Laser surface hardening of H13 steel in the melt case

Laser heating caused a melting layer to form on the H13 steel, which usually has bad thermal conductivity and diffusivity. Therefore, the modified Ashby–Eastering heat-transfer equation was used to provide the temperature field for laser surface hardening in the melt. When the laser hardened H13 steel through surface melting, the basic microstructure of the dendrites was surrounded by an extremely fine lamellar structure in the melt layer. It is clear that the contours of the melting point isotherms and the critical phase transition temperature of H13 in the quenched and as-received conditions were comparable in the temperature distribution field under different laser energy densities. When the laser moves on, the phase transition temperature of H13 is raised and it becomes higher than the A₁ temperature because the heating rate during laser processing is usually >10⁴ °C/s. The larger the grain size or the more heterogeneous the structure, the higher the temperature and the longer the duration required for transforming the steel into austenite.     

Particulate migration behavior and its mechanism during selective laser melting of TiC reinforced Al matrix nanocomposites

A transient three-dimensional model for describing fluid flow characteristics and particles migration behavior within the melt pool during selective laser melting (SLM) of TiC/AlSi10Mg nanocomposites was developed. The powder-solid transition, variation of thermophysical properties, and surface tension were considered in the model. The influence of laser energy per unit length (LEPUL) on heat and mass transfer, melt pool dynamics, and particles rearrangement was investigated. It showed that the Marangoni convection became more vigorous with an increase of LEPUL, accordingly enhancing the thermal capillary force. The high laser energy input induced a sufficient liquid formation and an improved wettability, lowering the friction force exerting on TiC solids. Under this condition, the reinforcing particles can be well mixed within the matrix. The experimental study on the distribution state of TiC reinforcement in the SLM-processed Al matrix was performed. The results validated that the dispersion of TiC reinforcement changed from a severe aggregation to a uniform dispersion in the matrix with the increase of LEPUL. The TiC reinforcement experienced a microstructural variation from the standard nanoscale structure with a mean particle size of 70–90 nm to the relatively coarsened submicron morphology with an average particle size of 134 nm as LEPUL increased.     

Theoretische Überlegungen zum Erstarrungsverhalten schnell abgeschreckter Schmelzen

Theoretical Considerations on the Solidification Behaviour of Rapidly Quenched Melts The rapid heating, cooling and solidification processes on laser surface melting are calculated. The influence of the laser operating parameters e.g. beam energy density, melting speed and the resultant depth of melt on the solidification parameters cooling rate, solidification rate and temperature gradient is shown. By combining the cooling curves and TTT (CT) curves an attempt can be made to predict the resulting grain size. The use of the programmes is demonstrated for other quenching methods e.g. melt spinning, powder atomizing.     

Surface melt dynamics and super lateral growth regime in long pulse duration excimer laser crystallization of amorphous Si films

In this work, the excimer laser induced crystallization of a-Si films on SiO₂ was investigated, using a long pulse duration (200 ns) XeCl source. The microstructural analysis of the laser irradiated area, for incident energy densities comprised between the surface and full melting thresholds of the a-Si layer, respectively, was performed by scanning electron microscopy. A numerical simulation of the surface melt dynamics was also presented and compared to the experimental observations.     

Development of a theoretical process map for laser cladding using two-dimensional conduction heat transfer model

In blown powder laser cladding process, the powder travels across the laser path, gets heated up by absorbing laser energy, and finally melts on the substrate under the intense laser beam; as the substrate moves away this melt pool solidifies to form a continuous built-up layer. In the present study a two-dimensional conduction heat transfer equation has been solved using finite volume method to develop a theoretical process map for laser cladding. The developed process map indicates a range of scanning speed and powder feed rate for the feasibility of the process; the lower limit is dictated by the maximum melt pool temperature, and the higher limit by poor bonding due to lack of melting of the substrate (i.e. low dilution). Parametric regions for thick and thin cladding with low dilution can be decided from the process map. It is found that the process range expands with the increase in total absorbed power as well as power directly absorbed by the powder. Correlations for maximum melt pool temperature and dilution are presented. A process map for identifying the form and scale of the microstructure in the solidified layer is also presented.     

