Laser Energy Input Research Articles

Influence of laser exposure time and point distance on 75-\u03bcm-thick layer of selective laser melted Alloy 718

A systematic matrix with 25 samples, using five different point distances and five laser exposure times, depositing 75-μm-thick layers of Alloy 718 has been studied. The work has concentrated on defects formed, hardness of the deposits, and the microstructure. Relatively large amount of defects, both lack of fusion and porosity, was found in several of the specimens in the deposits. The defects were never possible to fully eliminate, but a significant decrease, mainly in the lack of fusion, was seen with increasing laser exposure time. The gas porosity on the other hand was not affected to any larger degree, except for the lowest laser energy input, where a slight increase in porosity was seen. A small increase in hardness was noted with increasing laser energy input. The width of the deposited beads increased with increasing laser energy, while the depth of deposits was more or less constant. However, for the lowest combination of point distance and laser exposure time, quite deep and narrow beads were formed. A comparison was made with deposition of 50-μm-thick layers, with quite similar laser energy input, but with some variation in detailed deposition parameters. It was found that the 75-μm-thick layers contained less lack of fusion, particularly for small point distances. The amount of porosity was also less, but that did not vary with deposition parameters.

Numerical investigation of nanosecond laser induced plasma and shock wave dynamics from air using 2D hydrodynamic code

A two-dimensional axis symmetric hydrodynamic model was developed to investigate nanosecond laser induced plasma and shock wave dynamics in ambient air over the input laser energies of 50–150 mJ and time scales from 25 ns to 8 μs. The formation of localized hot spots during laser energy deposition, asymmetric spatio-temporal evolution, rolling, and splitting of the plasma observed in the simulations were in good agreement with the experimental results. The formed plasma was observed to have two regions: the hot plasma core and the plasma outer region. The asymmetric expansion was due to the variation in the thermodynamic variables along the laser propagation and radial directions. The rolling of the plasma was observed to take place in the core region where very high temperatures exist. Similarly, the splitting of the plasma was observed to take place in the core region between the localized hot spots that causes the hydrodynamic instabilities. The rolling and splitting times were observed to vary with the input laser energy deposited. The plasma expansion was observed to be asymmetric for all the simulated time scales considered, whereas the shock wave evolution was observed to transfer from asymmetric to symmetric expansion. Finally, the simulated temporal evolution of the electron number density, temperature of the hot core plasma, and the temperature evolution across the shock front after the detachment from the plasma were presented over the time scales 25 ns–8 μs for different input laser pulse energies.

Effect of laser processing parameters on porosity, microstructure and mechanical properties of porous Mg-Ca alloys produced by laser additive manufacturing

Laser additive manufacturing (LAM) was used to fabricate porous Mg-Ca alloys and the effect of laser processing parameters on porosity, surface morphology, microstructure, microhardness and compression performance were investigated. The results indicate that the porosity and surface morphology of the LAMed samples depend on the laser energy input. When energy density is between 875J/mm3 and 1000J/mm3, the porosity of porous magnesium alloy is between 18.48% and 24.60%, and at this time, the samples with better surface quality can be obtained. The LAMed porous Mg-Ca alloy shows overlapped cladding lines and periodic morphological features. With the farther away from the molten pool, the grains show a transition from equiaxed crystal to columnar crystal and the grain size increases. All the LAMed samples only consist of α-Mg phase and a lesser degree of MgO phase. The microhardness of LAMed samples is between 60 HV and 68 HV and the microhardness is superior to that of as-cast pure magnesium, which is mainly attributed to the grain refinement and solid solution strengthening. As laser energy input increases, the porosity decreases and the compression performance enhances meanwhile. The LAMed porous Mg-Ca alloy is a promising biodegradable material for future clinical application.

