I'm in the process of melting all of my scrap into ingots and I'm keeping track of the sources, like ingots made from extruded aluminum, ingots made from car rims, pistons, etc. I read someplace that extrusion alloys don't cast well. Are there known limitations on the others?
If it's a scrap casting, it'll likely work better for casting with than if it's not. A lot of auto castings are A356 which works well for a lot of casting applications because of its silicon content that reduces the occurrence of a lot of common defects. Some casting alloys are probably better for making certain types of castings though. So say if you melt down a bunch of particularly thin walled castings to pour something much more thick, you could maybe see some weird unwanted things happen that could be hard to explain. But generally if you stick to cast scrap you'll be better off than if you go and melt just any old scrap aluminum. Foil and cans and thin sheet metal are probably the worst not only because they lack helpful amounts of beneficial alloying ingredients, but also because they're all surface area, can turn mostly into dross before they even add any liquid to the melt, not to mention they are already significantly made up of a large percentage of preexisting surface oxide skins compared to chunkier scrap. Jeff
To add to that, when I was running classes at a local school for a few months, all we had to work with was extrusion aluminum cutoffs the owner got cheap from a local.machine shop. For casting decorative widgets it worked ok. Fine, even. Shrinks more, and we kept away from anything that had really thin sections, but you can use bigger feeders to compensate for the shrink, etc. However I don't think it would have worked as well for machine(d) parts. Jeff
Circumstances can vary a lot, but if that scrap can be cut to crucible size pieces, the process of reducing it all to ingots can be a wasted exercise. You lose your ability to identify the metal, plus the fuel used. But, again, circumstances may warrant it. I’ve sometimes pre melted rims to skim the lacquer off or computer hard drive bodies to get the paint off, or castings with embedded steel, etc. A whole other topic for reducing large stuff is the “stack melter”. That’s been a popular subject here over the years. I’ve used that process for tire rims, lawn mower bodies (beware of magnesium!), separating steel from whole lawnmower engines, etc. Some folks get the idea that it might be an advantage at the scrapyard, but it will devalue the load because of the identification issue. But if a guy has a reason to do it, even just for the hell of it, then have at it. I’ve done it and will probably do it again. Jeff has outlined some pretty good information above. Mystery defects are frequent when using mystery metal. Most of my castings are decorative, not structural, and I don’t do any heat treating. If you’re doing any of that stuff then I’d advise buying certified ingots because you really need to know what you’re working with. Otherwise know that with aluminum, generally, the more silicon, the better the flow and less shrinkage. However, in the absence of heat treating, it is gummy as hell on the lathe or mill and can be very frustrating to file. If I’m casting machining blanks, I use extruded alloy (wrought). You have to pick your battles. As an aside, I’ve learned that cast pistons have a very high silicon content so I’m saving them to mix with aluminum wire to potentially yield a useable mixture (total crapshoot). I’ll let you know how it goes. Pete
It has been awhile since I have posted, I process between 5500 and 7000 lb of near 356 alloy, all scrap is clean aluminum mostly automotive power train castings, and structural suspension castings, I do add pistons if the amount of other casting material would have a lower silicon content. when tested it comes out on the high end of silicone spec.. the advantage of silicon is a reduction of surface tension in the liquid form allowing for thinner sections of a casting, It can also be heat treated to T6. It typically doesn't gum up cutters if running a little slower cut or having a coolant on the cutters. I use a soap s0llution with a mist delivery system. It also doesn't require degassing unless the casting needs to be watertight or for structural integrity. If the casting needs to be degassed a sodium hypochlorite (Pool Shock) solution can be added. Proper ventilation is required as it off gasses chlorine. Due to age, I'm in my Mid 70s I'm in the process of getting the business to a condition that it is salable, the biggest issue is the computer burner control system is not user friendly as set-points etc are integral in the code, I'm in the process of generating a menu driven system with set-points being easily changed and visible system monitoring. it will also be able to switch to manual operation. the code will also be function driven. I'm hoping once I am no longer tied to the business then I will have time for other activities like model engines, fishing etc, Art B
