Effect of a copper filler metal on the microstructure and mechanical properties of electron beam welded titanium–stainless steel joint

Cracking in an electron beam weld of titanium to stainless steel occurred during the cooling process because of internal thermal stress. Using a copper filler metal, a crack free joint was obtained, which had a tensile strength of 310 MPa. To determine the reasons for cracking in the Ti/Fe joint and the function of the copper filler metal on the improvement of the cracking resistance of the Ti/Cu/Fe joint, the microstructures of the joints were studied by optical microscopy, scanning electron microscopy and X-ray diffraction. The cracking susceptibilities of the joints were evaluated with microhardness tests on the cross-sections. In addition, microindentation tests were used to compare the brittleness of the intermetallics in the welds. The results showed that the Ti/Fe joint was characterized by continuously distributed brittle intermetallics such as TiFe and TiFe(Cr)2 with high hardness (~ 1200 HV). For the Ti/Cu/Fe joint, most of the weld consisted of a soft solid solution of copper with dispersed TiFe intermetallics. The transition region between the weld and the titanium alloy was made up of a relatively soft Ti–Cu intermetallic layer with a lower hardness (~ 500 HV). The formation of soft phases reduced the cracking susceptibility of the joint.