盐球渗透铸造法制备开孔泡沫铝的孔结构及压缩性能

使用圆盘造粒机制备近球形的NaCl颗粒,并将其用于渗透铸造制备开孔泡沫铝。盐球的平均抗压缩强度为3.9 MPa,在超声波清洗机中可在5 min内完全塌陷。通过控制热压烧结时间为0.5～2 h,热压温度700℃,可制备堆积密度在0.66～0.83 g/cm³的预制体。延长热压烧结时间会使开孔泡沫铝的孔径从0.48 mm增加到1.16 mm,孔隙率从64%增加到82%。压缩实验结果表明,不同孔隙结构下泡沫体的宏观变形特征基本相同,均表现出逐层塌陷的变形特征。此外,泡沫铝的致密化应变值、弹性模量、平台屈服应力和能量吸收能力均随着孔隙率的增加而降低。当孔隙率为64%时,能量吸收能力最大(15.0 MJ·m^-3)。     

SiO2包覆CaCO3在无保护气氛下制备闭孔泡沫镁（英文）

在熔体发泡法工艺中,发泡剂的分解速度和浸润性直接影响泡沫金属的孔结构和孔隙率。为减缓泡沫镁发泡剂CaCO3的发泡速度并提高与镁熔体的浸润性,采用非均匀形核法,以硅酸钠为原料,盐酸为酸化剂,在CaCO3表面包覆SiO2钝化膜。采用TGA-DTA、XRD、SEM等方法对包覆后CaCO3的热稳定性和包覆层的微观结构进行分析。结果表明:包覆后的CaCO3分解温度提高;包覆层中的SiO2为无定形态;在CaCO3颗粒表面形成网络状结构。对比实验表明:包覆后的CaCO3发泡速度平稳。同时,采用合金化阻燃工艺在无气体保护条件下制备出较大尺寸的泡沫镁试样,并且试样孔径细小,孔结构均匀,孔隙率在60%-70%。     

Effects of carbamide shape and content on processing and properties of steel foams

Spherical and irregular carbamide particles covered with Fe–1.5% Mo steel powder have been employed to produce foams using space holder-water leaching technique in powder metallurgy. Foams having porosities between 49.2% and 71.0% were produced after sintering at 1200 °C for 60 min. The effect of carbamide shape and content on the foams’ processing, microstructure and mechanical properties has been evaluated. Using irregular carbamide particles and increasing its content decreased leaching time. The extents of specimens’ volumetric expansion after carbamide leaching and volumetric shrinkage after sintering also depended on volume fraction and carbamide shape. Final porosity was directly related to the added fraction of carbamide. Pore shape was similar to initial carbamide particle shape. Foams having irregular pore shape were observed to have compressive yield strengths between 20 MPa and 92 MPa and Young's moduli between 0.45 GPa and 2.69 GPa. The corresponding values for foams having spherical pore shape varied between 25 MPa and 112 MPa and between 0.71 GPa and 2.91 GPa, respectively. The foams’ strength increased with increasing relative density. Microstructure having spherical pores resulted in better compressive behavior.     

Wire based additive layer manufacturing: Comparison of microstructure and mechanical properties of Ti-6Al-4V components fabricated by laser-beam deposition and shaped metal deposition

The microstructure and the mechanical properties of Ti-6Al-4V components, fabricated by two different wire based additive layer manufacturing techniques, namely laser-beam deposition and shaped metal deposition, are presented. Both techniques resulted in dense components with lamellar α/β microstructure. Large ultimate tensile strength values between 900 and 1000 MPa were observed. The strain at failure strongly depends on the orientation, where highest values up to 19% were obtained in direction of the building direction. Heat treatment increased the highest strain at failure up to 22%. The fatigue limit was observed to be higher than 770 MPa.     

