Microstructural evolution and mechanical property of TC4/304 stainless steel joined by CMT using a CuSi3 filler wire

Abstract

Abstract To obtain excellent spreadability under lower heat input and optimize interfacial microstructure, the spreading limited factors were analysed in wetting of liquid CuSi3 on 304ss and TC4 plates. At the CuSi3/TC4 joining side, the spreading model changed from diffusion-limited to reaction-limited with the interfacial temperature decreases which caused by the decrease of heat input. In the reaction-limited spreading, a continuously thin reaction layer (∼61 μm) was observed, consisting of Cu + Ti2Cu3 + TiCu4 + Ti5Si3, and TiCu + Ti2Cu + Ti5Si3. In the diffusion-limited spreading, a thin reaction layer was transformed to a thick (∼125 μm) one with intricate phases, containing primarily of Ti5Si3 + TiFe2+Ti2Cu3 + TiCu4, Ti5Si3 + TiCu + AlCu2Ti, and TiCu + Ti2Cu therein. The spreading energy provided by the interfacial reaction (26 kJ/mol) was higher than that provided by the diffusion of Ti atoms in copper (21 kJ/mol), i.e. the spreading rate under 65 A welding current with a 0.783 KJ/cm heat input was greater than that under 70 A with a 0.853 KJ/cm heat input. The rapid spreading with a short residence time at a high temperature could reduce the thickness of the interface layer. At the CuSi3/304ss joining side, CuSi3 droplet could not wet 304ss when the heat input was lower than 0.783 KJ/cm. The wettability was gradually improved with increasing heat input, and the spreading driving force was provided by the diffusion of Fe in liquid Cu. A typical solid solution interface without crack defects presented at the CuSi3/304ss. Under the dual effects of reaction-promoted spreading at TC4 side and diffusion-promoted spreading at 304ss side, a wrap-around joint with high strength (∼407.6 MPa) was formed.