激光选区熔化TC4钛合金点阵结构的压缩性能

通过增材制造成型复杂晶格结构,实现航空航天结构件的轻量化设计得到越来越多的应用,然而点阵结构设计及其性能评价仍欠缺。本研究采用金刚石结构为基体,以点阵密度、结构形式为目标,设计不同几何参数试样,并以TC4钛合金为对象进行激光选区熔化(SLM)成型。对成型试样进行压缩试验,研究这种结构不同尺寸试样压缩性能的差异。结果表明:金刚石点阵结构试样受到载荷后应力在连接节点位置集中,产生断裂。提高晶胞密度可以缓解应力集中现象,提高比强度。减小晶胞尺寸和添加外壳都可以使应力均匀化,提高性能稳定性。小晶胞尺寸试样对球化现象、孔隙等冶金缺陷较为敏感,造成强度的下降。     

选区激光熔化成形点阵结构的各向异性对力学性能的影响

选区激光熔化技术（selective laser melting, SLM）成形的金属点阵结构由于具有结构设计自由度大、轻量化、缓冲吸能等优势,在航空航天等领域具有广泛的工程应用前景,然而对其力学性能的研究不够充分。本研究设计了不同方向的体心立方（body-centered cubic, BCC）和金刚石（Dia）两种晶胞点阵结构,基于SLM技术成形了AlSi10Mg点阵结构,并对成形试样进行了压缩试验,结合有限元分析（finite element analysis, FEA）研究了点阵结构的各向异性对其压缩响应和吸能特性的影响。结果表明,两种点阵结构均存在明显的各向异性。在相对密度基本一致的情况下,点阵结构方向从0°到45°,随着角度的增大,屈服强度明显增大,BCC点阵结构的各向异性对其压缩屈服强度的影响更加明显,Dia点阵结构的屈服强度明显高于BCC点阵结构。不同方向点阵结构的比吸能(specific energy absorption, SEA)存在明显差异,点阵结构方向从0°到45°,随着角度的增大,SEA明显增大,Dia点阵结构的SEA明显高于BCC点阵结构。不同方向点阵结构的碰撞载荷效率(crash load efficiency, CLE)存在明显差异,BCC点阵结构在0°方向取得最大值1.07,并随着点阵结构角度的增大逐渐减小,Dia点阵结构CLE随着点阵结构角度的增大而增大,并在45°方向上取得最大值1.01。     

ZL101半固态微压缩实验研究

对ZL101合金进行不同温度和保温时间的重熔处理,获得不同半固态组织状态,并进行常规尺寸(Φ5 mm)和微观尺寸(Φ1 mm)试样的等温压缩实验,得到不同变形温度和保温时间下试样的流变应力曲线,通过两种尺寸的力学性能比较,获得半固态金属在尺寸微型化的过程中尺度效应的存在条件与存在形态。研究表明,ZL101铝合金在常温和高固相率条件下均存在着流动应力增加的尺度效应现象;在高固相率条件下流动应力增大的现象更加明显;增加保温温度和保温时间,固相球化率增加,可以削弱流动应力增加的尺度效应。     

脉冲磁场强度对GH99镍基合金力学和微动磨损性能的影响

为研究不同磁场强度对镍基合金力学性能和耐磨性能的影响规律,在脉冲强磁场设备上对GH99镍基合金试样进行脉冲磁处理。通过观察显微结构,分析了GH99镍基合金的磨损机理和强化机制。结果表明：外加脉冲磁场可改善材料位错分布,减小试样表面残余应力的分散性;在磁场强度为10 T时残余压应力达到最大值（-223.45 MPa）,且此时材料拉伸断口的特征主要表现为韧性断裂,脉冲磁场处理合金产生亚结构位错胞有助于发挥细晶强化作用;在0~15 T范围内,随磁场强度增大,材料表面显微硬度和耐磨性能呈现先增强后减弱的规律,在脉冲磁场作用下合金材料内部的位错发生增殖致使位错密度增大,产生类似加工硬化现象,但磁场强度过大会导致位错塞积从而造成晶胞点阵畸变严重,出现材料性能恶化。     

温度和应力对不锈钢焊接接头蠕变行为的影响

用Cryo-Cracking试验方法研究了0Cr18Ni12Mo2Ti不锈钢焊接接头在不同温度和应力下蠕变试样中的空洞现象，结果表明，随着温度的提高，蠕变试样中的空洞尺寸趋于增大，空调密度增加，脆性断裂面的比例降低。应力对空洞尺寸和密度的影响与温度的影响类似，但温度对空洞尺寸和密度的影响大于应力的影响。     

