Laser Energy Input Research Articles

Laser shock compression induced crystallization of Ce3Al metallic glass

Laser shock compression studies on Ce3Al metallic glass performed using a 3 J Nd:YAG laser indicate shock-induced crystallization, evidenced by the presence of a two-wave/stepped particle velocity profile and structural changes observed via X-ray Diffraction (XRD) analysis of recovered material. A direct shock-compression setup was designed with 25 μm thick Ni driver foil, 40 μm thick Ce3Al metallic glass ribbon, and 3 mm thick poly(methyl methacrylate) (PMMA) backer window for use with input laser energies varying from 100 to 2000 mJ and corresponding estimated peak pressures of 1.4 to 4.1 GPa in Ce3Al. At shock pressures below ∼1.8 GPa (300 mJ laser input energy), samples were recovered showing no obvious deformation or structural changes evidenced via XRD analysis. At higher laser energies and shock pressures above the elastic limit, samples were recovered showing visible deformation and crystallization evidenced by Rietveld analysis of diffraction patterns. The corresponding velocity profiles also showed a stepped wave structure, increasing in magnitude with energy. The overall results reveal possible densification of the glass due to delocalization of 4f electrons in Ce at lower laser shock pressures and increased crystallization with preferred orientation and distortion of the nanocrystals at higher shock compression conditions.

Microhole Drilling by Double Laser Pulses With Different Pulse Energies

Previous investigations on “double-pulse” nanosecond (ns) laser drilling reported in the literature typically utilize double pulses of equal or similar pulse energies. In this paper, “double-pulse” ns laser drilling using double pulses with energies differing by more than ten times has been studied, where both postprocess workpiece characterizations and in situ time-resolved shadowgraph imaging observations have been performed. A very interesting physical phenomenon has been discovered under the studied conditions: the “double-pulse” ns laser ablation process, where the low-energy pulse precedes the high-energy pulse (called “low-high double-pulse” laser ablation) by a suitable amount of time, can produce significantly higher ablation rates than “high-low double-pulse” or “single-pulse” laser ablation under a similar laser energy input. In particular, “low-high double-pulse” laser ablation at a suitable interpulse separation time can drill through a ∼0.93 mm thick aluminum 7075 workpiece in less than 200 pulse pairs, while “high-low double-pulse” or “single-pulse” laser ablation cannot drill through the workpiece even using 1000 pulse pairs or pulses, respectively. This indicates that “low-high double-pulse” laser ablation has led to a significantly enhanced average ablation rate that is more than five times those for “single-pulse” or “high-low double-pulse” laser ablation. The fundamental physical mechanism for the ablation rate enhancement has been discussed, and a hypothesized explanation has been given.

Mechanical behaviour of additively manufactured lunar regolith simulant components

Additive manufacturing and its related techniques have frequently been put forward as a promising candidate for planetary in-situ manufacturing, from building life-sustaining habitats on the Moon to fabricating various replacements parts, aiming to support future extra-terrestrial human activity. This paper investigates the mechanical behaviour of lunar regolith simulant material components, which is a potential future space engineering material, manufactured by a laser-based powder bed fusion additive manufacturing system. The influence of laser energy input during processing was associated with the evolution of component porosity, measured via optical and scanning electron microscopy in combination with gas expansion pycnometry. The compressive strength performance and Vickers micro-hardness of the components were analysed and related back to the processing history and resultant microstructure of the lunar regolith simulant build material. Fabricated structures exhibited a relative porosity of 44–49% and densities ranging from 1.76 to 2.3 g cm−3, with a maximum compressive strength of 4.2 ± 0.1 MPa and elastic modulus of 287.3 ± 6.6 MPa, the former is comparable to a typical masonry clay brick (3.5 MPa). The additive manufacturing parts also had an average hardness value of 657 ± 14 HV0.05/15, better than borosilicate glass (580 HV). This study has shed significant insight into realising the potential of a laser-based powder bed fusion additive manufacturing process to deliver functional engineering assets via in-situ and abundant material sources that can be potentially used for future engineering applications in aerospace and astronautics.

