Ticket To A Leak-Proof Casting
Heavy-duty hydrogen fuel cells can pack up to 100kW of compressed energy into modular, stackable units for the rooftop of a city bus, but for Vancouver-based fuel cell maker Ballard, they also store up considerable opportunity. When a performance limitation with one of its key components surfaced, finding the best and quickest way to convert a five-billet part to a one-piece casting became an urgent priority.
The opportunity hub of focus was Europe, where fuel cell buses have emerged into mainstream public transportation as countries including Spain, Italy, France, Britain and Ireland, the Netherlands, Germany, and more are actively replacing diesel buses with cleaner, more fuel-efficient hydrogen-powered electric counterparts. Ballard is on board, fueling the transition with its FCveloCity and FCmove technology. The company partners with three different bus manufacturers, and Ballard hydrogen fuel cells currently power nearly 200 buses across Europe with about 650 more in development.
With so much riding on timely delivery of its products, a compliance issue with an international standard was an unwelcome discovery. The aluminum enclosure that housed Ballard’s hydrogen fuel cell did not meet the International Electrotechnical Commission’s ingress protection standard, IP6K9K, a guideline published in the European Union by the European Committee for Electrotechnical Standardization. In its original machined and assembled form, the enclosure was not fully sealed against dust and water, and that shortfall triggered immediate action.
But the fast-track to convert the component to a solid casting initially turned out to be a maze dotted with dead-ends and potholes. By the time the job landed at AFS Corporate Member Tooling & Equipment International (TEI) in Livonia, Michigan, Ballard’s team had worked with multiple separate suppliers—including ExOne to print a sand mold, a different foundry to (unsuccessfully) produce the casting, as well as an outsourced machine shop—and problems seemed to follow the project at various turns.
“The biggest thing here was speed,” said TEI Technical Sales Director Anthony D’Agostini. “Their time window went from something manageable to something extremely tight––they were really up against it and needed a product fast.
“So, they went back to ExOne and said, ‘Look, you sell you these [3D sand printing] machines—who do you sell them to? We need to go to somebody that can use this technology and deliver us a complete product.’ And ExOne recommended us.”
Receiving only a machined part model of the fuel cell enclosure, TEI cast, machined, and delivered the first component to Ballard within six weeks. The now-lighter 78.9-lb. enclosure is cast in A356-T6 and measures 33.5 in. x 22.8 in. x 11.8 in.
“The beauty of this is, if I have a casting problem, I’m the casting source and I’m the machining source—I can quickly make whatever changes are necessary and continue the process,” D’Agostini said. “I don’t have to call my customer to say ‘Hey, I need another casting or I’ve got a problem with machining.’ I contain that all under one roof, and that’s extremely powerful when you’re talking about high quality parts on compressed lead times.”
It took TEI just a week to review, quote, make a few recommendations, and receive the green light from Ballard to get started as the prototype supplier for the redesigned fuel cell enclosure casting conversion. Another vendor will serve as the production foundry, although to date, TEI has produced not only an initial run of 20 but is also currently fulfilling the second of two follow-on orders of 30 units. For most jobs, TEI isn’t the production supplier; instead, it guides its customers during the prototype stage so they’re able to easily replicate the process when they get to production, according to D’Agostini.
“Speed and quality are the major selling points of the service we offer our customers,” he said. “We work very fast at producing high-quality components. So, within that week’s time period, I had conversations with their engineering department on a few issues to get it to a state where we could hit the ground running.”
Start to finish, with the exception of a special coating to promote electrical conductivity, TEI created everything required for the part, from upfront engineering, all solidification modeling for the mold design, printing the molds, and, of course, casting and machining. The foundry owns three sand printers—two voxeljet VX 4000 machines, which are the only two in the U.S. among a mere 11 in existence worldwide; plus an ExOne S-Max Pro.
“Between those three machines, we have a tremendous amount of print capacity, and I’d say we’re the largest user of 3d printed sand in North America,” said D’Agostini.
