Casting Wrought Alloys
By Shea Gibbs
The notion of “casting wrought alloys” can elicit some disagreement among metallurgists.
During a recent roundtable, several scientists with different backgrounds grappled with the idea. Wrought alloys have to be produced in a certain way, otherwise they’re not wrought, one would say. Yes, but if the specification can be approximated through the casting process, it makes no difference, another would retort.
Fortunately, a disparate group of metalcasters from various corners of the industry have decided to proceed with research on casting wrought equivalents with or without the metallurgists’ blessing. Some of them are succeeding. Others are on the brink of breakthroughs.
For designers of metal castings, this means yet another group of materials are available (or soon will be available) for use with the casting process—regardless of what you want to call them.
Not All That New
Arvin 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 casting 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.
Cast Vs. Wrought
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 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 metalcasting 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 create non-dendritic semi-solid slurries for casting a wide selection of alloys.
In the process, a 10-20% solid fraction alloy yields higher fluidity and cavity filling capacities than its fully molten brethren. Because the alloy already is in solid condition after filling, shrinkage porosity caused by the variation of volume during transformation from a liquid to a solid is reduced.
These advantages directly counteract the main concerns when casting wrought alloys, which have low fluidity and are prone to incomplete fills 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 fluidity. We can make the material go where we want it to go.”
But Montes admits there are drawbacks to being able to cast wrought materials in only the true centrifugal process (which is not the same as either 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.
“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).
Table 1. Comparison of Comparable Wrought and Cast Alloys
Table 1. Comparison of Comparable Wrought and Cast Alloys
|UTS (ksi)||UYS (ksi)||Elongation (%)|
|Wrought Aluminum 7075-T6||83||73||11|
|Cast Aluminum 206-T7 permanent mold||63||50||11.7|
|Cast Aluminum A357-T6 permanent mold||50||40||10|
|Wrought Stainless Steel 316 ASTM 240||94||44||35|
|Cast Stainless Steel CF8M ASTM 743||70||30||30|
|Wrought Stainless Steel 304 ASTM 240||75||30||40|
|Cast Stainless Steel CF8 ASTM 743||70||30||35|
The Economics of Change
Several sets of researchers currently 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 Solidification method of casting wrought chemistries. The process plays off the traditional types of wrought casting in which semisolid material is used to overcome the high melting points, unique solidification curves and lack of fluidity of wrought materials.
According to Apelian, the method he and his team have found successful 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 flow 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 solidifies, so you have a casting through diffusion not through heat flow.”
A former student of Apelian, Shankar believes the Controlled Diffusion Solidification 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 [beyond that of traditional casting alloys],” he said. “Similarly, with cast versions of the alloys, once you have the near-net-shape casting, you still must heat treat to gain the elevated properties that are comparable to the wrought alloy.”
Dave Weiss, Eck Industries, Manitowoc, Wis., also is working with a research team to develop a cost effective method of casting wrought alloys. Weiss would like to develop a method that can easily slide into current metalcasting process flows.
“[Wrought aluminum] alloys typically contain very low levels of silicon, so they have fairly wide freezing ranges,” Weiss said. “If you just allow them to freeze on their own, it is difficult to feed them. That’s why the hot tearing occurs. You can avoid those [defects] by appropriate risering and cooling rates.”
Casting wrought alloys should be no different than pouring other difficult-to-cast materials, Weiss said. For example, many of the principals used in the casting of 200 series aluminum can be applied to the wrought compositions.
“In [200-series] aluminum casting, typically we use aluminum chills or iron chills. [With wrought] alloys, we use copper chills, and in some cases we water-cool those chills,” Weiss said.
According to Weiss, Eck also uses more extensive risering to achieve the steeper thermal gradients that are necessary to produce the strongest cast version of the wrought material possible.
Clearing the Way for Casting
Metal casting purchasers often ask their suppliers to produce wrought products. They simply aren’t aware the materials aren’t available in the form of a cast metal component. The designer knows only the properties he or she desires.
So when a metalcaster embarks on a mission to cast the alloys, it first must show the materials in fact offer comparable mechanical properties to traditional wrought alloys. The next step would be to have the alloys cleared by manufacturing’s many governing bodies and standards committees. The materials can’t just be passed off to customers as exact equivalents to wrought chemistries.
“There is no shortcut in terms of developing the alloys,” Montes said. “It isn’t starting over—you borrow some things from the wrought materials, so it is a nice starting point. But it only gives you the ballpark.”
Johnson Brass has succeeded in having its materials recognized in the Aluminum Association’s Pink Sheets, but the alloys are narrowly defined as centrifugally cast approximations of wrought chemistries. The developers of other types of cast-wrought alloys will have to go through similar steps to obtain recognition of their alloys.
“The alloys are going to have to be fully characterized as casting alloys,” Weiss said. “What we don’t know yet is how the fatigue performance differs from the wrought to the casting and what kind of corrosion issues they might have. It depends on the application, but certainly before I would use it on something I was designing, I would need more information.”
Still, Weiss agrees with Shankar that the alloys could be available in cost-effective, production-ready forms in two to three years. His research team has shown that in some cast aluminum equivalents, yield strengths in the 70 ksi range can be achieved, better than any of the traditional cast alloys.
The experiences of Johnson Brass don’t suggest quite as rapid a timeline. While the company began trying to perfect its properties and gain certification for its alloys when Montes was 28, he’s now 38, and only in the last half decade has it found a way to make the proprietary alloys understandable for customers.
“After we went to the Aluminum Association four to five years ago, things started happening quickly,” Montes said. Metal
This article originally ran in the Sept/Oct 2010 issue of Metal Casting Design & Purchasing