W6Mo5Cr4V2/A100复合材料的制备及摩擦性能

为了提高A100钢的摩擦学性能,采用气雾化法制备纯A100钢粉末及分别添加10%、17%、23% W6Mo5Cr4V2高速钢的W6Mo5Cr4V2/A100合金粉末,再利用粉末冶金工艺制备W6Mo5Cr4V2/A100复合材料。研究不同含量W6Mo5Cr4V2钢对复合材料力学性能和摩擦性能的影响,并探讨其磨损机理。结果表明,随着W6Mo5Cr4V2钢含量的增加,复合材料的硬度先上升后下降,且摩擦因数和磨损率都有不同程度的降低,但是材料的密度和韧性都略微下降。当W6Mo5Cr4V2钢含量为17%时,复合材料的摩擦性能最优,这与其优异的微观结构和力学性能有着密不可分的关系。     

工艺参数设置和厚度对电子束熔融加工Ti-6Al-4V合金微观结构的影响

研究了试件尺寸和工艺参数(电子束强度,扫描速度,焦点偏移量和扫描长度)对电子束熔融(EBM)加工Ti-6Al-4V合金微观结构的影响。结果表明,可以观察到EBM加工的Ti-6Al-4V合金的微观结构由原始β相的柱状晶粒组成。在柱状晶粒内部观察到典型的(α+β)结构,即魏氏体α片和在细小的α晶粒的界面上形成的杆状β相。还发现沿原始β柱状晶粒的晶界形成的α层晶界。随着试件厚度、电子束能量密度和扫描长度的增加,先前的β柱状晶粒的直径增大,并且生长的方向与加工方向一致。同时,柱状晶粒直径随着试件高度的增加而减小。随着试件厚度和电子束能量密度的增加,α片会变得更粗大。     

6H-SiC脆性切削声发射响应的分子动力学研究

采用分子动力学方法研究了6H-SiC脆性切削的声发射响应。研究了原子尺度下6H-SiC的微变形和裂纹形核,同时对加工过程中的声发射源进行了识别,分析了其相应的声发射特征。结果表明,6H-SiC在77 nm切削深度下的脆性变形过程简单但不寻常;在6H-SiC切削过程中位错不会连续扩展,变形后的工件在刀具挤压作用下被分割成块,并由位错的快速扩展引发裂纹。对于影响声发射源特征的因素研究发现：初始压应力会导致声发射功率的下降;频率-能量分析中可见的3种声发射源分别是晶格振动、位错扩展和裂纹扩展。此外,在1 K温度下,2次明显的位错传播的声发射响应比晶格振动具有更高的频率特性,但总能量水平最低。相反地,裂纹扩展的声发射响应具有更为明显的频率分布特性和能量特性。     

利用选区激光熔化技术制备梯度结构的Ti-6Al-4V合金

采用选区激光熔化技术（SLM）制备了梯度Ti-6Al-4V合金。随着样品沉积厚度的逐渐增加,控制激光功率或扫描速率逐步升高或降低,以此研究了梯度结构Ti-6Al-4V合金的相变及结构演变。结果表明：由于选取激光熔化过程中的高冷却速率,SLM制备的Ti-6Al-4V合金的主要组织结构为初始β柱状晶里的针状马氏体。β柱状晶会随着激光功率的增加或扫描速率的降低而增宽。激光功率和扫描速率的变化会引起马氏体择优取向的变动。最后,由于局部不同的能量变化,随样品沉积厚度增加而逐步升高或降低的扫描速率会引发2种不同的空洞缺陷。     

Microstructure and mechanical properties of Ti-6Al-4V/Ti-22Al-25Nb joint formed by diffusion bonding

Ti-6Al-4V (wt.%) and Ti-22Al-25Nb (at.%) were joined by diffusion bonding at 950 °C and 15 MPa for 100 min, and the microstructure and mechanical properties of the resulting joints were investigated. The composition of the diffusion layer is B2/discontinuous α/α₂ layer/necklace-shaped β+α′ layer, where the content of any element at a given point mainly depends on the distance of the point from the interface and the phase type at the point. The tensile strength of the joint is 894 MPa, which is almost the same as that of the Ti-22Al-25Nb base alloy. The fracture surfaces on both sides of the joint are composed of two main regions. One region displays a relatively flat surface and fractures along the bonding interface. The other is composed of a moderate number of irregularly-shaped cavities on the Ti-6Al-4V side and many irregularly-shaped bulges on the Ti-22Al-25Nb side. Both regions result from fracture along the boundaries between β+α′ layers and α_p grains or from the transcrystalline fracture of α_p grains.     

Shear localization behavior in hat-shaped specimen of near-α Ti−6Al−2Zr−1Mo−1V titanium alloy loaded at high strain rate

The microstructure characteristics in early stage shear localization of near-α Ti?6Al?2Zr?1Mo?1V titanium alloy were investigated by split Hopkinson pressure bar (SHPB) tests using hat-shaped specimens. The microstructural evolution and deformation mechanisms of hat-shaped specimens were revealed by electron backscattered diffraction (EBSD) method. It is found that the nucleation and expansion of adiabatic shear band (ASB) are affected by both geometric and structural factors. The increase of dislocation density, structure fragment and temperature rise in the deformation-affected regions provide basic microstructural conditions. In addition to the dislocation slips, the extension twins detected in shear region also play a critical role in microstructural fragmentation due to twin-boundaries effect. Interestingly, the sandwich structure imposes a crucial influence on ASB, which finally becomes a mature wide ASB in the dynamic deformation. However, due to much larger width, the sandwich structure in the middle of shear region is also possible to serve as favorable nucleation sites for crack initiation.     

