Rosemont, IL, 1993.
