Moving Laser Source Research Articles

Improving Laser Powder Bed Fusion Printability of Tungsten Powders Using Simulation-Driven Process Optimization Algorithms.

This study applies numerical and experimental techniques to investigate the effect of process parameters on the density, structure and mechanical properties of pure tungsten specimens fabricated by laser powder bed fusion. A numerical model based on the simplified analysis of a thermal field generated in the powder bed by a moving laser source was used to calculate the melt pool dimensions, predict the density of printed parts and build a cost-effective plan of experiments. Specimens printed using a laser power of 188 W, a scanning speed of 188 mm/s, a hatching space of 80 µm and a layer thickness of 30 µm showed a maximum printed density of 93.2%, an ultimate compression strength of 867 MPa and a maximum strain to failure of ~7.0%, which are in keeping with the standard requirements for tungsten parts obtained using conventional powder metallurgy techniques. Using the optimized printing parameters, selected geometric artifacts were manufactured to characterize the printability limits. A complementary numerical study suggested that decreasing the layer thickness, increasing the laser power, applying hot isostatic pressing and alloying with rhenium are the most promising directions to further improve the physical and mechanical properties of printed tungsten parts.

A Systematic Study on the Processing Strategy in Femtosecond Laser Scribing via a Two-Temperature Model.

Balancing quality and productivity, especially deciding on the optimal matching strategy for multiple process parameters, is challenging in ultrashort laser processing. In this paper, an economical and new processing strategy was studied based on the laser scribing case. To reveal the temperature evolution under the combination of multiple process parameters in the laser scribing process, a two-temperature model involving a moving laser source was developed. The results indicated that the peak thermal equilibrium temperature between the electron and lattice increased with the increase in the laser fluence, and the temperature evolution at the initial position, influenced by subsequent pulses, was strongly associated with the overlap ratio. The thermal ablation effect was strongly enhanced with the increase in laser fluence. The groove morphology was controllable by selecting the overlap ratio at the same laser fluence. The removal volume per joule (i.e., energy utilization efficiency) and the removal volume per second (i.e., ablation efficiency) were introduced to analyze the ablation characteristics influenced by multiple process parameters. The law derived from statistical analysis is as follows; at the same laser fluence with the same overlap ratio, the energy utilization efficiency is insensitive to changes in the repetition rate, and the ablation efficiency increases as the repetition rate increases. As a result, a decision-making strategy for balancing quality and productivity was created.

Measurement on a thin plate of stepped thickness using zero-group velocity effect of an S1 mode Lamb wave induced by a moving laser source.

The desired narrowband mode of a Lamb wave can be generated efficiently, as long as the laser's moving speed matches the mode's phase velocity, which is a typical characteristic of the moving continuous wave (CW) laser excitation method. In this paper, S 1 mode zero group velocity (ZGV) waves can be generated efficiently when the laser's moving speed matches the phase velocity of the S 1 mode at the ZGV point, while the two fundamental Lamb modes, i.e.,A 0 and S 0 waves, can be avoided. Meanwhile, measurements on thin plates of stepped thickness were carried out by using the zero-group velocity effect of the S 1 mode. Those simulation results demonstrated that the S 1-Z G V resonance can be used to measure the thickness of thin plates by tracking the resonance peak as the sample was scanned, and good accuracy can be achieved, since the measurement was reduced to a simple frequency gauging of sample resonances in the ultrasonic domain, which provides a reliable method to measure a thin plate of varying thickness.

The generation of large-amplitude surface acoustic waves induced by a moving laser source

This letter focuses on the non-contact generation of surface acoustic waves by using a continuous wave laser moving along the sample surface. The desired large-amplitude surface acoustic waves can be generated efficiently if the laser moving speed matches the Rayleigh value, which is a typical characteristic of the continuous laser moving excitation method. The different amplitudes of surface acoustic waves were generated at different laser moving speeds, and the largest amplitude occurred when the laser moving speed approached the surface acoustic wave velocity of the material. Subsequently, the influence of laser moving distance on the wave’s amplitude under Rayleigh resonance conditions was investigated. The surface acoustic waves’ amplitude grew linearly with the laser’s moving distance in the elastic range. In brief, this paper provides a novel method to excite large amplitude surface acoustic waves by simply changing the moving speed and moving distance of the light.

