Composition control for laser solid forming from blended elemental powders

Laser solid forming (LSF) from blended elemental powders is a powerful tool for the synthesis of novel materials. Accurate composition control is critical for the application of this technique. It is found that the chemical composition of as-deposited sample can be controlled the same as the premixed elemental powders by keeping the identity of the divergence angles of the elemental powder streams. A mathematical model was established to describe the powder delivery process during LSF from blended elemental powders. Based on the consistency condition for divergence angles of different elemental powder streams, the match condition among the elemental powder characteristics (including particle size and density) can be obtained, which ensures the consistency in composition between the laser deposits and the premixed elemental powders. LSF experiments were carried out using a blend of Ti, Al, and V powders and the composition analysis was performed on as-deposited samples. The experimental results demonstrate the validity of the developed mathematical model.     

Microstructure evolution of laser solid forming of Ti-Al-V ternary system alloys from blended elemental powders

Morphology evolution of prior β grains of laser solid forming (LSF) Ti-xAl-yV (x 11,y 20) alloys from blended elemental powders is investigated. The formation mechanism of grain morphology is revealed by incorporating columnar to equiaxed transition (CET) mechanism during solidification. The morphology of prior β grains of LSF Ti-6Al-yV changes from columnar to equiaxed grains with increasing element V content from 4 to 20 wt.-%. This agrees well with CET theoretical prediction. Likewise, the grain morphology of LSF Ti-xAl-2V from blended elemental powders changes from large columnar to small equiaxed with increasing Al content from 2 to 11 wt.-%. The macro-morphologies of LSF Ti-8Al-2V and Ti-11Al-2V from blended elemental powders do not agree with CET predictions. This is caused by the increased disturbance effects of mixing enthalpy with increasing Al content, generated in the alloying process of Ti, Al, and V in the molten pool.     

Process monitoring of powder pre-paste laser surface alloying

Instead of the continuous powder delivery method using a powder feeder for thick layer laser cladding, pre-pasting of the alloying powder on the substrate is a widely used method to supply the coating powders into the melt pool for LSA. A method to monitor the process of laser surface alloying based on the infrared emission from the melt pool using infrared photodiodes was developed. The technique is solely aimed at the process of laser surface alloying using pre-paste metal powder on the substrate surface prior to laser melting. This monitoring technique is able to distinguish the existence or the absence of the pre-paste powder and the consistency of the laser surface alloying process. The technique is of low cost and is simple to implement into the process.     

Microstructure study of direct laser fabricated Ti alloys using powder and wire

A compositionally graded material has been fabricated using direct laser fabrication (DFL). Two types of feedstock were fed simultaneously into the laser focal point, a burn resistant (BurTi) alloy Ti-25V-15Cr-2Al-0.2C powder and a Ti-6Al-4V wire. The local composition of the alloy was changed by altering the ratio of powder to wire by varying the feed rate of the powder whilst maintaining a fixed feed rate of wire-feed. For the range of compositions between about 20% and 100% BurTi only the beta phase was observed and the composition and lattice parameter varied monotonically. The grain size was found to be much finer in these functionally graded samples than in laser fabricated Ti64. Some samples were made using the wire-feed alone, where it was found that the microstructure is different from that found when using powder feed alone. The results are discussed in terms of the power requirements for laser fabrication of powder and wire samples.     

NANOSTRUCTURED AND AMORPHOUS MATERIALS BY MECHANICAL ALLOYING

Mechanical Alloying is particularly attractive because it can be used to process structurally uniform and isotropic bulk quantities of powdered materials that have a fine length scale. Furthermore, it appears to be applicable to many combinations of elements. Here experimental results regarding the synthesis and characterization of several binary and ternary nanocrystalline and amorphous alloys of Ti, Zr, Hf, V, Nb and Fe with other transition elements and with B, C and Si are reported. The nanocrystalline and amorphous powders, with effective particle size between 7 and 30nm, were prepared by mechanical alloying in a high-energy ball-mill using commercial elemental powders and/or alloy powder.     

Structure and Tribological Property of TiBN Nanocomposite Multilayer Synthesized by Ti-BN Composite Cathode Plasma Immersion Ion Implantation and Deposition

A Ti-BN complex cathode is made from Ti and h-BN powders by the powder metallurgy technology,and TiBN coating is obtained by plasma immersion ion implantation and deposition with this Ti-BN composite cathode.The TiBN coating shows a self-forming multilayered nanocomposite structure while with relative uniform elemental distributions.High resolution transmission electron microscopy images reveal that the multilayered structure is derived from different grain sizes in the nanocomposite.Due to the existence of h-BN phase,the friction coefficient of the coating is about 0.25.     

