Parametric study to improve laser hole drilling process

In the laser drilling process, the quality of the drilled holes is the main task. A method of studying the influence on the quality of the main process variables needs to be developed, which seeks to improve the quality and explains the drilling mechanism. In the present study, the effect of the laser parameters and the material properties on the hole quality when drilling is examined. A statistical approach, referred to as factorial design, is employed to test the significance level of the factors that affect the hole quality. Three materials, stainless steel, nickel and titanium, are considered. The experimental study yields tables of significance of each factor on the aspects that determine the quality of the holes. The hole geometry is evaluated by assigning marks for each geometric feature, the marking scheme being conducted relevant to the importance of the hole feature.     

Comparative statistical analysis of hole taper and circularity in laser percussion drilling

The present study investigates the relationships and parameter interactions between six controllable variables on the hole taper and circularity in laser percussion drilling. Experiments have been conducted on stainless steel workpieces and a comparison was made between stainless steel and mild steel (IMechE Part B: J. Eng. Manufact. (in press)). Laser peak power, laser pulse width, pulse frequency, number of pulses, assist gas pressure and focal plane position were selected as independent process variables. The central composite design (CCD) was employed to plan the experiments in order to achieve required information with reduced number of experiments.The process performance was evaluated in terms of equivalent entrance diameter, hole taper and hole entrance circularity. The ratio of minimum to maximum Feret’s diameter was considered as circularity characteristic of the hole. The models of these three process characteristics were developed by linear multiple regression technique. The initial models were computed according to the least squares procedure and were finalised by stepwise regression method. The significant coefficients were obtained by performing analysis of variance (ANOVA) at 1, 5 and 7% levels of significance. The final models were checked by complete residual analysis and finally were experimentally verified.It was found that the pulse frequency had a significant effect on the hole entrance diameter and hole circularity in drilling stainless steel unlike the drilling of mild steel where the pulse frequency had no significant effect on the hole characteristics.     

A closed form solution for laser drilling of silicon nitride and alumina ceramics

A closed form solution has been obtained for laser drilling of silicon nitride (Si₃N₄) and alumina ceramics (Al₂O₃). The drilling of workpiece is performed by using a TEM₀₀ 10 ns pulse Nd:YAG laser. It is assumed that the phase transition from solid to vapor occurs without melting. Considering the absorption of plasma plume formed on the surface of the ceramics, the one-dimensional thermal model is developed in order to describe the drilling process. The governing equation of heat-diffusion is solved analytically using a Laplace transformation method. Erosion depth per laser pulse obtained from the closed form solution agrees with the available experimental data.     

Deep hole drilling

Deep hole drilling processes, which differ significantly from conventional drilling processes, are relevant for a lot of different applications where holes with high length-to-diameter-ratios and very good qualities are necessary. This paper gives an overview of different methods, which are established to produce bore holes with demanding aspects related to diameter, length-to-diameter-ratio, bore hole quality, workpiece materials and complex internal contours. Beside the detailed explanation of the deep hole drilling methods and tools also the fundamentals of the deep hole drilling principle are explained and completely new developed figures and tables summarize the state of the art. In addition for the most important areas the latest results of process and tool development are included.     

Spatter-free laser percussion drilling of closely spaced array holes

Spatter is one of the inherent defects commonly associated with holes produced with laser drilling. This work reports on a method of spatter prevention, workable for a wide range of process parameters. The method is based on the application of a specially developed anti-spatter composite coating (ASCC), containing a mixture of ceramic filler particles embedded in a silicone elastomer matrix, on the workpiece surface prior to laser percussion drilling. Experiments were conducted using a fibre-optic delivered 400 W Nd:YAG laser for the drilling of closely spaced through holes (2 mm hole pitch) in Nimonic 263 alloy sheets. The work revealed that the ASCC effectively prevented the deposition of spatter such that laser drilled holes were produced whilst maintaining the as-received surface characteristics of the Nimonic 263 alloy for all the assist gases tested (O₂, air, N₂ and Ar). The process characteristics and spatter prevention mechanism associated with the use of the ASCC have been investigated using optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, contact angle analysis and high-speed photographic imaging. Comparative studies were also made with the case of uncoated laser drilling.     

