Function Of Scanning Speed Research Articles

Development and Validation of a One‐Dimensional Finite Difference Simulation Scheme for Polymer Laser Powder Bed Fusion with Application to the Effect of the Inter Layer Time

The division of the polymer laser powder bed fusion process polymers (PBF‐LB/P) into temporal regimes originating from laser motion, thermal diffusion, viscous flow, crystallization kinetics, and powder application is exploited by considering only the building direction. The reduction of dimensionality enables fast simulations which are used to investigate the influence of the inter layer time on the final part density. Numerical simulation results are validated by experiments using polyamide 12 (PA 12) with good agreement in terms of part density. It is shown that an inter layer time of 90 s leads to nearly dense PA 12 parts while a time of 45 s leads to less dense parts. The cooling effect of the applied powder is identified as a cause for insufficient densification of the previous layer. The simulation tool is quantitatively validated against experimental results for the surface temperature of PA 12 as function of scan speed and hatch distance, for the melt pool depth of polyamide 6 as function of scan speed and for the melt pool depth of polyetherketoneketone as function of laser power. The presented simulation method enables rapid process parameter adjustment for new polymer materials in the PBF‐LB/P process.

Effect of line energy on microstructures and mechanical properties of Co-Cr-Mo alloy single-track fabricated by DED method

The additive material built-up by the directed energy deposition (DED) method is affected by the melting and solidification reaction with the substrate material. This work investigated the effect of line energy on microstructures and mechanical properties of Co-Cr-Mo alloy single-track fabricated by the DED method. The line energy is a function of scanning speed and laser power, which are the main process parameters for the DED method. The Co-Cr-Mo alloy single-track with low line energy has a columnar grain microstructure with a strong (0 0 1)γ texture parallel to the build direction. As the line energy increased, the single-tracks had mixed microstructure, including fine columnar grains and coarse equiaxed grains with random texture. In addition, the secondary dendrite arm spacing of the single-track depends on the line energy due to the rapid cooling rate. The ε-Co phase in Co-Cr-Mo alloy single-track increased with increasing line energy throughout the isothermal γ → ε martensitic transformation. The effect of the ε-Co phase on the mechanical properties of the Co-Cr-Mo alloy single-track is insignificant. The diffusion of alloying elements from the substrate material throughout the fusion zone occurred and influenced the mechanical properties of the Co-Cr-Mo alloy single-track. The elastic modulus and nano hardness decreased with increasing line energy because of the diffusion of alloying elements from the substrate material.

Enhanced mechanical properties by laser powder bed fusion using cost-effective hydride-dehydride titanium powders

The high cost of powder feedstock has emerged as a critical issue in additive manufacturing of titanium (Ti). In this work, pure Ti parts were successfully produced via laser powder bed fusion (LPBF) by employing cost-effective hydride-dehydride (HDH) powders modified through gas-soild fluidization. The microstructures and tensile properties were comprehensively investigated as a function of scanning speed. The pure Ti parts made under the optimized processing parameter exhibited an ultimate tensile strength of 826.1 MPa with a fracture strain of 22.3% at room temperature, superior to those made by the expensive atomized powders. The fatigue performance of the LPBF-made pure Ti using the HDH powders is comparable to those made by rolling. The excellent mechanical performance can be attributed to the effects of grain refining, texture evolution, dislocation recovery, and cyclic softening by dislocation cells and tangles generated. This work provides an effectively low-cost route to fabricate pure Ti by LPBF with excellent mechanical properties, which also serves as a useful guidance on the processing window setup using the HDH powder.

Characterization of an amplified piezoelectric actuator for multiple-reference optical coherence tomography.

The characterization of an amplified piezoelectric actuator (APA) as a new axial scanning method for multiple-reference optical coherence tomography (MR-OCT) is described. MR-OCT is a compact optical imaging device based on a recirculating reference-arm-scanning optical delay using a partial mirror that can enhance the imaging depth range by more than 10 times the reference mirror's scanning amplitude. The scanning amplitude of the used APA was varied between 30μm and 250μm, depending on the scanning frequency of between 0.8kHz and 1.2kHz. A silver-coated miniature mirror was attached to the APA via ultraviolet-cured optical adhesive, and the light source was a super-luminescent diode with 1310nm center wavelength and 56nm bandwidth. The sensitivity was measured with and without the partial mirror in the reference delay line as a function of scan speed, frequency, and range, therefore providing results for MR-OCT and TD-OCT modes. It was found that the APA provides more than twice the mechanical scanning range compared to other opto-mechanic actuators, but results indicate degradation of signal-to-noise ratio and sensitivity at larger imaging depths. In conjunction with MR-OCT, the scan range of maximum 200μm can be enhanced up to 1-1.5mm by using a reduced amount of orders of reflections, which could be of interest to increase sensitivity in the future.

