spiridij
Mechanical
- Apr 23, 2015
- 37
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?