Paper Review

30. EXPERIMENTAL AND NUMERICAL STUDY OF MULTI-PULSE PICOSECOND LASER ABLATION ON 316L STAINLESS STEEL.

TONG ZHOU1,2,3 YUCHAO HONG1 ZHENG FANG2 WALTER PERRIE2 YANG FEI2 YOUYOU HU1,4 STUART EDWARDSON2 AND GEOFF DEARDEN2

An experimental and numerical study on 10 ps laser ablation of 316L stainless steel up to 400 hundred pulse exposure has been carried out. In this simulation, the material removal threshold temperature has been carefully discussed depending on the different ablation driving mechanisms. The influence of the instantaneous material removal has also been considered which will affect the calculation of the next pulse’s absorption. For single-pulse ablation, the simulated ablation threshold Fsim=0.26 J/cm2 is close to the fitted experimental result F0th=(0.29±0.01) J/cm2. For multi-pulse ablation, the simulated ablation rate Rsim=11.4nm/pulse is close to the fitted experimental result Rexp=(12.4±0.1) nm/pulse under 0.9 J/cm2 fluence, while the simulated ablation rate Rsim=19.8nm/pulse is slightly larger than the fitted experimental result Rexp =(16.1±0.7) nm/pulse at 2.7 J/cm2, providing good agreement between theory and experiment for both single and multi-pulse ablation.

29. Numerical simulation and experimental study of the shape variation influence on stainless steel drilling with picosecond laser

Zehui Gu, Yuyang He * , Jinghu Ji, Yonghong Fu

School of Mechanical Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China

The laser beam tilt induces significant changes in the shape of irradiation on the material surface, thereby affecting the ablation contour. However, the influence of tilted laser beams on micro-hole machining remains confined to a simplistic approach. This paper combines experimental and finite element simulation studies to investigate the impact of the laser angle of the incident (AOI) on micro-hole machining with picosecond lasers. A two-dimensional finite element model, including the two-temperature equation and the deformable geometry module, is established to simulate the ablative process of tilted picosecond laser beams on stainless steel. Single- pulse and multiple-pulse ablations under different AOIs are simulated using the model.

28. Microstructural changes induced by ultrashort pulsed lasers in microdrilling of fuel nozzles

L. Romoli a, G. Lovicua , C.A.A. Rasheda G. Diniaa, , M. De Sanctisaa ,M. Fiaschib b

aDepartment of Civil and Industrial Engineering, University of Pisa bContinental Automotive Italy S.p.A
Microholes for gasoline direct injection (GDI) nozzles were obtained in martensitic stainless steel AISI 440C with conventional micro-EDM and two laser based processes: water-jet guided s-laser and fs-laser. Since the analyzed drilling methods heavily differ for their thermal input on materials, the three processes were compared from the perspective of the microstructural changes induced in the bulk material after drilling. Moreover, the sharpness of the edges was taken into account as distinctive feature for a comparison among the three processes, being the spray atomization maximized by a cavitation process inside the microhole. A tailored procedure was optimised to prepare the samples for Scanning Electron Microscopy (SEM) and metallographic analyses.

27. Femtosecond laser high-quality drilling of film cooling holes in nickel-based single superalloy for turbine blades with a two-step helical drilling method

Meng Li a, Zhi - xun Wena , Ping Wangb, Yu -xing Liua Zhen -wei Lia,b Zhu -feng Yuea

aSchool of Mechanics, Civil Engineering and Architecture, Northwestern Polytechnical University, Xi’an 710072, PR China bSchool of Aerospace Engineering and Applied Mechanics, Tongji University, Shanghai 200092, PR China
A femtosecond laser two-step helical drilling method was proposed, which realized the extremely high-quality machining of film cooling holes (FCHs) on nickel-based single crystal superalloy for turbine blades. The two- step helical drilling increased the exhaust capacity of machining residues, steam and laser-induced plasma, so it avoided the damage and defects, such as recast layers, microcracks and microvoids caused by laser energy shielding and heat accumulation. Furthermore, since more aggressive parameters can be adopted in the first step, and the amount of material removal in the second step was reduced, the two-step helical drilling improved the machining efficiency compared with one-time helical drilling. After two-step helical drilling, sub-wavelength nano-stripes were formed on the hole wall at a certain regular inclination angle along the hole depth direc tion. This characteristic feature was caused by the combined action of the periodic distribution of energy due to the interference of the laser and the plasmon, and the high-speed rotation and downward movement of laser beam.

