Showing papers by "Harbin Institute of Technology published in 2001"

Three-dimensional hole drilling of silica glass from the rear surface with femtosecond laser pulses.

Abstract: By moving silica glass in a preprogrammed structure, we directly produced three-dimensional holes with femtosecond laser pulses in single step. When distilled water was introduced into a hole drilled from the rear surface of the glass, the effects of blocking and redeposition of ablated material were greatly reduced and the aspect ratio of the depth of the hole was increased. Straight holes of 4‐μm diameter were more than 200 μm deep. Three-dimensional channels can be micromachined inside transparent materials by use of this method, as we have demonstrated by drilling a square-wave-shaped hole inside silica glass.

Optical image encryption with multistage and multichannel fractional Fourier-domain filtering.

Abstract: We present a novel image-encryption algorithm that employs multichannel and multistage fractional Fourier-domain filtering architecture. We perform the encryption and decryption by randomly filtering the spatial frequency of the image and then recombining the information from the algorithm in a multistage fractional Fourier domain with pure random-intensity-encoded masks and their complements in a multichannel scheme. The algorithm can be implemented iteratively in an electro-optical setup. Numerical simulations have verified the validity of the algorithm.

Effect of interfacial reaction on mechanical behavior of SiCw/AZ91 magnesium matrix composites

Abstract: SiCw/AZ91 magnesium matrix composites with acid aluminum phosphate (Al(PO3)3) binder and without binder were fabricated by a squeeze casting process. The interfacial microstructure, tensile properties and fracture behavior of the composites were studied. For the composites with no binders, the SiCw-AZ91 interfaces are very clean, whereas for the composites with acid phosphate binders, there exist fine, uniform and discrete interfacial MgO reaction products at the interface. The MgO particles are formed from the reaction between the binder and the molten Mg. There is a definite crystallographic orientation relationship between the reaction product MgO and SiCw, which leads to an interface with good lattice matching. The fine discrete interfacial MgO reaction products increase the whisker–matrix interfacial bonding strength, which is beneficial to the load transfer from the matrix to whisker, thus leading to improved Young's modulus and tensile strength of the composites with Al(PO3)3 binder over the composites without binder.

Kinetic theory of fluidized binary granular mixtures.

Abstract: Balance laws and constitutive relations for a binary granular mixture with unequal granular temperature are derived. The complete pair distribution function for two colliding spheres was assumed to be the product of Maxwellian velocity distributions for each phase. The constitutive relations together with the transport equations form a model for a binary granular mixture with unequal granular temperature. An analytical expression for viscosity of each phase shows the effect of different masses and granular temperatures of particles

Comparison of laser-clad and furnace-melted Ni-based alloy microstructures

Abstract: Optimization of process variables for laser cladding of Ni-based alloys was performed using the pre-placed powder method. The microstructure of laser clad, under optimal processing conditions, and furnace melted, under near equilibrium conditions, Ni-based alloys has been comparatively investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. Comparison of the microstructures of the laser-clad and furnace-melted alloys revealed a remarkable difference. The microstructure of the laser cladding is complex, composed of blocky CrB type chromium carbon borides, orthorhombic structured Cr 7 C 3 type dendritic carbides, cellular-dendritic ν-Ni solid solution, different interdendritic eutectics and amorphous phases along grain boundaries. The interdendritic eutectics, either ν-Ni+M 23 C 6 or ν-Ni+Ni 3 B (Ni 2 B), can form depending on the local composition. ν-Ni+Ni 3 B stable solidification and ν-Ni+Ni 2 B metastable solidification exist simultaneously because of the non-equilibrium rapid solidification involved during the laser cladding. In contrast, the microstructure of furnace-melted Ni-based alloy under near equilibrium solidification is composed of hexagonal structured Cr 7 C 3 type carbides with hexagonal prism morphologies, near-equiaxed ν-Ni solid solution dispersed with fine Ni 3 Si precipitates, ν-Ni+Ni 3 B near lamellar eutectic and near-spherical Ni 3 B compound. ν-Ni is the main microstructural constituent in both the laser clad and furnace melted alloys. From the grain size, it was evident that the former is one to two orders finer than the latter.

Elevated temperature ablation resistance and thermophysical properties of tungsten matrix composites reinforced with ZrC particles