ZrO2陶瓷颗粒在激光作用下组织形貌演变规律

采用激光表面处理技术制备ZrO_2复合热障涂层可使陶瓷颗粒与金属之间实现冶金结合,增加结合强度,但由于陶瓷颗粒和金属热膨胀系数之间的差异,制备大面积复合热障涂层仍存在比较严重的开裂问题.为降低复合材料层的开裂倾向,更好地控制ZrO_2陶瓷颗粒的离散规律,本文针对激光熔注过程中陶瓷颗粒的熔化问题,采用高速摄像、扫描电子显微镜研究了氧化锆陶瓷颗粒进入熔池前后激光对其形貌与离散规律的影响规律.结果表明,在激光能量密度较低的情况下,激光直接作用于氧化锆陶瓷颗粒会导致单个ZrO_2陶瓷颗粒发生裂解现象;当激光能量密度高于2.5 J/mm~2时,ZrO_2陶瓷颗粒熔化严重并发生球化现象.在激光熔注过程中,由于受到激光和高温熔池的双重作用,氧化锆陶瓷颗粒内部晶粒长大,高能量大尺寸晶界促进了Ti熔体向ZrO_2陶瓷颗粒内部扩散.     

激光选区熔化316L不锈钢凝固特征与微观组织结构

对不同工艺参数下激光选区熔化(Selective Laser Melting, SLM)成形316L不锈钢微观组织结构进行表征,研究不同工艺参数下SLM成形316L不锈钢微观组织结构演化规律、单熔化道凝固特性。结果表明,SLM成形316L不锈钢具有跨尺度、非均质凝固组织特征,包括微米尺度柱状晶粒、小角晶界、熔池界面和纳米尺度亚结构。单熔化道的稳定成形是三维块体成形的基础,熔化道稳定性由激光工艺参数与金属粉体物理特性共同决定。不同的激光工艺参数显著影响SLM成形316L不锈钢微观组织结构,通过改变激光参数可实现微观组织结构的调控,在不同的激光逐层旋转角度下,SLM成形316L不锈钢晶粒尺寸随着扫描间距的增大而增大。强制定向热流使得外延生长机制主导凝固晶粒的生长,在不同的激光工艺参数下,沿增材方向的柱状晶粒形貌普遍存在。     

Methods of Increasing the Measurement Accuracy during the Experimental Determination of the Melting Point of Graphite

The experimental procedure involving pulsed laser heating is realized, which enables one to investigate phase transformations in graphite and eliminates the homogeneous condensation of carbon vapor over a sample. Data are obtained on the melting point of graphite and the temperature of crystallization of liquid carbon at a pressure of 15 MPa. The absence of a difference between the melting point of graphite at a heating rate of 100 MK/s and the temperature of crystallization of liquid carbon at the cooling rate of the melt bath of 1.6 MK/s is revealed, which is indicative of the quasi-equilibrium of the melting process. In view of this, the temperature data are averaged by a unified value of 4800±100 K. It is shown that, during the crystallization of carbon melt, highly ordered graphite with crystallites up to 250 #x00B5;m in size is formed in a liquid film 5 #x00B5;m thick on a graphite substrate.     

Optimization of the LENS Rapid Fabrication Process for In-Flight Melting of Feed Powder

A model for in-flight melting of feed-powder particles propelled through a laser beam in the Laser-Engineered Net Shaping (LENS) process has been developed. The model is next incorporated in an optimization analysis to determine optimum LENS process parameters (laser power, particle velocity, and the angle between the laser-beam axis and particle trajectory), which maximize the probability for in-flight particle melting while ensuring the absence of melting of the surface of the substrate. A simple model, based on solution of the thermal energy conservation equation, is also developed to determine the laser-power threshold for melting of the substrate surface. The optimization analysis is then applied to Inconel 625 Ni-Cr-Mo superalloy. The results show that by maximizing the laser power and the residence time of the particles in the laser beam (increases with reductions in particle velocity and particle trajectory angle), the probability for in-flight particle melting can be greatly increased, i.e. relatively coarse (–30/+40 mesh size) particles can be melted by propelling them through the laser beam.     