Laser additive manufacturing of Zn metal parts for biodegradable applications: Processing, formation quality and mechanical properties

Zn based metals have exhibited promising applications for biodegradable implants. Only a handful of very recent reports were found on additive manufacturing of Zn metal by selective laser melting (SLM). This work provided a systematic study on densification, surface roughness and mechanical properties regarding SLM of Zn metal. Single track surface after laser melting was rugged and twisted with a large amount of attached particles due to severe Zn evaporation. For SLM produced solid parts, the relative density was more than 99.50%; the surface roughness Sa was 9.15–10.79 μm for as-melted status and 4.83 μm after sandblasting, both comparable to optimal results obtained by SLM of common metals. The microstructure was made up of fine columnar grains. The average values of hardness, elastic modulus, yield strength, ultimate strength and elongation were 42 HV, 23GPa, 114 MPa, 134 MPa, and 10.1% respectively, better than those obtained by most manufacturing methods. The superior mechanical properties were attributed to high densification and fine grains resulted by optimal processing control of Zn evaporation and laser energy input. Cardiovascular stents were printed out to demonstrate additive manufacturing ability for complicated structures. All the results indicate the encouraging prospect of additive manufacturing of Zn based metals for biodegradable applications.

Experimental Evidence of Kinetic Effects in Indirect-Drive Inertial Confinement Fusion Hohlraums.

We present the first experimental evidence supported by simulations of kinetic effects launched in the interpenetration layer between the laser-driven hohlraum plasma bubbles and the corona plasma of the compressed pellet at the Shenguang-III prototype laser facility. Solid plastic capsules were coated with carbon-deuterium layers; as the implosion neutron yield is quenched, DD fusion yield from the corona plasma provides a direct measure of the kinetic effects inside the hohlraum. An anomalous large energy spread of the DD neutron signal (∼282 keV) and anomalous scaling of the neutron yield with the thickness of the carbon-deuterium layers cannot be explained by the hydrodynamic mechanisms. Instead, these results can be attributed to kinetic shocks that arise in the hohlraum-wall-ablator interpenetration region, which result in efficient acceleration of the deuterons (∼28.8 J, 0.45% of the total input laser energy). These studies provide novel insight into the interactions and dynamics of a vacuum hohlraum and near-vacuum hohlraum.

Production and Applications of Spin-Polarized Isotopes of Noble Gases

The development of a new direction of research on the production and application of spin-polarized isotopes of noble gases, 3He and 129Xe, is overviewed. Methods of laser hyperpolarization, problems of enhancing the efficiency of laser energy input, and methods of storing hyperpolarized (HP) isotopes are described. Examples and advantages of using HP isotopes in fundamental physics, engineering, medicine, and biology, as well as the progress in the creation of biosensors on hyperpolarized noble gases, are discussed. It has been shown that the study of protein structures and host–guest molecular complexes can prove useful in searching for means of the targeted delivery of radioactive isotopes (radiopharmaceuticals) in nuclear medicine. It is concluded that the progress in modern technologies for producing miniature electronic devices is suggestive of an imminent emergence of small-size scanners for human brain research. At the same time, a high sensitivity of the method is expected to provide the possibility of studying not only the structure of tissues and bloodstream, but also the response of the brain to various stimuli, and even cognitive functions.

Effect of laser energy density on microstructures and mechanical properties of selective laser melted Ti-6Al-4V alloy

Abstract Ti-6Al-4V samples were fabricated by selective laser melting (SLM) with a series of different laser energy inputs. The microstructure presented a feature of fine acicular martensite due to the high cooling rate of the SLM process. All the relative density, micro-hardness and tensile properties initially increased and then decreased with increasing laser energy density. The fracture morphology exhibited a mixed mode of complex ductile and brittle fracture. All the results indicated that the laser energy density has a crucial effect on the microstructures and mechanical properties of the SLMed Ti-6Al-4V alloy. The correlations between the laser energy density and the microstructures and mechanical properties of the SLMed Ti-6Al-4V alloy were studied.

The dependence of external focusing geometries and polarization in generation of supercontinuum by femtosecond laser pulse in air

Supercontinuum (SC) generation in ambient air using different focusing geometries and laser energy under linear and circular polarization was investigated experimentally. Under the same input laser energy, the SC spectra in the visible range became wider when smaller f-number convex lens are used and the linearly polarized laser is used as the pump source, the main nonlinear mechanism for asymmetric spectral broadening of femtosecond laser pulses towards the higher frequencies in isotropic media is that the cascade generation with GHz spectral shift for gases and the four-photon parametric wave mixing processes. Besides, the temporal differential of electron density Ne(t) play a more important role in extending the SC spectra to the blue side than the interaction length l. Under the same magnitude of the optical field and the critical power for self-focusing of linearly and circularly polarized beams, the SC spectra range is wider when circularly polarized beams as the pump source.