Investigation of crystal growth mechanism during selective laser melting and mechanical property characterization of 316L stainless steel parts

A fundamental investigation of the development of grain structure of 316L stainless steel fabricated by selective laser melting (SLM) was conducted. Finite element analysis (FEA) was carried out in order to reveal the growth mechanism of grains under rapid solidification conditions. From this analysis, the crystal growth patterns were determined as a function of the temperature gradient along grain development orientation. A detailed discussion of preferred crystal orientation of dendrites was performed by geometrical analysis in conjunction with experimental findings. In addition, the dendrite spacings were measured, and the variation in spacings as a function of scan speed was studied. It was found that, the rapid solidification induced by high-speed laser scanning brought about sub-micron grains within the final solidified microstructure and a high volume energy density ω could cause the increase of primary dendrite spacing. Furthermore, the result also indicated that both grain size and densification level dependent on ω could affect the mechanical properties significantly. As the ω was settled at the optimal value of 125.00J/mm3, a high Vicker hardness of 281.6HV0.1, a large tensile strength of 590MPa and an attendant elongation rate of 21.1% were obtained.

Determination of selectivity and specific area related to oxygen reduction reaction as a function of catalyst loading on non-noble metal based electrocatalyst porous electrodes: an example on nitrogen doped carbon nanotube

In a recent paper, model porous electrodes based on carbon nanotube and platinum organically grafted electrocatalysts were used to establish the determination of a specific surface area related to oxygen reduction reaction (ORR) in HClO4 1M, using cyclic voltammetry as a function of scan speed. Identical behavior was found for the trends of this parameter called S-AO2 and the selectivity of the ORR as a function of catalyst loading, for different model porous electrode structures. In the present paper, we report such a study on porous electrodes based on non-noble metal electrocatalysts built from nitrogen doped carbon nanotubes (N-CNT). The trends observed for S-AO2 and the selectivity of the ORR as a function of N-CNT loading are exactly the same as those observed for model porous electrodes. This suggests the interest of these approaches and in particular the determination of the S-AO2 parameter for the comparison of potentialities of different non-noble electrocatalysts, for example based on nitrogen doped carbon structures towards the ORR. Indeed, although the amount of chemical species supposed to be responsible for the ORR can be estimated using various methods, their accessibility at the level of the nano-object itself and more globally in the porous structure of an active layer is essentially unknown. The S-AO2 parameter value is presumably directly related to the active sites which are accessible to the oxygen reduction in a porous structure.

Fabrication and optical characterization of microstructures in poly(methylmethacrylate) and poly(dimethylsiloxane) using femtosecond pulses for photonic and microfluidic applications

We fabricated several microstructures, such as buried gratings, surface gratings, surface microcraters, and microchannels, in bulk poly(methylmethacrylate) (PMMA) and poly(dimethylsiloxane) (PDMS) using the femtosecond (fs) direct writing technique. A methodical study of the diffraction efficiency (DE) of the achieved gratings was performed as a function of scanning speed, energy, and focal spot size in both PMMA and PDMS. An optimized set of writing parameters has been identified for achieving efficient gratings in both cases. The highest DE recorded in a PDMS grating was ∼10% and ∼34% in a PMMA grating obtained with an 0.65 NA (40X) objective with a single scan. Spectroscopic techniques, including Raman, UV-visible, electron spin resonance (ESR), and physical techniques, such as laser confocal and scanning electron microscopy (SEM), were employed to examine the fs laser-modified regions in an attempt to understand the mechanism responsible for physical changes at the focal volume. Raman spectra collected from the modified regions of PMMA indicated bond softening or stress-related mechanisms responsible for structural changes. We have also observed emission from the fs-modified regions of PMMA and PDMS. An ESR spectrum, recorded a few days after irradiation, from the fs laser-modified regions in PMMA did not reveal any signature of free radicals. However, fs-modified PDMS regions exhibited a single peak in the ESR signal. The probable rationale for the behavior of the ESR spectra in PMMA and PDMS are discussed in the light of free radical formation after fs irradiation. Microchannels within the bulk and surface of PMMA were achieved as well. Microcraters on the surfaces of PMMA and PDMS were also accomplished, and the variation of structure properties with diverse writing conditions has been studied.

