High Scanning Velocity Research Articles

Numerical microstructure prediction by a coupled finite element cellular automaton model for selective electron beam melting

Selective Electron Beam Melting (SEBM) refers to an additive manufacturing process of building near net shaped components layer wise by iteratively melting of metal powder using an electron beam. Due to the preheating of the material and very high scan velocities of the electron beam, the process allows large processing windows defined by different process strategies. This variety enables the adjustment of part properties by, e.g., targeting certain microstructures. However, this variety has its disadvantage in the need for costly trial and error experiments. Therefore, numerical predictions are inevitable in locating promising process parameter combinations.We present the weak coupling of a Finite-Element (FE) model with a cellular automaton (CA) model to predict the microstructure evolution during SEBM. We use an FE model for the computation of the heat input, since the temperature history mainly influences the subsequent microstructure evolution. The FE model provides precomputed temperature fields for our sophisticated CA-based crystal growth model, which successfully predicts bulk material microstructures. The compound is validated by accurately reproducing the microstructure of an additively build cylinder from CMSX-4 powder, a nickel based superalloy. Special emphasize is laid on the accurate prediction of the microstructure both in the shell as well as the core of the cylinder.

High-speed laser interferometric distance sensor with reference mirror oscillating at ultrasonic frequencies

Abstract Optical measurement systems are an important part of the portfolio of 3D topography sensors. By precise, contactless and rapid measurements these sensors constitute an alternative to tactile instruments. In this contribution the principle of a laser interferometric distance sensor is presented, which in combination with lateral scan axes acts as a topography sensor and also as distance sensor for the compensation of vibrations in a coherence scanning Linnik interferometer. An advantage of this distance sensor is its high acquisition rate of height values, which in case of working as a topography sensor enables high scan velocities as it is demonstrated at a chirp standard measured with a scan velocity of 80 mm/s. This is much higher than the scan velocity of tactile instruments, which are typically limited up to 1 mm/s. In addition, the compensation of vibration disturbances demonstrates the capability of the fast distance measurement. In contrast to other existing high-speed point sensors the relevant components are mass products. This keeps the costs of the sensor setup in a limited range. Furthermore, the sensor shows potential of much higher measurement rates than 116 kHz provided by the sensor used here.

Loss Mechanisms of Optical Waveguides Inscribed in Fused Silica by Femtosecond Laser Direct Writing

Optical waveguides directly written in fused silica using a femtosecond laser were characterized from 350 to 1750 nm to gain insight on the waveguide's loss mechanisms and their dependence on processing parameters, such as pulse energy, scan velocity, and annealing temperature. Two major loss mechanisms were identified. In the range of parameters tested, high pulse energy was seen to improve coupling losses at long wavelengths, while high scan velocity has a negative effect in both Rayleigh scattering and coupling losses at long wavelengths. Thermal annealing of the waveguides demonstrated an improvement of the Rayleigh scattering at a cost of higher coupling losses at long wavelengths. Wavelength independent Mie scattering was also observed, evolving negatively with pulse energy. A minimum Rayleigh scattering coefficient of ≍0.5 dB⋅cm−1⋅μm4 (≍0.08 dB⋅cm−1⋅μm4 for thermally treated waveguides) together with a Mie scattering coefficient of ≍0.2–0.65 dB/cm are reported.

An experimental study of micro-machining of hydroxyapatite using an ultrashort picosecond laser

Bio-ceramics typified by hydroxyapatite (HAP) are drawing more and more attention due to their unique properties including bioactivity, biocompatibility and proper mechanical hardness and strength. However, the machinability of HAP by using conventional methods is poor. An exploration of micro-grooving of HAP employing ultrashort pulsed laser is carried out in this study, and orthogonal experimental method is used with various factors considered including laser scanning times, high voltage level, pulse frequency, scanning velocity and focal plane position. Range analysis is conducted and the significance of selected factor on the groove profile is discussed. It has been found that microgrooves of width and depth varying between 20∼45 μm without obvious thermal damage can be achieved when the parameters are properly selected, demonstrating the feasibility of the ultrashort picosecond laser in micro-processing of HAP. Meanwhile, cracks, recast layer and edge fracture occur in few cases, suggesting that the material removal mechanism in this process is not purely phase explosion as for femtosecond laser. The energy dispersive spectroscopy (EDS) comparison results between ablated and non-ablated surfaces illustrate an obvious change in the composition ratio of element oxygen (O), indicating that there may exist chemical reaction at the laser irradiation zone. Based on the analyses on groove profile and thermal damage, recommendations are made on processing parameters selection for micro-grooving of HAP.

