Varying Laser Power Research Articles

Influence of temperature and beam size on weld track shape in laser powder bed fusion of pure copper using near-infrared laser system

Additive manufacturing of copper using laser powder bed fusion enables the production of highly complex components with excellent heat and electrical conductivity. However, the processing of copper by means of near-infrared laser radiation, which is commonly used, is challenging due to its high reflectivity. Nevertheless, it has been demonstrated that high densities and electrical conductivities can be achieved using high-power laser systems. In order to process pure copper with reliable quality with different machines, it is essential to understand the conditions at which a continuous weld track is formed. For this purpose, weld tracks with varying laser power and scan speeds were welded on a copper substrate plate with an applied powder layer. The preheating temperature of the substrate plate and the beam size were varied to test different process conditions. The melt pool depths and widths were measured, and a relationship was elaborated. Based on these results, cube samples with discrete weld tracks on top were manufactured. The melt pool depth was measured and compared with the predicted melt pool depth to investigate the transferability of the elaborated relationship from the substrate to process conditions. It was found that with rising preheating temperature and for larger beam diameters at the same peak intensity, the weld width and weld depths increase. Furthermore, continuous weld tracks formed reliably in the keyhole welding regime. A good agreement between the weld depth of weld tracks on the substrate and the elaborated relationship was revealed. However, the weld tracks were shallower than predicted.

Research on the effect of the variation of laser power on the contour control and performance control of laser cladding Ni60CuMo/IN718 coating on RuT300

To improve the wear resistance of Vermicular cast iron (RuT300) surface and overcome the crack inhibition bottleneck of high-hardness coating, a Ni60CuMo/IN718 composite coating was fabricated on RuT300 surface. Results indicate the cracking-free coating with a maximum hardness of 475 HV0.5 at 800 W. In addition, the hard phases MC (M=Nb, Ti), Cr7C3, CrB and the self-lubricating phase Cu3.8Ni were in-situ synthesized in the coating. Increasing laser power raised C content from substrate in the molten pool, providing more C elements for in-situ synthesized Cr7C3. Along with the consumption of Cr by carbide generation, the Cr content used for in-situ synthesized CrB decreases. Moreover, the hardness of Cr7C3 is lower than that of CrB, and the low-hardness Fe in the substrate intensifies the coatings dilution with the increasing power. Therefore, these effects result in a non-positive correlation between the increase of laser power and the coatings hardness. Due to the better anti-wear effect of the high hardness coating, the hardest coatings have the lowest wear rates at room temperature (RT). Abrasive wear is the primary wear mechanism for these coatings at RT. Fe2O3 generated by high temperature friction plays a lubricating effect, but the coating with the low hardness softened by high temperature is not conducive to the compaction and forming of the oxide film. As a result, coatings that produce a moderate oxide film at 300 °C and 500 °C have a lower wear rate in the same environment. The main wear mechanism for the coatings is oxidative wear.

Interplay of laser power and pore characteristics in selective laser melting of ZK60 magnesium alloys: A study based on in-situ monitoring and image analysis

This study offers significant insights into the multi-physics phenomena of the SLM process and the subsequent porosity characteristics of ZK60 Magnesium (Mg) alloys. High-speed in-situ monitoring was employed to visualise process signals in real-time, elucidating the dynamics of melt pools and vapour plumes under varying laser power conditions specifically between 40 W and 60 W. Detailed morphological analysis was performed using Scanning-Electron Microscopy (SEM), demonstrating a critical correlation between laser power and pore formation. Lower laser power led to increased pore coverage, whereas a denser structure was observed at higher laser power. This laser power influence on porosity was further confirmed via Optical Microscopy (OM) conducted on both top and cross-sectional surfaces of the samples. An increase in laser power resulted in a decrease in pore coverage and pore size, potentially leading to a denser printed part of Mg alloy. X-ray Computed Tomography (XCT) augmented these findings by providing a 3D volumetric representation of the sample internal structure, revealing an inverse relationship between laser power and overall pore volume. Lower laser power appeared to favour the formation of interconnected pores, while a reduction in interconnected pores and an increase in isolated pores were observed at higher power. The interplay between melt pool size, vapour plume effects, and laser power was found to significantly influence the resulting porosity, indicating a need for effective management of these factors to optimise the SLM process of Mg alloys.

