Paper Review

Selective Laser Sintering (SLS) is a rapidly growing additive manufacturing process, because it has the capacity to build parts from a variety of materials. However, the dimensional accuracy of the fabricated parts in this process is dependent on the ability to control phenomena such as warpage and shrinkage. This research presents an optimization algorithm to find the best processing parameters for minimizing warpage. The finite element method was used to simulate the sintering of a layer of polymer powder, and the warpage of the layer was calculated. The numerical model was verified through comparison with experimental results. A back-propagation neural network was used to formulate the mapping between the design variables and the objective function. Results of 40 simulation cases with various input parameters such as scanning pattern and speed, laser power, surrounding temperature, and layer thickness were used to train and test the neutral network. Finally, The Genetic Algorithm was employed to optimize the objective function, and the influence of parameters on warpage was investigated.

Laser ablation micro-machining tests are conducted on silicon wafer, both in air and under flowing water stream, with the use of 355 nm-X AVIA laser. Effects of laser pulse frequency, power level, scan velocity and focal plane position on the associated laser spatter deposition (in air), irradiated areas (under flowing water film) and taper are investigated. It shows that low frequency, i.e. 30–40 kHz, and high peak power result in smaller spatter and irradiated areas, and the hole taper decreases with increase in pulse frequency. Increase in the laser fluence broadens both the areas and increases the hole taper. Both areas enlarge with the increase of scanning velocity of more than 3 mm s−1. The scan velocity has no effect on hole taper in air environment but inconsistent hole taper is obtained under flowing water stream. Furthermore, moving the focal plane position below the workpiece surface contributes relatively smaller areas of spatter deposition, irradiation and taper in comparison to zero focal plane position. Finally, the differences between laser ablation in air and under water are identified. The reduction in the spatter deposition and irradiated areas around the perimeter of the ablated hole and a smaller taper with the use of laser trepan drilling method in air and under water machining are investigated in this paper.

We investigate the feasibility of cutting and drilling thin flex glass (TFG) substrates using a picosecond laser operating at wavelengths of 1030 nm, 515 nm and 343 nm. 50 μm and 100 μm thick AF32®Eco Thin Glass (Schott AG) sheets are used. The laser processing parameters such as the wavelength, pulse energy, pulse repetition frequency, scan speed and the number of laser passes which are necessary to perform through a cut or to drill a borehole in the TFG substrate are studied in detail. Our results show that the highest effective cutting speeds (220 mm/s for a 50 μm thick TFG substrate and 74 mm/s for a 100 μm thick TFG substrate) are obtained with the 1030 nm wavelength, whereas the 343 nm wavelength provides the best quality cuts. The 515 nm wavelength, meanwhile, can be used to provide relatively good laser cut quality with heat affected zones (HAZ) of <25 μm for 50 μm TFG and <40 μm for 100 μm TFG with cutting speeds of 100 mm/s and 28.5 mm/s, respectively. The 343 nm and 515 nm wavelengths can also be used for drilling micro-holes (with inlet diameters of ⩽75 µm) in the 100 μm TFG substrate with speeds of up to 2 holes per second (using 343 nm) and 8 holes per second (using 515 nm). Optical microscope and SEM images of the cuts and micro-holes are presented.

The femtosecond laser ablation process for cutting thin aluminoborosilicate glass sheets of thickness 100 μm was investigated with emphasis on effective cutting speed (Veff) and mechanical strength of diced samples. The process parameters including the laser fluence (F), overlap ratio (r) of the laser beam and polarization direction were varied at a fixed pulse repetition rate f = 1 kHz to find the optimal process condition that maximizes Veff and edge strength. A three-point bending test was performed to evaluate the front-side and back-side bending (edge) strength of the laser-cut samples. Veff was proportional to F unless r exceeded a critical value, at which excessive energy began to be delivered at the same spot. The front-side edge strength was bigger than the back-side strength because of the back-side damages such as chipping. Good edge strength, as high as ∼280 MPa (front-side) and ∼230 MPa (back-side), was obtained at F = 19 J/m2, r = 0.99, with laser polarization vertical to the cutting path.

