Degassing aluminum

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Re: Degassing aluminum

Post by Harold_V » Thu Dec 23, 2010 12:17 am

Greg,
While it may not be oxygen that's the source of the inclusions, my friend, Red, made it clear that he did not pour on rainy days, an effort to avoid the inclusions, so that's in keeping with what you said.

I'm not a chemist, but I get the idea that excessive moisture is one of the sources of the problem. I expect that water reacts with heat to liberate oxygen, which is then combined with traces of aluminum, leaving hydrogen behind to be absorbed by the molten mass. While that sounds a little bit crazy, that's pretty much what happens when water is introduced to a magnesium fire.

Regards the integrity of the resulting castings, I have to go way back, when I was employed at a job shop. Interestingly, it was Red's brother, Marvin, now deceased, who cast a few hundred parts (defense work) from 6061 aluminum (yes, I realize it's not a casting alloy). I don't recall the details now, but the parts were cast in lieu of making them from bar stock. They had minor inclusions, yet were not rejected by the Navy---for whom they were made. I machined the parts, and have a reasonable memory of the appearance. I later spoke with Marv in regards to the inclusions. That may well be when I witnessed degassing by nitrogen. At any rate, it sounds like the way to go, assuming a guy has it available, as I do.

Thanks for your comments. It all helps in my learning curve. Maybe I can pick up the slack and do some casting in the future. :-)

Harold
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steamin10
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Re: Degassing aluminum

Post by steamin10 » Thu Dec 23, 2010 12:43 am

This is part of the arguement that scraps should be pigged, into house ingot, instead of poured into a molded part. The extra step seems wasted, but allows a gas off that reduces the problems of gassy metal.

It is also an argument for good wire venting to the casting to prevent steam blows back up the sprue, that adversly effects the pour, as steam perking through the pour is exactly opposite of what we want. It is also the reason that Oil bonded sands have taken over for Green sands that have more moisture in them to make them work. It reduces steam exposure to near zero.
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Re: Degassing aluminum

Post by todd goff » Thu Dec 23, 2010 7:38 am

I have never used the green sands but believe that the petro bond sands are far superior. Not trying to get off topic here, but I don't think that I have ever vented a casting using petro bond sand (well I did initially and tried it without and got the same end result). I can't remember the rule of thumb on sand but believe that the coarser the afs grade of sand the better it is. Coincidentally, I am trying to find a way to make this sand cheaper and have found the following; I can buy 50# bags of afs 100 sand from florida for 2.80 in ltl; bentone is the clay used in petro bond (bentonite used in green sand) and it is used in oil well drilling; I think that I have found a source that supplies oil well drillers with this and have yet to hear back. The p-1 catalyst in petro bond can be substituted by using methanol alcohol and also I am trying a retinol 100 oil to see how it will do in this process ( I got a 1 gallon sample from a chemical company). I am going to try this and will let everyone know how it works. Getting back to the hydrogen problem; an excellent book on this is aluminum melting and it was printed in 1925 and I got it from lindsay publications. It goes above and beyond on problems with aluminum; let's face it, these guys back then were probably ahead of some of the foundries today in technology. Fact is that hydrogen does pick up from the combustion process; it is a nasty little by product of propane or fuel oil melting. The faster that you melt the metal the less hydrogen that you pick up so in fact time is not on your side in this instance. I believe in gassing the molten aluminum even in the smallest furnace that I run which is a #16. Here lately, I have been making some adapter plates to mount turbochargers to engines on sports cars and I feel that this should definitely warrant me degassing the aluminum. It has in fact paid off as when I ran the casting on a disc sander it had zero pinholes in the aluminum so this worked great. The only problem that I did have with the casting was that it had one place on it that had nominal shrinkage but the customer said that there was enough meat on the casting so that he could overcome this when he machined it. I had done some experimentation with this and think I know why this happened; the basin for the casting actually was about 3.5" in diameter and was wood, it was damaged so I replaced it with another basin that was probably 2.5" in diameter. I put an adequate sprue for this casting though and figured that it would work (1.5") as these were some fairly heavy castings at 9# each. Only thing I can figure is that this decrease in the pouring basin caused it as I watched the temp. of the metal. A good friend of mine that runs a foundry does not hardly ever use a pyrometer; he told me that a good way to judge when the aluminum is ready is when it is (explicit pink) it should be good to go. I have been doing this lately and having good results; but anyway, who knows why things sometimes do what they do? I did get a compliment from my customer on the castings though; he does work for the military and told me that these castings were as close to an investment casting that he had ever seen; kind of makes you feel good knowing that people think of your work like that. Oh well, enough rambling on that: sorry to keep everybody here all day reading this. :wink:

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BobS
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Re: Degassing aluminum

Post by BobS » Thu Dec 23, 2010 11:59 am

I also use petrobond for my casting and wouldn't go back to greensand even with higher cost involved in purchasing Petrobond ll. It's gone up in the last couple years and would like to know a source of the bentone needed to make up some petrobond in house.

I have purchased the oil for making up molding sand from a local oil distributor rather then my foundry supplier for half the price they had been charging me. It went under the name Base oil 40. It's a refined 40#wt. crude with no additives added yet. The foundry name on their barrel said "Amoco 150". You might get a local distributor to do a cross reference to make a close match to one they have in stock.

I can get a ton of 110 grain sand from local quarry for $20 a ton it's just the main ingredient bentone that stopped me from making it all in house.

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I blow nitrogen in my aluminum melts when I need to but only when it's going to be highly polished after and then it's only done as a precaution. I only use good clean aluminum scrap that originally was also a casting before which removes a lot of guess work on what the outcome will be. :wink: Bob

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Re: Degassing aluminum

Post by ghornbostel » Thu Dec 23, 2010 3:44 pm

Oh boy, oh boy!!!! Isn't this getting to be fun. Its gone from how to test for gas to what is the best sand system. The wheel is still round. You need a furnace, no matter what and you need a mould to pour the metal into. These are the two main ingredients in the foundry formula. You can mix them many different ways to achieve the same result; a casting. For sand I use what is called "stove plate" by many foundrys in the Kansas/Nebraska area. This is sand that is natrual bonded and occurs in pockets and strada in the area. It screens out at 180 afs with about 20% kaolin of which about 12% is active. Unless I'm casting bronze I leave it alone. At higher temps I'll add a small amount of sodium but find that it really causes bridging of the sand grains and inhibits flowability of the sand on compaction. 5% H2O and proper mulling at each use plus discarding the sand that is on the face of the casting keeps the permability up. I have tried to replicate this sand but nothing comes close and yes I've tried PETRO-BOND. I find that stuff smelly at best and and never had the time to let my molds cool down enough to keep them from igniting. Petro-bond is proprietary. You will not get one of the three types of clay to work like it unless you have Petro-Bonds process. Western (sodium), southern (calcium) and fire clay (kaolin) are the simplest bond systems avaliable and we have all the information you need to make them work. You just have to have the proper equipment to do it. And I've been through the oil thing too and all I can say there is "water is water" and it doesn't get any simpler than that. :wink: But then I also agree that a Ford is not a Chevy and we are entitled to our opinions there too. So you still have gas and the only way you are going to eliminate it is by going through a process of change in the way you are casting metal. Without going to the next stage of what might be wrong all I can suggest is that you only change one thing at a time. :D It really doesn't matter where you start, whats important is what you end up with.
Once again, just an old man rambling on out in the middle of nowhere.
Greg Hornbostel

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Re: Degassing aluminum

Post by steamin10 » Thu Dec 23, 2010 6:31 pm

How do you keep your bronzes from skinning or sinking into the sand, use red iron oxide? Or do you even have that problem?
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
It is not getting caught in the rain, its learning to dance in it. People saying good morning, should have to prove it.

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Re: Degassing aluminum

Post by ghornbostel » Fri Dec 24, 2010 1:14 pm

Dave, its been a long time since bronze and from what I remember it depends on several factors. If you are degassing with phos-cu you probably should slack back a bit as this really adds to the fluidity of the melt. The other and I know it sounds like a echo but gating and pouring technique. You know that time when you had only one mould left and thought you didn't have enough metal to run it but what the hell. Well it just filled the mould and the casting turned out to be about the best one you've done. Mox (Martin Schultz) would have said "you pour it hard and cut it short". Mox had about a million years experience doing this so it was really easy for him but what it amounts to is pour it hard to get the gating filled and shut the metal off just when the mould is full. This is exactly what a overflow pouring basin does and it takes all the guess work out of it. Another thing that really helps is a 3/4 inch riser off the back or far side of the casting. We used to use a lot of washes before we got smart and mulled our sand. After that all the fur balls, buckles, sand wash and scabs went away.