26. Microdrilling of Through-Holes in Flexible Printed Circuits Using Picosecond Ultrashort Pulse Laser

Wanqin Zhao1,2, Lingzhi Wang1

1School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, China 2Shanghai Collaborative Innovation Center of Laser Advanced Manufacturing Technology, Shanghai 201620, China
High-density and high quality interconnects are necessary for the preparation of miniaturized and lightweight electronic products. Therefore, small-diameter and high-density through-holes in FPCs (Flexible Printed Circuits) are required. However, the current processing methods cannot further decrease the diameters and improve the quality of through-holes. Comparatively, ultrashort pulse laser is a good choice. In this paper, the processing technology for the microdrilling of through-holes in FPCs using a 10 ps pulse laser was systematically studied. The effects of laser parameters, including the wavelength, energy, pulses and polarization, on the drilling of through-holes were investigated. The various processing parameters were optimized and the plausible reasons were discussed. Finally, the desired small-diameter and high-density through-holes in FPCs were obtained. The experimental results showed that, through-holes with diameters of less than 10 m and inlet interconnection pitches of 0~2 m could be successfully drilled in FPCs using ultrashort pulse laser.

25. A review of modern advancements in micro drilling techniques

Mahadi Hasan, Jingwei Zhao, Zhengyi Jiang

School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Australia

The demand for micro drilling with a diameter in a range of a few microns to several hundred microns is increasing in industries such as electronics, aerospace, medicine and automobiles, due to a significant uptake in the use of miniaturised products and devices. In order to satisfy the demand, a number of differ ent micro drilling techniques have been developed. There has been, however, no report which explains, compares and contrasts all of these micro drilling techniques. This study examines the lasts micro drilling methods and techniques, categorises them into different groups, highlights recent developments and new trends, and depicts the future requirements in the field of micro drilling. Both conventional and non-conventional micro drilling techniques used in modern age applications are categorized.

24. Dual-path micro-holes process for 0Cr17Ni7Al stainless steel thin plate with picosecond laser

Zehui Gua, Yuyang Hea, Jie Yanga , Yonghong Fua, Jingu Jia, Yanhu Zhangb, Jian Lic, Gaohui Liua

aSchool of Mechanical Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China bInstitute of Advanced Manufacturing and Modern Equipment Technology, School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, China cSchool of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China

Ultrafast laser is widely used in drilling micro-hole because of its excellent processing quality. However, when machining micro-holes with an aperture of fewer than 100 μ m, the ultrafast laser drilling shows some drawbacks, such as poor roundness and taper of holes. This paper proposed a dual-path laser scanning strategy to solve the above-mentioned issues. A micro-hole with a diameter of about 90 μ steel sheet with a thickness of 80 μ m was successfully fabricated on a stainless m. In the dual-path laser scanning strategy, concentric circles were scanned layer-by-layer on the stainless steel sheet, quickly forming a through-hole. Then, a helical scanning path was conducted to reduce the taper of the hole. The effects of different laser ablation paths and angles of incidence (AOI) on micro-holes roundness, taper, and sidewall quality were illustrated.

23. Characterization of Micro-Holes Drilled Using a UV Femtosecond Laser in Modified Polyimide Flexible Circuit Boards

Lijuan Zheng 1,2 , Shuzhan Lin1,2,Huijuan Lu1,2 , Bing Huang1,2 , Yu Liu3, Jun Wang3, Xin Wei1,2, Jun Wang1,2 and Chengyong Wang1,2

1Institute of Manufacturing Technology, Guangdong University of Technology, Guangzhou 510006, China 2State Key Laboratory for High Performance Tools, Guangzhou 510006, China 3Zhuhai Jingwang Flexible Circuit Co., Ltd., Zhuhai 510006, China

Modified polyimide (MPI) flexible printed circuits (FPCs) are used as chip carrier boards. Thequality of the FPCdirectly affects the reliability of the integrated circuit. Furthermore, micro-holes are critical components of FPCs. In this study, an ultraviolet (UV) femtosecond laser is used to drill micro-holes in double-layer flexible circuit boards with MPI as the substrate. The morphology of the micro-hole wall in the copper foil and MPI layer is observed, and the effects of the laser processing parameters on the diameter and depth of the micro-holes are analyzed. The drilling process and mechanism of micro-holes obtained using a UV femtosecond laser in MPI FPCs are discussed. The results show that the morphology of femtosecond laser-machined copper is closely related to the laser energy, and a periodic structure is observed during the machining process.

22. Geometry assessment of ultra-short pulsed laser drilled micro-holes

Matthias Putzer 1,2 Norbert Ackerl2, Konrad Wegener2

1inspire AG, Technoparkstrasse 1, Zurich, Switzerland 2Department of Mechanical Engineering, IWF, ETH Zurich, Leonhardstrasse 21, Zurich, Switzerland
Ultra-short pulsed laser ablation enables a defined generation of micro-holes. A parameter study on the ablation characteristics of copper clearly reveals a benefit for green wavelength with lower threshold fluence, simultaneously increasing the Rayleigh length. The use of a circular drilling method allows a defined manufacturing of micro boreholes and micro through-holes with 35 μmdiameterofupto165μm and 300 μm length. Introducing high-resolution micro-computed X-ray tomography studying the micro-hole evolution and adjacent geometrical transformations reveals micrometer resolution and high usability. The conical geometry evolving up to an aspect ratio of 5:1 fits well to established models known for percussion drilling.

