Experimental and finite element analysis of ultrasonic vibration−assisted continuous-wave laser surface drilling

In pulsed laser drilling with co-axial assisted gas, material removal mechanism (surface vaporization and melt expulsion) determines the machining/drilling rate and quality of the drilled holes. Incomplete melt expulsion is one of the major causes of laser drilling defects. To improve the drilling efficiency and quality of holes, a novel ultrasonic vibration?assisted continuous-wave laser drilling (UVLD) approach is proposed. The application of ultrasonic vibrations (of frequency of 20?kHz and vibration displacement of 23?µm) during laser surface melting facilitates the melt expulsion in the form of sideways melt flow and droplet ejection from the drilling front. A systematic experimental study on the ultrasonic vibration-assisted laser drilling of AISI 316 stainless steel is performed to investigate the effect of working distance on the geometric features and surface quality of the holes. The experimental results based on high-speed photography indicate that the melt expulsion under the influence of ultrasonic vibrations initiates after the laser melted pool reaches a critical size/volume. Based on this underlying mechanism, a simplified finite element analysis is performed for the UVLD process to predict the hole volumes for the investigated working distances.     

On machining of hard and brittle materials using rotary tool micro-ultrasonic drilling process

The present paper reports on a recently developed rotary tool micro-ultrasonic drilling (RT-MUSD) process. The RT-MUSD process was utilized for machining of micro-holes in zirconia, silicon and glasswork materials. The effects of work material properties on the performance characteristics (material removal rate (MRR), depth of hole and hole overcut) of RT-MUSD process were investigated by varying the power rating, rotation speed, abrasive size and slurry concentration. Additionally, machined micro-holes and tool surface were analyzed considering microscopic images. The experimental results revealed that the MRR and depth of hole increased by increasing the power rating. An increase in rotation speed up to 300 rpm, abrasive size up to #1200 mesh and concentration up to 20% increased the MRR, depth of hole and decreased hole overcut. The maximum machining rate and hole overcut were observed during machining of silicon followed by glass and zirconia. The fracture toughness and hardness of the work material affected the MRR and tool wear, respectively. Pure brittle fracture mode of material removal was observed in all the work materials during RT-MUSD process. Eventually, the RT-MUSD process was optimized using desirability approach and a micro-hole of depth 4355 µm was achieved using optimal parameter settings.     

LASER SHOCK SURFACE TREATMENT OF Ni-BASED SUPERALLOYS

Laser shock surface treatment could be envisaged to harden and induce compressive stresses within materials thus improving their fatigue and fretting-fatigue resistance. Both laser beam optical phenomena and deformation mechanisms are studied for applications of laser shock treatment to Ni-based mono- and polycrystalline superalloys. Deformation microstructure is studied using TEM applied to the monocrystalline superalloys. Macroscopic mechanical phenomena are determined by superficial or in-depth residual stress measurements using X-ray diffraction technique or the incremental hole drilling method. A fairly elementary analytical model involving surface release wave mechanisms is developed to describe the formation of residual stresses. Understanding of these phenomena leads to improving the laser treatment for application to processing of fatigue test specimens by overlapping of impacts.     

Enhancement of ceramic-to-metal adhesive bonding by excimer laser surface treatment

A silicon nitride (Si₃N₄) based ceramic cutting material and a structural alloy steel (SAE 4340) were surface-treated using a 800 mJ KrF excimer laser with an aim to improve the ceramic-to-metal bond strength. For these two materials, the effects of laser energy density and the number of pulses upon the surface morphologies of the laser-treated surfaces to be joined were examined. Conical surface microstructures were generally observed on the laser-treated surfaces of the Si₃N₄ material, and the number of conical features was found to be significantly influenced by the laser energy density. Moreover, the results of XPS have shown that the surface chemistry of the ceramic was altered after being laser treated. On the other hand, excimer laser treatment had caused the alloy steel surfaces be melted and resulted in some “beach-mark” features. However, the laser energy density has little effect on the overall surface morphology and the roughness of the treated surfaces. Shear tests were performed on adhesive bonded samples of the laser-treated ceramic and alloy steel. Significant improvement in adhesion shear strength was obtained for the laser-treated samples as compared with those tested in the as-received and mechanically abraded conditions. The required laser operation condition for achieving good shear joint strength was discussed.     

