Predictive model and process parameters optimization of Nd:YAG laser micro-turning of ceramics

The present work highlights laser micro-turning operation of 10-mm diameter cylindrical-shaped alumina (Al₂O₃) ceramic using pulsed Nd:YAG laser. The paper also addresses development of mathematical models for correlating the various micro-machining parameters such as laser beam average power, pulse frequency, workpiece rotational speed, assist air pressure, and Y feed rate with the response criteria such as surface roughness and deviation in turned depth for achieving desired surface quality as well as dimensional accuracy during micro-turning operation using Nd:YAG laser system. Response surface methodology-based design of experiments has been adopted for the experimentation. This investigation also highlights the various test results that confirm the validity and correctiveness of the developed mathematical models through analysis of variance test. The test results were analyzed through various response surface plots to study the effect of the process parameters on the aforementioned responses. The results of validation experimentation show a good agreement for the developed empirical models. Sensitivity analyses of the developed models have been done to find out the variation in the output with respect to variations in the significant input process parameters. Moreover, multi-performance optimization has been done to find out the optimal parametric setting for achieving the desired process performances. Analysis also has been made based on scanning electron microscopy micrographs of the laser micro-turned surface achieved during machining at multi-criteria optimization setting.     

Investigation of laser and process parameters for Selective Laser Erosion

The process of Selective Laser Erosion (SLE) was investigated to study the effects of different process and laser parameters on the process outputs such as surface quality and erosion rate. The SLE process is a direct method to remove material in a layer-by-layer fashion due to high energy densities provided by the laser beam. In addition to its direct use as a subtractive manufacturing method, SLE may be used in combination with layer-additive techniques such as Selective Laser Melting (SLM). Such combination mainly makes sense when both processes can be performed with the same laser. However, one of the major problems involved in SLE process is the high number of the laser and process parameters (laser power, pulse frequency, scan speed, scan spacing, ambient atmosphere, etc.) and the complexity of the relations between them which has not yet been investigated completely.This paper presents an overview of the laser erosion process with nano-second Nd:YAG laser pulses and the results of several single-factor experiments that were carried out to determine the influence of the major parameters on the depth of erosion per layer and surface roughness. Additionally, the relations between the parameters are studied to investigate the interactions between them. The results from single-factor experiments showed that some relations were highly governed by the power intensity of the laser beam and also that cross interactions between the parameters play an important role on the output characteristics. The paper explains how multiple parameters (spot size, pulse frequency, scan speed, scan spacing) can be combined to define two indirectly controlled geometrical parameters, namely the scan and pulse overlap factors. Those two parameters allow calculating the number of hits of the laser beam on a same location on the workpiece possible which is the first step in physical modeling the topography of the surface left behind.     

激光加热辅助铣削氮化硅陶瓷试验研究

激光加热辅助切削是加工工程陶瓷材料的一种有效方法,关于辅助车削的研究较多,而激光加热辅助铣削的研究较少。本文搭建了激光加热辅助铣削试验系统,对氮化硅陶瓷进行了加工试验研究。随着激光能量升高,切削力降低,刀具磨损减少,切屑尺寸增加。工件表面粗糙度、表面形貌、表面组织与表面硬度的测量结果表明加工质量好、激光对工件不造成损伤。此外,研究了加工过程中产生的边缘碎裂现象,分析了工艺参数对碎裂宽度的影响规律。     

A new CNC turning canned cycle for revolved parts with free-form profile

Laser micromilling technique is a thermal machining process which is used to remove material on the target geometry and has been widely employed in mold and die making industry. In this technique, the control factors of process such as scan speed, scan direction, frequency, and fill spacing play major affect on the surface quality. The selected quality characteristics are the mean surface roughness and milling depth. The main objective of this study is to determine the optimal milling conditions based on machining direction for minimizing the surface roughness and maximizing the milling depth. Therefore, L18 orthogonal array is constituted and subsequently signal/noise ratio and analysis of variance were employed to investigate the optimal levels of process parameters. The analysis results show that the scan speed has the highest effect on the surface roughness of which percentage contribution is 39.68% and also the beam scan direction and fill spacing have significant effects which contribute 19.67% and 16.09%, respectively. The experimental result for optimal condition is 2.23?μm. The results for milling depth show that only scan speed and fill spacing have significant effects which contribute 69.08% and 19.21%, respectively. Moreover, the scan direction has the least effect on the milling depth which can be neglected. The frequency has no effect on both surface roughness and milling depth. The result obtained from experiment at the optimal condition is 121.4?μm.     