A novel combined laser cutting process (CLCP) has been successfully employed for the first time, as far as our knowledge extends, to enable the low-damage, high-speed cutting of interior holes in thick glass materials. To generate a precise weakening zone, a comprehensive technique utilizing the full ablation capability of a picosecond laser has been utilized. The cutting surface is split by CO2 laser-controlled crack propagation, and the final separation of the glass substrate is assisted by the temperature field. In this work, round, square, and irregular rectangular holes were cut in 6 mm soda-lime glass by CLCP. At present, the cutting speed is 150 mm/s, the minimum diameter is 60 mm, the edge chipping of two surfaces is lower than 3.5 µm and the maximum roughness of the side wall is 1.03 µm. This study proposes a new way of ultrafast laser cutting inner holes in thick glasses.

Novel anode gas diffusion layers (AGDLs) with both hydrophobic and hydrophilic pathways are created to enhance transfer of both methanol and CO2. Such AGDLs are created by perforating PTFE-treated AGDLs with laser, so that the original pores/pathways in the AGDL are hydrophobic and the laser perforations are hydrophilic, thus providing easy transport paths for both the liquid methanol solution and CO2. One of the novel AGDLs has increased the cell performance by 32% over the non-perforated AGDL. Results of electrochemical impedance spectroscopy (EIS) show that the main reason for the performance enhancement is due to the reduction in mass transfer resistance. Additionally, there is a reduction in charge transfer resistances due to the enhanced methanol transfer to the catalyst layer. The results of linear sweep voltammetry (LSV) show that the perforations increase methanol crossover, thus if perforation density of the AGDL is too high, the cell performances are lower than that of the virgin AGDL.

Bipolar plates are one of the key parts of a PEMFC (polymer electrolyte membrane fuel cell). The flow fields of the bipolar plates distribute reactant gasses on the electrode surfaces. The flow channels should have an appropriate design to decrease the mass transport loss at a minimum pressure drop. Among different flow channels, serpentine flow channel received considerable attention for application in electrochemical cells. In this work, a three-dimensional numerical model is proposed and applied for studying the effect of bend sizes on a PEM (polymer electrolyte membrane) fuel cell. The obtained results show that as bend size increases from 1 mm to 1.2 mm, not only does the over potential reduce significantly but temperature gradient is also alleviated. These welcome effects are largely due to a more even distribution of electrolytes over the electrode surface which eventually increases the power density of the fuel cell about 1.78% compared to channels with 0.8 mm square bend size. Moreover, it is shown that the serpentine flow channels with 1.2 mm square bend size act successfully in preventing secondary flows internal thereby decreasing pressure drop about 90.6% compared to serpentine flow channels with a bend size of 0.8 mm.

The wettability of the flow channel surface has a great influence on proton exchange membrane fuel cell performance. A modification method using polytetrafluoroethylene (PTFE) of improving hydrophobicity of graphite plate without significant resistance rise is obtained and the contact angle with 8  μL deionized water reaches 135°. The cathode plate is modified with this method and PTFE is removed from the crest. Then single-cells with the modified cathode plate and unmodified plate are assembled and tested, respectively. The peak power density increases by 6.37% at 30% cathode inlet relative humidity and decreases by 7.79% and 16.9% at 50% and 100% cathode inlet relative humidity. Based on this, we propose the explanation that the repulsive effect of hydrophobic flow channel surface on water can alleviate the membrane dryness at low relative humidity conditions, but hinder the gas from entering the gas diffusion layer at high relative humidity conditions.

