High Aspect Ratio Micro-holes Research Articles

Enhancing Surface Integrity of Micro-Holes Machined on Inconel 718 using Shaped Tube Electrochemical Machining via Hydroxide Mixing

This research explores enhancing surface integrity of high aspect ratio (AR>5) micro-holes machined on Inconel 718 using Shaped Tube Electrochemical Machining (STEM) by mixing limited amount of metal hydroxides in the different electrolyte solutions. Typically, STEM employs hazardous acid solutions. These solutions cause machine corrosion, environmental risks, and harm to operators. This research aims to explore eco-friendly and safe alternatives. The efficacy of adding NaOH or KOH to eco-friendly neutral aqueous solutions of NaCl, KBr, and NaNO3 on machined surfaces is expounded. We assess electrolyte mixing parameters, including concentration and composition, and explore the effects of varying electrolyte temperature, voltage, and tool feed rate. Surface assessment uses SEM, EDX, XRD, and profilometry. Results reveal that adding hydroxides to neutral salt solutions improves surface roughness and reduces pitting. Hydroxide ions aid in dissolving oxide films during STEM of Inconel 718. Under specific conditions (9 g l−1, 18°C, 8 V, 0.8 mm min−1), NO3│OH yielded the lowest Ra (1.64 μm), while Br│OH resulted in the highest Ra (2.14 μm) with pitting, cracks, and deposits. NO3│OH exhibited a 25% decrease in Ra when a hydroxide concentration rise from 3 g l−1 to 15 g l−1. This research supports eco-friendly STEM for improved surface quality without process performance compromise.

Fabrication of high aspect ratio micro-holes on 304 stainless steel via backside-water-assisted laser drilling

Backside-water-assisted laser drilling (BWALD) is a viable and effective alternative of laser direct drilling (LDD) to fabricate micro-holes. However, it still has its own limitations, primarily due to the generated air bubbles and debris during the drilling process and the involvement of various processing parameters, especially when fabricating micro-holes with high aspect ratios on the non-transparent materials. In this study, a two-step processing strategy based on the sequence of punching through and modification was developed to fabricate the micro-holes on 304 stainless steel. Based on this strategy, two modification methods, namely single circle modification (SCM) and concentric circle modification (CCM), were proposed to amplify the advantages of BWALD during the modification process. Experiments testified that the two-step processing strategy was capable of reducing the taper and improving the roundness during BWALD process. Furthermore, a new strategy of using flowing water (backside-flowing-water-assisted laser drilling, BFWALD) was proposed to solve the problem that the generated air bubbles and debris prevent the water from penetrating the through-hole. It was found that the BFWALD could effectively improve the stability during the modification process and achieved the fabrication of micro-holes with an aspect ratio of 10:1 and a taper of only 0.38° successfully.

Investigation of hole roundness-error using different electrolytes in STED process

ABSTRACT Shaped-Tube Electrolytic Drilling (STED) is an electrochemical dissolution-based process able to fabricate high aspect-ratio (AR) holes rapidly on the superalloys. The electrolyte flowing in the side gap between the tool’s lateral surface and hole wall in the presence of an electric field during the entire process duration leads to non-uniform dissolution and stray-cutting. The localization of anodic dissolution mitigates the stray cutting, which contributes to the non-uniformity of micro-holes. The enhanced localized anodic dissolution is attainable by using neutral electrolytes mixed with the alkaline electrolytes that promote passivation on the surface of Inconel 718. The hole roundness error is crucial for the dimensional accuracy of micro-holes due to the overcutting constraints. Therefore, the present research investigates the feasibility of using electrolyte solutions of dissimilar anionic activity mixed with alkaline electrolytes to mitigate hole roundness error in high AR micro-holes. The effects of process parameters, such as applied voltage, tool feed rate, supplementary electrolyte concentration, and electrolyte entry temperature on the hole roundness error were studied to understand the dissolution behavior with different electrolyte solutions. The investigation results revealed that adding alkaline electrolytes in NaCl and NaNO3 based electrolyte solutions mitigate hole roundness error, whereas the KBr-based electrolyte solution performs unsatisfactorily.

