Abstract: Purpose Lab-on-a-chip devices, where small channels and reaction chambers have to be covered with a sealing lid, laser beam welding has already been proven to show considerable potential for standard material combinations. To avoid absorbing additives, the wavelength of the laser has to fit the absorption bands of the polymers. For it to achieve a localized weld seam, the laser beam has to be strongly focused. Methods To achieve the desired temperature distribution, we establish and discuss the heat rate generated by a focused beam as a function of the absorption coefficient and the Rayleigh length. Thermal simulation of the weld process reveals the dependence of the temperature on Rayleigh length and absorption coefficient. Experiments were carried out with a tunable laser source emitting between 700 and 1000 nm on polycarbonate samples doped with Lumogen 788 dye. Microtome cuts of the welded samples reveal the dimensions of the heat-affected zone and, therefore, of the seam. Results Experimental and simulation results show that for typical welding conditions a localized seam inside a sample can be achieved with an appropriate focusing of the laser beam and tuning of the wavelength. The dimensions of the heat-affected zone decrease as absorption coefficient decreases and welding velocity increases. The discussion of the functional dependence of the heat rate on absorption coefficient and Rayleigh length shows that Rayleigh length and sample thickness have to be smaller than the optical penetration depth. PubDate: 2019-03-08
Abstract: Abstract NiTi shape memory alloys are well-known due to their outstanding functional properties including superelasticity (SE) and Shape Memory Effect (SME). Laser welding is a viable technique for the joining of NiTi wires, which are employed in the design of smart structures. Joining of dissimilar wires can provide better flexibility using various alloys and, consequently, lead to multi-functional properties. However, this is really challenging due to the effect of temperature on microstructure and composition of the welded joints, since the thermal process alters the material microstructure and subsequently the transformation temperature of the heat affected area. Therefore, it is of utmost relevance to find the optimal laser operational parameters for this process. In this context, this study investigates the dissimilar laser welding of NiTi wires. The experimental investigation is based on several interconnected analyses that include optical microscopy (OM), Scanning Electron Microscopy (SEM), and Energy Dispersive X-ray Spectroscopy (EDS) analysis. Thermal analysis by Differential scanning calorimetry (DSC) is also performed to check the matching between the thermal transition of the welded joints compared to the reference materials. Highest energy delivered during welding led to welded joints of better quality. Lastly, the identification of the optimal operational parameters of the laser welding process, such as laser power and scan speed, was found to be crucial to preserve microstructural properties of the welded wires, thermal transition temperatures and therefore avoid to affect the functionality of the materials. PubDate: 2019-03-04
Abstract: Abstract The nonlinear absorption behavior of thick polystyrene sample was experimentally investigated and theoretically analyzed. As polystyrene is transparent to the applied laser wavelength, the absorption was mainly through nonlinear absorption by the bulk material. The effective second order nonlinear absorption coefficient (β) was determined with the z scan technique. The nonlinear behavior at different laser powers: 5.2 mW, 10.4 mW, 14.4 mW and 23.5 mW were investigated. The transmittance of laser energy was measured and a significant change was observed with different sample distance from the laser focal plane. By treating the thick polystyrene sample as a stack of thin layers, the effective nonlinear absorption coefficient was determined to be 0.000695 m/W with a standard deviation of 0.000026. PubDate: 2019-03-01
Abstract: Abstract In the present study, dissimilar joining of AISI 420 Stainless Steel (SS 420) to Kovar alloy using a pulsed Nd:YAG laser welding was carried out. The effect of laser beam position with respect to the joint on the microstructure of dissimilar weldmetal was investigated through taking advantage of optical and scanning electron microscopy. Moreover, hardness measurement and fractography were performed to investigate the mechanical properties of weldmetal. A detailed effect of laser beam position on crack susceptibility of fusion zone is elaborated in this research work. The results showed promotion of susceptibility of fusion zone to cracking with offsetting laser beam position to SS 420 base metal. Also martensitic structure was formed in the weldmetal when off-set laser beam shifted 0.2 mm toward the SS 420 base metal. The microhardness test revealed that moving laser beam position toward SS 420 resulted in increasing weld hardness. PubDate: 2019-03-01
Abstract: Abstract Laser alloying of the surface of Ti-5.5Al-2Zr-1Mo-1 V titanium near-α-alloy was carried out in order to improve the surface properties. The surface of the titanium alloy was being melted by pulsed laser radiation while TiC alloying powder component was being supplied to the laser irradiation zone. As a result of laser alloying, partial melting of the powder particles of titanium carbide and mixing them with the substrate metal resulted in the formation of new disperse TiC phases in the form of dendrites and highly dispersed phases. It is shown that laser alloying of Ti-5.5Al-2Zr-1Mo-1 V titanium near-α-alloy by pulsed laser radiation leads to a 1.4 times increase in hardness of the surface of the titanium alloy and reduces friction coefficient 1.2 times. PubDate: 2019-03-01
