Mig gas for degassing

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mite5255
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Re: Mig gas for degassing

Post by mite5255 »

dallen wrote:
Harry wrote:
dallen wrote:The more you agitate the melt the more gas it will absorb, also running your furnace with it fuel rich will help to keep gas out, but will do nothing for oxide inclusion's which can also cause porosity.
That should be fuel lean for aluminum shouldnt it? I have always read the aluminum like to absorb hydrogen and brass/bronze like to absorb oxygen. That is why you use a charcoal cover over brass but not aluminum. I run very lean with aluminum and have very few gas issues. I have noticed on some stuff I have been milling though that there are sometimes some very tiny inclusions near the surface so I am going to do some testing with degassing.

From what I have seen on using an inert gas you do want as fine of bubbles as you can get, plunge to the bottom and stir then skim and pour.
I have always had problems with aluminum with porosity in aluminum, some of it I have narrowed down to using car wheel without first melting them into ingots, the other is that when I first got into this hobby I wasn't using any type of chemicals for gas or dross. and now I think some of it may be from having the burner on propane. I will have to figure out a way to throw together a 1.250 diameter oil burner.

since I started degassing every melt I have noticed that the porosity has almost disappeared, but its still there. theres too many ways for to to happen to say which is causing it till I eliminate them one at a time. or just start my whole process over from start. but when I only have two pounds to melt it seems kinda dumb to fire up a furnace that will melt a hundred pounds of iron. So I guess I will have to stick with some of the processes I have in place and just be more careful in my procedures.

.
For a degassing wand I got a piece steel fuel line that would fit the tube on my mig gas bottle, bent about 3 inchs at right angle crimpted the end then drilled about 8 small hole's in the bent bit for the gas to flow out,seems to works ok.I use car wheels and I also melt to ingots first
IMG_0081[1].JPG
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HT1
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Re: Mig gas for degassing

Post by HT1 »

I cannot think of any reason why using propane would cause gas, that oil would not also. unless someone else chimes in, I think yoiu can rule that out as a problem.

can we see picktures of your defects. it will help with the trouble shooting

gassy metal defects unless they are HUGH, normally dont show till you start machining. While sand issues will be obvious on breakout. a picture or two will help us diagnose what you got.

to me it sounds like sand is the problem. back when I was working professionally we never used green sand for aluminum unless the mold would not support itself. in other wordds a HUGH casting in a hugh Flask otherwise we always used petrobond to avoid sand/moisture issues
mite5255
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Re: Mig gas for degassing

Post by mite5255 »

Every thing that I had made has been remelted, but do have this photo, after milling there was a lot under the surface but I have no photos of that

Mike
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Harry
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Re: Mig gas for degassing

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Mike, those three black spots, are those holes that showed in the milling? That is a problem that needs addressed. What did you use for the alloy? You are using wheels arent you? A356?

I will get some super small gas inclusions and only some times using A356 melted into ingots and oil burner running very lean. Melt to 1350* F (measured with non contact just above the surface of the melt on the crucible), skim and pour... no degas and no flux. I do end up skimming a lot of metal because I am almost pouring too cold. I kind of figure if you are not getting a cold shunt from time to time you are probably pouring hotter than you need to. Of course this changes for every mold and I will adjust the temperature up 50* or so fir thinner section pours but most everything I do has nice thick sections throughout with a minimum being 1/4".

I do need to work on setting up degassing for milled parts so I dont get even the tiny bubbles and parts can even be polished like the excellent work Bob S has posted in the past. One thing that comes to mind is Murray always telling us long ago is degassing on the fall, pull the pot and degass as it begins to cool rather than while it is still rising in temperature. I would imagine this could also be done while in the furnace by turning down the heat which would work well for me since I use the same tongs to both pull and pour.
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HT1
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Re: Mig gas for degassing

Post by HT1 »

That is definately gas, but it is not very bad, a degasser tablet plunged to the bottom should clear that right up
mite5255
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Re: Mig gas for degassing

Post by mite5255 »

Harry wrote:Mike, those three black spots, are those holes that showed in the milling? That is a problem that needs addressed. What did you use for the alloy? You are using wheels arent you? A356?

I will get some super small gas inclusions and only some times using A356 melted into ingots and oil burner running very lean. Melt to 1350* F (measured with non contact just above the surface of the melt on the crucible), skim and pour... no degas and no flux. I do end up skimming a lot of metal because I am almost pouring too cold. I kind of figure if you are not getting a cold shunt from time to time you are probably pouring hotter than you need to. Of course this changes for every mold and I will adjust the temperature up 50* or so fir thinner section pours but most everything I do has nice thick sections throughout with a minimum being 1/4".

I do need to work on setting up degassing for milled parts so I dont get even the tiny bubbles and parts can even be polished like the excellent work Bob S has posted in the past. One thing that comes to mind is Murray always telling us long ago is degassing on the fall, pull the pot and degass as it begins to cool rather than while it is still rising in temperature. I would imagine this could also be done while in the furnace by turning down the heat which would work well for me since I use the same tongs to both pull and pour.
Yes Harry thats after drilling the oil gallery in the sump extension, I made 4 and the porosity was in the same place each time. I'm using alloy wheels that have been remelted into ingots and I pour at about 1350 and sometimes hotter, I do need to just concentrate on one thing at a time, I do at times if everything is moulded and I'm waiting on the melt, start another job and keep going back to check the melt and on a few occasions it has got a bit hot.
I have done two melts using mig gas and no more porosity , I then started to have problems with steam( see Photo )I will as HTI suggested face heat the mould next time which will be about Tuesday at the earliest

IMG_0087[1].JPG
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dallen
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Re: Mig gas for degassing

Post by dallen »

Mike I was setting here looking at the above picture and got to wondering about how your clamping the part into the mill for milling. This is a sump extension right? theres nothing in it that would stop you from say placing a cross bar in what would be the bottom is there?. if you could do that say a couple bars right were those I'll assume what looks like bolt bosses are you would eliminate the chance of overtightening the part in the vise and would be able to get a say really good clamp on it then, I just hate it when I think I got everything all bolted down all nice and tight stick the cutter it it and next thing you know cutters broke and parts all scarred up from it coming loose, and when your doing it to sell there goes your profit, anyway to cast the bottom on it so you only have to work one side? that would save you some work also, more profit.
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Harry
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Re: Mig gas for degassing

Post by Harry »

Casting the bottom on it is a great idea David, eliminates a gasket too. Would be an extended sump rather than a sump extension.
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