Optimisation and Experimental Investigation of Material Removal Rate and Tool Wear Rate of Micro Electro Discharge Machining (MEDM) of Hastelloy C276

ABSTRACT With rapid development in micromachining, high aspect ratio (AR > 10) micro-holes are essentially required in applications such as spinneret holes, biomedical needles, fuel injectors, cooling channels, etc. Machining such high aspect ratio micro holes on superalloys are challenging for micromachining processes. A high aspect ratio micro hole can be machined via. Micro Electrical Discharge Machining (MEDM) process with optimum levels of operating parameters. Work reported in this paper is aimed to investigate the effect of machining parameters of MEDM to produc micro-holes in Hastelloy C276. Extensive experimentations based on Taguchi Designs of Experiments (DOE) were conducted to identify the most influential processing parameters while machining micro holes of more than 10 aspect ratios. The experimental results are discussed which quantify an individual and combined effect of different operating parameters on the overall performance of MEDM like material removal rate (MRR) and tool wear rate (TWR). It is found that capacitance and spindle rotation speed are the most important parameters controlling the stability of the process. A direct relation of TWR has been found with discharge energy (capacitance and voltage). At optimum process parameter conditions, the aspect ratio (depth/diameter) of 12.5 is achieved while machining holes with 400 µm diameter.

Electrochemical Micromachining of Copper Alloy through Hot Air Assisted Electrolyte Approach

The industries like automobile, electronics, medical and biomedical sectors provide scope for various types of micro products/features made up of copper material. Due to the higher ductility of copper the electrochemical micro machining (EMM) is a prominent technique owing it’s no tool wear, high accuracy, burr free and non-contact machining surface. Although enhancing of EMM performance is significant work in the micro components manufacturing industry. Hence, in this present work the hot air is mixed with aqueous citric acid (C6H8O7) electrolyte (HAME) to machine high aspect ratio micro-holes (HAR micro-hole) and the results are compared with the dry air mixed electrolyte (DAME). The process parameters such as machining voltage (Mv), Duty cycle (Dc), and electrolyte concentration (Ec) are taken into account in evaluating the machining performance. The electrolyte temperature (Et) and frequency are kept at a constant level as 36 ± 1°C and 85 Hz respectively. The machining performances are estimated by means of material removal rate (MRR), overcut (OC) and taper angle of the HAR micro hole (TAHAR-micro-hole).The MRR shows 1.43 times improvement for HAME at the parametric combination of 20 g/L, 13 V, and Dc at 90% and reduced OC by 30.4% over DAME. The TAHAR-micro-holes are found to be 21.67° and 28.19° for the DAME and HAME respectively for the above parametric combinations. At parameters combination of 30 g/L, 13 V, and 90%, the MRR shows 2.87 times higher MRR compared to DAME. Furthermore, the use of citric acid as an electrolyte and mix of hot air contributes for higher MRR and reduced OC.The field emission scanning electron microscope (FESEM) analysis is done to understand the effect of hot air mixed electrolyte on the surface topography and inner machined surfaces of a micro-hole.

Non-diffraction-length Bessel-beam femtosecond laser drilling of high-aspect-ratio microholes in PMMA

Bessel beam femtosecond lasers being non-diffractive are well-suited for drilling high aspect ratio microholes with minor thermal impact. Hereby, the Bessel beams with different non-diffraction lengths using axicon lenses were generated with different taper angles for analyzing the drilled microholes to optimize the aspect ratio. The experimental results show that the depth of microhole is increasing with the pulse number accumulating for low energy laser pulses. With the increasing of energy laser pulse, the increment of microhole diameter is faster than that of the depth. The large aspect ratio microholes with the maximum value of 348:1 have been formed by Bessel femtosecond laser micromachining.

