Strategy of manufacturing components with designed internal structure by selective laser melting of metallic powder

Application of selective laser melting for manufacturing three-dimensional objects represents one of the promising directions to solve challenging industrial problems. This approach permits to extend dramatically the freedom of design and manufacture by allowing, for example, to create an object with desired shape and internal structure in a single fabrication step. The design of the part can be tailored to meet specific functions and properties (e.g. physical, mechanical, chemical, biological, etc.) using different materials. Metallic objects were manufactured by Phenix PM 100 machine from Inconel 625 powder. The objective was to analyze the influence of the manufacturing strategy on the internal structure and mechanical properties of the components manufactured by selective laser melting technology. Anisotropy of the internal structure and mechanical properties of the fabricated objects were studied.     

Alumina-zirconium ceramics synthesis by selective laser sintering/melting

In the present paper, porous refractory ceramics synthesized by selective laser sintering/melting from a mixture of zirconium dioxide, aluminum and/or alumina powders are subjected to optical metallography and X-ray analysis to study their microstructure and phase composition depending on the laser processing parameters. It is shown that high-speed laser sintering in air yields ceramics with dense structure and a uniform distribution of the stabilizing phases. The obtained ceramic-matrix composites may be used as thermal and electrical insulators and wear resistant coating in solid oxide fuel cells, crucibles, heating elements, medical tools. The possibility to reinforce refractory ceramics by laser synthesis is shown on the example of tetragonal dioxide of zirconium with hardened micro-inclusion of Al₂O₃. By applying finely dispersed Y₂O₃ powder inclusions, the type of the ceramic structure is significantly changed.     

Manufacturing of fine-structured 3D porous filter elements by selective laser melting

Selective laser melting (SLM) allows manufacturing porous 3D parts with customized near-net shape and internal geometry designed at the stage of their computer modeling. The relations between laser operational parameters, computer design of the manufacturing object, composition and microstructure of the obtained fine porous structures are discussed. A series of experiments are carried out on PHENIX PM-100 machine to analyze the influence of the manufacturing strategy on anisotropy and regularity of the internal structure of samples from stainless steel, nickel alloys and metal-polymer powders. The issues of accurate reproduction of the parts geometry, strategy of manufacturing thin-walled 3D filters and filters with customized pattern of the micron-sized channels are addressed. Effect of the porous structure on the material filtering performance is analyzed in order to optimize and diversify design of the porous materials for a given application and to improve their operational behavior.     

Transient effects in pulsed laser irradiation

Theoretical analysis of the influence of the temporal profile (rectangular, triangular, Gaussian) of the laser pulse on heating/cooling and phase transition velocities and quantity of ablated material was performed on the basis of a multifront Stephan problem. Modeling showed that material removal under stationary conditions (that correspond to long pulses) is entirely controlled by specific heat and material density, while in the case of transient regimes (short pulses) thermal conductivity and heat capacity play a predominant role. Interaction of the melting and evaporation fronts characterized by an evaporation front velocity far exceeding the melting front one is one of the examples of the transient nature of the phenomena influenced by the laser pulse parameters.     

Generation of nanospikes via laser ablation of metals in liquid environment and their activity in surface-enhanced Raman scattering of organic molecules

The formation of dense arrays of nanospikes occurs under laser ablation of bulk targets (Ag, Au, Ta, Ti) immersed in liquids such as water or ethanol. The average height of spikes is 50 nm and their density on the target amounts to 10¹⁰ cm⁻². The effect is observed with sufficiently short laser pulses. In particular, either a 350 ps or a 90 ps Nd:YAG lasers are used in their fundamental harmonics. The nanospikes are characterized by UV-Visible reflection spectrometry and atomic force microscopy. The oscillations of electrons within nanospikes result in a permanent coloration of the surface and a modification of the optical reflection spectra of the metal. Scanning the laser beam along the metal surface allows its nanostructuring over extended areas (∼1 cm²). The nanostructured Ag surface shows enhanced Raman scattering of acridine molecules at a concentration of 10⁻⁵ M/l, whereas the initial Ag targets do not show any signal within the accuracy of measurements.     

