Research On Compaction Mechanism Of Multicomponent Mixed Metal Powders Based On Ag Based Filler Metals

Due to the strong electrical conductivity and thermal conductivity,good corrosion resistance and brazing wettability,silver(Ag)based filler metals with melting temperature of 400?600? have been widely used as best candidates for joining ferrous and non-ferrous materials in aerospace,military industry,power electronics and home appliances and other industries.Usually,high proportions of Zn,Sn,In,Ga and other rare earth elements are added into the cadmium-free silver based filler metals,which result in high brittleness and poor plastic processing performance of the alloys,and the cadmium-free silver based filler metals are too difficult to be made into thin sheets after melting.While the powder metallurgy method that combines the compaction forming and sintering is effective in avoiding this processing problem.Using the powder metallurgy method to fabricate the silver based filler metal thin sheets,the properties of final product significantly depend on the quality of green compact.And the green compact quality is determined by the compaction densification behaviors of mixed metal powders during the compaction process.Because the compaction mechanism of the mixed powder is much more complicated than that of the pure powder compaction,the research on the theory of powder compaction forming is mostly limited to the pure powder.On the other hand,various industrial processes in practical production involve the compaction of mixed metal powders,such as metal ceramic composites,particle reinforced composites based on ceramics,metals and polymers,and pharmaceutical tablet production.Generally,these mixed powders are mixtures of particles with different mechanical features and properties,and then undergo the powder compaction and sintering processes to produce the products with the desired properties and sizes.Therefore,it is of great practical value and theoretical significance to investigate the compaction behaviors and densification mechanisms of the multicomponent mixed metal powders and then to optimize the compaction process,especially for enriching powder metallurgy theory and improving product performance and quality.In this work,four kinds of multicomponent mixed metal powders were used to fabricate thin sheets of the silver based filler metals by the powder metallurgy forming method,which include the Ag50Cu50 binary mixed powder,Ag60Cu30Sn10,Ag35Cu32Zn33 ternary mixed powder and Ag57.6Cu22.4Sn10In10 quaternary mixed powder.The soil plasticity mechanical constitutive equation Drucker-Prager Cap(DPC)model was introduced to model the compaction of multicomponent mixed metal powders,and the constitutive equation parameters were measured by designing appropriate test methods and devices based on the equivalent density method.The established constitutive model was verified by finite element modeling with a user subroutine,and the deformation behaviors and densification mechanisms of multicomponent mixed metal powders during the compaction process were analyzed.Meanwhile,the influence mechanism of the friction on powder compaction behavior was analyzed with the simulation,and a new comprehensive compaction equation theory considering the friction influence was established.The main contents and results of this paper are as follows:An instrumented die compaction test system of mixed metal powder was designed and assembled,an indirect method for measuring the radial stress between the powder and die during the compaction process is presented.Using the thin-wall cylinder theory combined with the finite element simulation,the radial stress between the powder and die can be indirectly determined according to the circumferential strain measured in the powder uniaxial die compaction test.In order to ensure the accuracy of measurement data,an accurate measurement method considering the influence of deformation of upper and lower punch,pressure sensor and other experimental devices in the test system on the measurement results of powder displacement was proposed.The density-dependent modified DPC models of four kinds of multicomponent mixed metal powders were established by a series of uniaxial compression,diametrical compression tests of powder compact and powder die compaction test,and the material model parameters are functions of the relative density(RD).A linear elasticity law was used to determine the elastic parameters of the mixed metal powders,the Elastic modulus E and poisson's ratio ? both increased with the increase of relative density,and the functions of powder elasticity parameters and relative density are established.The function relations between the relative density and vickers micro-hardness of four kinds of mixed metal powder compact were established by using the vickers micro-hardness test.The vickers micro-hardness of powder compact increases with the increase of relative density,and the relative density distribution in the powder compact was measured with the hardness test.The finite element simulation of compaction process of the multicomponent mixed metal powders was performed in ABAQUS software by using the USDFLD user subroutine,and the simulation results were compared with the experimental results to verify the established Drucker-Prager Cap material model.The theoretical and experimental analysis results show that,based on the equivalent density method,it is feasible and applicable to treat the multicomponent mixed metal powder as a pure powder with the equivalent density and introduce the soil plastic mechanics Drucker-Prager Cap constitutive model to model and analyze the mixed powder compaction densification behavior.The influence of friction on the compaction densification mechanisms of the four multicomponent mixed metal powders was analyzed by the finite element modeling of the powder die compaction.(1)The friction between the powder and die leads to the density gradient distribution inside the powder compact,the relationship functions between the relative density deviation rate and the relative density and friction coefficient is established.(2)Due to the effect of friction in the powder compaction,the maximum stress appears in the top area of powder compact close to the die wall,while the minimum stress appears in the bottom area.It leads to convective powder flows along the radial direction in the powder compact,which induces the relative density close to the die wall is relative high at the top part and the relative density is relative low at the bottom part in compact.(3)The friction action results in the axial deformation displacement gradient in the powder compact along the height and radial direction,and close to the die,the axial deformation of powder decreases.(4)After unloading,the friction induces non-uniform residual elasticity distribution in powder compact,which causes intensive stress bands in powder compact,and could induce cracks and capping in powder compact during the ejection.The compaction mechanisms of the four multicomponent mixed metal powders under different friction conditions were analyzed by using several linear and nonlinear compaction equation theories.(1)The fitting precision of Paelli and Balshin equations is higher in linear compaction equation,and the fitting precision of Gerdemann-Jablonski equation is the highest in nonlinear compaction equation,and the new comprehensive compaction equation theory considering the friction influence was established.(2)The densification process of mixed powders is mainly achieved through the plastic deformation mechanism of powder particles.With the increase of friction coefficient,the contribution of the powder particle rearrangement mechanism increases,while the contribution of the powder particle deformation mechanism decreases,and the loading period of the powder particle rearrangement and deformation mechanism increased.