21. Two-step approach to improving the quality of laser micro-hole drilling on thermal barrier coated nickel base alloys

Rujia Wang a,b,c, Xia Dong a,b,c, Kedian Wang a,b,c, Xiaomao Sun a.b.c , Zhengjie Fan a,b,c, Wenqiang Duan a,b,c

aState Key Laboratory for Manufacturing System Engineering, Xi’an Jiaotong University, Xi’an 710054, China bShaanxi Key Laboratory of Intelligent Robots, Xi’an Jiaotong University, Xi’an 710049, China cSchool of Mechanical Engineering, Xi’an Jiaotong University, Xi’an 710049, China
To improve the quality and processing efficiency of laser micro-hole drilling on thermal barrier coated nickel base alloys, a two-step approach is proposed. Through-holes were initially fabricated by a millisecond laser and were subsequently refined by a femtosecond laser. In step one, the influence of different pulse shapes on hole characteristics, such as taper and delamination was investigated using a temporal pulse train modulation (TPTM). In step two, femtosecond laser trepanning with an axial feed method was used to remove the recast layer. The results showed that a ramp-down pulse shape with a short time interval could prevent the vapor pressure from suddenly dropping, which was conducive to the continuous upward flow of the melt. This pulse shape led to holes with a small taper. The delamination location was independent of pulse shape, and with the increase of maximum peak power in the pulse train, it would shift from the bond coat (BC)/substrate interface to the thermal barrier coating (TBC)/BC interface. The preset trepanning radius can be precisely determined based on the measurements of the prefabricated hole diameter and the thickness of the recast layer. Finally, the recast layer and delamination cracks could be completely eliminated by femtosecond laser trepanning.

20. Dual-Method Characterization and Optimization of Drilling Parameters for Picosecond Laser Drilling Quality in CFRP

Zhao Zheng, Yao Ma, Zhonghe Wang , Siqi Liu and Chunting Wu

Jilin Key Laboratory of Solid-State Laser Technology and Application, Changchun University of Science and Technology, Changchun 130022, China

Carbon fiber-reinforced polymer (CFRP), known for its light weight, high strength, and corrosion-resistant properties, is extensively used in the lightweight design of satellite components, the optimization of electronic device casings, and the processing of high-performance composite materials in the defense sector. This study employs picosecond laser drilling technology for the precision machining of CFRP, demonstrating its advantages over traditional mechanical drilling and other unconventional methods in significantly reducing the heat-affected zone (HAZ) and enhancing hole wall quality. The optimization of laser power, scanning speed, and fill times via response surface methodology (RSM) significantly reduced the hole wall taper to 4.160◦ and confined the HAZ to within 18.577 µm, thereby enhancing machining precision.

19. Investigation of hole quality in drilled Ti/CFRP/Ti laminates using CO2 laser

Dhiraj Kumar, Suhasini Gururaja

Dept. of Aerospace Engineering, Indian Institute of Science, Bangalore, Karnataka, India

The machinability of titanium (Ti) and carbon fiber reinforced plastic (CFRP) (Ti/CFRP/Ti) laminates using CO2 laser is presented in this work. The effect of line energy and laser frequency on output responses such as heat affected zone (HAZ), taper angle (TA), metal composite interface (MCI) damage, surface roughness, dross height, and circularity were investigated. Line energy- the most influential parameter- demonstrated a threshold be havior; no drilling was observed below a certain line energy. Scanning acoustic microscopy (SAM), scanning electron microscopy (SEM), micro-computed tomography (μ-CT), and other imaging techniques were used to establish a correlation between laser parameters and CO2 laser machined damage in Ti/CFRP/Ti laminates.

18. Experimental and theoretical investigation of the drilling of alumina ceramic using Nd:YAG pulsed laser

M.M. Hanon a, E. Akmanb, B. Genc Oztoprakb, M. Gunesc ,Z.A. Tahaa, K.I. Hajima, E. Kacarb , O. Gundogduc, A. Demirc

aInstitute of Laser for Postgraduate Studies, University of Baghdad, Baghdad, Iraq bKocaeli University, Laser Technologies Research and Application Center, 41275 Yenik¨oy, Kocaeli, Turkey cKocaeli University, Electro-Optic Systems Engineering, 41380 Umuttepe, Kocaeli, Turkey
Alumina ceramics have found a wide range of applications from semiconductors, communication technologies, medical devices, automotive to aerospace industries. Processing of alumina ceramics is rather difficult due to its high degree of brittleness, hardness, low thermal diffusivity and conductivity. Rapid improvements in laser technologies in recent years make the laser among the most convenient processing tools for difficult-to-machine materials such as hardened metals, ceramics and composites. This is particularly evident as lasers have become an inexpensive and controllable alternative to conventional hole drilling methods. This paper reports theoretical and experimental results of drilling the alumina ceramic with thicknesses of 5 mm and 10.5 mm using milisecond pulsed Nd:YAG laser. Effects of the laser peak power, pulse duration, repetition rate and focal plane position have been determined using optical and Scanning Electron Microscopy (SEM) images taken from cross-sections of the drilled alumina ceramic samples.