A review of electrochemical macro- to micro-hole drilling processes

Electrochemical machining processes provide a viable alternative for drilling macro- and micro-holes with exceptionally smooth surface and reasonably acceptable taper in numerous industrial applications particularly in aerospace, electronic, computer and micro-mechanics industries. Advanced hole-drilling processes like jet-electrochemical drilling have found acceptance in producing large number of quality holes in difficult-to-machine materials. This paper highlights the recent developments, new trends and the effect of key factors influencing the quality of the holes produced by these processes. A comparative study of electro jet drilling with another non-traditional hole-drilling process (laser percussion drilling) has been presented which shows the potential and versatility of the electrochemical hole drilling processes.     

Investigation of delamination mechanisms during a laser drilling on a cobalt-base superalloy

Temperatures in the high pressure chamber of aircraft engines are continuously increasing to improve the engine efficiency. As a result, constitutive materials such as cobalt and nickel-base superalloys need to be thermally protected. The first protection is a ceramic thermal barrier coating (TBC) cast on all the hot gas-exposed structure. The second protection is provided by a cool air layer realized by the use of a thousand of drills on the parts where a cool air is flowing through. The laser drilling process is used to realize these holes at acute angles. It has been shown on coated single crystal nickel-base superalloy that the laser drilling process causes an interfacial cracking (also called delamination), detected by a cross section observation. The present work aims at characterizing interfacial cracking induced by laser drilling on coated cobalt-base super alloy. On the one hand, this work attempted to quantify the crack by several microscopic observations with regards to the most significant process parameters related as the angle beam. On the other hand, we studied the difference of the laser/ceramic and the laser/substrate interaction with real time observation by using a fast movie camera.     

Recent advances in the hole drilling method for residual stress measurement

The main activities in the hole drilling residual stress measurement technique recently developed at the University of Pisa are reviewed and presented. Particular attention was paid to developing tools for increasing the limits indicated by the presently applied standard procedures for residual stress evaluation. For residual stresses that were assumed to be uniform through-thickness, the effect of plasticity was numerically analyzed and results formed the basis for a procedure that allows an increase in the maximum measurable residual stress up to 0.9 of the material yield strength. For nonuniform through-thickness residual stress, accurate analytical influence functions are proposed by which arbitrary interpolation of the influence coefficients is avoided and all the experimentally obtained strains, with no regard to their number, can be used as input for residual stress evaluation.     

盲孔法测量高残余应力的可靠性与精确度

钻孔法测量残余应力的范围是应力不超过材料屈服极限的一半。但是在许多情况下,钻孔法是用于测量焊接残余应力,而该应力峰值往往接近材料的屈服极限。本文探讨了盲孔法测量高残余应力的可靠性与精确度。为了模拟工程应用中测量残余应力的实际过程,本文采用标定的方法,即在试验机保持一定载荷不变的条件下进行贴片与钻孔操作,测得释放应变值。试验结果表明,用测得释放应变值计算出的应力值与标定应力值吻合较好,最大绝对误差仅为21 MPa。     

Effect of beam angle on HAZ, recast and oxide layer characteristics in laser drilling of TBC nickel superalloys

Industrial applications of laser drilling include the production of cooling holes at acute angles in certain parts of the aero-engine components. These parts are often covered with ceramic thermal barrier coatings (TBC) to protect them from reaching excessive temperatures in hot engine environments. Acute angle TBC drilling brings three major simultaneous complications to the process. These are: (i) multi-layer drilling, (ii) non-symmetrical geometry and melt ejection, and (iii) increased depth of drilling. In a previous investigation by the authors, delamination of TBC was found as a main problem of angled drilling and mechanisms involved were studied. In the present study, implications of these difficulties on the hole quality is investigated through a comparative study of vertical and acute angle drilled holes. Characteristics of recast layer, heat-affected zone (HAZ), oxide layer and TBC delamination are investigated. Variation of these metallurgical characteristics with the depth of the hole is evaluated. Results for vertical and inclined holes are compared. The extent of HAZ, recast layer and oxide layer is seen to vary significantly with location and is found increasing with decreasing drilling angle to surface. Numerical simulation of pulsed laser heating of TBC Nimonic 263 was carried out for acute angle drilling with assist gas considerations. Results from the simulation suggested that the total heat transfer rate is higher on the leading edge side than the trailing edge of the heated region. Experimentally observed larger HAZ on leading edge side and larger recast layer on trailing edge side are explained by the analysis of heat flow characteristics obtained with the model.     