Quantitative analysis of cement powder by laser induced breakdown spectroscopy

Determination of elemental composition of cement powder plays an important role in the cement and construction industries. In the present paper, Laser induced breakdown spectroscopy (LIBS) is used for measuring the concentration of cement ingredient. Cement powder samples are pressed into pellets. Laser pulses are focused on the surface of pellets. A microplasma is formed in the front of samples. The plasma emission contains information about the elemental composition of the samples. By assumption of local thermodynamic equilibrium (LTE) and using several standard cement samples, a calibration curve is prepared for each element. The major and minor elements of cement such as Ca, Si, K, Mg, Al, Na, Ti, Mn and Sr are qualitatively and quantitatively determined. For verification of LTE conditions, plasma parameters such as plasma electron temperature and electron density are computed. According to the obtained results, the LIBS technique could be a suitable method for determination of elemental composition in the cement production industries.     

粉末状态对激光立体成形Zr55Cu30Al10Ni5块体非晶合金晶化行为的影响

基于金属熔体结构的遗传性, 激光熔池的快速熔凝导致粉末的晶化状态可能会对最终成形件的晶化产生重要影响, 理清其影响规律对于制备大块非晶合金具有重要意义. 本文选取等离子旋转电极法所制粉末和1000 K退火态粉末为沉积材料, 采用激光立体成形技术沉积Zr₅₅Cu₃₀Al₁₀Ni₅块体非晶合金, 考察了粉末中已有晶化相对熔池及热影响区晶化行为的影响. 结果发现, 原始粉末组织由非晶相及粗大的Al₅Ni₃Zr₂相组成; 当激光线能量较低时, 相应熔覆层的熔池和热影响区皆含有Al₅Ni₃Zr₂相; 随着线能量的提高, 熔池中Al₅Ni₃Zr₂相消失, 保持了非晶态, 但热影响区晶化加重, 并有大量Al₅Ni₃Zr₂相析出; 当采用退火态粉末时, 即使线能量较小, 相应熔覆层仍主要由非晶构成, 几乎无Al₅Ni₃Zr₂相析出. 这是由于原始粉末在退火时其微观结构发生重排, 与Al₅Ni₃Zr₂相关的原子短程/中程有序结构减少, 导致已沉积层非晶区的热稳定性提高, 不利于Al₅Ni₃Zr₂相析出. 可见, 提高线能量将会加剧非晶沉积体的晶化, 而粉末中的Al₅Ni₃Zr₂团簇相状态对Zr₅₅Cu₃₀Al₁₀Ni₅合金沉积层的晶化有重要影响.     

Laser sintering of green compacts

Green Compact Laser Sintering (GCLS) is a new technique for sintering powder metallurgical components by laser irradiation. After mixing; powders are pressed into a green compact; which can then be sintered by laser irradiation. The properties of powder metallurgical alloys for GCLS and conventional sintering are compared.     

Yttria catalyzed microstructural modifications in oxide dispersion strengthened V–4Cr–4Ti alloys synthesized by field assisted sintering technique

Abstract

The present work deals with synthesis, characterisation and elevated temperature mechanical property evaluation of V–4Cr–4Ti and oxide (yttria = 0.3, 0.6 and 0.9 at%) dispersion strengthened V–4Cr–4Ti alloy processed by mechanical alloying and field-assisted sintering, under optimal conditions. Microstructural parameters of both powder and sintered samples were deduced by X-ray diffraction (XRD) and further confirmed with high resolution transmission electron microscopy. Powder diffraction and electron microscopy study show that ball milling of starting elemental powders (V–4Cr–4Ti) with and without yttria addition has resulted in single phase α–V (V–4Cr–4Ti) alloy. Wherein, XRD and electron microscopy images of sintered samples have revealed phase separation (viz., Cr–V and Ti–V) and domain size reduction, with yttria addition. The reasons behind phase separation and domain size reduction with yttria addition during sintering are extensively discussed. Microhardness and high temperature compression tests were done on sintered samples. Yttria addition (0.3 and 0.6 at.%) increases the elevated temperature compressive strength and strain hardening exponent of α–V alloys. High temperature compression test of 0.9 at% yttria dispersed α–V alloy reveals a glassy behaviour.     

激光立体成形中熔池凝固微观组织的元胞自动机模拟

本文通过采用自适应网格技术, 将激光立体成形的宏观温度场模型和凝固微观组织的低网格各向异性元胞自动机模型(cellular automaton, CA)结合, 建立了适用于激光立体成形的集成数值模型. 模型包括基材的温度场分布, 熔池形貌和熔凝过程的凝固微观组织. 模拟了激光扫描速度为15 mm/s时, 激光作用在Fe-C单晶基材上形成熔池的形状以及熔池内凝固微观组织. 计算结果揭示了熔池内固液界面从平界面失稳到胞\枝晶的非稳态凝固过程, 并得到了平界面组织形成的白亮带. 白亮带上方形成了外延生长的枝晶列.     

