A model for prediction of dimensional tolerances of laser cut holes in mild steel thin plates

Dimensional variation in laser cutting of sheet metals depends on two factors: the accuracy and repeatability of the positioning system, and the thermal effects of the laser beam on the workpiece material. In this work, a physical model was formulated to estimate the dimensional accuracy of holes produced with laser cutting. The model included the assumption that the layer adjacent to the hole is plastically deformed and contains residual stresses up to the yield strength. The model was used to calculate the size of the hole and cut-out disk of varying radii in steel plates with thicknesses of 3.2 mm and 6.4 mm. The model-predicted data were verified with the experimental data obtained using a 1 kW continuous wave CO₂ laser. Results indicated that there is an excellent correlation between the model and the experimental data especially for smaller diameter holes.     

Transformation structures and strengthening mechanisms of laser processed Fe-Cr alloys

The microstructures and microhardness of laser processed Fe-Cr surface alloys were investigated as functions of composition (5 to 50 wt % Cr) and melt penetration depth (100 to 1500 m). The transformation structures were characterized by optical metallography and thin foil transmission electron microscopy. The microstructures were ferritic irrespective of compositions and melt depths. The alloys containing chromium up to 12 % (within the -phase field) exhibited a massive ferritic morphology while the alloys containing chromium more than 12% (beyond the -phase field) showed an equiaxed ferritic morphology. Transmission electron microscopy studies revealed that the substructure of ferrite consisted of dislocations, the dislocation density increased with increased chromium content. Melt depth, unlike composition, did not have a significant effect on the morphology and substructure of ferrite grains. Small amounts of -carbide and M₃C carbide phases were observed in these alloys. Both the carbides were found to decrease with an increase in the chromium of the fusion zone. Microhardness measurements indicated that there was an increase in hardness with an increase in the chromium content of the alloy.     

Crack separation mechanism in CO2 laser machining of thick polycrystalline cubic boron nitride tool blanks

An improved method for cutting thick polycrystalline cubic boron nitride (PCBN) tool blanks is explored because current methods of pulsed Nd:YAG laser cutting and wire electrical discharge machining (EDM) are constrained by low speed and low precision. We present a CO₂ laser/waterjet (LWJ) process to cut 4.8-mm-thick PCBN tool inserts by a crack separation mechanism. In LWJ, the PCBN blank is locally heated using a high-power continuous wave CO₂ laser to cause phase transition from cubic to hexagonal followed by water quenching to generate thermal stresses and form boron oxide leading to increased brittleness, subsequent cracking, and material separation. A 2³ fractional design of experiment (DOE) approach was employed to determine the factors of laser power, cutting speed, and waterjet pressure on the responses of phase transformation depth, taper, and surface roughness. A numerical heat flow model, based on Green’s function, was used to calculate the temperature distributions along the depth. Surface profilometer, scanning electron microscopy, and Raman spectroscopy were utilized to analyze the phase transformation and crack zones. Results from LWJ compared with pulsed Nd:YAG laser and laser microjet? methods indicate LWJ cuts 30 times faster; this was attributed to a nonconventional material removal (crack separation) mechanism. When LWJ was compared against nitrogen-assisted CO₂ laser cutting, improved cut quality (less taper and smaller heat-affected zone) was observed due to a greater control on phase transformation and crack propagation. DOE analysis revealed laser power and waterjet pressure, and the interactions among them are more significant factors than others.     

Dual-beam CO2 laser cutting of thick metallic materials

A dual-beam technique involving two CO₂ gas lasers with a power capacity of 1.5 kW each, was used to cut steel and superalloy. A comparison with single-beam CO₂ laser cutting showed that dual beams were capable of enhancing the cutting thickness and speed without deteriorating the quality of cut. Heat-conduction models, assuming the laser beams as line sources, were used to estimate the cutting thickness and speed as a function of distance between the two laser beams. Experimental data, coupled with theoretical modelling, have provided a new concept, namely stretching the width of the laser beam in the direction of cutting to cut thicker section solids at moderate speeds.     

Nano-holes in silicon wafers using laser-induced surface plasmon polaritons

Surface Plasmon Polaritons (SPPs) have been explored for a multitude of applications including sub-wavelength lithography, data storage, microscopy and photonics. In this paper, we report the use of SPPs for nanomachining silicon in massively parallel fashion. A Q-switched Nd:YAG laser beam was impinged on gold-thin film deposited, porous alumina membrane (PAM) that contains periodic 2-D array of thousands of nano-holes. The silicon substrate was placed in close proximity with PAM. The formation of SPPs and their coherent interference at the exit of PAM holes created strong nanoscale electrical fields which in turn produced 50-70 nm diameter holes in silicon.     

无需调谐的“砖墙式”低通音频滤波器

当系统规格需要具有陡峭的频率截止特性的低通滤波器时,工程师可以选择具有锐过渡频带的“砖墙式”滤波器设计。例如,在调频立体声广播系统中,基带音频的左右声道中的低通滤波器应该具有至少15kHz的-3dB截止频率和低于0.3dB的通带纹波,以及至少19kHz的阻带起始频率、大于50dB的阻带衰减和对两个声道完全相同的相位响应。     

Nanosecond pulsed excimer laser machining of chemically vapour-deposited diamond and graphite: Part II Analysis and modelling

Analysis of the experimental data presented in Part I of this paper and those available in the literature revealed that the mechanism of material removal in laser machining of chemically vapour-deposited diamond is a two-step process: diamond transforms to graphite, and subsequently graphite sublimates. The energy fluence required for the formation of graphite is much lower than its removal by sublimation, and both are sensitive to the wavelength of the laser beam, the impurities present in the film and the environment during machining. When a 248 nm excimer laser beam interacts with diamond, there is an energy loss of 20% by reflection and 10% by transmission. The remaining 70% energy is used for heating the diamond, converting diamond to graphite, and sublimating graphite. Graphite is removed mostly by physical ablation and to some extent by chemical oxidation with the ambient.A theoretical calculation based on bond strength estimates that the threshold energy fluence for the ablation of diamond is 0.37 J cm-2. The experimental energy fluence was 0.8 J cm-2. Experimental results on the material removal rates as a function of energy fluence closely follow the Beer–Lambert equation, suggesting that physical ablation is the determining mechanism. Temperature calculations showed that both diamond and graphite tend to oxidize in a single laser pulse that contributes to the material removal.