Modelling and simulation of electron beam melting

In recent years, the scientific and industrial relevance of additive layer manufacturing (ALM) has grown. In the metal area of ALM, the capacity of laser technologies is noticeably limited. This is mainly due to an inertial beam deflection device, which is also referred to as a mirror galvanometer. In contrast, the electron beam technology offers high power density as well as considerable scanning rates. Therefore, electron beam melting (EBM) seems to be suitable for processing a broad variety of alloys in an economic way. In particular, the enormous scanning rates which can be realized by use of an electron beam enable an economic manufacture of high quality parts. However, profound expertise is required in order to establish EBM as an industrial production technology. By means of mathematical–physical modelling, process stability of the melting step is being increased. Moreover, by solving a detailed thermal model using the finite element method (FEM), substantial knowledge of adequate parameter settings in dependence of the utilised material is developed. Finally, a process window as a function of scan speed and beam power is developed based on experimental results.

The effects of Prandtl number on wavy weld boundary

The effects of Prandtl number on two-dimensional thermocapillary convection and molten pool shape induced by negative surface tension coefficient in welding or melting with a time-dependent and distributed incident flux are numerically predicted in this study. This work is also applicable for predicting the quasi-steady three-dimensional thermocapillary convection. In this model, the time-dependent incident flux is specified as a function of scanning speed and energy distribution parameter. The computed flow patterns and molten pool shapes under the flat free surface are found to have distinct regions for different Marangoni and Prandtl numbers. Prandtl number indicates the thickness ratio between momentum and thermal boundary layers, whereas Marangoni number with negative surface tension coefficient induces an outward surface flow. Rather than the enhanced pool depth resulted from melting from an induced vortex cell near the bottom in the case of Prandtl number much less than unity, the molten pool shape for Prandtl number much greater than unity can produce a thin and narrow edge. The variations of Peclet number and dimensionless beam power with flow and temperature fields and fusion zone shapes are similar. The dimensionless peak surface speed versus product of Marangoni and Prandtl number, which involve predicted peak speed and temperature and molten pool width on the surface, agree with scale analysis and experimental data provided in the literature.

Simulation of ion beam induced micro/nano fabrication

The shrinking critical dimensions of modern technology place heavy requirements on optimizing feature shapes at the micro and nano scale. Ion beams are increasingly used for technology development in the nano-scale world. In recent years, many approaches and research results have indicated that re-deposition of sputtered target atoms is the most serious problem in fabricating micro and nano devices. A simulation tool is essential to reduce unnecessary time and efforts spent in process development. In this paper, we present two-dimensional string-based simulation software for ion milling and focused ion beam direct fabrication, AMADEUS-2D (advanced modeling and design environment for sputter processes). We discuss the numerical model for considering sputtering and re-deposition fluxes. In addition, we investigate sputtering yield and sputtered atom distributions as obtained from several binary collision simulation codes. The newly developed simulation code is validated by comparison with experimental data on single-pixel hole milling, on the width and dose dependence of trench formation and on the effective sputtering yield as a function of scan speed.

Friction and Wear of Laser Irradiated Amorphous Metals

Tribological properties are found to change with microstructure. In Ni-P amorphous alloy, annealing conditions were varied with laser irradiation parameters such as scanning speed and laser power. The increase in hardness affected by scanning speed. A peak of hardness was observed as the function of scanning speed. This is because, the formation of nanoscopic composite structure by distribution of crystalline Ni3P compounds in amorphous matrix is the hardest structure.