Investigation of laser percussion hole drilling by use of speckle correlation

In this paper speckle correlation is introduced as a tool to investigate the heat-influenced area during material processing with laser light. Two materials were investigated, a pure silver sheet and a sheet of SiC-diamond composite. The processing laser used in the experiments was a diode-pumped acousto-optical Q-switched Nd:YAG laser that allowed percussion hole drilling to be performed using green light through a second-harmonic crystal. The measurements were performed using a continuous-wave He-Ne laser and a digital camera. The experimental results show that the heat-influenced area is approximately 5000 times larger than the actual hole being drilled and that it reaches a steady-state condition toward the end of the processing cycle.     

Evaluating Process Capability for Geometrically Toleranced Parts: A Practical Approach

The practice of geometric tolerancing has gained in industry popularity since the 1990s. This approach has advantages over conventional tolerancing in defining both geometry and associated tolerance and, thus, generating a more realistic “acceptable design space.” There have been a number of highly mathematical treatments of the subject over the years which have not found their way into popular usage. We look at a specific example of geometric tolerancing and derive a simple approach based on the geometry of the situation and standard C_p and C_pk calculations. The study describes an approach based on understanding the limiting conditions of acceptable operation. In the example of a pin and clearance hole, we derive the limiting condition as a zero radial gap for a hole and a perfectly centered pin at maximum metal condition. We then performed a standard C_pk calculation with the limiting condition acting as the effective tolerance limit. Because we are dealing with radial gaps, we have an effective one-sided tolerance with a minimum acceptable value of 0 for the radial gaps. We tested the approach using Normally distributed simulated data and found that it provides an accurate evaluation of process capability and projected scrap levels. With minor cautions, we conclude that this methodological approach could be extrapolated to other geometrically toleranced situations.     

Effect of abrasive jet process parameters on machining glass fibre reinforced polymer composite

An experimental and theoretical research work on abrasive jet machining of glass fiber reinforced polymer composite materials was conducted using abrasive jet machining setup fabricated in our workshop. The objective of this research work is to machine holes on the glass fiber reinforced polymer composite using an abrasive jet machine under various levels of process parameter. The material removal rate and hole geometry (kerf analysis) were observed as a part of the investigation. Four factors five levels central composite rotatable design matrix was used for optimizing the required number of experiments. The objective of the present investigation is to develop mathematical models using the response surface methodology. The adequacy of the models has been checked using the ANOVA technique. Use of the developed mathematical models, material removal rate and hole geometry of the machined glass fibre reinforced polymer composite helps prediction at 95% confidence level.     

Study on small diameter drilling in GFRP

In small diameter drilling with the drilling machine, the drilled hole quality of the printed wiring board (PWB), made of GFRP, is evaluated by investigating the surface roughness and the internal damage to the hole. An investigation of the damage is conducted by changing the number of drilled holes. Additionally, in laser drilling using a CO₂ source and a YAG source, the laser drilling process is clarified by observing the damage to the hole, and moreover the relation between the drilling conditions and the damage of the hole is obtained. In conclusion, it is shown that, in the case of the present drilling machine, the anisotropy of the hole surface roughness is able to be developed by the tool tip profile, and as the new process is of no contact drilling, the application of a laser beam machine using a CO₂ laser source is effective in the smaller diameter drilling in PWB's.     

