Taking Off

Brian Sandalow, Associate Editor

One of the first challenges product engineer John Nish and Wellman Dynamics (Creston, Iowa) faced during the design and manufacture of a high-pressure boost pump housing for a rocket engine turbo system was also a fundamental one:

How to manage a new alloy.

In the past, similar products were made of A357 aluminum. But for safety and environmental reasons, the choice was made to use F357, which subtracts beryllium.
“The beryllium really helps scavenge a lot of little problems out of there. It’s essentially the same alloy but that little tiny ion makes a lot of difference. That was one of the first challenges,” Nish said. “We’re changing from a standard alloy for high-pressure rocket design, changing from an A357 to an F357. We knew there would be challenges with getting properties, especially on castings with thick walls.”

But that was far from the only challenge Nish and his team had to overcome to deliver the 150-lb. aluminum casting to customer Aerojet Rocketdyne (Sacramento, California).

Multiple Challenges
The design incorporates a specialized, internal vane system with an intricate flow path. Due to the high-pressure applications, a large portion of the casting wall must be several inches thick, while several of the surrounding features and walls are very thin. Part of the nature of this casting relates to the transitional areas between varying thick and thin sections of the product, while achieving proper metal flow for optimal solidification that results in a quality component. The mass of some sections within the casting presented another element of difficulty, in conjunction with achieving material properties that reached a certain standard. This configuration gave the foundry the chance to investigate and implement new nondestructive testing methodologies.

To overcome these issues, Wellman developed unconventional gating-engineering methods. AFS Corporate Member Liberty Pattern Company (New Liberty, Iowa) played a key role in this part of production. Nish said Liberty has a lot of experience with many configurations of castings. The foundry, meanwhile, has people in its own pattern shop and engineering group with experiences with various types of castings. All told, everybody involved in the gating had over 110 years of foundry experience, which helped as the component was converted from a nobake sand casting to a less-traditional hybrid that saw both printed and traditional cores used.

Together, everybody involved in the project looked at the mass and dimensions of the different parts of the casting they would have to make. They would have liked to do 3D modeling, but the quick turnaround precluded that. The product had to be completed quickly because Aerojet was competing with other companies for a contract.

“We took a variety of different areas of expertise from all of these places and we sat down and drew up a number of different gating schemes and then eliminated the ones that would produce lower properties and potential problems,” Nish said.

The foundry started on this project in early-to-mid-2016 and the first casting was poured within seven months of a first team meeting. Within the next five or six months, the metalcaster was making production castings.

“Liberty Pattern, they’re hard workers,” Nish said. “When they take on a task, they deliver. As soon as the tooling was approved and we sent the designs, they were able to make the tooling fairly rapidly.”

Good Thinking
The production of a part with this level of intricacy was coordinated with Liberty, a company with whom the foundry has a longstanding business relationship. Nish said Liberty played a vital role in the design and development of the hybrid conventional-additive manufacturing tooling capable of achieving complex sand molds by showcasing printed sand cores.

“We worked with the materials people at Aerojet and our metallurgy people here, and the team at Liberty Pattern,” Nish said. “We tried to make sure that we were taking into account that with all of these smaller features, if we put a large number of loose pieces or we tried to put a large number of different cores together inside, that was going to be a recipe that would not work too well. We can’t clean inside. There’s a lot of places we can’t get to.”

The 3D printed sand cores replaced 20 coreboxes with only two conventional coreboxes in the mold assembly, making the mold assembly much cleaner and easier than originally imagined.

“These geniuses at our pattern shop said ‘Why don’t we just try to design something that we can use with additive manufacturing? We might as well print the core instead of trying to make all of these small cores, try to paste them together, try to make sure the paste is dry,’” Nish recalled. “The Aerojet people and I just kind of looked at each other and we turned around and said ‘Yeah. Why don’t we do that?’”

Replacing conventional cores with printed cores eliminated the need for lap joints, pasting, core seams, and brought the reduction of core-on-core generation of sand particulate. These changes resulted in reduced variation, making the process much less prone to metallurgical and dimensional defects. The approach to core coating combined with improved 3D printing techniques was also utilized to enhance the surface finish on the internal flow passageways.

“We ended up with a hybrid casting of some standard cores as well as printed cores being inserted in there,” Nish said. “Any time you put cores together you know you’re going to have to file them down to get them to fit sometimes, so that’s extra sand that might be generated.

“It really took care of a large number of problems and not just the intricacy of fitting all these internal cores together, but also the cleanliness and the overall quality of the casting, where we were able to achieve a casting that didn’t have so much internal cleaning to worry about,” Nish added.  

Click here to see this story as it appears in the July/August issue of Metal Casting Design & Purchasing