Research On The Effect Of Ultrasonic Vibration On The Structure And Properties Of 316L/L415 Dissimilar Metal Welded Joints

Aiming at the problem of dissimilar metal welding joint problems such as different components and physical as well as chemical properties between different materials,uneven distribution and evolution of microstructure and poor performance in mechanics and corrosion resistance,this paper puts forward the method of ultrasonic-assisted welding of dissimilar metals 316L/L415,making full use of sound cavitation and sound flow effects as well as other nonlinear effects in liquid by ultrasonic,then the ultrasonic vibration on the welding joint microstructure and properties of 316L/L415 dissimilar metals were investigated;the theory problems of the effects of ultrasonic vibration were discussed,which provides a new idea to solve the difficulties of dissimilar metal welding.In order to explore the feasibility of ultrasonic vibration-assisted welding,this paper carried out an ultrasonic welding test,which showed that the ultrasonic vibration can refine the weld grain,increase the depth and width of weld.Then three groups of dissimilar metal welding of 316L/L415 were set up,including the common TIG welding(TIG),ultrasound vibration loading on the L415 side(U-L415)and ultrasonic vibration loading on the 316L side(U-316L).Universal testing machine,impact testing machine,microhardness tester and electrochemical workstation were used to evalute the effect of ultrasonic vibration on the 316L/L415 dissimilar metal welding joint properties;the results showed that the ultrasonic vibration was good for improving the mechanical properties and corrosion resistance;the average tensile strength of TIG,U-L415 and U-316L welding joints were 613MPa,650MPa and 610Mpa,resprctively;and the weld seam average impact absorbing energy was 70J,88.9J,96.3J,respectively;the hardness distribution trend of three groups of joints are basically the same,however,the weld hardness distribution changed a little after introducing ultrasonic vibration in the weld processing;it was found that,in this paper,the best corrosion resistance of weld was U-316L weld while the worst one was common TIG weld according to the investigations of the effect of ultrasonic vibration on weld corrosion properties of dissimilar metals 316L/L415 welded joints.By using optical microscope(OM),scanning electron microscope with energy spectrometer(SEM,EDS)and other analytical methods,the inffluence of ultrasonic vibration on the microstructure of dissimilar metals 316L/L415 joints were also studied.The results showed that ultrasonic vibration can refine the weld metal grain and the heat affected zone of L415,reduce the number and size of delta precipitation in the fusion zone of 316L,and decrease weld segregation degree whose mechanism was discussed in this paper from the the perspectives such as solute distribution and nucleation behavior of the weld pool.Based on electron backscatter diffraction(EBSD)techniques,the effect of ultrasonic vibration on crystal orientation of the heat affected zone of the joints at L415 side was investigated,the results showed that there existed great oriention difference in the U-L415 HAZ while the crystal orientation difference is relatively small in common TIG HAZ.The large angle grain boundary content in HAZ of U-L415 and common TIG joints were 20.3%,17.2%,respectively;while the content of small angle grain boundaries were 79.7%,82.8%,respectively;the average grain size in the HAZ of U-L415 and common TIG joints were 1.12?m and 2.33?m,respectively;for the conventional TIG welding HAZ,MA islands are mainly distributed in the grain boundaries in the chain-typed;when it comes to U-L415 HAZ,however,the MA islands are small size and mainly distributed inside the grains;the content of MA islands incommon TIG HAZ and U-L415 HAZ were 0.443%,0.071%,respectively;coincidence site lattice(CSL)in heat affected zone are mainly about ?3??17a and ?25.The content of ?3??17a and ?25 in U-TIG and common TIG joints at L415side are 3.21%and1.06%,0.34%and 0.24%,0.53%and 0.42%,respectively.Based on the EBSD analysis of weld metals of U-L415 and TIG group,it was found that the ferrite was distributed along the columnar austenite grain boundary with strong directivity in the common TIG weld;after applying ultrasonic vibration,the ferrite was dispersed with no obvious direction.The content of delta ferrite in U-L415 and TIG weldmetal is 3.57%and 4.08%,respectively;after calculation using HKL Channel 5.