在强热冲击下铝合金材料内部的微观损伤特征

分析了在强热冲击下2A12铝合金材料内部的微观损伤特征.采用激光在圆片试样中心打孔方式,来实施对铝合金强热冲击实验.应用扫描电子显微镜,观察了激光穿孔附近纵、横截面上的金相组织,并得到了热影响区内材料损伤的显微照片.这些电镜图片显示出穿孔周围有微孔洞群、沿晶开裂的微裂纹、三叉晶界处萌生的微孔、破碎状裂隙带和白色亮条等微...     

Extension to the blind hole drilling technique for residual stress determination with airabrasive hole forming

For an austenitic clad ferritic steel plate it was necessary to determine the residual stress state between the austenitic surface and a region below the austenitic/ferritic interface.
The thickness of claddings was 6 mm and to determine the stress variation between the surface and 8 mm an incremental analysis is required. Any material removal en masse in conjunction with small hole residual stress measurements would have resulted in a stress redistribution and hence a distortion in the stress field being measured. In conjunction with strain change measurements on the other face the sensitivity would not have been sufficient. These two options assume that a thin layer of material over relatively large areas can be removed by a non-stress inducing technique.
A blind hole residual stress measurement using a 13 mm diameter hole 13 mm deep will facilitate accurate incremental analysis to a depth of 8 mm.
The end mill method of hole forming cannot be used with stainless steels because of the random damage effect which is induced in the readings. The airabrasive method which induces no damage effect was used.
The standard airabrasive method of hole forming produces a 2 mm diameter and a 2 mm deep hole.
The modification of the standard airabrasive system is discussed which enables 13 mm diameter and 13 mm deep holes to be formed.
A deviation from the standard hole/gauge layout geometry has been investigated due to non-availability of suitable standard rosettes, and a significant increase in sensitivity has been obtained.
The incremental stress results for the austenitic layer and the austenitic/ferritic interface are discussed using the results from 1.6 mm, 3.2 mm and 13 mm diameter/deep holes.     

The Effect of Laser Trimming on Properties of Ti-6A1-4V Sheet

The effect of laser trimming on the tensile strength and fatigue resistance of Ti, 6 wt% Al, and 4 wt% V specimens was investigated. Due to the nature of laser processing, the microstructure of the titanium alloy was altered in areas local to the cut known as the kerf and the heat affected zone (HAZ), respectively. Experimental cutting was conducted in order to optimize laser process parameters to minimize dross adhesion and the width of the HAZ. Test specimens of two thicknesses, n“ and 0.125” were laser cut at three industrial laser sheet metal cutting laboratories at parameter values determined to be the optimum by each respective laboratory, with the criteria of no dross adhesion. Extreme care was taken to meaningfully compare the results of tensile strength and fatigue life data to those of mechanically machined and polished specimens, as a material reference using statistical techniques.     

Characterization of Properties of Cryogenically Treated Al-SiC Composites Fabricated by Powder Metallurgy

The basis of this research was an exploration of the fundamental phenomena that determine the response of silicon carbide-reinforced aluminium composite material to thermal cycling between cryogenic and ambient temperatures. This analysis began with a phenomenological approach that investigated the role of the production, processing, and machining of composite materials, and led to study of their mechanical behavior at cryogenic temperatures. Electric discharge machining was done on the composite specimens and mathematical models were developed for predicting the machining parameters such as metal removal rate, tool wear rate, and surface roughness. A five-level factorial design was chosen for experimentation and mathematical models were developed using the software DOE-PC IV. An analysis of variance technique was used to calculate the regression coefficients and to check the significance of the models developed. This approach provided an understanding of how temperature and vol.% of SiC influence composite machining behavior. The hardness, wear resistance, and tensile property are high for cryo-treated specimens and these properties reduce with increase in temperature. The properties also increase with increasing % of SiC reinforcements. The microstructures of the wear specimens show worn-out layers and grooves formed in the debris. The cryo-treated and the higher reinforced specimens exhibit less material removal and tool wear rate and this increases with increase in temperature. There is a relatively higher surface roughness when there is greater material removal.     