Predicting surface quality of γ-TiAl produced by additive manufacturing process using response surface method

Electron beam melting (EBM) has been found to be a promising technology for producing complex shaped parts from gamma titanium aluminide alloys (γ-TiAl). The parts produced by this process are projected to have dimensions very close to the desired final shapes. However, the surface roughness of the parts produced by EBM is excessively rough. In many applications, it is necessary to improve the quality of manufactured parts using a convenient post process. This paper determines process parameters of end milling when it is used as a post process for the parts produced by EBM. Design of experiments has been used to study the effect of the selected input parameters of end milling (spindle speed, feed rate, depth of cut and coolant type) on the surface roughness of γ-TiAl parts. Response surface methodology is used to develop a predictive model for surface roughness. Effects of the selected milling process are investigated. This paper also optimizes the selected process parameters to minimize the value of the obtained surface roughness.     

Review on mechanism and process of surface polishing using lasers

Laser polishing is a technology of smoothening the surface of various materials with highly intense laser beams. When these beams impact on the material surface to be polished, the surface starts to be melted due to the high temperature. The melted material is then relocated from the ‘peaks to valleys’ under the multidirectional action of surface tension. By varying the process parameters such as beam intensity, energy density, spot diameter, and feed rate, different rates of surface roughness can be achieved. High precision polishing of surfaces can be done using laser process. Currently, laser polishing has extended its applications from photonics to molds as well as bio-medical sectors. Conventional polishing techniques have many drawbacks such as less capability of polishing freeform surfaces, environmental pollution, long processing time, and health hazards for the operators. Laser polishing on the other hand eliminates all the mentioned drawbacks and comes as a promising technology that can be relied for smoothening of initial topography of the surfaces irrespective of the complexity of the surface. Majority of the researchers performed laser polishing on materials such as steel, titanium, and its alloys because of its low cost and reliability. This article gives a detailed overview of the laser polishing mechanism by explaining various process parameters briefly to get a better understanding about the entire polishing process. The advantages and applications are also explained clearly to have a good knowledge about the importance of laser polishing in the future.     

Parametric analysis and optimization of Nd:YAG laser micro-grooving of aluminum titanate (Al2TiO5) ceramics

Pulsed Nd:YAG Laser offers an excellent role for various micro-machining operations of a wide range of engineering materials such as ceramics, composites, diamond etc. The micro-machining of ceramics are highly demanded in the present industry because of its wide and potential uses in various field such as automobile, electronic, aero-space, and bio-medical engineering applications etc. Aluminum titanate (Al₂TiO₅) has tremendous application in automobile and aero engine industry due to its excellent thermal property. The present research paper deals with the response surface methodology based mathematical modeling and analysis on machining characteristics of pulsed Nd:YAG laser during micro-grooving operation on a work piece of aluminum titanate. In this present study, lamp current, pulse frequency, pulse width, assist air pressure and cutting speed of laser beam are considered as machining process parameters during pulsed Nd:YAG laser micro-grooving operation. The response criteria selected for analysis are deviation of taper and deviation of depth characteristics of micro-groove produced on a work piece made of aluminum titanate (Al₂TiO₅). The analysis of variance test has also been carried out to check the adequacy of the developed regression mathematical models. The optimal process parameter settings are assist air pressure of 1.3 kgf/cm², lamp current of 20.44 amp, pulse frequency of 1.0 kHz, pulse width of 10% of duty cycle, and cutting speed of 10 mm/s for achieving the predicted minimum deviation of taper and deviation of depth of laser micro-groove. From the analysis, it is evident that the deviation of taper angle and deviation of depth of the micro-groove can be reduced by a great extent by proper control of laser machining process parameters during micro-grooving on aluminum titanate (Al₂TiO₅).     

