Surface amorphous and crystalline microstructure by alloying zirconium using Nd : YAG pulsed laser

A novel composite coating was synthesized by laser alloying of zirconium nanoparticles on an austenite stainless steel surface using a pulsed Nd:YAG laser. The coating contained duplex microstructures comprising an amorphous phase and an austenitic matrix. A discontinuous zirconium-containing region formed at a depth of 16 mum below the surface. The amorphous phase was present in the zirconium-rich region, with the composition of zirconium ranging from 7.8 to 14.5 at. pet. The formation of the amorphous phase was attributed to the zirconium addition. The hardness, corrosion, and wear-corrosion resistance of the irradiated coating were evidently enhanced compared to those of the stainless steel.

Application of enthalpy method for moving boundary problem in laser surface melting

A numerical analysis was carried out to study the moving boundary problem in the physical process of pulsed Nd-YAG laser surface melting prior to vaporization. The enthalpy method was applied to solve this two-phase axisymmetrical melting problem Computational results of temperature fields were obtained, which provide useful information to practical laser treatment processing. The validity of enthalpy method in solving such problems is presented.

Anomalous Thermal Stability of Nd-Fe-Co-Al Bulk Metallic Glass

The thermal stability of Nd60Fe20Co10Al10 bulk metallic glass (BMG) has been studied by differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), isochronal dilatation and compression tests. The results show that the glass transition of the BMG takes place quite gradually between about 460 and 650 K at a heating rate of 0.17 K/s. Several transformation processes are observed during continuous heating with the first crystallization process beginning at about 460 K, while massive crystallization takes place near the solidus temperature of the alloy. The positive heat of mixing between the two major constituents, Nd and Fe, and, consequently, a highly inhomogeneous composition of the attained amorphous phase are responsible for the anomalous thermal stability in this system. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Microstructure and Magnetic Properties of Nd60Al10Fe20Co10 Glass-forming Alloy

The microstructural variations of the Nd60Al10Fe20CO10 melt-spun ribbons and the as-cast rod were studied by high resolution transmission electron microscopy (HRTEM), x-ray diffraction (XRD) and differential scanning calorimetry. Nano-clusters in glassy m

Microstructure and wear resistance of in situ TiCp composite coating by laser cladding

A new in situ method was realized by one step laser cladding to produce Ni-base alloy composite coating reinforced by in situ reacted and gradiently distributed TiCp particles. The submicron TiCp particles were formed and uniformly distributed because of the in situ reaction and trapping effect under the rapid solidification condition. And, TiCp particles were of gradient distribution on a macro scale and their volume fraction increased from 1.86% at the layer/substrate interface to a maximum 38.4% at the surface of the layer. Furthermore, the in situ generated TiCp/gamma-Ni interfaces were free from deleterious surface reactions. Additionally, the clad coating also revealed a high microhardness of gradient variation with the layer depth and the superior abrasive wear resistance.

Micro-shock waves generated inside a fluid jet impinging on plane wall

An Nd:glass laser pulse (18 ns, 1.38 J) is focused in a tiny area of about 100-mum diam under ambient conditions to produce micro-shock waves. The laser is focused above a planar surface with a typical standoff distance of about 4 mm, The laser energy is focused inside a supersonic circular jet of carbon dioxide gas produced by a nozzle with internal diameter of 2.9 mm and external diameter of 8 mm, Nominal value of the Mach number of the jet is around 2 with the corresponding pressure ratio of 7.5 (stagnation pressure/static pressure at the exit of the nozzle), The interaction process of the micro-shock wave generated inside the supersonic jet with the plane wall is investigated using double-pulse holographic interferometry. A strong surface vortex field with subsequent generation of a side jet propagating outward along the plane wail is observed. The interaction of the micro-shock wave with the cellular structure of the supersonic jet does not seem to influence the near surface features of the flowfield. The development of the coherent structures near the nozzle exit due to the upstream propagation of pressure waves seems to be affected by the outward propagating micro-shock wave. Mach reflection is observed when the micro-shock wave interacts with the plane wall at a standoff distance of 4 mm, The Mach stem is slightly deflected, indicating strong boundary-layer and viscous effects near the wall. The interaction process is also simulated numerically using an axisymmetric transient laminar Navier-Stokes solver. Qualitative agreement between experimental and numerical results is good.

Intensity Distribution Design for Laser-Induced Thermal Loading Based on Numerical Simulation

To accomplish laser-induced thermal loading simulation tests for pistons,the Gaussian beam was modulated into multi-circular beam with specific intensity distribution.A reverse method was proposed to design the intensity distribution for the laser-induced thermal loading based on finite element(FE) analysis.Firstly,the FE model is improved by alternating parameters of boundary conditions and thermal-physical properties of piston material in a reasonable range,therefore it can simulate the experimental resul...

