Research On Process And Mechanism Of TLP Bonded Cobalt-based Superalloy DZ40M

The cobalt-based superalloy DZ40 M has a higher initial melting temperature,better high-temperature strength,and better resistance to oxidation and corrosion than the traditional X-40 superalloy.It is an ideal for aeroengine turbine blades and combustion chambers material.However,because turbine blades are in complex stress conditions and high temperature environments,they often fail due to oxidation,cracking and other problems.Due to the high manufacturing cost of aeroengines,the defective parts are usually repaired to extend their service life,which can save a lot of costs.Therefore,this paper will use the transient liquid phase diffusion bonding technology to achieve the repair of DZ40 M cobalt-based superalloy.The intermediate layer used is a self-made Ni Cr Co WB intermediate layer.Through the melting of the intermediate layer at the welding temperature and the mutual diffusion between the MPD element and the alloying element,the intermediate layer is isothermally solidified.After a certain holding time,the joint is homogenized to obtain a joint with excellent high temperature durability.In this way,the influence mechanism of the bonding process on the joint interface structure and mechanical properties was studied.Layer thickness change at the weak position of the joint was analyzed through mathematical modeling,and the completion time of isothermal solidification and homogenization were predicted.Appropriate PBHT was used to homogenize the joint,which improves the joint performance.TLP bonding to DZ40 M at different welding temperatures(1120℃～1200℃)were conducted for 30 min.When bonding at 1120 ℃ the interface was divided into four region: EZ,ISZ,DAZ and BM.EZ mainly included Ni4B3,Cr5B3,Ni-rich boride and Ni-based solid solution.The phases in DAZ mainly included Co B,Co2 B,Cr2B,Cr5B3,etc.The half of the typical interface structure of the joint is: DZ40 M / Co B + Co2 B + Cr2 B + Cr5B3 /(Ni,Co,Cr)s,s / Ni-based solid solution + Ni4B3 + Cr5B3.As the bonding temperature increased,the eutectic decreased.Until 1160℃,the eutectic phase disappeared completely,and the joint realized full isothermal solidification.Different holding time was achieved at 1160℃.The more the holding time,the more isothermal solidification of the joint,and fully isothermal solidification was achieced at 30 minutes.The tensile test and fracture analysis of the joints showed that the joints kept at 1160℃ for 30 minutes completely eliminated the eutectic phase,and the number of needle-like boride in DAZ was small,the joint strength reached the highest,487 MPa.This reached the 88.6% of base metal strength.By analyzing the four stages of the TLP diffusion welding process,it was found that the isothermal solidification stage had a great impact on the joint interface structure.If the holding time was not long enough,the joint could not be isothermally solidified,and a brittle eutectic phase was formed at the center of the joint.Therefore,it is necessary to accurately control the completion of isothermal solidification and eliminate the brittle eutectic phase to improve the mechanical properties of the joint.The diffusion of B elements in the TLP process was calculated and simulated using the one-dimensional semi-infinite diffusion model.The higher the temperature,the faster the concentration of B element in the center of the joint dropped.At the same time,the decrease rate of eutectic layer thickness increased with increasing temperature.Since the concentration gradient of element B reached a local equilibrium after a certain period of time,the isothermal solidification rate also dropped to a stable value.The completion time of isothermal solidification predicted by the model was consistent with the experimental results.Post-bond heat treatment of DZ40 M TLP joint was conducted at 1120℃ for 5h.There was a small amount of needle-like phase and granular phase in the DAZ of the joint held for 5 minutes.Only the granular phase was left in the joint that has been held for 30 minutes.The boride was completely eliminated in the joints that held for 120 min,and the interface achieves a homogenized structure.It was found that the needle-like phase has a process of dissolving and growing during the isothermal solidification stage.In the process of PBHT,the outermost acicular phase began to dissolve until it disappeared.The B,Co,Cr and other elements where the boride was decomposed diffused into the base material along the grain boundary and reacted to form black M23(C,B)6 particle phase,which played a role in precipitation strengthening of the base material near the fracture zone.In addition,the calculation results showed that the required homogenization time at 1120℃ was 22～24h.In addition,the homogenization time required to completely eliminate the boride in the DAZ area of the joint is 3.22～4.15 h.