Neural network modeling and analysis of the material removal process during laser machining

To manufacture parts with nano- or micro-scale geometry using laser machining, it is essential to have a thorough understanding of the material removal process in order to control the system behaviour. At present, the operator must use trial-and-error methods to set the process control parameters related to the laser beam, motion system, and work piece material. In addition, dynamic characteristics of the process that cannot be controlled by the operator such as power density fluctuations, intensity distribution within the laser beam, and thermal effects can significantly influence the machining process and the quality of part geometry. This paper describes how a multi-layered neural network can be used to model the nonlinear laser micro-machining process in an effort to predict the level of pulse energy needed to create a dent or crater with the desired depth and diameter. Laser pulses of different energy levels are impinged on the surface of several test materials in order to investigate the effect of pulse energy on the resulting crater geometry and the volume of material removed. The experimentally acquired data is used to train and test the neural network's performance. The key system inputs for the process model are mean depth and mean diameter of the crater, and the system outputs are pulse energy, variance of depth and variance of diameter. This study demonstrates that the proposed neural network approach can predict the behaviour of the material removal process during laser machining to a high degree of accuracy.     

三维激光烧蚀加工的试验研究

针对激光烧蚀加工出三维表面所存在的几个问题,即沿光轴方向的尺寸精度,必要的表面粗糙度和表面完整性,设计了一种扩束-无球差聚焦光路系统,并配合适当的激光焦斑峰值功率密度,大大改善了直接切削的尺寸精度,从传热学角度分析了激光烧蚀的能量传输,获得了对激光烧蚀过程的了解,以陶瓷为例的激光烧蚀实验证实了这一分析的正确性,从而为改善激光烧蚀加工的表面粗糙度提供了依据。     

Dimensional analyses and surface quality of pulsed UV laser micro-machining of STAVAX stainless steel mold inserts

Laser micro-machining is a new, precise, and very flexible process in micro-mold manufacturing, especially for difficult to machine material, i.e., hardened steel. The aim of the work reported in this paper was to utilize response surface methodology to optimize the dimensional accuracy and surface finish for STAVAX stainless steel mold inserts in the pulsed UV laser micro-machining. The influence of laser machining parameters on the ablated depth and surface roughness of the machined mold inserts have been experimentally investigated. The parameters of insert quality are analyzed under varying laser power, pulse frequency, hatched spacing, scan rate, and number of passes. The settings of the laser micro-machining parameters are determined by using design of experiments method. The analysis of variance, and regression analyses are employed to find the optimal levels and to analyze the effect of the parameters on the depth accuracy values and surface finish. Confirmation experiments with the optimal levels of micro-machining parameters are carried out in order to illustrate the effectiveness of the multi-optimization method. The validity of regression approach to process optimization is well established.     

Experimental investigation and numerical analysis for machinability of alumina ceramic by laser-assisted grinding

Alumina (Al₂O₃) ceramic has been widely used in various fields, but it has certain difficulties in machining as a hard and brittle material. While laser-assisted grinding (LAG), an alternative and novel method for fabrication of alumina ceramic, can utilize laser beam to locally heat the workpiece before the ceramic is removed, thereby reducing fracture toughness and keeping the surface integrity. In this paper, a thermal model is established to study and understand the processing mechanism of the LAG process. Meanwhile, an orthogonal experiment is designed and implemented to optimize the grinding process. Then, by analyzing the surface topography, the advantages of LAG are strongly proved. It is found that the temperature modelling results matches experimental results well. The processing parameter that has greatest impact on surface roughness is laser power, followed by grinding depth and wheel speed, and feeding speed at last. The optimal surface roughness value can be obtained by certain processing parameters. Also, compared to conventional grinding (CG), the removal method of alumina ceramics alters from brittle fracture to plastic fracture. Overall, this study clearly elucidates that LAG of alumina ceramic is a very promising machining method, and can be potentially utilized for various industrial, aerospace and automobile applications.     

Influences of laser welding parameters on the geometric profile of NI-base superalloy Rene 80 weld-bead

In the present study, autogenous laser butt joint welding parameters of Ni-base super alloy Rene 80 has been investigated by using a continuous wave 2.2?kW CO₂ laser. The experiments were performed based on the response surface methodology as a statistical design of experiment approach in order to investigate the effect of parameters on the response variations, achieving the mathematical equations and predicting the new results. Laser power (1,000?C2,200?W), welding speed (120?C360?cm/min), laser beam focal point position (?0.5?C0.5?mm) and inert gas pressure (0.2?C1?bar) were considered as the input process variables while welding surface width (W1), welding pool area (A), width of the weld-bead at the middle depth (W2), undercut welding and drop of welding were considered as the five process responses. Analyzed by statistical techniques, the results show that the welding bead profile is influenced by the laser heat input and input laser process parameters. Welding speed is known as the most important parameter with the reverse effect on process outputs. Inert gas pressure is the next significant parameter, and higher gas pressure causes welding geometry defects. Laser power has a direct influence on all investigated responses.     

