Study On TLP Joining Process And Mechanism Of Directionally Solidified Cobalt-base Superalloy DZ40M

Aero turbine blades are usually made of high-strength,high-temperature nickel-based or cobalt-based superalloys containing many refractory and precious metal elements,which are expensive.This topic is based on the background of blade repair,using method of Transient Liquit Phase?TLP?to connect DZ40 M directional solidification superalloy.The basic process of TLP is to use a thin intermediate alloy with a lower melting temperature as the connecting alloy,place it between the welding surfaces,apply little pressure or no pressure,and heat under vacuum,because the melting point of the intermediate layer is reached or It is due to the mutual diffusion of the intermediate layer and the base metal to form a eutectic reaction product,resulting in the formation of a low-melting liquid-phase alloy forming a thin liquid phase intermediate layer.The liquid intermediate alloy wets the base material between the welding surfaces.A uniform liquid film is formed.After a certain holding time,diffusion occurs between the intermediate layer alloy and the base metal,especially through the melting point element continuously diffused into the base metal,so that the melting point of the residual liquid phase continuously rises,thereby solidifying under the isothermal condition,and finally obtaining a dense Connector.The Al/Co/Al sandwich foil was chosen as the intermediate layer for the TLP connection.The main elements of DZ40 M,Co,Cr,Ni,and W,diffused into the intermediate layer and metallurgically reacted.The melting point of the reaction product was higher than the melting point of Al itself.Interconnection between DZ40 M base metals at a temperature slightly above the melting point of Al.There are many kinds of elements in DZ40 M,and the direct welding system is complicated.In this thesis,the interfacial reaction between Al-DZ40 M and Al-Co system was studied firstly.The study of the two methods provided guidance for the process and microstructure analysis of the final Al/Co/Al composite interlayer TLP connected to DZ40 M.Aiming at the interfacial reaction of Al-DZ40 M system under different process parameters,the results show that under the parameters of welding temperature T=820?and holding time t=30min,Wetting Promotion Area?WPA?begins to appear in the interface.The presence of WPA can effectively promote the further wetting and spreading of Al on DZ40 M.The main phase composition of WPA is?Ni,Co?₃Al₄ single crystal,and on the other hand,the matrix phase?Ni,Co?₃Al₄ also has nano-scale dispersion.Slender strip-like Cr-based solid solution?phase,wherein the main solution element in the?phase is W.The analysis shows that the growth of WPA is mainly related to the diffusion of Ni element.The growth rate is positively correlated with the temperature and the time has little effect on it.In the case of WPA in the interface,with the increase of temperature,Al fully spreads into a thin layer on the DZ40 M,but the further increase of the temperature leads to a large heat input,and the phase change of the base metal affects the mechanical properties;at the interface In the absence of WPA,the macroscopically good wetting of Al and DZ40 M was achieved by prolonging the holding time,and the interface reaction layer was mainly composed of Al-Co compounds without defects such as cracks.Under the parameters of welding temperature T=940?and holding time t=30min,the microstructure of the Al-Co system is divided into three layers:pure Co in the base metal zone,intermediate solid solution in the AlCo phase near the pure Co zone,and on the other side.Al₅Co₂ phase solid solution,both of which can solidly dissolve certain Al and Co,have good ductility and toughness,and are good for joint mechanical properties.Tests at temperatures T>940?and t=30 min show that the heat input is high when the welding temperature is too high,and the reaction layer appears off and the interface appears due to the large linear expansion coefficient between Al and Co.A large number of cracking phenomena;these defects are well avoided by controlling the reaction temperature between 680?and 720?.Al/Co/Al composite interlayer TLP was used to connect the directionally solidified Co-base superalloy DZ40 M.The process parameters were obtained under pure Al foil thickness 50?m,pure Co foil thickness 50?m,welding temperature T=700?,holding time t=30min.The best joints.The interface microscopic composition of the joint is:DZ40M/Al₉Co₂+?/Al₁₃Co₄+Al₉Co₂/Al₁₃Co₄/AlCo/Co?s,s?/AlCo/Al₁₃Co₄/Al₁₃Co₄+Al₉Co₂/Al₉Co₂+?/DZ40 M,and the shear strength at room temperature is 32 MPa.The analysis of the fracture surface shows that the fracture site is between the Co intermediate layer and the DZ40 M.There are both dimples and smooth fracture surfaces on the fracture surface.The dimple is an Al-enriched zone and the smooth fracture surface is Al₉Co₂+Al₁₃Co₄ brittle phase.Both are weak links for joints.