ing designers and buyers. And Johnson Brass is not alone. A number of unique processes are available that are capable of casting wrought alloy compositions with comparable mechanical proper- ties to the worked versions (at a price), and more are on the way.
Cast Vs. Wrought Cast versions of wrought alloys will
never be 100% interchangeable with their rolled counterparts, which gener- ally exhibit higher mechanical proper- ties than traditional cast alloys. After all, wrought alloys (which begin as cast ingot) gain their strength in the me- chanical deformation process of rolling, and near-net-shape castings cannot be rolled after pouring and solidification. But approximations of the alloys
give designers the ability to specify them in applications that also draw on the inherent benefits of the metal- casting process (e.g. complex shapes produced with metal only where the end-user needs it). Various specialty casting processes,
namely rheocasting, thixomolding and semi-solid squeeze casting, have been producing the alloy formulations for years. The rheocasting process typically involves the rapid cooling of an alloy and applied convection (stirring) to
Table 1. Comparison of Comparable Wrought and Cast Alloys Material
Ultimate Wrought Aluminum 7075-T6
Cast Aluminum 206-T7 (permanent mold) Cast Aluminum A357-T6 (permanent mold) Wrought Stainless Steel 316 ASTM 240 Cast Stainless Steel CF8M ASTM 743
Wrought Stainless Steel 304 ASTM 240 Cast Stainless Steel CF8 ASTM 743
create non-dendritic semi-solid slurries for casting a wide selection of alloys. In the process, a 10-20% solid frac-
tion alloy yields higher fluidity and cav- ity filling capacities than its fully molten brethren. Because the alloy already is in solid condition after filling, shrink- age porosity caused by the variation of volume during transformation from a liquid to a solid is reduced. These advantages directly coun-
teract the main concerns when casting wrought alloys, which have low fluidity and are prone to incom- plete fills and hot tearing. Likewise, the centrifugal process used by Montes and Johnson Brass offers
83 63 50 94 70 75 70
Tensile Strength Yield Strength (ksi)
Ultimate (ksi)
73 50 40 44 30 30 30
Elongation (%)
11
11.7 10 35 30 40 35
many of the same advantages. “There is a unique material set with
[the centrifugal] process,” Montes said. “You can get away with alloys that have very poor fluidity. We can make the material go where we want it to go.” But Montes admits there are draw-
backs to being able to cast wrought materials in only the true centrifugal process (which is not the same as ei- ther the semi-centrifugal or centrifuged casting processes). First, only roughly tubular products can be produced. And second, a significant tooling investment is required for casting in a centrifuge. The other specialty processes also are cost prohibitive for many applications.
September/OctOber 2010
metal caSting DeSign anD purchaSing
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