Low Laser Energy Density Research Articles

Study on forming process and performance of nano-La2O3 reinforced tungsten-based composites by selective laser melting

In this study, La2O3 nanoparticles were incorporated into a tungsten (W) matrix to enhance the microstructure and properties of pure W. The selective laser melting (SLM) process for W-2wt% La2O3 was optimized, and the influence of SLM processing parameters on the relative density and internal defects of the samples was examined. Challenges such as uneven powder layering and layer-by-layer thickening during the SLM process were addressed by incorporating a homogeneous material support formed with low laser energy density beneath the sample. This approach widened the process window for sample formation, resulting in a 2.38 % increase in the average relative density of supported samples compared to unsupported ones.With optimized parameters of 240W laser power, 200 mm/s scanning speed, and 80 μm hatch spacing, the sample achieved a relative density of 98.32 % with minimal internal pores, though some microcracks remained. Further investigation revealed that surface roughness decreased with increasing laser power and scanning speed. High laser power combined with low scanning speed yielded good surface quality. Under conditions of high relative density (over 96 %), samples processed with low laser power and high scanning speed (200W, 200 mm/s) exhibited fine and uniform grain morphology, few internal defects, and excellent mechanical properties, with a microhardness of 474HV and a compressive strength of 2237 MPa.

Atomistic insight into laser shock peening response of α-titanium: an experimentally verified simulation study

Laser shock peening (LSP) can induce severe plastic deformation in the surface zone of metallic materials, which potentially enhances their mechanical properties. Although it is widely recognized that the improvement of mechanical properties can be attributed to microstructure evolution, the microstructural characteristics and response under different LSP parameters are complex, leading to distinct mechanical behaviors of materials at the macroscale. Besides, the exceedingly brief duration of LSP also poses considerable challenges to capture the microstructure evolution within the surface layer of materials. Therefore, to grasp a first in-situ atomistic insight into LSP response of α-titanium, molecular dynamics method is applied to simulate laser incidence at the [101¯0] crystallographic orientation with various energy densities. The results show that the lattice reorientation and dislocation slip dominate the plastic deformation at low laser energy density, while stacking faults and crystal disordering are the primary microstructural features at high laser energy density. The above shock-induced microstructures are verified by transmission electron microscopy (TEM) observations. Further, varied microscale plastic deformation modes under varying shock energies are proposed based on Schmid factors and energy barriers. The findings bridge laser energy to post-processing microstructure of α-titanium, which paves the way for improving mechanical properties of materials by optimizing LSP treatment parameters.

Heat treatment protocol for additively manufactured nitinol shape memory alloys in biomedical applications

Additive manufacturing (AM) is getting significant attention in realizing customizable nitinol (NiTi) devices for biomedical applications. However, due to the change in composition and constituent phases occurring during printing and the microstructural inhomogeneity in the printed parts, obtaining a desired set of phase transformation behaviors and superelastic properties is a major challenge. In this work, laser powder bed fusion (LPBF) was used to fabricate NiTi at a very low laser energy density (35 J/mm3) on a titanium base plate while keeping these behaviors and properties within desirable ranges without any structural defects such as crack and delamination. Our measurements showed that the as-printed NiTi exhibited distinct one-step phase transformation with the austenite finish (Af) temperature of 2.1 °C. To increase the Af temperature to 30.2 °C (within the recommended range of Af temperature for biomedical applications), a heat treatment protocol was developed, which includes a solution cycle (at 900 °C for 1 h) followed by an aging cycle (at 450 °C for 30 min). The heat treatment protocol enabled us to attain the homogenized microstructure while creating ultrafine metastable Ni-rich precipitate, Ni4Ti3, which facilitated the desirable phase transformation behavior with the increased Af temperature. The heat-treated sample showed narrower and sharper two-step martensitic phase transformation with the formation of intermediate R-phase. The presence of both Ni4Ti3 and the R-phase was confirmed by the transmission electron microscopic (TEM) analysis. In the superelasticity test at the body temperature, these samples, starting from the 2nd cycle, demonstrated a recovery ratio of more than 90% and a recoverable strain of more than 6.5%. After 10th cycles, the stable recoverable strain was 6.52% with a recovery ratio of 96%, which is the highest superelasticity reported for the LPBF processed NiTi to the best of our knowledge. After the initial deformation process, we expect these NiTi samples to attain near full superelasticity during service. The micro-hardness study also showed that the hardness of the heat-treated samples is less affected by the cyclic loading. Therefore, this paper presents a viable heat treatment protocol, which provides an important processing roadmap in making NiTi implants by AM while maintaining excellent functional properties of NiTi for biomedical applications.

