Fabrication of carbon nanotubes reinforced AZ91D composites by ultrasonic processing

Magnesium matrix nanocomposite reinforced with carbon nanotubes(CNTs/AZ91D) was fabricated by mechanical stirring and high intensity ultrasonic dispersion processing.The microstructures and mechanical properties of the nanocomposite were investigated.The results show that CNTs are well dispersed in the matrix and combined with the matrix very well.As compared with AZ91D magnesium alloy matrix,the tensile strength,yield strength and elongation of the 1.5%CNTs/AZ91D nanocomposite are improved by 22%,21%and 42%respectively in permanent mold casting.The strength and ductility of the nanocomposite are improved simultaneously.The tensile fracture analysis shows that the damage mechanism of nanocomposite is still brittle fracture.But the CNTs can prevent the local crack propagation to some extent.

Shape formation of closed-cell aluminum foam in solid–liquid–gas coexisting state

The mold pressing process was applied to investigate the formability of closed-cell aluminum foam in solid–liquid–gas coexisting state.Results show that the shape formation of closed-cell aluminum foam in the solid–liquid–gas coexisting state was realized through cell wall deformation and cell movement caused by primary α-Al grains that slid,rotated,deformed,and ripened within cell walls.During formation,characteristic parameters of closed-cell aluminum foam were almost unchanged.Under proper forming conditions,shaped products of closed-cell aluminum foam could be fabricated through mold pressing.

Optimization of Cross-sectional Shapes of the Bi-2223/Ag Wires before Flat Rolling

Rolling process plays an important role in the manufacture of Bi-based high temperature superconductor tapes, and the plastic flow regularities of the superconducting wires during deformation will directly affect the ultimate quality of the tapes. In order to investigate the effect of cross-sectional shapes before fiat rolling on the performance and homogeneity of the tapes, some numerical models of Bi-2223/Ag wires with different cross-sectional shapes including circular, square, elliptical and racetrack cross-sections are constructed during the rolling process. By comparing the relative density, logarithmic strain ratio and length-width ratio on the filaments, it is revealed that Bi-2223/Ag wire with special-shaped cross-section can achieve better conductivity than the round wire, in particular, the racetrack cross-sectional wire has the second best performance among four wires. Based on material processability and experimental condition, tri-pass racetrack drawing technique is employed to optimize the process and obtain racetrack cross-sectional wire. The rolling process of Bi-2223/Ag wire with racetrack cross-section causes more intensive deformation of filaments in the center of the tape and achieves the filaments with larger length-width ratio. Also, the deformation distribution of filaments verifies the numerical results. Consequently, the racetrack drawing technique can be utilized for a reference during the mechanical processing and to increase the current transmission capacities of Bi-2223/Ag tapes.

Development of Gesture-Changeable under-actuated Humanoid Robotic Finger

Robotic fingers, which are the key parts of robot hand, are divided into two main kinds： dexterous fingers and under-actuated fingers. Although dexterous fingers are agile, they are too expensive. Under-actuated fingers can grasp objects self-adaptively, which makes them easy to control and low cost, on the contrary, under-actuated function makes fingers feel hard to grasp things agilely enough and make many gestures. For the purpose of designing a new finger which can grasp things dexterously, perform many gestures and feel easy to control and maintain, a concept called ＂gesture-changeable under-actuated＂ （GCUA） function is put forward. The GCUA function combines the advantages of dexterous fingers and under-actuated fingers： a pre-bending function is embedded into the under-actuated finger. The GCUA finger can not only perform self-adaptive grasping function, but also actively bend the middle joint of the finger. On the basis of the concept, a GCUA finger with 2 joints is designed, which is realized by the coordination of screw-nut transmission mechanism, flexible drawstring constraint and pulley-belt under-actuated mechanism. Principle analyses of its grasping and the design optimization of the GCUA finger are given. An important problem of how to stably grasp an object which is easy to glide is discussed. The force analysis on gliding object in grasping process is introduced in detail. A GCUA finger with 3 joints is developed. Many experiments of grasping different objects by of the finger were carried out. The experimental results show that the GCUA finger can effectively realize functions of pre-bending and self-adaptive grasping, the grasping processes are stable. The GCUA finger excels under-actuated fingers in dexterity and gesture actions and it is easier to control and cheaper than dexterous hands, becomes the third kinds of finger.

Densification of in situ prepared mesocarbon microbead/carbon nanotube composites by hot-press sintering

In situ prepared mesocarbon microbead/carbon nanotube(MCMB/CNT) composites are potential precursors of high density carbon materials for various applications. Integrated MCMB/CNT composites were successfully fabricated by hot-press sintering at 550 ℃ under 30 MPa. After further calcination at 900 ℃, the hot-press sintering fabricated MCMB block has an apparent density of 1.77 g/cm3 and the open porosity 5.1%. With the addition of 5%(mass fraction) CNTs, the density of the composite block is elevated to 1.84 g/cm3, and its open porosity is reduced to 3.5%. The flexural strength of composite block with 5% CNTs is elevated to 116 MPa. Through the hot-press sintering, pores of 10-50 nm in the calcinated bulks are remarkably eliminated. The interstice between microbeads in the composite blocks is filled up by CNTs together with β-resin and quinoline-insoluble spheres, which can further contribute to the densification.