Quantification of a Ti(CxN1−x) based multilayer by Auger Electron Spectroscopy

Auger Electron Spectroscopy (AES) is an analytical technique sensitive to the surface of materials and providing elemental and chemical composition of conductive samples. The excellent spatial resolution and its quantification possibilities, even for light elements, make AES a commonly used technique to investigate surface and interfaces.TiN-like materials have a wide range of applications depending on their stoichiometry, but their composition is still complex (or at least not straightforward) to determine because of a strong overlapping of the Ti LMM with the N KLL Auger transitions. This quantification problem can be circumvented using computerised calculations as target factor analysis (TFA) to estimate the different nitrogen and titanium contributions in this peaks overlap. However, a more simple method, based on the study of Ti LMM and Ti LMV area ratio of pure TiN and TiC reference samples, is described in this paper and can be used to obtain the atomic composition of any titanium nitride based compound, whatever the complexity of the material. This method is an alternative to easily quantify TiN-like compounds by AES.As an illustration, a Ti(C_xN_1−x) based multilayer deposited on a hardmetal substrate was investigated. This quantification method was successfully used to evidence three different layers and the diffusion phenomenon at the interfaces between the layers. This study was completed with a quantitative SIMS depth profile that the high sensitivity and depth resolution allowed to measure the small variations of composition lower than the uncertainty of AES.     

Sintering behavior and PTCR properties of stoichiometric blend BaTiO3

Donor doped positive temperature coefficient of resistivity barium titanate is highly sensitive to minor changes in processing conditions, Ba/Ti ratio, and dopant concentration. This leads to a lack of reproducibility of properties and microstructure. This study was performed in an effort to obtain a more microstructurally stable PTCR material. Barium titanate ceramics were prepared by blending Ba-excess BaTiO₃ powder with Ti-excess powder, in different ratios. Such donor modified blended systems display uniform, medium grain size (4-6 μm), high-density microstructures which are more stable to changes in processing parameters. The microstructures are characterized by flat grain edges, large grain-to-grain contact area and high degree of domain coherence across grain boundaries. The PTCR effect was, however, measured to be nominal in these samples. This has been attributed to the presence of a smaller barrier potential, and such microstructural features as strong domain coherence across grain boundaries, large grain-to-grain contact area, and high density. It was found that the simple act of blending donor doped BaTiO₃ powders of different Ba/Ti ratios drastically changes both microstructure and electrical properties. Blending results in the suppression of liquid-phase induced anomalous grain growth, suppressing grain growth processes and allowing sintering processes to go to a greater degree of completion. The proposed mechanism whereby this happens is that the presence of the donor in blended systems either changes the kinetics of liquid-phase formation and/or the wettability of grains, affecting liquid-phase distribution.     

Comparison of ball milling of Fe/Cr powders and Fe-Cr crystalline alloy

Using the mechanical attrition technique (MA), we have prepared a Fe-Cr alloy starting with a mixture of elemental iron and chromium powders with a nominal composition of 28 at% of Fe and 72 at% of Cr. MA was also performed on solid solutions of Fe₂₈Cr₇₂ crystalline alloy. The Mössbauer effect of the mechanically alloyed powder from Cr and Fe metals has been compared with that from crystalline alloy.     

类Ne钛X射线激光输出特性实验

 在星光激光装置上, 使用X射线多层膜反射镜、铝滤片及X射线CCD探测器组成高灵敏度空间分辨探测系统, 对用预脉冲技术产生的类Ne钛单线 (J=0-1, 3p-3s跃迁)软X射线激光输出特性进行了测量。由实验得到了类Ne钛X 射线激光束近场与距靶端32mm处(准远场)强度的空间分布, 给出了该激光束在近场强度最大区域的空间尺度(FWHM)约为72μm×31μm, 以及在垂直靶面与平行靶面方向的束发散角分别约为4mrad与9mrad。     

Experiment study of powder flow feed behavior of laser solid forming

A photographic system for the powder feed process of laser solid forming (LSF) was developed using a high speed camera, and the powder feed behaviors (the particle speed and the powder flow concentration) were described based on the powder flow images. The influences of the powder feed parameters and the distance below the nozzle exit plane on the powder feed behaviors were discussed, and the influences of the powder feed behaviors on the deposited layer quality were also investigated. It can be seen that the smooth finish of the deposited layer surface was improved remarkably by increasing the particle speed, and the deposited layer height decreases with the increase of the particle speed. It can also be found that the variation of the deposited layer height with the increase of the distance between the deposited surface and the nozzle exit plane is similar to that of the powder mass concentrations on the vertical symmetry axis.     

