22
MOLTEN METAL FLOW IN HIGH INTEGRITY DIE CASTING PROCESSES
the die, while nonplanar filling occurred in Figure 2.10b. These differences in metal flow were made possible by adjusting
machine-controlled process parameters. Be that as it may, for complex component geometries, nonplanar fill may be unavoida-
ble.
Example Calculation 2.5
Utilizing a common commercial aluminum alloy, a component is manufactured using squeeze casting technology. Parameters for
this process include a gate velocity of 500 cmsec and circular gate diameter of 10 mm. Determine if the liquid metal flow
through the gate is laminar. The density and viscosity of liquid aluminum are 2.7 gcm
3
and 1 ⫻ 10
⫺3
gcm 䡠 sec, respectively.
2
Solution Using Equation 2.1, the Reynolds number may be used to deter-
mine if the metal flow is laminar at the gate. Since the gate ge- ometry is circular, the characteristic length is the diameter of the
gate
3
: Dv
Re ⫽
3
1.0 cm 500 cmsec 2.7 gcm ⫽
⫺3
1 ⫻ 10 gcm 䡠 sec
⫽ 1,350,000 For fluid flow through a circular cross section, the transition from
laminar to turbulent flow is completed when the Reynolds number reaches 3000.
3
Fluid flow at the gate is not laminar. Flow is tur- bulent for this squeeze casting example.
2.6 METAL FLOW IN SEMI-SOLID METALWORKING
Semi-solid metalworking is often incorrectly sighted as exhibiting laminar flow when filling the die cavity.
4–6
This misconception has been proliferated in the sales and marketing of semi-solid
2.6 METAL FLOW IN SEMI-SOLID METALWORKING
23
metalworking related products. Regardless of the increased vis- cosity of semi-solid metal mixtures, the high flow rates encoun-
tered when filling the die cavity under production conditions result in turbulence. Turbulence, however, does not cause gases to be
entrapped in the metal. Entrapment of gases occurs at the metal fill front.
Semi-solid metalworking exhibits planar metal flow as the die cavity is filled. This is illustrated in Figures 2.3 and 2.4. Planar
filling in semi-solid metalworking processes is the mechanism that reduces the entrapment of gases, not laminar flow. Although pla-
nar filling of the die cavity may solve a chronic problem encoun- tered with many casting processes, this phenomenon can lead to
unique defects, which will be presented in Chapter 11.
Example Calculation 2.6
When producing an aluminum component using a semi-solid met- alworking process, the gate of the die cavity is circular with a
diameter of 0.5 cm. Process parameters were optimized with a metal flow velocity of 500 cmsec at the gate. The density and
viscosity of semi-solid aluminum are 2.7 gcm
3
and 1 ⫻ 10
⫺1
gcm 䡠 sec, respectively.
2
Determine if the liquid metal flow through the gate is turbulent or laminar.
Solution In order to determine if the metal flow is laminar or turbulent, the
Reynolds number for the system must be calculated using Equa- tion 2.1. The characteristic length is the diameter of the gate:
Dv Re ⫽
3
0.5 cm 500 cmsec 2.7 gcm ⫽
⫺1
1 ⫻ 10 gcm 䡠 sec
⫽ 6750 The turbulent flow is present in fluid flow through a circular
crosssection when the Reynolds number exceeded 3000.
3
In this case, the metal flow is turbulent.
24
MOLTEN METAL FLOW IN HIGH INTEGRITY DIE CASTING PROCESSES
2.7 PREDICTING METAL FLOW IN HIGH INTEGRITY
DIE CASTING PROCESSES
Over the last decade vast improvements in computer hardware and software technology have made complex simulations of physical
phenomena possible. Today engineers and designers have availa- ble an ever-growing and continually refined set of tools to aid in
product development. Mathematical models using both finite el- ement and finite difference techniques have been developed to
simulate various manufacturing processes. Specific to high integ- rity die casting processes, mathematical models have been devel-
oped to simulate several elements, including
mold filling, air entrapment,
liquid metal surface tracking for predicting inclusion loca-
tions, solidification thermodynamics,
material properties after solidification, shrinkage porosity, and
part distortion.
Process modeling can be used to predict and design desired flow conditions within the die cavity. This can yield significant returns
on investment by optimizing the manufacturing process prior to building dies.
When utilizing computer models, one must consider that two phases are present in semi-solid metalworking. 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.
7
REFERENCES
1. Reynolds, O., ‘‘An Experimental Investigation of the Circumstances Which Determine Whether Motion of Water Shall Be Direct or Sinuous and of the
REFERENCES
25
Law of Resistance in Parallel Channels,’’ Transactions of the Royal Society of London
, Vol. A174, 1883, p. 935. 2. Flemings, M., ‘‘Behavior of Metal Alloys in the Semisolid State,’’ Metallur-
gical Transactions, Vol. 22B, June 1991, p. 269.
3. Gaskell, D., An Introduction to Transport Phenomena in Materials Engi- neering,
Macmillan, New York, NY, 1992. 4. 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.
5. Young, K., ‘‘Semi-solid Metal Cast Automotive Components: New Markets for Die Casting,’’ Paper Cleveland T93-131, North American Die Casting
Association, Rosemont, IL, 1993. 6. Siegert, K. and R. Leiber. ‘‘Thixoforming of Aluminum,’’ SAE Paper Number
980456, Society of Automotive Engineers, Warrendale, PA, 1998. 7. 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.
