Research On Microstructure Evolution And Properties Of Friction Stir Processed 2507 Duplex Stainless Steel

Defects such as stress corrosion cracks will be formed in the engineering components due to load-bearing and environmental factors in service.To improve the safety and reliability of engineering components and extend their service life,the cracks needs to be repaired before instable development.There might be several problems using conventional welding to repair duplex stainless steel components,such as the unbalance of phase ratio and the precipitation of harmful phase,which could deteriorate the properties of components.However,friction stir processing could effectively avoid these problems since the nature of solid phase repair.In this research,the friction stir processing was conducted on 2507 duplex stainless steel with the thickness of 5mm.The tool for high melting point material was designed and optimized.The wear behavior of tool was revealed.The effects of processing parameters on the microstructure,mechanical properties and corrosion resistance of processing zone were analyzed.And the mechanism of microstructure evolution and its relationship with properties were also studied to provide theoretical guidance for obtaining high quality processing zones and further repairing.Based on the material characteristics of duplex stainless steel,the optimum design of tool material and structure was finished to achieve process and reduce tool wear.Divided design was adopted with W-25 Re being the tool material.The thermo-mechanical interaction between tool and material varied with the variation of processing parameters,which influenced the distribution of temperature filed and spindle torque.The peak temperature increased with the increase of thermodynamic interaction,which increased the plasticization degree of material.Therefore,the spindle torque decreased correspondingly.The tool wear mainly occurred during processing,and the tool shape gradually self-optimized in the subsequent processing with the main wear of diffusion and adhesive wear.Well-formed processing zone with few burrs was obtained with the processing speed of 50-200 mm/min and tool rotation speed of 200-600 rpm.The processing zone could be divided into thermo-mechanically affected zone and stir zone due to the different thermo-mechanical effects.The defect free processing zone was only obtained under the parameters with sufficient thermoplastic flow.The microstructure of stir zone mainly depends on strain rate and peak temperature during processing.The grain size decreased with the increase of strain rate and the decrease of peak temperature.The ratio of phases varies little with the variation of thermal cycle and stress deformation,and the ferrite fraction remains between 40% and 60%.Through the control of strain rate and peak temperature,the precipitation phase could be effectively avoided with only few precipitation phases exist at the bottom of stir zone at the heat input.The recrystallization mechanism of ferrite phase and austenite phase was different owing to the different stacking fault energy of phases,and the dynamic recrystallization of ferrite phase is prior to austenite phase.The mechanical properties and corrosion resistance of stir zone were obviously related to the microstructure.The microhardness distribution of processing zone presents basin shape with the maximum hardness located at the bottom of advancing side of stir zone which corresponding to the smallest grain size of stir zone.The grain boundary provided by grain refinement in stir zone inhibits dislocation slip during tensile process,which makes the tensile strength of stir zone higher than that of base metal with the elongation decreased.And the tensile strength increased with the decreased grain size,while the elongation decreased.The refined grains in stir zone promoted the diffusion of elements and enhance the stability of passive film.Thus,the corrosion resistance of stir zone is obviously better than that of the base metal.Based on the relationship between microstructure and properties,the properties of processing zone could be effectively controlled by changing the processing parameters to control the microstructure of stir zone.