Study On The Orientation Matching Relationship Of Nickel-based Single Crystal Superalloy During Brazing

The experimental single crystal superalloy CMSX-4 with three different orientation deviation of 0°+0°,0°+45°and 0°+90°were brazed by Ni-based filler alloy at 1210°C.The brazing time was set as 30 min and a typical post-weld heat treatment was then carried out.The effects of orientation deviation and post-weld heat treatment regimes on the microstructure and mechanical properties of the brazed single crystal alloys were investigated.The microstructure ev olution,element distribution and orientation features in the joint were further analyzed to clarify the formation mechanism of the brazed joint.The results illustrated that the complete and uniform brazed joints could be obtained after the brazing process of single crystal superalloy CMSX-4 by JSSNi60A Ni-base filler alloy,while the joints were primarily made up of three typical parts:the filler alloy zone（FAZ）,the interfacial bonding zone（IBZ）and the diffusion affected zone（DAZ）.The FAZ was formed by the solidification of liquid filler alloy and consisted ofγ-Ni,M₃B₂ boride,CrB,γ+γ′eutectic,ternary eutectic phase of（γ-Ni+Ni₃B+CrB）as well as a small amount of（Ni,Nb,Co）₃Si phase at the center of the joint.The IBZ was originated from the isothermal solidification during the holding time,which was mainly composed ofγ-Ni.The DAZ was the substrate close to the interface bonding zone which was controlled by the element diffusion behavior,and it consisted ofγmatrix and uniformly distributedγ′phase.The microstructure evolution,element distribution as well as orientation characteristic of the joint brazed by single crystal superalloy CMSX-4 with different orientation deviations were investigated.It was observed that the microstructure of the joints with orientation deviations of 0°+45°and 0°+90°was similar to that of the joints without orientation deviation,while no significant changes related to the phase composition was found.The melting point depressant element B had not significantly diffuse to the substrate on both sides but mainly concentrated in the center of the joint,and would form stable boride with Cr,Mo,W,etc.The melting point depressant element Si was uniformly distributed in the joint with only a small proportion diffusing to the substrate on both sides.After heat treatment,a homogeneous diffusion process had taken place in the brazing joints,while the content of Cr and Co are higher than that of Al,Mo and W in the filler alloy zone of the joint.Given that there exists no orientation deviations between the single crystal substrates,the brazed joint can also exhibit a single crystal feature.However,when there exists some orientation deviations in the substrate,a relatively continuous grain boundary can be formed in the center of the joint,and the crystal orientations on both sides of the grain boundary are consistent with that of the adja cent substrate.The tensile properties at room temperature and high temperature of three different orientation deviation brazed joints were tested.The failure fractures were observed and the fracture mechanisms were analyzed.The results showed the orientation deviation has no significant effect on the tensile strength of the joints at room temperature and high temperature,while the average tensile strengths of these three single crystal brazed joints with different orientation deviations were similar.Compared with the brazed joints without post-weld heat treatment,the tensile strengths of these joints without orientation difference after post-weld heat treatment increased at room temperature and high temperature.However,the joints with orientation deviation demonstrated a higher strength at room temperature and the high temperature strength decreased.The observation of fracture morphology showed that the fracture positions of the joints with three different orientation deviation were all in the filler alloy zone.For the brazed joint without post-weld heat treatments,the cracks mainly occurred at the position of hard brittle phases in the joints at room temperature condition,and they could further propagate and give rise to the final fracture.At elevated temperature,however,the cracks were originated from the regions where the eutectic phase with lower melting point remelted or became softened.Moreover,for the brazed joints after post-weld heat treatments,no typical grain boundary with poor adhesive strength was formed in the joints without orientation deviation.The cracks initiated at the areas where the hard bri ttle phases had not been completely eliminated,resulting in the final fracture of the specimens.Nevertheless,for the joints without orientation differences,the specimen fractured at the continuous grain boundaries with poor adhesive strength.