Numerical analysis of combustion of a hydrogen–air mixture in an advanced ramjet combustor model during activation of O2 molecules by resonant laser radiation

This paper presents a numerical study of the combustion of a hydrogen–air mixture in a model ramjet combustor with separate hydrogen and air supply during activation of O2 molecules by resonant laser radiation at a wavelength of 762.3 nm and 193.3 nm. The calculation is made using the parabolized Navier–Stokes equations taking into account chemical reactions, laser irradiation, and the nonuniformity of air parameters at the combustor inlet due to the complex gas-dynamic structure of the flow in the air intake. It is shown that the combustion completeness at the combustor outlet can be increased by a factor of 2.8 by redistributing the hydrogen supply through the system of fuel tank pylons. Further increase in the combustion completeness can be obtained by exposure of a narrow flow region to resonant laser radiation, more effectively at a wavelength of 193.3 nm. The combination of laser exposure with hydrogen supply redistribution increases the combustion efficiency by a factor of more than 4.7 compared to the base case. In this case, this provides a 95% increase the longitudinal force component in the portion of the internal engine duct that provides a positive contribution to the thrust. Estimation of the energy efficiency of using laser radiation shows that the laser energy input required to achieve this effect is 40–80 times (depending on the fuel supply method) less than the increase in the chemical energy (compared to the case of no laser exposure) released due to fuel combustion.

An investigation on selective laser melting of Al-Cu-Fe-Cr quasicrystal: From single layer to multilayers

In this study, the effects of processing parameters on the microstructure of Al-Cu-Fe-Cr quasicrystalline (QC) coatings fabricated by selective laser melting (SLM) are investigated. A qualitative analysis on the XRD patterns indicates that the phase composition for the SLM processed coating mainly consisted of Al-Cu-Fe-Cr quasicrystals and α-Al (CuFeCr) solid solution, and with increasing laser energy input or coating thickness, the volume fraction of QC i-Al91Fe4Cr5 reduced and those of QC d-Al65Cu20Fe10Cr5 and crystalline θ-Al2Cu increased. The formation of cracks during the coating building procedure from single layer to multilayers is also discussed. For the coatings with the same layer number, the pores and balling particles diminish as laser power increases, due to the growth of melting degree. At the early stage of fabrication, with increment of layer number (or coating thickness), pores and balling particles decrease considerably because the molten pool solidified more “slowly”. However, after the layer number increases continuously from 10 to 20, the porosity no longer decreases, and some big size pores, microcracks and fractures appear, especially for the sample obtained at lower laser power. A wavy-like pattern composed chiefly of Al and QC phases, is formed at the interfacial region between substrate and coating due to Marangoni effect.

Characteristics of radio-frequency emission from nanosecond laser-induced breakdown plasma of air

The radio-frequency (RF) emissions in a range from 30 MHz to 800 MHz from the plasma, which is produced by the nanosecond laser (532 nm, 8 ns) induced breakdown of atmospheric air, are presented. A spectrum analyzer which can scan over a spectral range of 9 kHz-26.5 GHz is used to record the RF-range radiation intensities of the emission from the plasma. RF electromagnetic radiations from the laser induced breakdown of atmospheric air are obtained for different input laser energies. A half-wave plate and a Glan prism are used to vary the input laser energy. Experimental results show that the intensities of RF radiation in a range of 30-200 MHz increase with the increase of laser energy, but the intensities of RF radiation in a 360-600 MHz frequency range decrease. To study the effect of input laser polarization on the RF radiation, we adopt the input lasers with vertical and horizontal polarization respectively. When the polarizations of the input laser and the antenna are the same, the RF radiation intensity is relatively high, and the frequency lines are relatively abundant. The changing relationship between the total power of RF radiation and the energy of the input laser is calculated and analyzed. It is observed that the total power of RF radiation first increases and then decreases with the increase of input laser energy. The influences of the plasma electron density on the plasma frequency and the plasma attenuation coefficient are investigated to explain the relationship between the total power of the RF radiation and the laser energy. The RF radiation is caused by the following processes. The generated electrons and ions are accelerated away from the core by their thermal pressures. This leads to charge separation and forming the electric dipole moments. These oscillating electric dipoles radiate electromagnetic waves in the RF range. Furthermore, the interactions of electrons with atomic and molecular clusters within the plasma play a major role in RF radiation, and the low frequency electromagnetic radiation takes place from the plasma that is far from fully ionized state. Further study of the characteristics of RF electromagnetic radiation is of great significance for understanding the physical mechanism of the interaction between laser and matter.