17. Vibration-Assisted Femtosecond Laser Drilling with Controllable Taper Angles for AMOLED Fine Metal Mask Fabrication

Wonsuk Choi1,2, Hoon Young Kim 1,2, Jin Woo Jeon2, Won Seok Chang1,3 and Sung-Hak Cho 1,2

1Department of Nano-Mechatronics, UST, Korea University of Science and Technology, 217 Gajeong-Ro, Yuseong-Gu, Daejeon 34113, Korea
2 Department of Laser & Electron Beam Application, KIMM, Korea Institute of Machinery and Material, Yuseong-Gu, Daejeon 34113, Korea

3Department of Nanomechanics, KIMM, Korea Institute of Machinery and Material, 156 Gajeongbuk-Ro, Yuseong-Gu, Daejeon 34103, Korea

This study investigates the effect of focal plane variation using vibration in a femtosecond laser hole drilling process on Invar alloy fabrication quality for the production of fine metal masks (FMMs). FMMsare used in the red, green, blue (RGB) evaporation process in Active Matrix Organic Light-Emitting Diode (AMOLED) manufacturing. The taper angle of the hole is adjusted by attaching the objective lens to a micro-vibrator and continuously changing the focal plane position. Eight laser pulses were used to examine how the hole characteristics vary with the first focal plane’s position, where the first pulse is focused at an initial position and the focal planes of subsequent pulses move downward. The results showed that the hole taper angle can be controlled by varying the amplitude of the continuously operating vibrator during femtosecond laser hole machining.

16. Study of hole characteristics in laser trepan drilling of zta

Surendra K. Sainia, Avanish K. Dubeya, B.N. Upadhyayb, A. Choubey b

aMechanical Engineering Department, Motilal Nehru National Institute of Technology, Allahabad 211004, Uttar Pradesh, India
bSolid State Laser Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, Madhya Pradesh, India

Zirconia Toughened Alumina ceramic is widely used for aerospace components, combustion chambers, heat exchangers, bearings and pumps mainly due to its improved mechanical and thermal properties. To make holes in thick section Zirconia Toughened Alumina ceramics is a major challenge due to its unfavorable machining characteristics. Recent research have explored that laser machining can overcome the machining limitations of advanced materials having improved mechanical properties. In present research, authors have analyzed the effect of Laser Trepan Drilling on hole characteristics of 6.0 mm thick Zirconia Toughened Alumina. Effect of significant process parameters on hole characteristics such as hole circularity at top and bottom, hole taper, and spatter size have been studied. The optimum ranges of these parameters have been suggested on the basis of empirical modeling and optimization.

15. Triple rings trepanning technology for holes ablated using nanosecond pulse laser in al2o3 ceramics substrate

Linzheng Ye1, 2, Wanqi Zhang 1,2, Xijing Zhu1,2, Yao Liu1,2, Shida Chuai1,2 Boyang Lv1,2 and Tengwei Li1,2

1School of Mechanical Engineering, North University of China, Taiyuan 030051, China
2Shanxi Key Laboratory of Advanced Manufacturing Technology, North University of China, Taiyuan 030051, China

The drilling pattern significantly impacts the quality of the holes and the efficiency of laser holes processing. This study utilized triple rings laser trepanning technology to process holes in Al2O3 ceramics substrates, which were 0.25 mm thick, using a fiber nanosecond laser. The effects of the number of laser scans, laser scanning speed, the amount of defocusing, and the laser power on the geometrical features of the holes such as the hole diameter, hole roundness, and taper angle were studied. The results show that in the case of unsaturated holes, both the entrance and exit diameters expanded as the number of laser scans increased, and the taper angle reduced. In contrast, the diameter and taper angle of saturated holes remained relatively stable as the number of laser scans increased. The diameter of the holes gradually decreased as the laser scanning speed rose.

14. Optimization of trepanning patterns for holes ablated using nanosecond pulse laser in alumina ceramics substrate

Wanqin Zhao and Xuesong Mei

State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, China

Trepanning pattern is an important factor in laser hole machining, affecting both the hole quality and process efficiency. The influence of laser trepanning patterns on the hole ablating using nanosecond pulse laser in Al2O3 ceramics substrate was studied. Two laser trepanning patterns were evaluated, filled spiral trepanning and multiple rings trepanning, with the optimized laser machining parameters. In conjunction with the studies, the hole saturated taper and the saturated processing time were taken as the primary criteria for evaluation of the hole quality and the machining efficiency, respectively. Finally, the trepanning patterns were optimized aiming for the high hole quality; the process was based on the saturated hole tapers. The hole high qualities and machining efficiencies were obtained based on the saturated processing time, which was proven to have a great significance when using the nanosecond pulse laser to machine Al2O3 ceramics substrate.

