This subject comes up frequently, so I thought I’d make a sticky for reference and leave it open for others to post their questions and experiences. It's a fairly involved discussion and a more difficult problem to control than it may at first appear. I posted here in the Lost Foam sub forum because many attribute porosity and defects they experience to lost foam casting process when it most often is due to poor melt management practices, which apply to all aluminum casting processes. No doubt H2 is the scourge of aluminum casters. Very few gases are soluble to any appreciable degree in molten aluminum except H2 and when it comes out of solution as the metal freezes, the captured gas creates porosity which diminishes the mechanical properties of the metal. Hobbyists try all kinds of things and in the end, you need to do what seems to work for you. For me it's been 9 years home/hobby casting mostly aluminum machine parts. I designed/spec’d castings for a long time professionally and have lifelong friends that owned foundries so have been able to observe professional practice as well. It’s a different ball game when you need to control it yourself. My early lost foam castings were often very gassy and the melts had lots of dross. Initially I thought it was the lost foam process. It was not. It was my melt practice. When I started hobby casting, like many, I used scrap/recycled castings. Mostly recycled automotive parts. The stock wasn’t always very clean, and some had paint. I often overheated the melt to get successful pours. Casting Stock Best to cast with recycled castings as opposed to wrought stock. No soda cans! Using oily, painted, dirty stock has little effect IF you initially charge your cold crucible with it and don’t add it to a partial melt. The reason being, the hydrocarbons/H2 in the oil and paint are long gone and turned to carbon well before the aluminum melts. You may have the residue floating on top of the melt but that is harmless. However, if you add dirty metal to a melt, you are directly introducing a rich source of H2 from the volatilized hydrocarbons bubbling through your melt…..and this is very bad. For a bulk scrapping processes, I’m sure it is much more practical to treat and degas the melt than it is to clean the metal feed stock, and you’d likely have to degas anyway. Sources of H2 Exposure I've used a resistive electric furnace from day one so there was/is no issue with combustion gas hydrocarbons but fuel fired furnaces are not only a source of H2 because of the hydrocarbons in the fuel and tune of the burn, but because of the large amount of water vapor in the combustion air, and in a typical melt, you will run thousands of cubic feet of superheated heated water laden air through your furnace. Best to melt on a cold winter day or desert conditions because there is little water in the air. Worst is a hot, humid, summer or rainy day. Over/superheating the melt is terrible because the solubility of H2 in Aluminum is a strong (nonlinear) function of temperature. If you have very gassy metal, you can actually degas aluminum to a degree just by holding it near the melt temperature, which can often be easier said than done. This is when H2 is the least soluble in molten aluminum. The degree of solubility is governed by Ficks law (search it), and is a function of the partial pressure of H2 above the melt. Blanketing the melt with an inert gas will make that H2 partial pressure very low, the partial pressure of H2 will otherwise be that in the ambient air or furnace atmosphere thus the importance of lean burn, but you can’t escape the water in combustion air. Degassing The most common home brew degassing compound mentioned in hobby casting circles, is a Eutectic of NaCl (Sodium Chloride aka table salt) & KCL (potassium chloride, aka safe or low sodium salt). The Eutectic mix lowers the melt point of the salt so it melts at aluminum casting temps. I've never used CaCl2 or Ca(OCI)2/pool shock, though the latter is chemically similar to the commercial degassing tablets which I’ve also never used so can't comment. Maybe others will offer their experience with those. They’re all in layman’s terms salts and all have a strong affinity for absorbing water from the atmosphere, so store them in a tightly sealed container. There is a myriad of other things hobbyists have tried. In my experience, I think the NaCl & KCl was a better flux then degassing agent, meaning it did better at clumping oxides/dross than liberating H2. They are very corrosive and there use will cause a lot of rusting on your rig and nearby equipment. Still, when shoved into the melt, it would bubble and produce more dross and this causes hobbyists to conclude that because it is producing more dross it was working to degass......but this is not necessarily so. All of these salts have a very high affinity for water and very short, recommended shelf life. Many times it's the water being released that causes the bubbling. Agitating the melt exposes it to O2 and that will instantly combine with Al to make Al2O3 (aka aluminum oxide/dross). If you have a very gassy melt, a lot of things can improve it, but that doesn't mean it results in a good quality melt, just better than it was. To my point, you can stick a tube into your melt, blow perfectly dry air through it, and it will produce dross on the surface and over time will eventually turn all of the melt into Al2O3. Again, agitating the aluminum melt exposes it to O2 and causes it to oxidize forming Al2O3 aluminum oxide, which in general is very bad because it creates bifilms (oxides) in the melt. Commercial practice is to bubble Argon or N2 through the melt with rotary degassers and blanket the top of the melt with that inert gas. The bubbles of inert gas have zero partial pressure of H2 so as the Argon buoyantly bubbles up through the melt, H2 diffuses into the inert gas as a function of Ficks law and gets carried away as it leaves the surface of the melt. The rotary degassers make a very fine dispersion of bubbles so they have lot’s of surface area for diffusion. This is surprisingly hard to do as a hobbyist, especially in a crucible. Most try by making a wand and produce bubbles from a single tube using N2 or Argon welding gas. The gas is ok but this is largely ineffective because the large bubble have little surface area, contact very little of the melt, and usually creates strong