=manufacturing =cars =metallurgy =institutions
the giga press
Tesla
decided to use large aluminum castings ("gigacastings") for the frame of
many of its vehicles, including the Model Y and Cybertruck. This approach
and the "Giga Press"
used for it have been praised by many articles and
youtube videos, repeatedly called revolutionary and a key advantage.
Most cars today are made by stamping steel sheets and spot welding them
together with robotic arms.
Here's video of a
Honda factory. But that's outdated: gigacasting is the future! BYD
is still welding
stamped steel sheets together, and that's why it can't compete on price with
Tesla.
Hold on, it seems...BYD prices are actually lower than
Tesla's? Much lower? Oh, and Tesla is
no longer planning single unitary castings for future vehicles?
I
remember seeing analysis
like this from some people with car manufacturing
experience, concluding that unitary cast aluminum bodies could have a cost
advantage for certain production numbers, like 200k cars, but dies for
casting wear out sooner than dies for stamping steel, and as soon as you
need to replace them the cost advantage is gone. Also, robotic arms are
flexible and stamped panels can be used for multiple car models, and if you
already have robots and panels you can use from discontinued car models, the
cost advantage is gone, but Tesla was expanding so they didn't have available
robots already. So using aluminum casting would probably be slightly more
expensive, but not make a big difference.
"That seems reasonable", I
said to myself, "ふむふむ".
And I previously pointed that out, eg
here. But things are actually worse than that.
aluminum die casting
Die
casting of aluminum involves injecting liquid aluminum into a die and
letting it cool. Liquid aluminum is less dense than solid aluminum, and
aluminum being cast doesn't all solidify at the same time. Bigger castings
have aluminum flowing over larger distances. The larger the casting, the
less evenly the aluminum cools: there's more space for temperature
differences in the die, and the aluminum cools as it's injected.
As a
result, bigger castings have more problems with warping and voids. Also, a
bigger casting with the same curvature from warping has bigger position
changes.
Tesla has been
widely criticized for stuff not fitting together properly on the car
body. My understanding is that the biggest reason for that is their large
aluminum castings being slightly warped.
As for voids, they can
create weak points; I think they were the reason the cybertruck hitch
broke off
in this test. When aluminum doesn't have weak points, it stretches
before breaking. Defects from casting are the only explanation for that
aluminum breaking apart cleanly that way. If you want to inject more aluminum
as solidification and shrinkage happens, the distance it has to travel is
proportional to casting size - unless you use multi-point injection, which
Tesla doesn't, and that has its own challenges.
Somehow I thought
Tesla would have only moved to its "Giga Press" after adequately dealing
with those issues, but that was silly of me.
One approach being
worked on to mitigate warping of large aluminum castings is "rheocasting",
where a slurry of solid aluminum in liquid aluminum is injected, reducing
the shrinkage from cooling. But that's obviously more viscous and thus
requires higher injection pressures which requires high die pressures.
aluminum vs steel
Back when
aluminum established its reputation as "the lightweight higher-performance
alternative" to steel, 300 MPa was considered a typical (tensile yield)
strength for steel.
Typical cast aluminum can almost match that, and
high-performance aluminum for aircraft can be >700 MPa. Obviously there are
reasons it's not always used: high-strength aluminum requires some
more-expensive elements and careful heat-treatment. Any hot welds will ruin
the heat-treatment and thus be weaker.
But now, 1000+ MPa steel is
common and used in many cars, and it's possible to get
higher strengths than that. Aluminum alloys have had much less progress.
I suppose that's because a lot of research went into aircraft materials in
the past, and because steels are overall more complex.
Aluminum also
has worse heat & fatigue resistance than steel, but it resists corrosion as
well as stainless steel (which is weaker and more expensive than normal
steel). Steel exposed to water needs to be painted. On the other hand,
aluminum is more expensive, and most people want it painted anyway.
When a car is made from stamped steel sheets welded together, if one area
gets dented, it's sometimes possible to cut that part off and weld on a
replacement. People don't do such repairs as much as they used to, due to
the relative cost of manufacturing vs labor changing, and design changes
that make repair harder, but it's at least a theoretical advantage.
