Casting Aluminum over copper tubes 2

[tinsnano]

Dear All,
I have a project involving casting aluminum block where copper tubes are embedded. Unfortunately i found in the internet that liquid aluminum dissolve copper. My aluminum block is 30cmx30cmx6.5cm and Refrigerator copper 12mm OD. But again in the internet discussion i found some one saying what if we pre saturate the liquid aluminum with copper will it dissolve additional copper? what do you guys think. if his idea is right then how much kg of copper should i dissolve in 11 kg of liquid aluminum?

 

[racookpe1978]

Copper = 1984 F
AL = 1,221.



Dissolved into one another when only one is melted? Maybe he is thinking of a silver solder or brazing type union at the metal-metal surfaces.

The two will tend to merge (will move through each other's crystal lattices over time at high temperatures short of the melting point) but that's little compared to sag and creep at modest temperatures of 300 - 500 deg F. Loss of structural strength at those temperatures is a bigger concern if the whole thing droops or deforms than chemical changes.

How long does the assembly need to operate under what temperatures?

 

[tinsnano]

He is not the only guy who say liquid(molten) aluminum dissolve solid copper many others forum member have the same idea.

first i am planning melting aluminum at its melting point then pour this liquid into a mold which has copper tubes after the aluminum solidify i am intending this system to operate between around 0 degree centigrade

 

[winstonsk]

What about electroplating the Cu--with say, Ni?

 

[EdStainless]

Refrigeration (ice maker) people do this all of the time.
Plating the Cu may improve wetting, but the reason that the Cu tubes will not totally dissolve is kinetics.
Cool it quickly and you should be fine.

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P.E. Metallurgy, Plymouth Tube

 

[arunmrao]

You can give a thin layer of Plumbago coating on copper tube. I have used similar techniques but for different applications.


"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[tinsnano]

yes the project is ice maker machine.

plating Ni on the copper tube has two difficulty for me. the first Ni is not good conductor of heat. the second i can not find Nickel in my country.

I am not sure cooling could be effective the total mass of the molten aluminum is around 11kg

Dear arunmrao, i don`t understand Plumbago plating


I am thinking thickening the copper tube thickness using electrolysis of copper sulphate with one electrode the copper tube and the other with any copper source as the conventional way of purifying copper. But i am not sure if the plated copper tube will have the same ability of heat conduction. By doing this i reduce the risk of copper tuber rupture due to dissolving in the aluminum since it have more thickness.

 

[arunmrao]

Please see the definition below. Essentially means graphite coating applied on the copper tube.

Full Definitions of
plumbago
1
n used as a lubricant and as a moderator in nuclear reactors
Synonyms:
black lead, graphite
Types:
lead, pencil lead
mixture of graphite with clay in different degrees of hardness; the marking substance in a pencil
pencil
graphite (or a similar substance) used in such a way as to be a medium of communication
Type of:
C, atomic number 6, carbon
an abundant nonmetallic tetravalent element occurring in three allotropic forms: amorphous carbon and graphite and diamond; occurs in all organic compounds



"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[EdStainless]

Don't try to plate them, the plated layer will be porous and have poor properties.
A thin graphite wash would work (very fine graphite dust in a solvent, just dip and dry).
You have to make sure that you don't over heat the Al, just hot enough to pour well.

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P.E. Metallurgy, Plymouth Tube

 

[tinsnano]

so the graphite coating will not block heat transfer significantly? if it does not have such significant influence on the heat transfer issue then it is fine. but you guys tell me how to apply graphite coating (i don`t understand how to apply graphite coating, for example what kind of solvent is needed for graphite?) beside i need to ask what type of graphite can i use. for example i can get graphite from dry cell and in laboratory items shop i have once purchased a graphite for electrode. Yes overheating the liquid aluminum could be a problem.

 

[arunmrao]

Discuss with your FOSECO representative for more information, about application methods etc.


"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[EdStainless]

FOSECO is a good idea. This is used as a wash to protect molds in casting. It is very thin (runny like hot water) and it leaves a very thin coat.

