Curved drilling via inner hole laser reflection

In this paper, we describe curved hole drilling via the reflection of a laser beam off the sidewall of the drilled hole. A slightly offset laser beam forms a tilted surface at the bottom of the hole, controlling the angle of curvature. An ultraviolet laser beam operating at a wavelength of 266 nm was used. To visualize the hole formation process, borosilicate glass was used as the laser workpiece. This method was able to drill a curved hole with an average angle of ∼3° with curvature beginning at a depth of 400–600 μm. A curved hole with a diameter of <50 μm was achieved. A branched hole was also demonstrated by using the reflection of the tilted sidewall. The curved hole formation process was recorded with a high speed camera. Once the ablated sidewall reached a certain depth, drilling ceased as the laser energy fell below the ablation threshold. Ultimately, judicious selection of an appropriate laser fluence and sidewall angle allow the formation of curved holes.     

RESPONSE SURFACE ANALYSIS OF SLICING OF SILICON INGOTS WITH FOCUS ON PHOTOVOLTAIC APPLICATION

Polycrystalline silicon wafers are widely used in Photovoltaic (PV) industry as a base material for the solar cells. The existing silicon ingot slicing methods typically provide minimum wafer thickness of 300–350 μm and a surface finish of 3–5 μm R_a while incurring considerable kerf loss of 35–40%. Consequently, efficient dicing methods need to be developed, and in the quest for developing new processes for silicon ingot slicing, the wire-EDM (electric discharge machining) is emerging as a potential process. Slicing of a 3′′ square silicon ingot into wafers of 500 μm in thickness has been performed to study the process capability. This article analyzes the effect of processing parameters on the cutting process. The objective of the experimental study is improvement in slicing speed, minimization of kerf loss and surface roughness. A central composite design-based response surface methodology (RSM) has been used to study the slicing of polycrystalline silicon ingot via wire-EDM. A zinc-coated brass wire, 100 μm in diameter, has been used as an electrode in the slicing experiments. It has been observed that the optimal selection of the process parameters results in an increase of 40–50% in the slicing rate along with a 20% reduction in the kerf loss as compared to the conventional methods. The machined surfaces on the sliced wafer were free of micro-cracks and wire material contamination, thereby making it useful for electronic applications.     

氧化铝陶瓷基板紫外纳秒激光打孔工艺研究

氧化铝陶瓷基板作为雷达微波组件的核心部件,其硬脆特性使得传统加工方法在导通孔加工中存在很多限制。激光加工作为一种非接触式高能束加工方法,是氧化铝陶瓷表面孔加工的最优选择。文中主要研究了紫外纳秒激光氧化铝陶瓷表面孔加工中的激光加工参数（包括激光平均功率、扫描速度和扫描次数等）对孔的特征尺寸（包括入口直径、出口直径和锥度）的影响规律,并分析了各种规律产生的相关机理。此研究为雷达微波用电子陶瓷基板的导通孔加工提供了有力的理论依据和技术支持。     

Laser hole cutting in Kevlar: modeling and quality assessment

Machining of Kevlar laminates with conventional methods results in poor end-product quality and excessive specific energy requirement for machining. However, laser machining has considerable advantages over the conventional methods due to precision and rapid processing. In the present study, laser hole cutting into Kevlar laminates with different thicknesses and properties is carried out. The laser output power, frequency, and cutting speed are varied during the hole-cutting experiments. The specific energy requirements for cutting, thermal efficiency of the cutting process, and kerf width are formulated and predicted for various laser parameters and Kevlar properties. The cut quality is associated with the damage size around the holes cut and statistical analysis is carried out to examine the affecting parameters on the damaged size. It is found that specific energy requirement is significantly lower than that of the conventional drilling method. The damage size is affected significantly by the laser irradiated power. The quality of holes, as judged by the percentage of damage size around the cut edges cut by a laser beam, is considerably improved compared to the conventional methods.     

