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SEMI-SOLID METALWORKING
TABLE 5.1 Freezing Ranges for Common Die Cast Aluminum Alloys
Aluminum Alloy Designation Approximate Solidification Range
⬚C 319
356 357
380 383
390 604–516
613–557 613–557
593–538 582–516
649–507
Source: From Ref. 4.
Figure 5.1 Aluminum billet in the semi-solid state. Courtesy of Formcast,
Inc.
5.1 SEMI-SOLID METALWORKING DEFINED
69
Semi-Solid Metalworking Gravity Permanent Mold Casting
Low Pressure Die Casting Medium Pressure Die Casting
Squeeze Casting Conventional High Pressure Die Casting
Casting Pressure bar Gate V
elocity msec
Figure 5.2 Comparisons of casting pressures and gate velocities for numerous
die casting processes.
is a graph illustrating the process windows for numerous casting processes with respect to casting pressure and gate velocities. Gate
velocities during semi-solid metalworking are comparable to those achieved during squeeze casting.
A controlled injection system meters a partially solid–partially liquid metal mixture into a permanent metal die to manufacture
net-shape products. Semi-solid metalworking processes are more costly than traditional high pressure die casting. This is due to the
equipment and energy costs associated with preparing semi-solid metal. However, the integrity of components manufactured with
semi-solid technology is much improved in comparison to con- ventional die casting components. Semi-solid metalworking ex-
tends the capabilities of conventional die casting by
1. reducing the amount of entrapped gases, 2. reducing the amount of solidification shrinkage, and
3. modifying the microstructure of the alloy.
All strategies discussed in Chapter 1 for stretching the die casting process are addressed by semi-solid metalworking.
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SEMI-SOLID METALWORKING
5.2 MANAGING GASES IN THE DIE
Gas porosity is attributed to physical gas entrapment during die filling, to the gasification of decomposing lubricants, and to gas
dissolved in the liquid alloy, which evolves during solidification. Semi-solid metalworking exhibits planar metal flow due to the
highly viscous behavior of semi-solid metal combined with larger gate cross-sectional areas and slower shot speeds in comparison
to conventional die casting. Planar metal flow allows vents to re- main open throughout much of cavity fill. Slower shot speeds also
allow more gases to escape from the die before compression of the gases occurs.
Semi-solid metalworking is often incorrectly sighted as exhib- iting laminar flow when filling the die cavity.
2,5,6
This misconcep- tion has proliferated in the sales and marketing of semi-solid
metalworking related products. Regardless of the increased vis- cosity of the semi-solid metal mixture, the high flow rates en-
countered when filling the die cavity under production conditions results in turbulence. This turbulence, however, does not cause
gases to be entrapped in the metal. Entrapment of gases occurs at the metal fill front.
5.3 MANAGING SHRINKAGE IN THE DIE
When utilizing semi-solid metalworking, a reduction in solidifi- cation shrinkage porosity is realized as a result of injecting metal
that is already partially solid into the die. Also, the amount of heat, which must be removed to complete solidification, is reduced
for the same reason. This allows cycle times to be shortened in comparison to high pressure die casting while simultaneously re-
ducing the magnitude of thermal cycling to costly dies.
As with all die cast processes, high metal intensification pres- sures are maintained throughout solidification. Due to larger gate
areas in comparison to conventional and vacuum die casting, pres- surized metal is fed to the die cavities, further reducing solidifi-
cation shrinkage.
5.4 MICROSTRUCTURES IN SEMI-SOLID METALWORKING
71
5.4 MICROSTRUCTURES IN SEMI-SOLID
METALWORKING
Unlike products manufactured using traditional casting methods, the microstructure of products manufactured using semi-solid met-
alworking is not dendritic. During processing, the dendritic struc- ture is broken up and evolves into a spheroidal structure. The
mechanical properties of the spheroidal microstructure is superior to those found in castings with dendritic microstructures as re-
ported in numerous case studies.
1,5,7
In many cases, the strength of products produced using semi-solid metalworking rivals that of
forgings. Numerous variants of the semi-solid metalworking process ex-
ist. However, all of the processes can be grouped into one of two categories: direct processing and indirect processing. As the name
implies, indirect semi-solid metalworking does not immediately produce a component. Stock material must first be manufactured
with a spheroidal microstructure. This is accomplished by casting bar stock while stirring the solidifying metal mechanically or with
magneto-hydrodynamic technology. The stock material is then re- heated to the desired forming temperature and injected into the
die while in the semi-solid state. Direct processes avoid the pro- duction and reheating of stock material. The semi-solid metal mix-
ture is produced on demand and injected directly into the die. This greatly reduces the total cycle time. Processing cycle comparisons
and microstructural comparisons are presented in Figure 5.3 be- tween a direct semi-solid metalworking, b indirect semi-solid
metalworking, and c conventional casting processes.
Typical as-cast microstructures for an aluminum alloy produced using the direct and indirect semi-solid metalworking are shown
in Figure 5.4 and 5.5, respectively. The round white objects in the microstructures are primary aluminum spheroids, which make up
the solid fraction of the material during manufacture. The sur- rounding matrix in the microstructure formerly the liquid portion
during manufacture is composed of fine primary aluminum den- drites and the eutectic phase. During indirect semi-solid metal-
working, liquid metal may become entrapped within the solid fraction of the material. This entrapped liquid appears as dark