单胞构型和结构参数对SLM成形梯度点阵结构压缩性能的影响

梯度点阵结构由于压缩时具有优秀的吸能能力,目前常作为吸能组件被应用于航天、国防和医疗等领域。但随着现代工业的发展,工程领域对其压缩性能提出了更高的要求,为使其进一步优化,有必要探讨单胞构型、结构参数和压缩性能之间的关系。因此本研究通过选区激光熔化(Selective laser melting, SLM)成形了两种梯度差的AlSi10Mg变杆径梯度体心立方(Body-centered cubic, BCC)和金刚石(Diamond, Diam)结构,以研究梯度差对压缩性能的影响,并对两种单胞构型进行对比。准静态单轴压缩实验和有限元分析(Finite element analysis, FEA)的结果表明,在同相对密度下,当单胞构型相同时,随着梯度差的增加单位体积吸能量明显增加。而梯度差相同时,Diam梯度点阵结构的压缩模量、屈服强度、抗压强度和最大峰值应力均高于BCC,同时其单位体积吸能量和吸能效率也高于BCC。     

硅对可锻铸铁共晶渗碳体显微结构的影响

采用Ｘ－射线衍射仪、透射电镜研究了可锻铸铁白口组织中硅对共晶渗碳体点阵参数、晶胞体积及晶体缺陷的影响。结果表明，提高可锻铸铁中的含硅量，白口组织中强制固溶于共晶渗碳体中的硅量增加，使共晶渗碳体点阵参数、晶胞体积及位错密度增加。分析了硅对渗碳体石墨化的影响，指出，硅可削弱渗碳体中键的结合强度，降低了渗碳体的稳定性，从而促进了可锻铸铁固态石墨化过程。     

桁架类与极限曲面类点阵结构的力学性能

多孔钛合金的设计与力学性能是生物医学领域的研究热点。通过SLM技术设计并制造了2种均质和梯度的Gyroid极小曲面单胞结构,通过对其进行静态压缩试验和拉伸试验并与传统的桁架类单胞结构做对比,建立了5种不同点阵结构的准静态压缩模型。通过Hypermesh与ABAQUS联合仿真的方式,对它们进行了网格划分与分析计算,通过应力应变云图、塑性应变云图以及压缩实验过程的观察,综合分析了空心立方、G7、bcc、均质Gyroid和梯度Gyroid 5种多孔结构失效形式和变形机制,将仿真得出的应力应变曲线与试验结果进行了对比,发现该仿真方法可以较好预测出不同多孔结构的最大抗压强度。压缩和拉伸试验结果表明,Gyroid点阵材料的最大抗拉性能远高于桁架类结构,抗压性能也更优越。其中,G梯度结构的综合力学性能最优。     

应用纳米金刚石提高碳化硅基复合材料的强度

本文对高温高压下 ,用硅与金刚石微粉的浸渗压制及其相互作用所制得的复合材料的密度、强度和结构进行了研究。研究表明 ,往浸渗层中添加纳米金刚石的成分 ,可提高符合技术条件要求的试样的合格率 ,同时(在从 1.7- 2 .2GPa的压缩实验中 )可增加复合材料极限强度的平均值。硅与金刚石颗粒相互作用的产生与碳化硅的分散度的提高 ,和复合材料中没有结合的硅数量的减少有关     

应用纳米金刚石提高碳化硅基合材料的强度

本文对高温高压下,用硅与金刚石微粉的浸渗压制及其相互作用所制得的复合材料的密度、强度和结构进行了研究,研究表明,往浸渗层中添加钠米金刚石的成分,可提高符合技术条件要求的试样的合格率,同时（在从1.7～2.2GPa的压缩实验中）可增加复合材料极限强度的平均值。硅与金刚石颗粒相互作用的产生与碳化硅的分散度的提高,和复合材料中没有结合的硅数量的减少有关。     

Evaluation of light-weight AlSi10Mg periodic cellular lattice structures fabricated via direct metal laser sintering

Aluminium alloy porous structures are highly demanded for many applications such as light-weight aerospace and heat exchanger products. Conventional manufacturing methods such as casting, however, faces difficulty in making aluminium alloy complex periodic cellular lattice structures with designed unit cell shape and size and volume fraction. This study evaluates the manufacturability and performance of AlSi10Mg periodic cellular lattice structures fabricated via direct metal laser sintering (DMLS). Various lattice structures at different volume fractions and unit cell sizes are designed by repeating a unit cell type called “diamond”. Due to the self-supported feature of the diamond unit cell, low volume fraction (7.5–15%) AlSi10Mg periodic cellular lattice structures can be fabricated by the DMLS process with the unit cell sizes ranging from 3 mm to 7 mm. A good geometric agreement is found between the original design structure models and the DMLS made structures, but the strut sizes of the DMLS made structures are slightly higher than the designed values and thus pore sizes decrease. There is clear relationship between the compressive modulus and strength of the structures and their volume fraction and unit size. Hence, this study shows that light-weight aluminium structures can be designed and made with the controlled unit size and volume fraction and the predicted mechanical properties.     

