12.5 METAL MATRIX COMPOSITES FOR USE WITH DIE PROCESSES
173
result in accelerated corrosion at the defective location. Research is being conducted to address this problem.
Current commercial magnesium alloys are limited to near- room-temperature applications due to decreased creep resistance
as the temperature is elevated. Some magnesium alloys containing rare earth elements are available that can operate at temperatures
as high as 150⬚C with a significant cost penalty. Past research has shown that additions of aluminum and alkaline rare earth elements
barium, strontium, and calcium in magnesium form fine precip- itates of Al
11
E
3
at grain boundaries.
4,5
These precipitates inhibit creep at elevated temperatures. Of the alkaline metals currently
researched, calcium is the least costly as well as the least dense with the lowest total ‘‘cost per atom’’ of the possible choices. With
this knowledge, several magnesium alloys containing aluminum and calcium are under development.
12.4 EMERGING ALLOYS FOR USE WITH HIGH
INTEGRITY DIE CASTING PROCESSES
Aluminum and magnesium are the dominant metals utilized in high integrity die casting. However, efforts are underway to har-
ness the economic benefits of high integrity die casting processes with nontraditional die casting alloys.
Vacuum die casting is being utilized to produce components in titanium, -titanium, titanium aluminide, nickel-based alloys,
amorphous metallic glasses, and stainless steel.
6,7
The die casting dies utilized in with these alloys are fabricated from high tem-
perature refractory metals such as tungsten or molybdenum. Prod- ucts manufactured include airfoils, fan blades, structural hardware,
golf club heads, and automotive valves. Current production quan- tities are low with limited shot sizes.
12.5 METAL MATRIX COMPOSITES FOR USE WITH
HIGH INTEGRITY DIE CASTING PROCESSES
Within the materials community, composite materials are a subject of considerable research. As a result of these efforts, discontinu-
174
FUTURE DEVELOPMENTS IN DIE CASTING
Figure 12.2 Motor cycle sprocket die cast using an SiC particulate reinforced
aluminum matrix composite. Courtesy of Alcan, Inc.
ously reinforced metal matrix composites are under development for commercial use with high integrity die casting processes. Dis-
continuous reinforced casting alloys are a class of metal matrix composites in which a metal alloy matrix is reinforced with ce-
ramic particles or whiskers. Figure 12.2 is an illustration of a component produced using such a composite with the die casting
process. Discontinuously reinforced composite materials exhibit improved characteristics when compared to traditional casting al-
loys, including
reduced structural weight, increased tensile modulus,
improved yield strength, increased ultimate tensile strength,
improved fatigue limit, improved dynamic response, and
enhanced wear resistance.