Fabrication of surface relief gratings on transparent dielectric materials by two-beam holographic method using infrared femtosecond laser pulses

Fabrication of surface relief-type gratings in transparent dielectrics, which are hard to machine, has been achieved by a holographic technique using two infrared femtosecond (fs) pulses from a mode-locked Ti:sapphire laser. The present method can be applied for a variety of transparent dielectrics, Al₂O₃ (sapphire), TiO₂, ZrO₂, LiNbO₃, SiC, ZnO, CdF₂, MgO, CaF₂ crystals, and SiO₂ glass. It is found that the grating formation is due primarily to laser ablation processes. Planar surface relief gratings can be fabricated by colliding two fs laser pulses on the surface of substrates which move at a constant speed, synchronized with the laser repetition rate. Received: 1 March 2000 / Published online: 7 June 2000     

Laser induced melting and superheating in Te and in films for optical data storage

Thin solid tellurium and indium films on a substrate used for optical data storage show superheating during exposure to laser pulses shorter than 10 μs. The times for melting of both Te and In indicate that the speed of the melting front is limited by the availability of vacancies in the solid film material.     

A Simple Model for Ignition of Magnetized Target Fusion

A simple model for magnetized target fusion is proposed. Self-consistent equations are made as equivalent circuit is included. Ignition conditions and physical process are analyzed with practical parameters. It is shown that system temperature rises as external work is equal to or greater than the loss of Bremsstrahlung, and ignition of target happens as thermonuclear reaction energy is equal to or greater than the radiation loss from the target.     

Nanosecond pulse laser melting investigation by IR radiometry and reflection-based methods

Experimental system for nanosecond laser melting investigation was developed containing three independent noncontact methods: infrared radiometry, time-resolved reflectivity of He-Ne laser and sample surface reflected KrF heating laser pulse. The system was applied to the investigation of laser melting of Cu, Mo, Ni, Si, Sn, Ti, steel ?SN 15330 and stainless steel ?SN 17246 samples. For metallic samples the IR radiometry signal was transformed to temperature. Obtained surface temperature and reflectivity spectra in nanosecond time scale (10-1000 ns) for wide range of energy densities (100-5500 mJ cm⁻²) are presented. Interesting evolutions were found. Melting thresholds and melting durations were determined from the measured curves. The applicability of the methods is evaluated.     

Effect of irradiation history on the preparation of laser induced periodic microstructure on polyimide surface

Although UV laser is proved to be an effective tool to prepare microstructure on polymer surface, laser ablation accompanied by the formation of laser induced periodic surface structure (LIPSS) limits its application in many fields. The purpose of this report is to investigate the effect of pre-irradiation in advance, using a low-fluence laser, on the LIPSS formation. The properties of pre-irradiated PI films were characterized by X-ray photoelectron spectroscopy (XPS), surface tension based on the contact angle measurements and UV-vis spectra. It was found that pre-irradiation at low fluence led to the changes in surface property such as chemical components though no LIPSS was formed. As a result, threshold of LIPSS formation on such pre-irradiated PI film decreased and fine LIPSS with deeper amplitude was obtained.     