选区激光熔化制备多孔结构的成形偏差及力学性能与压缩失效分析

因多孔结构轻质高强度、力学性能可调节的特点,被广泛用于骨骼医疗、航空航天等领域. 为了探索多孔结构选区激光熔化(Selective Laser Melting, SLM)成形误差与压缩失效性能,以钻石型晶格和六孔开口球形两种多孔结构为例,采用理论预测与试验测试研究SLM制造多孔结构的压缩力学行为,使用ANSYS软件对所研究的多孔结构进行准静态压缩模拟,并对SLM成形的多孔结构进行单轴压缩试验,最后结合仿真和试验,观测和分析它们的变形过程和失效机制. 对比后发现数值设计的多孔结构尺寸与最终制造的结构存在偏差,导致力学性能理论值与试验值存在一定差异,但应力应变场变化规律一致. 试验结果表明,在孔隙率50% ~ 80%时,钻石型晶格结构屈服强度为31.85 ~ 182.13 MPa,弹性模量为1.45 ~ 2.30 GPa;六孔开口球形结构屈服强度为35.19 ~ 130.64 MPa,弹性模量为1.59 ~ 2.90 GPa,不同多孔结构随孔隙率的增大,力学性能变化趋势不一致.     

结构尺寸对Ti6Al4V多孔结构力学性能的影响

采用激光选区熔化技术制造了不同单元结构尺寸（1~6 mm）、孔隙率（40~80%）的拓扑优化多孔阵列结构,研究了单元结构尺寸对其压缩形变规律和弹性性能的影响。结果表明,多孔阵列结构的抗压强度、弹性模量均与单元结构尺寸成反比,抗压强度在126~199 MPa,弹性模量在3.5~55.47 GPa;压缩应力-应变曲线与单元结构尺寸有关,分别遵循弹性、弹脆性和脆性多孔材料三种应力应变规律;通过数值模拟多孔阵列结构的压缩形变过程,解释了两种45°断裂带的成因,力学性能与实验结果基本吻合;利用Gibson-Ashby模型评价多孔结构的稳定性,稳定性参数C与单元结构尺寸成反比;给出Gibson-Ashby拟合方程,特征参数n随单元结构尺寸增加而增大;建立了单元结构尺寸、相对密度和相对弹性模量的三维曲面数学模型,提出骨植入体的设计区域。     

激光增材制造Ti6Al4V点阵结构的抗压吸能特性

人体骨骼受到碰撞后的断裂过程伴随着能量吸收,多孔骨植入体的设计需考虑结构的抗压吸能特性。在空间尺寸（20 mm×20 mm×30 mm）内,通过拓扑优化设计和激光增材制造技术制备不同胞元尺寸和相对密度的Ti6Al4V点阵结构,采用熔池监控、单向压缩实验和有限元仿真方法,探究了点阵结构的表面质量、断裂形变规律和吸能特性。结果表明,点阵结构的结构参数受熔池温度场和粉末支持力的影响;点阵结构的抗压行为遵循弹脆性变化规律,断裂带与制造方向呈45°;点阵结构的断裂机制为韧性断裂,裂纹沿内部微孔洞分布方向扩展;能量吸收能力随着相对密度增大呈递增趋势,随着胞元尺寸增大呈递减趋势;能量吸收效率随着相对密度增大呈递减趋势,随着胞元尺寸增大呈递增趋势。     

Deformation, structural changes and damage evolution in nanotwinned copper under repeated frictional contact sliding