Evaluation of two-phase sample transport upon ablation of gelatin as a proxy for soft biological matrices using nanosecond laser ablation – inductively coupled plasma – mass spectrometry

BackgroundRecent papers on LA-ICP-MS have reported that certain elements are transported in particulate form, others in gaseous form and still others in a combination of both upon ablation of C-based materials. These two phases display different transport behaviour characteristics, potentially causing smearing in elemental maps, and could be processed differently in the ICP which raises concerns as to accuracy of quantification and emphasizes the need for matrix-matching of external standards. This work aims at a better understanding of two-phase sample transport by evaluating the peak profile changes observed upon varying parameters such as laser energy density and wavelength. ResultsIt is demonstrated that upon ablation of gelatin, elements are transported predominantly in particulate phase, but already at low laser energy density, a significant fraction of some elements is transported in the gaseous phase, which is even more expressed at higher energy density. This behaviour is element-specific since the ratio of the signal intensity for the analyte element transported in gas phase to the total signal intensity was 0 % for 23Na, 43 % for 66Zn and as high as 95 % for 13C using a 193 nm laser. The results also suggest an effect of the laser wavelength, as all elements show either the same or higher amount of gas phase formation upon ablating with 213 nm versus 193 nm. It was even established that elements that fully occur in particulate form upon ablation using 193 nm laser radiation are partly converted into gaseous phase when using 213 nm. SignificanceThis work provides a thorough investigation of the underexposed phenomenon of two-phase sample transport upon ablation of biological samples upon via LA-ICP-MS. It is shown that for some elements a fraction of the ablated material is converted and transported in the gas phase, which can lead to significant smearing effects. As such, it is important to evaluate element-specific peak profiles on beforehand and, if necessary, adapt instrument settings and slow down data acquisition.

Control of resistive switching type in BaTiO3 thin films grown by high and low laser fluence

A ferroelectric memristor has attracted much attention due to convenient controlling by polarization switching, but the resistive switching has been attributed to the drift or charge trapping of defects. To distinguish the resistive switching mechanism between ferroelectric polarization switching and the normal resistive switching mechanism such as the drift or charge trapping of defects, BaTiO3 (BTO) thin films were grown on a (001) Nb:SrTiO3 single crystal substrate by pulsed laser deposition with high and low laser energy density. Based on a piezoelectric force microscope, ferroelectricity is found in BTO thin films grown at high laser energy density. X-ray photoelectron spectroscopy further confirms the existence of defects in the BTO films grown at low laser energy density. The high energy sample with low density of defects exhibits a resistance hysteresis loop but little current hysteresis loop, while the low energy sample with high density of defects shows a significant resistance and current hysteresis loop simultaneously. These results provide a deep understanding about the resistive switching from ferroelectric polarization switching and the drift or charge trapping of defects.

