Tianfeng Zhou

Effect of Low-Frequency Pulsed Magnetic Treatment on Micro-Hardness of High Speed Steel

The effect of low-frequency pulsed magnetic treatment on micro-hardness of W9Mo3Cr4V(W9) high speed steel (HSS) has been investigated. A low-frequency pulsed magnetic treatment system was designed and applied to treat the tool material. Experiments were conducted at different magnetic field intensities and pulse frequencies. Experimental results show that the low-frequency pulsed magnetic treatment can effectively increase the micro-hardness of W9 HSS in a proper treatment parameters. The micro-hardness change rate first increase and then stabilize as the magnetic field intensity increases. Pulse frequency has little effect on the treatment effect.

Investigation of diamond wheel topography in Elliptical Ultrasonic Assisted Grinding (EUAG) of monocrystal sapphire using fractal analysis method

HighlightsThe fractal dimension of wheel topography is highly correlated to wear mechanisms.An increase in cutting edge density results in an increase of fractal dimension.The fractal dimension decreases as the grain pullout and wheel loading increase.Wheel topography changes from isotropic to anisotropic as grinding passes increase. ABSTRACT The wear behaviors of grinding wheel have significant influence on the work‐surface topography. However, a comprehensive and quantitative method is lacking for evaluating the wear conditions of grinding wheel. In this paper, a fractal analysis method is used to investigate the wear behavior of resin‐bonded diamond wheel in Elliptical Ultrasonic Assisted Grinding (EUAG) of monocrystal sapphire, and a series of experiments on EUAG and conventional grinding (CG) are performed. The results show that the fractal dimension of grinding wheel topography is highly correlated to the wear behavior, i.e., grain fracture, grain pullout, and wheel loading. An increase in cutting edge density on the wheel surface results in an increase of the fractal dimension, but an increase in the grain pullout and wheel loading results in a decrease in the fractal dimension. The wheel topography in EUAG has a higher fractal dimension than that in CG before 60 passes due to better self‐sharpening behavior, and then has a smaller fractal dimension because of more serious wheel loadings after 60 passes. By angle‐dependent distribution analysis of profile fractal dimensions, the wheel surface topography is transformed from isotropic to anisotropic. These indicated that the fractal analysis method could be further used in monitoring of a grinding wheel performance in EUAG.

Fabrication of a Micro-Lens Array Mold by Micro Ball End-Milling and Its Hot Embossing

Hot embossing is an efficient technique for manufacturing high-quality micro-lens arrays. The machining quality is significant for hot embossing the micro-lens array mold. This study investigates the effects of micro ball end-milling on the machining quality of AISI H13 tool steel used in the micro-lens array mold. The micro ball end-milling experiments were performed under different machining strategies, and the surface roughness and scallop height of the machined micro-lens array mold are measured. The experimental results showed that a three-dimensional (3D) offset spiral strategy could achieve a higher machining quality in comparison with other strategies assessed in this study. Moreover, the 3D offset spiral strategy is more appropriate for machining the micro-lens array mold. With an increase of the cutting speed and feed rate, the surface roughness of the micro-lens array mold slightly increases, while a small step-over can greatly reduce the surface roughness. In addition, a hot embossing experiment was undertaken, and the obtained results indicated higher-quality production of the micro-lens array mold by the 3D offset spiral strategy.

A novel constitutive model for Ti–6Al–4V alloy based on dislocation pile-up theory

ABSTRACT The mechanical properties of the titanium alloy Ti–6Al–4V, which vary with the specimen size under different temperatures, are studied through the Split Hopkinson Pressure Bar (SHPB) test and the quasi-static tensile test to determine the parameters for the classical Johnson-Cook (JC) constitutive model. Based on the dislocation pile-up theory, the classical JC constitutive model is modified by adding a grain strain term Δσ to consider the influence of grain size. The SHPB and tensile tests are analysed using a finite element method simulation. Compared with the experimental results, the simulation results based on the modified JC model exhibit a much higher calculation accuracy than that of the classical JC model.

The Effects of Ultrasonic Vibration in Hot Pressing for Microgrooves

Microgrooves with a pitch at wave length level are increasingly needed in the optical system. Conventional, the microgroove forming accuracy is low due to the incomplete filling of the material in the cavity of microgroove mold, and surface flaws occur easily due to the adhesion of the resin material to the mold surface. In this research, the response behavior of resin material subjected to alternating stress is resolved based on Generalized Maxwell model. Finite Element Method (FEM) simulation is carried out to test the microgroove forming effects under the pressing condition without and with ultrasonic vibration. An ultrasonic assisted pressing machine is developed and used to fabricate microgrooves on methacrylic resin surface. Form accuracy and surface quality of microgrooves are confirmed to be improved by comparing the ultrasonic assisted pressing with the conventional forming.