Analytical Model of Heating an Isotropic Half-Space by a Moving Laser Source with a Gaussian Distribution

This study presents the solution of the transient spatial problem of the impact of a moving source of heat flux induced by laser radiation on the surface of a half-space using the superposition principle and the method of transient functions. The solution is based on the Green’s function method, according to which the influence function of a surface-concentrated heat source is found at the first stage. The influence function has axial symmetry and the problem of finding the influence function is axisymmetric. To find the Green’s function, Laplace and Fourier integral transforms are used. The novelty of the obtained analytical solution is that the heat transfer at the free surface of the half-space is taken into account. The Green’s function that was obtained is used to construct an analytical solution to the moving heat-source problem in the integral form. The kernel of the advising integral operator is the constructed Green’s function. The Gaussian distribution is used to calculate integrals on spatial variables analytically. Gaussian law models the distribution of heat flux in the laser beam. As a result, the corresponding integrals on the spatial variables can be calculated analytically. A convenient formula that allows one to study the non-stationary temperature distribution when the heat source moves along arbitrary trajectories is obtained. A numerical, analytical algorithm has been developed and implemented that allows one to determine temperature distribution both on the surface and on the depth of a half-space. For verification purposes, the results were compared with the solution obtained using FEM.

Modeling layer-by-layer laser melting and solidification of binary alloy powder bed

A model for simulating layer-by-layer melting and solidification of a binary alloy powder bed due to a moving laser source is presented in this paper. The model uses a modified enthalpy-porosity approach to capture simultaneous melting and solidification of a powder bed. The effects of surface tension driven Marangoni convection and thermal and solutal buoyancy driven convection are incorporated in the model. Multiple layer formation is modeled by shifting the domain in the vertical direction to include the new layer. Simulations are performed for Al-Cu alloy to see the effect of different process parameters on the melt pool evolution, solute transport and segregation, and thermal transport with the primary focus on quantifying the nonhomogeneity in the final species distribution. Fixed melt pool results show that the effect of Marangoni convection is dominant resulting in considerably increased segregation in the solidified region. Simulation of layer-by-layer melting and solidification of the entire domain show that the solidified region can be divided into three zones based on solute distribution—initial low concentration zone, middle zone with slightly higher concentration, and the final zone with very high concentration. Subsequently, parametric studies are done that show that the nonuniformity in solute distribution can be reduced by reducing the laser power, increasing the laser spot radius, increasing the initial solute concentration, or decreasing the layer thickness.

The development of grain structure during additive manufacturing

Additive manufacturing of structural alloys results in the formation of complex microstructures, often with long, columnar grains that grow epitaxially from existing grains. A phase-field modeling framework is presented that considers solidification along a single track of 316L stainless steel in a regime where the solid-liquid interface is moving sufficiently fast that there is absolute interfacial stability with negligible composition variations. By coupling to a thermal field of a moving laser source, the model captures the trajectory of grains solidifying around a melt pool. The effect of interfacial kinetic anisotropy on the predicted as-solidified microstructures is examined through three-dimensional simulations. Through a combination of qualitative and quantitative analyses, we find that kinetic anisotropy has the largest impact along the center of the laser track and that the grain morphology in the early stages of solidification is predominantly due to the shape of the melt pool.

Spatial distribution of the phononic crystal modes excited by a moving laser source

In this paper, we have presented the dispersion curve of Lamb waves in an aluminum plate with a periodic rectangular corrugation experimentally. A mode selection method was applied to generate Lamb waves in the region of the corrugated periodic structure using a moving continuous wave laser, and the signal can be detected at any point in the excitation region. The relationship between the phase velocity and the frequency is observed. It is found that the intensity distribution of the dispersion curves in the experimental results is related to the slope of the dispersion curves as well as the selection of the detection point. Vibrations at a series of locations were detected, reflecting the change in energy that Lamb waves undergo in the process of propagation through the periodic structure, which is different from the existing method of single-point excitation and detection. The spatial distribution of the mode amplitude at the resonances points, in particular induced by the Bragg reflection, has been studied.

Generation of negative group velocity Lamb waves by a moving laser source

Analytical existence conditions for negative group velocity (NGV) of an arbitrary mode Lamb waves in elastic plate was obtained. Experimentally, NGV waves were excited by means of a thermoelastic laser source moving across the sample plate with a controllable velocity. Such source is coupled with modes which phase velocity component parallel to the surface coincides with the source speed. Additional mode selection was performed by choosing a quasiperiodic shape of the laser spot, which specifies the range of the wavenumber k. This technique was successfully applied for generation of backward (NGV) waves. By tuning the speed of the source the propagation direction was switched from forward to reversal.

