Laser-metal Interaction Research Articles

Temperature measurement in laser–metal processing using optical sensing: Radiometric calibration and validation of a multispectral camera

In laser–metal processing such as Laser-based Powder Bed Fusion of Metals (PBF-LB/M), the quality of the final product is significantly influenced by the thermal behaviour of the melt pool. Accurate temperature measurements are challenging due to high temperature gradients and rapid cooling. This study introduces Multispectral Imaging (MSI) as a highly effective thermometry technique to address these demanding conditions. MSI captures multiple bands of radiation, enabling the simultaneous determination of absolute temperature and surface emissivity. This capability is crucial for ensuring high-quality outcomes in laser–metal processing such as PBF-LB/M, contingent upon robust radiometric calibration.To enhance reliability, two radiometric calibration models were developed: an empirical model based on linear sensor data correlations and an analytical model leveraging sensor behaviour. An efficient calibration was found for both models across a range of signal-to-noise ratios in the black body calibrator, where the analytical model even performs robust under a high noise level.Additionally, the temperature accuracy in the solid-state case was tested with a thermo-mechanical simulator. Results showed that the empirical and analytical models achieved mean relative errors of 2.0% and 1.6%, respectively, outperforming the state-of-the-art. Further, laser irradiation experiments highlighted the analytical model’s ability to accurately determine the emissivity decrease during the solid-to-liquid transition, allowing for accurate melt pool temperature estimations. Conversely, the empirical model struggled to estimate temperature effectively in the liquid phase.This study not only proposed a robust methodology of MSI in temperature measurement but also confirmed the accuracy and reliability of the system, broadening the scope for further investigation into the intricate thermal dynamics involved in laser–metal interactions.

Minimizing Defects in Additive Manufacturing and Laser Welding Through Energy Coupling Experiments and Dynamic Modelling

Additive manufacturing is a revolutionary technology that produces complex components as a single piece, replacing traditional assemblies and reducing waste. Laser powder bed fusion, a type of additive manufacturing, allows for limitless design freedom by building intricate parts layer-by-layer through melting and fusing metal powder. However, build quality remains inconsistent, with deformations like pores and spattering occurring. To improve modelling of the behaviour of melted metal, thus minimizing defects, an experiment which determined the metal’s absorptance of laser energy through integrating sphere radiometry was conducted. The integrating sphere, which spatially integrated a fraction of measured light to calculate full radiance, had to sit above the build plate and not interfere with powder spreading. I designed two parts which attached the integrating sphere to the motorized powder spreading blade. Utilizing small ridges in the enclosure as weight-bearing tracks, the blade’s torque was reduced while allowing precise movement of the sphere. Through implementing these parts, the experiment was conducted, whose results will aid in our understanding of laser-metal interactions. Defects like spattering also occur during laser welding, due to the vaporization of unstable liquid metal. To combat this, novel beam shapes have been seen to increase the stability of the keyhole. To model this effect, I created a phenomenological, dynamic model of the keyhole depth during a copper weld. Two differential equations were created to model the keyhole depth’s rate of change using a dual mode and single mode laser, depending on absorptance and incident laser intensity. The dynamic depth of the keyhole for both modes was numerical solved in Python and iterated using existing copper weld data. Through better quantitative understanding of energy coupling and numerical modelling of the resulting dynamics, defects in laser welding and additive manufacturing can be minimized.

Lagrangian perspective on the expansion dynamics and shielding effect of femtosecond laser-induced copper plasma plumes