超声冲击对SMA490BW钢焊接接头超高周疲劳性能的影响

采用超声冲击工艺对转向架用SMA490BW钢对接接头焊趾表面进行冲击处理,研究了超声冲击对接头超高周疲劳性能的影响。借助金相显微镜、SEM和TEM研究了冲击层金相组织、冲击前后焊趾处的形貌及表层金属晶粒细化程度。运用有限元软件计算焊接接头的应力分布,采用X射线应力仪对冲击前后焊趾表层的应力进行了测量和分析。结果表明,在5×10~6循环周次下,冲击态接头和焊态接头的疲劳强度分别为206 MPa和153 MPa,经冲击处理后疲劳强度提高了34.6%。在1×10~8循环周次下,冲击态接头的疲劳强度为195 MPa,与焊态接头的141 MPa相比提高了38.3%。在240 MPa的应力水平下,焊接接头经超声冲击处理后的疲劳寿命提高了7倍。经冲击处理的焊趾部位的应力集中系数下降了19.1%,其表面的残余拉应力得到消除,并转变为有益的残余压缩应力,焊趾表层组织得到明显细化,这3个方面均对提高焊接接头疲劳性能起到了积极贡献。     

高强度碳化硅泡沫陶瓷的制备及其抗压强度研究

选用Al2O3、Y2O3作为烧结助剂,通过有机模板复制法及多次浸渍涂覆工艺制备出高强度碳化硅泡沫陶瓷材料。系统地研究了原料组成、烧结温度等工艺参数对制得的碳化硅泡沫陶瓷物相组成、宏观结构、微观结构的影响,同时对陶瓷的气孔率、力学性能等进行了测试。结果表明:通过选取不同PPI值的有机泡沫模板,泡沫陶瓷宏观孔径可控;随着涂覆次数的增加,陶瓷体孔径减小、孔棱直径增加;随着烧结温度的提高,孔棱致密度增加,抗压强度显著提高;在1700℃下获得了20PPI值,气孔率为77%,抗压强度达2.48MPa的碳化硅泡沫陶瓷。     

The role and effect of residual stress on pore generation during anodization of aluminium thin films

The role and effect of residual stress on pore generation of anodized aluminium oxide (AAO) have been investigated into anodizing the various-residual-stresses aluminium films. The plane stresses were characterised by X-ray diffraction with sin²ψ method. The pore density roughly linearly increased with residual stress from 64.6 (−132.5 MPa) to 90.5 pores/μm² (135.9 MPa). However, the average pore size around 40 nm was not changed significantly except for the rougher film. The tensile residual stress lessened the compressive oxide growth stress to reduce AAO plastic deformation for higher pore density. The findings provide new foundations for realizing AAO films on silicon.     

Red mud and fly ash-based ceramic foams using starch and manganese dioxide as foaming agent

Ceramic foams were prepared using red mud and fly ash as raw materials with sodium borate as sintering aid agent, starch and MnO₂ as foaming agent, respectively. The influence of the amount of starch or MnO₂ on the crystalline phase, pore morphology and physical–chemical porosities was studied. The results showed that the main crystal phases of samples with starch addition and MnO₂ addition were sodalite phase Na₆ (AlSiO₄)₆ and Na₈(SiAlO₄)₆MnO₄, respectively. The SEM images showed that the variation of porous structure was mainly dominated by the addition of foaming agent. With the increase of foaming agent, the samples exhibited better comprehensive properties: bulk density of 0.59–0.96 g/cm³, porosity of 41.82%–63.51%, water absorption of 3.16%–9.17%, compressive strength of 4.22–8.38 MPa, flexural strength of 2.44–5.82 MPa, acid resistance of 95.59%–99.60%, alkali resistance of 99.82%–99.99%. Based on these properties, the ceramic foams can be used in building field.     

