Metal working: Quick thoughts

A few successful furnace cycles have been run, and a handful of lessons have resulted. I was given a whole bunch of machine shop turnings to melt down (13.6 lbs of them) and learned that it takes a little more care than melting cans. Turnings (also called chip) have very large surface area compared to their volume, and their surfaces are saturated with crystal structure defects due to the machining. Taken together this means that they very readily react and oxidize at elevated temperatures. Not thinking about any of that, I went ahead and tossed a bunch of chip into a fresh steel can and fired up the furnace.

Failed Crucible
Steel crucible failed catastrophically

As evidenced in the above photo, this didn't work out great. A second attempt was made later with three thoughts in mind. 1) Allowing the crucible to reach temperature would thicken the protective oxides on the steel that prevent reaction with aluminum, 2) an existing bath of liquid aluminum would prevent rapid oxidation of the aluminum turnings if they could be effectively submerged, and 3) the residual machining oil on the chip might be a contributor to the reaction. After allowing a new crucible to reach temperature, and observing that a foil-and-salt flux packet readily melted into a bead, cans were fed in until a good layer of liquid aluminum was apparent. At that point I was able to feed in foil-wrapped packets of turnings which seemed to melt readily without adverse reaction. I haven't come up with an easy way to remove all the machining oil, but in the short term it doesn't seem to be an issue. It will take me quite a while to melt down all the turnings, as it doesn't look like I can just feed them in non-stop, but there is no shortage of cans.


Metal working: First firings

Before I dive into my weekend's efforts, a quick note on nomenclature (so exciting, I know). I've been misappropriating a few terms as I've gone along, and should clarify. Smelting is extracting metal from ore, and inasmuch as soda cans aren't really "aluminum ore", I'm really just melting things. Also, I may have mistakenly conflated furnace (for melting) with forge (for heating to working temperature).

The first time we fired it up, starting the coals in a borrowed coal starter, the crucible seemed like it just couldn't reach a high enough temperature. After cooling and cleaning I noticed that the foil we'd thrown in as a test had melted, but only partially. Given the amount of time (and fuel) we gave it, this seemed strange. The body and lid of the furnace visibly darkened in color during use, leading me to believe that a good deal of the heat went into driving off residual water in the plaster. The body did have a week to set, but it was very thick and the lid had only set for a few days. A second reason that the aluminum did not melt may have been due to the crucible. As it is made of a silica-based ceramic, it doesn't conduct heat well and is likely better suited to a less directional electrical-element based furnace. Hoping that the initial firing had burned off any residual water, I decided to work on finding a steel crucible as well.

A second (better documented) attempt was made the following day. I thought for a while about where to find a serviceable steel crucible at short notice, and eventually decided to buy a large-mouth can of soup. The soup was tasty, and the can was steel. I've been calling them "tin cans" my entire life, but the magnet doesn't lie. While it looked bad enough after one use for me to throw it out, at a dollar or so a firing it's a reasonable approach in the short term.

Steel "Crucible"
Steel "Crucible"

I gathered together all of the supplies and got a friend over just in case of emergency then started in.

Casting Supplies
Casting Supplies. Left to right: Bag of scrap aluminum, face shield and gloves, crucibles, long steel spoon, tongs, furnace and blower, steel muffin pan, coal starter, fireplace matches, and emergency water.

After dumping the started charcoal into the furnace, placing the crucible and replacing the lid, we turned the blower on and added the flux. For flux I obtained some Morton's Lite Salt, which is half KCl and half NaCl, which lines up well with the recommended flux for aluminum. Rather than pouring it in (the airflow kept tossing it back out), a teaspoon of the salts was folded into a pouch of aluminum foil and dropped in as a packet. After just a few minutes a metallic bead of aluminum was apparent at the bottom of the crucible, so we commenced loading in the scrap aluminum. A few charcoal briquettes were added when the melting slowed down, but far less than were burnt through the first time.



The air exits the furance vent fast enough to juggle bits of aluminum!