Process adapted structure optimization of deep hole drilling tools

In order to machine boreholes having a high length-to-diameter ratio, deep hole drilling methods are applied. Especially when drilling ductile materials, the removal of cooling lubricant and chips can be hindered by long chips which block the inside of the chip mouth. In order to increase the volume flow rate of the cooling lubricant and chips, the chip mouth cross-section of a commercially available state-of-the-art BTA (Boring and Trepanning Association) boring tool was analyzed. Structure topology optimization was applied to reduce the drill head material without the drill head losing its stiffness to maximize the chip mouth cross-section. Finally, new drill head design was developed and verified through experimental investigations.     

转炉吹氩孔在线加工技术

为了增设转炉底吹氩搅拌装置,实现旧式转炉的顶底复吹冶炼效果,设计制作了一种深孔加工设备,并利用该设备在转炉“非传动侧耳轴”上在线钻削出3对轴向和径向吹氩深孔,从而完成了氧气顶底复吹转炉技术改造。     

Automation of centered micro hole drilling using a magnetically levitated table

This paper presents the automation of centered micro hole drilling, using a magnetically levitated table. Centered micro hole drilling, an example of which is nozzle outlet hole drilling, has previously been performed manually by skilled craftsmen. If a micro hole is drilled when the center line of the drill and the center line of the guide hole are not aligned, the misalignment may cause drill breakage. By using a magnetically levitated table, a workpiece can be aligned frictionlessly. When the horizontal support stiffness of the table is set small, by lowering the drill slowly, centering can be performed due to the contact force between the drill tip and the conical surface of the nozzle. Spinning nozzles were used as experimental workpieces, and 0.1–0.5 mm diameter drills were used.     

Analysis of hole quality characteristics in the electro jet drilling process

The modern trend towards miniaturization has given a new impetus to the development of nontraditional small hole drilling techniques. Electro jet drilling (EJD) is one such promising technique, which is finding ever-increasing applications in several industries including aerospace, medical, automobile and microfabrication (electronic and computers). This paper reports experimental findings on the effects of important process parameters such as applied voltage, capillary outside diameter, feed rate, electrolyte concentration and inlet electrolyte pressure on the productivity and the quality of small holes (<800 μm dia) produced by using the EJD process. Roundness error and surface roughness have been used as the response parameters for evaluating the quality of hole whereas material removal rate has been used as the response parameter for evaluating the process productivity. The experiments were performed on SUPERNI 263A material. An analysis of variance (ANOVA) performed to test the significance of the variables at the 5% level indicates that applied voltage and electrolyte concentration significantly affect the response parameters.     

An experimental and numerical study on laser percussion drilling of thick-section alumina

A major challenge in laser percussion drilling of thick-section ceramics is to obtain a low taper and low spatter deposition hole leading to high quality post-processing. In order to achieve the fine hole drilling, it is important to understand the mechanism of laser percussion drilling. In this paper, an experimental and numerical study on laser percussion drilling was carried out. A two-dimension (2D) axisymmetric finite element (FE) model for simulation of temperature field and proceeding of hole formation during percussion drilling was developed. The FE model was validated by the corresponding experiment. Furthermore, a theoretical model for evaluation of temperature at melt front and velocity of melt ejection was presented in order to further validate the FE model and study the spatter deposition. The effects of laser peak power, pulse duty cycle and pulse repetition rate on hole diameter and spatter deposition were investigated by the developed models and experiments, in which the simulated results were in good agreement with the experiments. The study indicated that the size and temperature of the melt front significantly affected the hole diameter formation and spatter deposition during laser percussion drilling. The characteristic of melt front was mainly determined by the employed laser peak power, pulse repetition rate and pulse duty cycle. Based on the experimental and numerical study, the process parameters were optimised and a drilled-hole with low taper and low spatter deposition was obtained using a 3.5 kW CO₂ laser. A microstructural and element compositional study was also performed in this work, by which the characteristics of microstructure and element composition in HAZ around laser drilled hole were revealed.     