Manufacturability study in laser powder bed fusion of biomedical Ti alloys for orthopedic implants: an investigation of mechanical properties, process-induced porosity and surface roughness

PurposeSince the biomedical implants with an improved compressive strength, near bone elastic modulus, controlled porosity, and sufficient surface roughness, can assist in long term implantation. Therefore, the fine process tuning plays its crucial role to develop optimal settings to achieve these desired properties. This paper aims to find applications for fine process tuning in laser powder bed fusion of biomedical Ti alloys for load-bearing implants.Design/methodology/approachIn this work, the parametric porosity simulations were initially performed to simulate the process-induced porosity for selective laser-melted Ti6Al4V as per full factorial design. Continually, the experiments were performed to validate the simulation results and perform multiresponse optimization to fine-tune the processing parameters. Three levels of each control variable, namely, laser power – Pl (180, 190, 200) W, scanning speed – Vs (1500, 1600, 1700) mm/s and scan orientation – ϴ{1(0,0), 2(0,67°), 3(0,90°)} were used to investigate the processing performance. The measured properties from this study include compressive yield strength, elastic modulus, process-induced porosity and surface roughness. Finally, confirmatory experiments and comparisons with the already published works were also performed to validate the research results.FindingsThe results of porosity parametric simulation and experiments in selective laser melting of Ti6Al4V were found close to each other with overall porosity (less than 10%). The fine process tuning was resulted in optimal settings [Pl (200 W), Vs (1500 mm/s), ϴ (0,90°)], [Pl (200 W), Vs (1500 mm/s), ϴ (0,67°)], [Pl (200 W), Vs (1500 mm/s), ϴ (0,0)] and [Pl (200 W), Vs (1500 mm/s), ϴ (0,0)] with higher compressive strength (672.78 MPa), near cortical bone elastic modulus (12.932 GPa), process-induced porosity (0.751%) and minimum surface roughness (2.72 µm). The morphology of the selective laser melted (SLMed) surface indicated that the lack of fusion pores was prominent because of low laser energy density among the laser and powder bed. Confirmatory experimentation revealed that an overall percent improvement of around 15% was found between predicted and the experimental values.Originality/valueSince no significant works are available on the collaborative optimization and fine process tuning in laser powder bed fusion of biomedical Ti alloys for different load bearing implants. Therefore, this work involves the comprehensive investigation and multi-objective optimization to determine optimal parametric settings for better mechanical and physical properties. Another novel aspect is the parametric porosity simulation using Ansys Additive to assist in process parameters and their levels selection. As a result, selective laser melted Ti alloys at optimal settings may help in examining the possibility for manufacturing metallic implants for load-bearing applications.

Correlation of preparation conditions of SrRuO<sub>3</sub> ultrathin films with topological Hall effect

As one of the magnetic transition metal oxides, SrRuO<sub>3</sub> (SRO) has received much attention in recent years, which is mainly due to its unique itinerate ferromagnetism and the unusual electrical transport properties–behaving as Fermi liquid at low temperature and bad metal at high temperature. In the growth of SRO thin films, there are many factors that can affect the quality of thin films. In this work, we study various factors affecting the growth and quality of SRO thin films by using laser molecular beam epitaxy (laser MBE), including laser energy density, substrate temperature and target surface conditions, and explore their influences on the topological Hall effect (THE) in SRO. For thin films grown at high laser energy density and high temperature, we found that there are large trenches at the edge of steps, which deteriorate the transport properties of the thin films. When using low laser energy density, extra SrO may exist in the films, which also suppresses the conductivity. Films grown at low temperature tend to have poor crystallinity while films grown at high temperature exhibit island structures. The ablation degree of the target surface increases the decomposition of SRO to SrO, Ru and volatile RuO<sub>4</sub>, resulting in Ru defects in the grown thin film. The SRO thin film grown under the optimal conditions (1.75 J·cm<sup>–2</sup>, 670 ℃, fresh target surface) exhibits the optimal conductivity and the strongest THE. For non-optimal growth conditions that favors thickness inhomogeneity or Ru defects in the film, THE becomes weaker or even disappears. Therefore, we believe that the THE is due to the Dzyaloshinskii-Moriya interaction (DMI) resulting from the interfacial inversion asymmetry and the associated chiral spin structures.

