Theoretical Assessment of Dissimilar Metal Joint of Titanium to Stainless Steel

Direct joining of titanium to stainless steel (SS) poses some unique challenges because of a lack of metallurgical compatibility and the formation of brittle intermetallic compounds between the two metals.  Over the past decades, a variety of joining methods such as diffusion bonding, rotary friction welding, brazing and explosive joining have been developed.  Different interlayer metals have been utilized including copper, silver, nickel, just to name a few.  Although good mechanical strength of dissimilar Ti to SS joint has been reported in the published work, there is a lack of understanding on the in-service performance of those joints.  The theoretical assessment in this study consists of two parts.  The first part is the calculation of in-service stress distribution in the dissimilar metal joint using a finite element method (FEM) based model.  The inputs into the FEM model include (1) part geometry, (2) thermo-mechanical properties of materials (Ti, SS and interlayer metals), (3) temperature, and (4) mechanical loadings such as pressure and gravity.  For simplicity, the residual stresses from the joining process are not considered.  It is found that the mismatch of coefficient of thermal expansion (CTE) between Ti/SS and interlayer metal has a prominent effect on the in-service stresses.  The second theoretical assessment of the dissimilar Ti to SS joint pertains to the chemical stability of the joint at the elevated temperature based on the evaluation of inter-diffusion of alloying elements. 

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