Effect of spatial distribution and aging of ZVI on the reactivity of resin–ZVI composites for arsenite removal

A zero-valent iron (ZVI)–resin composite (D201–ZVI) has been proven as an effective arsenic removal material. Here, the effect of ZVI distribution and aging on the reactivity of the hybrid composite was investigated by comparing the As(III) removal performances of freshly synthesized and aged D201–ZVI composites. The ZVI distribution and structures of these composites were characterized using X-ray diffraction, X-ray photoelectron spectrometry, and scanning electron microscope equipped with an energy-dispersive X-ray analyzer (SEM-EDX). After aging in aerated water for 96 h, the ZVI distribution in the aged composites did not change significantly, which was confirmed by SEM-EDX. However, the Fe⁰ content decreased as the aging time increased. Among the as-prepared composites with variant ZVI distributions, the hybrids with more uniform ZVI distribution exhibited higher removal efficiency and faster reaction rate. Experimental results show that the D201–ZVI gradually lost reactivity with an increase in aging time from 24 to 96 h. The effects of aging time on the speciation of As suggest that the reduced As(III) removal efficiency was attributable to the decrease of the Fe⁰ content. Furthermore, the importance of the ZVI distribution is proposed to explain the aging effect.     

Perovskite solar cells: recent progress and strategies developed for minimizing interfacial recombination

Organometallic perovskite is a new generation photovoltaic material with exemplary properties such as high absorption co-efficient, optimal bandgap, high defect tolerance factor and long carrier diffusion length. However, suitable electrodes and charge transport materials are required to fulfill photovoltaic processes where interfaces between hole transport material/perovskite and perovskite/electron transport material are affected by phenomena of charge carrier separation, transportation, collection by the interfaces and band alignment. Based on recent available literature and several strategies for minimizing the recombination of charge carriers at the interfaces, this review addresses the properties of hole transport materials, relevant working mechanisms, and the interface engineering of perovskite solar cell (PSC) device architecture, which also provides significant insights to design and development of PSC devices with high efficiency.     

Electric discharge hole grinding in hybrid metal matrix composite

Moving toward a hybrid approach, a hybrid process, electric discharge hole grinding (EDHG) was used to machine a hybrid metal matrix composite (MMC) (Al6063/SiC/Al₂O₃/Gr). Here, holes were drilled and ground in a single step process (EDHG) using a novel tool electrode. The experiments were designed using response surface methodology (RSM). The objective of this study was to investigate the effect of electric discharge diamond hole grinding operation on the surface roughness (SR) of the hole. The input process parameters were current, duty factor, tool speed and flushing pressure. It was found that the process is very effective in producing a finished hole. A comparison of surface roughness was made between electric discharge drilling (EDD) and electric discharge diamond hole grinding, thereby showing the effectiveness of the electric discharge diamond hole grinding process. The grinding action of the process is clearly visible in the scanning electron microscopic (SEM) image. It was observed that the craters, globules of resolidified material and micro cracks, which are normally seen on surfaces machined by electric discharge machining (EDM), are completely ground off by electric discharge diamond hole grinding.     

Modeling laser drilling in percussion regime using constraint natural element method

The laser drilling process is the main process used in machining procedures on aeronautic engines, especially in the cooling parts. The industrial problematic is to reduce geometrical deviations of the holes and defects during manufacturing. The interaction between a laser beam and an absorbent metallic matter in the laser drilling regime involves thermal and hydrodynamical phenomenon. Their role on the drilling is not yet completely understood and a realistic simulation of the process could contribute to a better understanding of these phenomenon. The simulation of such process induces strong numerical difficulties. This work presents a physical model combined with the use of the original Constraint Natural Element Method to simulate the laser drilling. The physical model includes solid/liquid and liquid/vapor phase transformations, the liquid ejection and the convective and conductive thermal exchanges. It is the first time that all these phenomena are included in a modelling and numerically solved in a 2D axisymmmetric problem. Simulations results predict most of measurements (hole geometry, velocity of the liquid ejection and laser drilling velocity) without adjusting any parameters.     

