Research On Manufacture Of Cu-Mn-Ni-Co Filler Metal And Brazing Technology Of MGH956Superalloy

Oxide-dispersion-strengthened (ODS) MGH956is one kind of superalloy fabricated bymechanical alloying. Because the alloy exhibits excellent mechanical properties, goodoxidation and corrosion resistant at high temperature, they have been applied in the aviation,aerospace, energy resource and other fields. As a new type of structural material, it is necessaryto solve the problem of welding for expanding the application of MGH956superalloy.Compared with other welding methods, brazing can keep the organization and structure ofMGH956superalloy. So it is a more effective method to solve the problem of welding forMGH956superalloy. Currently Ni based filler metal is usually used to braze MGH956superalloy, but because of its high brazing temperature, the base metal will dissolve heavily,which weakens the performance of the brazed joint. When the welded joint of MGH956superalloy is used in the environment temperature that is not too high, using Cu based fillermetal can effectively reduce the influence of brazing tmeperature on ODS alloy. Moreover, Cubased filler metal also has the advantages of good processing performance and low price.In this paper, according to the performance requirement of the brazed joint of MGH956superalloy, five new kinds of Cu-Mn-Ni-Co filler metals were designed and developed. Theeffects of Mn and Ni contents on the microstructures and properties of the filler metals wereinvestigated. By DSC analysis and spreadability experiment, the filler metal with optimalbrazing characteristic was found and selected to braze MGH956superalloy. The effects ofbrazing temperature and holding time on the microstructures and properties of the joints wereinvestigated.The new kind of Cu-Mn-Ni-Co filler metal consists of three phases: γ-Cu based solidsolution matrix, orderly η′-Mn based solid solution second phase distributed along the grainboundary or in the interdendritic and σ-MnCo massive/acicular phase dispersed in the matrix.σ-MnCo isa hard and brittle phase and will reducethe plasticity of the filler metal. By reducingMn and Co contents in the filler metal and increasing the cooling rate in casting, σ-MnCo phasecan be inhibited from precipiting and growing up.Among the five new kinds of Cu-Mn-Ni-Co filler metals, the filler metal containing30wt.%Mn and10wt.%Ni has the optimal brazing characteristic because of the more narrow melting range and larger spreading area. The sound joints can be obtained at1000-1050℃for20min or1050℃for10-40min. The brazed joints consist of center zone of brazed seam anddiffusion reaction zones on both the sides. The center zone of brazed seam consists of Cu-Mnbased solid solution matrix,(Mn,Ni)-Si phase distributed along grain boundary and σ′phasenear the interface between the filler metal and the base metal. The diffusion reaction zoneconsists of Fe-Mn based solid solution matrix and σ-FeCr acicular phase. The (Mn,Ni)-Si phasecan be reduced and σ′phase will increase by improving the brazing temperature or extendingholding time. At the same time, the width of diffusion reaction zone will increase. During thebrazing process, the Y2O3particles precipitated from base metal and the Al2O3particles fell offfrom the surface of base metal enter into the liquid metal. The two kinds of particles will stayin the brazed seam by the subsequent solidification. In preparing sample for optical microscopyanalysis, the Y2O3and Al2O3particles tend to fall off owing to the grinding force and the weaklinkage to the matrix of brazed seam. Thus, a small amount of microvoids are obsevered in theoptical microscopic photos.With the increase of brazing temperature and holding time, The RT tensile strength ofjoint increases gradually and then decreases. The RT tensile strength can reach up to564.3MPa,about78%of the base metal. With the increase of environmental temperature, the tensilestrength of the brazed joint and MGH956superalloy decrease gradually. At500℃, the tensilestrength of the joint is428.7MPa, about95%of the base metal at the same temperature andthis shows that the brazed joint has the good performance at the using temperature of500℃.The tensile fracture of the joint occurs in the center of brazed seam, and the crack mainlyextends along the interface between the matrix phase and the brittle phase. Intergranular brittlefracture is the main fracture mode, accompanying with local transgranular fracture.