Abstract: In order to improve the elevated temperature ablation resistance of the copper infiltrated tungsten materials (Cu/W) which are used as the rudders of the rocket motors at present, a new kind of tungsten matrix composites reinforced by 30 vol% zirconium carbide particles (ZrCp/W) was developed, and the elevated temperature ablation resistance and thermophysical properties of the new materials were investigated. The values of the mass ablation rate and linear ablation rate of ZrCp/W were much lower than that of Cu/W, which indicated that the addition of ZrC particles in tungsten matrix remarkably increased the ablation resistance. Thermal thermochemical oxidation of tungsten and ZrC particles was the main ablation mechanism of ZrCp/W. A self-made dynamic responding multi-wavelength pyrometer was employed to measure the ablated surface temperatures and a thermal couple was employed to measure the back surface temperatures. The important temperature curves of the ablated surface and back surface of the specimens were successfully detected online, which indicated that ZrCp/W has good thermal shock resistance. The present of ZrC particles in tungsten matrix remarkably decreased the thermal conductivity and thermal diffusivity of the ZrCp/W composites, which is very useful for increasing the elevated temperature ablation resistance of the composites. The excellent elevated temperature ablation resistance of ZrCp/W is attributed to the lower thermal conductivity, lower thermal diffusivity, oxidation resistance and high melting points. The good properties of ZrCp/W makes it be suitable for the rudders for rocket motors instead of Cu/W.

Design of a smart functionally graded thermopiezoelectric composite structure

Abstract: In this paper, a smart functionally graded piezoelectric structure is analyzed. The smart structure consists of three layers: one layer of metal, one layer of piezoelectric material (PZT) used as an actuator, and a graded metal/PZT layer between the metal layer and the PZT layer. A finite element code is developed in the programming environment MATLAB and FORTRAN, with each finite element having varied material properties through space coordinates. The results reveal that both the stress discontinuity and thermal deformation of the structure can be controlled. By introducing a functionally graded layer between the PZT actuator layer and the metal beam layer, both stress discontinuity and the edge local stresses can be essentially reduced. The proposed method is expected to be useful for functionally graded thermopiezoelectric composite structures under operating environments where the thermal and piezoelectic effects are important.

Laser cladding of Ti-6Al-4V alloy with TiC and TiC + NiCrBSi powders

Abstract: Laser cladding of Ti-6Al-4V alloy with TiC and TiC+NiCrBSi powders was carried out, and microstructure, as well as microhardness profile, in the clad layers was examined. The results showed that, in the TiC clad layer, TiC was melted and solidified to form dendrites in the clad zone, and dissolved into the melted Ti-alloy substrate and precipitated to form well-developed dendrites in the dilution zone. With increasing specific laser energy, the dilution effect of the Ti-alloy substrate was enhanced, and the microhardness decreased in both the clad and the dilution zones. In the TiC+NiCrBSi laser clad layers, TiC particles dissolved into the melted Ni-based alloy (binder material) in the clad zone. With increasing specific laser energy, the degree of solution of TiC particles was increased. During cooling, fine spherical particles and dendrites of TiC precipitated from the Ni-based alloy. When the TiC volume fraction increased to more than 50%, clustering of TiC particles was observed in the clad zone. The clustering of TiC particles resulted in a decrease in the homogeneity of the microstructure and microhardness distribution in the clad zone. The dilution zone of the TiC+NiCrBSi clad layers is a mutually melted region of the Ni-based alloy and titanium-alloy substrate and presents a microstructure of dendrites.

Experimental analysis of sea clutter in shipborne HFSWR

Abstract: The spreading of the dominant first order Bragg lines in shipborne HFSWR (high-frequency surface wave radar) severely obscures targets with Doppler frequencies within the spreading domain. STAP (space-time adaptive processing) is one of the effective methods for solving the problem. The experimental one-dimensional spreading spectrum and two dimensional space-time spectrum of sea clutter in shipborne HFSWR are given. The eigenvalue distribution of the space-time covariance matrix is also obtained. All experimental results show good agreement with the spreading model of sea clutter and theoretical analysis of the eigenspectrum for a random process, which is of advantage for sea clutter suppression and target detection in shipborne HFSWR with STAP.

Rapid prototyping and combustion synthesis of TiC/Ni functionally gradient materials

Abstract: TiC–Ni functionally gradient materials (FGM) parts were fabricated by laminated object manufacturing (LOM), one of rapid prototyping (RP) techniques and combustion synthesis technique. The microstructure and phases of TiC–Ni FGM were analyzed with SEM and XRD. TiC–Ni FGM had anisotropic mechanical properties. It was stronger in the direction parallel to the thickness than in the direction perpendicular to the thickness. The maximum strength was 950 MPa in the TiC–20wt.%Ni region. The hardness of TiC–Ni FGM was larger than HRA 84 and the density was larger than 5.2 g cm −3 . When the content of Ni raised the density increased, then there was the largest relative density in the TiC–20wt.%Ni region.

Al2O3 Ceramics Preparation by LOM (Laminated Object Manufacturing)

Abstract: Al 2 O 3 ceramics green tape of 0.7 mm thickness was fabricated by a roll-forming technique. Ceramic green parts were formed by LOM (laminated object manufacturing), a kind of RP (rapid prototyping) technique. Final products were produced by a pressureless sintering process. Thermal analysis and SEM microstructure analysis of green tape were carried out to decide on the best process for removing the binder. Using a SEM, the microstructure of the ceramic parts was analysed. The mechanical properties of the parts were also analysed.