Microstructure and mechanical properties of a modified 316 austenitic stainless steel alloy manufactured by laser powder bed fusion

A 316 austenitic stainless-steel alloy, with modified alloy composition, manufactured by laser powder bed fusion (L-PBF) has been investigated. The modification of the alloy composition included addition of niobium (Nb), tungsten (W) and copper (Cu), together with a reduction in the amount of molybdenum (Mo) and an increased amount of carbon (C). To find suitable process parameters, a parameter study by varying laser power, hatch distance and scan speed was performed, centered on typical parameters used for normal 316 L. As-built material from a selected parameter configuration was then subjected to different stress relief annealing heat treatments and ageing heat treatments. The effectiveness of the stress annealing was ranked using a deformation-based method. Microstructural characterization, hardness and room temperature tensile testing were done to evaluate the effect of stress relief and aging heat treatments.It was found that a higher volumetric energy was needed to build dense material, about ∼50 % higher compared to the volumetric energy input for normal 316 L. A subsequent aging heat treatment at 725 °C for 3 h increased the strength and hardness of the material. A reinforcement of the cellular microstructure by precipitation of carbides in between the cells is believed to be the main reason for this. To completely alleviate the residual stresses it was necessary to carry out a stress relief annealing process at 950 °C, which resulted in a removal of the cellular structure and a lower strength material.

Plastic deformation mechanisms and their threshold pressures of Ti6Al4V thin-walled structures induced by laser peen forming

To explore the minimum energy required for laser peen forming (LPF) and broaden the application scope of the process, plastic deformation behaviors of Ti6Al4V thin-walled structures induced by laser shock loading were investigated. LPF experiments varying laser power densities on thin-walled specimens with thicknesses of 0.5 mm and 3.0 mm were conducted. Finite element methods were used to analyze the evolutions of stress and strain with time to understand plastic deformation mechanisms, including the stress gradient mechanism (SGM) and shock bending mechanism (SBM). Experimental results showed that the threshold power density for LPF to deform a 0.5 mm-thick specimen is much lower than to deform a 3.0 mm-thick one. Simulated time/position diagrams of strain reveal that plastic deformation occurs once the shock loading acts on the material in the deformation mechanism of SGM. While in the deformation mechanism of SBM, plastic deformation does not occur until the whole section exhibits an overall bending, which presents an apparent sequential relationship after SGM. Finally, a criterion concerning the threshold pressure and thickness to determine plastic deformation of thin-walled structures under laser shock loading is proposed.

Implemented in-situ heat treatment process for controlling the residual thermal stresses during the fabrication of Ti-6Al-4V titanium alloy through additive manufacturing

The aim of this study is to eliminate the conventional heat treatment process and save on associated equipment, time, energy, and professional labor costs. In this work, an in-situ heat treatment scanning strategy has been implemented to control the thermal residual stress produced during the fabrication of Ti-6Al-4V titanium alloy through the selective laser melting (SLM) manufacturing process. The laser scanning was performed twice: first, it was used for melting and bonding the powder to build the samples, and second, it was used to remove the residual stress. Additionally, the laser power variation impact was also analyzed from 100 W to 300 W. It has been found that the strength and ductility both improved by 20–25 %. The microstrain and dislocation density were also analyzed and found that the significant reduction in microstrain and dislocation denticity after in-situ heat treatment.

