Laser machining is widely applied in manufacturing processes thanks to the laser oscillator’s improved stability and to the emergence of new laser types. Laser machining has gone from microscale applications, such as semiconductor dicing to large-scale applications such as automobile-body welding, and laser power now ranges from several watts to several kilowatts. Machining tasks using lasers have expanded from conventional drilling, cutting, and welding to additive manufacturing, the internal machining of transparent materials, and surface texturing. Understanding these processes comprehensively requires that we study individual elements such as oscillators, focal optics, scanners and stages, and numerical control.
This special issue features 13 research articles – one review and 12 papers – related to the most recent advances in laser machining. Their subjects cover the various machining processes of drilling, deposition, welding, photo curing, texturing, and annealing on the latest laser machines and in the newest applications.
We deeply appreciate the careful work of all the authors and thank the reviewers for their incisive efforts. Without these contributions, this special issue could not have been created. We also hope that this special issue will trigger further research on laser machining advances.
The history of the developments of the laser processing technologies in Japan was analyzed. During past 50 years of the processing technologies were divided of first half of 25 years and the latter half, and they were compared. During the first half, the laser was evaluated as an alternative to conventional heat-processing technology. In the recent 25 years, the diversification of laser application was spread by development of laser equipment and materials processing technologies. And surface functionalization with short pulsed laser and 3D Additive Manufacturing were developed widely.
Ultrashort pulse laser processing that facilitates high-speed and fine processing of glass materials has received considerable attention in recent years, despite mechanical processing or etching having been the mainstream methods. However, the physical mechanisms of this technique and the influence of various parameters, such as the processing conditions and physical properties of the processed material, on generated shapes are not yet fully understood. In this work, we comprehensively investigated the influence of various parameters of ultrashort pulse lasers on the processing mechanisms through experiments conducted by changing the wavelength, pulse width, repetition rate, and pulse energy over a wide range. The physical effects of the laser parameters on the reflection of light and heat generation were discussed by analyzing the experimental results, and the influence of the parameters on the generated shapes, processing speed, and saturated depth was clarified. In addition, cracks around the ablated area, which are one of the problems concerning glass processing with ultrashort pulse lasers, were observed, and the influence of the pulse energy on the cracks was evaluated. It is expected that this research will allow for a thorough understanding of the laser parameters that are suitable for glass processing and widen the range of laser processing applications.
In laser cutting and drilling process, molten material was scattered as spatter, which deteriorates the surface integrity of a workpiece because of the thermal damage. It is expected that the control of assist gas flow can reduce the adhesion of spatter. In order to investigate the improvement method of thermal damage due to the adhesion of spatter, it is required to clarify characteristics of spatter. Therefore, a method was developed to collect and analyze spatter based on the use of high-speed video cameras in the laser micro-drilling process, and the characteristics of spatter movement were numerically investigated by CFD analysis. The scattering velocity and angle of the spatter were investigated by recognizing and tracking spatter with the high-speed video observation. The movement of spatter was observed by using two high-speed video cameras, and analyzed by using a two-direction tracking method, in which the 3D tracking lines of spatter particles were reconstructed in the forward and backward frames, and the actual trajectory of individual spatter particle was obtained by averaging those tracking lines. These measurements revealed that the initial velocity of spatter was mainly distributed from 52 m/s to 200 m/s with an average velocity of 129 m/s. The initial angle of spatter was mainly distributed between 0 and 30 degrees from the workpiece surface in the upward direction. There was little correlation between the initial velocity and angle of spatter. The diameter of spatter was mainly distributed from 1 μm to 4 μm with an average diameter of 3.7 μm. It is important to use the processing conditions achieving the smaller spatter diameter in order to reduce the thermal damage caused by spatter. Although coaxial assist gas flow has an influence on the spatter behavior, that time period is very short. Therefore, it is important to control the spatter behavior outside of the coaxial assist gas flow by using an additional gas flow to prevent the thermal damage to the workpiece surface.