Femtosecond laser ablation of metals generates a strongly ionized plasma plume near the irradiated surface. The resulting plasma shielding effect can reduce subsequent laser energy deposition and lower nanomachining efficiency, especially during multi-pulse irradiation. Understanding the spatiotemporal evolution of the laser-induced plasma and its associated shielding effect is, therefore, crucial. A hybrid two-temperature and direct simulation Monte Carlo (TTM-DSMC) computational model is developed in this study, which synergistically couples the ultrafast laser–metal interaction physics and the plasma collisional transport. The model simulates the plasma properties including electron density, temperature dynamics, reflectivity, and energy attenuation throughout the plume expansion process from femtosecond to nanosecond timescales. A complex “penguin-shaped” plasma plume with internal shockwaves is observed due to the effects of double-pulse irradiation. Significantly enhanced plasma reflectivity and reduced laser energy deposition demonstrate the accumulated shielding effect, which increases with higher plasma density accumulation when the pulse separation is insufficient. Our model provides valuable theoretical guidance for optimizing processing parameters to enhance efficiency and precision in femtosecond laser machining. The integrated TTM-DSMC approach could also facilitate the study of laser-induced plasmas in other contexts like material characterization and nanoparticle synthesis.

Accelerating process development for 3D printing of new metal alloys

Addressing the uncertainty and variability in the quality of 3D printed metals can further the wide spread use of this technology. Process mapping for new alloys is crucial for determining optimal process parameters that consistently produce acceptable printing quality. Process mapping is typically performed by conventional methods and is used for the design of experiments and ex situ characterization of printed parts. On the other hand, in situ approaches are limited because their observable features are limited and they require complex high-cost setups to obtain temperature measurements to boost accuracy. Our method relaxes these limitations by incorporating the temporal features of molten metal dynamics during laser-metal interactions using video vision transformers and high-speed imaging. Our approach can be used in existing commercial machines and can provide in situ process maps for efficient defect and variability quantification. The generalizability of the approach is demonstrated by performing cross-dataset evaluations on alloys with different compositions and intrinsic thermofluid properties.

Pulsed laser activation method for hydrogen storage alloys

The activation procedures of metals and alloys, crucial for hydrogen absorption, pose a significant challenge in the large-scale application of metal hydrides. This study introduces, for the first time, the Pulsed Laser Activation (PLA) method for hydrogen storage alloys. We demonstrated that the hydrogen storage ability of an aged (exposed to air for 30 days) Ti11V30Nb28Cr31 body-centered cubic alloy can be restored by scanning the sample with a nanosecond pulsed laser for only 3 min. X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS) analyses were conducted to investigate structural features that changed in the Ti11V30Nb28Cr31 samples after ageing and after the PLA treatment. Surface remelting, the formation of oxide layers, and crack formation appear to be factors influencing the hydrogen storage ability of the Ti11V30Nb28Cr31 alloy activated via PLA. While the mechanisms involved in the PLA are not clear yet, this procedure paves the way for the development of activation methods based on laser-metal interactions that can be easily applied to metals and alloys for hydrogen storage systems.

Numerical study on multiphase evolution and molten pool dynamics of underwater wet laser welding in shallow water environment

Underwater wet laser welding (UWLW) is a promising and labor-saving in-situ repair technique in aquatic environments, which simplifies the arduous dismantling and transportation processes of repairing underwater equipment on land. However, the thermo-physical mechanism during UWLW is still unclear. This study presents, for the first time, a thermal multi-phase flow model to study the heat transfer, fluid dynamics, and phase transitions involved in the UWLW process at shallow water column depths (water depth ≤ 4 mm). A dual-cone volumetric heat source model considering the laser energy distribution in both the water and metal is proposed. The driving forces, including gravity, surface tension, and the water/metal vapor recoil pressure are taken into account. The simulated weld profiles are in good agreement with the experimental results. The results indicate that in the UWLW process, laser heating creates a water keyhole, forming a localized dry region (LDR) above the molten pool. The LDR isolates the water from the molten pool and protects the laser-metal interactions, thereby making the welding environment similar to that of in air laser welding (IALW). Due to the water evaporation, the upper rear and sides of the molten pool during the UWLW pocessess a higher cooling rate compared to the IALW case, resulting in a higher temperature gradient and smaller molten pool dimensions. The results contribute to a better understanding of the complex interactions in UWLW process.