This study focuses on the parametric analysis of Proton Exchange Membrane Fuel Cells (PEMFC) to enhance its performance used in Fuel cell vehicles. The study involves fabrication of membrane electrode assembly at 40% Pt/C loading and experimenting with different parameters, viz, cell temperatures, oxygen and hydrogen flow rates and cathode and anode humidification temperatures. The results show that cell temperature has significant effect on the performance of the PEMFC, whereas other parameters produce variation only in the activation polarization region and in the concentration polarization region. A prototype model of FCV, indigenously powered by a fuel cell stack and run continuously without any auxiliary power supply is developed as a viable model for a higher power vehicle. The vehicle’s performance is studied by conducting various load tests.

Bipolar plates include separate gas flow channels for anode and cathode electrodes of a fuel cell. These gases flow channels supply reactant gasses as well as remove products from the cathode side of the fuel cell. Fluid flow, heat and mass transport processes in these channels have significant effect on fuel cell performance, particularly to the mass transport losses. The design of the bipolar plates should minimize plate thickness for low volume and mass. Additionally, contact faces should provide a high degree of surface uniformity for low thermal and electrical contact resistances. Finally, the flow fields should provide for efficient heat and mass transport processes with reduced pressure drops. In this study, bipolar plates with different serpentine flow channel configurations are analyzed using computational fluid dynamics modeling. Flow characteristics including variation of pressure in the flow channel across the bipolar plate are presented. Pressure drop characteristics for different flow channel designs are compared. Results show that with increased number of parallel channels and smaller sizes, a more effective contact surface area along with decreased pressured drop can be achieved. Correlations of such entrance region coefficients will be useful for the PEM fuel cell simulation model to evaluate the affects of the bipolar plate design on mass transfer loss and hence on the total current and power density of the fuel cell.

The microstructures on a diamond surface have attracted extensive attention in microelectronics, ultra-precision machining tools, and optical elements, etc. In this work, microgrooves were fabricated on a single-crystal diamond surface using ultraviolet nanosecond or infrared picosecond laser pulses. The surface and internal morphologies of the microgrooves were characterized. The chemical composition and phase transition of the diamond after laser irradiation were analyzed. Furthermore, the ablation threshold, ablation rate, and material removal rate of the diamond processed by nanosecond or picosecond lasers were also calculated. In addition, the temperature distributions of the diamond ablated by nanosecond or picosecond lasers were simulated. Finally, the material removal mechanisms of a single-crystal diamond processed by nanosecond or picosecond lasers were revealed. This work is expected helpful to provide a guidance for the laser fabrication of microstructures on diamond.

This study presents the investigation results of micro-channel fabrication in nickel-based super alloy (Inconel 718) by Nd:YAG laser beam machining. The effects of laser parameters on the machining performance characteristics over dimensional sizes are evaluated. Three number of laser parameters have been reserved as predictors to the naming of laser intensity, pulse frequency, and laser scanning speed. The channel’s top width, bottom width, depth, and taperness are considered as the process responses. Micrographs by SEM have been organized to study and measure the micro-sized dimensions of fabricated channels. The results showed that the selection of channel size is critical to achieve desired machining results. Wider-sized channels (for example 200 × 100 μm and 1000 × 500 μm) can more confidently be machined than narrower-sized channels (50 × 50 μm). The possible reasons behind the failure of narrow-sized micro-channel fabrication are identified. The appropriate combination of parameters that can yield the better results for 100 × 100 μm channel size are the laser intensity of 92.7 %, repetition rate of 30 kHz, and scan speed of 300 mm/s. At this combination, the channel geometries of wider-sized channels are more close to the designed geometries as compared to narrow-sized channels. Microstructures of the machined channels are also studied showing the recast layer with lamellar grain structure and phase transformation near the edges of micro-channels. The channel edges and their adjacent areas show variation in hardness relative to bulk material. This has been validated via micro-hardness profiles of the close vicinity of machined micro-channels.