Mechanisms and characteristics of spatter generation in SLM processing and its effect on the properties

Selective Laser Melting (SLM) is a powder bed based technology to fabricate metal parts through laser melting, it provides excellent mechanical properties and freedom. The authors study the influence of laser energy on spattering, the investigation analyzed the formation principle, appearance and compositions of spattering. Results indicate that as the laser energy input increases from 0.32×105W/cm3 to 1.30×105W/cm3, the intensity and the quantity of spattering increases, the metal liquid jetted out even reach to the height of 11cm. Major sources of spattering included three types, which were mainly caused by recoil pressure, Marangoni effect and heat effect in molten pool, these three different sources of spattering leading to three types of spattering morphology correspondingly. The solidified spattering particles have an average size of approximately 162μm, much larger than the original powder size of 32μm, and these spatter particles present various appearances. The compositions of spattering powers are almost the same as the original powders, but the contents of O, Si and C increase dramatically. The spattering particles are embedded into the surface and interior of the SLM-fabricated parts. These results are helpful in controlling the intensity of spattering, improving stability and repeatability of the SLM fabrication process.

RESIDUAL STRESS IN TI6AL4V OBJECTS PRODUCED BY DIRECT METAL LASER SINTERING

Direct Metal Laser Sintering produces 3D objects using a layer-by- layer method in which powder is deposited in thin layers. Laser beam scans over the powder fusing powder particles as well as the previous layer. High-concentration of laser energy input leads to high thermal gradients which induce residual stress within the as- built parts. Ti6Al4V (ELI) samples have been manufactured by EOSINT M280 system at prescribed by EOS process-parameters. Residual stresses were measured by XRD method. Microstructure, values and directions of principal stresses inTi6Al4V DMLS samples were analysed.

Sintering densification behaviors and microstructural evolvement of W-Cu-Ni composite fabricated by selective laser sintering

The sintering densification behavior and microstructural evolvement of W-Cu-Ni alloys (90 wt% W, 5 wt% Cu, 5 wt% Ni) were investigated via selective laser sintering (SLS). Through the analysis of the surface morphology and microstructure obtained by different laser sintering parameters, revealing that the sintering mechanism evolves from solid state sintering to liquid phase sintering depends on the laser input energy. The sintered density initially increases with the laser input energy then decreases; the poring and balling phenomenon cannot be eliminated drastically. The SLS of W-5Cu-5Ni (wt%) alloys is a complete process of molten liquid phase spreading and solidifying, which is determined by the intrinsic properties of W/Cu and the laser sintering parameters. Setting a moderate temperature for the molten pool would restrain the phenomenon of balling and poring, so as to achieve a sintered sample which is nearly densified. Combined with composition and phase analysis, Ni-Cu is acting as the binder phase, adding Ni additive promotes partially dissolution of W, meanwhile most nonmelting W is acting as structural phase.

Optimizing Adhesion of Laser Structured Plated Ni-Cu Contacts with Insights from Micro Characterization

An improved understanding of the interface situation between metal and laser-structured semiconductor for nickel-copper plated solar cell contacts is created by micro characterization using SEM and Raman spectroscopy. A special focus is set on laser ablation with high energy density and the resulting mechanical contact adhesion. This aims to increase the flexibility of the process towards variable cell and emitter designs. The analysis revealed two neglected aspects. First, on laser openings that were created with high laser energy input the thickness of the native oxide exceeds the typical dimension of 1nm. The oxide layer allows metal plating, but hinders the growth of nickel silicide and thus affects the mechanical properties of the contacts. As shown in this work the adhesion can be improved by a more thorough etching of the oxide before plating. Second, Raman spectroscopy revealed that the tension under the laser processed surfaces differs for varying energy inputs only after thermal silicidation was performed. Inadequately chosen laser parameters causes high tension in the laser treated silicon causing decreased adhesion. This work reveals the value of microscopic analysis for the optimization of laser processes for metallization.