An analytical modeling of heat transfer for laser-assisted nanoimprinting processes

Laser-assisted direct imprinting (LADI) technique has been proposed to utilize an excimer laser to irradiate and heat up the substrate surface through a highly-transparent quartz mold preloaded on this substrate for micro- to nano-scaled structure fabrications. While the melting depth and molten duration are key issues to achieve a satisfactory imprinting pattern transfer, many material property issues such as crystalline phase alteration, grain size change and induced film stress variation are strongly affected by transient thermal response. With one-dimensional simplification as a model for the LADI technique, the present paper has successfully derived an analytical solution for the arbitrary laser pulse distribution to predict the relevant imprinting parameters during the laser induced melting and solidification processes. The analytical results agree quite well with the experimental data in the literature and hence can be employed to further investigate the effects of LADI technique from laser characteristics (wavelength, fluence and pulse duration) and substrate materials (silicon and copper) on the molten duration, molten depth and temperature distributions. Three kinds of excimer laser sources, ArF (193 nm), KrF (248 nm) and XeCl (308 nm) were investigated in this study. For the silicon substrate, the melting duration and depth were significantly dictated by the wavelength of laser used, indicating that employing the XeCl excimer laser with longer pulse duration (30 ns in the present study) will achieve the longest molten duration and deepest melting depth. As for the copper substrate, the melting duration and depth are mainly affected by the laser pulse duration; however, the wavelength of laser still plays an insignificant role in LADI processing. Meanwhile, the laser fluence should properly be chosen, less than 1.4 J/cm² herein, so as to avoid the substrate temperature exceeding the softening point of the quartz mold (~1950 K) and to make sure that the mold can still maintain the original features.     

Nanoscale rapid melting and crystallization of semiconductor thin films

The current study details nanosecond laser-based rapid melting and crystallization of thin amorphous silicon (a-Si) films at the nanoscale using two different optical near-field processing schemes. Both apertureless and tapered fiber near-field scanning optical microscope probes were utilized to deliver highly confined irradiation on the target surface. The various modification regimes produced as a result of the rapid a-Si melting and crystallization transformations were shown to critically depend on the applied laser fluence. Consequently, the crystallized pattern morphology and feature size could be finely controlled. High energy density was observed to impart ablation surrounded by a narrow melt ring. At much lower incident laser energy density, single nanostructures with a lateral dimension of approximately 90 nm were defined.     

Parameters for the laser processing of tool and carbon steels

Surface melting of various steel systems, including M2 tool steel and specialty grade carbon steels, by CO₂ lasers was investigated to determine the relative effects of the processing variables such as the laser power P, the spot size d and the interaction time τ (τ = d/v where v is the translation speed of the sample) on the efficiency of the laser melting process. The cross-sectional area A of the laser-melted zone is found to vary linearly with the energy per unit length incident on the surface of the sample (A = P/v). Comparison with the calculated energy required to melt a specified volume of sample provides a means of evaluating the efficiency of a given set of laser processing variables. The area of the melted zone is found to vary proportionately with P/v within certain ranges of conditions, although the power density at the surface is also found to be a critical parameter. The work was made necessary by the need to control the size and geometry of the melted zone to achieve controlled concentrations in a laser alloying experiment, as well as to optimize the conditions for the highest process throughput. The laser processing conditions were 0.45–3.6 kW of laser power, an interaction time of 1–60 ms and a spot size of 0.15–1.25 mm, with power densities ranging from 0.19 to 2.83 MW cm^-2.     