Ultrafast laser ablation, known for its high precision and the depressed thermal influence during processing, is fevered in micro channel fabrication on the glass bulk. In present work, a micro channel structure fabricated on fused silica bulk by a picosecond pulsed laser is parametrically investigated with different processing environments, scanning velocities, pulse energies, hatch distances and repetitive scans. The best processing parameters are determined through the material remove rate and the bottom surface roughness of the channel. Moreover, the fabrication defect attributed to the heat accumulation effect is analyzed by Raman spectrum. A scanning strategy is put forward to compensate the asymmetry in the cross section of the micro channels.

The quality of micro-features in various technologies is mostly affected by the choice of the micro-fabrication technique, which in turn results in several limitations with regard to materials, productivity, and cost. Laser beam micro-machining has a distinct edge over other non-traditional methods in terms of material choices, precision, shape complexity, and surface integrity. This study investigates the effect of laser fluence and pulse overlap while developing microchannels in alumina ceramic using an neodymium-doped yttrium aluminum garnet (Nd:YAG) laser. Microchannels 200 µm wide with different depths were machined using different laser peak fluence and pulse overlap (percentage of overlap between successive laser pulses) values. It was found that high pulse overlaps and fluences should be avoided as they give rise to V-shaped microchannels i.e., 100% bottom width errors. The optimal peak fluence range was found to be around 125–130 J/cm2 corresponding to 3–5 µm depth per scan. In addition, channels fabricated with moderate pulse overlap were found to be of good quality compared to low pulse overlaps.

Microchannel structure as catalyst support has been widely used to construct numerous microreactors for hydrogen production. In this work, the laser micro-milling technique was introduced into the fabrication process of microchannels with different geometry and dimensions. The effects of varying scanning speed, laser output power and number of scans on the surface morphology and geometrical dimension of microchannels have been investigated based on SEM observations. It is found that the change of scanning speed and laser output power significantly affected the surface morphology of microchannel. Moreover, the depth of microchannel was increased when the laser output power and number of scans were increased. Subsequently, the microchannels on copper sheet fabricated by the laser micro-milling technique were used as catalyst support to conduct the methanol steam reforming reaction. The better reaction performance of methanol steam reforming in microchannels indicates that laser micro-milling process is probably suitable to fabricate the microchannel reactor for the commercial application.

Carbon nanotube (CNT) film can be used as thin film electrodes and wearable electronic devices due to their excellent mechanical and electrical properties. The femtosecond laser has the characteristics of an ultra-short pulse duration and an ultra-high peak power, and it is one of the most suitable methods for film material processing. The ablation and patterning of CNT film are performed by a femtosecond laser with different parameters. An ablation threshold of 25 mJ/cm2 was obtained by investigating the effects of laser pulse energy and pulse number on ablation holes. Raman spectroscopy and scanning electron microscope (SEM) were used to characterize the performance of the pattern groove. The results show that the oligomer in the CNT film was removed by the laser ablation, resulting in an increase in Raman G band intensity. As the laser increased, the ablation of the CNTs was caused by the energy of photons interacting with laser-induced thermal elasticity when the pulse energy was increased enough to destroy the carbon–carbon bonds between different carbon atoms. Impurities and amorphous carbon were found at and near the cut edge while laser cutting at high energy, and considerable distortion and tensile was produced on the edge of the CNTs’ groove. Furthermore, appropriate cutting parameters were obtained without introducing defects and damage to the substrate, which provides a practical method applied to large-area patterning machining of CNT film.

