粉体增塑挤压—烧结制备金属蜂窝材料的工艺、性能和理论研究

【作者】 周芸；

【导师】 孙加林；

【作者基本信息】 昆明理工大学 ， 材料学， 2007， 博士

【摘要】 金属蜂窝是金属骨架和蜂窝孔（形状与蜜蜂的巢穴极其相似）相间的一种新型多功能复合材料。金属蜂窝除具有金属特性（如良好的导热性、导电性、强韧性等）外,还具有高比表面积（可达3m²/dm³或以上）、大开口度、超轻质高比刚度、散热、渗透流通等结构特性。由于金属蜂窝具有的特殊结构和功能特性,最近几年,金属蜂窝得到多方面的研究、开发与应用。应用已涉及到交通运输、航空航天、环境保护、新型能源、机械化工、电子电工、建筑工程等领域。目前,金属蜂窝的研究、开发与应用已初步形成一种高新技术产业,尤其是大批量制造的汽车尾气排气净化催化剂的金属载体。目前作为载体使用的金属蜂窝为波纹薄带卷绕型金属蜂窝,存在制备工艺复杂、成本高、金属蜂窝壁表面光滑与催化剂的结合性能不佳等问题。针对以上问题,本论文提出一种新型金属蜂窝制备方法,即采用粉末增塑挤压-烧结工艺直接制备整体蜂窝制品,以期获得低成本、高性能的金属蜂窝载体。论文从材料选择设计,到材料制备过程的各个环节进行了工艺与基础理论研究,主要研究内容与结果如下:论文以载体材料选材原则为依据,选择高温抗氧化性能良好的Fe-Cr、Fe-Cr-Al两种材料体系为研究对象。Fe-Cr体系以410L不锈钢粉为原料;Fe-Cr-Al体系以纯铁、铬、铝粉为原料。实验将各体系的粉末与粘结剂按一定配比混合均匀制成挤压料,研究粉末形态粒度、成分配比、粘结剂、挤压速率、挤压圈大小等参数对蜂窝成形性的影响,并对挤压缺陷进行分析。研究表明:（1）颗粒表面光滑呈球形,粒度分布范围在1-40μm的颗粒构成的挤压料成形性最好,颗粒表面形态不规则,偏离球形越多,成形性越差,410L系挤压料成形性优于FeCrAl系。（2）挤压速率适中,太快太慢都不利于成形质量好的蜂窝制品,最佳的挤压速率为30-60mm/min。（3）配料比越高,即粉末装载量越大,挤压料的流动性越差。对于配料比低的挤压料,尽管挤压料的流动性好易于成形,但挤压蜂窝制品太软,保形稳定性差。对于410L不锈钢挤压料,配料比为80%-82.5%时成形性最佳:对于Fe-Cr-Al合金挤压料,配料比为65%-67.5%时成形性最佳。（4）挤压入料圈大小必须与挤压料配合适当,入料圈直径过小会使物料进入模具前,外层流速小于内层,而出现表面缺料;入料圈直径过大时与此相反,外层流速大于内层,会出现表面褶皱。每一种配比的挤压料都有对应的最佳入料圈,配料比越高相应的入料圈直径越大。（5）以水溶性有机物为粘结剂,粘结剂与水配比适当才能保证挤压料具有较好的挤压蜂窝成形性。（6）要求模具设计合理,加工精度高。挤压料流变特性在很大程度上影响着挤压蜂窝制品的成形性、稳定性,为了获得成形质量好的蜂窝制品,借助流变学理论对挤压料流变特性进行研究。实验采用毛细管流变测量装置对各种体系不同配比的挤压料的流变参数（表观粘度、最大剪切应力）进行计算;建立表征挤压料流变特性的流变学本构方程;测量获得410L不锈钢体系和Fe-Cr-Al体系的最大金属粉末装载量φ_max,并将悬浮分散系相对粘度模型与实验测试结果比较。研究结果表明:（1）410L不锈钢体系和FeCrAl体系挤压料均属于剪切稀化流体,其流变学本构方程为:τ=kγⁿ,随着配比提高,即金属粉末装载量提高,剪切稀化指数n和稠度系数k增大,流体的剪切稀化作用降低。（2）在金属粉末装载量不变的条件下,Fe粉中加入粗颗粒的Cr粉,剪切稀化指数n略有提高,而稠度系数k明显降低。说明在细粉中加入一定量的粗颗粒可以降低挤压料流体粘度。（3）在Fe-Cr-Al体系中加入少量润滑剂SA可以使挤压料的剪切稀化指数n和稠度系数k降低,改善挤压料的流动性。（4）通过实验测量获得410L不锈钢体系和Fe-Cr-Al体系的最大金属粉末装载量φ_max分别为69%和64%。（5）将悬浮分散系相对粘度模型与实验测试结果比较可知,Dougherty—Krieger模型与实验数据吻合较好,对于410L不锈钢体系,K_H=2.0;对于Fe-Cr-Al体系,K_H=2.0～2.5。由相对粘度η_r随金属粉末装载量φ变化曲线可以确定挤压蜂窝成形性最佳的挤压料配比。挤压蜂窝体经干燥后形状完整无裂纹的蜂窝体,可作为烧结坯体。烧结在高温气氛炉中进行,通过研究烧结工艺参数和条件（温度、时间、粉末颗粒形态、气氛）的影响,获得以下结果:（1）对于410L不锈钢体系,随着烧结温度升高或烧结时间的延长,体积收缩率增大,密度提高。为达到同样的烧结目的,提高烧结温度比延长烧结时间更有效。当烧结温度为1200℃时,配料比为80%的挤压蜂窝,烧结后体积收缩率可达55%、密度为2.7 g/cm³。