An artificial neural network approach for the control of the laser milling process

Laser milling (LM) can be classified as a layer manufacturing process in which the material is removed by a laser beam by means of the ablation mechanism. It is a laser machining process which uses a laser beam to produce 3D shapes into a wide variety of materials. It is also known as laser ablation. It shows clear advantages versus the traditional milling such as the unlimited choice of materials, the direct use of computer-aided design structure data, the high geometric flexibility, and the touchless tool. LM requires the selection of optimal machining parameters for the job. Unlike the mechanical milling and the mechanical incision, the depth of the single removed layer is chosen at the beginning as input parameter of the process. In LM, the ablated depth depends from the process parameters such as laser power, scan speed, pulse duration, and pulse frequency. This work aims to develop an algorithm that can predict the parameters necessary to execute the material removal with a preset ablation depth. Using the results of an experimental campaign, the laser milling process was modeled by means of a back-propagation artificial neural network. Then, an iterative algorithm, based on the previous trained neural network, permitted to calculate the scanning velocity and pulse frequency that approached for the best the preset ablation depth. The developed approach represents a mean for the rational selection of laser ablation process parameters. It can be performed in an intuitive manner since it uses simple artificial intelligence like the artificial neural network.     

Investigating material removal rate and surface roughness using multi-objective optimization for focused ion beam (FIB) micro-milling of cemented carbide

Cemented carbide has been investigated as a useful material for the fabrication of micro devices. Focused ion beam (FIB) micro-milling has been found to be one of the most appropriate methods for the fabrication of micro devices. The experimental FIB micro-milling on cemented carbide have been conducted according to the L₁₆ orthogonal array of Taguchi technique. Beam current, extraction voltage, angle of beam incidence, dwell time and percentage overlap between beam diameters have been considered as process variables of FIB micro-milling in experimental design. Material removal rate (MRR) and surface roughness have been determined experimentally for FIB micro-milling of cemented carbide and beam current has been identified as the most significant parameter. The minimum surface roughness of 5.6 nm has been reported on cemented carbide, which is not a usual practice to achieve on such polycrystalline material, and hence it may be considered as a significant research contribution. Maximum MRR of 0.4836 μm³/s has been reported. Moreover, genetic algorithm toolbox of MATLAB has been utilized for multi-objective optimization between MRR and surface roughness. The corresponding optimum values of MRR and surface roughness for multi-objective optimization have been represented by pareto optimum solution generated by genetic algorithm. The research work presented in this paper determines the setting of process parameters of FIB micro-milling for achieving a specific combination of MRR and surface roughness on cemented carbide.     

Modeling and optimization of kerf taper and surface roughness in laser cutting of titanium alloy sheet

Laser cutting of titanium and its alloys is difficult due to it’s poor thermal conductivity and chemical reactivity at elevated temperatures. But demand of these materials in different advanced industries such as aircraft, automobile and space research, require accurate geometry with high surface quality. The present research investigates the laser cutting process behavior of titanium alloy sheet (Ti-6Al-4V) with the aim to improve geometrical accuracy and surface quality by minimizing the kerf taper and surface roughness. The data obtained from L₂₇ orthogonal array experiments have been used for developing neural network (NN) based models of kerf taper and surface roughness. A hybrid approach of neural network and genetic algorithm has been proposed and applied for the optimization of different quality characteristics. The optimization results show considerable improvements in both the quality characteristics. The results predicted by NN models are well in agreement with the experimental data.     