Aluminum is less dense than steel, which at the same strength-weight ratio
increases bending strength, but casting also can't produce thin walls or the
complex ridge patterns that stamped steel can. Aluminum is also harder to
weld, and die casting can't produce hollow tubes like stamp + weld can.
this shouldn't be a thing
The chassis of cars is a relatively small fraction of their cost. The cost
of aluminum die casting and stamped steel is, on Tesla's scale, similar.
Yet, there were so many articles saying gigacasting was a major advantage of
Tesla over other companies.
I don't really care if Tesla cars have
some panel gaps or some cars cost 3% more to make. If you're interested in
minor improvements to car manufacturing, my friends have thought of more
interesting ones, such as:
- Tires are
filled with carbon black, so tire wear produces a significant amount of
particulate pollution. There's a way to precipitate CaCO3 so it can be a
nontoxic and slightly-better-performing alternative.
- Tires use
vulcanized rubber. Most thermoplastic elastomers have too low a melting
point and higher losses than that rubber, but there's a novel thermoplastic
elastomer with slightly lower tan δ than polybutadiene rubber, much better
wear resistance, and sufficient heat resistance. That would make tires
longer-lasting and somewhat recyclable.
- A cost-effective active
suspension, which would greatly reduce vibration and bumps while driving.
- A better electric motor driver design for (car-sized) axial-flux motors
(which have low inductance).
- Oh yeah, the battery chemistry that I have
a patent for.
As for how my car-manufacturing-related predictions have gone so far, some years back, I remember being optimistic about the practicality of:
- factories
using AGVs instead of conveyor belts
- using high-strength boron steels
with hot stamping in cars
- power skiving of internal gears for planetary
gearsets
- DLC coating of gears and engine components
- oil-soluble
ionic liquids for lubricants for steel
- using ball screws instead of
hydraulics for some presses
- EconCore-type panels for trucks
-
electric motors on turbochargers ("e-turbos")
So, the reason I'm writing about
this isn't because of how much I care about die casting vs stamping. What
concerns me is the failure of institutions and cultural systems.
When
magazines talked about, say, "microservices" or "the cloud" being the
future, it actually made them happen. There are enough executives that are
gullible or just want to be given something to talk about and work on that
it established an environment where everyone wanted to get "microservices"
or whatever on their resume for future job requirements, and it was
self-sustaining. Gigacasting isn't at that self-sustaining point, but it's
another example of how shallow the analysis behind the decision-making
processes of American businesses is.
What's worse is that something
actually good could've gone in that cultural space. MIT press releases full
of BS, Forbes 30
lists full of scammers, stupid TED talks...these things occupy the spaces
that actually-good ideas needed to succeed.
why was this a thing
Why
were there so many articles that wrongly decided the Giga Press was of
paramount importance and wrongly assessed its relative advantages vs
existing systems? I think there were a few reasons.
Money is a
factor, of course; PR agencies drive a lot of the articles in media. I
assume Tesla pays some PR firms and people there presumably decided to push
the Giga Press.
There are many fans of Tesla in general and Elon Musk
personally. They want to see positive news about Tesla - especially news
about Tesla pushing technology forwards, because that's what they like about
Elon. In the current American economy, where the limiting factor for a lot
of investment is the legibility of technical expertise to investment
managers, someone like Elon provides what's missing.
The theory
behind managers specializing in "management in general" was that business
executives don't need technical understanding because they can get expert
advisors, but it doesn't work. Elon actually provides some examples of why:
there were engineers at SpaceX and Tesla who knew why the Hyperloop concept
was flawed and the Giga Press would be somewhat worse, but did they go tell
Elon that? I don't think so. Elon, despite understanding engineering much
better than the median US executive, wasn't able to cut through the
sycophancy and BS.
Even having the decisions made by people with PhDs
doesn't always work. I've talked to a number of business executives with
scientific PhDs, and I'd sometimes look up their thesis and figure I'd make
some brief but intelligent comment to show a basic level of scientific
understanding. Some of them forgot everything about the topic they studied,
and some of them...in retrospect, I think they had their thesis
ghostwritten.