De-gassing ally melt.

Ive started to melt things..... built myself a furnace and crucible and things are going well. Mainly melting Ally at the moment to repurpose for other projects.

Some of the billets have been machined and I noticed lots of small air bubbles in the casting.

Ive heard of de-gassing which I assume will help this problem but not sure what this entails.

Advice greatly accepted.

You need to use a kind of flux - salts that often contain chlorine or fluorine. Here is a Youtube video.

Thor

Edited By Thor 🇳🇴 on 21/10/2022 11:30:46

As the term De- gassing implies it is more likely to be absorbed or disolved gasses possibly hydrogen. The first thing is your melting technique, keep the temperature just above melting point by constant feeding until you have enough metal melted, THEN raise to your pouring temp and pour IMMEDIATLY ! DO NOT hold at elevated temps !

Temperature control is vital, use a dip pyrometer !

Your pouring or mold design must avoid entraining air !

These points may well avoid the need for a covering flux which brings it's own problems.

You do not say what method of melting your using ?

Noel.

Many people melt aluminium and end up with excellent results without any degassing. Depends on how good your original scrap is. I did a piece of 50mm x 125mm 2 weeks a go and machined it yesterday and it looks great. Use good known stock (not cans) Ali wheels & extrusions are excellent for the job. Dont overheat it when casting.

regards

Have you watched some of the early myfordboy videos. He has good results and shows you what to do.

I'm using a gas burner - but as yet no way to tell the correct temp - i'm just going till it melts and then pour.

The sodium carbonate seems logical but no indication as to volume needed.

Scrap is good quality old engine parts and wheels so I don't think that would be the problem. I erring towards the overheating.

Starch works as a degasser (for aluminium) or footpowder. If you have turbulence when pouring you will get gas pickup especially if you have too much superheat.

Cast these recently to make my bike a little more comfy.

If you use magnesium (by accident) do it at night, the neighbourhood will enjoy the show - even if you don't !

I watched a guy do ali casting today and the crucible was as hot as I would use for brass(1020c) WAY to hot, so no wonder he was having trouble. A lid on the crucjble will help to reduce hydrogen absorbtion, and pre heat tools before putting in the metal. Noel.

All advice noted - thanks.

There is a guy over here who sells aluminum ingots, and he processes aluminum engine blocks by the ton.

He told me what a good degasser is, and I don't think it was the highly toxic chlorine stuff.

I will dig that out.

I generally don't use a degas agent with aluminum though.

Bring the melt up to pour temperature as fast as possible.

Pour temperature is 1,350 F.

Don't overheat the aluminum.

Don't let the aluminum sit after it reaches 1,350 F; pour immediately.

Never ever stir an aluminum melt.

The myfordboy washing soda method is not a valid one.

You can verify this by baking the washing soda in the oven to dry it, and then you will see that it does nothing in an aluminum melt except sit on top of the melt.

Olfoundryman is the guy on youtube who worked in foundries, and he is the ultimate aluminum casting guy.

356 alloy aluminum is what is used for castings, not extrusions, and not cans.

Wheel rims seem to be 356.

Edited By PatJ on 21/10/2022 15:41:37

Nice bar risers Luker, cost me a fair bit when I had to buy some for my Blackbird a few yeads ago.

Another trick I found out about is you can judge pour temperature of aluminum by watching the meniscus.

When the meniscus goes flat, you are at pour temperature.

I watched an individual use this method, and it is very accurate.

I have not tried it myself.

Supposedly, a good degass mixture is Sodium Chloride and Potassium Chloride, 1/2 and 1/2 mixture, 1 teaspoon full per 13 lbs of aluminum. I am not sure about the safety of the fumes that this creates, so use caution.  I have not tried this, but the person who uses it process 1,000 lbs of aluminum per month.

And using a water quench and dropping the aluminum castings in water as soon as they are solid helps the aluminum machine more cleanly, with less bit loading.

A two step process for some really nice aluminum (356) is 8 hours in a kiln at 1,000 F, followed by a water quench, and then 4 hours at about 400 F, for an approximate T6 hardness.

.

Edited By PatJ on 21/10/2022 19:21:39

Hi, in my old employment many years ago, the company developed and made a few different grades of tablets for degassing aluminium alloys. these tablets were about 36mm in diameter and 13mm thick and were packed in tens in some specially backed foil, they were used in machines made for the company, I don't know what was in them as that was only on a need to know bases, and I wasn't privy to know, but I do know the stuff rusted steel and you had to make sure you cleaned your tools after maintenance on the machines. The tablet making machine was a bit of a brute, and the twin die and rams had to be changed once in a while, as the chrome plating would wear down and so they had to go away to be refurbished, Refitting the fresh die and rams was a pig of a job, although the die was fairly heavy, it locked into the carriage in only one position, but you had to push the carriage by hand towards the rams to line them up correctly, I can't remember the clearance, but it was less than the thickness of a human hair. After a few years, the whole plant was sent to China where they built a brand-new plant for making the powder and other commodities plus the whole of the research and development department.

Regards Nick.

Thanks Bezzer. Made my bike a dream to ride! I redesigned the risers completely. They moved back with less of an angle backwards, raised and the vertical angle is different aswell. These were cast from Mag wheels, and I didn't get any gas issues or inclusions. I didn't de-gas any of the castings done on this day. Cost me nothing to do, even the wheels were free from a friend that moved...

Sand additives and dusting was normal flour.

So presumably the problem gas is H2, the fumes are HCl and the sodium and potassium has dissolved into the aluminium?
I wonder then if washing soda (KOH) dissociates as readily and produces steam. Only thing being I thought one source of the H2 was moisture in the scrap in the first place reacting with the aluminium (to produce the oxide) liberating hydrogen to dissolve.

You are falling for the basic non sequitur fallacy.

Its like saying that you have seen hundreds of videos of British people driving blue cars, and so all people driving blue cars are British.

It does not follow.

Washing soda absorbs moisture from the air, and dumping it into molten aluminum causes the moisture to bubble vigorously.

If you bake washing soda in the oven at 400 F, and then dump it into molten aluminum, it does absolutely nothing; no bubbling, no disolving, nothing; it just lays on top in a powder, which you then have to laboriously scrape off.

So in order to prove that washing soda works, you have to make a casting without it, and then make a casting with it. The castings I make without washing soda are exactly like the castings I make with washing soda.

Testing things using proven scientific methods by making castings is a much better indicator of what actually works.

Make some alumium castings, try some things, and you will see what I mean.

.

Edited By PatJ on 22/10/2022 22:23:59

As I understand it, aluminum at molten temperatures will readily and quickly absorb hydrogen.

The higher the temperature above pour temperature you overshoot, the more prone and faster the aluminum seems to absorb hydrogen.

If you dump whatever material into the melt, the chemical reaction has to remove the hydrogen from the melt.

I can't elaborate on it more than that, since that is the limit of my understanding.

If you leave any powdered substance on the shelf for a while, it will absorb moisture, and if you dunk it into molten aluminum, it will vigorously bubble exactly like washing soda.

If you bake some powered substance perfectly dry, then it may or may not react with the aluminum.

.

Edited By PatJ on 22/10/2022 22:26:11