An equivalent domain integral method for computing crack-tip integral parameters in non-elastic, thermo-mechanical fracture

The crack-tip parameters, such asJ; T^*, ΔT^* etc, which quantify the severity of the stress/strain fields near the crack-tip in elastic-plastic materials subject to thermo-mechanical loading, are often expressed as integrals over a path that is infinitesimally close to the crack-tip (front). The integrand in such integrals involves the stress-working density, stress, strain and displacement fields arbitrarily close to the crack-tip. In a numerical analysis, such data near the crack-tip are not expected to be very accurate. This paper describes simple approaches and attendant computational algorithms, wherein, the “crack-tip integral” parameters may be evaluated through “equivalent domain integrals” (EDI) alone. It is also seen that the present (EDI) approaches form the generic basis for the popular “virtual crack extension” (VCE) methods. Several examples of thermo-mechanical fracture, including: (i) thermal loading of an elastic material, (ii) arbitrary loading/unloading/reloading of an elastic-plastic material, containing a single dominant crack, are presented to illustrate the present approach and its accuracy.     

A theory for granular materials exhibiting normal stress effects based on Enskog's dense gas theory

Granular materials exhibit phenomena such as normal stress differences, which are typical of materials whose response is non-linear. For example, when a non-linearly elastic slab is sheared, its motion is not determined by the shear force but by the normal forces that manifest themselves due to the shearing (Poynting effect). Another example is a non-linear fluid which exhibits normal stress differences that lead to phenomena like “die-swell” or “rod-climbing,” which is again a manifestation of the stresses that develop orthogonal to planes of shear.

In this paper, an expression for the stress tensor of a granular material that can exhibit normal-stress effects due to a solids fraction gradient is derived from both continuum and kinetic models. The continuum model motivates and develops the form of the stress tensor, but introduces undetermined coefficients. The kinetic model evaluates those coefficients using Enskog's dense gas theory. The dependence of the granular stress tensor on the solids fraction gradient arises by requiring that the correlating factor that links the two-particle distribution function to the two single-particle distribution functions be the contact value for the radial distribution function of a non-homogeneous, hard-sphere fluid. A representation for that contact value is found by developing the generalized van der Waals theory expression for a stress tensor element of a nonhomogeneous fluid (a fluid that exhibits a density gradient) in equilibrium, and comparing it to the exact expression. That representation of the contact value is introduced into the two-particle distribution function, and its contribution to the stress tensor is found. The resulting stress tensor expression is applied to a simple shear flow problem in which a linear, solids-fraction profile is transverse to the flow. The resulting normal-stress effects increase with the solids-fraction and its gradient.     

Electrical Discharge Machining of Functionally Graded 15-35 Vol% SiCp/Al Composites

A functionally Graded 15-35 volume% silicon carbide particulate (SiC_p) reinforced Al359 metal matrix composite (SiC_p/Al MMC) was drilled by electrical discharge machining (EDM) to assess the machinability and workpiece quality. The machining conditions were identified for both the machining performance and workpiece quality of the EDM process, including some aspects of material removal mechanisms, material removal rate (MRR), electrode tool wear, and subsequent drilled hole quality including surface texture and roundness by using surface profilometry, coordinate measuring machine (CMM), and scanning electron microscopy (SEM). It was observed that the material removal rate increases with increasing peak current and pulse-on-time up to the optimal points and drops drastically thereafter. Higher peak current and/or pulse-on-time result in both the greater tool wear and the larger average diameter error. As the percentage of the SiC particles increases, MRR was increased and electrode wear was found to be decreased. At the EDM machined subsurface layer, the fragmented and melted SiC particles were observed under the SEM and EDX-ray examination.