Fabrication and Structure Characterization of Alumina-Aluminum Interpenetrating Phase Composites

Alumina-Aluminum composites with interpenetrating networks structure belong to advanced materials with potentially better properties when compared with composites reinforced by particles or fibers. The paper presents the experimental results of fabrication and structure characterization of Al matrix composites locally reinforced via Al₂O₃ ceramic foam. The composites were obtained using centrifugal infiltration of porous ceramics by liquid aluminum alloy. Both scanning electron microscopy (SEM + EDS) and x-ray tomography were used to determine the structure of foams and composites especially in reinforced areas. The quality of castings, degree of pore filling in ceramic foams by Al alloy, and microstructure in area of interface were assessed.     

Effect of Hydrogen on Mechanical Properties of 23Co14Ni12Cr3Mo Ultrahigh Strength Steel

In order to evaluate the effect of hydrogen on mechanical properties of 23Co14Ni12Cr3Mo ultrahigh strength steel, the specimens were electrochemically hydrogen charged for different times. The tensile property, fatigue fracture behavior, fatigue crack growth (FCG) behavior, and threshold stress intensity (ΔK _th) of the samples were studied. The fracture morphology was characterized by scanning electron microscopy. It was shown that tensile strength decreases from 2300 to 2000 MPa, critical fatigue stress from 577 to 482 MPa, and ΔK _th from 27.4 to 14.3 MPam^0.5 with the increasing hydrogen contents from 0.0001 to 0.0008 wt.%. Hydrogen enhances the FCG rate from 2.4 × 10^?3 to 3.6 × 10^?3 mm/cycle at ΔK = 80 MPam^0.5 in the hydrogen-charging range. Microscopic observation showed that the tensile fracture is a combination of overload microvoids and some intergranular regions for 0 h, and isolated areas of transgranular (TG) fracture are observed with brittle cleavage for 24-72 h. The fatigue fracture is ductile for the uncharged specimens, while the hydrogen-charged specimens show mainly brittle TG fracture. These results suggest that hydrogen degrades the fracture behavior of 23Co14Ni12Cr3Mo ultrahigh strength steel.     

Investigation of electroless copper plating on polyurethane foam,as an initial step of open cell foam production process

Abstract

Coating of polymeric foams is known as a method for production of metallic foams, which produces foams with high volume of porosity and controllable pore size. In this research, this method was employed to produce open cell copper foam by use of polyurethane foam with an average pore size of 0.4?mm as the substrate. Since polyurethane foam as a non-conductive material is not able to be coated directly by electrolytic deposition, the substrate was initially metallised by electroless copper plating. In the electroless plating process, the effects of the main parameters such as bath chemical composition, solution pH and temperature on deposition rate and thickness of the coatings obtained were investigated. The results showed that the optimum condition of the process is obtained when CuSO₄ concentration in the deposition bath is 12?g?L^??1, pH is 13 and plating temperature range is 55–60°C.     

Fabrication of lotus-type porous Mg−Mn alloys by metal/gas eutectic unidirectional solidification

Lotus-type porous Mg–xMn (x=0, 1, 2 and 3 wt.%) alloys were fabricated by metal/gas eutectic unidirectional solidification (the Gasar process). The effects of Mn addition and the fabrication process on the porosity, pore diameter and microstructure of the porous Mg−Mn alloy were investigated. Mn addition improved the Mn precipitates and increased the porosity and pore diameter. With increasing hydrogen pressure from 0.1 to 0.6 MPa, the overall porosity of the Mg−2wt.%Mn ingot decreased from 55.3% to 38.4%, and the average pore diameter also decreased from 2465 to 312 μm. Based on a theoretical model of the change in the porosity with the hydrogen pressure, the calculated results were in good agreement with the experimental results. It is shown that this technique is a promising method to fabricate Gasar Mg–Mn alloys with uniform and controllable pore structure.     