The purpose of this study is to develop novel cutting tools with micro- or nanoscale textures on their surfaces. Texturing micro- or nanoscale features on a surface allows us to control the tribological characteristics of the tool. For this research, textures were applied to end mills with a diameter of 0.5 mm using a femtosecond laser, and milling experiments were conducted on aluminum alloy to evaluate the developed tools. The applied texture decreased the cutting forces. This effect depends on the shape of the texture: groove textures are more effective for reducing friction and the resultant cutting forces. Periodic textures fabricated through the interference of the laser were effective at reducing the adhesion of the work material. A larger effect was obtained for shallow and large pitch textures. The results indicate that the proposed method is effective at improving the machining performance of small-diameter end mills.
Molding technology is widely used to manufacture optical components because of its high efficiency. Along with the quick development of miniaturization in industry, the detrimental effects of previously negligible burrs and defects on mold surfaces have become significant to the performance of components, so these problems should be minimized. In this study, a laser assisted micromachining method was developed to solve this problem during the fabrication of periodic microstructures on a molding material of electroless nickel-phosphorus (NiP) plating. The transient temperature distributions of the workpiece under laser irradiation and the change in the maximum shear stress during the laser assisted micromachining process were simulated to set appropriate experimental conditions. Then, periodic micropyramid structures were fabricated by both conventional cutting and the laser assisted cutting processes. Results show that defects largely decreased on machined structures with the assistance of laser irradiation. The decrease in specific cutting force and the change of chips’ morphology were also utilized to analyze the reasons for this improvement.
Recently, studies on three-dimensional (3D) formation technology, which is capable of forming components directly, have become increasingly popular as a part of additive manufacturing technologies. However, a very limited amount of information has been published about its processing conditions or settings. In this study, we have prototyped a wire-feeding type 3D laser deposition equipment to publish information about adjusting the deposition conditions and examining the deposition characteristics, which can be used as a reference by engineers who carry out 3D formations. We hope that this study can contribute significantly to the progress in additive manufacturing technologies. Using the proposed equipment, we determined the deposition conditions under which the melting of a specimen can be limited to a shallow depth, while making depositions of sufficient height on the specimen. These deposition conditions are a laser power of 2.0 kW, a laser traveling speed of 1.5 m/min, and a wire-feeding speed of 7.7 m/min. Further, we confirmed that slight tuning of these deposition conditions even allows for 10-layer depositions and depositions containing curved lines.
This paper deals with micro-welding of glass substrates using a YAG laser to prevent cracks. In order to fuse the glass substrates precisely using a YAG laser, a new glass welding method was proposed and developed in the previous report. In the method, two glass plates were made to overlap and the welded area of the glass surface was coated with absorbent. The YAG laser irradiated the surface, and the laser was absorbed in the coated area only. Therefore, only the coated area can be welded and the glass surface is expected to be clear. However, in the previous report, some micro cracks generated by the thermal shock of the YAG laser appeared on the fused area of the glass substrates. In this study, it is proposed that pre-heating the substrates decreses the heat shock, producing a crack-free surface of the fused area. In the pre-heated welding experiments, the substrate temperature and laser power are changed, and the welding surface and welding strength are evaluated. It is clarified from the experiments, that the glass plates can be welded without a crack using pre-heated welding.
Photopolymer parts with short ferromagnetic fibers are fabricated by laser stereolithography. A magnetic field is applied to a liquid photopolymer that contains the short ferromagnetic fibers, with the axes of the short fibers aligned in the direction of the magnetic field. Then, the photopolymer is solidified with UV laser irradiation to get the desired shape. After fabricating the part, it is magnetized by applying magnetic field to it. When the magnetic field is applied in the direction of the aligned short fibers in the fabricated part, the residual magnetic flux density of the magnetized part in the direction of the aligned fibers becomes approximately twice that in the other directions. The magnetized part can be bent by applying an external magnetic field.
A furnace is used in the conventional quenching of small work pieces. When quenching is performed using a furnace, the electricity consumption is high because the heating is performed over a long time period. Moreover, the working environment is unsatisfactory because quenching oils and salt baths are used in the process. Laser quenching has attracted considerable attention as a method for addressing these issues. In previous studies of this method, a laser was used to perform whole quenching on a sheet. However, problems such as deformation and tempering can arise during whole quenching on a thin plate. Furthermore, it is difficult to determine appropriate feed speeds and scanning orders. Therefore, in this paper, we propose a quenching method that uses a semiconductor laser to reduce these thin plate quenching problems. Additionally, an evaluation function is provided for quantitatively assessing the feed speeds. We strive to determine the appropriate laser scanning order from an understanding of decreases in hardness and transformation. We then used the evaluation function to appropriately set the feed speed for the whole quenching of a thin plate using a laser. As a result, we were ultimately able to select a suitable feed speed at which whole quenching can be performed.