Using discarded cans as the sole source of aluminum did generate a lot of dross, as seen piled on the brick in the photo below, though a bit of additional flux tossed in at the end did seem to free it from the liquid. The two lumps of reclaimed aluminum are visible in the muffin tin. They were allowed to cool for about 20 minutes while my friend ran to the grocery for hot dogs and marshmallows, as the coals still had a good deal of heat left in them.

Dross and Ingots
Dross and Ingots

I'm planning on collecting all the supplies and their costs into a table for reference, just in case anyone else is thinking about taking a stab at this but is being held back by cost concerns. The next steps will be to build a mold flask for green sand casting, mixing up some green sand, and picking a few good objects to cast. I've got some ideas already!

Metal working: Background bits

I've got a lot of materials science study behind me, but never really embraced the core activities of metallurgy, namely smelting and casting. It's been on my mind for a while now, but I kept waving it off by reasoning that it would be far too expensive and dangerous to attempt on a residential scale. Recently I've come across a good number of videos and write-ups by people who've undertaken so-called backyard blacksmithing (by no means limited to iron) and the costs of their furnaces have been comparatively cheap. Some are definitely more professional looking than others, to be sure, but they're all a factor of 2-5 cheaper than the comparable commercial models. As for safety, I guess I qualify as "adult supervision", so as long as I keep all precautions in mind, everything will be fine.

So, with a couple aluminum projects in mind, I went about the task of comparing designs based on price, capability, aesthetic (it's silly, but everyone loves a sharp dressed forge), and ease of construction. There were some fun looking designs for electric arc and gas-powered furnaces, but I decided to limit my first foray to a forced-air and wood/charcoal affair. Capability was also an interesting beast, because I hadn't thought about the relative melting temperatures of various metals (excepting outliers like lead, mercury, and tungsten) in many years. I've thrown together a table for quick comparison below.

Metal Symbol(s) Melt (C) Melt (F)
Tin Sn 232 450
Lead Pb 327 621
Zinc Zn 419 786
Aluminum Al 659 1218
Magnesium Mg 670 1240
Brass (85 Cu 15 Zn) Cu+Zn 920 1688
Bronze (90 Cu 10 Sn) Cu+Sn 925 1197
Silver Ag 961 1762
Gold Au 1063 1946
Copper Cu 1083 1981
Cast Iron C+Si+Mn+Fe 1260 2300
Manganese Mn 1260 2300
Steel-High Carbon Cr+Ni+Mn+C 1353 2500
Stainless Steel Cr+Ni+Mn+C 1363 2550
Inconel Ni+Cr+Fe 1393 2540
Silicon Si 1420 2588
Medium Carbon Cr+Ni+Mn+C 1427 2600
Nickel Ni 1452 2646
Low Carbon Cr+Ni+Mn+C 1464 2700
Iron Fe 1530 2786
Chromium Cr 1615 3034
Titanium Ti 1795 3263
Tungsten W 3000 5432

Based on the numbers that others have posted, it looks like the design I selected (unless upgraded to gas or electric elements) will probably reach past 700-900 C depending on air flow and insulation. That being said, if everything works out with the first iteration, with an upgrade I should be able to work with metals up through copper without issue. I do plan on obtaining a thermocouple thermometer and logging the ramp rate and peak temperatures for a few configurations of fuel and air flow, just to see how far it can be pushed.

The general idea that I went with was a plaster and sand refractory cast inside of a steel bucket, with an air intake drilled in from the side. The initial build went quite quickly, allowing a full day for all the setting of the plaster parts. Over all, the materials for the furnace cost $51 at Home Depot, and left me with extra sand, plaster, and buckets. That doesn't include the hair dryer I already had on-hand that will become the blower on the air intake, but those can be found cheap.

Furnace Supplies
Furnace Supplies

The body and lid are already made, safety supplies and a good crucible are headed my way from Amazon! I'll post another update with the details of the build and, barring any unforeseen snags, photos from the first melt.