电路板复合材料微小孔加工技术

介绍树脂基复合材料印刷电路板的机械钻削和激光钻削微小孔加工技术,分析了影响钻削质量的因素以及加工过程中易出现的各种缺陷和改进措施。机械钻削微小孔时,轴向力和切削扭矩是导致各种加工缺陷的主要因素,低进给量、高主轴转速可明显提高钻削质量。激光钻削时,选择合适的激光功率及减少激光照射时间可提高钻削质量。     

An investigation of the hole machining processes on woven carbon-fiber reinforced polymers (CFRPs) using abrasive waterjets

An investigation of the hole cutting and drilling processes on woven carbon-fiber reinforced polymer sheets using abrasive waterjet (AWJ) is presented. The drilling process uses a stationary AWJ to impinge a target material to make a hole, while the cutting process requires an AWJ to penetrate the workpiece before moving in a circular path to cut a hole. It is found that the holes machined by both the processes exhibit similar geometrical features, where the diameter at the top is greater than at the bottom. It is further found that the holes from the drilling process have a better roundness than those from cutting process primarily due to the jet instability during cutting movement. Plausible trends of the hole characteristics (e.g., diameter and wall inclination) and defects (e.g., delamination) with respect to the process parameters are discussed. It is shown that water pressure is the major parameter affecting hole defects. The hole drilling process yields more severe defects than the cutting process because of the initial impact of the jet. Predictive models for machined hole diameter in both processes are developed. The model predictions are in good agreement with the experimental data under the corresponding conditions.     

Micro-drilling of Ti–6Al–4V alloy: The effects of cooling/lubricating

This paper presents a series of experimental investigations of the effects of various machining conditions [dry, flooded, minimum quantity lubrication (MQL), and cryogenic] and cutting parameters (cutting speed and feed rate) on thrust force, torque, tool wear, burr formation, and surface roughness in micro-drilling of Ti–6Al–4V alloy. A set of uncoated carbide twist drills with a diameter of 700 μm were used for making holes in the workpiece material. Both machining conditions and cutting parameters were found to influence the thrust force and torque. The thrust force and torque are higher in cryogenic cooling. It was found that the MQL condition produced the highest engagement torque amplitude in comparison to the other coolant–lubrication conditions. The maximum average torque value was obtained in the dry drilling process. There was no substantial effect of various coolant–lubrication conditions on burr height. However, it was observed that the burr height was at a minimum level in cryogenic drilling. Increasing feed rate and decreasing spindle speed increased the entry and exit burr height. The minimum surface roughness values were obtained in the flood cooling condition. In the dry drilling process, increased cutting speed resulted in reduced hardness on the subsurface of the drilled hole. This indicates that the surface and subsurface of the drilled hole were subject to softening in the dry micro-drilling process. The softening at the subsurface of drilled holes under different cooling and lubrication conditions is much smaller compared to the dry micro-drilling process.     

On acoustic emissions in percussion laser drilling

Laser drilling is a well established sheet metal processing method. The development of a monitoring system capable of assessing the dimensions of holes is the subject of this work. This paper investigates the applicability of an acoustic-based monitoring system for the percussion laser drilling process. Correlation between the sensor output and the hole's geometry, determined by its depth and upper diameter, is investigated and the results are presented. In general, the results indicate that a correlation exists between the acoustic signal output and the depth of the hole.     

Drilling,bonding, and forming conductive path in the hole by laser percussion drilling

Several studies have focused on the electrical connections between the front and rear surfaces of stacked substrates in order to improve device performance. The fabrication and mounting process of the substrates involves three steps: (1) through-hole drilling, (2) formation of a conductive path inside the hole, and (3) physical bonding and electrical wiring connection of the substrates. In this paper, we demonstrate a technique for performing the process above simultaneously by laser percussion drilling. A borosilicate glass sample was used as the substrate, while copper was used as the wiring material. The substrate was drilled to a diameter of ~30 μm by laser radiation, while the copper was evaporated and deposited in a ~12-μm-thick layer on the inner surface of a glass-copper hole. Hence, a conductive path was formed inside the glass hole, facilitating bonding and conduction between the glass substrate and the copper sheet. The conductivity and bonding strength per 100 points between the glass surface and the copper sheet were ~5 Ω and ~1 N respectively. Furthermore, gaps were observed between the glass substrate and the copper sheet by energy dispersive X-ray analysis using a scanning electron microscope. However, the glass substrate and the copper sheet were bonded by the formation of a redeposited layer on the inner surface of the hole and in the gap between the glass and copper surfaces.     

HIGH THROUGHPUT DRILLING OF TITANIUM ALLOYS

The experiments of high throughput drilling of Ti-6Al-4V at 183 m/min cutting speed and 156 mm3/s material removal rate using a 4 mm diameter WC-Co spiral point drill are conducted. At this material removal rate, it took only 0.57 s to drill a hole in a 6.35 mm thick Ti plate. Supplying the cutting fluid via through-the-drill holes and the balance of cutting speed and feed have proven to be critical for drill life. An inverse heat transfer model is developed to predict the heat flux and the drill temperature distribution in drilling. A three-dimensional finite element modeling of drilling is con-ducted to predict the thrust force and torque. Experimental result demonstrates that, using proper machining process parameters, tool geometry, and fine-grained WC-Co tool material, the high throughput machining of Ti alloy is technically feasible.     

