Ultrafast Laser Drilling of Crack-free, Debris-free and Heat Affected Zone (HAZ)-free Blind Holes in Al2O3 with Flat Bottom and Reduced Taper Angles
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This paper deals with the electron beam drilling of alumina green sheet. The following results were obtained through some experiments by multi-pulse drilling under various duty factors and a thermal calculation. (1) The number of beam pulse does not affect the diameter and taper of drilled hole. Furthermore, hole taper is kept constant for various value of workpiece thickness when drilling are performed in same conditions. (2) Hole diameter and taper increase dramatically at high duty factor even though drilling conditions are kept constant. (3) Material removal in the drilling of green sheet is taken place through scattering of surrounding material by explosive vaporization of organic binding material in the green sheet. (4) The increase in hole diameter and taper is due to the focal movement taken place through the space charge neutralization by positive ions generated by the ionization of vaporized gas.
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Temperature Gradient
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Laser crack-free cutting of thick and dense ceramics (thickness > 10 mm, relative density ≥ 98%) by single pass which is based on continuous piercing through the work-piece was presented in this paper. With two processing stages matching with appropriate time shot for every piercing, low cycle duty, low gas pressure and other processing parameters, the crack-free internal straight or curve shape cutting of the ceramics could be obtained by CO2 laser at higher cutting speed of about 25 ∼ 30 mm/min. Combining with the analysis of the power flowing intensity and heat accumulation effects around laser spot, it is concluded that the non-equilibrium heat accumulation was inhibited effectively by the single pass laser crack-free cutting technique. For cutting of 10 mm thick Al2O3 ceramic, the total time without power flowing between the fore-piercing and the next-piercing by the technique could be ∼600 ms, which combined with ejecting of melting materials, could supply enough cooling time to inhibit the non-equilibrium heat accumulation resulting in cracks.
Laser cutting
Intensity
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Microscale chemistry
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In order to study the effects of laser shock forming on the surface roughness,metal sheet of aluminum and 304 stainless steel of 0.5mm thickness are used as samples of laser shock forming,and laser beams with radius of 0.4mm,wavelength of 1054nm and pulse width of 23ns was applied in laser shock forming,while the energy for aluminum sheet was 15.4J and for 304 stainless steel was 18.92J.The measurement results after laser shock forming show that laser shock forming may improve the surface roughness to two roughness rank.There are two reasons for surface roughness improvement.On the one hand,it will generate a series of trasmission wasves and reflection waves when the stress wave spreads in tiny peak of sheet surface,and the tiny peak would rupture if the strength of reflection tensile wave is beyond the tensile strength of targets.On the other hand,a high pressure zone is formed in the air because of high speed motion of sheet,and plastic deformation of tiny peak would take place if the pressure of air acted on the back of sheet is greater than the dynamic bend intensity.So the altitude of tiny peak is decreased by the rupture and plastic deformation of tiny peak and it results in improvement of the surface roughness.
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In the present experimental study, percussion micro-drilling on quartz is carried out by Ytterbium (Yb3+) doped 50 W fibre laser system in order to find out the effects of various process parameters such as laser power, pulse frequency, duty cycle and assist air pressure (process parameters) on the response criteria, i.e., taper angle, circularity for the entrance holes and heat affected zone (HAZ) thickness. The larger and smaller diameters of the micro-holes at the entry side are measured as 124.13 µm and 68.17 µm respectively, whereas the larger and smaller diameters of micro-holes in exit side are 111.65 µm and 58.88 µm respectively. The smaller value of the taper angle (θ) is found at 0.187° along with the smaller HAZ thickness of 16.10 µm at the entry side. In addition to this, greater entrance hole circularity is measured as 0.938 during fibre laser percussion micro-drilling on quartz materials.
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Determination of hole taper and extent of heat affected zone is a challenging task for manufacturing engineers from the quality and accuracy point of view of the laser-drilled hole especially in reflecting materials like aluminum. In this article, an axisymmetric finite element method-based thermal model has been developed for the determination of the transient temperature distribution in the aluminum sheet, which is further used for determination of hole profile, hole taper and extent of heat-affected zone during laser beam percussion drilling. Simulation results showing the effect of pulse width, pulse frequency, and peak power on hole taper and extent of heat affected zone incorporating the temperature dependent thermal properties and phase change have been presented. The developed model is also capable of predicting the hole profile through the isotherm generated along the depth of hole due to repeated laser pulses. The predicted values of hole taper are compared with the experimental results. It is found that the effect of pulse width on hole taper is more prominent than the peak power and extent of heat affected zone increases with the increase of pulse width, pulse frequency but it decreases with the increase of peak power.
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Fiber laser cutting of thick mild steel plate with oxygen assist gas results in a large slope angle of kerf, and reduction of the slope angle of kerf is required to enhance the applications of fiber laser cutting. Therefore, the cutting characteristics of mild steel were discussed, and the improvement method of slope angle of kerf was experimentally and numerically investigated in the fiber laser cutting of mild steel.In the case of a large slope angle of kerf, heat is largely generated at lower part of thick plate, and reduction of the heat generation is necessary to achieve a small slope angle in fiber laser cutting of thick metal plate. When the tips of inner and outer nozzles in double nozzle configuration are arranged at the same level in a perpendicular plane to assist gas flow direction, the thickness of heat affected zone (HAZ) becomes small. A small slope angle can be achieved by using the nozzle, which can create higher and straighter gas flow.In fiber laser cutting of thick mild steel plate with oxygen assist gas, slope angle of kerf decreases with increasing the cutting speed. In other words, there is a large delayed distance of cutting front, which is the distance in scanning direction between tip and tail of front wall. In addition, a small delayed distance of cutting front indicates that reflected laser beam from the cutting front is absorbed on the both side walls at the rear of cutting front, while a large delayed distance leads to the reduction of reflected laser beam on the side walls. Computational fluid dynamics (CFD) analysis of assist gas flow suggests that sufficiently high flow velocity can push the molten material downward even in the case of large delayed distance of cutting front. In addition, oxygen density at lower part of plate decreases in the case of a large delayed distance, which can suppress the oxygen-combustion properly. Both phenomena can be achieved by good straightness of assist gas flow. Therefore, it is clarified that a small slope angle can be achieved by the combination of large delayed distance and assist gas supplying with good straightness of assist gas flow at lower part of thick plate.
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Laser beam machining
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Dendrite (mathematics)
Weld pool
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