| Metallic honeycomb is named by their linear cellular structure.Metallic honeycombs not only have important metal properties such as good heat conductivity,electrical conductivity,high strength,toughness,but also have structural characteristics such as large surface area,clear cross section,light weight high stiffness,heat sink and filtration.Due to their excellent structural and functional properties,they are becoming more prevalent in a wide range of applications.Some of these applications include transportation,aerospace, environment protect,new power,mechanical chemistry,electronics and construction.The research,development and application of metal honeycombs has lead to a advanced technology,especially in lager scale manufacturing metal supports of exhaust gas catalyst.At present,for the traditional metallic honeycomb processing,a metal strip is formed on special rollers into a corrugated sheet and then coiled up,along with a flat metal sheet.This processing method is costly and complex,and the wall of metallic honeycomb is smoothly and is not good for attaching catalyst.To aim at those problems,this paper provides a new process fabricating metallic honeycombs by powder extrusion forming.The process is low cost,and metallic honeycombs would have higher properties.The design of raw powders and the process of metallic honeycombs have been studied.The main researching work and results are below:Due to their excellent oxidation resistance,FeCr and FeCrAl were chosen as researching systems in the paper.For FeCr using 410L powder;For FeCrAl using pure Fe,Cr and Al powders.Metal powders were mixed with an addition to become a paste which can be extruded to honeycomb structure by a designed die. The effect of powder morphology,composition,binder,extruding velocity and extruding ring size on formability of metal honeycomb were studied.The defects of extruded metal honeycomb were discussed.The result show:(1)smoothly surface powders,ranging in 1-40μm are best for the formability of extruded honeycomb;the more irregular the powder morphology,the worse the formability. The formability of extruding 410L is better than FeCrAl.(2)The appropriate extruding velocity is 20-30mm/min,the higher or lower is not good for the formability of extruded honeycomb.(3)The more the powder loading,the worse the paste fluidity;although the paste fluidity of low loading is better than high loading,the stability of forming is not good.For 410L extruding paste,77.5% powder loading is optimal;For FeCrAl extruding paste,65%powder loading is optimal.(4)The size of entering ring changes with the powder loading.The higher the powder loading is,the bigger the size of entering ring.Entering ring is smaller, the cover of extruded honeycomb is shortage of paste;Entering ring is bigger,the outer side flow of extruded honeycomb is faster than center part.(5)an aqueous polymer was chosen as the binder which is best for the formability of extruded honeycomb.The ratio of binder to water is maximum dissolubility.(6)The extrusion die must be designed and machined preciously.The key to forming high quality,defect free extradite lies in the optimization of paste properties and is contingent on solids loading and fluid- phase rheology.The paste properties were studied by means of rheology principles.The rheology parameters of different solids loading paste were measured and calculated by means of the capillary rheometer;the rheology equations charactering paste property were built.The maximum solids loading(φmax)of 410L and FeCrAl extruded pastes was measured,and the experimental result and theory result of suspension models were compared.The result show(1)410L and FeCrAl extruded pastes belong to shear thinning behavior,and comply with the equationτ=k(?)n,n shear thinning index and k viscosity coefficient,increasing with solids loading.(2) at same solids loading,adding coarse Cr particle into Fe powder increase slightly n but decrease k,which show the viscosity of extrude pastes can be reduced by adding appropriate coarse particle into fine powder.(3)Adding some lubricate SA into Fe-Cr-Al mixture can reduce the n and k value,and improve the fluidity.(4) The maximum solids loading(φmax)of 410L and FeCrAl extruded pastes are 78% and 67%.