Aluminum Alloy with a Pliable Transition Zone Between Solid and Liquid?

[metalman8357]

Hi All,

I'm working on a project where our goal is to design an extrusion head for aluminum wire. Basically the goal is to feed 0.035" wire through a tapered ceramic nozzle that is wound in a water cooled copper induction coil so that when it exits the nozzle (tip diameter 0.015") it will be just above the melting temperature of the alloy. We are working on a somewhat 3D dimensional printer (mostly 2D traces) for aluminum and the deposition of the solid aluminum will need to be fairly accurate on the substrate. The problem with using a standard aluminum alloy like 6061 is that the metal transitions rapidly from solid to liquid and this will cause the metal to run and wet out when extruded from the nozzle. We are looking for something more similar to how a plastic would melt (think FDM 3D printing with plastic). A plastic can be extruded at the melting temperature and still keep it's shape while still fusing to the solid plastic material on the build table of a 3D printer. With metal you wouldn't be able to control the liquid molten metal accurately and it would most likely melt the solid base layers below or wet out away from the target solidification area.

My question is, is there an alloy of aluminum that exists that is more pliable or gummy like plastic when it melts? And if so, would it still be able to fuse to solid base material at this temperature assuming no issues with the oxide layers in a vacuum chamber? If using a different alloy isn't the solution, can anyone come up with any ideas (even crazy ones are welcome!) that would solve the issue of the liquid aluminum being too runny to control?

Thanks!
-M

 

[CoryPad]

It sounds like you want to process the metal similar to semi-solid casting, which is in the temperature range between solidus and liquidus (what you called the rapid transition from solid to liquid). Try looking at casting alloys like A356 that have a wider transition.

 

[metalman8357]

Hi CoryPad thank you for the response. Regarding semi-solid casting, how viscous would the material be in the liquid phase? In other words, if it were to be extruded as a liquid (assume 50% in the solid phase) through a 0.015" diameter nozzle onto a base plate of aluminum, would it instantly solidify and hold its place or would it wet out?

 

[MintJulep]

Perhaps you'll find something useful in here:

http://www.hindawi.com/journals/isrn/2013/679820/

Found with 'thixotropic casting aluminum' as a Google search. Plenty of other stuff too.

 

[metalman8357]

Thanks for the information. I am very curious about the A356 hypoeutectic alloy, it sounds like it might help my issue.

Has anyone worked with this alloy before? I'm curious about the fluidity of this alloy when it is just barely melted (right above the solidus temperature). How does the fluidity of a liquid like this (~50% solid/50%liquid) compare to the fluidity of 100% liquid aluminum? Is there really that much of a difference in fluidity? If I am extruding this material from my nozzle will the liquid phase just push out and separate from the solid phase? Ideally I am looking for a material that will behave more like a plastic when melted. In other words I am looking for an alloy of aluminum or a technique that will severally decrease the fluidity of the liquid.

 

[CoryPad]

You will find that the viscosity of a 50 % solid slurry will be MUCH higher than 100 % liquid. See the attached graph (source: Magnesium Injection Molding by Frank Czerwinski).

 

[metalman8357]

I'm thinking about developing a prototype extrusion nozzle using solder to prove the concept at a much lower temperature. Can anyone recommend a commercially available alloy of solder that has a larger temperature range between the solidus and liquidus? I'm shooting for at least 60% solid at the melting temperature to give the molten material high viscosity.

 

[1gibson]

Try it with a low temp aluminum brazing rod. Not sure on the details of the alloy, but for small parts in certain applications, it might even be a complete solution.

 

[berkshire]

Metalman8357,
The 50/50 tin lead solders have a wider plastic range than the 60/40 solders which have a sharp melt.
Body solder has the widest plastic range being designed to be troweled onto an auto body for contouring.
There are now lead free auto body solders, however I have no experience with them. This should give you a pointer for your search.
B.E.


You are judged not by what you know, but by what you can do.

 

[metalman8357]

Thank you Berkshire. I will definitely check out these alloys of solder for our prototype. Having a solder with a much wider plastic range will help our project immensely.

Another challenge that I cannot seem to figure out has to do with aluminum side of things. I know that aluminum rapidly oxidizes in air so by using a shielding gas on the aluminum extrusion nozzle I should be able to extrude and lay down a molten bead of aluminum without it oxidizing (similar to welding). The bigger challenge will be getting this molten aluminum to fuse to the aluminum layer beneath it (TIG welding is usually done with AC current which cleans the base metal of its oxide layer allow the molten weld puddle to bond). If the base layer is also aluminum,it will already have an oxide layer and the molten aluminum will not stick to this substrate. I will also have the same problem when trying to 'print' more molten aluminum on top of already 'printed' aluminum.

Basically I need a way to locally remove the oxide layer from the solid substrate aluminum that will be bonded to the extruded molten aluminum so that they will form a metallurgical bond. I could print the aluminum in a ultra high vacuum but this solution is cost prohibitive. I'm thinking that using chemical etchants like an acidic flux might work but I dont know how fast that would remove the oxide layer and it might cool the molten aluminum too much to fuse (also, fumes).

