I am using TIG welding method to join SS304 and CP Titanium plates having dimension of 150mm*50mm*2mm. For this process I am using copper as a buttering layer in between these two plates while joining them as a butt joint. While observing welded samples I noticed cracks in the Ti-Cu zone. Is this a nucleation cracks?
Probably the welding speed was high and a thermal junction was obtained at which internal stresses were large resulting in soil cracks.
Ti has poor thermal conductivity, and Cu has a very good thermal conductivity. This difference results in internal voltages during cooling (cooling). To avoid this unfavorable effect, the bodies are heated to tempterium without any phase processes in the structure. After welding, it is slowly cooled to evenly distribute the stress after welding.
You should analyse this by using SEM and EDX. You may detect some intermetalic phases, which makes your joint more brittle. By 1 optical micrograph i cant say much. But let me explain the residual stress mechanism.
You have two different material which have different thermal expansion coefficient (TEC) curves. You heat and cool them extremely fast which makes the TEC differences produce high residual stresses. Cu can actually absorb a lot of stresses because of its plastic deformation behaviour, but in this process the cooling phase is too fast. As i said before you should examine them by using SEM. The area around the crack, and the different contrasted area in Cu. You should check with EDS if you see something different, at least the diffusion profile.
Good Luck!
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