Numerical simulation and experimental validation of ultrafast laser ablation on aluminum

In this paper, a numerical study of the ultrashort single pulse ablation process in aluminum has been conducted using the two-temperature model, taking into consideration the several dependencies of the parameters on temperature. The evolution of temperatures for each point within the material over time is obtained, allowing to comprehend the distribution of energy in the laser-metal interaction and its subsequent diffusion within the material. This study allows to gain a more precise understanding of the thermophysical and optical properties throughout the material heating and ablation process. The ablation process has been experimentally validated using a femtosecond laser on a polished aluminum film for a wide range of fluences. The developed theoretical model and the experimental values are in good agreement for both ablation depth and diameter across the entire range of powers studied, enabling reliable determination of the threshold fluence, laser spot size and material absorption coefficient. These parameters are crucial for achieving optimal micromachining processes.

Laser-Metal Interaction with a Pulse Shorter than the Ion Period: Ablation Threshold, Electron Emission and Ion Explosion.

The laser energy per unit surface, necessary to trigger material removal, decreases with the pulse shortening, becoming pulse-time independent in the sub-picosecond range. These pulses are shorter than the electron-to-ion energy transfer time and electronic heat conduction time, minimising the energy losses. Electrons receiving an energy larger than the threshold drag the ions off the surface in the mode of electrostatic ablation. We show that a pulse shorter than the ion period (Shorter-the-Limit (StL)) ejects conduction electrons with an energy larger than the work function (from a metal), leaving the bare ions immobile in a few atomic layers. Electron emission is followed by the bare ion's explosion, ablation, and THz radiation from the expanding plasma. We compare this phenomenon to the classic photo effect and nanocluster Coulomb explosions, and show differences and consider possibilities for detecting new modes of ablation experimentally via emitted THz radiation. We also consider the applications of high-precision nano-machining with this low intensity irradiation.

Dynamic simulation on laser-metal interaction in laser ablation propulsion considering moving interface, finite thermal wave transfer, and phase explosion

Despite extensive investigations on the mechanism of laser-irradiated solid propellants, formulating an accurate model that characterizes more details of laser-target interactions remains a challenging task. In this paper, a thermal model for nanosecond pulsed laser ablation of aluminum was developed using the finite volume method which considers the instant recession on the propellant surface and the non-Fourier effect during heat conduction. The robust coupling iteration scheme adopted in the model tracks the dynamic boundary condition due to instant propellant removal. In particular, the pre-qualified threshold is used as discriminator for phase explosion and the thermal relaxation time is introduced to quantify the non-Fourier effect on heat conduction. In addition, the temperature-dependent physical and optical parameters of the propellant were also regarded in the proposed model. The numerical implementations for Gaussian laser ablation of aluminum were performed at different laser fluences and thermal relaxation times. The theoretical computation of the ablation depth coincides well with the experimental results to validate the feasibility of the model.

Modeling laser interactions with aluminum and tantalum targets using a hybrid atomistic-continuum model

A hybrid atomistic-continuum method can model the microstructure evolution of metals subjected to laser irradiation. This method combines classical molecular dynamics (MD) simulations with the two-temperature model (TTM) to account for the laser energy absorption and heat diffusion behavior. Accurate prediction of the temperature evolution in the combined MD-TTM method requires reliable accuracy in electron heat capacity, electron thermal conductivity, and electron–phonon coupling factor across the temperatures generated. This study uses the electronic density of states (DOS) obtained from first-principle calculations. The calculated electron temperature-dependent parameters are used in MD-TTM simulations to study the laser metal interactions in FCC and BCC metals and the phenomenon of laser shock loading and melting. This study uses FCC Al and BCC Ta as model systems to demonstrate this capability. When subjected to short pulsed laser shocks, the dynamic failure behavior predicted using temperature-dependent parameters is compared with the experimentally reported single-crystal and nanocrystalline Al and Ta systems. The MD-TTM simulations also investigate laser ablation and melting behavior of Ta to compare with the ablation threshold reported experimentally. This manuscript demonstrates that integrating the temperature-dependent parameters into MD-TTM simulations leads to the accurate modeling of the laser–metal interaction and allows the prediction of the kinetics of the solid–liquid interface.

