Effects Of Microalloying On Microstructure And Properties Of Low Silver & Cd-free Ag-Cu-Zn Filler Metal

Ag-Cu-Zn brazing filler metal has been widely used in household appliances,automobiles,motors,refrigeration and instrumentation.Based on the requirement of environmental protection,many countries have issued the ban on the use of Cd-containing Ag-Cu-Zn brazing filler metal in electronic devices.At the same time,the rising price of precious metal Ag increases the production cost of Ag-containing filler metal highly.Therefore,it is of great significance to develop an environmentally friendly filler metal with low cost and high performance.In this paper,microalloying effects on microstructure and properties of low silver Ag-Cu-Zn filler metal were studied and a novel brazing filler metal 21Ag-Sn-P-In was developed that was expected to substitude 26B-Sn-In commercial production with 5wt.% Ag content reduction.The research contents are divided into three main parts.In the first part,the comprehensive influence of Sn,P,Ni elements on the melting characteristics and strength as well as hardness properties of the alloy were studied.Taking the melting characteristics of commercial 26B-Sn-In as the targets,21Ag-Sn-P filler metal was developed by alloying Sn,P,Ni elements.Experimental results showed that 21Ag-Sn-P sample was superior to 26B-Sn-In in melting temperature,wetting property and tensile strength properties,but inferior in deformation ability due to its higher hardness which led to the failure of conventional drawing process.In the second part,the reason why 21Ag-Sn-P alloy was hard to be continuously deformed during drawing process was discussed by comparing the metallographic differences between 21Ag-Sn-P metal and 26B-Sn-In commercial brazing filler metal,Results showed that Sn addition led to the increase of Sn content in intra-granular and inter-granular phases.Due to the precipitating strengthen of Sn,the deformation property of brazing filler metal was deteriorated.However,the metallographic characteristic,with boundary phase structure,was maintained,just as that of the ternary brazing filler metal.It was noteworthy that small amounts of P element significantly changed the microstructure characteristic of Ag-Cu-Zn brazing filler metal.Samples with P content over 0.5 wt% have no continuous and resilient inter-granular phase.Therefore,it is prone to harden quickly during the drawing process.Based on the above results,the lower alloying contents and other additives such as Ni and In elements are suggested for(Sn+P)combined alloying formula to balance the brazing temperature and forming ability.In the third part,a novel brazing filler metal 21Ag-Sn-P-In with both satisfactory melting properties and excellent deformation performance was obtained by using the recommended formula in part two,which was also applicable to online trial production.Experimental results showed that 21Ag-Sn-P-In brazing filler metal had the similar melting temperature range and hardness as that of 26B-Sn-In brazing filler metal.When used in the brazing connection of copper,brass and carbon steel,it showed the superior welding performance.The mechanical properties of three kinds of brazing joints all met the requirements of 26B-Sn-In commercial metal with joint strength even byond the tensile strength of three base metals.In summary,21Ag-Sn-P-In alloy,as the cadmium-free and low-silver brazing filler metal,is a promising alternative of 26B-Sn-In commercial metal with good economy and practicability.