Influence of laser fluence and irradiation timing of F2 laser on ablation properties of fused silica in F2-KrF excimer laser multi-wavelength excitation process

A collinear irradiation system of F₂ and KrF excimer lasers for high-quality and high-efficiency ablation of hard materials by the F₂ and KrF excimer lasers’ multi-wavelength excitation process has been developed. This system achieves well-defined micropatterning of fused silica with little thermal influence and little debris deposition. In addition, the dependence of ablation rate on various conditions such as laser fluence, irradiation timing of each laser beam, and pulse number is examined to investigate the role of the F₂ laser in this process. The multi-wavelength excitation effect is strongly affected by the irradiation timing, and an extremely high ablation rate of over 30 nm/pulse is obtained between -10 ns and 10 ns of the delay time of F₂ laser irradiation. The KrF excimer laser ablation threshold decreases and its effective absorption coefficient increases with increasing F₂ laser fluence. Moreover, the ablation rate shows a linear increase with the logarithm of KrF excimer laser fluence when the F₂ laser is simultaneously irradiated, while single KrF excimer laser ablation shows a nonlinear increase. The ablation mechanism is discussed based on these results. Received: 16 July 2001 / Accepted: 27 July 2001 / Published online: 2 October 2001     

Phase transitions in femtosecond laser ablation

In this study we simulate an interaction of femtosecond laser pulses (100 fs, 800 nm, 0.1-10 J/cm²) with metal targets of Al, Au, Cu, and Ni. For analysis of laser-induced phase transitions, melting and shock waves propagation as well as material decomposition we use an Eulerian hydrocode in conjunction with a thermodynamically complete two-temperature equation of state with stable and metastable phases. Isochoric heating, material evaporation from the free surface of the target and fast propagation of the melting and shock waves are observed. On rarefaction the liquid phase becomes metastable and its lifetime is estimated using the theory of homogeneous nucleation. Mechanical spallation of the target material at high strain rates is also possible as a result of void growth and confluence. In our simulation several ablation mechanisms are taken into account but the main issue of the material is found to originate from the metastable liquid state. It can be decomposed either into a liquid-gas mixture in the vicinity of the critical point, or into droplets at high strain rates and negative pressure. The simulation results are in agreement with available experimental findings.     

Tritium Burn-up Depth and Tritium Break-Even Time

Similarly to but quite different from the xenon poisoning effects resulting from fission-produced iodine during the restart-up process of a fission reactor, we introduce a completely new concept of the tritium burn-up depth and tritium break-even time in the fusion energy research area. To show what the least required amount of tritium storage is used to start up a fusion reactor and how long a time the fusion reactor needs to be operated for achieving the tritium break-even during the initial start-up phase due to the finite tritium breeding time that is dependent on the tritium breeder, specific structure of breeding zone, layout of coolant flow pipe, tritium recovery scheme, extraction process, the tritium retention of reactor components, unrecoverable tritium fraction in breeder, leakage to the inertial gas container, and the natural decay etc., we describe this new phenomenon and answer this problem by setting up and by solving a set of equations, which express a dynamic subsystem model of the tritium inventory evolution in a fusion experimental breeder （FEB）. It is found that the tritium burn-up depth is 317g and the tritium break-even time is approximately 240 full power days for FEB designed detail configuration and it is also found that after one-year operation, the tritium storage reaches 1.18kg that is more than the least required amount of tritium storage to start up three of FEB-like fusion reactors.     

Multi-material two-temperature model for simulation of ultra-short laser ablation

We investigate the interaction of 100 fs laser pulses with metal targets at moderate intensities (10¹² to 5 × 10¹³ W/cm²). To take into account effects of laser energy absorption and relaxation we develop a multi-material two-temperature model based on a combination of different approaches. The backbone of the numerical model is a high-order multi-material Godunov method in a purely Eulerian form. This formulation includes an interface-tracking algorithm and treats spallation at high strain rates and negative pressures. The model consistently describes the hydrodynamic motion of a two-temperature plasma and accounts for laser energy absorption, electron-phonon/ions coupling and electron heat conductivity. In particular, phase transitions are accurately taken into account by means of a wide-range two-temperature multi-phase equation of state in a tabular form. The dynamics of the phase transitions and the evolution of the heat-affected zone are modeled and analyzed. We have found that a careful treatment of the transport coefficients, as well as consideration of phase transitions is of a great importance in obtaining reliable numerical results. Calculation results are furthermore compared for two metals with different electron-phonon coupling parameters (Au and Al). We have found that the main part of ablated material results from fragmentation of melted phase caused by tensile stresses. A homogeneous nucleation mechanism alone does not explain experimentally observed ablation depth.     

