is a signifi cant cost consideration. T e near-net-shape advantage and the greater freedom of design that is inherent with the investment casting process provide opportunities to greatly reduce or eliminate machin- ing. T e successful implementation of cast MMC can depend on possessing the necessary engineering skills to design products that take advantage of the investment casting process. For engineers, the near-net-
shape advantage means only critical surfaces need be machined. Raised pads and undercuts can be employed to relieve the surrounding area of the part. Only minimal amounts of ma- chine stock must be added to these machined features to assure cleanup. Investment casting also is known
for its ability to generate savings by reducing the part count of an engineered assembly. Fig. 8 is an ex- ample of a part design that used the investment casting process to reduce the part count. T is capability is particularly benefi cial when design- ing for MMC as the fewer assembly points of a unitized structure also correlates to a reduction in the cost for machining. Additionally, prices for invest-
ment castings are less sensitive to the complexity of a structure than in other metalcasting processes. Features such as undercuts, cored holes, cooling fi ns and pins, con- nector holes, etc. are commonly investment cast without the need to consider a draft angle. Engineers are encouraged to incorporate this detail into their cast MMC confi gurations because these design nuances come at minimal additional expense. C onfigurations for aluminum
alloy/SiCp MMC castings gener- ally are subject to the same basic design considerations as those for the investment casting of any aluminum alloy. A standard linear tolerance of ±0.005 inch per inch (±.1 mm/20 mm) of length can be applied to MMC designs without a draft angle allowance. T e lack of elongation in the MMC also limits the amount of post-processing that can be per- formed on a casting. In addition
Nov/Dec 2014 | METAL CASTING DESIGN & PURCHASING | 35
to the linear tolerance, a minimum allowance for a surface fl atness of 0.005 inch per inch (0.1 mm/20 mm) should be incorporated in any alumi- num alloy MMC design. Optimum wall thicknesses for
an investment cast aluminum alloy MMC are between 0.080 in. to 0.120 in. (2 to 3 mm). Walls have been cast in MMC down to 0.060 in. (1.5 mm) without becoming a signifi cant cost driver and as low as 0.040 in. (1 mm). T e tendency for heavy sections in MMC to form inter-granular shrink makes wall thicknesses exceeding 0.25 in. (6.4 mm) increasingly diffi cult to invest- ment cast. Fillet radii and smooth transi-
tions between features take on an increased importance with MMC castings. In addition to helping moderate the turbulence in the casting, fi llet radii also mitigate the buildup of stresses around sharp in-
ternal corners. A minimum fi llet radius of 0.060 in. (1.5 mm) is recommended when designing aluminum alloy MMC confi gurations and more as the wall thickness increases. T e surface fi nish of an aluminum
alloy MMC investment casting averag- es approximately 125 root mean square (RMS) although the cast surface may vary between 60-200 RMS. Datums and datum points are
recommended for any casting design. We suggest that datum selection be coordinated with both the foundry and machining source to enhance the over- all manufacturability of the product. T e special properties of aluminum/
silicon carbide MMCs can be of ben- efi t and are available to solve a myriad of design challenges for weight reduc- tion, stiff ness, vibration, heat transfer, wear and/or thermal expansion. T e investment casting process off ers a unique capability for the economical manufacture of MMCs.
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