A


rvin Montes was just 28 years old when he was tasked with selling the world on his company’s ability to cast wrought alloys.


At the time, the company, Johnson


Brass and Machine Foundry Inc., Saukville, Wis., used a several page brochure to explain to customers on a case-by-case basis what the alloys could do. The confusion was that wrought alloys can’t be cast—so how could a metalcaster be selling a wrought alloy? “It sort of grew as a trade name,”


Montes said. “We had internal data sheets, and there would be a lot of hand holding in terms of selling it to a customer. But about 10 years ago, we started thinking bigger. We started thinking, ‘how can we sell more of this without having to explain it all to the customer?’ That’s when we went looking for a spec.” The centrifugal casting company has


since been working to make its cast approximations of wrought alloys more attractive and accessible to metal cast- 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 properties to the worked versions (at a price), and more are on the way.


Not All That New Cast versions of wrought alloys


will never be 100% interchangeable with their rolled counterparts, which generally exhibit higher mechanical properties than traditional cast alloys. After all, wrought alloys (which begin as cast ingot) gain their strength in the mechanical deformation process of rolling, and near-net-shape cast- ings cannot be rolled after pouring and solidifi cation. But approximations of the alloys give metalcasters another way to compete with other metal manufacturers, and approximations do exist. 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 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 fl uidity and


MODERN CASTING / August 2010


cavity fi lling capacities than its fully molten brethren. Because the alloy already is in solid condition after fi lling, shrinkage 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 cast- ing wrought alloys, which have low fl uidity and are prone to incomplete fi lls and hot tearing. Likewise, the centrifugal process used by Montes and Johnson Brass offers 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 fl uidity. 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 signifi cant tooling investment is required for casting in a centrifuge. The other specialty processes also are cost prohibitive for many applications. “They already do thixocasting [of


wrought chemistries] commercially,” said Sumanth Shankar, a professor at McMaster Univ. who has done extensive work on casting wrought alloys. “But it is very expensive. Some automotive guys make really high integrity castings [that way], but they pay a premium.” The goal now is to come up with


new ways to cast wrought materials so they can compete on the open market in applications that require properties not available with cast alloys (Table 1).


The Economics of Change Several sets of researchers current-


ly are working to make cast-wrought alloys a prudent purchase for casting designers, and each is developing a different means to achieving that end. Diran Apelian, a professor at the


Worcester Polytechnic Institute, has developed what’s known as the Controlled Diffusion Solidifi cation method of casting wrought chem- istries. The process plays off the traditional types of wrought casting in which semisolid material is used to overcome the high melting points, unique solidifi cation curves and lack of fl uidity of wrought materials. According to Apelian, the method


he and his team have found suc- cessful is to begin with two types of material—one investment casting- type cavity of iron powder with very low carbon content, and one crucible of molten iron with a high carbon content (roughly 4.3%). With both materials heated to the same temperature, the team then pours the molten material into the heated powder, which melts not due to heat fl ow but due to carbon diffusion from areas with higher carbon content to areas of lower carbon content. “It’s like brining a chicken,” Apelian


said. “It diffuses in. But as soon as the carbon leaves the liquid metal, it so- lidifi es, so you have a casting through diffusion not through heat fl ow.” A former student of Apelian, Shan-


kar believes the Controlled Diffusion Solidifi cation of casting wrought alloys could be commercially available in two to three years. The researchers have cast several viable samples of 2000, 6000 and 7000 series wrought alloys; all that’s left is to optimize the process and determine how best to heat treat the materials. But that’s no small step, Shankar said. “In wrought alloys, you have a cast


billet that is then rolled out. That is when the properties are elevated [be-


Table 1. Comparison of Comparable Wrought and Cast Chemistries 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


83 63 50 94 70 75 70


Tensile Strength Yield Strength Elongation (ksi)


Ultimate (ksi)


73 50 40 44 30 30 30


(%) 11


11.7 10 35 30 40 35


25


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