High Aspect Ratio Machining of Carbon Fiber Reinforced Plastics by Electrical Discharge Machining Process

Abstract Micromachining of carbon fiber reinforced plastics (CFRPs) is essential for numerous applications in several industries such as aerospace, automotive, defense, shipping, sporting goods, and biomedical industries. The major challenge in machining CFRP by electrical discharge machining (EDM) is due to the nonconductivity of epoxy material which is used as a binder for manufacturing these CRFPs. This study attempts a novel, yet simple, approach to ensure the conductivity of the work piece through the entire machining process. Experiments were carried out as a part of this work to assess the feasibility of machining high aspect ratio microholes in CFRP by micro-EDM. The effect of process parameters such as voltage and feed-rate on the hole quality was studied. Using optimal process conditions, microhole of 2500 μm deep with an aspect ratio of over 11 was achieved.

Desktop Micro-EDM System for High-Aspect Ratio Micro-Hole Drilling in Tungsten Cemented Carbide by Cut-Side Micro-Tool

Tungsten cemented carbide (WC-Co) is a widely applied material in micro-hole drilling, such as in suction nozzles, injection nozzles, and wire drawing dies, owing to its high wear resistance and hardness. Since the development of wire-electro-discharge grinding (WEDG) technology, the micro-electrical discharge machining (micro-EDM) has been excellent in the process of fabricating micro-holes in WC-Co material. Even though high-quality micro-holes can be drilled by micro-EDM, it is still limited in large-scale production, due to the electrode tool wear caused during the process. In addition, the high cost of precision micro-EDM is also a limitation for WC-Co micro-hole drilling. This study aimed to develop a low-cost desktop micro-EDM system for fabricating micro-holes in tungsten cemented carbide materials. Taking advantage of commercial micro tools in a desktop micro-EDM system, it is possible to reach half the amount of large-scale production of micro-holes. Meanwhile, it is difficult to drill the deep and high aspect ratio micro-holes using conventional micro-EDM, therefore, a cut-side micro-tool shaped for micro-EDM system drilling was exploited in this study. The results show that micro-holes with a diameter of 0.07 mm and thickness of 1.0 mm could be drilled completely by cut-side micro-tools. The roundness of the holes were approximately 0.001 mm and the aspect ratio was close to 15.

EDM of high aspect ratio micro-holes on Ti-6Al-4V alloy by synchronizing energy interactions

ABSTRACT Electrical discharge machining (EDM) of high aspect ratio (AR) micro-holes on materials like Ti-6Al-4 V alloy is challenging. The ineffective removal of the machining by-products from and replenishment of fresh dielectric medium in the inter-electrode gap (IEG) limit the penetration depth. The approaches suggested in existing literature either complicate the EDM process or increase the operational costs. Moreover, the effectiveness of energy interactions in the IEG affecting the process performance has not been adequately investigated. This research attempts to synchronize effects of energy interactions between the tool electrode, dielectric, and work material in the IEG by controlling the input discharge energy through current and pulse-on-duration, and dissipated energy through pulse-off-duration and lift settings. Experimental indicators like tool penetration rate, machining time, debris morphology, gas-bubble dynamics, and discharge waveforms are employed to expound the non-linear dynamic behavior of the process. Accordingly, the machining of high AR micro-holes is defined into four zones: usual micro-EDM, transition, high-AR EDM, and critical zones. The maximum depth (10 mm) and AR (17.4) ever reported during the micro-EDM of Ti-6Al-4 V alloy was achieved without employing any additional assistance. Thereby, the capabilities of the conventional die-sinking EDM process are improved for machining high AR micro-holes on Ti-6Al-4 V alloy.

Processing capabilities of micro ultrasonic machining for hard and brittle materials: SPH analysis and experimental verification

Micro ultrasonic machining (micro-USM) is an unconventional micromachining technology that has capability to fabricate high aspect ratio micro-holes, intricate shapes and features on various hard and brittle materials. The material removal in USM is based on brittle fracture of work materials. The mechanical properties and fracture behaviour are different for varied hard and brittle materials, which would make a big difference in the processing capability of micro-USM. To study the processing capability of USM and exploit its potential, the material removal of work materials, wear of abrasive particles and wear of machining tools in USM of three typical hard and brittle materials including float glass, alumina, and silicon carbide were investigated in this work. Both smoothed particle hydrodynamics (SPH) simulations and verification experiments were conducted. The material removal rate is found to decrease in the order of glass, alumina, and silicon carbide, which can be well explained by the simulation results that cracking of glass is faster and larger compared to the other materials. Correspondingly, the tool wear rate also dropped significantly thanks to the faster material removal, and a formation of concavity on the tool tip center due to intensive wear was prevented. The SPH model is proved useful for studying USM of different hard and brittle materials, and capable of predicting the machining performance.