13. COMPOUND CONNECTION MECHANISM OF ALUMINA CERAMIC AND TC4 TI ALLOY WITH DIFFERENT JOINING MODES

Yan Zhang 1, 3, Yankun Chen1, JianPing Zhou1, DaQian Sun2, HongMei Lia2

1 School of Mechanical Engineering, Xinjiang University, Wulumuqi 830000, China 2 Key Laboratory of Automobile Materials, School of Materials Science and Engineering, Jilin University, Changchun 130022, China3 State Key Laboratory for Manufacturing Systems Engineering, Xian 710000, China

In this paper, laser welding-brazing of TC4 Titanium (Ti) alloy and Al2O3 ceramic dissimilar material was carried. The results showed that the Ti alloy and Al2O3 were joined by melting filler metal when the laser was concentrated in the Ti alloy side of the joint. The joint with one fusion weld and one brazed weld separated by remaining unmelted Ti alloy. Laser beam offset the Ti alloy 1.5 mm, Ti alloy would not be completely melted in joint. Through heat conduction, the filler metal melted occurred at the Ti-ceramic interface. A brazed weld was formed at the Ti-ceramic interface with the main microstructure of β-CuZn + Ti2Zn3, β-CuZn and Al2Cu + β-CuZn. The joint fractured at the brazed weld with the maximum tensile strength of 169 MPa.

12. femtosecond laser drilling 100 MICRONS DIAMETER MICRO HOLES WITH ASPECT RATIO > 20 IN A NICKEL BASED SUPERALLOY

Nan Zhang a,1, Manshi Wang a.b,1, Mingxia Bana, Lanjun Guoa, Weiwei Liua*

a Institute of Modern Optics, Institute of Ophthalmology, Nanki University, Tianjin Key Laboratory of Micro-scale Optical Information Science and Technology, Tianjin, 300350, China b Department of Mechanical and Automation Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong SAR, China

Micro holes with high aspect ratios are essential for the cooling performance of aero-engines. Challenges persist in deep hole drilling due to the escalating stringent requirements in terms of hole diameters, aspect ratios, and 3D profiles. In this study, we propose a two-step femtosecond laser drilling method to produce cylindrical micro holes with diameters of ~100 μ m, aspect ratios >20, and near-zero taper angle in Inconel 718 plates. In the first drilling period, the laser polarization trepanning method is used, yielding through-holes with aspect ratios >20 but undesired tapers and irregular shapes at the hole exit. A new method called laser spinning is then introduced in the second drilling period, proficiently decreasing taper angles to be < 0.05 ◦ and improving the irregular exit profiles into circular contours. Employing a rotating inclined laser beam, the laser spinning method facilitates deep hole drilling by enhancing laser penetration, solely resulting augmented hole diameters at the exit without compromising hole aspect ratios. The efficacy of this drilling approach is achieved through modifying the incident laser reflection on internal hole walls.

11. Femtosecond laser layered ring trepanning of stainless steel sheets with and without transverse magnetic assistance

Tao Li a, Naifen Rena, Houxiao Wanga, Wei Zhoub, Nai En Wuc,d, Kaibo Xiaa , Ye Xua, Jianan Tiana

aSchool of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu, China bSchool of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore c Precision Laser Solutions Pte. Ltd., 280 Woodlands Industrial Park E5, #09-39, Singapore 757322, Singapore c Suzhou Dacheng Laser Micro-Nano Technology Co. Ltd, No.38 Bei Guan Du Road, Wuzhong Economic Development Zone, Suzhou, China

A novel femtosecond laser layered ring trepanning (LLRT) technology assisted by external transverse magnetic f ields is reported for improving laser trepanning efficiency and quality while reducing hole defects. The influence of key operating parameters on blind-hole geometry trepanned by altering magnetic flux density was discussed. The element compositions were compared and analyzed for the corresponding domains near the entrances, middles and bottoms of the blind-hole sidewalls trepanned with and without magnetic assistance. The surface morphology and roughness together with local microscale irregularities and defects of the blind-hole sidewalls trepanned with and without magnetic assistance were also analyzed based on corresponding comparisons via altering scanning speed, magnetic flux density, pulse repetition rate and feed distance. It was demonstrated that the magnetic field-enhanced plasma motion induced by Lorentz force scoured hole sidewall and weakened the multi-shock effect on the hole wall. A stronger transverse magnetic field was more effective to reduce/avoid the local hole-wall defects such as micro cracks together with micro irregularities for improving the formation uniformity/quality of the hole wall. With the increase of magnetic flux density, the hole depth and aspect ratio increased while the hole diameter and taper angle decreased.

