Mechanical Alloying Synthesis And Characterization Of Ag-Cu Based Intermediate Temperature Brazing Metals

Silver-based alloys have become the most widely used brazing filler metals due to their characteristics of perfect wettability,higher strength,excellent corrosion resistant as well as the good electrical conductivity and thermal conductivity.Silver-based intermediate temperature brazing metals are developed by adding the low melting element such as Zn,Sn,In etc.to reduce the melting point of hard solder on the basis of Ag-Cu binary alloy.However,the addition of these elements tends to form the brittle intermetallic phases due to their low solid solubility in silver and copper,which is not good forthe brazing joint and makes the forming of strip or wiresolder difficult.In this paper,mechanical alloying method was adopted to prepare the non-equilibrium Ag-Cu based alloypowder withthe supersaturated solid solution ormicrocrystalline intermetallics phases in order to improve the bondingof the jointand to solve the problem of forming by mixing it into solder paste which is economical,efficient and easy to apply.The influences ofball-milling parameters onthe morphology,phase structure,melting properties and the alloying process of Ag-Cu binary powders were firstly studied.On these bases,ternary system Ag-Cu-Zn and Ag-Cu-Sn powders were prepared by mechanical alloying method,and the researches of the thermal stability of non-equilibrium ternary alloy was also carried on by characterizing their phase structure,melting properties and the microstructure before and after the heat treatment.At last,Ag-Cu-Sn as-milled powder was mixed into solder paste and the performance was evaluated by the spreading test,bonding strength as well as microstructure of joint interface.The main research contents are as follows:1.The effects of ball-milling parameters were systematic studied.The results showed that Ag-Cu solder powders could be completely alloyed and the supersaturated solid solution alloys could be obtained under the optimized ball-milling parameters:40 h milling time,20:1 ball-to-powder weight ratio,400 r/min milling speed,ethyl alcohol as PCA.For the ternary system,the formation of supersaturated solid solution was not only related with the ball-milling parameters,but also affected by the type and content of the third element.By controlling the ball-milling parameters,the metastable Ag（Cu）supersaturated solid solution as the main phase composition of Ag60Cu30Sn10 could be obtainedafter 40 h milling.2.Thermos-stability of Ag-Cu-Sn as-milled powderwas investigated.The results indicated that during vacuum annealing,Ag-rich solid solution appeared obvious crystallization while Cu-rich solid solution was easy to decompose to Cu₃Sn intermetallic compound.Meanwhile Ag-Sn compound transformedinto Ag-based solid solution and Sn-rich phase.Accordingto the transformation order of Cu₆Sn₅→Cu₃Sn→Cu₄₁Sn₁₁→Cu_5.6Sn with the annealing temperature risingand Cu/Sn ratioincreasing,Cu-Sn compounds decomposition products was a variety of mesophases.At the same time,the average size of alloy powders became very tiny due to the reconstruction of grainsresulted from the solid phase transformation.3.Solder paste was prepared with Ag60Cu30Sn10 metastable as-milled powders and the brazing performanceswere evaluated.The suitablebinder and solvent were firstly selected by sintering and dissolving experiments,and then mixed with the ball-milling powders and the brazing fluxto make solder paste.Following that,Spreading test on stainless steel substrate was carried out.The results indicated that the spreading area increased with the increasing content of Tin.For the same composition powder,40 h-milled sample showed almost the same spreading areawith that of 60 h-milled sample,but the better fluidity.The analysis of microstructure for interface between stainless steel and filler metals also supported the 40 h-milled Ag60Cu30Sn10 sample because its uniformly metallographic structure,composing with Ag-based solid solution phase,Ag-Cu eutectic structure and Cu-rich bulk phase,eliminatedthe typical dendritic eutectic phaseof 60 h-milled sample and formed2³μm continuous transition layer between the junction with the substrate,which was helpful to increase the joint strength.