We investigated the effect of different solvents on the Cu micropatterns formed via femtosecond laser reduction patterning. Solvents such as ethylene glycol, 2-propanol, and glycerol were mixed with CuO nanoparticles and polyvinylpyrrolidone. The degree of reduction and the resistivity of the fabricated micropatterns depended on the solvent. Glycerol was the most effective reducing agent. This solution was used to fabricate Cu/Cu2O composite micro-temperature sensors. Cu-rich electrodes and Cu2O-rich sensors were selectively formed by controlling the laser scanning speed at 5 mm/s and 0.5 mm/s, respectively, when the pulse energy was 0.53 nJ. The temperature sensor exhibited a negative temperature coefficient of the resistance, which was consistent with the value for Cu2O.
Combining an optical laser scanning system with mechanical axes in a redundant configuration and synchronised control allows the separation of scanning motions according to the strengths of the two systems. Assigning the highly dynamic movement part to the agile optical axes reduces the acceleration and jerk of the mechanical axes. The mechanical axes enable precise motion over the whole workspace, that cannot be achieved by optical systems. The determination of the ideal trajectory separation among the two redundant systems poses an optimisation problem. This study proposes a method for the calculation of the optimal trajectory separation and for productivity increases. Furthermore, a windowing technique is introduced to limit the required computational power. The operation of the optimisation algorithm is demonstrated based on example geometries. It is shown that the machining time is decreased, and the jerk of the solution is minimised. The method is verified using a laser scanning system.
The demand for minimally invasive injection needles or needle-shaped tools is growing from those who carry out medical practices such as blood or insulin injections. Applying the mosquito biomimetic, we have used a femtosecond laser to fabricate minimally invasive microneedles out of ultrafine hollow SUS304 pipes, 50 μm in outer diameter and 20 μm in inner diameter. When such a stainless steel needle tip is angled at 15°, it has the lowest penetration resistance, two and a half times lower than that of the finest hollow needle that is commercially available. A blood suction experiment with a newly developed microneedle has demonstrated that 2.8 μℓ of blood can be drawn out in 20 seconds. Such stainless steel microneedles fabricated by femtosecond lasers have great potential as minimally invasive and mass-producible blood sampling needles to be used for diabetic inspections.
The demand for the development of low-invasive injection needles or needle-shaped tools for applications in medical practices, such as blood sampling and blood sugar level tests, are growing. We observed a mosquito’s penetration motions and fabricated low-invasive microneedles from 30-μm-thick stainless steel foil, imitating a pair of the mosquito’s maxillae. In our earlier studies, we attempted to fabricate needle tips with jagged portions by using a specially ordered machine tool and sharpening them by electrolytic etching, but found it difficult to maintain the needle tips in a jagged shape. However, in this study we successfully fabricated microneedles (70 μm in width and 2.2 mm in length) with three-dimensionally sharpened tips (angled at 15° on the upper surface and sides) by machining 30-μm-thick stainless steel foil by femtosecond laser. Femtosecond laser machining can be applied to any type of material and can fabricate any desired three-dimensional structures by changing the angles at which the materials are set.
This study aims to demonstrate that dielectric particles can be stacked to produce electromagnetic functions. By drawing up a hydrophilic substrate from an aqueous suspension in which fine particles are dispersed, monolayer closed-packed or ordered structures can be obtained by utilizing the meniscus attraction between the particles during the drying process. By repeating this process, particles of different sizes and materials can be stacked layer by layer. By changing the permittivity and/or diameters of the particles through layers, the reflectance of light at a particular wavelength can be increased or decreased. Aiming at selective reflection of the near infrared light, silica (SiO2) particles of φ200 nm and titanium oxide (TiO2) particles of φ33 nm were chosen and two layers were stacked on a silicon wafer. The reflectivity was measured with an original setup. Monochromatic light was focused on the sample at various angles of incidence, and the reflection intensity was measured at specified angles independent of the angle of incidence. By scanning the wavelength of the light, the reflection spectrum at specific incident and reflection angles was obtained. It was confirmed that reflectivity increased by 1.7 μm when the TiO2 oxide layer covered the SiO2 layer, as derived from theory.