Micro hole fabrication in TiN-Al2O3 ceramic composite by SiC powder assisted micro- EDM

Considering the importance of ceramics in many microdevice applications, a modified micro-electrodischarge process in which the addition of silicon carbide powders in the dielectric was used to fabricate high aspect ratio microholes in dense and hard TiN-Al2O3 ceramic composite has been described in the present paper. TiN-Al2O3 ceramics has been prepared by reactive hot pressing technique. Response Surface Methodology (RSM) has been employed to design and optimize the experimental input parameters like voltage, capacitance, and tool rotation speed and for evaluating the responses in terms of Material Removal Rate (MRR), Electrode Wear Rate (EWR), Radial Overcut (ROC) and Taper Angle (TA). SiC powder addition has shown the improvement in MRR by 6.5 times, reduction of TA by 2 times and reduction in ROC by 1.5 times. However, EWR has increased by 1.8 times. Improvement in the machining performance has been analysed with respect to the early breakdown of the dielectric fluid by the addition of SiC powder and the enlargement of the discharge gap. Topographies of the machined surfaces analysed from the Scanning Electron Micrographs (SEM) show smoothening of the surface by SiC powder addition and reduction of tool rotation speed. Chemical compositions of the machined surfaces determined by energy dispersive spectral analysis (EDS) showed the formation of amorphous Ti-Al-O-N phase by the discharge. Thermal micro-cracking of the amorphous solidified melt, spalling, micro-bubble formation and accumulation of spherical electrode debris have been observed during the machining of TiN-Al2O3 ceramic-composite.

High-aspect ratio mechanical microdrilling process for a microhole array of nitride ceramics

In semiconductor industry, wafer testing requires a microhole array constructed by high-density holes below ϕ 100 μm for stabilizing the microprobe used in conduction testing. This study conducted mechanical microdrilling experiments of a nitride ceramic sheet to investigate the effects of drilling parameters on quality characteristics of the microhole drilled. Fabrications of a high-aspect ratio microhole must be implemented by segmented machining. A center pilot hole was first made and then drilled the microhole by a two-segment process. Drilling factors analyzed in this study included depth of the center pilot hole, depth ratio of the hole segments, and machining parameters of cutting speed (spindle rotation rate), tool feed rate, and step in peck drilling in the microdrilling. Because both diameter and position of the microhole affect the functionality of hole array directly, this is a multi-objective problem. This study proposes a new decision method for deriving the optimal parameter level set in the multi-object problem based on the level effects obtained from Taguchi’s analyses. In addition, this study used a scanning electron microscopy (SEM) to observe the influences of drilling stroke on flank wear of the microdrill. This study implemented two series experiments of ϕ 69 μm and ϕ 55 μm with aspect ratios of 15.36 and 11.64, respectively. Experimental results indicate that applying the optimal parameter level set derived in ϕ 55 μm experiment could obtain excellent performances after the drilling stroke of 384 mm, 600 holes drilled. The microdrills have fulfilled a high use efficiency.

Characteristics of micro-hole formation during fibre laser drilling of aerospace superalloy

High aspect ratio micro-holes are nowadays increasingly used in various applications including aerospace, medical and automotive. This research investigates the characteristic of quasi-CW fibre laser micro-hole drilling (∼100 μm) of nickel superalloy using experimentation and numerical modelling. Micro-holes of diameter around 100 μm was drilled using percussion technique over a 2 mm thick nickel superalloy material. The main investigation focuses on understanding the micro-hole formation during the laser interaction with the material. Finite element modelling tool and high-speed imaging camera had been used to assist in understanding the basic-fundamentals of micro-hole formation during the millisecond laser drilling process. During the micro-hole drilling process, irrespective of the laser parameters, the melt ejections start after around 80 μs, and the total melt ejection duration depends on laser energy and pulse duration. The experimental results showed that it takes 15 ms to drill a micro-hole of diameter around 100 μm using a laser pulse duration of 0.1 ms, laser pulse energy of 1 J and 15 number of percussion pulses.

