Degassing aluminum

[todd goff]

I have recently bought a rats unit to test aluminum for hydrogen porosity and am fixing to get a rotary degasser. I realize that this is probably a waste of money but am looking to get into casting on a larger scale and think that this would be a selling point(all about quality control). Is this a waste of money; I mean have I lost my mind or is this something worthwhile?

[steamin10]

Quality in casting is always the first objective. It is as important as shape, the reason to cast in the first place.

I have always used innoculants to degass and treat an aluminum heat before pour. Pigging scrap metals before use can reduce hydrogen, as it is gassed off upon solidification. Keep paint, oils, and moisture away from molds and melt metals, dont hold a heat. Get to temp and pour as quickly as possible. FUN-di-mental rules.

I have no experience with those particular machines, or for testing other than pouring a test bar and breaking it, to get an idea of grain structure. Perhaps a bit cave man, but pin tests for carbon in steel, and iron can be very telling to a trained eye. So can a wedge bar for iron, to see if it chills for white iron. All primitive, all work.

[Harold_V]

Not familiar with the rats unit you speak of, but I recall that a friend that operated a foundry used to bubble nitrogen through his molten aluminum for a period, prior to pouring. He used rather large crucibles, say #150's, and would generally superheat the aluminum slightly, then pour when the temperature dropped to the desired level. Nitrogen ran until that time.

Harold

[steamin10]

Harold: do you know the 'Superheat' temp? 1600?

I try for 1350 pour for small castings, 1300 for larger heavy sections. I try not to go above 1450, I get bad blows and other problems.

Any body?

[Harold_V]

When I witnessed the process it was something I had never seen. I recall asking how much hotter, and think I was told by only 100°, but I wouldn't stake my life on it. Considering the size of the heat (#150 crucible, maybe larger), cooling wasn't all that fast.

I haven't talked with this guy in years. He was older by a fair margin, and I've been gone from Utah for almost 15 years. Pretty good chance he is no longer alive.

Harold

[steamin10]

Ok, that clicks with the numbers I have posted, they are close. I was wondering about confirmation. I am getting old, and have lost all those that knew the 'secrets'. Like Merlin, when he is gone, I am but the apprentice, trying to remember the intonations and advice.

[Harold_V]

Ok, that clicks with the numbers I have posted, they are close. I was wondering about confirmation. I am getting old, and have lost all those that knew the 'secrets'. Like Merlin, when he is gone, I am but the apprentice, trying to remember the intonations and advice.

This guy (Marvin) had two brothers. One of them, George, died while I still lived in Utah. The other, Red (Lamar) was alive and well when I left. I had known these guys from my teen years, when I'd stop by their foundry and watch them work. At that time, they all worked for their father.

Red started his own foundry and was doing well. In fact, he was the source of the inoculants I brought with me, inoculants for making ductile iron from scrap steel. Anyway, he may still be alive, although likely retired. He has sons that worked in the foundry, so, barring the business going under because of recent events, it most likely is still in existence. If so, I'll see if I can contact Red and get more information.

Harold

[steamin10]

That would be a good thing, getting the real info. Most of what I run into is book larnin' types that profess, but never got sunburn running a furnace. I used to trim my hair and eyebrows regularly. No, really! If there was a way to ork things up, I found it. I dont know if I am just fast, or have my personal guardian Angel.

You know Angels watch over Children, Idiots, and Drunks. I am not a child in body, so that leaves a debate!

[Harold_V]

Well, thanks to a good prodding, I called Red today. It was fun talking with him, having seen him last about 15 years ago. He's still active at age 67, running not only his foundry, but a machine shop as well as running a salvage yard. They started buying scrap years ago, a source of metals for the foundry. He said business in the foundry and shop have slowed, but the salvage business is booming.

In talking, he told me that his brother Marv died three years ago. I wasn't surprised, although not thrilled to hear the news. Marv was a real rebel, but a good guy, and a good guy to know.

In the course of talking, I asked Red about using nitrogen for degassing aluminum. My memory of events wasn't totally correct in that he said that they don't superheat the aluminum, just take it to the temperature they hope to use when they pour. That varies according to the castings, which I'm sure you'd know. He talked about heating to 1350°F for small items, and as low as 1,300°F for heavy castings.