Methods and Artifacts for Comparison of Scanning CMM Performance

Coordinate measuring machines (CMMs) with continuous-contact scanning capabilities are experiencing more and more use in a wide variety of discrete-part manufacturing industries. Many users of these CMMs, if asked, will say that their metrology requirements include high density scans at high speeds with high accuracy. These requirements are in conflict, as there will be some point at which the accuracy of the scanned data begins to decrease with an increase in the scanning speed. This paper addresses the effects of scanning speed on the performance of different CMMs using contact sensors. The use of a family of artifacts is proposed in order to evaluate the relative scanning performance as a function of scanning speed and the direction of the scan within the CMM volume. The artifacts that are developed for these tests have a sinusoidal waveform superimposed on a flat surface. This paper describes a series of experiments that utilize these artifacts and assess the ability of these tests to capture how a scanning CMM will perform when measuring actual parts.

Nanoscale velocity–drag force relationship in thin liquid layers measured by atomic force microscopy

The relationship between velocity and drag force acting on a nanoprobe has been measured with an atomic force microscope (AFM). A special nanoprobe “whisker” was partially submerged in thin layers of glycerol–water mixtures and moved by using the AFM in scanning mode. The viscous drag force-caused torsion of the cantilever probe was recorded as a function of scanning speed and submersion depth. A linear drag force–velocity function was determined for cylindrical bodies with diameters of the order of 50nm. The experimental results were supported by calculations for the torsional force exerted on an AFM probe dragged through a viscous medium. The viscosity was calculated for each experiment assuming no slip conditions and was in agreement with the macroscopically determined values. With some refinements, this offers a possible means of determining viscosity in thin liquid layers.

Focused ion beam sputter yield change as a function of scan speed

In many of the applications of focused ion beams, such as integrated circuit sectioning and TEM sample preparation, considerable volume of materials may need to be removed. Thus optimizing the sputter yield is important. For very rapid scan speeds at normal incidence, each pass of the beam removes a thickness of material which is much smaller than the beam diameter. In this case, this milling yield corresponds to the yield at normal incidence. However, if the scan speed is slowed down so that the thickness removed per pass is comparable to the beam diameter, then locally under the beam the ions are not normally incident even though the beam is normal to the surface. The milling yield of Si and SiO2, for example, increases by a factor of seven to eight in going from normal incidence at 0° to 75°–85°. Thus the material removal rate can be significantly increased by reducing the scan speed. We have measured the milling yield of Si and SiO2 as a function of scan speed in one axis by milling boxes, typically 7 μm×9 μm using 30 keV Ga+ ions. In the other axis, the scan speed is many orders of magnitude faster so that the beam can be thought of as a sheet or “blade.” To measure the dependence of yield on scan speed, we milled the boxes with a single scan in the slow direction, and then measured the depth of the boxes with an atomic force microscope. The beam currents used were 1 and 2.9 nA. At the slowest single-axis speeds, the sputter yield increased by a factor of two. The observed dependence on scan speed agrees with existing models, which assume a single local angle of incidence under the beam. We also measured the redeposition on the bottom of the pit and found that it increased as the scan speed was decreased. In many applications, a small amount of redeposition would be unimportant.

Atomic Force Microscopic Imaging in Liquids: Effects of the Film Compressed between the Substrate and the Tip

Measurements are made of the forces acting on the tip of an atomic force microscope when the sample and cantilever are in air and also immersed in polar solvents like water and DMSO. For large tip/substrate separations ( >1000 nm) the liquid drag force can be modeled using a classical hydrodynamic drag force expression. For separations between the tip and substrate smaller than around 100 nm in a water medium, the force due to the effective viscosity increase of the compressed films for high tip/substrate relative velocity is comparable to the contribution of the double-layer repulsion and the van der Waals attraction. After tip/substrate contact these compressed films produce an attractive force that is a function of the liquid medium. In the DMSO medium the attractive force between tip and substrate shows an adhesive force that has at least two components indicating a multilayered structure between the tip and substrate during the tip/substrate separation. The scanning of a surface immersed in water and DMSO with a tip in contact produces distinct AFM images which depend on the liquid. These images show diverse symmetries and spacings between features which presumably correspond to the solvated atomic structure and are only obtained with atomic resolution for a scanning speed of 2 nm/s. It is possible to estimate the viscous relaxation time of the compressed layer value to be 250 ms from the observed resolution of substrate structures as a function of scanning speed.