Experiment on Glass Microgroove Molding by Using Polycrystalline Nickel Phosphorus Mold

This paper aims at increasing the forming accuracy of microgrooves by using electroless plated Nickel Phosphorus (Ni-P) as the mold material in glass molding press (GMP) process. The mechanical characteristics and the performance of amorphous Nickel Phosphorus (a-Ni-P) plating layer were experimentally tested. In order to bring down the deformation of the a-Ni-P mold and increase the forming accuracy of microgroove molding, polycrystalline Nickel Phosphorus (c-Ni-P) was developed by heat treatment. With the increase of the hardness after crystallization, the plating layer shape changes due to the heat treatment, and the strategy to deal with the deformation and the techniques to produce c-Ni-P mold are generalized. Finally, the performance of c-Ni-P mold was experimentally tested, which confirms that c-Ni-P is an excellent mold material for glass microgroove forming.

Effects of interface thermal resistance on surface morphology evolution in precision glass molding for microlens array

To study the effects of the interface thermal resistance on surface morphology evolution in precision glass molding (PGM) for microlens array with different mold materials, including Tungsten carbide and heat-resistant stainless steel, the glass-mold interface thermal resistance is calculated, and heat-transfer simulation of PGM based on an interface thermal resistance model at the heating stage is conducted correspondingly. The effect of flattening behavior on the glass-mold interface is explained. Then, experiments evaluating the relationship between heating time and glass surface roughness are carried out, and the glass adhesion phenomenon appearing on the heat-resistant stainless steel mold is observed and analyzed. Finally, the microlens array is fabricated on the nickel phosphorous plating layer on the heat-resistant stainless steel substrate by diamond-ball nose-end milling, and experiments of PGM for the microlens array are carried out to verify the interface thermal resistance model. The result shows that a high-quality surface can be obtained by the combination of a smooth mold and rough glass. Compared with the microlens array fabricated with the rough glass preform, using the smooth glass preform achieves higher form accuracy without defects or blurs.

Surface defect analysis on formed chalcogenide glass Ge_22Se_58As_20 lenses after the molding process

Chalcogenide glass (ChG) is increasingly used in infrared optical systems owing to its excellent infrared optical properties and scalable production using precision glass molding (PGM). However, surface scratches affected by the molding temperature and microdimples on the lens surface caused by gas release seriously impair the quality of the formed lens. To reduce these surface defects when molding Ge22Se58As20 ChG, the temperature effect must be studied, and the gas generation must be minimized, while the gas escape must be maximized. In this work, we studied the effect of temperature on the surface defects. Additionally, we studied the influences of the roughness and curvature of the contact surfaces, as well as the pressing force on the formation of the microdimples. It was found that the molding temperature should be approximately 30°C higher than the softening temperature (Ts) to avoid surface scratches. The gas generation could be inhibited by increasing the pressing force and decreasing the roughness of the mold surface, and finally, increasing the curvature difference between the mold and glass preform surfaces improved the gas escape.

Thermal analysis of laser irradiation of fused silica

To grind fused silica in ductile mode, surface and subsurface micro cracks (SSMC) on ground fused silica should be repaired by CO2 laser irradiation before ultra-precision grinding. In this paper, 2D thermal analysis of single pass laser irradiation of fused silica was conducted, and the simulation results were discussed by comparing with the experiment results. To repair SSMC and decrease the surface roughness of ground fused silica simultaneously, the maximum temperature on the surface during laser irradiation should be controlled higher than 3280 K and lower than 3550 K.

A Review of the Precision Glass Molding of Chalcogenide Glass (ChG) for Infrared Optics

Chalcogenide glass (ChG) is increasingly demanded in infrared optical systems owing to its excellent infrared optical properties. ChG infrared optics including ChG aspherical and freeform optics are mainly fabricated using the single point diamond turning (SPDT) technique, which is characterized by high cost and low efficiency. This paper presents an overview of the ChG infrared optics fabrication technique through precision glass molding (PGM). It introduces the thermo-mechanical properties of ChG and models the elastic-viscoplasticity constitutive of ChG. The forming accuracy and surface defects of the formed ChG are discussed, and the countermeasures to improve the optics quality are also reviewed. Moreover, the latest advancements in ChG precision molding are detailed, including the aspherical lens molding process, the ChG freeform optics molding process, and some new improvements in PGM.