Nanotube size-dependent melting of single crystals in carbon nanotubes

Received: 15 April 1997/Accepted: 16 April 1997     

Tailored ablation processing of advanced biomedical hydroxyapatite by femtosecond laser pulses

The micromachining of hydroxyapatite (HAp) is highly important for orthopedics and dentistry, since human bone and teeth consist mainly of HAp. We demonstrate ultrashort Ti:sapphire laser ablation of HAp, using pulse-widths of 50 fs, 500 fs, and 2 ps at a wavelength of 820 nm and at 1 kpps. The crucial medical issue is to preserve the chemical properties of the machined (ablated) surface. If the chemical properties of HAp change, the human bone or tooth cannot re-grow after laser processing. Using X-ray photoelectron spectroscopy, we observe chemical properties of HAp ablated in air. The HAp is ablated at laser fluences of 3.2 J/cm² (6.4×10¹³ W/cm² at 50 fs), 3.3 J/cm² (6.6×10¹² W/cm² at 500 fs), and 9.6 J/cm² (4.8×10¹² W/cm² at 2 ps), respectively. As a result it is found that the ablated surface is unchanged after laser ablation over the pulse-width range used in this experiment. Received: 7 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-45/566-1533, E-mail: obara@obara.elec.keio.ac.jp     

Growth and melting behaviour of thin in films on Ge(100)

Growth and melting behaviour of thin indium films on Ge(100) have been investigated by Auger-electron spectroscopy (AES), atomic force microscopy (AFM) and perturbed angular correlation (PAC) spectroscopy, respectively. At room temperature inidium is found to grow in three-dimensional islands even at submonolayer coverages. A very rough film surface is observed for thicknesses up to 230 ML. The melting behaviour of such films has been studied by PAC. A reduction of the melting temperature T _m as well as a strong supercooling of the films is observed. The electric field gradient for ¹¹¹In(¹¹¹Cd) in the indium islands is determined as a function of temperature and is used to monitor the local crystalline order of the films up to temperatures just below the melting point.     

Statistical mechanics of melting mediated by two types of defects

We propose a defect-mediated melting theory based on the statistics of two types of lattice defects, the point defects and dislocation pairs. The model predicts a first-order phase transition. Based on the model, phase transition temperature, latent heat and other thermodynamic functions are derived. Melting occurs due to discontinuous growth of point defects into dislocation pairs. The calculated phase transition temperature for five alkali metallic crystals are in fair agreement with measured melting temperatures, and the Richards' rule is derived by the model also.     

Vibrational spectroscopy of ice interfaces

Vibrational spectroscopy via infrared-visible sum-frequency generation was used to probe interfaces of hexagonal ice(0001) with vapor, a hydrophobic solid, and a hydrophilic solid. The OH stretch vibrational modes for hydrogen-bonded and free dangling OH bonds were measured at various temperatures below bulk melting. At the vapor/ice interface, the dangling OH mode provided information about surface melting. Surface structural disordering appeared to set in at ∼200 K and increased dramatically with temperature. Received: 15 January 2002 / Published online: 11 June 2002     

Modelling of the solid–liquid interface during laser processing using an inverse methodology

This study presents a methodology for estimating the melt depth during laser processing of solid materials. The determination of the melt depth is treated as an inverse heat conduction problem, which includes the solid and liquid phases. The conjugate gradient method is applied to treat the inverse problem using the available temperature measurements. Without the inverse methodology the melt depth is very difficult to obtain with precision. The proposed method can also be applied during microthermal machining to determine the location of the solid–liquid interface and the temperature distributions of the two phases by using scanning thermal microscopy.