Harley-Davidson aluminum cylinder head casting defects

Example use case for HIP densification — referenced as the parts visible in the wrecked Grafton building after the explosion.

There is something called hot isostatic pressing, developed in the 50s and used more and more extensively since. You can take a casting — for example, Harley-Davidson aluminum cylinder heads for their engines, which are a critical component. Why are they critical? You're sitting right on top of it; it blows up, okay. You take a pressure vessel — this could be a pretty good-sized pressure vessel — it has end caps that are just screwed in, so it's a big cylinder.

Harley HIPs every cast aluminum cylinder to eliminate porosity — critical because of cylinder location between rider's knees. Tom learned this when he saw a rack of Harley cylinders inside the failed North Andover HIP vessel.

Take complex shapes. Anybody here ride Harleys? The Harley engine block is aluminum, with all those fins. They cast it. You'd like to know it's not going to blow up when those little explosions are going on between your knees. It turns out each one of those cylinders is hot isostatically pressed. That's why you pay a little more for that engine. They don't bother on an aircraft piston engine, which is also a finned aluminum cylinder head — if you lose one cylinder on an aircraft, well, it's a big deal, but you can glide down. On a motorcycle it's a little more critical.

HIP application example — porosity in cast aluminum cylinder heads is dangerous because the cylinder sits between the rider's legs. Used to underline why criticality determines whether HIP is worth the cost.

They do it in argon. The high temperatures and argon isostatic pressure just squeeze the pores out of the material. You take a superalloy you're going to make a turbine disc out of for a jet engine, and you squeeze the pores out. Same for Harley-Davidson aluminum cylinders, because you don't want one to fracture. Where's that cylinder when you're riding the motorcycle? Right between your legs. If it blows up, not a good day. More critical than the piston engine in your car, because if it blows up in your car you'll probably slow down, hopefully no accident. But on a motorcycle, you're intimately involved. For critical things, we take the time to squeeze out all the porosity.

How do you re-heat-treat it? You don't have exactly the same chemistry. You take something that's very similar to it. You can't weld on a prolongation on something like this. So you just take a piece of the material and put it in next to it in the furnace. But that doesn't cool exactly the same way as this piece. This one's 200 tons; this one might be two tons, and they cool differently, they get different grain sizes. The second time, the heat rate, it passes. Great — ship it. They did, and it goes out. It's a hot isostatic press. They used to put Harley-Davidson — I remember walking around the rubble of the building from the explosion, because it just wiped out the whole building. They had a bunch of Harley-Davidson aluminum cylinders. They were cast cylinders and they would hot-isostatic-press to get rid of all the shrinkage porosity from the casting, because guess what — when you're on a Harley-Davidson motorcycle, guess what's right between your knees. If it blows up, not a good day. So they have very stringent quality control standards for these cylinders.