Thermal behavior and grain growth orientation during selective laser melting of Ti-6Al-4V alloy

Directional solidification causes a columnar grain microstructure with orientation deviating from building direction to the laser scanning direction which leads to anisotropic mechanical properties of deposited parts in selective laser melting (SLM) additive manufacturing (AM). However, compared to directed energy deposition (DED), the tilt angles between the grain growth orientation and the building direction in SLM are relatively small and hard to investigate due to the high scanning velocity and small facula during processing. This paper investigates the grain growth orientation in the SLM process via finite element analysis and experiments. The maximum heat flow directions and temperature gradients at different deposition heights are studied in order to discuss the local grain growth orientation. Further investigations into the differences in the grain growth orientation under different process parameters have also been carried out. In addition, the research provides a deeper understanding of the grain structures and is expected to help control the solidification texture, which is of vital importance to properties of components.

Effect of laser treatment parameters on surface modification and tribological behavior of AISI 8620 steel

In this study, the effect of different shielding gas and laser scanning velocity on the resulting retained austenite (RA) content and tribomechanical properties of AISI 8620 steel were investigated. RA generation was comparatively lower at higher scanning velocity and with argon as the shielding gas due to lower beam interaction time and inert environment. Varied heat input and differing reaction volume resulted in different hardness levels on these samples. Air shielded samples showed slightly higher friction coefficient than the other samples due to higher surface roughness and shearing of oxide layers. The nitrogen shielded samples exhibited the best wear resistance among all samples, which was attributed to nitride formation on surface and to high hardness resulting from the laser treatment.

Distribution Characteristics and Reinforcing Behavior of (Ti, Nb)C Reinforced Particle in the Coating Fabricated by Laser Rapid Cladding

(Ti, Nb)C reinforced Fe-based laser coatings were prepared with normal and high scanning velocities of the laser beam. The distribution characteristics of reinforced particles in the coatings were investigated. The mechanical properties of coatings were tested. The results showed that the morphologies of the microstructure and the reinforced particle changed dramatically at high solidification rate due to rapid laser processing compared with that prepared by normal processing. Two kinds of particles were observed in the coating. One was (Ti, Nb)C multiple carbide particle with the size of micron and sub-micron scales, in which a mass of dislocations were found. Another was nano-sized particle including α-Fe and (Ti, Nb) C obtained by rapid solidification. The microstructure of the coatings was highly refined and a large number of twin crystals were found in matrix. The results of mechanical properties test revealed that the wear resistance of the coating was improved by rapid laser processing, compared with that of the coating prepared with normal speeds. The above-mentioned conclusion indicated that rapid laser cladding can promote not only the processing efficiency but also the mechanical properties of the coating.

Process Monitoring at Laser Welding of Thermoplastics

AbstractA pyrometer, integrated into a 3D‐scanner, offers the possibility to measure the weld seam temperature at quasi‐simultaneous laser transmission welding. Experimental studies have shown that gaps located in the joining zone can be identified by a temperature rise even at a high scanning velocity. This enables the implementation of algorithms for observation and control strategies.

Herschel Hi-GAL imaging of massive young stellar objects

We used Herschel Hi-GAL survey data to determine whether massive young stellar objects (MYSOs) are resolved at 70$\mu$m and to study their envelope density distribution. Our analysis of three relatively isolated sources in the l=30{\deg} and l=59{\deg} Galactic fields show that the objects are partially resolved at 70$\mu$m. The Herschel Hi-GAL survey data have a high scan velocity which makes unresolved and partially resolved sources appear elongated in the 70$\mu$m images. We analysed the two scan directions separately and examine the intensity profile perpendicular to the scan direction. Spherically symmetric radiative transfer models with a power law density distribution were used to study the circumstellar matter distribution. Single dish sub-mm data were also included to study how different spatial information affects the fitted density distribution. The density distribution which best fits both the 70$\mu$m intensity profile and SED has an average index of ~0.5. This index is shallower than expected and is probably due to the dust emission from bipolar outflow cavity walls not accounted for in the spherical models. We conclude that 2D axisymmetric models and Herschel images at low scan speeds are needed to better constrain the matter distribution around MYSOs.

