In this paper, the microstructure optimization and mechanical property of joints are studied when the Cu, CuSi3, and CuNi10 wires are used to Cold Metal Transfer arc-braze TC4 to 304L. Based on the microstructure analysis and thermodynamic calculation, the existing state of Ti and Fe atoms are transferred from high brittleness Ti-Fe to less brittle Ti-Fe-Si intermetallics when the welding wire is changed from Cu to CuSi3 due to the lower Gibbs free energy of Ti-Fe-Si intermetallics. Furthermore, the (Fe, Ni) solution and Ti-Ni intermetallics with the lowest hardness are formed when CuNi10 wire is used. Thus, three Ti-Fe suppression mechanisms are concluded: suppressed by forming intermetallics and diffusion inhibition, suppressed by forming intermetallics, and sup

pressed by forming intermetallics and solution. Based on the microstructure optimization, the max hardness of intermetallics layer decreases from 769 to 654 Hv and the max tensile strength increases from 186.4 to 319.4 MPa. However, all cracks of the joints with different wires will initiate from the bottom of the seam and

propagate in the TC4/seam transition zone due to the high Schmid factor. Besides, the crack propagation rate of more than 200 m/s can be in-situ measured by high-speed camera with 200000 fps.