Precision machining of micro tool electrodes in micro EDM for drilling array micro holes

Micro electro discharge machining (micro EDM) is suitable for machining micro holes on metal alloy materials, and the micro holes can be machined even to several microns by use of wire electro discharge grinding (WEDG) of micro electrodes. However, considering practicability of micro holes <Φ100 μm in batch processing, the controllable accuracy of holes’ diameter, the consistency accuracy of repeated machining and the processing efficiency are required to be systematically improved. On the basis of conventional WEDG method, a tangential feed WEDG (TF-WEDG) method combined with on-line measurement using a charge coupled device (CCD) was proposed for improving on-line machining accuracy of micro electrodes. In TF-WEDG, removal resolution of micro-electrode diameter (the minimum thickness to be removed from micro electrode) is greatly improved by feeding the electrode along the tangential direction of wire-guide arc, and the resolution is further improved by employing negative polarity machining. Taking advantage of the high removal resolution, the precise diameter of micro-electrode can be achieved by the tangential feed of electrode to a certain position after diameter feedback of on-line measurement. Furthermore, a hybrid process was presented by combining the TF-WEDG method and a self-drilled holes method to improve the machining efficiency of micro electrodes. A cyclic alternating process of micro-electrode repeated machining and micro holes’ drilling was implemented for array micro holes with high consistency accuracy. Micro-EDM experiments were carried out for verifying the proposed methods and processes, and the experimental results show that the repeated machining accuracy of micro electrodes was less than 2 μm and the consistency accuracy of array micro holes was ±1.1 μm.     

激光诱导空化微孔冲裁试验研究

针对微尺度下金属箔模具冲孔存在模具挤压磨损、对中困难,激光打孔存在烧蚀、吸收层无法补偿等问题,提出通过激光诱导空泡对金属箔进行加载来实现冲裁小孔的方法。研究了不同激光焦点位置(H=0~4 mm)、激光能量(E=10.3~50.8 mJ)和铜箔厚度(T=20~70μm)对铜箔冲孔的影响,发现激光焦点位置影响明显,当铜箔变形平均深度达到147.0μm后,铜箔发生剪切断裂,可实现冲裁,并且制备的小孔边缘正表面无烧蚀、毛边、裂纹和卷边等缺陷。同时,利用高速摄影仪对激光诱导空化微孔冲裁过程进行研究,结果表明激光诱导空化微孔冲裁过程是激光等离子体冲击波、空泡溃灭冲击波和微射流共同加载的过程。     

Electrical discharge machining of submicron holes using ultrasmall-diameter electrodes

We have carried out the electrical discharge machining (EDM) of submicron holes using ultrasmall-diameter electrodes. Two types of electrode were used: tungsten electrodes fabricated by the combination of wire electrodischarge grinding and electrochemical machining, and silicon electrodes originally designed as probes for scanning probe microscopes. The diameters of the former and latter were 1 μm or less, and less than 0.15 μm, respectively. Holes were drilled using a relaxation-type pulse generator at an open-circuit voltage of less than or equal to 20 V with the machine's stray capacitance as the only capacitance. Using tungsten electrodes, holes of less than 1 μm in diameter and more than 1 μm in depth were successfully drilled. A 1.3-μm-wide slot was also fabricated by drilling many holes with a small pitch. It was possible to drill holes of approximately 0.5 μm diameter using silicon electrodes because the electrode diameter was less than those of the tungsten electrodes. These holes have the smallest reported diameter for holes drilled by EDM, indicating the possibility of submicron- and nanoscale machining by EDM.     

High efficiency fine boring of monocrystalline silicon ingot by electrical discharge machining

This article deals with high efficiency and high accuracy fine boring in a monocrystalline silicon ingot by electrical discharge machining (EDM). In manufacturing process of integrated circuits, a plasma-etching process is used for removing oxidation films. This process has recently been examined for use of monocrystalline silicon as the electrode to minimize the contamination. However, it is difficult to machine silicon accurately by the conventional diamond drilling method, because the material removal is due to brittle fracture. The machining force in the EDM process is very small compared with that in conventional machining, therefore, the possibility of high efficiency and high accuracy boring holes in silicon ingot by EDM is experimentally investigated. The removal rate of monocrystalline silicon by EDM is much higher than that of steel, while the electrode wear is extremely small. The improvement method leads to a better hole without chipping at the exit of hole or sticking of the insulator on the wall of hole. Furthermore, it is proved that even a high aspect ratio of about 200 boring is possible.     