Laser-deposited CoCrMo alloy: Microstructure, wear, and electrochemical properties

Abstract

Contact Force and Scanning Velocity during Active Roughness Perception

Haptic perception is bidirectionally related to exploratory movements, which means that exploration influences perception, but perception also influences exploration. We can optimize or change exploratory movements according to the perception and/or the task, consciously or unconsciously. This paper presents a psychophysical experiment on active roughness perception to investigate movement changes as the haptic task changes. Exerted normal force and scanning velocity are measured in different perceptual tasks (discrimination or identification) using rough and smooth stimuli. The results show that humans use a greater variation in contact force for the smooth stimuli than for the rough stimuli. Moreover, they use higher scanning velocities and shorter break times between stimuli in the discrimination task than in the identification task. Thus, in roughness perception humans spontaneously use different strategies that seem effective for the perceptual task and the stimuli. A control task, in which the participants just explore the stimuli without any perceptual objective, shows that humans use a smaller contact force and a lower scanning velocity for the rough stimuli than for the smooth stimuli. Possibly, these strategies are related to aversiveness while exploring stimuli.

Influence of the pulse frequency and water cooling on the femtosecond laser ablation of bovine cortical bone

Ultrafast lasers are extremely promising tools for minimally-invasive orthopedic surgery, but the ablated volumes per pulse are low, so a high pulse frequency is necessary to reach practical ablation rates. The purpose of this work was to study in vitro the influence of the pulse repetition rate on the ablation rate, surface topography and surface composition of bone using of bovine cortical femur as a model. The tests were carried out by scanning the laser beam in relation to the sample, using pulse frequencies between 50 and 3000Hz, scanning velocities from 0.5 to 10mm/s and average pulse energy of 650μJ. The experiments were performed in dry conditions and with water irrigation. The higher ablation rates were obtained at high scanning velocity without water irrigation but severe thermal effects such as resolidification, cracking and, eventually, carbonization occurred in these conditions due to heat accumulation in the tissue. Thermal damage was avoided for all the laser processing parameters ranges tested by using water cooling. The highest ablation rate achieved was 1.4mm3/min for a scanning velocity of 10mm/s at 2kHz pulse repetition rate under water irrigation.

Microstructure and Transformation Behavior of in-situ Shape Memory Alloys by Selective Laser Melting Ti–Ni Mixed Powder

The selective laser melting (SLM) of Ti–Ni mixed powder with atomic ratio of 1:1 was performed in the present work in order to elaborate shape memory alloy (SMA). The martensite phase of Ti–Ni alloy can be found by X-ray diffraction (XRD) analysis under temperature field, moreover, the Ti2Ni phase at a high scanning velocity. The crystalline phase images also show that the synthesized Ti–Ni alloy possessed a refined martensite microstructure. In order to evaluate the mechanical properties, the microhardness and porosity were measured. The microhardness is relatively high about 400HV0.2 with champ temperature. Besides, the porosity is quite low due to the excellent laser energy absorptivity and meltability of Ni element. The differential scanning calorimetry (DSC) analysis shows that the transformation temperature from austenite phase to martensite phase is relatively high and stable.

Microstructure evolution and density behavior of CP Ti parts elaborated by Self-developed vacuum selective laser melting system

This work mainly focuses on the evolution of density behavior and microstructure of commercial pure (CP) Ti parts elaborated by SLM under vacuum system (1×10−4bar) developed at the IRTES-LERMPS laboratory. The mechanism of melt and evaporation process during selective laser melting (SLM) under vacuum environment is also presented in this paper. The surface quality, density and microhardness of Ti samples were measured as a mechanical property. Fine hexagonal crystal structure of α can be found with low scanning velocity, martensitic formation α′ can be observed with high scanning velocity, the method of scanning twice can enlarge the of grain size. A systemic SLM process under vacuum is proposed to calculate the maximum temperature of the molten pool and reveals the evolution of the solidification of melting pool under the laser beam irradiation.

