Microstructure,mechanical Properties And Corrosion Behavior Of Friction Stir Welded Similar And Dissimilar Joints Of Q235 And SS304 Steels

Friction stir welding（FSW）technology has been widely applied to the welding of low-melting metals such as aluminum and magnesium.However,few research about welding of high-melting metals such as steel has been reported.In this work,the industrial Q235 steel and 304 stainless steel（SS304）were used as basal metals（BM）,the similar and dissimilar welding of Q235 and SS304 with well-formed and non-defective were successfully processed by FSW.Combining experiments and numerical simulations,the microstructure characteristics in different zones of similar and dissimilar of Q235 and SS304 FSW joints were characterized,the microstructure factors affecting the recrystallization mechanism were revealed,and the relationship between the microstructure,mechanical properties and corrosion properties of joints was established.In addition,the finite element model of galvanic corrosion of joints was constructed by COMSOL Multiphysics（?）software,and the influence of galvanic corrosion on the corrosion performance between different regions of joints was discussed.The main conclusions were as follows:Both the heat-affected zone（HAZ）at the retreating side(HAZ_RS)and the HAZ at the advancing side(HAZ_AS)recovered under the heating cycle.Pearlite and acicular ferrite were generated in the thermo-mechanically affected zone（TMAZ）at the retreating side(TMAZ_RS),the stir zone（SZ）and the TMAZ at the advancing side(TMAZ_AS)due to phase transformation,which content depended on the peak temperature.The continuous dynamic recrystallization（CDRX）occurred in all three zones,whereas the grains were refined.The SZ mainly exhibited D₁{（?）2}<111>,D₂{11（?）}<111>and F{110}<001>shear texture,and the TMAZ_ASonly exhibited a F{110}<001>shear texture.Due to the difference of temperature field and coefficient of thermal expansion caused by phase transformation in each zone,the residual stress distribution was W-shaped for Q235 FSW joint.The hardness and tensile strength of joint were improved by fine-grained microstructure,pearlite and acicular ferrite.The ultimate tensile strength（UTS）was 479 MPa,which was 1.3%higher than that of BM.The uniform elongation（UE）was only 16%,which was decrease by 33%.The fracture presented ductile fracture characteristics.Besides,the FSW joint showed an excellent bending performance.The corrosion resistance of different areas in the joint was decreased in the following order:HAZ_RS,BM_Q235,HAZ_AS,TMAZ_RS,SZ,TMAZ_AS.The results showed that the content of pearlite and the distribution uniformity of laminar spacing of pearlite were the main factors to affect the corrosion resistance in each zone.The higher the pearlite ratio,the worse the corrosion resistance.The corrosion products mainly consisted ofα-Fe₂O₃andα-Fe OOH.The fine grains occurred in the SZ₃₀₄and TMAZ₃₀₄at SS304 side of Q235/SS304dissimilar FSW joint,and the discontinuous dynamic recrystallization（DDRX）and twin-induced dynamic recrystallization（TDRX）were the main grain refinement mechanisms.The recovery occurred in the HAZ.At Q235 side,the peak temperature in SZ_Q235exceeds Ac₃,which result in the formation of a high fraction of acicular ferrite.The peak temperature of TMAZ_Q235was between Ac₁and Ac₃,and the fraction of acicular ferrite decreased.CDRX and DDRX were two major recrystallization mechanisms.TMAZ₃₀₄mainly presented B{1（?）2}<110>and （?）{（?）}<（?）0>shear texture,and SZ₃₀₄exhibited B{1（?）2}<110>,（?）{（?）1（?）}<（?）0>and A*₁{112}<（?）1（?）>,A*₂{111}<111（?）>shear texture,and SZ_Q235exhibited D₁{（?）2}<111>,D₂{11（?）}<111>shear texture.Q235/SS304 dissimilar FSW joint showed mechanical and metallurgical bonding.The difference in expansion coefficient and microstructure between the two steels resulted in the formation of residual compressive stress in SZ.Grain refinement and phase transformation strengthening were responsible for the improvement of hardness and tensile strength.The tensile strength of joint was 493 MPa,which was higher than that of BM_Q235about 4%.However,the UE was only 15%,which was decreased by about 38%.The fracture surface showed ductile fracture characteristics.In addition,the bending strength of FSW joint was significantly improved.The corrosion resistance of different zones in the joint was decreased in the following order:BM₃₀₄,TMAZ₃₀₄,HAZ₃₀₄,SZ,BM_Q235,HAZ_Q235,TMAZ_Q235.The results showed that the corrosion resistance at SS304 side was related to grain boundary characteristics,and the corrosion resistance at Q235 side was mainly affected by microstructure.The corrosion products mainly includedα-Fe₂O₃,γ-Fe₂O₃,α-FeOOH,Cr₂O₃and FeCrO₄.The corrosion resistance of Q235/SS304 dissimilar steel FSW joint was better than that of Q235 steel similar FSW joint.The results of model prediction showed that the anode area of the joint was more susceptible to corrosion than the cathode area.The larger the pearlite size and area ratio of the joint,the worse the corrosion resistance,and the corrosion depth increased with time.The simulation results were in agreement with the experiment results.