Research On Microstructure And Properties Of The DD98M Single Crystal Superalloy Joints Brazed With Pd-Si Composite Filler

The aeroengines are the key components in the aircraft.It is of strategic significance to improv the manufacturing process of aeroengines.The turbine blade,as the core component of an aeroengine,is mainly fabricated by the nickel-base single crystal superalloy.Considering the hollow structure for compound cooling of the turbine blade,the internal structure of model cavity is too difficult to directly cast the turbine blade.In order to solve the depoling problem in cast,the two-halves structure is usually adopted in practice.Thus,it is important to achieve the high-quality connection of two parts of single crystal blade.The Pd-Si composite brazing filler metal was used to braze the DD98M nickel-base single crystal superalloy in current work.The joint with perfect high-temperature performance and impact resistance was obtained.The different phases in the joint were confirmed by SEM and EDS.The influence of brazing temperature and holding time on the microstructure and mechanical property of the joint was studied.The brazing parameters were optimized by shearing tests.The mechanical properties of joints were evaluated by the tests of high-temperature stability and impact toughness.The main achievements can be concluded as follows:(1)The brazing joint can be classified into DD98M/Zone 1/Zone 2/Zone 1/DD98M.Zone 1 was mainly composed of Al2Pd5 and Ni[Cr,Pd].Zone 2 constituted of Pd4Si,Ni-Pd[Cr,Co]and Pd[Ni,Ti,Al].Ni-Co[Cr]and Ni[Cr,Pd]were produced in the zone adjacent to the brazing joint.As the temperature increased to 1000 ℃,the Pd-Si brazing filler metal was melted.The elements in based metal and brazing metal started to diffuse into each other.Near the boundary,Cr and Co were dissolved in Ni to form Ni[Cr,Co]solid solution.Al and Pd formed Al2Pd5.In the center,Pd reacted with Si forming Pd4Si.Ti,Ni and A1 were dissolved into Pd to form Pd[Ni,Ti,Al]solid solution in the area without Si.At the same time,Cr and Co were dissolved in Ni and Pd to form Ni-Pd[Cr,Co]solid solution.(2)As the brazing temperature elevated,the precipitate phases of Al2Pd5(Zone 1)and Pd4Si(Zone 2)and solid solution of Ni-Pd[Cr,Co](Zone 2)were coarsened.With the holding time lasting longer,Zone 1 became wider.The microstructural evolution in Zone 1 with longer holding time can be described as below:Pd[Ni,Ti,Al]transformed to Pd4Si,Al2Pd5 grew coarser,and the number of Ni-Pd[Cr,Co]increased initially and reached a maximum value.The shear strength of the joint firstly increased and then decreased with the higher brazing temperature and longer holding time.The maximum value of shear strength,338 MPa,was obtained at 1060 ℃ for 90 min.The joints with different brazing parameters were fractured at the brazinrg seam in the shear tests.(3)The thermal stability of joints was elevated by the heat-treated experiments at 600 ℃and 700 ℃ for 240 h.The results showed that there was no obvious solid-liquid phase transition and new phase formation in the joint.The distinct change in the joint was the coarsening of all phases.The shear strength of the joint after heat-treatment kept stable compared to that of the joint without heat-treatment.(4)The results of impact toughness testing indicated that the impact energy of joint was 0.49 and 0.46 of the base metal at ambient temperature and 600 ℃,respectively.The impact toughness of joint showed a decrease compared with the base metal.Pd-Si composite brazing filler metal was innovatively utilized in the work to braze DD98M single crystal superalloy.The high-quality joint was obtained by adjusting the brazing parameters.