在同样的烧结条件下,配料比提高,密度提高。（2）对于Fe-Cr-Al体系,随着烧结温度升高或烧结时间的延长,体积收缩率增大,而密度则出现增大到一定值后而下降的现象。当温度超过1265℃或温度为1250℃、时间超过50min时,密度开始降低。对于配料比为70%的挤压蜂窝,烧结后体积收缩率最大可达53%、密度为1.68 g/cm³。（3）410L不锈钢体系最佳烧结工艺参数为:温度1120℃、烧结时间20min;Fe-Cr-Al体系最佳烧结工艺参数为:温度1250℃、烧结时间30min。（4）颗粒细小的粉体比颗粒粗大的粉体烧结致密化程度高。Fe-Cr-Al烧结体系中,采用+300目的Cr粉的挤压料,其烧结蜂窝致密度比采用-300的目Cr粉的低。（5）常见的烧结蜂窝缺陷主要是形状不规则、中心网格断裂、椭圆形蜂窝、大小头蜂窝、液滴渗出、里外颜色不一致现象等。在烧结过程中,要控制好烧结温度、烧结时间、烧结气氛,以保证获得尽可能均匀的温度场。通过研究410L和Fe-Cr-Al体系在烧结过程中的组织结构变化和机理可知:（1）410L不锈钢体系,属于单元系烧结,烧结过程大体分为烧结颈形成、烧结颈长大、孔隙缩小和消失三阶段。在低温阶段烧结机制主要是表面、界面扩散机制,高温阶段以体积扩散和塑性流动机制为主。烧结完成后组织为α-Fe和少量析出相Fe（Cr）₂Si。（2）Fe-Cr-Al体系,属于多元瞬间液相烧结,烧结过程主要包括液相流动与颗粒重排阶段、固相溶解和再析出阶段、固相烧结阶段。有液相存在时烧结机制以粘性流动为主;液相消失后,烧结机制主要是扩散机制。而扩散机制又分为:粉末颗粒接触初期,以表面扩散机制为主:当颗粒烧结连接形成骨架后,主要以晶界和体积扩散机制为主。烧结完成后组织为α-Fe（Cr、Al）固溶体,随烧结时间延长,α-Fe（Cr、Al）固溶体晶粒长大,成分均匀化。通过对烧结蜂窝体结构参数测量和计算,以及物理和力学性能测试可得:（1）410L烧结蜂窝体结构参数为:内壁厚t为0.15～0.20mm、孔密度n（1/in²）为329～354、开孔率ε为73%-79%、比表面积Sv（sq m/cu dm）为2.45～2.63;Fe-Cr-Al烧结蜂窝体结构参数为:内壁厚t为0.18～0.23mm、孔密度n（1/in²）为316～339、开孔率ε为70%-76%、比表面积Sv（sq m/cu dm）为2.35～2.52。（2）410L烧结蜂窝体的比热容C_p（J/g.K）为0.5～0.56、热导率κ（W/m.K）为10.20～11.35;Fe-Cr-Al烧结蜂窝体的比热容C_p（J/g.K）为0.6～0.65、热导率κ（W/m.K）为6.5～6.78。（3）烧结蜂窝由于表面凸凹不平并有许多微孔,导致表面粗糙度和比表面积显著提高,有利于涂层负载和涂层与基体的结合。Fe-Cr-Al烧结蜂窝体表面形成的Al₂O₃与以Al₂O₃为主要成分的涂层有天然的亲合性,因而涂层负载率高、涂层与基体的结合性好;410L烧结蜂窝体表面形成的Cr₂O₃与Al₂O₃涂层间存在润湿性和热膨胀过渡性问题,涂层负载率和结合性均较差。（4）410L烧结蜂窝体的横向压缩强度为30～40Mpa、纵向压缩强度为140～150Mpa;Fe-Cr-Al烧结蜂窝体横向压缩强度为7～8Mpa、纵向压缩强度为20～25Mpa。随烧结蜂窝体密度提高,蜂窝体的抗压缩强度提高。（5）410L和Fe-Cr-Al系烧结蜂窝体均具有比热容低、导热性好的金属特性。FeCrAl烧结蜂窝体不仅具有比陶瓷蜂窝好的力学性能,同时还具有良好的涂层表面附着性,是作为载体材料的理想选择;410L烧结蜂窝体力学性能远远高于陶瓷蜂窝,除了可以作为载体材料外,在轻型结构材料、散热材料、过滤材料等方面还具有潜在的应用价值。论文为制备金属蜂窝及蜂窝载体提供了一种新途径,为金属蜂窝载体的进一步开发应用奠定了良好的理论基础。更多还原

【Abstract】 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(φ_max)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(φ_max)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,K_H=2.0 for 410L and K_H=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 C_p（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 C_p（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.更多还原