W. Hou

Influence of cell shape on mechanical properties of Ti-6Al-4V meshes fabricated by electron beam melting method

Ti-6Al-4V reticulated meshes with different elements (cubic, G7 and rhombic dodecahedron) in Materialise software were fabricated by additive manufacturing using the electron beam melting (EBM) method, and the effects of cell shape on the mechanical properties of these samples were studied. The results showed that these cellular structures with porosities of 88-58% had compressive strength and elastic modulus in the range 10-300MPa and 0.5-15GPa, respectively. The compressive strength and deformation behavior of these meshes were determined by the coupling of the buckling and bending deformation of struts. Meshes that were dominated by buckling deformation showed relatively high collapse strength and were prone to exhibit brittle characteristics in their stress-strain curves. For meshes dominated by bending deformation, the elastic deformation corresponded well to the Gibson-Ashby model. By enhancing the effect of bending deformation, the stress-strain curve characteristics can change from brittle to ductile (the smooth plateau area). Therefore, Ti-6Al-4V cellular solids with high strength, low modulus and desirable deformation behavior could be fabricated through the cell shape design using the EBM technique.

The influence of cell morphology on the compressive fatigue behavior of Ti-6Al-4V meshes fabricated by electron beam melting.

Additive manufacturing technique is a promising approach for fabricating cellular bone substitutes such as trabecular and cortical bones because of the ability to adjust process parameters to fabricate different shapes and inner structures. Considering the long term safe application in human body, the metallic cellular implants are expected to exhibit superior fatigue property. The objective of the study was to study the influence of cell shape on the compressive fatigue behavior of Ti-6Al-4V mesh arrays fabricated by electron beam melting. The results indicated that the underlying fatigue mechanism for the three kinds of meshes (cubic, G7 and rhombic dodecahedron) is the interaction of cyclic ratcheting and fatigue crack growth on the struts, which is closely related to cumulative effect of buckling and bending deformation of the strut. By increasing the buckling deformation on the struts through cell shape design, the cyclic ratcheting rate of the meshes during cyclic deformation was decreased and accordingly, the compressive fatigue strength was increased. With increasing bending deformation of struts, fatigue crack growth in struts contributed more to the fatigue damage of meshes. Rough surface and pores contained in the struts significantly deteriorated the compressive fatigue strength of the struts. By optimizing the buckling and bending deformation through cell shape design, Ti-6Al-4V alloy cellular solids with high fatigue strength and low modulus can be fabricated by the EBM technique.

High power passively mode-locked Nd:YVO 4 laser using graphene oxide as a saturable absorber

We report on a passively mode-locked Nd:YVO4 laser using a novel graphene oxide saturable absorber fabricated by vertical evaporation method. An 880 nm LD pump source was used to reduce the thermal load of the laser crystal. At the pump power of 7.4 W, 1.2 W average output power of continuous wave mode-locked laser with optical conversion efficiency of 16.2% was achieved. To the best of our knowledge, this is the highest output power of passively mode-locked solid-state laser using graphene oxide saturable absorber. The repetition rate of passively mode-locked pulse was 88 MHz with the pulse energy of 13.6 nJ.

Passive mode-locked Nd:YVO 4 laser using a multi-walled carbon nanotube saturable absorber

We report a passive mode-locked Nd:YVO4 laser pumped by 880 nm LD using a transmission-type multi-walled carbon nanotube saturable absorber. At the pump power of 6.1 W, the average output power of 0.8 W of continuous wave mode-locked laser with optical conversion efficiency of 13.1% was generated. The repetition rate and pulse energy of the mode-locked pulse were 88 MHz and 9.1 nJ, respectively.

High power, high energy nanosecond pulsed fiber amplifier with a 20-μm-core fiber

We report on a master-oscillator fiber power amplifier (MOPA) system with a core diameter of only 20 μm and 6-m-long Yb3+-doped large mode area (LMA) double-clad fiber. Actively Q-switched Nd:YVO4 laser is used as a seed source of light pluses. The system works at the repetition rate from 40 to 100 kHz. Up to 77 W of amplified radiation with the pulse duration of 17.8 ns at the wavelength of 1064 nm and repetition rate of 40 kHz are generated, corresponding to pulse energy of 1.9 mJ and the slope efficiency of 73.5%. In reported literatures, the 77 W is the highest average power with a 20-μm-core LMA fiber to our knowledge. The stimulate Brillouin scattering (SBS) is observed and investigated.