Effect of α phase morphology on fatigue crack growth behavior of Ti−5Al−5Mo−5V−1Cr−1Fe alloy

Taking a Ti−5Al−5Mo−5V−1Cr−1Fe alloy as exemplary case, the fatigue crack growth sensitivity and fracture features with various tailored α phase morphologies were thoroughly investigated using fatigue crack growth rate (FCGR) test, optical microscopy (OM) and scanning electron microscopy (SEM). The tailored microstructures by heat treatments include the fine and coarse secondary α phase, as well as the widmanstatten and basket weave features. The sample with coarse secondary α phase exhibits better comprehensive properties of good crack propagation resistance (with long Paris regime ranging from 15 to 60 MPa·m^1/2), high yield strength (1113 MPa) and ultimate strength (1150 MPa), and good elongation (11.6%). The good crack propagation resistance can be attributed to crack deflection, long secondary crack, and tortuous crack path induced by coarse secondary α phase.     

Creep-rupturing of open-cell foams

A repeating element consisting of four straight and uniform-thickness cell struts in a pentagonal dodecahedron model is employed to analyze theoretically the creep-rupturing of open-cell foams. In the repeating element, the solid making up cell struts is assumed to follow power-law creep and the Monkman–Grant relationship. Consequently, the theoretical expressions for describing the steady-state creep strain rate and creep-rupturing time of open-cell foams are obtained. It is shown that the creep-rupturing of open-cell foams can also be described by the Monkman–Grant relationship. Moreover, the Monkman–Grant parameters m¹ and B¹ of open-cell foams depend on their cell structure and those of solid cell struts. The Monkman–Grant parameters determined from the existing experimental results on the creep-rupturing of open-cell aluminum alloy foams are compared to those calculated theoretically from the proposed pentagonal dodecahedron model. The difference between theoretically calculated and empirically determined B¹ is attributed to some pre-existing cell structural imperfections in open-cell aluminum alloy foams.     

Fabrication of silicon oxycarbide foams from extruded blends of polysiloxane,low-density polyethylene (LDPE), and polymer microbead

Silicon oxycarbide (SiOC) foams with porosities ranging from 77% to 90% and a cell density higher than 10⁸ cells/cm³ were made from polysiloxane, low-density polyethylene (LDPE), and polymer microbead blends. The polysiloxane, LDPE, and polymer microbeads were compounded directly using a counter-rotated twinscrew extruder. The obtained blends were foamed with gaseous carbon dioxide, cross-linked, and subsequently pyrolyzed. The process resulted in the production of highly porous, open-cell SiOC foams with a bimodal distribution of pore morphology, small spherical pores derived from polymer microbeads and relatively large elongated or equiaxed pores derived from foaming using carbon dioxide and decomposition of LDPE.     

Study of the effects of Zr addition on the microstructure and properties of Nb-Ti-Si based ultrahigh temperature alloys

The effects of alloying with Zr on the microstructure, mechanical and oxidation properties of Nb-Ti-Si based ultrahigh temperature alloys have been investigated in this study. The microstructures of the all alloys were comprised of primary γ(Nb,X)₅Si₃ blocks, Nbss and eutectic colonies, and the additions of Zr do not affect the microstructure and phase constituents of Nb-Ti-Si based alloys. Zr improves both the room-temperature toughness and the high-temperature strength. The alloy with addition of 8 at.% Zr shows the highest fracture toughness of 15.01 Mpa·m^1/2. The compressive strengths of the alloys are improved to 278.89–293.08 MPa for the Zr-containing alloys when compare with the Zr-free alloy (194.23 MPa). The oxidation resistance of the alloys was also obviously ameliorated with Zr addition, showing a reduced weight gain with the increase of Zr content.     

Structure and properties of aluminum alloy 1421 after equal-channel angular pressing and isothermal rolling

Sheets from the aluminum alloy 1421 with an ultrafine-grained (UFG) structure and a weak crystallographic texture were prepared by the method of equal-channel angular pressing (ECAP) through a die with channels of a rectangular cross section and by subsequent isothermal rolling. Both operations were carried out at a temperature of 325°C. It is shown that severe plastic deformation (SPD) leads to the formation of a completely recrystallized uniform microstructure with an average grain size of 1.6 µm in the alloy. At room temperature the alloy 1421 demonstrates high static strength (σ_u = 545 MPa, σ_0.2 = 370 MPa) in the absence of a significant anisotropy. At temperatures of hot deformation, the alloy showed ultrahigh elongations under superplasticity (SP) conditions. At a temperature of 450°C and initial deformation rate of 1.4 × 10^?2 s^?1 the maximum elongation at fracture was ～2700%. At static annealing at a temperature of SP deformation, the UFG structure formed in the process of SPD remains stable. The SP deformation is accompanied by an insignificant grain growth and pore formation.     