Abstract: Abstract The shape and size of a weld bead - consisting of outer weld surface and inner fusion boundary - are important quality and strength attributes in sheet metal welds. The asymmetricity coupled with additional controlling parameters makes it challenging to predict the bead shape in laser-arc hybrid fillet-lap joints with use of lower order nonlinear analytical mathematical functions. An artificial neural network is designed to address the challenge, considering the welding speed, wire feed speed, voltage, current, and laser power as inputs. The experimentally obtained weld bead profiles are digitized in polar coordinates (r, θ) and thereby many input-output pairs are made available for training even with a limited number of experiments. An optimized neural network topology is presented with an assessment of reliability of simulation results. A rational approach for determining the number of coordinate points needed to accurately map the weld bead profile is an important contribution from the present investigation. The parametric study elucidates the effects of input parameters on geometry of the weld beads. The neural network exhibits the capability of capturing the process physics - demonstrated through the analysis of the weld dilution obtained from the simulation results. The welding speed and wire feed speed signifyingly affect the bead shape while the laser power has a minor impact. The laser, even though with less power, improves the weld dilution due to preheating of the base plate and stabilization of the welding arc. PubDate: 2019-03-01
Abstract: Abstract The numerical investigations on experimentally determined cut front geometries presented in this paper show that the absorbed intensity reacts more sensitively to small local changes of the angle of the cut front than the gas velocity and the pressure. It is also found that the absorbed intensities near the top and the bottom of the cut front increase significantly when the initially regular process at high cutting speeds turns into the regime with interrupted striation patterns on the surface of the cutting edge. PubDate: 2019-03-01
Abstract: Abstract Nitinol structures were synthesized in a fully dense form using a laser direct deposition method. The pure elemental metal powders of nickel and titanium were used and powder ratios were controlled to arrive at the prescribed final chemical compositions of Nitinol. The transformation temperatures of synthesized Nitinol samples with different chemical composition and post heat treatment conditions were systematically analyzed and compared with those of conventional Nitinol. Compared to Nitinol parts produced by other techniques, the laser engineered net shaping (LENS) created the least amount of secondary phase, indicating the possibility of high corrosion resistance. Two step post processing of solution heat treatment and aging heat treatment was carried out to improve the homogeneity of the microstructure and to investigate its effects on phase transformation temperatures. The resultant phase transformation temperatures could be controlled by the heat treatment parameters. Compression test results showed mechanical properties of synthesized Nitinol samples are largely affected by its post heat treatment history while the effect of initial chemical composition was negligible. PubDate: 2019-03-01
Abstract: Abstract Stainless steel target was ablated under both of Hexane and water to examine the effect of liquid on the ablation mechanism in terms of surface heating and cooling. Experimentally, a narrow, a smooth and a deep ablation is observed under water. In addition, the pit due to the collapse of cavitation bubble under water is insignificant as compared with Hexane. The focused beam on the target under liquid was estimated from measuring the crater’s diameter from the optical image. The difference in focused beam sizes is attributed mainly to the refractive index of the liquid. Heat transfer equation is solved to estimate the temperature profile on the solid surface for a single pulse. By considering the estimated beam size under liquid, the solution of heat transfer equation suggests for the first time rapid heating of the target under water, but doesn’t confirm the rapid cooling rate under Hexane. The smooth ablation in water is attributed to the more confined plasma. PubDate: 2018-12-01
Abstract: Abstract In the recent development of the automobile, power generation and petrochemical industries the application of dissimilar welds are gaining importance. In the present work, pulsed Nd:YAG laser welding of austenitic stainless steel (304 L) and carbon steel (st37) of 1.4 mm thickness for butt joint configuration have been investigated for automotive industries. The effect of pulsed width on weld bead width (top bead width (TBW) and bottom bead width (BBW), depth of penetration (DOP) and heat affected zone (HAZ) and fusion zone area have been studied. The microstructure characterization of the fusion zone, HAZ and base metals have been compared at varying pulse width using optical microscope. DOP decreases with increase in pulse width beyond 5 ms and fusion zone area decreases with increase in pulse width. Bottom weld bead width decreases with increase in pulse width and HAZ width of st37 side decreases with increase in pulse width. The mechanical properties such as microhardness and tensile strength of the welded joints at varying pulse width have been investigated. Ultimate tensile strength of all the welded joint at different pulse width was equal to the base metal st37 and fracture location was away from the fusion zone. The average microhardness of the fusion zone decreases with increase in pulse width due to the presence of martensite at low pulse width. PubDate: 2018-12-01