agitation at the surface of the melt, which again creates dross as the melt contacts the air, so the hobbyist concludes it is working……when it’s probably doing as much harm as good. I made this degassing lance. It’s not rotary, but it does make fine bubbles. I can only use it in A20 and larger (better) crucibles or it creates too much agitation. Degassing Lance | The Home Foundry You will often see recommendations to leave the dross on top of the melt and just skim before pouring. This is because H2 is not soluble in Al2O3 so it acts as a diffusion barrier of sorts like commercial cover fluxes. This is helpful if you have clean metal underneath, but bear in mind, it can also keep in or at least reduce the rate at which H2 can escape if you add dirty metal to the melt to top up the crucible. Should you process scrap into ingots before using it as casting stock? Maybe. If the stock is dirty stock, it may be beneficial to do so for several reasons, especially if you can effectively degas it. If so, you can use these ingots to top up a melt while minimizing the exposure to H2 that plopping in dirty stock would create. However, if you can effectively degas, you are probably just wasting energy and would be no worse or better off just using the degassed melt in the first instance. Sometimes, the mere act of pouring the ingot liberates H2, especially if the cooling rate is slow because the ingot will release H2 at its surface right up until it freezes so your H2 level in this case becomes very close to equilibrium with the partial pressure of the H2 in the air. If it’s very dry environment like the desert or winter temperatures, this could be helpful. But introducing a clean ingot into a poor furnace atmosphere and/or melting practice will still produce a poor quality melt. Melting fast versus slow There seems to be a lot of pride taken in a furnace/burner that can melt fast. For aluminum, as far as minimizing H2, until the metal melts, it doesn’t matter…..have at it….blast away, but it will be harder on your furnace refractory and crucibles due to thermal shock. Once you approach melt temperature is does matter, and in a fuel fired furnace I’d recommend backing off to the leanest lowest flow rate burn that achieves the desired temp. The difference in melt time between just setting an optimal burn and blasting away is only a few minutes and practically speaking you just need to ask yourself if those few minutes are worth the effort. Should you preheat your furnace and crucible? Preheating your furnace might mean less time to get from melt to pour temp thus less exposure time but I think in the big picture any benefit with respect to H2 management is de minimis compared to the other factors mentioned here. Now the crucible manufacturers will recommend you ramp heat (especially on a new or not often used crucible), and also add a small amount of metal to form a heal, then add more to the melt throughout the heat. This minimizes thermal gradients and stress on the crucible but may not be the greatest for melt quality. IMO, Aluminum duty is not very challenging for a good crucible compared to higher melt metals and I’ll take melt quality over 5 % more crucible life. I get several hundred melts out of my crucibles and I do not return them to the furnace after a melt because it turns the residual aluminum into aluminum oxide that tenaciously adheres to the inner crucible wall. If I let it air cool, I can peal the foil and heal out and start the next melt with a pristinely clean crucible. Should you return the crucible to the furnace after a melt? IMO, Aluminum duty is not very challenging for a good crucible compared to higher melt point metals and I’ll take melt quality over a small improvement in crucible life. I get several hundred melts out of my crucibles and I do not return them to the furnace after a melt because it turns the residual aluminum into aluminum oxide that tenaciously adheres to the crucible walls. If I let it naturally air cool, I can peal the foil and heal out and start the next melt with a pristinely clean crucible. This undoubtedly creates somewhat more stress on the crucible but I like a clean crucible. For me, tool contact has a larger affect on (reducing) crucible life. What has worked for me I rarely degas these days. On my most important melts I use virgin ingot in my resistive electric furnace. I still use returns and remelt too. The furnace is well sealed with ceramic paper gaskets and I actually have low(er) oxygen atmospheres and zero (combustion) air flow. I resist opening the furnace vent until experience tells me it is nearing melt temp. I’m sure some atmospheric water infiltrates the furnace atmosphere (Ficks Law again) but the exposure to sources of H2 compared to a fuel fired furnace is very low. The melt rate in a resistive electric furnace is very controllable with experience very predictable within a minute or two from a cold start. Consequently, unless I become distracted, I never superheat the metal and am able to pour just as it reaches the desired pour temperature. Even so, I always measure the melt temp with an immersion/contact pyrometer to confirm metal temperature prior to the pour. Having good control of the melt temp is a very important to success and repeatability. When I skim a virgin ingot melt, there is only a very thin grey film on the surface. When I skim remelt from my previous castings, there is only slightly more dross. I try to cast on weather favorable (low humidity) days. I have a lift off furnace so I can snatch and pour in one motion with the same tool, usually within 5-10 seconds of removing the crucible from the furnace, minimizing heat loss, and allowing lower melt temperatures. I don’t have professional metrology equipment to directly measure H2 gas levels in casting samples, but I have polished and examined them and found them to be comparable or better to sand casting I’ve looked at in similar fashion. I’m not saying I have no porosity. Virtually all aluminum castings do. It’s just a matter of degree. Porosity can vary quite a bit in different locations of a casting, especially more massive areas that freeze last. Collectively, this is what produces the best quality melt for me. I could probably improve it a little more by blanketing my furnace with inert gas and more elaborate Argon degassing…..but I get satisfactory results for my purposes. Best, Kelly