Laser Welding Penetration Monitoring Based on Time-Frequency Characterization of Acoustic Emission and CNN-LSTM Hybrid Network

In-process penetration monitoring of the pulsed laser welding process remains a great challenge for achieving uniform and reproducible products due to the highly complex nature of the keyhole dynamics within the intense laser-metal interactions. The main purpose of this study is to investigate the feasibility of acoustic emission (AE) measurement for penetration monitoring based on acoustic wave characteristics and deep learning. Firstly, a series of laser welding experiments on aluminum alloys were conducted using high-speed photography and AE techniques. This allowed us to in-situ visualize the complete keyhole dynamics and elucidate the generation mechanism of acoustic waves originating from pressure fluctuations at the keyhole wall. Then, an adaptive time-frequency technique namely VMD (Variational Mode Decomposition) was proposed to characterize the acoustic energy distribution among the nine subsignals with low-frequency and high-frequency components under different welding penetrations. Lastly, a novel hybrid model combing CNN (Convolutional Neural Network) and LSTM (Long Short Term Memory) was designed to deeply mine the spatial and temporal acoustic features from the extracted frequency components. Extensive experiments demonstrate that our proposed approach yields a remarkable classification performance with a test accuracy of 99.8% and a standard deviation of 0.21, which obtains a high recognition rate. This work is a new paradigm in the digitization and intelligence of the laser welding process and contributes to an alternative way of developing an efficient end-to-end penetration monitoring system.

3D tracking velocimetry of L-PBF spatter particles using a single high-speed plenoptic camera

Particle spatter is an unavoidable by-product of the Laser-Powder Bed Fusion (L-PBF) process, as the high intensity of the incoming laser beam generates high vapor fluxes on the meltpool, allowing metal particles to be ejected into the process environment. This is detrimental to the final manufactured part, as it risks the incorporation of defects. It is therefore important to study this spattering behavior and to apply the knowledge gained to further improve the L-PBF process. This work introduces the use of a high-speed plenoptic camera to acquire 3D spatter particle trajectories generated via L-PBF line tracks. Spatter particles are tracked in the volume above the laser-metal interaction zone at 1000 fps and their velocity calculated. It is found that the calculated speeds of the spatter particles are within the expected range for this process, and that the behavior is a complex 3-dimensional process.

Understanding keyhole induced-porosities in laser powder bed fusion of aluminum and elimination strategy

Laser powder bed fusion (LPBF) technology has the potential to revolutionize the fabrication of complex metal components in the aerospace, medical, and automotive industries. However, keyhole pores may be induced during the rapid laser-metal interaction (∼10−5 s) of the LPBF. These inner porosities can potentially affect the mechanical properties of the fabricated parts. Here, based on the experimentally observed keyhole-penetration pore (KP-pore) led by the keyhole splitting of the molten pool in LPBF, a multi-physics finite volume model was established to reveal this mechanism, where keyhole pores were formed under the direct contact of keyhole and solid metal substrate, which is different from the previously reported gas–liquid interaction. The formation mechanisms of the KP-pore, rear-front pore (RF-pore), and rear pore (R-pore) could be attributed to different keyhole fluctuation modes. The effects of the powder on the characteristics of the keyhole, molten pool, and pore formation were explored. The increased pore counts and decreased size were owing to the powder-promoting keyhole and molten pool oscillation. In addition, a relationship map between the input energy density and pore number was built via a high-throughput simulation, providing a strategy to reduce or remove the pores in laser powder bed fusion.