Experimental study of conditions for the 3D laser cladding of nickel aluminide

The layerwise laser cladding of powdered alloy based on intermetallic gamma Ni3Al phase is studied. The effect deposition parameters have on the geometry of the deposited beads is shown. Microstructures are investigated and the cracking susceptibility of the deposited material is analyzed. The effective deposition parameters are determined within a range of specific laser energy inputs of (2–8) × 106 J kg−1 at beam scanning rates of (1.67–10) × 10−3 m/s and a powder feed of 6.3 × 10−5 kg/s−1.

Accurate propagation velocity measurement of laser supported detonation waves

One dimensional experimental data of a Laser Supported Detonation (LSD) wave is necessary for validation of the LSD propagation model. In this study, a LSD was driven in a pulsed CO2 laser beam whose equivalent diameter was 7.2 mm, which was sufficient to neglect lateral enthalpy dissipation in comparison with laser energy input. In order to evaluate this relation, an identical LSD wave propagation velocity was obtained with and without lateral flow confinement by a solid wall. In addition, accurate prediction of the LSD propagation velocities were obtained. These were same in a beam of 5.1 mm diameter while it was about twice as high as in a beam of 1.2 mm diameter. In the high laser intensity region, the LSD propagation velocity was higher than that of Chapman-Jouguet condition.

The influence of processing parameters on aluminium alloy A357 manufactured by Selective Laser Melting

The aim of this work is to investigate the effects of Selective Laser Melting (SLM) on the microstructure and mechanical properties of A357 aluminium alloys. The SLM processing parameters were optimised to achieve maximum density, corresponding to an extremely fine microstructure with very few pores. This translates to differences in mechanical properties compared to conventional cast alloy. Porosity in SLMed A357 Al samples was analysed based on relative density versus laser parameter and energy input curves. Substrate temperatures and the combination of laser parameters influence the mechanical properties via changes in melt pool morphology and eutectic Si cell characteristics. The anisotropy of SLMed Al samples is explained in relation to the directionality of the microstructure based on differences in the deformation response of horizontal and vertical tensile samples. Fractographic studies have been performed to understand tensile properties by comparing fracture surfaces of tensile samples with microstructural features in different planes. This has led to an explanation of why the tensile properties are better for the horizontal test samples than for the vertical ones in an as-SLMed material.

Effect of Laser Input Energy on the Laser Joining of Polyethylene Terephthalate to Titanium

Effect of Laser Input Energy on the Laser Joining of Polyethylene Terephthalate to Titanium

Numerical analysis of heat transfer during multi-layer selective laser melting of AlSi10Mg

A three-dimensional finite element model is developed to simulate multi-layer deposition of AlSi10Mg in selective laser melting (SLM) by using commercial ANSYS software. The temperature distribution, thermal history, molten pool depth, remelting depth, cooling rate and solidification morphology parameter in multi-layer buildup process are investigated. Furthermore, the effect of laser energy input on the melting and solidification process is evaluated. The results show that the temperature and molten pool depth gradually increases with new layers and energy input. The cooling rate increases progressively while the solidification morphology parameter decreases with the increase of energy input. It leads to obtain fine grains with low energy input. To verify the accuracy of simulated results, experiments are performed. Compared simulated results with experimental ones, good agreement is obtained.

Particle swarm optimization algorithm for ignition of hydrogen isotopes (deuterium–tritium) pellets

Recent improvements on high power lasers have made access to dense and hot plasmas possible realizing the inertial confinement fusion (ICF). Results from Osaka University experiments found that higher energy gains are reachable through ideal adiabatic shock-free volume ignition. Laser pulses limit the range of laser input energy. Thus, it becomes necessary to make efficient use of available energy to obtain the highest gain. The present paper aims to investigate the attainment of highest gain for D–T fuel using this energy with iteration model. It is shown that the maximum gain of 1.36 × 106 is obtained only at input energy of 7.27 × 1010 J, volume of 0.586 cm3 and 8133times of solid state density.