Fatigue properties of friction stir welded butt joint and base metal of MB8 magnesium alloy

The fatigue properties of friction stir welded(FSW) butt joint and base metal of MB8 magnesium alloy were investigated.The comparative fatigue tests were carried out using EHF-EM200K2-070-1A fatigue testing machine for both FSW butt joint and base metal specimens.The fatigue fractures were observed and analyzed using a scanning electron microscope of JSM-6063 LA type.The experimental results show that the fatigue performance of the FSW butt joint of MB8 magnesium alloy is sharply decreased.The conditional fatigue limit(2 × 10~6) of base metal and welded butt joint is about77.44 MPa and 49.91 MPa,respectively.The conditional fatigue limit(2 × 10~6) of the welded butt joint is 64.45%of that of base metal.The main reasons are that the welding can lead to stress concentration in the flash area,tensile welding residual stress in the welded joint(The residual stress value was 30.5 MPa),as well as the grain size is not uniform in the heat-affected zone.The cleavage steps or quasi-cleavage patterns present on the fatigue fracture surface,the fracture type of the FSW butt joint belongs to a brittle fracture.     

Fatigue strength and microstructure evaluation of Al 7050 alloy wires recycled by spray forming,extrusion and rotary swaging

Recycled high-strength aluminum alloys have limited use as structural materials due to poor mechanical properties. Spray forming remelting followed by hot extrusion is a promising route for reprocessing 7xxx alloys. The 7050 alloy machining chips were spray formed, hot extruded, rotary swaged and heat-treated in order to improve mechanical properties. Microstructures, tensile properties and fatigue strength results for a 2.7 mm-diameter recycled wire are presented. Secondary phases and precipitates were investigated by XRD, SEM, EBSD, TEM and DSC. As-swaged and heat-treated (solution and aging) conditions were evaluated. Mechanical properties of both conditions outperformed AA7050 aerospace specification. Substantial grain refinement resulted from the extensive plastic deformation imposed by rotary swaging. Refined micrometric and sub-micrometric Al grains, as well as coarse and fine intermetallic precipitates were observed. Subsequent solution treatment resulted in a homogeneous, recrystallized and equiaxed microstructure with grain size of 9 μm. Nanoscale GP(I) zones and η′ phase precipitates formed after aging at 120 °C, imparting higher tensile (586 MPa) and fatigue (198 MPa) strengths.     

Fatigue threshold and crack propagation in γ-TiAl sheets

The fatigue crack propagation behaviour of two different microstructures — a coarse-grained designed fully lamellar (DFL), and a fine-grained near γ (FG) — of a Ti–46.5 at.% Al–4 at.% (Cr, Nb, Ta, B) alloy was studied. Both the threshold of stress intensity range and standard long crack growth behavior were determined. A special technique was applied to separate the different mechanisms — intrinsic and extrinsic effects — and their changes with crack length. The fatigue crack propagation rate of long cracks is much smaller in the DFL microstructure than in the FG microstructure at the same stress intensity range. The effective threshold of stress intensity range of both microstructures is about 1.7 MPa√m. The threshold of stress intensity range shows a strong R-curve behavior. In other words the propagation–non-propagation conditions of cracks are significantly influenced by the crack extension. The long crack thresholds of stress intensity range at the stress ratio 0.1 are relatively large; they are about 4.5 and 8 MPa√m in the DFL and the FG microstructure, respectively. The differences in the crack growth behavior between the two microstructures are mainly induced by extrinsic resistance mechanisms.