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P.E. Metallurgy, Plymouth Tube

 

[Compositepro]

I seriously doubt that you have to worry about the tubes dissolving. Dissolving takes time, and the amount of time that the tubes will be contacting molten rather than solid aluminum should be very short (seconds). Flowing cooling air through the tubing will insure that the aluminum contacting the tubing stays solid. It should also insure that there are no shrinkage voids at the tube interface. Having the aluminum freeze from the tubing outward will cause a tight compression onto the tubing.
Aluminum will also dissolve iron but cast iron molds are often used. Graphite is a good release agent.

 

[tinsnano]

Thank you all for taking the time to help.

 

[WKTaylor]

Wow...

What about the thermal strain differences as aluminum cools in adhesive contact with the copper tubing [SAE AIR809, Metal Dimensional Change with Temperature]; and the high corrosion potential between highly dissimilar metals [MIL-STD-889 DISSIMILAR METALS, especially in sea-coast environments]???



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[EdStainless]

In commercial ice makers the Cu tubes are serpentine fully embedded in Al.
The Cu is soft enough that the thermal expansion is not a bother.
They usually seal these to the hopper, with one side exposed to the ice, and then fully seal the back side with closed cell foam.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[tinsnano]

what would be the thermal stress? Will a gap created between the copper tube and the aluminum interface or the copper tube tends to be crushed?

and what about pre saturating the liquid aluminum with copper such that the aluminum wouldn't dissolve any more copper

I am asking now and then continuously because i live in a place where i could not find any appropriate material secondly i only have limited budget for this project so i have to make sure in selecting the best way, so please try to understand me.

 

[tbuelna]

tinsnano-

You did not mention the aluminum casting alloy being used. There are some aluminum casting alloys, such as 2XX series, that already have a fair amount of copper.

You also did not mention what foundry process is used for your casting. The pour temp of the aluminum is usually well above the melting temp. The specific pour temp of the aluminum depends on factors like the type of tooling/mold used, the size and section characteristics of the cast part, etc. Parts that have complex geometry, thin wall sections, and/or large volumes use a higher pour temperature to ensure the molten aluminum completely fills the mold cavity before it begins to solidify.

As mentioned, one issue with overcasting copper tubes with aluminum is the lower CTE of copper versus aluminum (9ppm vs 12ppm). If the copper tubes are not pre-heated they will act like "chills", which causes the molten aluminum in direct contact with the tube surface to solidify more rapidly than the surrounding aluminum.

 

[tinsnano]

Here is summary of my idea/intention/question :- The only material that i can get in my country are pure aluminum, refrigerator copper and graphite. I have a block of gypsum mold in which i will put number of copper tubes side by side and a bit above the surface of the gypsum mold and pour liquid aluminum such that i will have a block of aluminum in which copper tubes are embedded. The mold is 30cmx30cm and 6.5cm height totally open above (roof less).

my major two problem are :-
1. Copper being dissolved by liquid aluminum while pouring aluminum
2. May be problem associated with different thermal expansion of copper and aluminum (worst if the interface gap between the two metals increase, which is barrier for heat transfer)


i am going to use the solution (apply thin graphite coat on the copper tubes) given in this forum if the next solution didn't work. what i got on the internet as a solution for such problem of copper being dissolved in aluminum is pre saturating the molten aluminum with copper such that i cant dissolve my copper tubes. this is like saturated salt solution cant further dissolve another salt. if you guys know this solution work then i apply this solution if not i will apply the graphite method.

 

[tbuelna]

OK. So you are going to make a plaster mold 30cm x 30cm x 6.5cm, that is embedded with lengths of copper tube, and open at one end. Then you will fill the open mold cavity with molten pure aluminum and let it solidify.

A couple things to consider. First, you'll want to bake the plaster (or gypsum) mold to make sure it contains no moisture. If there is any moisture in the plaster mold when you pour the very hot molten aluminum into it, it will result in a very hazardous situation. Second, given the large difference in mass, temperature, and CTE between the copper tubes and aluminum casting you describe, I imagine you will not get a satisfactory bond between the aluminum casting and copper tube surfaces.

You can minimize any potential reaction between the molten aluminum and copper tube surfaces by purging the mold space with an inert gas. But as others noted, the limited amount of time the molten aluminum is in contact with the copper tube surfaces is not likely to result in any appreciable loss of copper from the tubes.