In situ synchrotron X-ray imaging of 4140 steel laser powder bed fusion

An in situ synchrotron X-ray imaging technique was used to examine laser powder bed fusion (LPBF) of a chrome-molybedum, AISI 4140, ferrite–bainite steel, from a moving laser source. Since 4140 has received only minimal attention in the AM literature, focus here was on the effects of melt pool dynamics, vapor cavity depths, build layer height, and the origin of porosity on the 4140 as-built microstructure. Four build parameter sets, which enabled variation of laser power, scan speed, laser spot size, and specific energy density (SED), were applied. Vapor cavity and melt pool depths were measured for each single-track laser scan. Direct imaging of the LPBF process demonstrated that the primary source of porosity in the build originates from incorporation of entrapped gas within the powder irrespective of build parameter set. High resolution SEM and EBSD demonstrated that the as-built microstructure did not vary significantly with build parameter set over a broad range of SED. The fast solidification rates observed (0.06–0.08 m/s) lead to fine martensite packet/block size distributions (1–3 µm) with the potential for increased material strength.

Reduced order modeling via PGD for highly transient thermal evolutions in additive manufacturing

In this paper, a highly performing model order reduction technique called Proper Generalized Decomposition (PGD) is applied to the numerical modeling of highly transient non-linear thermal phenomena associated with additive manufacturing (AM) powder bed fabrication (PBF) processes. The manufacturing process allows for unprecedented design freedom but fabricated parts often suffer from lower quality mechanical properties associated with the fast transients and high temperature gradients during the localized melting-solidification process. For this reason, an accurate numerical model for the thermal evolutions is a major necessity. This work focuses on providing a low-cost/high accuracy prediction of the high gradient thermal field occurring in a material under the action of a concentrated moving laser source, while accounting for phase changes, material non-linearities and time and space-dependent boundary conditions. An extensive numerical simulation campaign shows that the use of PGD in this context enables a remarkable reduction in the total number of global matrix inversions (5 times less or better) compared to standard techniques when simulating realistic AM PBF scenarios.

Heat transfer in granular medium for application to selective laser melting: A numerical study

In this study, the heat transfer in a granular medium due to a moving laser source is investigated numerically. A numerical model on laser energy absorption and its transport in granular media is presented. This process is especially relevant to laser melting/sintering of metal powders. The non-continuum approach is used to consider the discrete nature of granular medium explicitly. The validation of each of the individual modules of the thermal model is presented. The simulation results demonstrate the ability of numerical approach to predict powder bed properties and influence of laser melting/sintering process variables (grain size, material emissivity, substrate property etc.) on laser energy distribution and transport in the powder bed.

Study of the Influences of Laser Parameters on Laser Assisted Machining Processes

Hybrid machining processes using additional energy sources such as laser assisted machining (LAM) have increased considerably during the last years. The benefits of LAM for reducing tool wear and cutting forces are well known, especially for superalloys. However, optimal machining results depend on both the laser parameters and the cutting process parameters. It is difficult to find optimal LAM settings due to the complexity of the influencing parameters and their mutual interactions. The aim of the paper is to characterize the laser heating process by detecting how the individual LAM parameters influence working temperature, heat affected zone (HAZ) extension and laser track width. A reliable application requires a localized and controlled continuous heating of the material within the machining zone directly in front of the tool contact area. In this research statistical and technological knowledge is fully involved in the experimental activities. Therefore, a statistical study based on design of experiment (DoE) was carried out in order to investigate the effects of laser process parameters and their interactions. In practice, two-level fractional factorial design and analysis of variance (ANOVA) were applied. The following process parameters were examined: laser power, scanning speed, defocus (the distance between focal point and workpiece surface), temperature controlled by pyrometer, and surface roughness. Furthermore, a Finite Element (FE) model was developed based on experimental results in order to find out the optimal parameters for modeling the laser heating process. Future FE simulations of the laser assisted cutting processes will be carried out using this model of the moving laser source.

FEM Simulation Technologies of Laser Cutting Wood Board Based on ANSYS

Laser cutting is an important application field of laser machining technology. It can improve machining quality and save time by using a computer to simulate the machining process to obtain the optimal process parameters. In the paper, the FEM simulation techniques for laser cutting are discussed by taking wood as an example and using the FEM software ANSYS as a simulation tool. The technologies include solid modeling, finite element meshing, loading of the moving laser source and secondary developing of APDL language. laser cutting; finite element method; simulation

Numerical Model for Multilayer Thin Films Irradiated by a Moving Laser Source

A two-dimensional transient analysis of the conjugate optical-thermal fields induced in a multilayer thin film structure on a glass substrate by a moving Gaussian laser source is carried out numerically. The workpiece is considered semi-infinite along the motion direction and its optical and thermophysical properties are assumed temperature dependent. The COMSOL Multiphysics 3.3 code has been used to solve the combined thermal and electromagnetic problem. The optical field is considered locally one dimensional and Maxwell equations are solved in order to evaluate the absorption in thin film. Results, in terms of transient temperature profiles and fields, are presented for different Peclet numbers and starting point of the heat source with respect to the workpiece boundary along the motion direction.