Abstract: Abstract Conventional material development of new compositions is expensive and time consuming. Thus, for material characterisation there is a demand to enable high through-put experimentation to analyse and develop new materials in a short time. In this context, a new experimentation method is presented, which is based on TEA CO2 laser-induced shockwaves. First, plasma is created with a nanosecond pulsed TEA CO2 laser on top of a spherical indenter. Further interactions of the plasma with the high intensity laser beam result in a shockwave. The pressure of the shockwave is used to force the indenter penetrate inside the test material. Indentations are created on different aluminium alloys and correlated with hardness. The influence of environmental conditions, indenter material and diameter are investigated. Additionally, an energy model is introduced, which describes the possible indentation strain energy in dependence of the indenter diameter and the shockwave energy transferred to the indenter. The experiments reveal that smaller indenter diameters are recommendable for higher impact efficiencies. Best indentation results are achieved in terms of reproducibility and depth with a 3 mm indenter. PubDate: 2018-12-01
Abstract: Abstract Laser polishing (LP) represents one of the finishing/superfinishing technologies that has experienced a rapid growth over the past two decades. However, while undeniable progress has been achieved on the experimental and/or practical side, the development of the theoretical/numerical models of the continue to be somewhat slower and still dominated by significant simplifying assumptions. Along these lines, the main goal of the present study was to collate the most important modeling developments that were proposed so far in an attempt to synthesize the current stateof-art in the field. While the current consensus is that no single model could provide a comprehensive and accurate picture of the phenomena taking place during LP, it can be asserted at this time that reasonable matches between modeling and experimental results can be obtained under certain conditions. Furthermore, the complexity of the overlapping thermophysical processes that occur during laser polishing combined with the relatively limited database of functional dependencies between material properties and temperature and the impossibility to adequately monitor/measure in real-time many of the process parameters will continue to pose significant modeling and/or validation challenges. Moving forward, it could speculated that additional progress in the latter two categories will likely translate in more accurate representations of the intrinsic mechanisms underlying laser polishing. PubDate: 2018-12-01
Abstract: Abstract The recent advances in the manufacturing technology have led to the development of miniature products in the field of automobile, biomedical implants, biomedicine, aerospace and robotics. The literature is filled with work performed by researchers in the field of laser microdrilling. However, as of today, no exclusive review on the state of the art in laser micro drilling is reported. An attempt has been made to summarize the current state of understanding in laser micro drilling. The review begins with the classification of laser micro drilling and development of various modeling approaches. A variety of issues like identification of key process variables, selection of parameter ranges, selection of laser type and work piece combination and cause and effect diagram for various performance parameters are critically analyzed and presented. A statistical analysis of the contribution of various process parameters to the performance variable is reported. An insight into the laser micro drilling and future growth is presented in this article. PubDate: 2018-12-01
Abstract: Abstract During laser surface treatment, the beam absorption can vary considerably depending on the surface quality, temperature and environment. The metallic surfaces exhibit higher absorptivity when irradiated with a focused laser light. Investigation of the dynamic variation of absorptivity of metals during laser processing is a topic of interest for manufacturing community. This paper focuses on a brief review of absorptivity measurement methods adopted for industrial metals such as steel, titanium, aluminium and copper during laser surface treatment processes. It provides state of art in analytical, numerical and experimental techniques for measuring the absorptivity in order to design an efficient laser control systems. PubDate: 2018-12-01
Abstract: Abstract Due to localized heating and cooling in the Selective Laser Melting based additive manufacturing process, a steep temperature gradient forms in the heat affected zone which induces unwanted thermal stresses in the component. In this work, a transient coupled thermo-mechanical model is developed to study the evolution of stresses at microscale on a spot during laser processing of Ti6Al4V powder layer in the heating and the subsequent cooling stage. The thermal model accounts for melting, solidification, fluid flow in the melt pool, variation of the material properties with different phases and volume reduction in the porous powder layer upon melting. The thermal model is validated by comparing the predicted melt pool size for different laser powers with the benchmark experimental data. The thermal model is fully coupled with the mechanical model, accounting plasticity, used to calculate the resulting stress field. The temporal evolution of temperature, various stress components and von Mises stress is described. During heating, compressive stresses are observed in the region below the melt pool. A zone of tensile stresses is developed in the surrounding region to balance these compressive stresses. During the cooling stage, tensile stresses are found in the solidified melt pool region and balancing compressive stresses are found in the region underneath. Determination of critical locations of excessive residual stress suggests that in the solidified melt pool and surrounding region in the solid substrate the residual stresses exceed the yield strength of the material. This implies yielding in that region which can cause material failure. It is quantified that the magnitude of residual stresses increases with laser power and laser interaction time and decreases with pre-heating of solid substrate. PubDate: 2018-12-01