This work demonstrates superhydrophobic behavior on nanosecond laser patterned copper and brass surfaces. Compared with ultrafast laser systems previously used for such texturing, infrared nanosecond fiber lasers offer a lower cost and more robust system combined with potentially much higher processing rates. The wettability of the textured surfaces develops from hydrophilicity to superhydrophobicity over time when exposed to ambient conditions. The change in the wetting property is attributed to the partial deoxidation of oxides on the surface induced during laser texturing. Textures exhibiting steady state contact angles of up to ∼152° with contact angle hysteresis of around 3–4° have been achieved. Interestingly, the superhydrobobic surfaces have the self-cleaning ability and have potential for chemical sensing applications. The principle of these novel chemical sensors is based on the change in contact angle with the concentration of methanol in a solution. To demonstrate the principle of operation of such a sensor, it is found that the contact angle of methanol solution on the superhydrophobic surfaces exponentially decays with increasing concentration. A significant reduction, of 128°, in contact angle on superhydrophobic brass is observed, which is one order of magnitude greater than that for the untreated surface (12°), when percent composition of methanol reaches to 28%.

In this research, the fabrication process of a super-hydrophobic metallic surface using laser ablation and electrodeposition was investigated. Re-entrant structure and surface roughness play an important role in forming a super-hydrophobic surface on intrinsically hydrophilic material. A micro pillar array with a re-entrant structure of copper on stainless steel was fabricated through a sequential process of laser ablation, insulating, mechanical polishing and electrodeposition. Spacing of the micro pillars in the array played a major role in the structure hydrophobicity that was confirmed by measuring the water contact angle. Surface morphology changed relative to the parameters of the laser ablation process and electrodeposition process. Under a gradual increase in current density during the electrodeposition process, surface morphology roughness was maximized for fabricating a super-hydrophobic surface. Finally, the super-hydrophobic surface was successfully fabricated on metal.

A lot of research efforts have been invested in the fabrication of superhydrophobic surfaces in recent years due to many protentional applications in science and industry including anti-icing, self-cleaning and anti-corrosive surfaces. Laser as a non-polluting, precise and flexible tool can be applied to replicate surface microstructures of extremely water repellent lotus leave surface. In this study, a common nanosecond laser source is used to fabricate a super/ultrahydrophobic surfaces with different microstructure designs, contact angles above 170° and sliding angles below 5°. The freshly processed surface is hydrophilic and becomes hydrophobic and superhydrophobic in a certain time period, which could be dramatically reduced by storing samples in high vacuum. The transformation in wetting properties are analysed with respect to surface geometry and surface chemistry.

Laser cleaning technologies have been attracting attention as a solution to the environmental problems caused by pre-treatment processes, but there are very few studies on the removal of paint using laser cleaning. In this study, laser cleaning was performed on steel painted with shop primer and epoxy paint using a Q-switching fiber laser, and the characteristics of the laser cleaned surfaces was compared in relation to the pulse overlap rate, as a main parameter. Experimental results showed that the number of scans to remove the paint decreased as the pulse overlap rate increased. At pulse overlap rates of 20 % and 50 %, the oxide layer was not removed from the surface. However, when the pulse overlap rate was increased to 70%, the oxide layer was completely removed. In addition, damage of the base material was reduced when the pulse overlap rate was increased, more precise laser cleaning was possible.

Strong metal/non-polar plastic dissimilar joints are highly demanded for the lightweight design in many fields, which, however, are rather challenging to achieve directly via welding. In this study, we designed a laser processing pretreatment on the Al alloy to create a deep porous Al surface structure, which was successfully joined to the polypropylene (PP) via friction spot welding. A maximum joint strength of 29 MPa was achieved, the same as that of the base PP (i.e. the joint efficiency reached 100%), much larger than ever reported. The joining mechanism of the Al alloy and the PP was mainly attributed to the large mechanical interlocking effect between the laser processed Al porous structure and the re-solidified PP and the formation of chemical bond at the interface. The deep porous Al surface structure modified by laser processing largely changed the Al‒PP reaction feature. The evidence of the Csingle bondOsingle bondAl chemical bond was first time found at the non-polar plastic/Al joint interface, which was the reaction result between the oxide on the Al alloy surface and thermal oxidization products of the PP during welding. This study provides a new way for enhancing metal-plastic joints via surface laser treatment techniques.