Effect of Injection Parameters on Immersion Liquid during Wafer Scanning

Double patterning improves the development of immersion lithography, but it brings new challenges including the higher scanning velocity of wafer. In this paper, considering two typical injection types, the three-dimensional computational fluid dynamics model is developed to investigate the effect of injection parameters on immersion liquid during wafer scanning. By analyzing the velocity distribution of immersion flow and normal stress on the lens, the proper parameters of injection are proposed under the typical conditions, and the appropriate parameters which lead to effective renovation with low lens distortion are also given.

High-speed atomic force microscopy in slow motion—understanding cantilever behaviour at high scan velocities

Using scanning laser Doppler vibrometer we have identified sources of noise in contact mode high-speed atomic force microscope images and the cantilever dynamics that cause them. By analysing reconstructed animations of the entire cantilever passing over various surfaces, we identified higher eigenmode oscillations along the cantilever as the cause of the image artefacts. We demonstrate that these can be removed by monitoring the displacement rather than deflection of the tip of the cantilever. We compare deflection and displacement detection methods whilst imaging a calibration grid at high speed and show the significant advantage of imaging using displacement.

Circular mode: A new scanning probe microscopy method for investigating surface properties at constant and continuous scanning velocities

In this paper, we introduce a novel scanning probe microscopy mode, called the circular mode, which offers expanded capabilities for surface investigations especially for measuring physical properties that require high scanning velocities and/or continuous displacement with no rest periods. To achieve these specific conditions, we have implemented a circular horizontal displacement of the probe relative to the sample plane. Thus the relative probe displacement follows a circular path rather than the conventional back and forth linear one. The circular mode offers advantages such as high and constant scanning velocities, the possibility to be combined with other classical operating modes, and a simpler calibration method of the actuators generating the relative displacement. As application examples of this mode, we report its ability to (1) investigate the influence of scanning velocity on adhesion forces, (2) measure easily and instantly the friction coefficient, and (3) generate wear tracks very rapidly for tribological investigations.

Studies of magnetic properties of permalloy (Fe–30%Ni) prepared by SLM technology

In the present study, a high permeability induction Fe–30%Ni alloy cubic bulk was prepared by the selective laser melting process. In order to reveal the microstructure effect on soft magnetic properties, the microstructure and magnetic properties of the Fe–30%Ni alloy were carefully investigated by scanning electron microscopy, X-ray diffraction and hysteresis measurements. The bcc-Fe (Ni) phase formation is identified by X-ray diffraction. Meanwhile, it was found that low bcc lattice parameter and high grain size could be obtained when high laser scanning velocity and low laser power were used. Moreover, the lowest value of coercivity is 88A/m, and the highest value of saturation magnetization is 565Am2/kg, which can be obtained at a low laser scanning velocity of 0.4m/s and high laser power input at 110W.

Effect of Forming Conditions on Crystallization in Laser Forming of Palladium Based Thin Film Metallic Glass

Pd77Cu6Si17 thin film metallic glasses of which thicknesses were 10, 20 and 28 µm were bent by laser forming. The working conditions were changed and crystallization of thin films was investigated. When the laser power was changed under fixing the scanning velocity to 40 mm/s and the Q-switch frequency to 3 kHz, crystallization was observed at laser irradiated surface and/or the reverse side to the laser irradiation. In a case of changing the scanning velocity under fixing the Q-switch frequency to 3 kHz and the laser power to 2.0 W for the film thickness of 20 µm and 3.0 W for the film thickness of 28 µm, the working conditions without crystallization were found at high scanning velocity. When the Q-switch frequency was changed under fixing the scanning velocity to 40 mm/s and the laser power to 2.0 W for the film thickness of 20 µm and 3.0 W for the film thickness of 28 µm, the working conditions without crystallization were found at low Q-switch frequency.

Design of an Optical system for the In Situ Process Monitoring of Selective Laser Melting (SLM)

Selective Laser Melting (SLM) is an Additive Manufacturing technology that enables the production of complex shaped individual parts with series identical mechanical properties. Areas of improvement are up to now quality and reproducibility of parts made by SLM due to different kinds of errors. Therefore the integration of a monitoring and control module into a SLM-machine is aspired. The design of such an optical system capable of monitoring high scanning velocities and melt pool dynamics is introduced as a first step.