Nanotwinned metals have the potential for use as structural materials by virtue of having a combination of high strength as well as reasonable ductility and damage tolerance. In the current study, the tribological response of nanotwinned copper has been characterized under conditions of repeated frictional sliding contact with a conical tip diamond indenter. Pure ultrafine-grained copper specimens of fixed grain size (∼450 nm), but with three different structural conditions involving relatively high, medium and negligible concentrations of nanotwins, were studied. The effects of twin density and number of repetitions of sliding cycles on the evolution of friction and material pile-up around the diamond indenter were studied quantitatively by depth-sensing instrumented frictional sliding. Cross-sectional focused ion beam and scanning electron microscopy observations were used to systematically monitor deformation-induced structural changes as a function of the number of passes of repeated frictional sliding. Nanoindentation tests at the base of the sliding tracks coupled with large-deformation finite-element modeling simulations were used to assess local gradients in mechanical properties and deformation around the indenter track. The results indicate that friction evolution as well as local mechanical response is more strongly influenced by local structure evolution during repeated sliding than by the initial structure. An increase in twin density is found to result in smaller pile-up height and friction coefficient. Compared to the low-density nanotwinned metal, high-density nanotwinned copper showed significantly higher resistance to surface damage and structural changes, after the initial scratch. However with an increase in the number of sliding passes, the friction coefficient and rate of increase of pile up for all specimens acquire a steady value which does not change significantly in subsequent scratch passes. The frictional sliding experiments also lead to the striking result that copper specimens with both a high and low density of nanotwins eventually converge to a similar microstructure underneath the indenter after repeated tribological deformation. This trend strongly mirrors the well-known steady-state response of microcrystalline copper subjected to uniaxial cyclic loading. General perspectives on contact fatigue response of nanotwinned copper are developed on the basis of these new findings.     

Effect of nano-vanadium nitride on microstructure and properties of sintered Fe-Cu-based diamond composites

With FeCu₃₀ pre-alloy powder as the main component of the bond, a new type of nano‑vanadium nitride (VN) additive with different concentrations was introduced into Fe-Cu-based diamond composites to investigate the effect of nano-VN on the microstructure and properties of Fe-Cu-based diamond composites. The hardness, relative density, bending strength and wear loss weight of the fabricated specimens were tested, and then the fracture surfaces and worn surfaces of those specimens were analyzed using scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The results show that the Fe-Cu-based diamond composites with nano-VN addition exhibited an improvement in the mechanical properties, plasticity and wear resistance, which can be attributed to the dispersion strengthening and grain refinement caused by nano-VN. And the nano-VN can also activate sintering, which can significantly improve the wettability of the binder to diamonds, resulting in more binder elements wetting and diffusion on the diamond surface during the sintering process. Besides, the diamond composites showed the best properties with the addition of 2% nano-VN. That is, the bending strength and the HRB hardness of the diamond composites increased by 25% and 20%, respectively, and the wear resistance of the matrix and holding force coefficient of the matrix to diamond were improved significantly. But an excessive amount of nano-VN was detrimental to the mechanical properties of Fe-Cu-based diamond composites.     

3D打印轻质高强钨材的力学性能研究

采用选择性激光熔化（SLM）3D打印方式成功设计和制造了具有点阵结构的钨材，结合有限元分析、扫描电镜、准静态单轴压缩试验探究了不同点阵结构下钨材力学性能的变化规律，分析了微观组织对力学性能的影响。结果表明圆弧型点阵结构可有效降低节点处的应力集中，保持点阵结构轻质、低孔隙率特性同时还维持着钨材的高强度力学性能，平均抗压强度达到535MPa，平均质量仅为1.25g，激光打印后圆弧点阵较立方点阵平均抗压强度提升93%，其中体心圆弧点阵（BCA）显示出更优抗压性能，极限抗压强度达到721MPa，结构致密度为理论值12.8%；力学性能指标接近于变形态。与立方点阵相比，圆弧点阵具有良好的能力吸收特性，后者相较前者总能量吸收值提升223%，圆弧点阵平均能量吸收达到1664J/cm3。此外，SEM图像显示圆弧点阵因其弧形特性，减少了打印中斜支柱的悬挂距离，成型效果优于立方点阵。     

The microanalysis of the bonding condition between coated diamond and matrix

Ti, Mo, and Cu were plated on the diamond surface by magnetron sputtering method. The compression strength of Ti-coated diamond was measured; the structure of the diamond surface after vacuum heat treatment was analyzed by XPS. By the SEM analysis and the bending stress testing experiments, the authors researched the interface bonding condition of the coated diamond. The research revealed that the coatings on the diamond surface can protect the diamond well, and they also avoided or decreased the erosion of the diamond by matrix. The bonding strength between the diamond and the matrix was improved after plated Ti and Mo, but it was greatly influenced by the matrix composition and the hot-pressing technics. The plated metal and diamond could form local chemical union at the hot-pressing temperature of 830–840 °C.