Study Of Ultrasonic Impact Treatment On The Microstructure Evolution And Recrystallization Behavior Of 321 Stainless Steel Fabricated By WAAM

Wire and arc additive manufacturing(WAAM)is a novel three-dimensional material preparation technology that has gained considerable attention due to its benefits,including high deposition efficiency,low cost,and high material utilization.However,WAAM faces challenges such as coarse columnar crystals and excessive residual stress due to multiple thermal cycles during deposition,which impede its application and development in additive manufacturing.Ultrasonic impact technology is a surface treatment technology that employs high-frequency and high-speed ultrasonic impact to plastically deform the material’s surface,break coarse grains,and introduce beneficial compressive stress,which enhances the material’s structure and performance.By incorporating ultrasonic impact treatment into the WAAM process,plastic deformation brought by ultrasonic impact and the thermal effect of arc fuse additive manufacturing multilayer deposition can be combined to change the microstructure and refine the grains’ original coarse columnar crystals,transforming them into equiaxed crystals.In recent years,research on the recrystallization behavior in ultrasonic impact-assisted arc fuse additive manufacturing has become one of the current research hotspots.This paper aims to investigate the effect of ultrasonic impact treatment on the microstructure and properties of 321 stainless steel deposition layers prepared using arc fuse additive manufacturing technology.Well-formed deposition layers were chosen for ultrasonic impact treatment with varying parameters.Microscopic characterization and performance tests were conducted to explore the impact of different ultrasonic impact parameters on the structure and properties of the deposited layers,aiming to optimize the ultrasonic impact and arc additive composite process.In-situ tests at high temperature and EBSD tests were employed to study the influence of different ultrasonic impact parameters on microstructure evolution and recrystallization.Additionally,the paper examines the mechanism by which ultrasonic impact promotes the recrystallization of deposited layers,which offers a basis for the ultrasonic impact composite arc fuse additive manufacturing technology promotion and contribution.The main research findings are summarized as follows:In order to investigate the impact of ultrasonic treatment on the arc-fused 321 stainless steel deposited layer,ultrasonic treatment was applied with varying parameters(ultrasonic power/external load/impact time).The deposited layer was post-processed,and metallographic observation and microhardness test were conducted to evaluate the deposited layer.The results indicated that the top layer structure of the deposited layer changed from equiaxed cellular structure to rheological structure after ultrasonic impact.As the three ultrasonic impact parameters increased,the depth of the plastic deformation layer increased,and the surface layer hardness of the deposited layer also increased.The effect of ultrasonic impact decreased with increasing distance from the surface,which was confirmed by the degree of plastic deformation and the depth of hardness.EBSD testing of the deposited layer before and after ultrasonic impact showed that the KAM value and strain of the deposited layer increased after ultrasonic impact,and the subgrain boundaries and small-angle grain boundaries of the deposited layer also increased.This study aims to investigate the microstructure evolution and recrystallization process of ultrasonic impact-assisted arc fuse 321 stainless steel during multi-layer deposition.The deposited layer after ultrasonic impact was used as a sample,and the high-temperature laser confocal microscope was utilized to simulate the thermal stress of arc fuse 321 stainless steel during multi-layer deposition.Dynamic high-temperature in-situ observation of the microstructure evolution and recrystallization process was carried out,and the mechanism of recrystallization of the deposited layer under heating after ultrasonic impact was explored in combination with EBSD tests.The test results show that increasing the three parameters of ultrasonic impact enhances the effect of grain refinement observed at variable temperatures.The impact time parameter has the most significant effect,followed by the applied load,and ultrasonic power.When observing the in-situ structure at constant temperature,numerous small recrystallized grains were detected near the grains.Compared to the variable temperature test,the in-situ test showed more recrystallized grains and complete structure evolution.The grain size of the ultrasonic impact deposition layer was smaller than that of the non-ultrasonic impact deposition layer.The ultrasonic impact + heat preservation deposition layer sample formed more recrystallized grains,and the grain size was finer,indicating a better effect.Additionally,the high-temperature in-situ dynamic observation results support the nucleation mechanism of recrystallization in ultrasonic impact-assisted arc additive manufacturing,which is grain boundary arch nucleation.The study compared the EBSD test results of the deposition layer before and after ultrasonic impact with ultrasonic impact+heating deposition layer samples.The findings revealed that ultrasonic impact increased the content of substructures,such as sub-grain boundaries and small-angle grain boundaries,and these substructures underwent recrystallization transformation under heating conditions.This led to the formation of new high-angle recrystallized grains,resulting in a decrease in the average grain size of the deposited layer.Through quantitative analysis of the recrystallization process of 321 stainless steel assisted by ultrasonic impact,the Avrami equation was used to establish the recrystallization kinetic model.The recrystallization activation energy Q was found to be 303836.5 J/mol.