Tesla Casts a New Strategy for Lightweight Structures

June 1, 2020

Bill Visnic

Aluminum is synonymous with “weight-saving” in most contemporary automotive-engineering reference points. But apart from a few applications – most notably Ford’s F-Series pickups – aluminum largely is deployed where steel can be readily displaced without performance loss or for comparatively small components that deliver comparatively small weight or process savings.

Electric-vehicle maker Tesla is readying the next step in aluminum use, however – one that effectively matches Ford’s “big gain” approach by specifying a massive piece of structural die-cast aluminum for the rear underbody of the recently launched Model Y crossover. According to Tesla CEO Elon Musk, this new aluminum application represents a radical step for its design and manufacturing advantages and its lightweighting potential.

Musk is renowned for outsized promises, but in the case of the Model Y’s die-cast rear underbody, his enthusiasm for this advanced use of aluminum – cast by a house-sized “giga press” – is supported by manufacturing experts who call it a game-changer. “It’s definitely an all-new look at how to do things,” asserted Laurie Harbour, president at Harbour Results Inc. manufacturing consultancy. “Elon Musk has always pressed his engineers to be creative.”

Reductions in – pretty much everything

Musk spoke in detail about the new casting process in an episode of the “Third Row Tesla Podcast” in April and made more than a passing mention of it in Tesla’s 1Q2020 financial results call in early May. “The current version of Model Y has basically two big high-pressure diecast [HPDC] aluminum castings that are joined and there’s still a bunch of other bits that are attached. Later this year,” he said on the podcast, “we’ll transition to the rear underbody being a single-piece casting that also integrates the rear crash rails.

“It gets better,” he continued. “The current castings, because you’ve got to interface with so many different things, we have to CNC-machine the interfaces and there’s a bunch of things that have to be joined; they have datums on them and that kind of thing. The single-piece casting has no CNC machining – it doesn’t even have datums. It took us a lot of iterations, by the way, to get there.”

Sandy Munro, CEO at Munro & Associates, the benchmarking and competitive analysis firm renowned for its highly analytic “teardowns” of popular and innovative vehicles, recently completed a teardown of a Model Y. A series of internet videos covering Munro’s assessment garnered more than 36 million impressions in little more than a month. Munro was particularly impressed by the current two-piece aluminum underbody structure – and openly offered admiration in an interview with SAE’s Automotive Engineering.

He said the current Tesla Model Y has “two of the biggest castings we’ve ever seen in a car. We’ve never seen them used in an automobile before of that size. There’s lots [of innovative aluminum applications] at Cadillac, BMW, Audi – they’ve all used castings. But nothing quite the size of this thing.” Munro also participated in the podcast in which Musk spoke of the coming single-piece casting. Moving to the “megacasting,” as Munro dubbed it, “definitely wins the prize,” he asserted. “That’s going to be the biggest casting for quite a while. Nobody’s exploring that.”

The mammoth machine is being supplied by IDRA Group, an Italian leader in HPDC equipment founded in 1946. Tesla is the first customer for IDRA’s hulking OL6100 CS (with upgraded locking force to handle the special Tesla casting), destined for installation in the company’s Fremont, California and Shanghai, China, plants. IDRA’s “Giga Press” measures some 64 feet (19.5 m) long and 17 feet (5.3 m) tall. Along with the higher clamping force is a maximum aluminum-alloy “shot” weight of 104.6 kg (231 lb). The OL6100’s output may be lightweight castings, but the machine itself is anything but light, weighing in excess of 410 tons.

The single-piece casting for Model Y will replace around 70 stampings, extrusions and castings that currently make up the same fabricated assembly in the Model 3, on which much of the Model Y is based. Musk described the Model 3’s rear structure as “a patchwork quilt – it’s not great. The complexity in the body shop is insane,” he said.

Harbour agreed. With such a large and inclusive casting, “Even with a big cycle time, you eliminate all the labor to assemble pieces and subcomponents,” she observed. “You’re saving on automation cells, you’re saving on people. It would be tough to put dollars to it, but think of multiple suppliers doing stampings, you could save maybe 20% on labor cost. And reduction in footprint is major. My guess is that it’s a net-net efficiency gain.”

Musk claimed the new single-piece casting design, and the goliath machine that will produce it, will deliver a 30% reduction in the size of the body shop. He added that the process probably will transfer to Model 3 production as well. “That’s the thing we want to bring to bear on the Model 3 over time,” he said.

Potential caveats

Munro said Tesla is pondering a similar strategy for the front of the Model Y. And a large casting probably makes changes to the affected structure a less-expensive proposition than “trying to change a bunch of stamping dies.” Alterations such as wheelbase changes also could easily be accommodated by such an architecture, he said. But it wouldn’t be the approach for low-volume production, Munro maintained. And “castings don’t repair very well,” he added. “If an impact was severe enough, the car’s a write-off,” he said. Of course, such is the case with many contemporary vehicle designs.

Munro said Tesla plans to assemble up to 1 million vehicles annually off the Model Y architecture, so the company’s Giga Press investment likely is a sound one. He and Harbour agree that the mega-casting approach is an example of Tesla being Tesla. “They do continuous improvement in design,” a practice most auto companies typically don’t embrace, Munro said.

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