Once heated, the crucible is removed from the furnace and the lance is introduced to the aluminum. As I suggested originally, he talked about using #150 crucibles, so there's a significant amount of mass present. He said that when the crucible is removed, it tends to be hotter than the charge, so they monitor temperature while nitrogen is being percolated through the charge. The residual heat in the crucible walls is absorbed, raising, slightly, the temperature of the charge. When it drops to pouring temperature, the nitrogen lance is removed, the crucible skimmed, and the castings poured. I asked him specifically about superheating the aluminum, but his response was that it burns up the aluminum, so they don't do any superheating.

It's nice to know folks that do things for a living.

Harold

[steamin10]

Well, it is good to confirm the information is still basically in the head as yet. I did not remember the Nitrogen for the degass, I never used it. Then again, my furnaces are school type small, so it is more toylike than a real foundry anyway.

Thanks much for the shout up the line, I am sure you enjoyed the old contact.

[Harold_V]

I'll likely give the nitrogen idea a go, assuming I get that far along. My plans for non-ferrous melting will revolve around either a #16 or a # 20 crucible. I have some of each, and I have a couple handling tools. Unfortunately, one for each size, and not the same tool. One is a pouring shank, the other a lifting tong. If they were both the same size, I'd be inclined to stick with which ever size they were.

Regards the use of nitrogen, considering I already have a (large) cylinder (for plastic welding), it wouldn't be much of a deal to give it a go. I expect that one could leave the crucible in the furnace and percolate for a couple minutes instead of removing the crucible. That would likely prevent excessive heat loss.

What do you do for gasses now? Do your castings come out without porosity?

Harold

[steamin10]

When I innoculate the melt with the chlorine packet, the swish, and turbulance, have a profound effect on the melt. I try to innoculate in the furnace, but that depends on the temperature reading. Of course once outside temps fall like a stone, but the final working can be done when you transfer from tongs to shank. A final stir, and skimming dross just before the pour.

I have seldom come out with airy castings. Of course this is process so I will layout the Not-to's that get into the system and create problems. Pitfalls include:

Casting stock. Get good metal. Inducing cans and junk with paint, plastics, and unknown metals, like diecast, will put you in the woods with content and performance. Without the reduction to pig of 'new scrap' you run the risk of inducing hydrogen into the melt that will cause fatal flaws and porosity. Dont allow rivets, bolts, screws, to get into your scrap feedstock, it raises the iron content, and cannot be removed, only diluted. Metals that are pasty willnot pour , get cold shuts, misruns, and gassy due to the mix. Silicon, allows greater fluidity, for detail, and can be preserved by fast heating, and low holding times. Small amounts can be added but it is more effective to save what is there. Items that were cast, are best cast again. Extrusions need more silicon, and can give a gummy casting to machine. There are a great many alloys of aluminum, and ID is nearly impossible by eye, so it is shake and bake for the home shop. A dip and a test bar cast from a heat is all I can do. It has caused me to pig and remelt, rather than waste a mold on 'bad' metal.

Pig stock is best, with alloy certificate from a supplier. Expensive? you bet. But in some situations the only way to get the quality you can guarentee.

Minding the sand, not too wet, or hard packed, that it cannot gas off, making blow holes in the casting. Water turns to steam and is undesirable in aluminum. Oil sands present a similar problem, as the oils smoke out, they react with the aluminum skin and pull hydrogen into the metal. It is all timing in that these reactions are small before the metal freezes and becomes inert.

Pattern design is crucial, and an art in itself. Traps and runners should be placed that erosion of sand into the casting is prevented. Most often a casting without a pouring runner will give poor performanc by taking sand into the void and eroding a fasace of the mold creating imperfections.

A larger pouring hole, it is argued, will pull more air with the metal, and has a degrading and cooling effect on the pour. A cup shape that can run full from the crucible is a better idea, pouring fast and full without interuption, is the goal. Keep the metal moving and rising in the casting to eliminate cold spots that freeze early before fill. Solid usable castings is the goal, instead of wasting time being 'that guy' down the street.

This is a quick shot at what has to happen in varying degrees, yea, its alot to remember while you work fast and steady to get the heat poured. But they write books that generally leave out the details that may not apply to the home shop guy. I strongly recommend writings of Amman, and others that are for home and small commercial shops. Gingery, while inspiring, is not the best reference here. As one gains experience, you can see why.

I hope I did not bore anyone with my 'Learned' opinions. Like a rainbow, many factors come into play for it to work well. Miss something, and you dont get the full effect.

I bid you peace.