Titanium warping - metallurgy help 2

[spiridij]

Hello All,

I'm having a very difficult time with a resistance braze joint that I'm trying to create between grade 2 Ti and cp silver. I'm stuck with those two materials for a number of reasons so I'm hoping someone can offer some assistance. Here's the issue. I have two grade 2 Ti "shells" that have a wall thickness of 0.015" that are currently machined (overall dimensions are ~0.9" long X ~0.5" wide and ~ 0.2" deep). The flange is about 0.040" wide and the shells are mirror images of each other in the region of the brazing operation. Between these two Ti shells, there is a silver membrane that serves as a filler material for the braze joint and a membrane to hermetically seal the two chambers. I use resistance welding because I can use the difference in electrical resistance to heat the flanges of the Ti to a temperature that first dissolves the oxide layer back into the Ti and then melt the silver. The two housings are squeezed together during the process, so I don't believe the oxide layer can form to a thickness that would cause a problem since there won't be any exposure to air. What I'm seeing is a perfect bond between the parts initially, and then after about 12 hrs, the Ti parts "pucker" and separate the braze joint resulting in a failure of the silver at that location. My theories are:

1. There's a residual stress in the Ti shells somehow resulting from the machining process
2. There's a residual stress in the system resulting from the fast heating and relatively fast (~0.5 secs) cooling of the assembly during the resistance brazing operation
3. The puckering resulting from something I'm missing such as an alloy that is created at the surface of the Ti that is causing the puckering of the Ti housings due to differential thermal expansion/contraction

Does anyone have some insight?

 

[spiridij]

Updated question. Through the process, the Ti goes from alpha phase to beta phase. Is it possible that the phase transformation is causing the warping of the parts? Could it be as simple as decreasing the temperature of the process such that the Ti remains in the alpha phase throughout the process? Or if that's not possible, is it possible to slowly decrease the temperature, once the Ti has reached the beta phase, such that it switches back into the alpha phase, without warping?

 

[EdStainless]

Have the Ti parts been stress relieved?
Have you looked at micros to see if there is an intermetallic phase forming?
I like the idea above, you need to respect the volume changes that go along with such transformations.

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P.E. Metallurgy, Plymouth Tube

 

[WKTaylor]

spiridij...

ASM has several texts, references and handbooks on titanium/Ti-alloys, all aspects. I have several of these ASM references for when I worked F-15s [about 40% Ti alloys]. Exceptionally useful/necessary for parts fabrication and assembly, and repairs, such as welding, brazing, cutting/machining, heat treating (all aspects) etc.

Hope this link works for you...

http://www.asminternational.org/mat...mode=view&p_p_col_id=column-1&p_p_col_count=1

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[spiridij]

EdStainless, I'm in the process of getting some micros done to see what's going on at the interface, hoping to shed some light on that aspect of the process this week. The Ti parts have not been stress-relieved after machining. I've tried to use a stress-relieving process on the part after the brazing at 650 deg C for 2 hrs and it did not relieve the stress. To clarify, you mention respecting the "volume changes that go along with such transformations". Does that mean that the total volume changes during the transformation? Is it possible that the volume change in transforming into the beta phase is locked in place by the silver and when it goes back to the alpha phase, it's trapped in that shape by the silver, causing the residual stress which then breaks the silver braze joint?

 

[EdStainless]

A lot of things are possible.
What temp are you going to? The a-b trans is about 880C.
The volume change would be enough that you should then sit at a temp below that (about 750C) for some time in order to allow stress relief.
However at these temps the formation of intermetallics may be making the joint so brittle that it is a moot point.
The shear strength of pure silver is not very high, so keeping the load low is critical.

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P.E. Metallurgy, Plymouth Tube

 

[spiridij]

Thanks EdStainless, I'm going to somewhere around 961C, the melt temp of silver. I'm considering using a silver braze alloy which brings the melt temp below the a-b trans temp. Will that eliminate the possibility of warping of the Ti?

 

[EdStainless]

It won't eliminate it, but the lower them sure would help.
I am still concerned about intermetallics.

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P.E. Metallurgy, Plymouth Tube