Interface Bongding Mechanism And Brazing Process Of MnCuAl Damping Alloy With 430 Stainless Steel

Mn Cu alloy has high damping property,excellent mechanical properties such as strength and plasticity,as well as good processing property.The corrosion resistance of Mn Cu alloy can be improved by adding appropriate amount of elemental Al.Therefore,MnCuAl alloy has a great development prospect in vibration and noise reduction materials in the fields of ship,rail transit,instrument manufacturing and so on.The high-performance bonding between high damping MnCuAl alloy and 430 stainless steel can make the two materials complement each other,which is of great significance to broaden the engineering application of MnCuAl alloy.Induction brazing is an effective way to join dissimilar metals,however,it is difficult to improve the mechanical properties of brazed joints because of the diversity of dissimilar metals.As the key factor of brazing,the melting characteristic and the wettability of the filler on base metals directly affect the metallurgical bonding strength of the brazed joint.The intermetallic compound（IMC）will make the joint structure uneven and then reduce the shear strength of the joint.Aimed at the above problems,related researches are carried out in this paper,which provide practical and theoretical references for the high-performance bonding between MnCuAl alloy and 430SS.The main contents and results are as follows:（1）The fillers with different Sn contents were designed and developed,and the effect and mechanism of Sn content on microstructures,mechanical properties and melting characteristics of Cu-34Mn-6Ni filler were investigated.The results show that,the saturated solid solubility of Sn in Cu-34Mn-6Ni alloy is about 6 wt.%at 720℃and holding for 10h.When the content of Sn exceeds 6 wt.%（including）,the Sn-rich BCC Cu₁₇Sn₃ phases form between the matrix phaseγ-（Cu,Mn）.The amount of this Sn-rich phases gradually increases with the increasing Sn content,and its proportion in the 10Sn filler will be higher than the matrix.The addition of Sn causes lattice distortion of the matrix phase.The lattice constant of the matrix gradually increases with the increasing Sn content,and the maximum swelling rate reaches 0.7253%.When the Sn element dissolves in the matrix,the tensile strength and plasticity of the filler are almost unaffected.However,the formation of Sn-rich phase promotes the tensile strength and lowers the plasticity of the filler significantly.With the increase of Sn content,the liquid point of filler decreases significantly.The liquid point of the Sn-rich phase is lower than that of the matrix phase,which makes the melting interval of the filler wider.The melting interval of the filler metal is the largest when the content of Sn is 6 wt.%,and then the width becomes smaller with the further increase of Sn content.（2）Through the wetting test of filler on 430SS surface,the spreading characteristic was analyzed.The elemental Mn permeates the oxide film with the help of brazing flux,and then the Mn rich（in 0Sn filler）or Sn rich phases melt,spread,then form precursor film.Finally,the matrix part of filler spreads.The wetting temperature is 50℃higher than the liquid point of the filler.The surface tension of the Sn-rich phase is obviously reduced because the liquid point of the Sn-rich phases is lower than that of the filler.Therefore,the Sn-rich phase promotes the spread of the filler on the base metal surface.The interfacial wetting mechanism of the filler on 430SS is due to element diffusion.The diffusion of Mn and Ni from the filler towards the base metal results in a phase transition in the interfacial layer,which changes from the BCCα-（Fe,Cr）phases to the FCCγ-（Fe,Mn）phases.Part ofγ-（Fe,Mn）particles break away from the interface to the filler metal.At the same time,intermetallic compound（Fe,Mn）₂B was formed during wetting.The phase transition,grain shedding,and IMC formation will increase with the temperature increasing,which makes the decrease of wetting angle slow down when the filler spreads at higher temperature.（3）The spreading behavior and wetting mechanism of the filler on MnCuAl damping alloy were investigated.Because the main components of the filler are similar to that of the base metal,the spread performance of the filler on the surface of MnCuAl alloy is excellent,the spread area is large,and the wetting Angle is close to 0°.When the filler is metallurgically combined with MnCuAl alloy,the phenomena of grain boundary penetration and co-crystallization appear.The elemental Sn diffuses into the base metal and concentrates at the grain boundary.The grain boundary will be liquefied when the concentration of Sn reaches to4 wt.%,that is,the grain boundary penetration occurs.Then the base metal grains are partially liquefied and inter-soluble with the filler metal.Finally,during cooling and crystallization,the molten liquid grows epitaxially based on the semi-liquefied base metal grains to form co-crystallization.（4）Furthermore,430SS and MnCuAl alloy were induction brazed.The microstructure and microhardness of the joints were observed and measured,the effects of temperature and Sn content on microstructure and hardness were analyzed.The analysis shows that:the brazing joint consists of seven zones,430SS,the diffusion-solution layer between 430SS and filler,（Fe,Mn）₂B compound layer at the side of 430SS,the center of brazing seam,（Fe,Mn）₂B compound layer at the side of MnCuAl alloy,the diffusion-solution layer between MnCuAl alloy and filler,and MnCuAl alloy are in order.At the same brazing temperature（880℃）,the Sn content does not change the characteristic of the joint,but the width of the brazing seam,the morphology and width of the incomplete wetting zone,and the hardness of the brazing seam center are changed.The width of the incomplete wetting zone using 6Sn,8Sn and 10Sn are 0μm,200μm and 460μm,respectively.The widths of brazing seams using 6Sn and 8Sn fillers are almost the same,while the width of brazing seam using 10Sn filler decreases obviously.The average hardness in the seam center increases gradually from 6Sn to 10Sn joint,which is consistent with the trend of the filler metal itself.（5）The shear strength of joints was tested,and the fracture positions and morphologies were observed to analyze the joint performance and weak area.The results show that the strength of the joints of 6Sn,8Sn and 10Sn fillers increases with the increasing temperature.At 880℃,the shear strength is 320 MPa,307 MPa and 293 MPa,respectively.The shear cracks penetrate the IMC layer on the MnCuAl side and the center of the brazing joint.It can be seen that the IMC layer is the main reason for the decrease of interfacial bonding performance in brazed joints.The formation mechanism of（Fe,Mn）₂B is as follows:reductive elements,such as Al and Si from the substrates,can reduce B₂O₃ to B at the brazing temperature.Elemental B then reacts withγ-（Fe,Mn）falling off from the 430SS side interface into the brazing seam to form compounds.（6）Electroplating Ni layer on 430SS surface can inhibit the formation of IMC in the joint,and thus improve the joint strength to a large extent.Brazed with 8Sn filler at 860℃,the shear strength of the joint without Ni plating is 242 MPa.When the thickness of Ni plating is 8μm and 20μm,the shear strength of the joint is 312 MPa and 315 MPa,which is 1.29 times and1.30 times of that without plating,respectively.（7）When the thickness of the plating is 8μm,the Ni plating completely dissolves into the filler,promoting the Fe element inγ-（Fe,Mn）phase diffusing into the seam,so that theγ-（Fe,Mn）phase is dissolved and then eliminated in the brazing seam.At the same time,the Ni plating promotes the wetability of the filler on the 430SS surface,enhances the fluidity of the filler,so as to promote the discharge of the brazing flux and its reaction products.Therefore,no B₂O₃was found to interact with the alloying elements to generate B atoms.The above factors result in the inhibition of IMC in the joint.Because the Ni layer with the thickness of20μm blocks the diffusion of Mn,theγ-（Fe,Mn）phase and IMC are not observed in the joint of 430SS and MnCuAl alloys.