Research On Microstructure And Mechanical Properties Of Brazed Joint Of Mn-Cu Alloy And 430 Stainless Steel

Mn-Cu alloys have promising prospects in the field of vibration and noise reduction because of their high damping property,excellent mechanical properties and good processability.The effective connection between Mn-Cu alloys and stainless steels can further expand the application of Mn-Cu alloy.However,due to the great difference in physical properties and chemical compositions between Mn-Cu alloys and stainless steels,brittle intermetallic compounds are easily formed in common welded joints.Therefore,the mechanical properties of joints are deteriorated.In this paper,a low melting point Cu-Mn based filler metal was designed,and the wettability of the filler metal on the surface of 430 stainless steel(SS)was studied.The brazing and semi-solid brazing of Mn-Cu alloy and 430 SS were carried out in a muffle furnace,and the effects of brazing parameters on the microstructure and mechanical properties of the joints were investigated.The liquidus temperature of Cu-34Mn-6Ni-10 Sn filler metal was 819 °C.The filler metal consisted of a Sn-poor(Cu,Mn,Ni)solid solution and a Sn-rich(Cu,Mn,Ni)solid solution.The filler metal exhibited a good wettability on 430 SS.With the increase of temperature and holding time,the wetting angle of filler metal on 430 SS surface was decreased gradually,and the wetting area was increased correspondingly.In the unfixed brazed joint of Mn-Cu alloy and 430 SS,a(Fe,Mn)diffusion layer was formed between 430 SS and brazing seam,and the interface between the diffusion layer and brazing seam was easy to crack.The Sn-rich solid solution was distributed at the grain boundary of Sn-poor solid solution in the brazing seam.The dissolution of 430 SS into the brazing filler metal resulted in the formation of a large number of needle-like Mn-Cr-Cu-Fe compounds in the brazing seam near the base metals.The penetration of Sn-rich solid solution along the grain boundary of Mn-Cu alloy promoted the dissolution of alloy.The brazing seam was widened and the Mn content in the brazing seam was increased accordingly.The maximum shear strength of the unfixed joint was 212 MPa,and the fracture was occurred in the center of the brazing seam.The solid-liquid interactions between 430 SS and filler metal were depressed by the zinc coating to some extent.The contents of Mn and Cu in the diffusion layer between 430 SS and brazing seam were decreased in the brazed joint of Mn-Cu alloy and galvanized 430 SS.And there was no crack formed at the interface between diffusion layer and brazing seam.The amount of needle-like compounds in brazing seam was decreased.The Sn-rich solid solution was finely distributed in the brazing seam.The penetration depth of Sn-rich solid solution along the grain boundary of Mn-Cu alloy was decreased,therefore,the dissolution amount of the alloy was reduced.The highest shear strength of Mn-Cu and galvanized 430 SS brazed joints was 266 MPa.The joint was mainly fractured in the center of the brazing seam,and the fracture crack was propagated to the needle-like compounds distribution area near the Mn-Cu alloy.In the semi-solid brazed joints obtained at 790 °C and 800 °C,the decrease of brazing temperature depressed the solid-liquid reactions between the base metal and the filler metal.The dissolution quantities of 430 SS in filler metal were decreased,and the penetration depth of Sn-rich solid solution along the grain boundary of Mn-Cu alloy was decreased evidently.The amount of needle-like compounds in the brazing seam was decreased sharply.At 810 °C,the dissolution of 430 SS and Mn-Cu alloy led to more compounds formed in brazed joint.The shear strength of semi-solid brazed joint was about 230 MPa,and the fracture of joint was mainly occurred in the concentration area of needle-like compounds.