Annealing Copper without oxidation - what is needed?

[irpheus]

Hi all,

I have an annealing chamber question that I hope one of you may be able to help with ...

As it is I will soon be in need of dead-soft annealing copper wires and to this end have built a small annealing chamber from Stainless Steel (SS) surrounded by a ceramic insulation.

Before starting the annealing process I pour Argon into the annealing chamber, place a SS plate on top of the chamber, and then start heating the argon from inside the chamber with a scrapped kitchen stove heater plate. This works fine in terms of being able to reach sufficient temperatures (~725 degr. C) ... BUT ... the copper plate is heavily oxidized when cooled down.

I reckon the reason why this oxidation happens is that the argon when heated "flows turbulently" inside the annealing chamber, and then, since the chamber is not gas tight on the transition between the bottom chamber box & the top SS plate, the argon slowly pours out into the surrounding air and - when cooling takes place - atmospheric air enters the chamber and here causes the oxidation (my guess ).

However, I now have on hand a vacuum pump capable of reaching a vacuum of 0.002 mmHg ... so I wonder if this would be sufficient to ensure a non-oxidizing annealing - in case the annealing chamber can be kept gas-tight? And, if not, what could be a "kitchen table" non-oxidizing annealing chamber solution? The annealing chamber needs not be that large - 20*20*10 cm suffices - but it should be able to reach temperatures that will also allow annealing of other metals as well (silver & gold mainly).

I would appreciate your feedback

Cheers,

Jesper

 

[irpheus]

@mdm55: Thank you also for suggesting ... Reading your post I came to think of if - assuming a vacuum chamber - the normal moisture absorbent "grains" used e.g. in a shed would be sufficient to eliminate the moisture issues with ceramic insulation? I.e. - if one places such a pouch of moisture absorbents inside the vacuum chamber when not in use - if that would be sufficient, or not ... ?

@EdStainless: Thanks Ed for following up ... I now can visualize what you suggest and it seems like a quite accessible approach as the sizes I need to anneal - at least in the beginning are just reasonably small. Sounds like a splendid idea with the Ti foil pieces - I suppose that if Argon + H gas is added this could be a very clean annealing process. And since I already have an Argon based annealing chamber this may be a simple way to go ...

Will just consider - thanks again for your help here!

Jesper

 

[irpheus]

... came to think of a P.S.:


... there isn't by any chance a flexible tube material with the same qualities as SS ... Just hoping here as it would be simpler ...

Cheers,

Jesper

 

[mdm55]

if one places such a pouch of moisture absorbents inside the vacuum chamber when not in use - if that would be sufficient, or not ... ?

Typical reusable dessicants have a high capacity for moisture, but they do not have a high affinity as evidenced by the rather low regeneration temperatures. Surface adsorbed species can be bound quite tightly on ceramic surfaces. I have a small 1200°C box furnace in the garage that "sweats" if I haven't used it for several months and it is run at >500°C.

As EdStainless mentioned, titanium is an excellent getter for oxygen, not only due to its affinity for oxygen, but it also has a high solubility. Titanium foil can be made as brittle as glass by heating in air.

If you could tell us what the ultimate use of your annealed copper wires is (electrical?) and the size of the wires it would help in determining a preferred method of annealing. There are other oxygen getters (e.g., graphite powder) that can be used, and various dips that can be used to remove residual oxides after annealing. For example, if the wires are small, they can be packed in a small diameter alumina tube with graphite and heated with a propane torch. One could use silicone and copper tubing attached to the ends of the alumina tube to connect a vacuum pump.

This is a bit off-topic, but a fully annealed piece of copper with smooth surfaces (e.g., electropolished or bright-dipped) will expose the grain structure on heating in air. The oxide growth rate is dependent upon the crystallographic orientation of the grains, so as the oxide grows, each grain will show a different interference color similar to the colors one sees in a thin oil slick on water. With continued heating and increased oxide thickness the interference effects are lost. I discovered this years ago when I impatiently removed previously bright-dipped copper pieces from a wet hydrogen pusher furnace before they were fully cooled.

 

[irpheus]

Hi ...

@mdm55: First, sorry for my late reply - for some reason I wasn't notified with a "new reply" email as I normally am (or I may have missed it, of course).

.. I think it sounds like a feasible way to go so I have ordered a bit of titanium foil and coincidentally I also have some graphite coming that should arrive soon.