Fabrication of SERS substrates by laser restructuring of interconnected Au nanostructures for urea detection

We discuss a facile, fast, simple, and reproducible fabrication method to modulate the structural morphology of the SERS substrate to improve its performance. The interconnected Au network structure named as nanoporous Au (NP–Au) is prepared by the dealloying method and further exposed with varied laser power in order to check the effect of power variation on the morphology as well as on the SERS performance. The effectiveness of the substrate has been tested by detecting Rhodamine 6G as a probe molecule. The Raman results show a good linear relationship with the laser power increment up to 3.2 mW and after that, the signal becomes saturated for a further increment in the power. The substrate is able to enhance the signal up to 102-fold as well able to show all the characteristic peaks without any damage to the probe molecule. Morphological identification reveals that different sizes and shapes of the pores (open, diffused, and interlinked) are responsible for the SERS signal enhancement. Subsequently, the sensitivity of SERS substrate has been examined and the result suggested that the optimal concentration of Rhodamine is 10−9 M. Furthermore, the substrate possesses good uniformity by representing a relative standard deviation value of less than 15 %. The substrate potential for real-world application has been examined by identifying urea (pesticide) molecules. The prepared nanoporous substrate is able to show a well-intensified signal with ∼16-fold enhancement compared to planar Silicon substrate.

Optimization of processing parameters for LPBF-manufactured CoCr alloys based on laser volume energy density

In this paper, the finite element method (FEM) and experiments were adopted to optimize the processing parameters of laser powder bed fusion-manufactured cobalt-chromium (CoCr) alloys on the basis of laser volume energy density. According to simulation results, both the maximum temperature, heat-affected zone and cooling rate of molten pool increased with the enhancement of laser power and scanning speed even under the same laser volume energy density. By conducting relevant experiments, it is found that the phase transformation, microstructure and mechanical properties were also influenced by the variation of laser power and scanning speed, and the LPBF-processed CoCr alloys with excellent forming quality could be acquired when the laser power was 160 W, and the scanning speed was 750 mm/s. Therefore, it is impracticable to optimize the processing parameters only on the basis of laser volume energy density, various methods should be employed to complete the whole optimization procedure.

Effect of laser power and heat treatment on microstructures and tensile properties of LPBF fabricated Hastelloy X superalloy

Laser powder bed fusion (LPBF) technic is suitable for manufacturing metal parts with complex shapes and internal channels. The energy input and post heat treatment have great influence on the formability, microstructure and properties of the metallic materials. In this work, Hastelloy X superalloy is fabricated by LPBF with a variation of laser power, and then post-treated by hot isostatic pressing and solution treatment. Influences of laser power and heat treatment on microstructure features of as-built sample are evaluated. The correlation among laser power, microstructure features and tensile properties of heat-treated samples are revealed. The results reveal that average grain size, aspect ratio, texture intensity increases while the proportion of high-angle grain boundaries decreases with the increase of laser power. Precipitates of Cr-rich carbides along grain boundary are observed after post heat treatment. Post-treatment promotes the twinned recrystallization, grain coarsening and texture degradation. Yield strength increases while tensile strength decreases with the increase in laser power. The higher vertical yield strength of sample with laser power of 320 W could be attributed to its larger aspect ratio and more twin boundaries. 280 W is the preferred laser power to obtain the best overall tensile properties. This study is helpful for the exploration of optimal LPBF process and post heat treatment, by which Ni-based superalloys with a good balance between strength and ductility can be produced.

An Empirical Model and Feedforward Control of Laser Powder Bed Fusion

Abstract Reliable process control for the laser powder bed fusion process, especially at the melt pool scale, remains an open challenge. One of the reasons for this is the lack of suitable control-oriented models and associated control design strategies. To address this issue, this paper (1) identifies an empirical control-oriented model of geometry-dependent melt pool behavior and (2) experimentally demonstrates melt pool regulation with a feedforward controller for laser power based on this model. First, the study establishes that the melt pool signature increases as the scan lines decrease in length. An empirical model of this behavior is developed and validated on different geometries at varying laser power levels. Second, the model is used to design a line-to-line feedforward controller that provides an optimal laser power sequence for a given geometry. Finally, this controller is validated experimentally and is demonstrated to suppress the in-layer geometry-related melt pool signal deviations for different test geometries.

