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Effect Of Ga And Ce On The Microstructures And Properties Of Ag17CuZnSn Filler Metals

Posted on:2018-05-10Degree:DoctorType:Dissertation
Country:ChinaCandidate:C L MaFull Text:PDF
GTID:1361330596950669Subject:Materials Processing Engineering
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Silver brazing filler metals are well known as one of the most suitable brazing materials for joining metals and alloys due to their high strength,advantages of electrical conductivity,thermal donductivity and oxidation resistance.In the traditional silver brazing filler metals,the Cd-contained silver brazing filler metals have been widely used in industries.However,owing to the inherent toxicity of cadmium,the Cd-contained silver brazing filler metals are restricted.Moreover,the high price of Ag also limits the further application of silver brazing filler metals.So reducing the Ag content without sacrificing the properties of silver brazing filler metals has been the topic of interest for many investigators.In this research,novel Ag17CuZnSn-xGa-yCe filler metals were first developed and used for brazing H62 brass and 304 stainless steel.The effect of Ga and Ce addition on the properties and microstructures of Ag17CuZnSn brazing filler metals and brass/stainless steel were studied in the dissertation.The effect mechanisms of Ga and Ce were discussed in this study.The differential scanning calorimetry test indicated that the solidus and liquidus temperature of Ag17CuZnSn brazing filler metals decreased with addition of Ga and Ce.When the content of Ga is 6wt.%,the solidus and liquidus temperature of filler metals are 721?and 791?,respectively.And the antioxidant capacity of Ag17CuZnSn brazing filler metals improved significantly with 2 wt.%Ga and 0.15 wt.%Ce addition.During brazing process,Ce tends to assemble on the surface of silver brazing filler metal,which hinders the oxidation of low silver filler metals.Spreading test revealed that there was a wetting ring on the front edge of the melting silver brazing alloys.The wetting ring consisted of white phase?silver-riched phase?,gray phase?copper-riched phase?and dark phase?stainless steel phase?.It is suggested that the wetting ring is helpful to the wetting process between the brazing filler alloys and base metal.The microstructure analysis showed that the addition of Ga and Ce into the Ag17CuZnSn brazing filler metals significantly improves its microstructure.The quantity and size of?CuZn compounds phase decreased obviously with the addition of 2 wt.%Ga.Ga prefer to form solid solution with Cu according to the atomic radius of Cu,Ag and Ga.The liquidus temperature of silver brazing filler metals decrease with addition of Ga.With the degree of subcooling increasing,more nuclei are formed in the liquid alloys,which lead the microstructure to be refined.And during the solidification process of the filler metal,as the temperature was decreased,Ce prefer to segregate along the solid-liquid interface and concentration gradient of Ce in the liquid alloy increased.As undercooling increased,the crystal growth of?-CuZn compounds phase changed from bulk dendrite to cellular dendrite.Ce combined Ag,Cu and Sn to form Ce20?Ag,Cu?40Sn40 compounds in the Ag17CuZnSn-0.5Ce filler metals.The anticorrosion ability of silver filler metal is improved with increasing Ce content.H62 Brass/304 Stainless Steel brazed joints showed higher shear strength and refined microstructure with appropriate Ga and Ce addition.The shear strength of Ag17CuZnSn2Ga and Ag17CuZnSn0.15Ce were 367.5 MPa and 341.5 MPa,which inproved 38.6%and 28.9%compared with the joint brazed with Ag17CuZnSn filler metal.The microhardness of brazing seam increase obviously with Ga and Ce addition in filler metals.There is a hardness increase in the interface between brazing seam and 304 stainless steel when Ag17CuZnSn-xCe was employed for brazing.During brazing process,a white layer was observed at the interface between brass and filler metal,which appeared to be a CuZn compounds phase.The EDS results indicated that there are obviously elements diffusion between brazing filler metals and stainless steel.And Ga has higher diffusion coefficient into the stainless steel than that of Cu,Zn and Ce.The content of Fe in the Ag-riched phase is higher than that of in Cu-riched phase near the interface between brazing filler metals and stainless steel.Most of the fracture occurred in the interface layer between the brazing seam and 304 stainless steel.With the appropriate addition of Ga and Ce in filler metals,the fracture dimples become narrower.The solidus and liquidus temperatures of Ag17CuZnSn decrease with the composite addition of Ga and Ce into filler metals.And the improvement of composite addition of Ga and Ce for the wettability of silver filler metals is better than addition of Ce.There were no big bulk?-CuZn compound phase found when the content of Ce in 0.1-0.5 wt.%range.However,some bright rare-earth compounds was found when the content of Ga and Ce is 6 wt.%and 0.5 wt.%,respectively.The shear strength results showed that the maximum strength was up to 376.8 MPa when the Ag17CuZnSn-2Ga-0.15Ce was employed.And the interdiffusion of the Ga and Ce at the interface between filler metals and 304 stainless steel during brazing process.The 200-400 nm reaction layer between 304 stainless steel and silver brazing filler metal was found at the interface which was Fe-Cu-Zn compounds.This reaction layer will act as the diffusion barrier for the Fe atoms into the brazing seam.Grain boundary penetration behavior between silver brazing filler metals and stainless steel occurs in the brazing process.Accelerated aging test was carried out at 150?,the mechanical properties and microstructure of the brass/stainless steel at the isothermal aging were studied in this research.The shear strength of brass/stainless steel brazed joints decreased with the increasing of aging time.The mechanical properties test results showed that the addition of Ga and Ce in the Ag17CuZnSn brazing filler metals enhanced the shear strength of the brazed joint.After brazing and aging at 150?.The average shear strength of Ag17CuZnSn2Ga0.15Ce brazed joint improved by 44%compared to the BAg17CuZnSn after aging for 600 h.The depth of grain boundary penetration in stainless steel increased as the aging time,which deteriorate the mechincal property of brazed joint.The addition of Ce restrains the groth rate of grain boundary penetration during aging process.
Keywords/Search Tags:Cadmium-free low silver brazing filler metals, Brass/Stainless steel brazing, Melting properties, Microstructure, Intermatallic compounds, Mechanical properties
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