Porous Cast Aluminum?

[Vrogy]

Strange thing here- we got a porous pour with the lost foam process today.

Here's the setup:
2X4 & plywood flask
fire clay, bentonite, and sand 'green sand', oil bonded with used motor oil
ground drywall texture slurry, dried one day, as investment
wall insulation foam from home depot, joined with hot glue.

Any ideas about what might cause the bubbles?

[awemawson]

Molten aluminium has an enormous affinity for hydrogen - it dissolves hydrogen very easily. If you have moisture present (H2O) then the molten aluminium will take up the hydrogen and release it as it cools. If you do a melt on a humid day the problem gets worse. All tools, flasks, crucibles etc that come near the melt need to be pre-heated, as does the ingot or scrap that you are melting.

The dissolved hydrogen can be removed by 'scrubbing' with a gas. Several methods are available. You can get purple tablets to plunge in the melt, that give of a gas that removes the hydrogen:

http://www.budgetcastingsupply.com/A...as_Tablets.php

I use an argon lance. This uses the gas from my TIG welder, applied via a length of stainless steel 15mm water pipe, which I have crimped at the end and drilled radial 1mm holes to form small bubbles. I previously used dry nitrogen which also works, but was able to give back the rental cylinder when I got the argon for the TIG.

[Vrogy]

Sweet, thanks for the advice, Andrew. I think we'll be going with the argon lancing, as we already have an argon supply on hand.

[Kipper]

I use Nitrogen to purge...You have the strangest method of casting Vrogy....Lost foam and green sand oil bonded......Try using kiln dried silica sand as the drywall slurry should have the mechanical strength to hold the shape and silica sand is a good refractory...Does the casting require investment casting or is it just simpler for you? Regarding Stainless steel and Al..Al is very "acidic" when molten and can dissolve varying amounts of stainless and you get a casting that is a ***** to machine...If you normally get a non porous casting go back to that method. Cool looking part btw it reminds of me of JD!

[metalworkz]

Hi Vrogy,
Do you do any venting with this motor oil bonded sand? Is it possible there may be trapped gases-steam in the mold released from the sand etc?
Regards,
Wes

[awemawson]

I pull the crucible from the furnace, and while it is on the floor sitting in the pouring shank I insert the argon lance and open the tap gently until bubbles appear and keep it so for for perhaps 15 seconds. Much more and you cool the metal too much, and much less doesn't scrub the hydrogen out. The smaller the bubbles the better ie lots of small bubbles are far better than a few big ones.

In theory you can contaminate the melt with the constituants of the lance being dissolved into it, but so far my thin walled lance shows no signs of erroding. Proper industrial practice would use a ceramic coated lance.

I understand that the action is a physical one and not a chemical one, with the hydrogen preferring to occupy the bubble space rather than the molten metal. It is not in chemical combination with the aluminium as I understand it.

[Big_d]

I think you would be able to get a little better casting if you did a few things to the mould. You may have some hydrogen in the metal but I think the holes are mostly due to not having enough venting in your mould. Cast it face down with a few small-medium risers on the back and this will improve the display face heaps. Degassing the metal wont do a lot for you as you add heaps of gas to the metal by burning the foam (Oily sand wouldn't help either). The only way to fix this is to vent it with risers and this will give the added bonus of keeping extra heat in the mould for a better finished casting. With some cylinder heads when cast %70 of the metal used is risers and runners.
Nice casting Vrogy.
awemawson
Degassing works by having as many bubbles (smaller the better) of gas swirling through out the metal. If there is hydrogen in the metal when the bubbles come into contact with it it will react and form dross. As dross is lighter than pure metal it floats to the top. You just scrape it off. Carbon is normally what is used for long term immersion it is usually reinforced with a ceramic sleeve on the metal surface as the molten metal and dross combination is just like lapping paste but hot. Magnetic furnace tools are fine but steel erodes to easily when hot and contaminates the metal. Most of the coatings used on furnace tools are ceramic as this is not especially for the heat but as an anti wear coating. Did you know that you can add hydrogen to aluminium for difficult moulds as when it expands in the mold it will help fill the entire pattern. You do have to really dump it in though for this to work. This is a very common industrial practice.
Cheers
Daza

[henryj1951]

some things that may help - make a plaster mold & dry it in an oven at 300 degrees minimum for 24 hours minimum, change the up surface, change from vertical to horizontal pour, monitor the pour speed ( I do not see any burn in so that is good) make proper gating so the metal enters at a moderate flow rate, add a metal screen or ceramic filter below the pour cup to promote laminar metal flow. don't be afraid to add extra vents or extra thickness so the defect can be machined away. be careful of degassing times because it removes magnesium and if it is depleted the aluminum cannot be heat treated. It will be gummy and not machine or finish well. good luck

[Janardhan]

Getting porous casting may be due to various reasons such has, high pouring temperature, not fully degassing the metal, metal mold reaction.
1) Check the volume of your foam mold.
2) Dry the mold properly before poring metal
3) Use degassing by means of tabs. Can use nitrogen. If you can handle chlorine then that is the best degasser for aluminium.
4) You may add .005% Berillium just before poring to prevent metal mold reaction which will do wonder.

[Vrogy]

Getting porous casting may be due to various reasons such has, high pouring temperature, not fully degassing the metal, metal mold reaction.
1) Check the volume of your foam mold.
2) Dry the mold properly before poring metal
3) Use degassing by means of tabs. Can use nitrogen. If you can handle chlorine then that is the best degasser for aluminium.
4) You may add .005% Berillium just before poring to prevent metal mold reaction which will do wonder.

Thanks for the concise response. I have a few questions-
re #1: Do you mean a large volume or a small volume is better?
#4: How does this work? I'm assuming a liquid solution.. doesn't it just boil off?

[Janardhan]

After mixing the volume should be double than that of the one before mixing.
This is done mainly to create a porous media so that the gases can escape easily. The unescaped gases are main causes for porosity. As suggested by
henryj1951, proper care should be taken while baking in a oven. All the moisture should be removed.

[beone]

Judging by the picture, your problem is not porosity. Porosity is many fine bubbles found thrughout a casting upon machining. You have surface blows caused by gasses evolved during pour-vend-vent-vent! You might have too much oil in the sand. Also-a pet peave- You can't have oil bonded green sand. You have oil bonded sand-- green sand is tempered with water.
Dave