Corrigendum to “Quantification of mass-specific laser energy input converted into particle properties during picosecond pulsed laser fragmentation of zinc oxide and boron carbide in liquids” [Appl. Surf. Sci. 348 (September) (2015) 22–29

Corrigendum to “Quantification of mass-specific laser energy input converted into particle properties during picosecond pulsed laser fragmentation of zinc oxide and boron carbide in liquids” [Appl. Surf. Sci. 348 (September) (2015) 22–29

Study of Selective Laser Melting of intermetallic TiAl powder using integral analysis

Abstract Selective laser melting (SLM) of intermetallic Ti–48Al–2Cr–2Nb powder is studied using different and complementary methods: (a) experimental parametric analysis (variation of laser power, beam scanning velocity, etc.); (b) metallography including X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM); (c) optical monitoring by infra-red (IR) camera, high speed pyrometer and (d) mathematical modelling. The advantage of the present approach is based on parallel application of diverse methods of analysis using the same SLM set-up and feedstock powder. Also the novelty is related to the preheating of the entire manufacturing module up to 450 °C which is applied before and during the manufacturing of the samples. XRD analysis shows domination of the α2-Ti3Al phase in the initial powder, heat treated powder before SLM and in the developed samples. Some minor peaks of γ-TiAl are identified in SLM parts. EDS analysis confirms the effect of Al evaporation, which intensifies with laser energy input. IR-camera is used for optical monitoring of a single track formation. The geometry of the thermal emission field for different processing parameters is compared with the track geometry. The intensification of hydrodynamic instabilities accompanied by material rejection from the zone of powder consolidation with beam scanning velocity is found. For the first time the evaporation of Al for elevated laser energy input is clearly detected by the IR-camera. Mathematical modelling shows that the same elementary volume of SLM sample can be remelted several times during its fabrication depending on the SLM processing parameters and beam scanning strategy. The maximum values of calculated cooling rates are in the order of 106 °C/s. For the first time the thermo-cycling during SLM parts fabrication is confirmed experimentally by pyrometer measurements.

Selective laser melting of stainless steel and alumina composite: Experimental and simulation studies on processing parameters, microstructure and mechanical properties

Metal matrix composites (MMC) find their uses as high performance materials. The selective laser melting (SLM) of a 316L stainless steel and Al2O3 MMC is presented in this paper. Agglomerate Al2O3 particles had shown to be an adequate powder choice with uniform dispersions in the resultant prints. Relative density, phase, microstructure and mechanical properties of all 1-, 2-, 3-wt% doped products were carefully analyzed. Finite element modeling model was developed to study the associated multi-physics phenomena with high efficiency for process parameter optimization. It is found that the change in SLM temperature profile with Al2O3 addition is mainly due to the change in optical properties rather than thermal. Hence, both simulation and experimentation revealed that higher laser energy input is needed for optimized melting. In addition, cellular dendrites were found to coarsen with increasing Al2O3 addition due to the decreased cooling rate. With hard particle strengthening effects, all samples showed improved hardness with 3-wt% up to 298HV and 1-wt% samples showing much improved yielding and tensile stresses of 579 and 662MPa from 316L. Corresponding microlattice built this way demonstrated a 30 and 23% increase in specific strength and energy absorption from that of 316L too.

Effects of Laser Energies on Wear and Tensile Properties of Biomimetic 7075 Aluminum Alloy

Inspired by the non-smooth surface of certain animals, a biomimetic coupling unit with various sizes, microstructure, and hardness was prepared on the surface of 7075 aluminum alloy. Following experimental studies were conducted to investigate the wear and tensile properties with various laser energy inputs. The results demonstrated that the non-smooth surface with biomimetic coupling units had a positive effect on both the wear resistance and tensile property of 7075 aluminum alloy. In addition, the sample with the unit fabricated by the laser energy of 420.1 J/cm2 exhibited the most significant improvement on the wear and tensile properties owing to the minimum grain size and the highest microhardness. Also, the weight loss of the sample was one-third of the untreated one’s, and the yield strength, the ultimate tensile strength, and the elongation improved by 20, 20, and 34% respectively. Moreover, the mechanisms of wear and tensile properties improvement were also analyzed.

