| Austenitic stainless steel(ASS)and duplex stainless steel(DSS)have good mechanical properties and corrosion resistance,and both widely applied in oil and gas transportation,petrochemical,shipbuilding and other fields,both have their own advantages in service performance and usage cost.Due to their own advantages in service performance and usage cost,it is inevitable to carry out dissimilar welding of ASS and DSS in some complex largescale equipment or projects,such as the large chemical tankers,acid oil-gas field exploitation,seawater desalination equipment,etc.,in order to reduce costs and meet the complex service environment.However,the traditional fusion welding processing for dissimilar welding of ASS and DSS is often accompanied by detrimental intermetallic phase precipitation,solidification cracks,element segregation,etc.Most of these problems associated with fusion welding would be suppressed or avoided by solid-state welding processes.Friction stir welding(FSW),as a typical solid-state welding method,has been widely used for welding aluminum alloys.Whereas,FSW is still lack of wide application in high melting point alloys because of the main problems such as large welding load and tool wear.In this paper,an electricity-assisted FSW device has been developed.Based on the proposed device,we have performed the experimental researches of 2205 DSS and 304 ASS dissimilar FSW and their electrically-assisted FSW.The main research works of this theme are listed as follows:(1)The welding load,thermal history and tool wear of the homogeneous FSW of 2205 DSS and 304 ASS with various welding parameters are studied.The microstructure evolution of the weld is deeply analyzed,the influence of welding thermal history on the microstructure evolution is revealed,and the relationship between microstructure and mechanical properties is explained.The stirring zone of the two kinds of stainless steel has undergone dynamic recrystallization,and both stainless steel welds have high strength and good plasticity.There are obvious gap defects in the 304 ASS welds,as well as an obvious wear of the stirring tool has occurred.The excessively high welding temperature in 304 ASS homogenous FSW is the main reason for the tool wear.The homogenous FSW experiments of the two materials under the same welding parameters are carried out,and the influence of the workpiece material on the welding load and thermal history is deeply analyzed.(2)The welding load,thermal history,material flow and weld formation in the dissimilar FSW of 2205 DSS and 304 ASS under different welding parameters are studied,and the material flow process in the stirring zone is revealed according to the microstructure of the weld cross-section and the weld horizontal-section of different thicknesses,and the corresponding material flow diagram is drawn.According to the experimental results,the formation mechanism of the large-area metallurgical mixing zone in the weld is revealed.Material position and tool offset have few effects on welding load and heat input,while significantly affect the quality of weld formation as well as the material flow and mixing in the weld.Rotation speed has a greater influence on welding load and less influence on welding heat input.Increasing the rotation speed cannot effectively improve the material flow in the weld.Appropriately reducing the welding speed can effectively increase the welding heat input and improve the quality of weld formation.Both the rotation speed and the welding speed have a significant effect on the peak welding temperature.Using a tool with a smaller pin can effectively increase the welding heat input and broaden the welding process window.Electrical assistance can effectively reduce the flow stress of the workpiece material,resulting in a reduction of welding load and heat input from the tool,as well as a reduction in the peak welding temperature and the weld width.(3)Through electron backscatter diffraction analysis(EBSD),scanning electron microscope(SEM)observation,energy spectrum(EDS)analysis and other methods,the microstructure and alloy composition of each characteristic area in the dissimilar weld are characterized.By analyzing the Fe-Cr-Ni alloy phase diagram and welding temperature data,the phase transition,grain size,recrystallization mechanism,etc.of the weld are researched.According to the microstructure,the stirring zone of the dissimilar welds can be divided into2205 DSS stirring zone,304 ASS stirring zone and metallurgical mixing zone.The phase ratios of the 2205 DSS stirring zone and the 304 ASS stirring zone are the same as their base metals,respectively.Dynamic recrystallization of both zones are observed,combined with significant refinement in the grain sizes.The phase ratio of the thermal mechanical affected zone has not changed,and its grain size is between the stirring zone and the base metal.The metallurgical mixing zone is formed through sufficient mechanical mixing and alloying elements diffusion between 2205 DSS and 304 ASS,its alloy composition and phase ratio are between the two base metals,and it has undergone complex phase transformations.Due to the pinning effect of second-phase,the grains of the metallurgical mixing zone and the 2205 DSS stirring zone are obviously smaller than the 304 ASS mixing zone.The dual-phase structure will hinder the migration of the phase boundary,and inhibit the growth of grains near the phase boundary.Therefore,the grains of the metallurgical mixing zone and the 2205 DSS stirring zone are obviously smaller than the 304 ASS stirring zone.Due to the difference in the lattice structure of the ferrite and austenite phases,the recrystallization mechanisms of the two are also different,and their microstructure characteristics such as grain size and grain boundary characteristics are obviously different.(4)The influence of welding parameters and electrical assistance on the alloy elements distribution and microstructure in the weld are studied through SEM and EDS analysis.The relationship between welding thermal history and microstructure evolution is explained.Tool offset and material position parameters affect the alloy composition of the metallurgical mixing zone by affecting the ratio of the two base metals involved in the material mixing,thereby affecting the microstructure in the metallurgical mixing zone.Welding speed,rotation speed and pin diameter mainly affect the grain size of each characteristic area in the weld by affecting the welding thermal history,and they have no obvious effects on the phase ratio of each characteristic area in the weld.Moreover,the influence of the welding peak temperature on the grain size of the weld is greater than the influence of the welding heat input.Increasing the tool pin diameter will reduce the welding heat input,thereby reducing the weld grain size.Electrically assistance can reduce the welding peak temperature,thereby refining the grain.(5)Through the microhardness test,tensile test and electrochemical corrosion test,the mechanical properties and corrosion resistance of the weld are studied,and the influence of welding parameters and electrical assistance on the weld performance is studied.The relationship between the microstructure of the weld and the mechanical properties and corrosion resistance is explained.The properties of welds firstly depends on the quality of weld formation,welding defects such as holes and tunnels will deteriorate the mechanical properties of welds.For welds with good forming quality,their mechanical properties mainly depend on their microstructure,and the microhardness of each characteristic area in the weld is negatively related to its grain size.The hardness of each characteristic area in the stirring zone is higher than that of its base metal.The hardness of the thermal mechanical affected zone is between the stirring zone and base metal.Tensile tests show that the yield strength and tensile strength of welds with good forming quality are higher than those of 304 ASS base metal,and they also have good plasticity.Welding parameters mainly affect the microhardness distribution of the weld by affecting the grain size and the distribution of each characteristic area in the stirring zone.The electrochemical corrosion test results show that the corrosion resistance of the dissimilar stainless steel weld is slightly better than that of the 304 ASS base metal,and the weld with a smaller grain size has better corrosion resistance. |
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