10. Experimental characterization of the inner surface in micro drilling of spray holes: A comparison between ultrashort pulsed laser and edm

L. Romolia, C.A.A Rasheda, M. Fiaschib

a Department of Civil and Industrial Engineering, University of Pisa, Pisa, Italy b Continental Automotive Italy S.P.A., Italy

In this research, the inner surface characteristics of micro-drilled holes of fuel injector nozzles were analyzed by Shear Force Microscopy (SHFM). The surface texture was characterized by maximum peak-to-valley distance and periodicity whose dimensions were related to the adopted energy. 180 mm diameter holes were drilled using an ultrashort pulsed laser process using pulse energies within the range of 10–50 mJ. Laser-ablated surfaces in the tested energy range offer a smooth texture with a peculiar periodic structure with a variation in height between 60 and 90 nm and almost constant periodicity. The Scanning Electron Microscopy (SEM) photograph of the Laser Induced Periodic Surface Structure (LIPSS) showed the co-existence of Low Spatial Frequency LIPSS (LSFL) and High Spatial Frequency LIPSS (HSFL). A comparative analysis was carried out between the highest laser pulse energy in the tested range energy laser drilling which enables the shortest machining time and micro-electrical Discharge Machining (m-EDM). On the contrary, results showed that surfaces obtained by electro-erosion are characterized by a random distribution of craters with a total excursion up to 1.5 mm with a periodicity of 10 mm. The mean squared surface roughness (Rq) derived from the scanned maps ranges between 220 and 560 nm for micro-EDM and between 50 and 100 nm for fs-pulses laser drilling.

9. Experimental study of nickel - based superalloy in792 with femtosecond laser drilling method

Fengyun Zhang a, Jin Wanga, Xi Wanga, Jian Zhang a,b, Yoshio Hayasaki c, Dongsik Kim d

a Shandong Collaborative Innovation of Laser Green Intelligent Manufacturing Technology and Equipment, Qingdao University of Technology, Qingdao 266525, China b College of Mechanical & Electrical Engineering, Wenzhou University, Wenzhou 325035, China c Center for Optical Research and Education (CORE), Utsunomiya University, 7-1-2 Yoto, Utsunomiya 321-8585, Japan c Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea

The improvement of aircraft engine performance resulted in the high demand of drilling micron-sized holes (<1 mm) into the blades. A two-step approach with an axial feed method for nickel-based superalloy IN792 femtosecond laser drilling was studied. The effects of different processing path parameters on the hole quality are described and discussed in detail. The drilling strategy contained a drilling through phase and an enlargement phase, which was able to produce cylindrical holes with diameters of 600 μm on 680 μm and 1300 μm thick plates without modification and reconstruction of the processing platform. According to the different material thicknesses, the corresponding drilling optimization strategy was proposed. The experiments showed that the hole quality processed using the two-step method improved obviously compared with the one-step method, including that the outlet circularity can be increased by 7.4% and the taper angle can be reduced by 4 degrees. Moreover, there is no obvious heat-affected zone (HAZ), recast layer or micro-cracks on the hole surface and wall. In conclusion, the two-step approach is an effective hole processing method that provides the possibility of high-efficiency and high-quality machining of film cooling holes.

9. Experimental study on the development of a MICRO DRILLING CYCLE USING ULTRASHORT LASER PULSES

L. Romoli a, R. Vallini b

a Department of Industrial Engineering, University of Parma, Italy b Department of Industrial and Civil Engineering, University of Pisa, Italy

Microholes for the production of high-precision devices were obtained by ultrashort pulsed laser machining of martensitic stainless steels. A micro-drilling cycle based on the sequence of drilling through phase, an enlargement and a finishing phase is proposed to solve the trade-off between process time and quality of the ablated surfaces without making use of complex design of experiments. The three phases were studied taking into account the evolution of the microhole shape as a function of the main process parameters (number of passes per phase, incidence angle and radius of the beam trajectory with respect to the hole's axis). Experiments testified that the drilling strategy was able to produce cylindrical holes with a diameter of 180 μm on a 350μm thick plate in total absence of burrs and debris within a drilling time of 3.75 s. Repeatability tests showed a process capability of nearly 99%. SEM inspection of the inner surface of the micro holes showed the presence of elongated and periodic ripples whose size and inclination can be controlled by adjusting the incidence angle of the beam over the tapered surface before the ultimate finishing phase.

8. Experimental study on micro hole drilling using ultrashort pulse laser radiation

Andreas Gruner a, Joerg Schille a, Udo Loeschner a

a Laserinstitut Hochschule Mittweida, Technikumplatz 17, 09648 Mittweida, Germany

This study investigated a drilled micro hole varied between 25 µm – 1 mm and the hole diameters, pulsed numbers for through bores by varying the polarization state and repetition rate. Percussion drilling was used and parameters such as polarization state, frequency, laser fluence, and material thickness were varied. Circular polarization resulted in more consistent and uniform hole geometries compared to linear polarization. Hole consistency decreased as material thickness increased. High repetition rates reduced drilling time but increased heat accumulation.

