Study On Microstructure Mechanical Properties Of Superalloy GH4169and GH738Vacuum Brazed And Diffusion Bonded Joints

In the recent rapid development of aviation and ships, there was a increasingdemand for the resistance to elevated temperatures of the key component materials. Nickelbased superalloy GH738and GH4169have been considered to be the perfect hightemperature materials with potential application in aviation and ships filed because of thehigh mechanical properties at high temperature, high lasting strength and high creepstrength, especially the high corrosion resistance at high temperature and have beensuccessfully used as gas turbine engines of the ships. The cost of the manufacturing ofengine parts of high-temperature alloy can be reduced and also get good performance byusing the connection method, the most commonly technique to connect the nickel-basedsuperalloy are brazing and diffusion brazing process. In this paper, the microstructure,element distribution and phase composition, microhardness distribution and mechanicalproperties of the brazed joints were investigated by optical microscope, scanning electronmicroscope, transmission electron microscope, electron probe microanalyzer, X-raydiffractometer, micro-hardness meter and electronic universal testing machine.It was observed that there were two main forms of brazed joint by differentparameters of the brazed joint: the first kind of the brazed joints was composed by thecompounds which were in the centerline of the brazed joints, the solid solution was locatedbetween the joint boundary and the centerline of the joint and the compounds in the basemetal near the brazed joints, the three-parts were generated by non-isothermalsolidification process, isothermal solidification process and the diffusion of the fillerelement to the base metal. The second kind of brazed joints was composed of two parts which are the homogeneous solid solution and the compounds in the base metal near thebrazed joints; it was the results that a complete isothermal solidification process wascarried out. Through the observation on the same joint in different locations, it was foundthat the solid solution was gradually reduced and the compounds were increased from theside of filler metal placed to the opposite direction.Through the different brazing process parameters on the microstructure and propertiesof the brazed joints have found that the diffusion of the elements of the filler metal into thebase metal was more sufficient, the solid solution in the brazed joints was graduallyincreased and compounds reduced as the rising of the brazing temperature or extending ofthe holding time or reducing of the joint gap size, and the joints brazed was entirelycomposed by uniform solid solution when the rising of the brazing temperature orextending of the holding time or the reducing of the joint gap size to a certain value, butthe compounds in the boundary of the joint still exist.There were compounds appeared inthe centerline of the brazed joint as the temperature continue to rise to1140℃instead ofthe joint with complete solid solution when the temperature at1110℃. The Ni-B, Cr-B andNi-Si compounds were detected in the center of the brazed joint, needle like and blockychromium boride were detected in the diffusion zone at the boundary of the brazed joint.The data of micro-hardness which perpendicular to the brazed joint shows that there weregreat values in the joint center compounds zone while small values int the solid solutionzone, the values rebounded at the boundary of the joint where were exist compounds andthe values decrease with increase of the solid solution in the brazed joints.The formation of brazed joints mainly made up of four stages, filler metal melting andthe generation of primary solid solution, the generation of boride, silicide and eutectic solidsolution in the center of the brazed joint, the generation of Si-rich solid solution in theprimary solid solution and the process of the diffusion of the elements. The first threestages were traditionally brazing process, the fourth stage of the process was thediffusion brazing.It was found that the optimal specification were brazing at1110℃for45min holding time with30μm gap size through the comparative analysis of the brazingprocess parameters on the microstructure and mechanical properties for the selectedparameters in this study.