NUMERICAL SIMULATION OF MECHANICAL BEHAVIORS OF SUPERCONDUCTING POWDER BSCCO (BiSrCaCuO)

An equivalent continuum method and a deformable discrete method to describe the mechanical behaviors of superconducting powder BSCCO （BiSrCaCuO） aggregate are studied syste-matically. The equivalent continuum model idealizes the aggregation of the powder as an equivalent continuum material. The powder aggregate yielding is caused by not only the deviatoric stress but also the hydrostatic stress and the modified Drucker-Prager/Cap model is adopted to describe the mechanical behaviors of BSCCO powder aggregate in continuum method. The deformable discrete model is known as a direct model, which considers the discrete nature of the powder particles. Its framework encompasses the local behaviors between the particles, such as particles contact, sliding and rolling. Based on commercial finite element software ABAQUS, the equivalent continuum model and the deformable discrete model are used to simulate the confined compression of superconducting powder BSCCO, and the numerical results show agreement with experimental results, which verify the correctness of these built models. Compared with the equivalent continuum model based on macroscopic statistics method, the deformable discrete model can present the microscopic information during processing and can describe the nature of mechanical behaviors of superconducting powder BSCCO. But from an industrial viewpoint, the equivalent continuum model has a definitive edge over the microscopic models in that the gross behavior of the powder mass can be modeled and simulated on an industrial scale.

Static recrystallization behavior of 25CrMo4 mirror plate steel during two-pass hot deformation

The static recrystallization behavior of 25CrMo4 mirror plate steel has been determined by hot compression testing on a Gleeble 1500 thermal mechanical simulation tester. Compression tests were performed using double hit schedules at temperatures of 950-- 1 150 ~C, strain rates of 0.01--0.5 s-1 , and recrystallization time of 1--100 s. Results show that the kinetics of static recrystallization and the microstructural evolution were greatly influenced by the deformation parameters （deformation temperature, strain rate and pre strain） and the initial austenite grain size. Based on the experimental results, the kinetics model of static recrystallization has been generated and the comparison between the experimental results and the predicted results has been carried out. It is shown that the predicted results were in good agreement with the experimental results.

Effect of Processing Parameters on Thermal Phenomena in Direct Laser Metallic Powder Deposition

Direct laser metallic powder deposition technique is widely used in manufacturing, part repairing, and metallic rapid prototyping. The ability to predict geometrical accuracy and residual stress requires an understanding of temperature distribution during the deposition process. This study presents a numerical model of three-dimensional transient heat transfer in a finite model heated by a moving laser beam. Thermal phenomena in the process were investigated. The complex solid-liquid problem and latent heat of fusion were treated by means of equivalent thermal conductivity and modified specific heat, respectively. Using method of birth and death of elements, the growth of additive layers and the shape of melt pool were obtained. The effect of processing parameters such as absorbed power, travel speed, and preheated temperature on melt pool sizes and cross-section of deposited layer profile was studied. The results show that the melt pool sizes increase with absorbed power and decrease with travel velocity. In addition, the preheated temperature contributes less to the melt pool size. The results are generally in a good agreement with experiments in published literature.

Rapid Prototyping and Manufacturing Technology:Principle, Representative Technics, Applications, and Development Trends

The rapid prototyping and manufacturing technology (RPM), is an integration of many different disciplines. It is based on an advanced dispersed-accumulated forming principle and originated from 1980s. It generates an entity by first forming a series of layers according to the dispersed section information of the digital model, and then piling the formed layers sequentially together. It is capable of forming parts with complicated structures and non-homogeneous materials. Traditional RPM techniques are mainly used as prototypes in product invention process, such as stereolithography, three-dimensional printing, laminated object manufacturing, and fused deposition modeling. Later, with the progress of material and enabling technology, many new RPM techniques emerged out and have been already applied in the fields such as rapid tooling/moulding, direct formed usable part, nano-/micro-RPM, and biomanufacturing. This high flexible digital manufacturing method has a likely ability to become an almighty forming technology.

High Boron Iron-Based Alloy and Its Modification

High boron iron-based alloy,a new kind of wear-resistant material was developed,and its microstructure and mechanical properties were studied. Moreover,the modification with V,Ti and RE-Mg was carried out as well. The results indicate that the high boron iron-based alloy comprises a dendritic matrix and interdendritic eutectic borides under as-cast condition. The matrix is made up of fine pearlite,and the eutectic boride has a crystal structure of M2B （M represents Fe,Cr or Mn）. The boride has a microhardness of HV 1 425 and is distributed in the form of continuous network,which is detrimental to the toughness of high boron iron-based alloy. Nevertheless,high boron iron-based alloy has a higher toughness than that of white cast iron,which is attributed to the matrix that is made up of lath-type martensite with high toughness. In order to increase the toughness further,V,Ti and RE-Mg were adopted to improve the morphology of boride. The results indicate that after modification the matrix grain is decreased in size by half,and the size of boride is also decreased,moreover,it is distributed more even though it is still continuous. After heat treatment,the boride network is break up and results in the further improvement of toughness of high boron iron-based alloy.

Automatic Defect Detection in X-Ray Images Using Image Data Fusion

Automatic defect detection in X-ray images is currently a focus of much research at home and abroad. The technology requires computerized image processing, image analysis, and pattern recognition. This paper describes an image processing method for automatic defect detection using image data fusion which synthesizes several methods including edge extraction, wave profile analyses, segmentation with dynamic threshold, and weld district extraction. Test results show that defects that induce an abrupt change over a predefined extent of the image intensity can be segmented regardless of the number, location, shape or size. Thus, the method is more robust and practical than the current methods using only one method.