Study On Brazing Filler Metal For The Free-Flux Brazing Process

As an important method in the processing of aluminum alloy,brazing plays an important role in the manufacture of products.with the development and progress of the society,the demand of green flux-free brazing technology is continuing to increase.In order to adapt to this trend,the new materials are emerging for the free-flux brazing process.These new materials have characteristics of removing oxide film and improving the strength of brazing joint.Based on the point,Cu,Ni and Mg elements are added into the brazing filler metal on the base of Al-10Si alloy,because the oxide film is removed by high vapor pressure of Mg element,the brazing temperature is decreased and the strength of brazing joint is increased by Cu element and the duffusion of Cu element into the matix is restrained and the corrosion resistance is increased by Ni telement.By the combined method of thermodynamic calculation and experimental research,the effects of alloying elements on the microstructure,melting point,wettability,corrosion resistance and the strength of the brazing joints are studied to opitimize the content of Cu,Ni and Mg in this paper.The results are as follows:(1)The liquidus temperature of alloys decreases with the increasing of Cu,Ni and Mg element content,and the effect of Cu element content on it is more significant and it can be reached to 553℃.(2)Non-equilibrium solidification structure of the alloys is mainly composed of alpha(Al)phase,Al-Si eutectic,Al3Ni phase,Al3Ni2 phase,Al7Cu4Ni and Al5Cu2MgsSi6 phase.However,when the content of Cu is 10wt%and 15wt%,the brittle phase Al2Cu will be produced in the matrix,which seriously affects the quality of the brazing joint and affects its fluidity.Moreover,the phase is easy to cause cracks during the solidification of the alloys.Therefore,it is appropriate that the content of Cu element is 5wt%for the brazing filler metal.(3)With the increasing of the cooling speed,the secondary dendrite arm spacing of alpha(Al)phase and the size of the Al-Si eutectic decreases remarkably;In the condition of the simulation of brazing(the cooling rate is about 10℃/s),the solidification structures of Al-10Si-5Cu-Ni-0.5Mg,Al-10Si-5Cu-Ni-Mg,Al-10Si-5Cu-Ni-1.5Mg,Al-10Si-5Cu-2Ni-1.5M g,Al-10Si-5Cu-3Ni-0.5Mg and Al-10Si-5Cu-3Ni-1.5Mg alloys does not include the primary Siphase so they can be used as filler metals;(4)Al-10Si-5Cu-2Ni-1.5Mg alloy has the lowest melting point which is 543.7℃,the most narrow melting range which is 38.4℃,and its wettability is strongest and the spreading area is 573.4mm2,and its liquidity is highest and length of its filling area is 15.4mm,and it has the strongest corrosion resistance and its corrosion potential is-198mV,and the strength of brazing joint is highest and its average shear strength of 60.2N/mm2;(5)Al-10Si-5Cu-2Ni-1.5Mg alloy has some advantages of low melting point,strong wettability,high liquidity,strong corrosion resistance and high strength of brazing joint;its weld structure is mainly composed of alpha(Al)phase,Al-Si eutectic,Al3Ni2 phase,Al7Cu4Ni phase and Al5Cu2Mg8Si6 phase,and its microstructure is fine and homogeneous and the secondary dendrite arm spacing can be reached to 23.2 μm;Therefore,it is suitable for the free-flux brazing process of aluminum alloy.