Analytical and numerical modelling of the direct metal deposition laser process

The direct metal deposition (DMD) laser process is a novel technique, well adapted for aeronautical applications, that allows the building of complex 3D geometries through the interaction between a powder nozzle system and a continuous laser beam. A three-step analytical and numerical approach was carried out to predict the shapes of manufactured structures and thermal loadings induced by the DMD process. First, powder temperature was calculated using a recent analytical model, then the geometry of walls was predicted by a combined numerical + analytical modelling using a discretization of the physical interaction domain, and finally, a finite element calculation was carried out on COMSOL 3.3 Multiphysics software to describe thermal behaviour during DMD of a titanium alloy.Our thermal model takes into account the moving interface during metal deposition with a specific function κ (t, x, y, z) allowing the conductivity front to move simultaneously with the moving laser source (with an appropriate spatial energy distribution), thus representing rather precisely the DMD process. This allowed us to provide an adequate representation of temperatures near the melt-pool, and to reproduce with a good accuracy thermal cycles and melt-pool dimensions during the construction of up to 25-layer walls. This was confirmed by comparisons with experimental thermocouple data T = f(t), and fast camera melt-pool recording.

Numerical modelling of the laser surface processing of magnesia partially stabilized zirconia by the means of three-dimensional transient finite element analysis

A numerical technique has been employed to use the ABAQUS finite element analysis (FEA) package in order to simulate the CO2laser surface processing of a magnesia partially stabilized zirconia (MgO-PSZ) bioinert ceramic. The transient FEA takes into account the heat radiation, heat convection and phase change during the laser processing. The heat source has been modelled as a stepwise moving laser source with small steps in the scanning direction to approximate continuous movement. It further extends and validates numerical methods by comparing experimental data of surface temperature for laser surface processing of the MgO-PSZ to the solution from the FEA model. Experiments involving CO2laser surface melting of the MgO-PSZ were also carried out using various laser process parameters, and the measured melt width and depth of laser-treated tracks were used to evaluate the validity of the models. In order to prevent the crack formation in the laser processing, pre- and post-heating were proposed by using the scanning of laser beam with the lower power before and after laser processing with high power to lower the thermal gradient.

Une modélisation de la genèse des contraintes résiduelles lors d'un traitement superficiel par une source laser mobile

The numerical simulation of the internal stresses genesis in processes of heat treatment holds, indeed, to the formulation of the coupled thermomechanics–metallurgic problem taking into account several and severe physical non linearities. This study aims to develop at least, a multipurpose model for prediction of the residual stresses state generated by a superficial heat treatment in solid phase without diffusion. For the generality of the models set up, to the sense of the thermomechanical history and other coupling terms importance, we consider the case of the treatment by a moving laser source and self quenching. Thus, the solution of this evolution problem, will lie in both the thermal and thermomechanical aspects as well as in the phase transformation effects. For the thermal analysis, we propose a non linear transient model allowing the simulation of the whole thermal history during the treatment process until the return to the thermal balance. The thermomechanical analysis is dealt with a non linear incremental model whereby we simulate the mechanical history since the beginning of the treatment until the residual stress state establishing. In the numeric scheme developed within a FEM code, the mechanical features are described in space–time evolutions depending on the temperature and the local instantaneous phases which are present throughout the domain. We introduce a global dilatometric coefficient to take into account the simultaneous and cumulated thermal and phase transformation strains. The comparison of numeric and experiment results reveals on the one hand a better qualitative consistency of the model, and tends on the other hand towards a new description of the structural transformations to reach a truly quantitative model.

Generation of elastic waves in a laminated solid by a moving photothermal source

The steady state response of an elastic layered plate (laminate) which is subjected to a moving laser source illumination is studied. The response of the laminate is obtained using the transfer matrix approach. The application of the photo-thermal source (laser) to the upper surface of the laminate is formulated as equivalent stresses applied at the illuminated boundary. The equivalent stresses are derived with the use of the causality principle. It is shown that the generated displacement field is sensitive to the variations of the laminate inner structure and also to the variations of the elastic properties of a bonded elastic half-space.

Distortion minimization of laser‐processed components through control of laser scanning patterns

Residual thermal stresses and distortion are frequently present in the parts built using a layer‐by‐layer solid freeform fabrication techniques assisted with a moving laser source. This study uses finite element analysis to investigate the effect of laser scanning patterns on residual thermal stresses and distortion. It is shown that the out‐of‐plane distortion of a layer, processed by a moving laser beam can be minimized with a proper selection of the laser scanning pattern. A scanning pattern having changes in its scanning direction frequently by 90° at every turn can lead to the cancellation of concave upward and downward distortions. As a result of this cancellation, very small distortion is present in the laser processed plate. It is also found that distortion is mainly caused by transient thermal stresses rather than residual thermal stresses.