A Merry Christmas to all of you
Greg Hornbostel

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steamin10
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Re: Degassing aluminum

Post by steamin10 » Sat Dec 25, 2010 5:12 pm

I get that, and I use what we refer to as HOCKY PUCKS opposite the pouring riser, to get the cooler metal out of the way so filling is more consistant. I use the Online copy of PETRO BOND, and use the bagged powder from the supply house. I am told it is red Iron powder and other Bentone base. When mixed with the #100 sand I use, it can be rammed too tight, so I always wire vent the cast a few places, and it seems to help. I have dressed molds with a wetted finger for sand grains, and actually left my finger print on the face of a part, the reproduction can be that good. You dont need the iron for Aluminum, but I am told it is the secret for preventing Bronze scabbing. I use the same mix for bronze,brass, and aluminum, out of dedicated vessels, and dont have all that many failures. Mostly #16 bilge for my projects. I do have a Cannon pattern that will need more material, so that will run from two furni, with a simultaneous pour. The local foundries around Chicago, last I asked, wont touch this thing.

I wont go into making 15 casts over two days, for model truck sideframes, and having nothing to show for all the time and expense. As best I can figure, it was heavy with returns, and I got too much iron into the melt, (my iron pot days), so even with another 100 in temp, the cast molds shrank badly, and failed to fill, only to be remelted, to ruin another batch.

There comes a time, when you just start at A, and move forward. Been there done that.

Added: Bob S. The real expense in the Petro is the synthetic oil. It does not burn like natural oils and really jells with the Alchohol. When I mix this stuff, a virgin batch will often stall my tiny mull, (mighty-mite) as it begins to react and develop bond strength. A coupla days later, it will relax somewhat as it creeps through the sand, and strength can be controlled on aging sand with very small additions of Alchahol and nothing else and remull. I have about #200 lb of mold sand that resides in a 30 gallon barrel. Some day, I will need more. For now it is enough to fill two of my largest flasks, with a bit left over. I always cut and turn my sand when I shake out, but dry spots force me to remull often. Virgin batches usually go for my small 6x8 mold boxes for details, that must be sharp. The mighty-mite only does about a good shovel full at a times. I have taken to using a 1/5 yard cement mixer with a coupla slugs of scrap to remix. That has given way to 3 shotput cannon balls to squish the sand along with the roll. A paddle mixer would be better, and is what the supplyer uses. I dont have one.

I am being called to go to another family dinner. Happy Holidays, wherever you are.

I made stuffed cabbage again by request. :?
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
It is not getting caught in the rain, its learning to dance in it. People saying good morning, should have to prove it.

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BobS
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Re: Degassing aluminum

Post by BobS » Sat Dec 25, 2010 9:18 pm

In all the years of casting I have never tried synthetic oil with my petrobond mixes. I will see if I can pick some up that doesn't have all the additives like the ones sold for motor oil. Both regular crude and synthetic oils refined for motor oils have detergents, modifiers as well wearing resistant additives which are of no value for petrobond bonding oil and add more to its cost as well as producing a breathing hazard when they are cooked out.

I usually buy my oil in 55 gallon drums so it might be as you say expensive but a 5 gallon pail might be worth a try if its available as a synthetic base oil with no additives added yet. I usually mix up a ton molding sand at a time and store what I don't keep on the floor. I now use Petrobond II which doesn't require the alcohol catalyst but I still give old sands a shot of alcohol every once and awhile to give the green strength a boost.

My muller is a homemade unit copied from a larger model and can do about 50 lbs at a time.
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steamin10
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Re: Degassing aluminum

Post by steamin10 » Sun Dec 26, 2010 1:04 pm

The additives are uneeded and unwanted.

The synthetics used in Petro Bond are based for 2 cycle use, therefore they must degrade cleanly without residue or ash content. The Supplier will want to sell you a catalyst at some $40 a pint, but methyl alchahol works just the same. New sand is REAL sticky, but settles down in a few days or a few casts, and lasts for months, without much change.
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
It is not getting caught in the rain, its learning to dance in it. People saying good morning, should have to prove it.

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