| Dissimilar metals joint between copper and stainless steel can combine the high thermal conductivity of copper and the superior mechanical properties of stainless steel,and has been widely used in many fields.However,in fusion welding of copper/steel,the significant mismatching in thermophysical properties between the base metals can cause cracks.Besides,Fe-Cu liquid phase separation in the molten pool makes defects readily form in the weld.Moreover,the joint is always the weak part of the composite structure which is vulnerable to load and corrosion.In this work,dissimilar copper/stainless steel joints were prepared by gas tungsten arc welding(GTAW)with Cu filler wire,307Si filler wire and NiCrMo-3 filler wire,respectively.Microstructure evolution,mechanical properties and corrosion resistance of joints achieved by different fillers were investigated.Dissimilar copper/stainless steel joints without defects were prepared by gas tungsten arc welding(GTAW)with Cu filler wire,307Si filler wire and NiCrMo-3 filler wire,respectively.All tensile specimens fractured at the heat-affected zone on copper side,and the tensile strength of each joints reaches at least 77%of Cu base metal.Dimples and stretched zone were distributed on the fracture surface,showing a ductile fracture mode.The microstructure formation mechanism of copper/steel dissimilar joint was analyzed based on the metallurgical reaction between Fe and Cu.When Cu filler wire was used,Fe-rich spherulites formed by primary liquid phase separation between iron and copper that distributed in the welds.Inside the Fe-rich spherulites,minority Cu-rich spheres formed by secondary liquid phase separation.However,when 307Si filler wire was used,the primary liquid phase separated Cu-rich spherulites showed no characteristic of secondary liquid phase separation.The difference in the liquid phase separation degree can be attributed to various cooling rate of molten pool achieved by the two filler wires,and the different undercooling threshold of Fe-rich phase and Cu-rich phase.Ni can narrow the miscibility gap between Fe and Cu,so the microstructure showed no character of Fe-Cu liquid phase separation when NiCrMo-3 filler was used.Instead,the microstructure consists of ? formed by Ni-Fe-Cr,and Cu rich phase ?-Cu.The macro-segregation mechanisms near fusion boundary in welds achieved by different filler wires were studied based on solidification theory of fusion-boundary.When filler wires were used,non-isothermal pool boundary formed at fusion boundaries.The liquidus temperature of bulk weld pool(TLW)is lower than that of steel base metal(TLB)in joint achieved by copper filler.Resultantly,a bulk molten pool cooler than the melted 316L base metal(BM)formed near the solidification front.Subsequently,the melted unmixed BM was carried into the cooler molten pool by the convection and agitation,and solidified quickly into a peninsula and island shape before it fully mixed with the bulk weld pool.On the other hand,the liquidus temperature of copper base metal is lower than that of bulk weld pool in joint achieved by 307Si filler and NiCrMo-3 filler.The copper BM was cooler than the bulk molten pool,resulting in the invasion of Fe rich and Ni rich phases into the Cu-based melted unmixed zone.Local galvanic corrosion occurred in the welds achieved by different filler wires in the immersion corrosion test.The Cu-rich phase acted as anode in all welds,while Fe-rich and Ni-rich phase served as cathodes.The results of immersion corrosion test and electrochemical experiment both indicated that the sequence of the corrosion resistance of the weld zone achieved by each filler wire was NiCrMo-3 filler wire>307Si filler wire>copper filler wire. |
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