Simulation Study on the Energy Utilization Efficiency of a Turbine Impeller Based on a Selective Laser Melting Process

In this paper, a simulation model for Selective Laser Melting (SLM) technology is established to simulate the additive manufacturing process of a turbine impeller for an aerospace engine. By utilizing the simulation model, variations in laser power and scanning speed are employed to obtain simulated results of thermal deformation for the turbine impeller under different laser power and scanning speed conditions. The results indicate that the thermal deformation of the component increases with the augmentation of laser power, decreases with the escalation of scanning speed, and eventually stabilizes. Based on the relationship between thermal deformation and energy, the energy utilization efficiency of the SLM process under different conditions is calculated. The findings demonstrate that, within a certain range of power, the synergistic effect of laser power and scanning speed allows for an increase in energy utilization efficiency and a reduction in processing time while ensuring the mechanical performance of the formed parts. Consequently, this approach proves effective in lowering production costs for complex components based on SLM technology.

Laser Cladding of Alloy 82 Powder for Corrosion Protection in Small Modular Reactors

There has been an increased demand for surface modification technologies to enhance corrosionresistance and wear-resistance. Among the representative surface coating technologies, laser cladding has attracted great attention because of its multiple advantages including low heat input, low dilution ratio, controllable clad height, etc. Recently, laser cladding has been considered as a surface coating technology for the next-generation small modular reactor, although submerged arc welding was utilized for the 3rd generation nuclear reactor. Cobalt-free materials are required as cladding materials in the nuclear reactor because cobalt has a long half-life. In this work, nickel based cobalt-free Alloy 82 powder was utilized. The main experimental parameters for laser cladding were intensively investigated by varying laser power, scan speed, powder supply, carrier and shield gas, overlap ratio, etc. Additionally, cross-sectional area calculation and EPMA analysis were carried out to examine the dilution ratio. Mechanical properties were also evaluated using microhardness tests and wear tests at high temperatures. Finally, corrosion tests were performed to compare a laser clad surface with an uncoated carbon steel surface. Our study indicates that the laser cladding using Alloy 82 powder is feasible for corrosion protection in next-generation small modular reactors.

A close look at temperature profiles during laser powder bed fusion using operando X-ray diffraction and finite element simulations

In laser powder bed fusion (LPBF), complex components are manufactured layer-by-layer via scanning the cross-sections of a 3D CAD model using a high intensity laser. Throughout this process, the material is exposed to temperature profiles that significantly differ from conventional manufacturing methods, and result in development of a unique and inhomogeneous microstructure and high levels of residual stresses in additively fabricated parts. The large temperature gradients and rapid cooling rates around the moving laser spot, and the overall heterogeneity of the temperature field need to be better understood in order to optimize the process parameters for increased production quality. In this study, operando X-ray diffraction (XRD) was employed to measure and compare temperature histories on the laser path under various processing conditions for Hastelloy X. Finite element thermal simulations were validated based on the acquired XRD data and then used as a supplementary tool to discuss the cooling behaviour and thermal heterogeneities across the geometry. The increase in the deposited energy density was qualitatively linked with higher temperature levels and slower cooling rates during LPBF. The melt-pool lengths showed strong sensitivity to the laser power and little variation with the scanning speed. Furthermore, even for a single set of parameters, large variations in the temperature field within the build were observed such that the cross-section edges located at higher build layers were exposed to markedly higher temperature levels.

