Titanium warping - metallurgy help
Titanium warping - metallurgy help
(OP)
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?
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?





RE: Titanium warping - metallurgy help
RE: Titanium warping - metallurgy help
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
RE: Titanium warping - metallurgy help
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/materials-resource...
Regards, Wil Taylor
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o We believe to be true what we prefer to be true. [Unknown]
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RE: Titanium warping - metallurgy help
RE: Titanium warping - metallurgy help
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
RE: Titanium warping - metallurgy help
RE: Titanium warping - metallurgy help
I am still concerned about intermetallics.
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P.E. Metallurgy, Plymouth Tube