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What kinds of aluminium scraps have I been melting to make castings and why

Simple answer: whichever you have already or can get for free! With a few caveats:

The kind of scrap you use will affect the properties of your casting. Fluidity when molten and machineability when solid are two that don’t get thought of as often as strength, but can be just as important. Lets take this little lost foam casting (below) as an example.

Lost foam castings

This lost foam casting is one of a pair, that will form wonderfully unusual coffee table feet. Note how the Aluminium has flowed into the cells of the foam and taken its surface appearance. The central square is where the sprue has been cut off with a hacksaw.

Don’t waste lots of time collecting aluminium beer and soda cans – they can work (forgive the pun) but there is a fairly strong consensus among casters that they don’t work well. The reason is that they have a huge surface area compared to their mass. Cans thus have a large layer of oxidised material and/or paint contaminants and your resulting melt will be of very poor quality.

In the past I have melted lots of scrap extrusions, like old window frames, ladders, poles etc. They are a plentiful source but don’t make ideal food for your crucible. Castings made from scrap extrusions tend to be difficult and ‘gummy’ during machining, and less fluid during the pour. The desirability of ‘fluidity’ – the ability of the material to flow into all the corners and details of your mold – depends on your part design. If the part is intricate, it’s particularly important.

Casting from scrap extrusions.

These odd castings, straight from the sand, will be part of the same coffee table. It is made from not-too-judicious selection of aluminium bits, some extrusion. Looks very poor finish now, but turns out OK in the end, which goes to show, it can be used for non critical parts.

Castings are best, but which?

The best aluminium source to look out for is scrap cast aluminium. As you are making a casting too, it’s a fair bet the alloy is going to be useful. Scrapped cars and other vehicles can be a great resource, look out for old cylinder heads, alloy wheels, pistons and connecting rods (probably the strongest and finest quality alloy for casting), cast housings etc. Also well worth saving are old hard disk drive casings.

Importance of machinability

The feet cast earlier are machined to accept four old aluminium tent poles (extruded aluminium scrap), which will make the table legs.

Alloy wheels have become my favourite, generally being easier to extract and clean than the oily engine parts, and often being Al 386 – great for casting! One cautionary note here: some expensive alloy wheels (which you aren’t likely to come across as scrap, but I suppose it is possible) are made from magnesium alloy. A magnesium fire in your crucible is very bad. To test, file a bit of the wheel off and sprinkle the filings in the furnace. An alloy very high in magnesium will burn a bit like a sparkler. Either avoid melting it altogether or use it very sparingly and carefully as an alloying element for other aluminium rich melts.

Cutting up a scrap alloy wheel

Cutting up a scrap alloy wheel. One downside to alloy wheels, and engine block sized scrap is the need to cut it into bits that will fit in your crucible.

Alloys with silicon, like 300 series aluminium are some of the strongest cast aluminium alloys. Many others that people think of as strong (such as 6000 & 7000 series alloys) are actually designed for extrusion or rolling, and do not really attain their best properties unless cold worked. Some DIY casters get really into making alloys, with magnesium and zinc being common additions. My advice however, is to try and start with good scrap castings that are already a desirable alloy. Of course, if you already have a big stash of scrap extrusion or other non cast Al, go ahead and learn about alloying it. In terms of finding breakdowns of casting alloys, matweb.com is an amazing resource (and not just for metal properties!).

Here’s just some of the details of Al 390 (sometimes used for pistons), for example:

Aluminum, Al 74.4 – 79.6 %
Copper, Cu 4.0 – 5.0 %
Iron, Fe <= 1.3 %
Magnesium, Mg 0.45 – 0.65 %
Manganese, Mn <= 0.10 %
Other, each <= 0.10 %
Other, total <= 0.20 %
Silicon, Si 16 – 18 %
Titanium, Ti <= 0.20 %
Zinc, Zn <= 0.10 %

Notice the very high silicon content, a characteristic of many good casting alloys.

In sum, assess what the final part is to be used for and if a very clean strong item is called for, use cut up castings to supply your crucible. For the CNC I’m building at the moment, I’m being really careful about the type of alloy I use, but for other stuff like crazy experimental table feet – just use what you have ;) Here’s how those earlier castings worked out on the coffee table and if you want to check out the finished table there is a full build log here.

Castings in use

You can see the feet at the back, and the other odd casting forms an insert at the top of the oak leg.

 

Responses to Which scrap aluminium to use for casting?

  1. tony

    i love the final outcome on the feet for the table. did you also pour some of the AL. for locking key on the table top. or is it some type of epoxy filler. ok, i am wrong on the coffee table top.
    just wondering if you can pour cast a locking pin into the leg & insert. without setting the leg on fire

  2. Bongo

    Hi Tony,
    Thanks for the comment. I did pour a casting for the locking key on the table top. But I didn’t pour it directly in. I made a foam key that fit, and cast that in the sand as usual (see the second pic of this article – that’s the casting).
    I have set enough sand filled flasks on fire to know that a pour directly onto the wood would involve a lot of charring (the flames could be controlled with very careful use of a water mister).
    Cheers, B.

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