(5)The experimental result was compared to the rheology models.The result show:Dougherty—Krieger model is best match,KH=2.0 for 410L and KH=2.0～2.5 for Fe-Cr-Al.The defect free extradites are sintered in a high temperature atmosphere furnace. The optimal parameters of processing and condition were studied.The result show: (1)the volume shrinkage and density increase with sintering temperature and time. In order to reach same sintering effect,raising sintering temperature is more effective than longing sintering time.For 80%solid loading 410L system,at 1200℃sintering,the volume shrinkage and density of sintering honeycombs are 55% and 2.7 g/cm3,the more solid loading,the higher density.(2)For Fe-Cr-Al system, the volume shrinkage increase with sintering temperature and time,but the density reaches a maximum at 1265℃,30min or 1250℃,50min,then decrease.The maximum volume shrinkage reaches to 53%,the density 1.68 g/cm3.(3)For 410 system,At 1150℃sintering 20min is optimal;For 410 system,At 1250℃sintering 30min is optimal;(4)the sintering density of fine powder is higher than coarse powder.The sintering density of using +300mesh Cr particle in FeCrAl system is lower than -300mesh Cr.(5)The typical defects of sintering honeycombs are irregular shape,broken wall,elliptical,liquid phase dropping out,and heterogeneous.In sintering process,in order to gain the high quality honeycomb,it is necessary to control carefully the sintering temperature,time,atmosphere. The evolution and mechanism of structure of 410L and Fe-Cr-Al sintering were studied.The result show:(1)410L belongs to unit- phase sintering;the processing step is classed as the forming sintering neck,neck growing,and interstice shrinking.Surface and boundary diffusion is main sintering mechanism at low temperature;volume diffusion and plastic flow become main effect.The structure of sintering honeycombs is composed ofα-Fe and Fe(Cr)Si2.(2)FeCrAl belongs to multi- component sintering with instant liquid.The process includes liquid flow, particle rearranging,solid dissolving and precipitating,solid sintering and grain growing.Plastic flow is main effect,while with liquid;the diffusion mechanism becomes main effects without liquid.The surface diffusion makes the main effect at the beginning of sintering;the boundary and volume diffusion make the main effect after the grain connecting.The structure of sintering honeycombs is composed ofα-Fe(Cr,Al),Al2O3.With sintering time extending,α-Fe grain growing and composition unifying.The structural parameters of and properties of sintered honeycombs were obtained by measuring and calculating:(1)for 410L sintered honeycombs,wall thickness t 0.15～0.20mm,cell number n(1/in2)329～354,clear cross sectionε(%)73～79, specific surface Sv(sq m/cu dm)2.45～2.63;for Fe-Cr-Al sintered honeycombs,wall thickness t(mm)0.18～0.23mm,cell number n(1/in2)316～339,clear cross sectionε(%)70～76,specific surface Sv(sq m/cu dm)2.35～2.52.(2)for 410L sintered honeycombs,specific heat capacity Cp(J/g.K)0.5～0.56,heat conductivityκ(W/m.K) 10.20～11.35;for Fe-Cr-Al sintered honeycombs,specific heat capacity Cp(J/g.K) 0.6～0.65,heat conductivityκ(W/m.K)6.5～6.78.(3)There are high specific surface due to a lot of pore and coarse surface on sintering honeycomb walls.This is benefit to coating on sintering honeycomb walls and binding of coats and substrates.For FeCr-Al sintering honeycombs,the affinity of Al2O3 forming on sintering honeycomb walls for Al2O3 coats is nature,so coating loading is higher,and binding of coats and substrates is stronger;for 410L sintering honeycombs,there are problems on affinity and dilative difference of coats and Cr2O3 film forming on sintering honeycomb wall, so coating loading and binding of coats and substrates are low.(4)For 410L sintered honeycombs,transverse compression strength 30～40 Mpa,longitudinal compression strength 140～150 Mpa;for Fe-Cr-Al sintered honeycombs,transverse compression strength 7～8 Mpa,longitudinal compression strength 20～25 Map.The compression strength of 410L sintered honeycombs is higher than Fe-Cr-Al,but The compression strength of Fe-Cr-Al sintered honeycombs is higher than ceramic honeycombs.The strength of sintered honeycombs increases with density increasing.(5)410L and FeCr-Al sintering honeycombs have metallic properties like low specific heat capacity and high conductivity.Fe-Cr-Al sintering honeycombs not only exhibit higher mechanical properties than ceramic honeycombs,but also superior coating adherence, so are ideal catalyst support;410L sintering honeycombs manifest much higher mechanical properties,except used as catalyst support,their potential applications including light structure,heat sink and filter.This paper provided a new method for fabricating metal honeycombs,and established the foundation of developing and applying metal honeycombs.
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