Yes, it is electrical and partly wires of likely 0.8mm diameter and a couple of meters in length. The wires can't be bent but must be coiled up in maybe 15 cm circles. I would like to cover these wires with various metals (electroplating - gold, silver, other metals) and then anneal these metals with - or into - the copper (without impurities as much as is possible).

Years ago I read that the copper + gold combination readily "diffuses" into eachother and this is what I would like to achieve so that the gold/silver/other metal diffuses/anneals into the copper. Not an entirely even diffusion from the outside to the inside - I would expect a diffusion gradient - but a degree of diffusion/annealing.

Additionally, I would need the copper wire to be very flexible - and preferably to stay this way, if possible.

Hope this may be a usable description of my use ... ?

Hmmmm .. and then your last paragraph makes me think that you seem to have quite a bit more experience in this field than I do ... and also that - as life most always shows me - there are usually unknown-of depths to most any subject if one allows the subject's Pandora's Box to be opened ...

Cheers,

Jesper

 

[mdm55]

Hi Jesper,

There are a couple of things to keep in mind for your project. The grain size distribution in the annealed copper will be a function of the annealing process, and the diffusion behavior will be a function of the grain size distribution.

When annealing at high temperatures the thermodynamic driving force for nucleation of new grains is small, and diffusivity is high. This results in fewer, larger grains. When annealing at lower temperatures, the nucleation driving force is large and diffusivity is lower, resulting in more, smaller grains.

Diffusion along grain boundaries is about an order of magnitude higher than diffusion within a grain.

I recommend that you do a preliminary anneal or stress relief prior to depositing a metal on the copper wire. This will give you a more unform starting condition so that you can evaluate the impact of the processing that you are doing. I also suggest that you familiarize yourself with the phase diagrams for the material combinations that you are going to use. Google "copper gold phase diagram" and "copper silver phase diagram" images. The phase diagrams will inform you as to the solubility limits and the formation of intermetallic phases.

MDM

 

[irpheus]

@mdm55: Hi MDM ... and thank you very much for considering and replying again! And then I again notice that I haven't received any notification email (even if it should be on according to settings) which is the reason I reply this late.


This sounds like good suggestions and I will look into this shortly ... I say "shortly" because I am currently building a lapping machine and - while doing this - getting to know my CNC mill and the correct speeds and feeds for various tools in different materials - is proving time consuming. But it sounds like a good idea to refresh the phase diagrams (the study years are years back in time ).

Cheers & thanks again,

Jesper

 

[mdm55]

Hi Jesper,

What CNC mill you are working with? At various times I have used full-sized mills and although I have acquired a Taig CNC mill, I have not set it up due to lack of space. In my experience, intricate machining of annealed OFHC copper is very difficult, which is why I got into post-machining annealing of copper. A severely deformed surface layer is almost unavoidable on soft metals.

MDM

 

[irpheus]

Hi MDM ... this time I got a notification ... fine ;-)

I use a mill that I have built myself out of G50 cast iron rectangular rods (with good guidance from an experienced CNC builder).

It is designed to not be too big so that it fits in a relatively small area of ~ 95*95 cms - yet still have a quite reasonable work cubic (330*470*150mm; C-beam type design) while being capable of working with "most all" materials. Weight likely is ~170 kg.

It is also mounted on a table with wheels and the CNC mill/router itself (don't really know which is what?) is placed at appr. 150 cm height so that mounting and working with it is simple - and doesn't bend my back unpleasantly. This elevated height does mean that it can be a little shaky if used at high feeds - but I much prefer running it slowly and then being able to work with everything almost at eye level. Very comfortable I think.

Additionally, and this has been the best design detail I at the beginning didn't realize I was adding, I have made a small, variable height & local, exhaust which in most cases removes >99% of the mill "chips". Also, it tends to remove most of the mill "dust" thus keeping the air in the CNC room quite pleasant.

Although a major bit of work I am quite satisfied with the CNC mill's overall functionality.


Hmmm ... I haven't tried milling annealed copper yet, and likely won't be doing so, since I - as I understand you now do - will anneal after machining the parts. But I have milled the "normally available" copper (plates) with quite good surface result (a surface almost like a mirror) using an 8mm HRC55 TiAIN coated end mill. The sides of the outer milling traces aren't entirely "sharp" but the milled surface itself I would say is almost mirror-like.

Hope this answers your semi-question ;-)

Jesper