Laser and focused ion beam combined machining for micro dies

We have developed a laser and focused ion beam (FIB) compound process for press mold dies of a micro lens array (MLA) and a micro needle array (MNA) in a glassy carbon (GC). The press mold die of the MLA was roughly fabricated by UV-YAG laser. After this process, we finished this surface by scanning FIB. As a result, higher accuracy and good roughness of surface profile can be realized. An optical glass is used to confirm the shape of lens. Moreover, we fabricated 6 × 6 through-holes in the GC by the spiral drilling in addition to the focus position movement of the UV laser for press mold die of the MNA. After the FIB process, we were able to make the needle die of surface and hole wall roughness less than 0.9 μm. A silicon rubber is used to confirm the shape of the holes.     

Statistical analysis of repeatability in laser percussion drilling

This study investigates the effect of six parameters in the repeatability of drilled holes in laser percussion drilling process by means of statistical techniques. Peak power, pulse width, pulse frequency, number of pulses, gas pressure and focal plane position were considered as independent process parameters. Experiments were designed with the aim of reducing the number of required experiments. The response surface method was used to develop the models for required responses. The significant factors in the process were selected based on the analysis of the variance (ANOVA). The experiments were conducted in mild steel sheet with a thickness of 2 mm. Each experiment was repeated 35 times in order to investigate the repeatability of the process. The equivalent entrance diameter, percentage of standard deviation of entrance diameter (%STD Eq Dia), circularity (ratio of minimum to maximum Feret’s diameter) and its standard deviation (STD circularity) were selected as process characteristics. The %STD Eq Dia and STD circularity, respectively, show the repeatability of equivalent diameter and circularity in the process. The results show that the process of drilling smaller hole diameters is more repeatable than drilling larger holes. Pulse width, gas pressure, focal plane position, peak power and number of pulses, respectively, have significant effect on the repeatability of hole diameter and circularity. Pulse frequency has no significant effect on the repeatability of the process.     

State-Of-Art,Challenges, and Outlook on Manufacturing of Cooling Holes for Turbine Blades

A cooling hole is important structure of turbine blades for high-performance aircraft engines. It is very challenging to manufacture cooling holes in superalloys including nickel-based and titanium alloys. This article aims to provide a critical assessment on the major types of machining processes for manufacturing cooling holes. The process mechanism, efficiency, form accuracy, and surface integrity of the state-of-art of four machining processes, i.e., mechanical drilling (MD), electrical discharge drilling (EDD), laser drilling (LD), and electrochemical drilling (ECD) have been thoroughly analyzed and compared in details. The future challenges and future potential research directions for the machining processes are also discussed.     

Micro-drilling of monocrystalline silicon using a cutting tool

The micro-drilling of monocrystalline silicon using a cutting tool was tested with the aim of fabricating three-dimensional and high aspect ratio micro-shapes. Micro-tools with a D-shaped cross-section and cutting edge radius of 0.5 μm were fabricated by wire electrodischarge grinding (WEDG). The results showed that, with a depth of cut of 0.1 μm, ductile-regime cutting was realized, and that a tool clearance angle larger than 0° was necessary to prevent fractures at the hole entrance. The smallest machinable hole was of 6.7 μm diameter, which is the smallest not just in the present study, but of all holes drilled using a cutting tool so far. Furthermore, an aspect ratio of more than four was obtained in the drilling of a 22 μm diameter and 90 μm deep hole.     

BTA深孔切削直线度分析

随着孔深的增加,孔的直线度将有所下降.针对BTA(Boring and Trepanning Association)深孔钻削中的轴线偏斜问题,从加工方式、导向条布置、长孔钻削三个角度进行研究,分别介绍了三者产生影响的原因,最后通过对切削方式的合理优化,从而得出最佳的钻孔轴向直线度.     

Prediction of aspect ratio of a micro hole drilled by EDM

Micro electrical discharge machining (EDM) has the ability to drill micro holes with high accuracy in metallic materials. The aspect ratio of a micro hole generated by micro EDM is usually higher than those by other processes such as etching, mechanical drilling, and laser. However, it was found that the drilling speed of micro EDM slows down and even stops when the aspect ratio of a micro hole reaches a certain value. To understand this phenomenon, a theoretical model is proposed based on the fluid mechanics and surface tension. Experiments under different machining conditions are carried out to verify this model. Experimental results agree with the theoretical values, which indicate the validity of the proposed model. The difference between the theoretical values and the real values might be caused by debris, temperature and rotating of electrode in the discharge gap, which are ignored in the model.