Abstract: Abstract In the present work, corrosion behavior of laser shock peened 316 L stainless steel was evaluated and compared with unpeened specimen in Hank solution (simulated body fluid) and chloride medium. Laser shock peening (LSP) was used for surface modification using Q-switched Nd-YAG laser. In the corrosion study, different electrochemical experiments such as open circuit potential (OCP)-time measurements, potentiodynamic anodic polarization, electrochemical impedance spectroscopy (EIS) measurements were performed in 0.5 M NaCl and Hank solution to examine the corrosion resistance of unpeened and laser peened 316 L SS specimens. The results of corrosion study demonstrated that LSP improved the pitting corrosion resistance despite marginally higher roughness (Ra = 1.01 μm) for laser peened surface than unpeened surface (Ra = 0.54 μm). Improvement in pitting corrosion resistance was indicated by ennoblement of OCP, more noble (anodic) value of pitting potential (Epit) and lower corrosion current density (Icorr). Conductivity and pH of NaCl and Hank solution increased after every polarization experiments which indicate the ionic dissolution during corrosion experiments. Qualitative analysis of Nyquist plots obtained from electrochemical impedance spectroscopy (EIS) results also showed higher arc radius (higher polarization resistance) for laser peened specimen than unpeened, which is indicative of more protective passive film and hence improved corrosion resistance. Further, microstructural examination after polarization experiments in both NaCl and Hank solution showed less pitting sites in laser peened specimens than unpeened surfaces. These microstructural observations are in line with the results obtained in polarization experiments. The results of the study are important with respect to medical implants and their biocompatibility in the human body. PubDate: 2018-09-01
Abstract: Abstract Origami-inspired design allows three dimensional systems to be made from two dimensional patterns typically far easier to create. We demonstrate a self-folding roll-to-roll process based on laser machining and folding. Using a low cost laser cutter, metal is cut, optically folded into 3D shapes and removed from an unpatterned roll of thin metal sheeting, all without manual handling. Folding is done through laser forming, localized laser heating to cause expansion and contraction for bending. Two different laser forming mechanisms, based on different temperature distributions within the workpiece, are characterized and demonstrated to allow both up and down bending using only the laser itself. These mechanisms are then used to prototype a series of parts using a roll-to-roll setup, with individual parts taken from a blank sheet of metal to a finished folded part in minutes. Our roll-to-roll laser origami approach is a major advance in production technology, allowing the use of a widely available 2D machine tool to create repeated custom 3D parts without human intervention. PubDate: 2018-09-01
Abstract: Abstract In this study, electromagnetic-force-assisted laser bending and straightening process is proposed, in which the external force is applied by a controlled force generated by an electromagnet. The process was found suitable for laser assisted bending and straightening. The experiments as well as simulations indicated that a large bend angle can be obtained by controlling the electric current and air gap between electromagnet and workpiece. A good agreement between simulation and experimental results was obtained. Edge effect was very less in case of strips that got attached with the magnet during laser bending. The spring-back effect was very less at high laser power, low scan speed and high current. The laser irradiated region had higher micro-hardness than base material. The micro-hardness of laser irradiated region depended on laser power, scan speed and magnetic force of attraction. Straightening of mechanically bent strip was also carried out. Results indicate that use of an electromagnet with laser irradiation is an effective way to straighten bent strips. PubDate: 2018-09-01
Abstract: Abstract Laser modification of nickel-aluminide (Ni3Al) coatings on low alloy medium carbon steel substrate was carried out with the help of industrial CO2 laser. The depth of the laser melted zone (modified surface) was controlled as a function of input energy by varying the laser beam travel speed. The laser treated specimens were characterized for microstructure, chemical composition, and micro-hardness profile with respect to the laser penetration depth. Microstructural examination revealed that the laser treated surface was composed of ultrafine grain structure which evolved as a result of laser re-melting and solidification (cooling) rate used in this experiment. Microhardness results showed that as the depth of the laser melted zone increases the hardness of the modified surface decreases and vice versa. Further, the wear behavior of the laser treated surface was also studied with the help of ball-on-disc tribometer and by selecting 2000 m sliding distance. Wear test showed that as the depth of laser melted zone increases the wear of the surface increases as well and vice versa. The possible reason behind this phenomenon was the migration of iron (Fe) atoms from substrate into the surface layer which resulted the dissolution of Ni3Al coating particles in the surface layer and this was confirmed by the EDX analysis. PubDate: 2018-09-01
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