How Not to Hate Casting Aluminum

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Our old friend and mentor Ironman posted a (I hope) humorously titled video on YouTube, "Why I Hate Casting Aluminum" :

And while the casting results were terrible enough to cause him to discard 200 kilos of donated scrap automotive castings, I think that's maybe throwing the baby out with the bathwater. While some donated automobile cast aluminum may be poor for DIY purposes, a lot of it will be good and some will be excellent

I recently had cause to melt an accumulated pile of donated scrap aluminum, and I decided to segregate it into related types for a sequence of small melts. I wanted to pour ingots to consolidate stock storage -- I wasn't casting an actual part. The ingots were poured into a muffin tin for a form.

An interesting result was seeing how the different ingots cooled. 

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 Here's a shot of some of the ingots poured. They are lined up in columns by type. The worst stuff is in the first column on the left. It was poured from donated X shaped extrusions, one of which can be seen at the head of the column.

Column number two had the best quality aluminum (in my opinion) for casting. It was from an automotive bell housing.

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Some closer views of the ingots:

Worst, Column 1, X extrusions. All of the muffin ingots were poured to the same degree of fill of the same mold. You can see how coarse the grain structure is, how great the shrinkage was, and the lower height, and deep shrink depression. Also how dull the metal looks.

Best, Column 2, automotive bell housing. Same degree of fill, same mold as column 1. The grain is almost imperceptible. The muffin has almost the same puffy shape as poured, far less shrinkage, minimal shrink depression, and as shiny as a new dime! 

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Ingots from columns 3 and 4:

Column 3 was a small finned utility engine cylinder -- might have been a lawnmower. The ingots have a good puffy shape indicating low shrinkage, and fairly fine grain structure. Good casting material.

Column 4 was an automotive front suspension control arm. Ingot shape is still reasonably good. The grain is coarser than column 3. Deeper shrinkage. It will probably be usable for general castings, not necessarily fine stuff, and will require more attention to shrinkage in risering, and also in part size shrink allowance.

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Finally, Ingots from column 4:

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This was a mixed bag of leftovers, including a couple of sprues, some bicycle brake levers, a lot of long thin extrusion scrap which I kept piling in through the top of the furnace, and even some spills from former casting sessions.

And yet, the ingots look pretty good. Probably the sprues and spills were high quality casting stock originally, and these mixed with extrusions produced muffins with qualities somewhere in between the others. Fairly good grain, reasonable shrinkage, rounded shape to the ingot. Definitely usable stock.

So, what's the conclusion?

well, if you don't want to hate casting aluminum, don't assume the stock you receive is going to work directly out of the scrap heap and cast  into a part. Instead cast it as ingots first. This will allow you to cull anything you don't like the looks of and avoid the disappointment of a failed casting session. It will also allow you to clean your stock by skimming, and remove any pieces of embedded steel, which some cast auto parts contain. It produces convenient size stock for melting, and allows you to reserve extra special quality ingots (like column 2) for a project that might need it.

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I did a little research online, and I believe that the extrusion which created the worst muffin ingots was called an 8020 1515 linear rail, and it seems these are typically 6063 -T6 alloy. Good to know in general.

My guess at the alloy from the bell housing that made the best looking, very low shrinkage muffins is either AlSi12, aka A413 (US)and D6 (GB), also used for pistons. Or AlSi18 aka A390, which has the lowest shrinkage I can find, of the aluminum casting alloys.

6063 has a thermal elongation coefficient of 23, A413 is 21, and A390 is 18. (I'm leaving out units here, but you get the idea.

This is all of interest to me presently because I'm considering trying a cast iron piston in an aluminum cylinder, and thermal behavior is what I think will make or break that combo.

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I did another set of consolidation melts today. I'm trying to get more room in my shed, so melting stuff down kills two birds with one stone. Plus it's fun. I thought you all might like to see the results from some other scrap types. Result, about 30 half pound ingots.

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Stock: I melted down a light aircraft wheel hub, a small Tecumseh engine, and a bunch of thin architectural extrusions, plus small odds and ends.

The Tecumseh engine was probably about 4 hp horizontal, which I removed most of the steel and iron from. Actually, see my Avatar up left? Well that's it! You're seeing an exhaust and intake port view of it. It's been hanging around in storage for many a year. It had thrown a rod, so not much use.

I don't know where I got the aircraft wheel hub from. It was conical shaped with internal ribs. I think that was its use.. not totally sure. 

The thin extrusions looked like window molding stuff.

Anyway here they are as muffins. The motor made beautiful shiny puffy versions with fine grain. The wheel, meh, not as nice, but usable, fairly coarse grain fair amount of shrinkage.

And the mixed scrap and extrusions turned out pretty good, considering. Maybe just a little better than the wheel aluminum, finer grain for sure.

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I've taken to marking muffins now for future reference when mixing, and matching for purpose.

Another way not to hate casting aluminum!

Endnote: BTW I test every major piece of casting material before melting, by taking a small sliver off of the piece, and subjecting it to my MAPP torch. We don't want to mistake magnesium for aluminum! Magnesium burns with an intense white flame. And as I found out a few years ago upon doing the same test, the nice big VW bug engine block I was given was made of magnesium. Darn!!!! that had a lot of material in it.

And this time I was glad I did, as well. An old carburetor found half buried in the ground from some ancient vehicle seemed suspiciously heavy. A little piece of it turned yellow in the torch flame -- zinc based pot metal. Not good to mix with aluminum. In fact I do wonder if the casting defects seen in Ironman's video were due to zinc contamination. Zinc has BIG shrinkage, and coarse crystalization, not unlike the appearance of those castings.