Originally Posted by
NC Cams
When it comes to "mission" or application critical strength or other required properties, by all means, BUY what you want/need - the chances of a non-metallurgically trained person to melt and pour exactly what they need are quite remote.
I know of some scrap dealers who won't accept "mixed" scrap that they can't readily identify the alloy of. Why? Because if you dump the wrong or contaminated scrap into a vat of molten aluminum (contaminated with copper, zinc, iron/steel, whatever) you end up with junk.
Comparing the metallurgical requirements of a transmission case to that of a piston, wing spar or whatever is hardly a suitable decision to be made by a hobby level metal worker.
Similarly, trying to home melt metal that will meet any/all of these requirements (from swarf, beer cans, whatever) is also a bit beyond the pale of the average DIY'er.
Regarding transmission cases or other devices that simply need cross sectional mass, one could probably cast up a reasonable home brew of alloy and it would work fairly well. Machining might be an issue as might porosity but it would probably work.
Now, lets say you want to machine up a connecting rod or some other device that needs to do more than just "hold up". When you are going to subject a part to repeated load reversals or high tensile loads, you need a material with RELIABLE metallurgical properties AND properties that are TAYLORED for that particular application.
There's nothing wrong with experimentation with aluminum foundry practices and casting. However, to simply cook up a batch of metal to pour into a mold in one's back yard may result in a material that simply doesn't have the properties suitable for the intended application.
Alloy content is but a small portion of what goes into creating a suitable material. Subsequent cold and hot forming along with heat treating are critical to the creation and tailoring of materials suitable to a task. Some of these are not readily duplicable by the DIY'er.
If you know what you're doing and can create the alloy structure that is appropriate for the task, go ahead and cast it. Otherwise, you might be better served by buying a piece of metal that is already alloyed and heat treated so that the bar has the properties that you need.
Some people call this semi-prefinished/formed piece of metal stock as "billet". Others call it bar, some call it plate - some might even call it "ally".