TEI uses a completely automated, low-pressure casting process to make the part, a method that often utilizes steel tooling—the foundry eliminated time, tooling, and cost at the front end by printing the mold. Low-pressure casting, in which molten metal is pumped into the mold, also eliminates the introduction of oxygen during the casting process, which can be detrimental to the integrity of a part, according to D’Agostini.
“We’ve taken low-pressure casting technology to where we can pump aluminum into a mold rather than pour aluminum into a mold,” he said. “And we do it slightly over atmospheric pressure, which allows for a very tranquil fill into the mold. We can control the time, the temperature, and the pressures during the entire casting process.
“All of our casting is computer controlled, as well,” he added. “Our fill profiles are generated from our casting simulation, and our computer-controlled casting processes are extremely repeatable. For example, we’re always casting within plus or minus one degree Celsius. And the actual control of the pressures in the fill is all done by the computer. It’s not done with manual input by a person.”
Ballard’s engineer, Rory Patterson, designed the enclosure part as a casting, and TEI provided only minor castability adjustments to their customer’s well-thought-out design.
“There was a slight lack of filleting on the casting,” he said. “When you have a surface that’s perpendicular to another surface, creating a 90-degree angle, you would naturally have a blend or a radius between those two surfaces. This helps the product in terms of reducing or eliminating stress risers. And it helps the casting process because it enables material to flow nicely from one corner to the next. One of the items we worked with them on was adding appropriate filleting into certain areas of the casting for material flow and protection of the part.”
Casting integrity was top of mind for the metalcaster, knowing that porosity would be a key issue. Porosity can result in a leaking casting, said D’Agostini, and achieving tight, flawless sealing was, after all, the major catalyst for converting the enclosure to a casting in the first place. TEI performed leak testing as well as X-ray and CT inspection of the first-off parts to ensure utmost quality.
Strength was going to be essential, too. In its former billet design, the enclosure had demonstrated excellent material properties simply due to the nature of its A6061 composition, and while Ballard didn’t bring stringent material properties to the casting specifications, TEI knew that A356 would provide the necessary strength needed for the component. The alloy also reinforces TEI’s goal to ensure smooth repeatability for the production supplier—it’s a very common 300 series alloy, said D’Agostini, is very castable, and flows well in sand.
Careful consideration was required when it came time for heat treating the casting, which was necessary for ensuring hardness. The process of heat treating can be brutal on a 1,000F-degree part as it’s plunged into water or a polymer, shocking the part and locking mechanical properties into place. The process can have the undesirable result of creating distortion in a component if it hasn’t been designed with sufficient stiffening and ribbing structure to keep it straight during heat treating.
“We have to work very hard to mitigate that distortion,” said D’Agostini. “And we can do that by creating fixtures to support the part during the heat treatment process.
“We had to do extensive laser scanning pre-heat-treatment and post-heat-treatment to understand the distortion. Then we had to put in place distortion mitigators in the form of fixturing—that was our biggest challenge with this part: keeping it in its shape so we could machine it accurately.
TEI called another brief time-out to rethink optimum outcome at the machining stage of the game. Some features requiring 90-degree heads were in awkward locations and would have been hard to machine, even with TEI’s latest, five-axis machining technology.
“When we have to put features in at extremely tight tolerances, 90 head work isn’t necessarily the right approach. Just because you can do it in CAD doesn’t necessarily translate to yes, you can do it in a part,” said D’Agostini. “What seems like a very simple feature of a hole on the side of a part can add cost and complexity to the manufacture of the components. We provided input to evolve these features so that just about anybody can do them with any piece of CNC equipment.”
For TEI, the best way to partner and get product done with speed and accuracy is getting the right people on the bus.
“It really works well if we have a small group comprised of an engineer [Patterson] and a buyer [Rhea Orbon], and that’s exactly what we had here with Ballard,” said D’Agostini. “And what was critical in terms of getting the product to them as quickly as possible was being able to have access to both commercial and engineering at the same time. We were able to make quick decisions.
“They were a team that was receptive to our feedback and guidance on getting them the absolute highest quality in the shortest amount of time,” he added. “It was really a good dynamic relationship between TEI and Ballard, because Ballard had a small team that was empowered to make decisions.”