Fabrication of Micro-Parts with High-Aspect Ratio Micro-Hole Array by Micro-Powder Injection Molding.

The present study investigated high-aspect ratio micro-hole array parts which were made by ZrO2 micro-powder with different particle sizes and micro-powder injection molding technology. It analysed the influence of particle sizes on feedstock, debinding and sintering of ceramic nozzles with multi-micro-holes. The forming quality of ceramic nozzles with multi-micro-holes was discussed in this paper. The results show that the two mixed ZrO2 feedstocks have fine uniformity. The average deviation of the feedstock made with 200 nm powder was −2%, and the average deviation of the feedstock made with 100 nm powder was −7.1%. The sample showed certain sintering characteristics which provided better strength (11.10 MPa) to parts after debinding. The linear shrinkage and the density of the two powder samples at different sintering temperatures increased as the sintering temperature increased. If the temperature continued to increase, the linear shrinkage and the density decreased. The highest hardness and flexural strength values of the ZrO2 sample with 200 nm powder used were: 1265.5 HV and 453.4 MPa, and the crystalline particle size was 0.36 μm. The highest hardness and flexural strength values of the ZrO2 sample with 100 nm powder used were: 1425.8 HV and 503.6 MPa, and the crystalline particle size was 0.18 μm. The ceramic nozzles with multi-micro holes shrunk to nearly the same axial, radial and circumferential directions during sintering. After sintering, the roundness of ceramic micro-hole met the user requirements, and the circular hole had a high parallelism in the axial direction. The micropore diameter was 450 ± 5 μm, and it was possible to control the dimensional accuracy within 1.5% after sintering. The study presented a superior application prospect for high-aspect ratio micro hole array parts in aerospace, electronics and biomedicine.

Non-diffraction-length, tunable, Bessel-like beams generation by spatially shaping a femtosecond laser beam for high-aspect-ratio micro-hole drilling.

Bessel beams are advantageous in high aspect-ratio microhole drilling because of their immunity to diffraction. However, conventional methods of generating Bessel beams result in poor adjustability of the nondiffraction length. In this study, we theoretically describe and experimentally demonstrate the generation of Bessel-like beams (BLBs) with an adjustable nondiffraction length by using a phase-only spatial light modulator. In this method, nondiffraction lengths varying from 10 to 35 mm can be achieved by changing the designed phase profile (curvature). High-quality, high aspect ratio (560:1) and length-adjustable microholes can be drilled by spatially shaping a femtosecond laser beam.

A review on process capabilities of electrochemical micromachining and its hybrid variants

Abstract Electrochemical micromachining (micro-ECM) is an unconventional micromachining technology that has capability to fabricate high aspect ratio micro-holes, micro-cavities, micro-channels and grooves on conductive and difficult-to-cut materials. Both academia and industry have the consensus that it offers promising machining performance especially in terms of high surface finish, no tool wear and absence of thermally induced defects. Furthermore in order to machine novel materials with extreme properties, novel hybrid electrochemical micromachining technologies are under development. With these hybrid micro-ECM technologies, capabilities of micro-ECM can be expanded by combining it with other processes. To fully exploit the potential as well as improve micro-ECM technology and related hybrid processes, a wide spectrum of multidisciplinary knowledge is needed. The present review systematically discusses process capabilities and research developments of electrochemical micromachining and its hybrid variants considering knowledge of both basic and applied research fields. After few introductory review articles in prior state of the art, this review fills an important gap in research literature by presenting first time an extended literature source with a wide coverage of recent research developments in electrochemical micromachining technology and its hybrid variants. This paper outlines the research and engineering developments in electrochemical micromachining technology and its hybrid variants, review of the related concepts, aspects of tooling, advanced process capabilities and process energy sources. It also provides new sights into technological understanding of micro-ECM technology which will be helpful in future engineering developments of this technology.