7. Laser micro drilling of 316L stainless steel orthopedic implant

S. Pattanayak a, S. Panda b, D. Dhupal a

a Department of Production Engineering, Veer Surendra Sai University of Technology, Burla, 768018, Odisha, India b Department of Mechanical Engineering, Veer Surendra Sai University of Technology, Burla, 768018, Odisha, India
It has recently been shown that there is a requirement of inclined micro hole with smooth side walls (without cracks and burr), high aspect ratio, low taper, reduced heat affected zone (HAZ) and thin recast layer on or thopedic implant. So with the aim of improving hole quality characteristics and functionality of implant multi-objective optimization of laser micro drilling with argon as an assist gas is addressed in this study. A hybrid statistical approach based on grey relational analysis (GRA) and coefficient of variation (COV) is proposed to analyze the experimental data. The key process variables responsible for quality improvement are identified. Four statistical tests are performed to compare the hybrid and GRA approach. Further, it was found that the proposed hybrid is superior to GRA in terms of computational time and ease of computation. It is also observed that focal point position and diode current have a striking effect on hole quality. An important implication of this paper is that a thin recast layer, small HAZ width and spatter area were observed when argon was used as an assist gas.

6. An INVESTIGATION ON THE HOLE QUALITY DURING PICOSECOND LASER HELICAL DRILLING OF STAINLESS STEEL 304

Hongyu Zhang a, Jianke Di b, Ming Zhou a, Yu Yan c, Rong Wang a

a State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China

b Center for Photon Manufacturing Science and Technology, School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China

c Corrosion and Protection Center, Beijing University of Science and Technology, Beijing 100083, China

Precision drilling with ultra-short pulse lasers (e.g., picosecond and femtosecond) has been advocated to significantly improve the quality of the micro-holes with reduced recast layer thickness and no heat-affected zone. However, a combination of high-power picosecond laser with helical drilling strategy in laser drilling has rarely been reported in previous studies. In the present study, a series of micro-holes with circular, triangular, rectangular, and rhombic shapes (diameter 0.6 mm) were manufactured on stainless steel 304 using a newly developed laser drilling system which incorporated a picosecond laser and a high-speed laser beam rotation apparatus into a five-axis positioning platform. The quality of the helical drilled holes, e.g., recast layer, micro-crack, circularity, and conicity, were evaluated using an optical microscope, an optical interferometer, and a scanning electron microscope. In addition, the microstructure of the samples was investigated following etching treatment. It was demonstrated that the entrance ends, the exit ends, and the side walls of the micro-holes were quite smooth without accumulation of spattering material and formation of recast layer and micro-crack.

5. Metallographical analysis of steel and hard metal substrates after deep-Drilling with femtosecond laser pulses

G Dumitru a, V Romano a, H.P Weber a, M Sentis b, J Hermann b, S Bruneau b, W Marine b, H Haefke c, Y Gerbig c

a Institute of Applied Physics, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland.

b LP3 FRE2165 & GPEC UMR 6631, CNRS, Faculte des Science de Luminy, Case 917, 13288 Marseille, France.

c CSEM Centre Suisse d'Electrotechnique et de Microtechnique, Jaquet Droz 1, 2007 Neuchatel, Switzerland.

Investigations on the evolution of the ablated depth over a large number of incident femtosecond laser pulses and the occurrence of structure and hardness changes near the laser-induced craters (for two sorts of steel and a hard metal substrate) are reported in this work. Experiments were performed in air with a Ti: sapphire laser (800 nm, 100 fs) at mean fluences of 2, 5, and 10 J/cm2. After the laser irradiation, cuts were made through the processed samples and pore cross-sections were obtained. Through their metallographic analysis, it was possible to evidence crystalline structure changes near the laser-induced microholes; the extent of modified material zones increased for higher laser fluences. In such zones, a chemical composition quasi-similar to that of the raw material was found and the performed nano hardness tests revealed hardness increases.

4. femtosecond laser dicing of ULTRATHIN si wafers with cu backside layer - a fracture strength and microstructural study

Michael Raj Marks a,b, Kuan Yew Cheong b, Zainuriah Hassan c

a Infineon Technologies (Kulim) Sdn. Bhd., Kulim Hi-Tech Park, 09000 Kulim, Kedah, Malaysia

b Electronic Materials Research Group, School of Materials & Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia

c Institute of Nano Optoelectronics Research and Technology (INOR), Universiti Sains Malaysia, 11800 Penang, Malaysia

This work systematically investigated the effect of femtosecond laser dicing on the fracture strength and sidewall microstructure of 20 μm Si dies with 0− 30 μm Cu backside layer. Using an improved three-point bending (3PB) test method, the intrinsic fracture strengths of the die sidewall were measured. The die types with Cu backside show an average of 46.5 % higher backside characteristic fracture strength, but an average of 6.6 % lower frontside characteristic fracture strength, compared to the die type without Cu backside. Fractographic analysis by scanning electron microscopy was conducted to determine the fracture initiation behaviour in the 3PB test samples. The microstructures, phases, and defects at the sidewall were characterized by transmission electron microscopy, and their effect on fracture strength is discussed. Based on the observed microstructural features, some suggestions concerning the femtosecond laser processing parameters are made to improve the die sidewall frontside and backside fracture strengths, compared to previous results from nanosecond and picosecond laser dicing.