A versatile SPH modeling framework for coupled microfluid-powder dynamics in additive manufacturing: binder jetting, material jetting, directed energy deposition and powder bed fusion

Many additive manufacturing (AM) technologies rely on powder feedstock, which is fused to form the final part either by melting or by chemical binding with subsequent sintering. In both cases, process stability and resulting part quality depend on dynamic interactions between powder particles and a fluid phase, i.e., molten metal or liquid binder. The present work proposes a versatile computational modeling framework for simulating such coupled microfluid-powder dynamics problems involving thermo-capillary flow and reversible phase transitions. In particular, a liquid and a gas phase are interacting with a solid phase that consists of a substrate and mobile powder particles while simultaneously considering temperature-dependent surface tension and wetting effects. In case of laser–metal interactions, the effect of rapid evaporation is incorporated through additional mechanical and thermal interface fluxes. All phase domains are spatially discretized using smoothed particle hydrodynamics. The method’s Lagrangian nature is beneficial in the context of dynamically changing interface topologies due to phase transitions and coupled microfluid-powder dynamics. Special care is taken in the formulation of phase transitions, which is crucial for the robustness of the computational scheme. While the underlying model equations are of a very general nature, the proposed framework is especially suitable for the mesoscale modeling of various AM processes. To this end, the generality and robustness of the computational modeling framework is demonstrated by several application-motivated examples representing the specific AM processes binder jetting, material jetting, directed energy deposition, and powder bed fusion. Among others, it is shown how the dynamic impact of droplets in binder jetting or the evaporation-induced recoil pressure in powder bed fusion leads to powder motion, distortion of the powder packing structure, and powder particle ejection.

Evolution of an industrial-grade Zr-based bulk metallic glass during multiple laser beam melting

Selective laser melting (SLM), taking advantage of its inherent rapid cooling rates and near-net-shape forming ability, has been employed to fabricate bulk metallic glasses (BMGs). However, crystallization is frequently triggered during the SLM process, which results in the loss of advantageous properties of BMGs, such as extremely high hardness and near-theoretical yield strength. Although many studies have been conducted to investigate SLM of BMGs, there is still a lack of knowledge about the microstructural and compositional evolution during the laser beam processing, particularly the micromechanical property response upon crystallization.In the present work, a systematic investigation is performed to gain a much better understanding about the evolution of microstructure and composition as well as the corresponding micromechanical property change during multiple laser beam melting. The material used in this study is an industrial-grade Zr-based BMG Zr59.3Cu28.8Al10.4Nb1.5 (AMZ4) with two different oxygen levels. AMZ4 demonstrates its good thermal stability by the fact that observable crystalline structure appears around the melt pool only after more than once laser beam treatment. The compositional stability of AMZ4 is manifested by the homogeneous elemental distribution on the melt pool area after even twenty-five laser beam remelting. The laser-metal interaction, melting and subsequent solidification are not effectively influenced by the emerging and expanding of crystallization zone (or heat affected zone, HAZ). Higher oxygen content results in not only a larger HAZ but also more quenched-in nuclei at the melt pool bottom. The HAZ does not exhibit a fully crystallized structure, but rather has a mixture of amorphous and crystalline phases. Crystallization of AMZ4 leads to an increase in hardness and Young’s modulus of the material.

Energy Relaxation and Electron-Phonon Coupling in Laser-Excited Metals.

The rate of energy transfer between electrons and phonons is investigated by a first-principles framework for electron temperatures up to = 50,000 K while considering the lattice at ground state. Two typical but differently complex metals are investigated: aluminum and copper. In order to reasonably take the electronic excitation effect into account, we adopt finite temperature density functional theory and linear response to determine the electron temperature-dependent Eliashberg function and electron density of states. Of the three branch-dependent electron–phonon coupling strengths, the longitudinal acoustic mode plays a dominant role in the electron–phonon coupling for aluminum for all temperatures considered here, but for copper it only dominates above an electron temperature of = 40,000 K. The second moment of the Eliashberg function and the electron phonon coupling constant at room temperature K show good agreement with other results. For increasing electron temperatures, we show the limits of the approximation for the Eliashberg function. Our present work provides a rich perspective on the phonon dynamics and this will help to improve insight into the underlying mechanism of energy flow in ultra-fast laser–metal interaction.