Metal-assisted chemical etching (MACE) is a site-selective etching process produced by a catalyst reaction at the interface between noble metal and silicon. This paper aims to make clear the applicability of sphere lithography and MACE to the fabrication of high aspect ratio Si nanostructures. The capacity to control the etched profiles and the scale extension are investigated. First, silica particles (e.g. φ1 μm) were self-assembled on a Si substrate. After the reduction of particle size via argon ion bombardment, a gold layer was deposited using the particles as a mask. The substrate was then etched with a mixture of hydrofluoric acid and hydrogen peroxide. It was found that an array of nanopillars with a regular pitch, good separation, and an aspect ratio of about 52 was produced. The effects of MACE conditions on final profiles were clarified. A limitation of this approach is the small (several millimeters) area fabricated due to the dependence on the vacuum technique (ion bombardment, Au deposition), and the size of the area limits its practical applications. Thus, Ag nanoparticles (e.g. φ150 nm) were applied. The relationship between the concentration of the Ag suspension, the Ag assembled layer, and the morphology of MACE structures was made clear. A spray method was applied to extend the deposited area of Ag particles up to φ100 mm. Finally, the effects of the cross-sectional profile on the contact angle of a water droplet were examined. By applying a high aspect ratio nanostructure on the substrate, the water contact angle increased up to 153 degrees while that without the structure is 58 degrees.
Normal vector measurements of the machining point and attitude adjustments of the end effector are key aspects to meet the technological requirements of hole verticality in auto-drilling and the residual wall thickness in mirror milling. In this paper, a surface normal on-machine measuring method using an EC sensor array is proposed. The influences of the object surface inclination and the sensor array arrangement on the performance of EC displacement sensors were investigated, and the sensor measuring errors from coupling interference were effectively eliminated. Moreover, a practical calibration algorithm was established in which the positions of the EC sensors in a normal vector calculation model were accurately corrected. The feasibility of the measuring method was validated through a calibration experiment, as well as a measurement experiment based on the calibration results. The accuracy of a normal vector measurement is improved when applying the accuracy compensation and position calibration algorithm of an EC array to engineering practices.
It has become important to consider energy-efficient optimization not only in a process design but also in the operations of manufacturing systems to promote sustainable and green manufacturing. This paper extends authors’ previous work to a more practical situation to demonstrate the applicability of the proposed framework of energy-efficient manufacturing operations based on a resource-constrained project scheduling problem (RCPSP). Both have varying resource requirements and multi processing modes, which can produce a suitable energy-load profiles for complete manufacturing systems. This study proposes a mathematical model for producing optimal energy-load profiles, and based on these profiles, each given operation is allocated to a machine tool with a specific processing mode. A processing mode refers to machining conditions for the corresponding operation, conditions that provide a predictive processing time and estimated electrical energy consumption. Through some cutting experiments on aluminum alloy performed on a three-axis machining center, we provide several possible processing modes for workpieces (operations), and we generate energy-load profiles by applying multi start local searches. We then discuss the applicability and capability of the energy-load profiles as an energy-aware production control.
Conventional methods often use ball end mills with a small diameter to finish machining of a steam turbine blade to satisfy accuracy requirements by using a small pick feed value. Thus, the cutting length increases, resulting in increased wear and a lower milling efficiency. Therefore, a new method using a tilt-taper end mill is proposed. This paper presents the validity of the proposed method used for milling planes by comparing the ball and square end mills through tool wear experiments. Factors including removal degree, surface roughness, tool wear, and machined surfaces are investigated with respect to the plane model. The experimental results show that tilt end mill can retard the tool wear remarkably to obtain a steady surface profile, and the maximum surface roughness value, using the tilt-taper end mill, is less than 6 μm until process completion.