From: Pete Albrecht
Newsgroups: rec.crafts.metalworking
Subject: Cast and forged aluminum alloys (was Alum car wheels?)
Date: 22 Oct 1998 21:02:42 GMT

Like two other posters say, 356 T6 sounds like the most likely culprit.

This is not a particularly strong alloy. One of my metallurgist friends said
that at his company (a division of Avco, Lycoming, or whoever owns it all now)
they regard cast aluminum in the 356 flavor as just one step above molded
plastic.

I researched this a few years ago when I considered making reproduction racing
brake drums for Porsche 356. New ones are no longer available. Somebody had
been making them but wasn't having much luck with them. Well, Porsche forged
them, and for a good reason. On a Porsche or VW drum brake car, the drum is
actually the center part of your wheel; instead of having lug bolts come out of
a forged steel hub and through the drum, as on most drum brake cars, the lug
bolts (Porsche) or threaded holes (VW) are put into the drum on quite a large
bolt circle. The drum houses the front wheel bearing races or a steel spline
for the rear halfshafts. Anyway, what this means is that in turns, the drum is
loaded in bending from cornering forces. This does not happen on conventional
drums, where the drum is basically just a big "washer" clamped between the
wheel and the (usually) forged steel hub, and the wheel takes higher bending
forces.

Knowing that the drum would be loaded in bending, Porsche had them forged. VWs
were cast iron, a stronger material to begin with. Of course, on a forged
aluminum drum, you need an iron wear surface for the brake shoes. So they press
fit an iron ring into the finished, machined forgings. I discovered all this by
cutting a scrap drum apart and etching it. Textbook forged grain structure
readily visible. Drum machined after forging to accept iron liner. (Note that
this was NOT the so-called "Alfin" process invented by Fairchild Aircraft for
cylinders, and used by many British and Italian drum brake makers after the
war; in Alfin, aluminum is cast around a specially prepared iron liner).

Now, a few years ago, somebody with more enthusiasm than experience went off
and decided to make repro drums. He had a foundry cast them out of the old
standby, 356 alloy. (The guy probably thought this was kinda cute -- 356 T6 is
also a Porsche model designation, for the last 64-65 models). The history of
the project's failures is interesting.

1. Attempt to press fit iron liner into cast drum. Drum bursts from excessive
interference fit.

2. Reduce amount of interference fit. Liner now spins inside drum.

3. Pin liner in place using four tangential threaded grubscrews. Hey, brakes
get hot, don't they, might as well use stainless screws, it's sexier.

4. Cast iron liners, which are thinner at each grubscrew location (screw goes
across width of liner) turn hot because heat won't conduct away fast enough
through stainless screw and various voids/contact asperities. There's a pretty
blue line and a crack marking the location of each grubscrew.

5. Use common old ordinary steel screws. Problem solved -- for now.

6. Customers who race cars with these brakes report a cute ring-shaped crack
halfway between the hub and the bolt circle. If the crack should propagate all
the way through, the tire, wheel, and most of the drum will take a leave of
absence from the car. Probably while it's the outside wheel in the middle of a
turn.

7. Cease production immediately, don't answer phone, don't honor warranty,
don't support installed base of drums (for which you took in $2000 a pair), do
the best you can not to be found.

I looked into this. I had the forging alloy analyzed. Looked up the
characteristics of the material. It is well known that aluminum behaves like
wet noodles at high temperatures; strength just drops to nothing. At drum brake
operating temperature, the forged alloy was about as strong as 356 T6 at room
temperature. The cast alloy started out weak and it got worse from there. Hence
the circular fatigue cracks. Also the grain structure of a forged piece is
infinitely superior to the anisotropic structure of a lump of cast metal. The
trains of the forged drums ran radially and made the bend at the lip, just a
perfect solution for the job.

Similarly, a now defunct but recently famous maker of custom wheels was raking
in the dough by CNC machining any style you wanted -- design your own wheel,
they'd make 'em your way. In some cheap cast alloy. No effort was made to check
the design for strength. They got a reputation for cracking. Company is no
longer in business.

Pete