Structural characteristics and high-temperature tribological behaviors of laser cladded NiCoCrAlY–B4C composite coatings on Ti6Al4V alloy

In order to improve the hardness and tribological performance of Ti6Al4V alloy, NiCoCrAlY–B₄C composite coatings with B₄C of 5%, 10% and 15% (mass fraction) were fabricated on its surface by laser cladding (LC). The morphologies, chemical compositions and phases of obtained coatings were analyzed using scanning electronic microscope (SEM), energy dispersive spectrometer (EDS), and X-ray diffraction (XRD), respectively. The effects of B₄C mass fraction on the coefficient of friction (COF) and wear rate of NiCoCrAlY–B₄C coatings were investigated using a ball-on-disc wear tester. The results show that the NiCoCrAlY–B₄C coatings with different B₄C mass fractions are mainly composed of NiTi, NiTi₂, α-Ti, CoO, AlB₂, TiC, TiB and TiB₂ phases. The COFs and wear rates of NiCoCrAlY–B₄C coatings decrease with the increase of B₄C content, which are contributed to the improvement of coating hardness by the B₄C addition. The wear mechanisms of NiCoCrAlY–B₄C coatings are changed from adhesive wear and oxidation wear to fatigue wear with the increase of B₄C content.     

Electrochemical dissolution and passivation of laser additive manufactured Ti6Al4V controlled by elements segregation and phases distribution

The electrochemical dissolution and passivation of laser additive manufactured Ti6Al4V were investigated through Tafel polarization, potentiostatic polarization and AC impedance measurements. The results show that the solution treatment−aging (STA) process aggravates the element micro-segregation compared to the annealing process, leading to varied Al and V contents of the phases from different samples. It is proven that either Al-rich or V-rich condition can highly affect the electrochemical dissolution behaviors due to thermodynamical instability caused by element segregation. The dissolution rate in the metastable passivation process is controlled by the stability of the produced film that is affected by phases distribution, especially the difficult-to-dissolve phase. And then, the dissolution rate of the phases in the transpassivation region is consistent with the rank in the activation process because the dense film is not capable of being produced. Compared to the annealed sample, the higher dissolution rate of the STA sample is beneficial to the electrochemical machining (ECM) of Ti6Al4V.     

Hot Deformation Behaviors in Ti-6Al-4V/(TiB + TiC) Composites

Thermal compression testing was investigated using the Gleeble 3800 thermal simulator,and thermal deformation behavior of particle-reinforced titanium matrix composites (TMCs) was studied under deformation temperatures of 750-900 ℃,strain rates of 0.001-1 s-1,and experimental deformation of 60％.According to obtained flow stress curves,the hot deformation characteristics were analyzed.Based on the Arrhenius hyperbolic sinusoidal model,the constitutive equation at high tempera-ture was established.Based on the theory of dynamic material models,a hot processing map of TMCs at high temperature was established,and the peak region of power dissipation rate and the instability region in the hot processing map were both determined.At the same time,the corresponding microstructures in the peak power dissipation rate and rheological instability regions were observed.The results showed that flow stress decreased with increasing deformation temperature and increased with increasing strain rate.The thermal deformation activation energy of titanium matrix composites was 301.8 kJ/mol.The Ti-6Al-4V/(TiB + TiC) composites possessed only one instability zone under high-temperature compression at a strain of 0.5,with corresponding temperatures at 750-840 ℃ and strain rates at 0.1-1 s-1.The optimal thermal deformation parameters included corresponding temperatures of 830-880 ℃ and strain rates of 0.001-0.05 s-1.The microstructures corresponding to optimal hot working parameters in processing maps were more homogeneous than the microstructures in the instability zone,including the distribution uniformity of reinforcement and the degree of dynamic recrystallization,and no instability phenomena including abnormal grain growth,microcracks or intensive fracture of reinforcements were found,indicating that the hot processing map had a positive guiding effect on the option of desirable material thermal-working parameters.     

3D打印Ti-6Al-4V钛合金表面纳米孔洞制备技术

目的 通过3D打印技术制得钛合金,并构建出微米级多孔粗糙表面,再通过阳极氧化表面处理技术在微米级多孔粗糙表面构建出纳米级结构.方法 首先,通过高压水处理与酸蚀处理相结合的方法,对3D打印钛合金表面进行前处理,去除不良结合的球形粉末颗粒,降低3D打印钛合金表面的粗糙度及各向异性.然后,通过阳极氧化处理,在3D打印钛合金表面制备出具有纳米级孔洞的TiO2类骨膜层.结果 3D打印钛合金表面存在较大的各向异性,导致后续的电化学过程中电场放电不均匀,形成的TiO2类骨膜层稳定性差,而阳极氧化的前处理方法可有效解决这些问题.此外,纳米多孔结构抑制了阳极氧化处理中不稳定放电现象的发生,保证了膜层表面颜色的均匀性,且使膜层与钛或钛合金结合牢固,可在3D打印钛合金表面构建纳米级膜层结构.结论 该工艺方法制备的3D打印钛合金表面膜层的成膜速率和稳定性较好,膜层呈三维网状结构,大孔内附有小孔.该结构有利于细胞在其表面更好地粘附、分化和增殖,是理想的医用植入材料.