Fabrication of NiCr alloy parts by selective laser melting: Columnar microstructure and anisotropic mechanical behavior

NiCr alloy, because of its wide applications in electrical elements and dental field was widely studied in the past. In this work, NiCr cubes and tensile specimens were fabricated by using a new processing technique-selective laser melting (SLM). Microstructural and mechanical behavior characterization of SLM-processed NiCr components was performed. An unusual columnar microstructural architecture composed of 〈1 0 0〉 texture (corresponding to (2 0 0) plane) oriented the building direction was observed. Moreover, it was found that the columnar grain growth across the melt pools occurred during the SLM process and the growth trend became stronger with the decrease of the laser scanning speed. Associated with the microstructural characteristic, an anisotropic mechanical behavior at different reference planes (i.e., at the horizontal and vertical surfaces) was demonstrated for the samples fabricated using different processing parameters. The results showed that with increasing the laser scanning speed, the microhardness at the horizontal surface decreased, while at the vertical surface it increased; an increase of the yield strength (YS) and the ultimate tensile strength (UTS) was observed.     

Effect of laser irradiation on microstructure and electromigration in aluminum films

Optical and transmission electron microscopy were employed to study the effect of ruby laser irradiation on the microstructure and electromigration properties of thin aluminum films. At low energy levels a single pulse creates multiple regions of localized grain growth; grain size in the irradiated area can be an order of magnitude larger than the matrix. Vaporization, melting, and grain growth are observed and related to the input laser energy. In some cases, the morphology of the laser-damaged film resembles that of cast ingots, i.e., a columnar structure radiating from an equiaxed boundary network and bounded by a circular chill zone.     

Microstructural features of as-cast A356 alloy inoculated with Sr, Sb modifiers and Al-Ti-C grain refiner simultaneously

This article focuses on the microstructural refinement of as-cast A356 alloy, obtained by melt inoculation of the same. The inoculants used are Sr (modifier), Sb (modifier) and Al-Ti-C (grain refiner). Microstructural characterization of A356 alloy reveals that the Secondary Dendritic Arm Spacing (SDAS) of α-Al dendrites and size/morphology of eutectic silicon decrease by treating the melt with pre-defined amounts of Sr, Sb and Al-5Ti-2C grain refiner. Microstructure of the inoculated as-cast A356 alloy shows that, modified eutectic-silicon exhibits fine fibrous morphology at shorter melt holding, while the same exhibits fine lamellar morphology on longer melt holding.     

The effect of microstructure on the toughness of ferritic nodular cast iron

The ductile-brittle transition behaviour in ferritic nodular cast iron appears to depend not only on chemical compositions but also on many microstructural variables. The ferrite grain diameter and the mean free path between graphite nodules are studied here as the main microstructural variables, and the effects of these on toughness are investigated mainly by the instrumented Charpy impact test. The results show that the transition temperature is lowered when the ferrite grain size or the mean free path is decreased. On the other hand, increase in the ferrite grain size or the mean free path leads to a rise in the upper shelf energy in the ductile fracture range. Equations to predict the dynamic lower yield load (P _y) and the maximum load transition temperature (T _rpm) are derived from the relation for steel dispersed with spheriodal carbides.     

Dynamics of melting and crystallization processes in CdTe under laser irradiation

Numerical modeling results indicate that cadmium vaporization caused by nanosecond pulses of a ruby laser has a significant effect on the dynamics of phase transitions in the near-surface region of CdTe and leads to surface cooling of the material, resulting in a nonmonotonic temperature profile, with a maximum at a depth of about 20 nm. At incident energy densities above the threshold for CdTe melting, the molten zone forming below the surface layer extends both toward the surface and into the bulk of the semiconductor. Cd vaporization and the diffusion of Cd and Te in the melt give rise to tellurium enrichment in the near-surface region. Taking into account the dependences of the crystallization temperature and the latent heat of the phase transition on the Cd and Te concentrations in the melt, we achieved reasonable agreement with experimental data on the effect of incident energy density on the time during which a molten layer is present in CdTe.     