One solution that I think might be possible is using a high powered laser to vaporize and remove the oxide layer. The spot size would be very small since I am only printing a 0.015" width bead of aluminum at a time. Basically the laser would be under CNC control and positioned just ahead of the extrusion nozzle. It would vaporize the oxide layer and expose bare aluminum just as the molten material is about to fuse to the base material. Is something like this feasible or am I away off base?

 

[berkshire]

Metalman8357,
The cleaning action on Aluminum welding is primarily done by the high frequency tracer which is run continuously during ac welding operation it serves two purposes one to assist in cleaning the surface and two to keep the torch lit during the current reversal of ac welding. If you could add a high frequency tracer it might help with your deposition.
B.E.

You are judged not by what you know, but by what you can do.

 

[metalman8357]

Thank you for the idea Berkshire. The problem with the HF tracer is that I won't be using an electrical arc since I do not wan't to melt the base material that is fusing to the molten aluminum exiting the nozzle. In addition, the HF will disrupt my motion control electronics.

The laser ablation idea to remove the aluminum oxide layer intrigues me. Can someone comment on the below thought experiment?

Suppose you dropped a molten aluminum bead onto a solid aluminum plate (at room temp). The bead will splatter on the plate and it will not fuse. This is because there is a thin protective oxide layer on the surface of the solid aluminum plate which prevents it from forming a metallurgical bond to the molten aluminum (not enough thermal energy here to break the oxide layer and form a bond).

Now, let's say you shot a 60W Laser with a de-focused 0.060" diameter spot onto the plate of aluminum. Theoretically this laser will vaporize the surface oxide layer on the aluminum and should expose pure aluminum at the surface as long as the laser is still on. Now imagine dropping that same molten bead of aluminum onto this laser spot while the laser is still on. The question is, will the molten aluminum now fuse to the base material because it's oxide layer has been removed? My intuition tells me it would but I'm not certain of how it would work in practice.

 

[ornerynorsk]

I'm no metallurgist, but does the high surface tension of molten aluminum have as much to do with adhesion difficulties as does oxide formation? It doesn't "wet" very easily unless the substrate being added to is also in liquid state. Anyone who has ever TIG welded has observed this. My thought is that, with aluminum, DMLS might be a much more viable process than an FDM-type extrusion process.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[metalman8357]

Ornerynorsk that is a very valid point about the surface tension of molten aluminum.

So I suppose that raises a more general question: In a high vacuum will liquid aluminum fuse to solid aluminum under the weight of gravity alone, or will it just solidify as a solid ball on the surface. In other words, is the surface tension keeping together a ball of molten aluminum higher than the energy needed to form a metallurgical bond?

 

[1gibson]

Dropping on a room temp plate? I can't see it sticking.

 

[metalman8357]

Well can't this thought experiment be answered with soldering and/or brazing? Solder uses flux to remove the oxide layer on the surface of the solid as well as the molten liquid. Without this, the solder will just ball up and not bond because of the oxide barrier. Once flux is applied, the molten solder will wet out and bond to the room temperature substrate.

Now the question is, does the same scenario work for aluminum?

 

[CoryPad]

Prior to your extrusion process, if you use a cleaning laser, e.g. a diode laser like those produced by CleanLaser:

http://www.cleanlaser.de/wEnglish/anwendungen/schweissvorbehandeln.php

Then I think there is a good chance the liquid droplet will adhere to the solid aluminium surface.

 

[Compositepro]

To 3D print aluminum you would need to keep the substrate at some temperature just below the melting point so that the bead can fuse to the substrate. This is done now with systems that print high temperature thermoplastics. It would be far more important to do this with aluminum because of its high thermal conductivity, which will freeze the applied bead without the substrate coming close to fusing with the bead. Inert gas in the chamber would prevent oxide formation so cleaning is not required, except perhaps for the first pass.

 

[metalman8357]

Thanks for the imput CompositePro.

So basically what you're implying is that the molten extruded aluminum will not fuse to the solid substrate because the molten aluminum will instantly solidify before bonding. I have come across someone who was able to successfully extrude molten solder and wet in successfully but his base plate was about 80% of the melting temperature of solder (on absolute Kelvin scale). For aluminum, this would mean that the build surface would need to be around 850F in order to fuse. This is no simple task.

Can anyone theorize an out-of-the-box solution to pre-heating the solid base aluminum (perhaps even locally) so that the molten aluminum will wet in properly and fuse together?

 

[berkshire]

This deposition of a plasticised metal onto a substrate is more akin to sintering than flowing liquid metal onto a solid substrate. one thing to examine would be the mechanism wherebye zinc based aluminum solders fuse to the base metal ( aluminum) without melting it, although they are liquid not plastic when they are applied..
B.E.

You are judged not by what you know, but by what you can do.