Study On Process And Mechanism Of Pressure-Assisted TLP Bonding Of DD407

Nickel-based single crystals have been widely used in the aerospace industry due to their excellent high temperature performance.In recent years,transient liquid phase?TLP?bonding has been widely used to welding nickel-based single crystals.Since the TLP bonding requires long-term holding time under high temperature conditions,it is easy to cause the transformation of the base metal structure and the inefficiency of welding.Many scholars have improved and optimized the TLP bonding process.This paper aims to improve the connection efficiency of the nickel-based single crystal by the method of pressure-assisted TLP bonding,and form an excellent joint during a short time.The BNi2 amorphous foil was used as the interlayer,and the DD407 nickel-based single crystal superalloy was bonded in three different ways.Firstly,using TLP bonding method,by studying the microstructure and properties of welds in joints under different holding time,combined with the connection mechanism of TLP,it provides ideas and references for the auxiliary role of pressure TLP in subsequent experiments.Secondly,the method of applying continuous pressure is used to assist the connection of the sample.The effects of different pressures and holding time on the microstructure and properties of the weld are investigated.The connection mechanism of continuous pressure-assisted TLP connection is analyzed.Finally,the sample is connected by instantaneous application of forging pressure.Combined with the results of the first two experimental methods,the auxiliary mechanism of the instantaneous forging pressure assisted TLP connection method is proposed,which provides an idea for the optimization of the TLP connection method.The relevant conclusions are as follows:The DD407 single crystal was connected by TLP connection.When the holding temperature was 1120°C,the tensile strength was increased by 1,15,60 and 105 min respectively.The tensile strength of the sample increased with the increase of holding time.The maximum tensile strength was 673 MPa.The Ni₃Si in the center of the weld and the Cr-rich boride disappear with the increase of the holding time.According to the tensile fracture and the evolution of the microstructure in the weld,the limitations of the TLP connection are analyzed:a shorter holding time will result in the formation of Ni₃Si in the middle layer liquid phase during the cooling process,which leads to the rapid expansion of the crack here and reduces the joint.Tensile properties.Since the time required to achieve complete isothermal solidification of the joint is related to the weld width,a continuous pressure is applied to reduce the width of joint.It was found that the tensile strength of the TLP joint can be effectively improved by continuous pressure assist.The isothermal solidification process of the middle layer liquid phase can be realized under the condition of 60 min of heat preservation,and the tensile properties of the sample are equivalent to the tensile strength of the base material.The tensile strength of the sample kept for 15 minutes was also higher than that of the TLP joint.Through the analysis of weld morphology and microstructure,it is found that the brittle phase such as Ni₃Si is irregularly distributed in the weld,and the reduction of brittle phase and irregular distribution are the reasons for the increase of joint strength.Combined with the same insulation condition,the width of the weld decreases with the increase of continuous pressure.The connection mechanism of continuous pressure-assisted TLP connection is proposed.Under the action of pressure,the liquid phase outflow gap of the intermediate layer is reduced,and the liquid phase solidification is reduced.The width of the weld formed later,while the flow of the liquid phase causes irregular growth of the gamma phase in the weld.According to the mechanism of action of the continuous pressure on the TLP connection,an instantaneous forging pressure is applied to the sample at different time points for connection.It was found that the tensile strength of the joint was higher than that of the 60 min sample under the continuous pressure for 15 min.When a forging pressure is applied at 1010°C,a large amount of boride is generated in the weld near the side of the base material,and when a forging pressure is applied at 1120°C,a weld completely in the?phase is obtained.According to this phenomenon,the connection mechanism of forging pressure assisted TLP connection is proposed.The forging pressure causes the intermediate layer liquid phase to be instantaneously extruded into the weld seam,and the residual small amount of liquid phase is in contact with the parent metal surface or the?phase layer front edge respectively,and the isothermal solidification forms the final joint.