PLC在深孔钻组合机床控制系统中的应用

本文简要介绍了深孔钻组合机床的控制原理，并给出其PLC控制系统的流程图和梯形图程序设计。     

激光打孔机床控制系统设计与分析

激光加工机床近年来广泛应用于航空、航天、兵器、电子、核工业等国防工业,以及汽车、机车、机械制造等民用行业。针对透平机械上的火焰筒气膜孔的加工及薄壁零件异形孔切割,介绍一种基于YAG固体激光器的激光打孔机的结构。该打孔机选用FAGOR数控系统、日本HARMONIC驱动器及力矩电机构成打孔机的五轴结构。详细分析系统调试参数对激光制孔工艺的影响。试验结果表明,机床结构加工性能良好,该控制模式具有很好的通用性,可用于其他类型的激光加工机床。     

The effect of vibratory drilling on hole quality in polymeric composites

The anisotropy of fiber-reinforced plastics (FRP) affects the chip formation and thrust force during drilling. Delamination is recognized as one of the major causes of damage during drilling of fiber reinforced plastics, which not only reduces the structural integrity, but also has the potential for long-term performance deterioration. It is difficult to produce good quality holes with high efficiency by conventional drilling method. This research concerning drilling of polymeric composites aims to establish a technology that would ensure minimum defects and longer tool life. Specifically, the authors conceived a new drilling method that imparts a low-frequency, high amplitude vibration to the workpiece in the feed direction during drilling. Using high-speed steel (HSS) drill, a series of vibratory drilling and conventional drilling experiments were conducted on glass fiber-reinforced plastics composites to assess thrust force, flank wear and delamination factor. In addition, the process-status during vibratory drilling was also assessed by monitoring acoustic emission from the workpiece. From the drilling experiments, it was found that vibratory drilling method is a promising machining technique that uses the regeneration effect to produce axial chatter, facilitating chip breaking and reduction in thrust force.     

Targeted suppression of vibration in deep hole drilling using magneto-rheological fluid damper

Under the concept of precision or targeted vibration suppression, a novel approach is developed to reduce the vibration of drilling tool system in deep-hole drilling process. The concept is to suppress the dominant vibration upon its emergence by variable damping or to force the harmful vibration morph into less harmful by different damper position. A magneto-rheological (MR) fluid damper is designed with adjustable damping capability to counteract emerging vibration. A series of experimental investigations are carried out on the BTA deep hole drilling process. The experimental results show that the adjustable MR damping brings down the vibration in all applications with different suppression effect. A relative optimal suppression can always be found. The suppressed vibration experiences a frequency shift which sheds light on the possible avoidance of matching of unwelcome frequency. Different damper locations have different but consistent suppression results which foresee a possibility of forcing the mode of vibration into less harmful one. A potential precision or targeted suppression of vibration upon its emergence is thus possible.     

Measurement of residual stress in thermal spray coatings by the incremental hole drilling method

The experimental measurement of residual stresses originating within thick coatings deposited by thermal spray processes onto solid substrates plays a fundamental role in the preliminary stages of coating design and process parameters optimization. The main objective of the present investigation was to determine the residual stresses by means of the incremental hole drilling method in order to perform the measurement of the stress field through the thickness of two different HVOF Nickel-based coatings. The holes through the coatings were carried out by means of a high velocity drilling machine (Restan). A finite element calculation procedure was used to identify the calibration coefficients necessary to evaluate the stress field. The Integral method was used for the analysis of non-uniform through-thickness stresses. The results for both coatings indicate that the nature of the residual stresses is tensile and their values are between 150-300 MPa.