High-temperature imprinting and superhydrophobicity of micro/nano surface structures on metals using molds fabricated by ultrafast laser ablation

Micro/nano surface structures show great significance in both fundamental research and practical applications. Imprinting technique is widely used to fabricate micro/nano surface structures on polymers at low temperature, but barely implemented on metals. In this paper, femtosecond (fs) laser was used to fabricate micro/nano surface structures on tungsten (W), which served as molds for subsequent solid state imprinting process to replicate the counter structures onto copper (Cu) surface at temperature up to 1000°C. Nano-ripples superimposed on micro-bumps array are fabricated without surface oxidation, and the height of the micro-bumps is adjustable by varying the laser energy input for W molds preparation. The as-prepared Cu surface structures are hydrophobic and turn to superhydrophobic after chemical modification. To demonstrate the feasibility and capability of this technique, similar surface structures were fabricated on A356 aluminum alloy (A356 AA) by liquid state imprinting at 740°C using M2 steel molds prepared by picosecond (ps) laser ablation. This approach combines the advantages of ultrafast laser ablation to fabricate molds with micro/nano surface structures and high-temperature imprinting process to replicate the micro/nano structures onto target metallic surfaces. It opens new possibility for the mass fabrication of functional micro/nano surface structures on metals.

The effects of electron thermal radiation on laser ablative shock waves from aluminum plasma into ambient air

The effect of electron thermal radiation on 7 ns laser ablative shock waves from aluminum (Al) plasma into an ambient atmospheric air has been numerically investigated using a one-dimensional, three-temperature (electron, ion, and radiation) radiation hydrodynamic code MULTI. The governing equations in Lagrangian form are solved using an implicit scheme for planar, cylindrical, and spherical geometries. The shockwave velocities (Vsw) obtained numerically are compared with our experimental values obtained over the intensity range of 2.0 × 1010 to 1.4 × 1011 W/cm2. It is observed that the numerically obtained Vsw is significantly influenced by the thermal radiation effects which are found to be dominant in the initial stage up to 2 μs depending on the input laser energy. Also, the results are found to be sensitive to the co-ordinate geometry used in the simulation (planar, cylindrical, and spherical). Moreover, it is revealed that shock wave undergoes geometrical transitions from planar to cylindrical nature and from cylindrical to spherical nature with time during its propagation into an ambient atmospheric air. It is also observed that the spatio-temporal evolution of plasma electron and ion parameters such as temperature, specific energy, pressure, electron number density, and mass density were found to be modified significantly due to the effects of electron thermal radiation.

Empirical investigation of environmental characteristic of 3-D additive manufacturing process based on slice thickness and part orientation

The significant amount of research has been done in improving the mechanical properties (compressive strength), dimensional accuracy (length, height and width), and build time of the components manufactured from the additive manufacturing process. In contrast to this, the research in the optimization of environmental characteristic i.e. energy consumption for the additive manufacturing processes such as selective laser sintering (SLS), and selective laser melting (SLM) needs significant attention. These processes intakes the significant portion of input laser energy for driving the laser system, heating system and other machine components. With world moving towards globalization of additive manufacturing processes, the optimization of laser energy consumption thus become a necessity from productivity and as well as an environmental perspective. Therefore, the present work performs the empirical investigation by proposing the optimization framework in modelling of laser energy consumption of the SLS process. The experimental procedure involves the computation of energy consumption by measuring the total area of sintering. The optimization framework when applied on the experimental data generates the functional expression for laser energy consumption which suggests that the slice thickness is a vital parameter in optimizing it. The implications arising from the study is discussed.

Low f-number photoacoustic lens for tight ultrasonic focusing and free-field micro-cavitation in water

We demonstrate a photoacoustic lens with a low f-number of 0.61 and a high focal gain of 220 at 15-MHz frequency for laser-generated focused ultrasound (LGFU), which enables free-field micro-cavitation in water. Due to tight ultrasonic focusing (90 μm in lateral and 200 μm in longitudinal spot widths at a distance of 9.2 mm), the lens produces a peak pressure of 20 MPa (positive) using an input laser energy of only 1 mJ/pulse (6-ns temporal width). Remarkably, we confirm single-pulsed micro-cavitation in a free-field condition by using this lens, which has not previously been achieved with LGFU. The free-field cavitation was monitored and characterized in terms of a bubble radius, a lifetime, and a probability. Our result demonstrates that LGFU amplitudes can be sufficiently higher than a threshold for free-field cavitation at a microscale spot, which is a crucial step for cavitation-based therapy with high precision.