Microstructure and mechanical properties of open cellular Ti–6Al–4V prototypes fabricated by electron beam melting for biomedical applications

Titanium and titanium alloys with open cellular structures and foams possess low Youngs modulus matching to human bone and the capability to provide space for bone tissue ingrowth to reach a better fixation, which have been thought as a good choice for the replacement of commercial dense implants. However, currently developed fabrication methods of titanium cellular alloys, such as powder sintering, reveal shortcomings like the low porosity and poor control over the size, shape and distribution of the pores. Recently, additive manufacturing using the electron beam melting method has been applied successfully to fabricate titanium cellular meshes and foams. Compared to other reported methods, this technique has the advantages of accurate control of internal pore architectures and complex cell shapes. In the present paper, the authors briefly review the fabrication, Youngs modulus, mechanical properties and biocompatibility of Ti–6Al–4V cellular structures fabricated by electron beam melting techniques, along with future development trends.

High efficiency passive mode-locked Nd:YVO(4) laser based on SESAM under direct pumping at 880 nm

We report on an 880 nm LD pumped passive mode-locked TEM00 Nd:YVO4 laser based on a semiconductor saturable absorber mirror (SESAM), with a high optical-to-optical conversion efficiency of 67.3%, and a slope efficiency of 71%. When the absorbed pump power was 11 W, 7.4 W average output power of 1064 nm continuous-wave mode-locked laser was achieved. To our knowledge, this is the highest optical-to-optical conversion efficiency among all the published reports of 880 nm LD pumped SESAM passive mode-locked lasers. The repetition rate of mode-locked pulse was 80 MHz with 26 ps pulse width. The maximum pulse energy and peak power were 92.5 nJ and 3.6 kW, respectively.

LD side-pumped 41 W high beam quality acousto-optical Q-switched single-rod Nd:YAG laser

We report a LD side-pumped high beam quality (Mx2 = 1.20 and My2 = 1.19) acousto-optic (AO) Q-switched single-rod Nd:YAG laser with a TEM00-mode dynamic stable cavity. At the pump power of 600 W, 41 W TEM00-mode 1064 nm laser was achieved with electro-optical conversion efficiency of 7%. The repetition rate and pulse width were 30 kHz and 102 ns, respectively with pulse energy of 1.4 mJ and peak power of 13 kW. Up to 24 W of 532 nm green laser was generated by external frequency doubling, corresponding to 59% optical conversion efficiency.

Fabrication of open-cellular (porous) titanium alloy implants: osseointegration, vascularization and preliminary human trials

In this study we describe the fabrication of a variety of open-cellular titanium alloy (Ti-6Al-4V) implants, both reticular mesh and foam structures, using electron beam melting (EBM). These structures allow for the elimination of stress shielding by adjusting the porosity (or density) to produce an elastic modulus (or stiffness) to match that of both soft (trabecular) and hard (cortical) bone, as well as allowing for bone cell ingrowth, increased cell density, and all-matrix interactions; the latter involving the interplay between bone morphogenetic protein (BMP-2) and osteoblast functions. The early formation and characterization of elementary vascular structures in an aqueous hydrogel matrix are illustrated. Preliminary results for both animal (sheep) and human trials for a number of EBM-fabricated, and often patient-specific Tialloy implants are also presented and summarized. The results, while preliminary, support the concept and development of successful, porous, engineered “living” implants.摘要本文采用电子束增材(EBM)制造技术制备了多种具有开放孔隙结构的多孔钛合金(Ti-6Al-4V)植入物, 包括网状和泡沫状结构. 该多孔钛合金植入物可以通过调节孔隙率(或密度)降低其弹性模量(或刚度)以减轻 “应力屏蔽” 效应, 实现与软(小梁)和硬(皮质)骨的弹性模量(或刚度)匹配; 同时还可以促进骨组织长入, 增加细胞密度和细胞外基质间的相互作用, 后者涉及骨形态发生蛋白(BMP-2)和成骨细胞功能之间的相互影响. 总结了在水性水凝胶基质中初级血管结构的早期形成和特征, 报道了EBM技术制备的个性化钛合金植体在动物(羊)和人体临床试验的初步结果. 本文结果为钛合金多孔材料作为组织工程“活性”植入物的应用可行性研究提供了有力支持.

A high average power single-stage picosecond double-clad fiber amplifier

In this paper, we report 38.8 W average power output through a single-stage fiber amplifier, with emission of 1064 nm wavelength with 80 MHz repetition and 35 ps pulse width amplified from a 2.15 W SESAM passively mode-locked Nd:YVO4 laser oscillator. The high power fiber amplification is through a coupled 60.8 W 976 nm backward unidirectional pump power into a 2 m long 30/250 μm Yb-doped inner cladding. No obvious nonlinear effects arise in the high power output. To our knowledge this is the highest average power output with 2 m 30/250 μm Yb-doped double-clad fiber in a single-stage picosecond fiber amplifier.