| Mar-M247 cast nickel-base superalloy has been widely used in aero-engine and heavy gas turbine hot-end parts(nozzle blade or moving blade)because of its excellent high temperature strength and microstructure stability.In the process of manufacturing and repairing the hot-end parts,it is often necessary to carry out the joining process.However,because of the high content of Al and Ti in Mar-M247,the welding crack sensitivity is very strong,so it is not suitable to adopt the welding technology.Transient liquid phase bonding(TLP)not only combines the advantages of high strength of solid phase diffusion bonded joints and pract practicable of brazing process,but also avoids the disadvantages of heat affected zone sensitivity and microstructure inhomogeneity of weld joint.TLP is generally considered as the best method for joining precipitation hardened nickel-based superalloys.MBF-80,a Ni-Cr-B amorphous foil,is widely used for TLP bonding superalloy as interlay.Firstly,the effect of process parameters on the microstructure and mechanical properties of Mar-M247 joints joined by MBF-80 interlayer alloy was studied.The microstructure of the joint was characterized by scanning electron microscopy(SEM)SEM and transmission electron microscope(TEM).The morphology,crystal structure and formation mechanism of precipitated phases in the diffusion zone(DAZ)were studied.Tensile tests at room temperature and high temperature and high temperature creep rupture tests of joints were carried out.The diffusion behavior of refractory alloy elements such as W and Co has a great influence on the high temperature mechanical properties of the joints.It is concluded that the composition design of the interlayer and its matching with the substrate alloy play a key role in the microstructure and mechanical properties of the joints.In view of the important role of interlayer alloys in TLP bonding process,a new multicomponent interlayer alloy has been developed.Firstly,by means of Thermo-Calc and DICTRA software,three kinds of interlayer alloys were screened out according to their amorphous ability,melting point and high temperature mechanical properties of the joints by these interlayers.The results of trial-manufacture,characterization and bonding tests show that DFB-1(Ni-7W-5Co-5Cr-2Ta-B)and DFB-10(Ni-10W-2Cr-2Ta-B)interlayer alloys can achieve the desired effect and can achieve high strength TLP joint of Mar-M247.The microstructure and mechanical properties of TLP joints made of two different compositions of self-made interlayer foil strips were tested and analyzed.The diffusion behavior of various alloy elements during TLP process were analyzed by means of electron probe analysis(EMPA).And the influence of diffusion behavior on the mechanical properties of joints was discussed.By comparing with commercial interlayer alloy MBF-80,it is concluded that the high W and Co content in the initial composition of interlayer alloy can effectively improve the creep rupture strength of joint.In addition,the W-rich phase precipitated from the center line of the bonding zone by the self-made interlayer can be eliminated by reasonable post-bonding heat treatment(PBHT).TLP bonding process at 1230 C/6H and PBHT treatment at 1230 C/24H-1250 C/4H,AC + 1080 C/4H and AC + 870 C/20 H were adopted,the high temperature creep rupture strength of TLP joints with DFB-1,DFB-10 and MBF-80 as interlayers have been greatly improved.Especially,the rupture time of DFB-10 interlayer joints with W content up to 10wt% is similar to that of the base metal Mar-M247.TLP bonding is a process of the alloy elements interdiffusion between the intermediate layer and the base metal,and post-bonding heat treatment is also a process of redistribution of alloy elements.In this paper,the method of evaluating the long-term microstructure stability of the alloy at high temperature according to the electron vacancy is adopted,and the microstructure stability of the joint is evaluated according to the measured results of chemical composition in the joint area obtained by EMPA.The evaluation results show that the TLP joints with homogeneous microstructure obtained by using MBF-80,DFB-1 and DFB-10 interlayer alloys have a low possibility of precipitating harmful phases and accelerating the performance degradation during long-term high temperature service.In this paper,the effects of composition of interlay,the commercial ternary Ni-Cr-B interlayer and self-made multicomponent interlayer,on the microstructure and properties of Mar-M247 TLP joint were systematically analyzed by means of element distribution,precipitation phase characterization and mechanical properties tests.The core of the TLP process is the diffusion and homogenization of elements,However,most of the previous studies focused on the diffusion law of B element and isothermal solidification mechanism,while the diffusion behavior of various alloying elements which determine the high-temperature strength of joint was not systematically studied.In this paper,the morphology,crystal structure and precipitation behavior of many kinds of boride precipitates in diffusion-affected zone are studied on the basis of comparative analysis of the compositional homogeneity differences of jonts bonded by three typical interlayer alloys with different initial compositions.The precipitation mechanism of various kinds of borides is revealed,and the control measures of precipitates are proposed.The high strength joining of Mar-M247 was obtained,and the high temperature strength of TLP joint has effectively improved.Two amorphous ribbon interlayer alloys DFB-1 and DFB-10 containing Co and W elements were successfully designed and manufactured according to the diffusion behavior of the elements in TLP process.The results show that the bonding effect of the developed interlayer alloy is better than that of the foreign similar products.In addition,the influence of Hf and W elements on isothermal solidification behavior was first discovered,and the TLP process idea of accelerating isothermal solidification rate by precipitation phase and eliminating precipitation phase to realize homogenization at specific homogenization temperature was put forward. |
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