Relationship between laser energy input, microstructures and magnetic properties of selective laser melted Fe-6.9%wt Si soft magnets

Selective Laser Melting (SLM), a powder-bed Additive Manufacturing technology, can be used in combination with high temperature post-annealing to produce high-silicon steel parts characterised by quasi-static magnetic properties comparable to those of commercial electrical steel. However, the role of the as-built microstructure on the magnetic properties is still unexplored. Therefore, in this study the effect of the energy input of the processing laser on the magnetic properties of the material is investigated. The magnetic properties are determined for 4 mm-high rings obtained using three laser energy input values that provide a good compromise in terms of porosity and crack formation. The best magnetic properties are obtained for the rings built using a value of energy input that produces a strong fibrous crystallographic texture, in which one of the crystallographic 〈001〉 axes is preferentially aligned along the build direction. Whether the magnetic properties change with sample direction as a result of the crystallographic texture will be the subject of future research.

Influence of phase error of optical elements on optical path design of laser facilities

Optical path design of high power laser facilities should consider several optimization measures such as those that are related to image transmission, ghost avoidance, and stray light management. According to the diffraction optical propagation theory, we study the the influences of wavefront characteristics of large aperture optical components on optimizing the design parameters of optical path in view of increasing the output load. The results show that the arrangement interval of the last stage optical drive can be very useful in improving the output load of the laser facilities if it is controlled to be over 6 m long. In general, a large aperture optical element with a phase error peak value of 0.34 can reduce the near field beam quality of a high-power laser by about 10% and give rise to a maximum decrease of about 21% when the phase error reaches 1.36. Superposition of multiple optical elements with different phase error distribution characteristics can reduce the negative effect of the mid frequency phase error. However, the nonlinear effect of large aperture optical components can aggravate the influence of the intermediate frequency phase error on the damage resistance capacity of the device. Under the premise that the damage threshold of the large caliber optical element is limited to 20 J/cm2, the using of a laser facility with a compact optical path, with an input laser energy density controlled to be below 16.8 J/cm2, will avoid damaging the optical components efficiently. A relatively flexible optical layout can further increase the average energy density of the final output laser and is very beneficial to the enhancing of the output load capacity of the laser facility.

Excitation-wavelength dependent terahertz wave polarization control in laser-induced filament

We examine the terahertz (THz) emission from air filament driven by two-color lasers with relatively longer wavelengths than 800 nm. The THz energy dependence on the input laser energy increases more rapidly with a longer laser wavelength, and the scaling laws of THz energy as a function of fundamental wavelength vary for different optical powers, which is theoretically validated by considering the optical wavelength-dependent ionization rate. Furthermore, the THz polarization undergoes a continuous rotation as a function of the laser wavelength, since the relative phase and polarization of the two pulses are adjusted through changing the excitation wavelength in the frequency doubling crystal. Our results contribute to the understanding of THz wave generation in a femtosecond laser filament and suggest a practical way to control the polarization of terahertz pulses for potential applications.

Integrated experimental and computational approach to laser machining of structural bone

This study describes the fundamentals of laser–bone interaction during bone machining through an integrated experimental-computational approach. Two groups of laser machining parameters identified the effects of process thermodynamics and kinetics on machining attributes at micro to macro. A continuous wave Yb-fiber Nd:YAG laser (wavelength 1070 nm) with fluences in the range of 3.18 J/mm2–8.48 J/mm2 in combination of laser power (300 W–700 W) and machining speed (110 mm/s–250 mm/s) were considered for machining trials. The machining attributes were evaluated through scanning electron microscopy observations and compared with finite element based multiphysics-multicomponent computational model predicted values. For both groups of laser machining parameters, experimentally evaluated and computationally predicted depths and widths increased with increased laser energy input and computationally predicted widths remained higher than experimentally measured widths whereas computationally predicted depths were slightly higher than experimentally measured depths and reversed this trend for the laser fluence >6 J/mm2. While in both groups, the machining rate increased with increased laser fluence, experimentally derived machining rate remained lower than the computationally predicted values for the laser fluences lower than ∼4.75 J/mm2 for one group and ∼5.8 J/mm2 for other group and reversed in this trend thereafter. The integrated experimental-computational approach identified the physical processes affecting machining attributes.