基于532nm半导体激光器的远距离传输高精度准直系统设计

半导体激光器在军事、工业、医学等许多方面有着重要的应用前景,但由于半导体激光器输出光束具有较大的发散角,因而在几乎所有要求较高的应用领域中,其输出光束都必须通过特殊的光学系统进行准直。扩束镜因其结构简单、材料便宜以及加工容易而在半导体激光束准直领域获得较多的应用。     

A statistical approach to the optimization of a laser-assisted micromachining process

The objective of this study is to optimize a laser-assisted micro-grooving process designed for micromachining of difficult-to-machine materials such as hard mold/die steels and ceramics. The process uses a relatively low power continuous wave laser beam focused directly in front of a micro-grooving tool to thermally soften the material thereby lowering the cutting forces and associated machine and tool deflections. However, the use of laser heating can produce a detrimental heat-affected zone (HAZ) in the workpiece surface layers. Consequently, the laser and micro-grooving parameters need to be optimized in order to achieve the desired thermal softening effect while minimizing the formation of a HAZ in the material. Although thermal and force models for the hybrid process have been developed for possible use in process optimization, they are computationally intensive and are not accurate enough to produce reliable results. We overcome these deficiencies using a statistical approach. First, easy-to-evaluate metamodels are developed to approximate the complex engineering models. Then, the metamodels are statistically adjusted using real data from the process to make more accurate predictions. The optimization is then carried out on this statistically adjusted metamodels. The optimization strategy is experimentally verified and shown to yield good results.     

Theoretical and experimental analysis of the remote welding process on thin, lap-joined AISI 304 sheets

In this study, a theoretical approach of the remote welding process has been developed and discussed. The study obtains numerically the melting boundaries of different heat source angles, based on an analytical calculation of the keyhole depth. The approach considers the dominant process parameters of the laser power, the welding speed and the inclination of the laser beam on the workpiece surface. The geometrical particularities of the beam spot, due to the different inclination of the laser beam upon the processing plane, have also been considered in a previous study of the authors. The theoretical results present good agreement when compared with experimental data obtained from a remote welding system (RWS) on lap welding of AISI 304 stainless steel, thin sheets.     

Numerical analysis of Nd:YAG pulsed laser polishing CVD self-standing diamond film

Chemical vapor deposited(CVD) diamond film has broad application foreground in high-tech fields.But polycrystalline CVD self-standing diamond thick film has rough surface and non-uniform thickness that adversely affect its extensive applications.Laser polishing is a useful method to smooth self-standing diamond film.At present,attentions have been focused on experimental research on laser polishing,but the revealing of theoretical model and the forecast of polishing process are vacant.The paper presents a finite element model to simulate and analyze the mechanism of laser polishing diamond based on laser thermal conduction theory.The experimental investigation is also carried out on Nd:YAG pulsed laser smoothing diamond thick film.The simulation results have good accordance with the results of experimental results.The temperature and thermal stress fields are investigated at different incidence angles and parameters of Nd:YAG pulsed laser.The pyramidal-like roughness of diamond thick film leads to the non-homogeneous temperature fields.The temperature at the peak of diamond film is much higher than that in the valley,which leads to the smoothing of diamond thick film.The effect of laser parameters on the surface roughness and thickness of graphite transition layer is also carried out.The results show that high power density laser makes the diamond surface rapid heating,evaporation and sublimation after its graphitization.It is also found that the good polish quality of diamond thick film can be obtained by a combination of large incident angle,moderate laser pulsed energy,large repetition rate and moderate laser pulse width.The results obtained here provide the theoretical basis for laser polishing diamond film with high efficiency and high quality.     

Fabrication of micro-end mill tool by EDM and its performance evaluation

Abstract

Micro-milling is a fast, cheap and controlled process compared to other micro-fabrication processes such as lithography, laser/electron/ion beam machining, etc. However, scarcity of cutting tools of very small dimensions often results in limited application of micro-milling. In the present study, electro discharge machining (EDM) is used for fabrication of micro-end mill tool. To ensure high dimensional accuracy of the tool, a parametric study is conducted by replicating the a tungsten carbide block to a tungsten carbide (WC) block. The relationships between the drilled cavities on the block and the features on the micro-tool are established. The influence of machining parameters (voltage, capacitance and spindle speed) on the response variables (entrance diameter, hole depth, material removal rate (MRR) and surface roughness) is reported. Capacitance is found more dominant as compared to other selected process parameters. Using optimized parameters from the parametric study, a WC micro-end mill tool of 100?µm diameter is fabricated. Channel of around 110 µm width, 40?µm depth and surface roughness of 70?nm is successfully fabricated on aluminum. The performance of the fabricated tool is compared with a commercial end mill tool by milling micro channels on stainless steel.