Re-envisioning laser sources and alloys for metal Additive Manufacturing

Metal powder-bed fusion (PBF) additive manufacturing (AM), while continuing to play an important strategic role across a diverse application space, still lacks the necessary control to obtain parts that meet strict performance-driven criteria for qualification and certification. Here, a science-based approach is described which can address this shortcoming and fundamentally transform metal PBF AM through development of a modeling and experimental framework that guides optimization of the beam shape dependent laser-metal interaction and the alloy composition. Using this framework new materials and local part properties can be fabricated, thus driving the potential for more flexible metal-based AM design capabilities compared to those that are currently available today.

Multiscale simulation of rapid solidification of an aluminium–silicon alloy under additive manufacturing conditions

At present, most multiscale simulation approaches to model the temperature evolution of the molten pool, and the resulting microstructure evolution for selective laser melting, assume an equilibrium freezing range and steady state solidification conditions. This is despite the solidification conditions being observed to be highly unsteady and non-equilibrium. These two assumptions lead to inaccurate predictions of the temperature evolution of the molten pool and thus microstructure predictions. To demonstrate this, an approach to scale-bridging computational models of the laser additive manufacturing process is presented, in which the temperature history is passed from a macroscale molten pool simulation to a microscale phase-field simulation. This linkage is achieved by volume mapping of the temperature field from the grid of the molten pool simulation to the grid of the microstructure simulation. To describe the system evolution at the scale of the molten pool, a computational fluid dynamics (CFD) method that captures the laser–metal interaction, vapour production, gas recoil pressure, fluid flow, surface tension, Marangoni flow, and heat conduction, convection, and radiation is applied. To capture the chemical kinetics of the phase-transition, a non-equilibrium CALPHAD-integrated phase-field (PF) model is applied. The discrepancy between the predictions of the solid front isotherm is quantified as ⩾ 100 K for an Al-10Si alloy under the large observed cooling rate. This leads to a spatial discrepancy in the solidification front between the CFD model, which assumes equilibrium freezing behaviour, and the PF model, which does not, of approximately 10 µm over 50 µs in the present case. Under these conditions, present formulations of multiphase CFD cannot accurately predict the solidification behaviour because of the assumption of equilibrium at the solid–liquid interface. Strategies for reconciling this discrepancy for materials that exhibit rapid solidification under large thermal undercooling will need to be developed for multiscale simulation of additive manufacturing to advance.

Measurement Uncertainty of Surface Temperature Distributions for Laser Powder Bed Fusion Processes.

This paper describes advances in measuring the characteristic spatial distribution of surface temperature and emissivity during laser-metal interaction under conditions relevant for laser powder bed fusion (LPBF) additive manufacturing processes. Detailed descriptions of the measurement process, results, and approaches to determining uncertainties are provided. Measurement uncertainties have complex dependencies on multiple process parameters, so the methodology is demonstrated on one set of process parameters and one material. Well-established literature values for high-purity nickel solidification temperature and emissivity at the solidification temperature were used to evaluate the predicted uncertainty of the measurements. The standard temperature measurement uncertainty is found to be approximately 0.9% of the absolute temperature (16 AC), and the standard relative emissivity measurement uncertainty is found to be approximately 8% at the solidification point of high-purity nickel, both of which are satisfactory. This paper also outlines several potential sources of test uncertainties, which may require additional experimental evaluation. The largest of these are the metal vapor and ejecta that are produced as process by-products, which can potentially affect the imaging quality, reflectometry results, and thermal signature of the process, while also affecting the process of laser power delivery. Furthermore, the current paper focuses strictly on the uncertainties of the emissivity and temperature measurement approach and therefore does not detail a variety of uncertainties associated with experimental controls that must be evaluated for future generation of reference data.

Pulse duration dependence of dry laser peening effects in the femtosecond-to-picosecond regime

We found an optimum pulse duration for dry laser peening in the femtosecond-to-picosecond regime, in which the laser intensity exceeds the air breakdown threshold. A pulse duration of 1 ps produced the most effective peening effects under conditions wherein the laser energy was constant; this was caused by a decrease in the laser fluence due to a beam expansion of less than 1 ps, in addition to an increase in the thermal effect above 1 ps. When the laser intensity exceeds the air breakdown threshold, it is necessary to select the pulse duration while considering laser–air and laser–metal interactions.