Comparative studies in electro-physical processes (ECM & EDM) for circular micro-holes drilling

Day by day interest is growing in drilling of high quality and high aspect-ratio micro-holes in various difficult-to-cut materials for many applications. Electrochemical micro-drilling (ECMD) and electrodischarge micro-drilling (EDMD) are the two major electro-physical process (ECM and EDM) based cost-effective techniques, provide a better alternatives in drilling micro-holes with reasonably accurate dimensions and good surface finish in difficult-to-cut materials for various industrial applications especially in computer, electronic, and aerospace industries. This paper presents a review of comparative studies in electro-physical processes on the developments of circular micro-holes drilling. It highlights the studies on aspect-ratio, over-cut and diameter-error (difference between inlet diameter and exit diameter) of circular micro-holes obtained by various experiments of ECMD and EDMD. For further research, it will open up various challenging opportunities in the field of (i) development of micro-tools and micro-electrodes, and (ii) development in innovative techniques for the purpose of drilling high quality and high aspect ratio micro-holes into hard-to-machine materials using ECMD and EDMD..

A study on effects of discharge energy on geometric characteristics of high aspect ratio micro-holes on TiN-Al2O3 ceramics

In recent years, the need for ceramics products containing micro-features has shown a pronounced and steady growth in several fields of industry application. For the development of micro-hole devices, one of the most important technologies is Micro Electro Discharge Machining (µ-EDM). Micro-EDM can be considered as an ideal process to obtain burr-free micron-size features with high aspect ratios. This work presents an innovative method for the µ-EDM of high aspect ratio (AR =25) micro holes. The investigation focuses on the influence of input parameters and discharge energy on material removal rate (MRR), electrode wear (EW) rate and more over Moreover, an analysis of the geometrical characteristics of the micro holes in terms of Taperness and diametrical overcut was carried out.

Two-Side Laser Processing Method for Producing High Aspect Ratio Microholes

Laser microprocessing is a very attractive option for a growing number of industrial applications due to its intrinsic characteristics, such as high flexibility and process control and also capabilities for noncontact processing of a wide range of materials. However, there are some constrains that limit the applications of this technology, i.e., taper angles on sidewalls, edge quality, geometrical accuracy, and achievable aspect ratios of produced structures. To address these process limitations, a new method for two-side laser processing is proposed in this research. The method is described with a special focus on key enabling technologies for achieving high accuracy and repeatability in two-side laser drilling. The pilot implementation of the proposed processing configuration and technologies is discussed together with an in situ, on-machine inspection procedure to verify the achievable positional and geometrical accuracy. It is demonstrated that alignment accuracy better than 10 μm is achievable using this pilot two-side laser processing platform. In addition, the morphology of holes with circular and square cross sections produced with one-side laser drilling and the proposed method was compared in regard to achievable aspect ratios and holes' dimensional and geometrical accuracy and thus to make conclusions about its capabilities.

Micro-EDM drilling of high aspect ratio micro-holes and in situ surface improvement in C17200 beryllium copper alloy

Beryllium copper alloys are the ideal materials to fabricate the supporting parts owing to their excellent physical and chemical properties. Many high aspect ratio micro-holes are often required to realize the special missions. However, they were not easily fabricated by the traditional machining methods owing to the materials' mechanical strength. Here, we presented a high efficiency method for drilling high aspect ratio micro-holes on C17200 beryllium copper alloy by combining deionized water based micro-EDM and in situ PMEDM with boron carbide additives. The machining parameters were systematically investigated and optimized to realize the ever high aspect ratio of 23.26 at the holes' diameter of 215 μm. The surface quality of the micro-hole was improved further by in-situ PMEDM, which was demonstrated by the decreasing of surface roughness (Ra) from 2.65 μm to 0.92 μm. Our method will open an avenue to fabricate high aspect ratio micro-holes on C17200 with high efficiency.