Thermal behavior and densification mechanism during selective laser melting of copper matrix composites: Simulation and experiments

Simulation of temperature distribution and densification process of selective laser melting (SLM) WC/Cu composite powder system has been performed, using a finite volume method (FVM). The transition from powder to solid, the surface tension induced by temperature gradient, and the movement of laser beam power with a Gaussian energy distribution are taken into account in the physical model. The effect of the applied linear energy density (LED) on the temperature distribution, melt pool dimensions, behaviors of gaseous bubbles and resultant densification activity has been investigated. It shows that the temperature distribution is asymmetric with respect to the laser beam scanning area. The center of the melt pool does not locate at the center of the laser beam but slightly shifts towards the side of the decreasing X-axis. The dimensions of the melt pool are in sizes of hundreds of micrometers and increase with the applied LED. For an optimized LED of 17.5 kJ/m, an enhanced efficiency of gas removal from the melt pool is realized, and the maximum relative density of laser processed powder reaches 96%. As the applied LED surpasses 20 kJ/m, Marangoni flow tends to retain the entrapped gas bubbles. The flow pattern has a tendency to deposit the gas bubbles at the melt pool bottom or to agglomerate gas bubbles by the rotating flow in the melt pool, resulting in a higher porosity in laser processed powder. The relative density and corresponding pore size and morphology are experimentally acquired, which are in a good agreement with the results predicted by simulation.     

Microstructural evolution and defect formation in a powder metallurgy nickel-based superalloy processed by selective laser melting

In this study,the selective laser melting(SLM)technology has been employed to manufacture a nickelbased superalloy which was conventionally prepared through powder metallurgy(PM)route.The microstructural features and defects were systematically investigated both prior to and after heat treatment and compared with the PM counterpart.Both solidification cracking and liquation cracking were observed in the SLM specimen in which the grain misorientation and low melting point(γ+γ')eutectic played a vital role in their formation mechanism.Columnar grains oriented along building direction were ubiquitous,corresponding to strong<001>fiber texture.Solidification cell structures and melt pools are pervasive and noγ'precipitates were detected at about 10 nm scale before heat treatment.After supersolvus solution and two-step aging treatments,high volume fractionγ'precipitates emerged and their sizes and morphologies were comparable to those in PM alloy.<001>texture is relieved and columnar grains tend to become more equiaxed due to static recrystallization process and grain boundary migration events.Significant annealing twins formed in SLM alloy and are clarified as a consequence of recrystallization.Our results provide fundamental understandings for the SLM PM nickel-based superalloy both before and after heat treatment and demonstrate the potential to fabricate this group of alloys using SLM technology.     

Growth of pseudocubic perovskite-type SrRuO3 thin films on quartz substrate using pulsed laser deposition method

Strontium ruthenium oxide (SrRuO₃) thin films have been grown using pulsed laser deposition technique on silicon, Pt coated silicon and quartz substrates. The effect of substrate temperatures on the structural, microstructure, and electrical properties of the SrRuO₃ films on quartz substrate has been investigated using XRD, SEM, AFM and four-probe method, respectively. The lowest resistivity at room temperature for the SrRuO₃ thin film on quartz substrate has been achieved at substrate temperature of 700 °C. Furthermore, the comparisons of SrRuO₃ thin films deposited on various substrates have been done with respect to structural, microstructural and electrical properties. XRD patterns exhibit that all thin films are a single phase, pseudo-cubic perovskite structure. Study of surface morphology shows that grain size and roughness varies with respect to substrate. It is observed that SrRuO₃ thin films yield larger grain size and root mean square roughness on Pt/Si substrate. Investigation of electrical properties shows that SrRuO₃ thin films can serve the purpose of the bottom electrode in dielectric and ferroelectric devices.