Development of Cu-based shape memory alloy through selective laser melting from elemental powder mixture: Processing and characterization

The Cu-based shape-memory alloy 81Cu-11Al-3Ni-3Mn (wt%) was processed by selective laser melting (SLM) and the parameters were optimized to obtain samples with a high relative density of up to 99 %. The study used ball milling of elemental powders to obtain the feedstock material and the process was developed for building dense SMA structures. The process optimization is performed to obtain dense structures by varying laser power and scan speed. Further, the parameters yielding higher density is selected to study the influence of the different laser power on the overall density, defect distribution, mechanical properties, and grain sizes. Cu-based SMA fabricated at different laser powers were characterized using Scanning electron microscopy (SEM), Differential scanning calorimetry, X-ray diffraction (XRD), tensile test, and microhardness tests. It was observed that the processing technique has a strong influence on the degree of porosity and the distribution of the pores, the grain size as well as the grain morphology. Dense and uniform samples are formed with elemental powder which shows a maximum hardness of 380 HV, and tensile strength of 1550 MPa. The transformation peak became sharp with respect to laser power increment and the transformation temperature increased with an increase in laser power. The study provides a path toward the deployment of SLM for building dense Cu-based SMA from elemental powders.

Laser-Induced Melting and Crystal Growth of Sodium Ion Conductive β-NaFeO<sub>2</sub>

Laser irradiation of β-NaFeO2 shows local melting and crystal growth from the melt pool and sodium ion conductivity. Local melting, densification, and crystal growth were confirmed on the β-NaFeO2 surface through laser irradiation with a wavelength of 1 µm. Scanning electron microscopy and electron backscatter diffraction analysis confirmed anisotropic grain growth and grain boundary shrinkage in the laser-irradiated area. X-ray diffraction results suggested that the laser-irradiated area was in a low crystalline state. In addition, the precipitation of Fe3O4 in laser-irradiated β-NaFeO2 indicates a reduction reaction, suggesting that the laser-irradiated area is heated to 1500°C or higher. We confirmed that the structure change region could be controlled down to a thickness of about 10 µm by laser irradiation with varying laser power to β-NaFeO2.

Prediction of Primary Dendrite Arm Spacing of the Inconel 718 Deposition Layer by Laser Cladding Based on a Multi-Scale Simulation.

Primary dendrite arm spacing (PDAS) is a crucial microstructural feature in nickel-based superalloys produced by laser cladding. In order to investigate the effects of process parameters on PDAS, a multi-scale model that integrates a 3D transient heat and mass transfer model with a quantitative phase-field model was proposed to simulate the dendritic growth behavior in the molten pool for laser cladding Inconel 718. The values of temperature gradient (G) and solidification rate (R) at the S/L interface of the molten pool under different process conditions were obtained by multi-scale simulation and used as input for the quantitative phase field model. The influence of process parameters on microstructure morphology in the deposition layer was analyzed. The result shows that the dendrite morphology is in good agreement with the experimental result under varying laser power (P) and scanning velocity (V). PDAS was found to be more sensitive to changes in laser scanning velocity, and as the scanning velocity decreased from 12 mm/s to 4 mm/s, the PDAS increased by 197% when the laser power was 1500 W. Furthermore, smaller PDAS can be achieved by combining higher scanning velocity with lower laser power.

Influence of powder size on defect generation in laser powder bed fusion of AlSi10Mg alloy

The particle size distribution (PSD) often changes as powders are cycled for recycled and different powder batches are used, challenging a critical aspect of quality assurance in the Laser Powder Bed Fusion (LPBF) process. It is therefore important to understand how powder PSD affects the processability of powder in the context of LPBF. In this work, a batch of AlSi10Mg powder was sieved into four sets of powder with controlled PSD (D50 ranging from 16 μm to 76 μm). Single-track and cuboid samples were made from powders with variations in laser power whiling fixing other laser processing parameters constantly. It is shown that coarser powders are much more sensitive to changes in laser power in terms of melt pool characteristics, defect population and size, showing significant improvements in relative density upon increasing laser power. The simulations reveal with the combination of low laser power and coarse powders, pores mainly originate from insufficient melting of powders at the sides of melt pools due to insufficient and inhomogeneous absorption of laser energy. It is possible to achieve nearly fully-dense parts when processing coarse powders given that the incident laser beam energy is sufficient.