Effects of La2O3 on Microstructure and Wear Properties of Laser Clad gamma/Cr7C3/TiC Composite Coatings on TiAl Intermatallic Alloy

The effects of La2O3 addition on the microstructure and wear properties of laser clad gamma/C(r)7C(3)/TiC composite coatings on gamma-TiAl intermetallic alloy substrates with NiCr-Cr3C2 precursor mixed powders have been investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive spectrometer (EDS) and block-on-ring wear tests. The responding wear mechanisms are discussed in detail. The results are compared with that for composite coating without La2O3. The comparison indicates that no evident new crystallographic phases are formed except a rapidly solidified microstructure consisting of the primary hard Cr7C3 and TiC carbides and the gamma/Cr7C3 eutectics distributed in the tough gamma nickel solid solution matrix. Good finishing coatings can be achieved under a proper amount of La2O3-addition and a suitable laser processing parameters. The additions of rare-earth oxide La,03 can refine and purify the microstructure of coatings, relatively decrease the volume fraction of primary blocky Cr7C3 to Cr7C3/gamma eutectics, reduce the dilution of clad material from base alloy and increase the microhardness of the coatings. When the addition of La2O3 is approximately 4 wt.%, the laser clad composite coating possesses the highest hardness and toughness. The composite coating with 4 wt.%La2O3 addition can result the best enhancement of wear resistance of about 30%. However, too less or excessive addition amount of La2O3 have no better influence on wear resistance of the composite coating.

Numerical simulation of an adaptive optics system with laser propagation in the atmosphere

A comprehensive model of laser propagation in the atmosphere with a complete adaptive optics (AO) system for phase compensation is presented, and a corresponding computer program is compiled. A direct wave-front gradient control method is used to reconstruct the wave-front phase. With the long-exposure Strehl ratio as the evaluation parameter, a numerical simulation of an AO system in a stationary state with the atmospheric propagation of a laser beam was conducted. It was found that for certain conditions the phase screen that describes turbulence in the atmosphere might not be isotropic. Numerical experiments show that the computational results in imaging of lenses by means of the fast Fourier transform (FFT) method agree well with those computed by means of an integration method. However, the computer time required for the FFT method is 1 order of magnitude less than that of the integration method. Phase tailoring of the calculated phase is presented as a means to solve the problem that variance of the calculated residual phase does not correspond to the correction effectiveness of an AO system. It is found for the first time to our knowledge that for a constant delay time of an AO system, when the lateral wind speed exceeds a threshold, the compensation effectiveness of an AO system is better than that of complete phase conjugation. This finding indicates that the better compensation capability of an AO system does not mean better correction effectiveness. (C) 2000 Optical Society of America.

Enhanced Plasticity of Zr-Based Bulk Metallic Glass Matrix Composite with Ductile Reinforcement

A composite material containing uniformly distributed micrometer-sized Nb particles in a Zr-based amorphous matrix was prepared by suction cast. The resulting material exhibits high fractured strength over 1550 MPa and enhanced plastic strain of about 29.7% before failure in uniaxial compression test at room temperature. Studies of the serrations on the stress-strain curves and the shear bands on the fractured samples reveal that the amplitude of the stress drop of each serration step corresponds to the extent of the propagation of a single shear band through the materials. The composite exhibits more serration steps and smaller amplitude of stress drop due to the pinning of shear band propagation by ductile Nb particles.

Application of the Conjugate Gradient Optimization Algorithm of Neural Network on Laser Machining

在应用激光技术加工复杂曲面时,通常以采样点集为插值点来建立曲面函数,然后实现曲面上任意坐标点的精确定位。人工神经网络的BP算法能实现函数插值,但计算精度偏低,往往达不到插值精确要求,造成较大的加工误差。提出人工神经网络的共轭梯度最优化插值新算法,并通过实例仿真,证明了这种曲面精确定位方法的可行性,从而为激光加工的三维精确定位提供了一种良好解决方案。这种方法已经应用在实际中。

Nonequilibrium microstructures and their evolution in a Fe-Cr-W-Ni-C laser clad coating

The laser-solidified microstructural and compositional characterization and phase evolution during tempering at 963 K were investigated using an analytical transmission electron microscope with energy dispersive X-ray analysis. The cladded alloy, a powder mixture of Fe, Cr, W, Ni, and C with a weight ratio of 10:5:1:1:1, was processed with a 3 kW continuous wave CO2 laser. The processing parameters were 16 mm/s beam scanning speed, 3 mm beam diameter. 2 kW laser power, and 0.3 g/s feed rate. The coating was metallurgically bonded to the substrate, with a maximum thickness of 730 mu m, a microhardness of about 860 Hv and a volumetric dilution ratio of about 6%. Microanalyses revealed that the cladded coating possessed the hypoeutectic microstructure comprising the primary dendritic gamma-austenite and interdendritic eutectic consisted of gamma-austenite and M7C3 carbide. The gamma-austenite was a non-equilibrium phase with extended solid solution of alloying elements and a great deal of defect structures, i.e. a high density of dislocations, twins, and stacking faults existed in gamma phase. During high temperature aging, in situ carbide transformation occurred of M7C3 to M23C6 and M6C. The precipitation of M23C6, MC and M2C carbides from austenite was also observed.