Influence of SLM scan-speed on microstructure, precipitation of Al3Sc particles and mechanical properties in Sc- and Zr-modified Al-Mg alloys

Additive processing via Selective Laser Melting (SLM) of Sc- and Zr-modified Al-Mg alloys (commercially known as Scalmalloy®) provides significant advantages over traditional 4xxx casting alloys. This is due to the high strength and ductility at very low mechanical anisotropy, which is a result of the alloys' very fine-grained microstructure combined with weak texture in the build-up direction. Next to their advantageous mechanical properties, the reliability of alloys processed by additive manufacturing is of great importance. Variations in microstructural features due to the use of different in processing parameters (e.g. laser scan speed) are well known for traditional alloys. This study analyses the influence of varying laser scan speeds on the static mechanical properties of SLM-processed Scalmalloy®, and discusses the evolving microstructure and precipitation of nm-sized Al3Sc particles at the correspondingly different energy inputs. It is found that by about doubling the laser scan speed the peak grain sizes in the fine-grained regions decrease from 1.1μm to 600nm, whereas in the coarse grained region almost no influence is observed. Al3Sc particles are only precipitated during processing at low scan speeds due to the corresponding high laser energy input, or by the intrinsic heat treatment from subsequent layers being built.

Investigation into the influence of laser energy input on selective laser melted thin-walled parts by response surface method

Selective laser melting (SLM) provides a feasible way for manufacturing of complex thin-walled parts directly, however, the energy input during SLM process, namely derived from the laser power, scanning speed, layer thickness and scanning space, etc. has great influence on the thin wall's qualities. The aim of this work is to relate the thin wall's parameters (responses), namely track width, surface roughness and hardness to the process parameters considered in this research (laser power, scanning speed and layer thickness) and to find out the optimal manufacturing conditions. Design of experiment (DoE) was used by implementing composite central design to achieve better manufacturing qualities. Mathematical models derived from the statistical analysis were used to establish the relationships between the process parameters and the responses. Also, the effects of process parameters on each response were determined. Then, a numerical optimization was performed to find out the optimal process set at which the quality features are at their desired values. Based on this study, the relationship between process parameters and SLMed thin-walled structure was revealed and thus, the corresponding optimal process parameters can be used to manufactured thin-walled parts with high quality.

Porosity formation mechanisms and fatigue response in Al-Si-Mg alloys made by selective laser melting

Defects in Al-7Si-Mg and Al-10Si-Mg alloys produced by selective laser melting are categorised into three types. The first type are large irregular-shaped defects with unmelted powder particles, formed due to a lack of fusion as a result of insufficient volumetric energy density. The second type are small round gas pores below 5µm in diameter, associated with high area energy density. These pores enlarge during solution heat treatment, but the enlargement is reduced significantly when the powder is pre-dried at 200°C for 16h under an argon atmosphere immediately before the build. The last type are large round keyhole type pores located at the base of melt pools. They can either form in contour scan regions, at the edges of core scans, or at island boundary overlap regions due to an excessive local energy density compared with the nominal energy density. Sub-surface porosity due to contour and core edge keyhole type defects can be more detrimental to the fatigue performance than net-shaped rough surfaces, but such sub-surface porosity can be minimised by either lowering the laser energy input for the contour scan and/or changing the way the laser turns between scan tracks.

The influences of melting degree of TiC reinforcements on microstructure and mechanical properties of laser direct deposited Ti6Al4V-TiC composites

This study is concerned with the influences of melting degree of embedded TiC reinforcements on microstructure and mechanical properties of laser direct deposited Ti6Al4V-TiC composites and a functionally graded material. The melting degree of embedded TiC was controlled by the input laser energy density and the added TiC content. The formation of detrimental primary dendritic TiC grains was successfully avoided by properly adjusting the deposition conditions and the particle size range of TiC reinforcements. The resultant compression test revealed the ultimate strength increasing from 1381±19MPa to 1636±23MPa as the premixed TiC content increased from 0 to 15vol% while a true strain of 0.141±0.002 was still retained for 15vol% TiC. The primary strengthening mechanism for composites with the most melting control of TiC is the solid solution strengthening induced by carbon, while that for the least melting control is dominated by the unmelted TiC particulates and the refined microstructure resulting from the resolidified carbides. The defect-free functionally graded Ti6Al4V-TiC with 0 to 40vol% TiC achieved an increased hardness from HRC ~39 to HRC ~65.