Process And Mechanism For Brazing Of TiAl And Ti₂AlNb

Due to the high entropy effect and hysteresis diffusion effect,the high entropy alloy can reduce the diffusion rate of elements and effectively hinder the diffusion of elements.Therefore,in order to prevent the increase of element diffusion and the increase of brittle phase in the joint and the decrease of mechanical properties due to the temperature increase of the brazed joint of Ti Al/Ti₂Al Nb alloy,two kinds of equiatomic ratio high entropy alloy solder foils were designed and prepared to braze the joint of Ti Al/Ti₂Al Nb alloy.The interface microstructure morphology,phase distribution and element distribution of the welded joint obtained by brazing with two kinds of high entropy solders were investigated respectively.Meanwhile,the effects of welding temperature and holding time on the interface microstructure,element diffusion and mechanical properties of the welded joint were also investigated.The optimal process parameters of the two kinds of solders were given.The design and preparation process of the filler metal is as follows.Firstly,the high entropy criterion is calculated for the selected equiatomic ratio Ti Zr Hf Co Ni Cu and equiatomic ratio Ti Zr Hf Ni Cu multicomponent systems,and it is confirmed that the selected two groups of multicomponent alloys are both high entropy alloys.The solder ingots with specific shapes are then smelted using a vacuum arc smelting furnace and a suction casting system.In order to avoid dendritic segregation caused by too slow cooling rate,the inner cavity of copper mold in the suction casting system is designed as fine rod shape to increase the cooling rate.The distribution of elements at the tip of the rod-like casting is uniform,which is in line with the standard of preparing filler metal foil.Finally,the tip of the casting is made into a solder foil with a thickness of about 200?m using a wire cutting machine.The Ti Al/Ti₂Al Nb alloy was brazed with Ti Zr Hf Co Ni Cu filler metal foil.The internal interface structure of the welded joint was as follows:Ti₂Al Nb/Ti0.5Al0.12Nb0.38/?₂+Ti0.5Al0.12Nb0.38+?₄+Ni₂Al Zr+?/?₂+?-Ti Al/?-Ti Al.The micro hardness and modulus of the diffusion layer on the side of Ti Al differ greatly from those of the adjacent areas,and the stress concentration tends to occur when subjected to stress,which is the weak link of the joint.The Zr,Hf,Co,Ni and Cu elements in the weld zone effectively delay the diffusion rate of Al and Nb elements into the weld.The content of dark phase in the weld is positively correlated with the diffusion of Nb element.The effects of welding temperature and holding time on the microstructure and mechanical properties of the welded joints were investigated.Under the process condition of 1100?/15min,the shear strength of the welded joints at room temperature and high temperature(650?)reached the peak value,which were 157MPa and 123MPa,respectively.In order to improve the mechanical properties of the welded joint,the contents of Co,Ni and Cu in the filler metal are reduced.The Ti Al/Ti₂Al Nb alloy is brazed by using isoatomic Ti Zr Hf Ni Cu filler metal foil.The microstructure of the internal interface of the welded joint is the same as that of the welded joint using isoatomic Ti Zr Hf Co Ni Cu filler metal foil.The Zr,Hf,Ni and Cu elements in the weld zone effectively delay the diffusion rate of Al and Nb elements into the weld.The content of dark phase in the weld is positively correlated with the diffusion of Nb element.The influences of welding temperature and holding time on the microstructure and mechanical properties of the welded joint were investigated respectively.The shear strength of the welded joint at normal temperature increased greatly,while the shear strength at high temperature(650?)did not change much.Under the condition of 1100?/15min,the shear strength of the welded joint reaches the peak value at room temperature and high temperature,which are182MPa and 129MPa,respectively.Contrast to use Ti-Ni solder,increased by 7.06%and 46.6%respectively.