Fig. 8. Cast in aluminum, the 2013 MCDP Casting of the Year electronics component is an example of how good casting design can reduce the part count of an engineered structure—particularly advantageous with MMCs.
chasing pre-alloyed ingot stock with the percent reinforcement desired and metallurgical bond between the parent metal and particles is recom- mended. The base alloy of cur- rently produced MMC is Aluminum 359, which can be heat treated for specific applications. In addition, the particle size of the SiCp can be varied for more design freedom (Figs. 1-7).
Although elongation and fracture toughness decrease with the SiCp in the MMC, the values often are better than other alternative methods to achieve stiff ness and thermal prop- erties. T is is because as the higher particle contents are cast, the material takes on more of a ceramic character with lower fatigue life and brittle fail- ure without plastic deformation. T e fl uidity of aluminum MMC
alloys decreases as the concentra- tion of SiCp increases; the diffi culty of successfully casting higher SiCp
concentrations increasingly becomes a challenge. T e tendency of aluminum alloy MMC to form shrinkage and gas porosity also becomes a greater concern with higher densities of SiCp. It is es- sential to minimize turbulence within the melt and during mold fi lling. Hot isostatic pressing higher percent SiCp MMCs sometimes is necessary to eliminate subsurface gas voids. The reinforcement of metals can
have many different objectives and opens up the possibility for appli- cation of these materials in areas where weight reduction is the top priority. The precondition here is the improvement of the component properties. The objectives for light metal composite materials are: • Increase in yield strength and tensile strength at room tempera- ture and above while maintain-
34 | METAL CASTING DESIGN & PURCHASING | Nov/Dec 2014
ing the minimum ductility or component toughness.
• Increase in creep resistance at higher temperatures compared to that of conventional alloys.
• Increase in fatigue strength, especially at higher temperatures.
• Improvement of thermal shock resistance.
• Improvement of corrosion resistance.
• Increase in Young’s modulus. • Reduction of thermal elonga- tion and engineering thermal conductivity.
Investment Casting Design Considerations
As it is with other hard alloys such
as inconel or even stainless steel, the secondary machining of MMC castings
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