3. A high-repetition-rate femtosecond laser for thin silicon wafer dicing

Krishnan Venkatakrishnan1, Nitin Sudani1 and Bo Tan2

1 Department of Mechanical and Industrial Engineering, Ryerson University, 350 Victoria Street, Toronto, M5B 2K3, Canada

2 Department of Aerospace Engineering, Ryerson University, 350 Victoria Street, Toronto, M5B 2K3, Canada

In this study, a high-power–high-repetition-rate femtosecond laser was investigated for singulation of silicon wafers. The femtosecond laser used for this investigation, unlike the previously used amplified system, is a compact unit that emits infrared ultrashort pulses at high repetition rates in the MHz range and an average output power of 11 W. A systematic study of the influence of the laser parameters on the kerf width, depth, and quality of machining was carried out. Some different experiments were performed using a silicon wafer of diameter 50 mm, P-type boron doped, and back grinded to a 250 µm thickness wafer with an orientation of 100. The experimental results show that the high-power–high-repetition-rate femtosecond laser can be a promising and competitive tool for thin wafer dicing. It is also the first time that the high-repetition-rate femtosecond laser has been demonstrated for real-world industrial applications for micromachining. The experiment demonstrated a cutting speed of 40 mm s−1 with acceptable quality of sidewalls, depth of cut, and kerf width, which can be considered when applying for industrial usage.

2. A HYBRID METHOD OF ULTRAFAST LASER DICING AND HIGH-DENSITY PLASMA ETCHING WITH WATER-SOLUBLE MASK FOR THIN SILICON WAFER CUTTING

Shih-jeh Wu

Department of Mechanical and Automation Engineering, I-Shou University, Kaoshiung City 840, Taiwan, ROC

Future silicon wafers are getting thinner in the semiconductor industry to satisfy the requirement of miniaturizing package size in mobile device applications. This paper proposes a novel method incorporating the convenience of laser ablation and speed of plasma etching to replace traditional mechanical dicing saws for thin wafer cutting. The main idea is to use a laser to create patterns on the silicon wafer coated with water-soluble protection material. The unprotected area is exposed to the high-density plasma which is later etched through. This study includes the development of the water-soluble protecting mask material, the design of inductively coupled plasma (ICP) and microwave plasma sources, pattern scribing utilizing ultrafast laser, and the trial of an etching (Bosch) process associated with the high-density plasma chamber to create high aspect ratio trench. The polyvinyl alcohol (PVA) based protecting material has excellent solubility and is extremely easy to rinse with water. With appropriately added chemicals, optical absorption is improved for laser ablation. A 532 nm 10 pico-second laser is used to perform surface scribing with minimal recast and heat-affected zone (HAZ) at the edges. Good plasma homogeneity is demonstrated in the designed 12″ microwave and ICP chambers. With the protection coating the silicon wafer can be etched at a 7.2 µm/min rate and 10.11 etch selectivity by the microwave and ICP sources in the chamber. Finally, etching process trials are performed, and the results show that deep and high aspect ratio trenches (~15 µm wide, ~95 µm deep) can be achieved which is fully acceptable for wafers under 100 µm thickness.

Investigation of through micropatterns fabrication on C194 copper foil by ultraviolet nanosecond pulsed laser micro drilling

Yujie Han a, Junjie Zhang a, Yuan Liu b, Min Sheng c, Xiaohui Wang d, Tao Sun a

a Center for Precision Engineering, Harbin Institute of Technology, Harbin 150001, Chinab School of Astronautics, Harbin Institute of Technology, Harbin 150001, Chinac Wuhan Maritime Communication Research Institute, WuHan 430205, Chinad School of Mechanical Engineering, University of Jinan, Jinan 250

While the lead frames of C194 copper alloy widely used in IC packages contain a high density of through micropatterns, the method for realizing high precision manufacturing of the micropatterns on copper foil with thickness down to a few hundred micrometers is highly demanded. This paper demonstrates the feasibility of manufacturing high-density through micropatterns on C194 copper foil with high precision and high uniformity using ultraviolet nanosecond pulsed laser micro-drilling. The absorption coefficient of C194 copper foil for 355 nm wavelength is experimentally derived, based on which the sublimation enthalpy of the material is calibrated by iteratively comparing predicted crater morphology by finite element simulations with experimental data. Then the multi-pulse laser ablation mechanisms of C194 copper foil are investigated jointly by finite element simulations and experiments, which derive the quantitative correlation of ablation crater depth with laser ablation parameters. Finally, high-density arrays of through micropatterns with high dimensional accuracy and high uniformity are fabricated on C194 copper foil with a thickness of 100 μm by the proposed ultraviolet nanosecond pulsed laser micro-drilling with theoretically predicted repeat scanning number. This work provides a feasible method for manufacturing copper alloy lead frames with high precision through micropatterns in an environmentally friendly manner.

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