Joining of titanium to 304L stainless steel by friction welding

The dissimilar metal joint of titanium (Ti) to 304L stainless steel (SS) is essential in the nuclear industry for the dissolution of spent fuel that is carried out in boiling nitric acid in the dissolver vessel (made of Ti) and the dissolved solution is transported through the 304L SS pipes to the other plant components made of 304L SS. Because of the radioactive environment, leak tightness and corrosion resistance of this dissimilar joint are important. In this work, friction welding process was attempted to join Ti to 304L SS. Direct friction welding of Ti to 304L SS results in a stronger weld in which failure occurs in the Ti base metal during tensile testing. However, the joints have almost zero bend ductility that has been attributed to the formation of intermetallics due to mechanical alloying, strain hardening of Ti near the joint interface and residual stresses. Post-weld heat treatment marginally increases the bend ductility to 5° because of relieving of the effects of strain hardening and of residual stresses at the joint interface. Corrosion test in boiling nitric acid as per ASTM A-262 Practice C shows that the average corrosion rate is 10 mpy with the joints remaining intact after the corrosion test. The details of mechanical tests, microstructure analysis using optical and scanning electron microscopy, are discussed.