Ultra-High Interfacial Thermal Conductance via Double hBN Encapsulation for Efficient Thermal Management of 2D Electronics.

Heat dissipation is a major limitation of high-performance electronics. This is especially important in emerging nanoelectronic devices consisting of ultra-thin layers, heterostructures, and interfaces, where enhancement in thermal transport is highly desired. Here, ultra-high interfacial thermal conductance in encapsulated van der Waals (vdW) heterostructures with single-layer transition metal dichalcogenides MX2 (MoS2 , WSe2 , WS2 ) sandwiched between two hexagonal boron nitride (hBN) layers is reported. Through Raman spectroscopic measurements of suspended and substrate-supported hBN/MX2 /hBN heterostructures with varying laser power and temperature, the out-of-plane interfacial thermal conductance in the vertical stack is calibrated. The measured interfacial thermal conductance between MX2 and hBN reaches 74 ± 25 MW m-2 K-1 , which is at least ten times higher than the interfacial thermal conductance of MX2 in non-encapsulation structures. Molecular dynamics (MD) calculations verify and explain the experimental results, suggesting a full encapsulation by hBN layers is accounting for the high interfacial conductance. This ultra-high interfacial thermal conductance is attributed to the double heat transfer pathways and the clean and tight vdW interface between two crystalline 2D materials. The findings in this study reveal new thermal transport mechanisms in hBN/MX2 /hBN structures and shed light on building novel hBN-encapsulated nanoelectronic devices with enhanced thermal management.

Weld-penetration-depth estimation using deep learning models and multisensor signals in Al/Cu laser overlap welding

Al/Cu laser-welded overlap joints, in which weld-penetration depth significantly influences both joint strength and electrical conductivity, are widely applied in automotive battery cells. In this study, a unisensor convolutional neural network (CNN) model that predicts penetration depth using coaxial weld-pool images as input and multisensor CNN models that utilize additional photodiode signals are proposed. The penetration depth was estimated using an optical coherence tomography sensor. The coefficient of determination values for the unisensor and multisensor CNN models were between 0.982 and 0.985, and their mean absolute errors were between 0.0278 and 0.0302 mm. The short-term Fourier transform multisensor model presented the best performance in terms of prediction of penetration depth when applied to the photodiode signal. The proposed prediction models were validated using a gradually varying laser power experiment, which demonstrated the efficacy of this approach and its potential use in automotive applications. Keywords: Laser welding, Al/Cu overlap joint, Penetration-depth estimation, Image sensor, Photodiode, CNN, Deep learning.

Fusing optical coherence tomography and photodiodes for diagnosis of weld features during remote laser welding of copper-to-aluminum

This study has been designed to investigate whether variations in the features of laser weldments can be isolated and diagnosed by fusing photodiodes and optical coherence tomography (OCT). Two manufacturing scenarios (variation in laser power and focal offset) have been considered during remote laser welding of 0.2 mm thick Cu foils on 2 mm thick Al 1050 plates with an adjustable ring mode laser integrated with a 1D oscillation head. The process was monitored by measuring weld penetration depth with OCT and by process emissions (plasma and back-reflection) via photodiodes. The acquisition frequency of all signals was 40 kHz. Strong correlations (r > 0.75) were shown between plasma, back-reflection, and OCT signals and measured depth and width of the weld. Weak correlations (r < 0.5) between voids, cracks, and sensor signals were observed. Although plasma is the predominant signal that carries most of the information about the process, and the OCT allows direct measurement of the penetration depth, their integration reached 87% classification accuracy of the tested welding scenarios. The main misclassification was observed between “good weld” and “over weld,” defined by the measured weld depth. Sensor fusion strategies with manufacturing implications are discussed throughout the paper.