Bob Puhakka on Bifilm theory

Yes, great video Tobho.
My video and Ironfarmer's shots look just like the cover view. If I hadn't seen it for myself I would never have believed there was that much splashing in the mold cavity. I am convinced that most of the bifilm I get is from the crucible and my re-use of sprues and risers.
I have always tried for a choked basin so I don't see bubbles in my castings, but the bifilms from multiple melts have ruined many of them. Campbell mentions that charging the crucible with ingot entrains the skin oxides in the melt. He describes a dry hearth furnace where the ingot skin remains on the hearth, and the molten aluminum runs to a molten basin. It is a more complex system, but would reduce the need to degas. The counter gravity method requires a three piece flask for any pattern with a midplane parting line, but the sub drag with the runner and vented swirl could be formed with sort of a match plate and adapted to loose pattern work.

I bent up a little flask to hold the offset basin in rammed sand using the pattern for the NaSi basin. It has the advantage of adding sprue height, but might require a basin above the swirl gate rather than just the vent to contain the volume above the cope.





I altered it to have a high aspect rectangular sprue which is similar to what Bob recommends.
Nothing to pour , so no test yet.

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High aspect? You mean rather deep compared to width? I am not sure what ratio you are referencing.

Dry Hearth? Can you expand on that a bit.

Sorry I am so dense (but better than being full of voids ;-] )

Denis

 

Could you elaborate on what you get in the way of a ruined casting from bifilm? I ask because I routinely remelt and don't think I have any failures from bifilm other than excessive porosity prior to keeping the sprue flooded.

 

I routinely remelt and don't think I have any failures from bifilm

I believe all the cavities you showed at magnification are unfurled bifilm. You would not be able to see hydrogen porosity.

This is on of the best examples since I wanted to machine and polish the outer face. When I did it I had mentioned being a little to casual with my charge and ended up with wheels I wouldn't even sell. They are full of unfurled bifilm cavities. The second pour was better.

 

Thanks! Do you have any closeups of the defects?

 

High aspect? You mean rather deep compared to width? I am not sure what ratio you are referencing.

In Campbell's Handbook, he shows an offset basin that Bob uses. There is a smaller basin separated by a weir feeding a full width rectangular sprue that tapers to possibly a square sprue. That is not clarified, but he does want to get away from the round sprues.
The dry hearth takes advantage of the oxide skin of the feedstock to protect the melting metal from exposure to the furnace atmosphere and has the advantage of separating iron and other tramp metals from being included in the melt before they are submerged. Somehow these oxide skins and anything else that does not melt are moved along in the process and collected somewhere other than in the ladle.
I just scan parts of the book until I find something that seems like it could help my casting and run one of my shade tree experiments. As we have seen some are quite surprising so on we go.

 

I took a shot as close as I could of the rim.



And then one of the runner from later tests. Much better feedstock and using an offset basin. There is one small unfurled bifilm as the spot that looks the same as your magnified cavities.

 

Thanks! That wheel is full of it, like my worst hammer head, or worse. And that was poured with a flooded sprue, not ingesting air?

 

So this just does not look like a gas bubble to me.



Sorry I can't listen to the video, but the surface finish on the samples they had cut would only show very large cavities. The previous one is big, 0.011" by 0.016", easily visible, but not like this next one (same scale in both photographs):



This is 0.003" porosity.



It is isolated but looks to have a long tail.

 

That is how I understand it. One side of the skin is atomically bonded to the melt, and the other side of it is Al2O3, the skin, and is dry. When Bob brushed the top of the large open pool his comment was that you never see molten aluminum because the surface immediately oxidizes. When the speed in the sprue, runners, gates and mold exceeds 1M/s for aluminum the oxide skin tears and bits and pieces become entrained in the casting. This is compounded if air is aspirated with poor basin and runner design, because it creates new oxide bits and trails that are then pulled into the casting.
I go back to the crumpled sheet of paper that both Bobs used and the lung volume (conical pouring basin) demonstration that is at the end of Campbells video.

 

I agree with the last two posts.
That is also my understanding.

Bob was reluctant to believe that some backyard rubes could understand and embrace bifilm theory quicker and better than the industry guys, but he was wrong.
I think industry can't/won't change because they are all set up to do it one way, and if they did change, the managers/owners would have to admit they had been doing it wrong all their lives, and most of them (but not all) will not do that (admit that they were ignorant of bifilms and their critical impact on getting good castings).

Even though we work in the back yard, that does not mean we can't think objectively and pick up new concepts quickly.
We have some rather sharp minds on this forum, and that is a very good thing for quality castings.
I think this forum group is light years ahead of most run-of-the-mill aluminum can / flower pot furnace backyard casting folks.
Concentrating all these great minds in one place and having discussions is also a great asset.
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My slant is what I think I recall Bob saying, which is don't degas to fix a bifilm problem, because you are wasting time and money.
If you eliminate most of the bifilms being churned into your metal, and you are not having porosity problems at that point, then don't start solving problems that you do not have by degassing, because the degasing can also churn the metal and do bad things (entrain biflms into the metal).

Fix one problem at a time.
And don't fix problems that you do not have.
I have seen some here that degas no matter what, as a matter of routine, even when it many not be necessary.

I have gotten some of the best aluminum castings I have ever made by bring the melt up to temperature quickly, NOT STIRRING the molten aluminum, not overheating the melt, and pouring immediately after I hit pour temperature, and with NO degassing.
I normally don't even consider degasing aluminum, and I routinely avoid degasing and still get castings with no visible porosity.

Overheating an aluminum melt and letting it sit around for a few minutes in the overheated state basically guarantees a lot of porosity in the castings, but this is what I often seen happen in videos, and I use to do it myself.
Stirring the aluminum is a disaster; this ain't soup we are making; don't touch the melt except to skim it, and skim it lightly.

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I dunno. He’s been talking at industry for a while, and then volunteered a bunch of videos for Perry swdweeb’s channel specifically to address the hobbyists.

There needs to be oxygen, not hydrogen, for an oxide layer to form.

Bob Puhakka’s foundry is set up to avoid the issues predicted by John Campbell’s bifilm theory and (IIRC) completely ignores the question of hydrogen (no degassing, etc.), and still he gets results without any of the troubles usually associated with dissolved hydrogen. He at least has good reason to suspect that dissolved hydrogen is benign in the absence of oxide bifilms.