Crystal growth, FTIR and thermal characterization of bis(ethyltriphenylphosphonium) tetrabromomanganate(II) dihydrate crystals

Single crystals of a novel compound, bis(ethyltriphenylphosphonium) tetrabromomanganate(II) dihydrate (BTP-Mn) were grown by solution growth-slow evaporation technique from aqueous solution of the compound at ambient temperature. The grown crystals were characterized by elemental analysis, powder X-ray diffraction, thermal analysis, nuclear magnetic resonance spectroscopy (NMR) and Fourier transform infra-red spectroscopy (FTIR) techniques. The chemical composition of the compound was revealed by elemental analysis and its crystallinity was confirmed by powder X-ray diffraction. Thermal analysis confirmed that the compound was stable up to 125°C. The various kinds of protons and carbons present in the compound were confirmed by ¹H NMR and ¹³C NMR technique respectively and the presence of phosphorous was confirmed by ³¹P NMR spectrum in the compound. The modes of vibration of different molecular groups present in the compound were identified by FTIR spectral analysis. The second harmonic generation behaviour was tested by Nd:YAG laser source.     

XPS study of porous dental implants fabricated by electro-discharge-sintering of spherical Ti-6Al-4V powders in a vacuum atmosphere

A single electro-discharge-sintering (EDS) pulse (0.7-2.0 kJ/0.7 g), from a 300 μF capacitor, was applied to atomized spherical Ti-6Al-4V powder in a vacuum to produce a porous-surfaced implant compact. A solid core surrounded by a porous layer was formed by a discharge in the middle of the compact. X-ray photoelectron spectroscopy was used to study the surface characteristics of the implant material. C, O, and Ti were the main constituents, with smaller amounts of Al, V, and N. The implant surface was lightly oxidized and was primarily in the form of TiO₂ with a small amount of metallic Ti. A lightly etched EDS implant sample showed the surface form of metallic Ti, indicating that EDS breaks down the oxide film of the as-received Ti-6Al-4V powder during the discharge process. The EDS Ti-6Al-4V implant surface also contained small amounts of aluminum oxide in addition to TiO₂. However, V detected in the EDS Ti-6Al-4V implant surface did not contribute to the formation of the oxide film. The small amount of N in the implant surface resulted from nitride material that was also found in the as-received Ti-6Al-4V powders.     

Nitrogen containing barium titanate: Preparation and characterisation

The purpose of this work is to study the incorporation of nitrogen into barium titanate at oxygen sites by ammonolysis. The nitrogen content of the resulting barium titanate powders strongly depends on the flow rate of ammonia and the ratio Ba/Ti. The amount of resulting oxygen vacancies is lowered by co-doping with fluoride and tantalum.By avoiding an excess of BaO and exchanging 10 mol% Ti by Ta it is possible to produce powders with nitrogen content of more than 1 wt%. Partial substitution of barium oxide by barium fluoride also increases the nitrogen content. The nitrogen containing powders are colored and show an additional absorption at 500 nm measured in the UV/vis reflectance spectrum. The concentrations of nitrogen and fluorine in the samples were determined using ion-sensitive electrodes. X-ray powder diffraction shows the composition of the powders. The thermal stability was investigated by thermogravimetric analysis, and in order to calculate the specific surface area the three-point BET method was used.     

显微红外反射光谱法快速鉴别珍珠粉真伪

珍珠是名贵的装饰品和中药材,在我国对其已有2000多年的应用历史。研究发现,贝壳内层的化学组成与珍珠相似,主要含有碳酸钙和各种氨基酸。贝壳粉也可在临床上做药用,但其药用价值远低于珍珠粉。然而,珍珠粉和贝壳粉外观特性极为相似,元素组成和形貌检测无法对其进行有效地鉴别。由于贝壳粉的成本低,常常冒充珍珠粉流入市场,侵害了消费者的利益。本研究通过珍珠粉和贝壳粉在不同温度下煅烧一定时间后显微红外反射光谱的差异实现珍珠粉真伪鉴别。实验结果表明,在一个大气压下经过400 ℃煅烧30 min后,珍珠粉中文石型碳酸钙部分转化为方解石型,而贝壳粉中文石型碳酸钙完全转化为方解石型。这种不同的相转变过程可以被显微红外反射光谱很好的区别。因此,可在此条件下利用红外反射光谱鉴别珍珠粉和贝壳粉相变差异。和传统的透射式红外光谱法相比,这种显微反射红外光谱法无需压片制样、快速灵敏。本研究也讨论了该方法针对其他常见伪品的适用性,如牡蛎粉和石决明粉,结果表明该方法能够简单、高效和准确地鉴别珍珠粉真伪。