Aluminium casting problem.

[100model]

I have a problem with aluminium casting that I cannot figure out why it is doing it? I made a video with closeups showing that defect.

 

[Richard Carlstedt]

Well, I am not an expert, even though I have done some casting (Alum ,Bronze ,CI)
i think mixing your scrap is a bad idea. For the best and easiest , stay with Automatic transmission housings (A 356) IMHO . Doing wheels and others is a crap shoot--again IMHO.
when I worked at the Die company, we had aluminum castings made that were critical air flow surfaces and NO Porosity was tolerated ! Our vendor did special things to produce these castings
like keeping the crucible covered and poring immediately after melt ( to minimize Hydrogen absorption ) and they told me the most important was never ever pour when there is high humidity or during rain weather.
just a thought
Rich

 

[GreenTwin]

I use aluminum ingots purchased from our member ArtB, and have not had any trouble with castings made from them.
I think Art uses automotive engine blocks, and perhaps heads also.

100model has a great pouring technique, and very good control of the shank/crucible.
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[Toymaker]

About 30+ years ago, while living in humid Florida USA, I poured a few aluminum castings using the lost wax technique. I used automotive pistons purchased from scrap yards for the aluminum and melted them in an electric furnace. My castings always had pits and voids, most likely caused by the high humidity found in Florida. The attached document explains how humid air allows molten aluminum to absorb (dissolve) hydrogen gas into the liquid metal, which after pouring, the dissolved hydrogen comes out of solution, becoming a gas again, and forming pits and voids in your casting as the liquid aluminum solidifies.

 

[Roger Taylor]

About 30+ years ago, while living in humid Florida USA, I poured a few aluminum castings using the lost wax technique. I used automotive pistons purchased from scrap yards for the aluminum and melted them in an electric furnace. My castings always had pits and voids, most likely caused by the high humidity found in Florida. The attached document explains how humid air allows molten aluminum to absorb (dissolve) hydrogen gas into the liquid metal, which after pouring, the dissolved hydrogen comes out of solution, becoming a gas again, and forming pits and voids in your casting as the liquid aluminum solidifies.

I taught high school metals shop for 40 years and we cast with mixed aluminum scrap using a aluminum flux and nitrogen degassing pellets plunged into the melt with a steel rod with a cup shaped end to push the pellet into the mix. With proper temperature, flux treatment and dry pouring area we achieved very acceptable results with scrap.

 

[master53yoda]

The pictures are showing entrained gases probably Hydrogen. This can also be caused by pouring too hot. I use calcium hypochlorite as a degassing agent. Below are the Flux recipes I use, this is part of the information I send out with all of my orders.
Flux recipes
Drossing Flux (used in our processing)
50% sodium chloride “normal salt, preferably without Iodine)
50% calcium chloride “NoSalt brand”
½ teaspoon per 10 -12 lbs.

Degassing and Drossing Flux “Caution releases chlorine gas, you must have adequate ventilation”
50% Drossing Flux
50% calcium hypochlorite “Pool Shock”
One tablespoon or three teaspoon per 12 lbs.

Depending on how large the casting is you may need to place a shrinkage sprue on the opposite side of the casting large enough to feed metal into the casting as it cools..

The problem mixing different types of scrap is that it varies the silicon content, A356 has about 8% silicon. The silicon reduces surface tension as well as allows for cleaner machining. Pistons can have anywhere from 9 to 15% silicon as well as other alloys including Magnesium. They make good castings but act differently then 356 does, I use only automotive power train and structural members in my ingots to maintain a consistency in the end product. My ingots typically have about .5% more FE than A356 standards. That is why I describe my ingots as NEAR 356.

I also process in a direct fired waste oil furnace at 150 lbs per pour which if I do have a piece of other then desired scrap it isn't enough to affect the end product. Bear in mind that any entrained gases are release when remelted. I remelt in my normal furnace when doing a casting that can't have entrained gases.

Wheels are 356 by law but they can be chrome plated which changes the end alloy.
There is a commercial foundry in Spokane that won't take chrome plated wheels because they can't meet 356 standards if they are in the melt.

I hope this information helps.

Art B

 

[Toymaker]

I taught high school metals shop for 40 years and we cast with mixed aluminum scrap using a aluminum flux and nitrogen degassing pellets plunged into the melt with a steel rod with a cup shaped end to push the pellet into the mix. With proper temperature, flux treatment and dry pouring area we achieved very acceptable results with scrap.

I should have taken your aluminum casting classes 30 years ago,... I was not aware of nitrogen degassing pellets or using aluminum flux.

 

[bluejets]

I have a problem with aluminium casting that I cannot figure out why it is doing it?

Do you use any cleaning flux or any hydrogen (not nitrogen) gas pellets in your crucible with the molten aluminium?

https://www.alalloycasting.com/degassing-of-molten-aluminum/

 

[100model]

Do you use any cleaning flux or any hydrogen (not nitrogen) gas pellets in your crucible with the molten aluminium?

Many years ago I used chlorine tablets but they are really bad for your lungs so I don't use any fluxes or degassing pellets.

 

[100model]

structural members

Can you please give some details of the structural members you melt?

 

[GreenTwin]

I have used ingots from ArtB to make a number of aluminum castings, and have never had any problems.

I melt the ingots as fast as possible, making sure I don't overshoot the pour temperature of 1,250 F, and pour immediately at 1,250 F.

I don't degass the ingots, and no problems so far.

I tried cutting up aluminum wheels, and also cutting up large ingots, and I found that to be very tough going.
ArtB's ingots make melting aluminum very convenient, and the ingots come from metal that has already been degassed.

Edit:
In the early days (DBP - days before pyrometer), use to get a very rough surface finish if I overheated aluminum using petrobond, and if I did not pour immediately a 1,350 F, I would get pinholes all over the casting, which were not a structural problem, but would be a gastight/watertight problem.

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