Influence of the different nanostructures of acrylonitrile butadiene styrene/carbon nanotubes nanocomposites on laser cutting properties: Machining and chemical aspects

Reported on in depth for the first time herein is the influence of the diverse nanostructures of acrylonitrile butadiene styrene (ABS)/multi-walled carbon nanotubes (MWCNTs) on its chemical, physical and electrical properties after laser cutting. Injection moulding was used to fabricate the nanocomposite samples in various structures with a thickness of 3 mm by adjusting temperature and pressure. The samples’ nanostructures were evaluated prior to cutting with a CO2 laser. Design of experiments (DoE) by a full-factorial method used three levels laser power 45, 55 and 65 W) and the cutting velocity at three levels (4, 8 and 12 mm/s) as independent variables. The findings from this work are significant and support new theories. It was revealed different surface damage modes such as shrink holes, cracks, decomposed smithereens and sink marks. These were affected by the laser energy density criterion which means that the ratio of power to cutting velocity was the governing factor, whilst the effect of primary nanostructures was negligible. Unlike surface damage, the width of heat affected zone (HAZ) was found to depend on the thermal conductivity, which directly relates to the samples’ nanostructure. Minimum HAZ was obtained at 0.45 mm for the sample with maximum thermal conductivity equal to 0.23 W/mK. Analysis of the post-laser cut surface and HAZ indicated that the MWCNTs were well dispersed with higher orientation and degrees of distribution. This, naturally, allows the inference that application of low laser energy density accounted for and governed oxidation of these regions. The results show that the nanotexture of the post-laser cut surface is completely changed in comparison with the as-moulded surface, leading to the lowest reduction in surface electrical resistivity to 3.2 kΩ for the sample produced at a temperature of 220 °C and a holding pressure of 70 bar.

Photoacoustic testing of shear viscoelastic properties of soft tissues using annular beam illumination.

This Letter reports a new, to the best of our knowledge, photoacoustic excitation method for evaluating the shear viscoelastic properties of soft tissues. By illuminating the target surface with an annular pulsed laser beam, circularly converging surface acoustic waves (SAWs) are generated, focused, and detected at the center of the annular beam. The shear elasticity and shear viscosity of the target are extracted from the dispersive phase velocity of the SAWs based on the Kelvin-Voigt model and nonlinear regression fitting. Agar phantoms with different concentrations, and animal liver and fat tissue samples have successfully been characterized. Different from previous methods, the self-focusing of the converging SAWs allows sufficient SNR to be obtained even with low pulsed laser energy density, which makes this approach well compatible with soft tissues under both ex vivo and in vivo testing conditions.

Effects of processing parameters on fabrication defects, microstructure and mechanical properties of additive manufactured Mg–Nd–Zn–Zr alloy by selective laser melting process

Mg–3Nd–0.2Zn–0.4Zr (NZ30K, wt.%) alloy is a new kind of high-performance metallic biomaterial. The combination of the NZ30K Magnesium (Mg) alloy and selective laser melting (SLM) process seems to be an ideal solution to produce porous Mg degradable implants. However, the microstructure evolution and mechanical properties of the SLMed NZ30K Mg alloy were not yet studied systematically. Therefore, the fabrication defects, microstructure, and mechanical properties of the SLMed NZ30K alloy under different processing parameters were investigated. The results show that there are two types of fabrication defects in the SLMed NZ30K alloy, gas pores and unfused defects. With the increase of the laser energy density, the porosity sharply decreases to the minimum first and then slightly increases. The minimum porosity is 0.49 ± 0.18%. While the microstructure varies from the large grains with lamellar structure inside under low laser energy density, to the large grains with lamellar structure inside & the equiaxed grains & the columnar grains under middle laser energy density, and further to the fine equiaxed grains & the columnar grains under high laser energy density. The lamellar structure in the large grain is a newly observed microstructure for the NZ30K Mg alloy. Higher laser energy density leads to finer grains, which enhance all the yield strength (YS), ultimate tensile strength (UTS) and elongation, and the best comprehensive mechanical properties obtained are YS of 266 ± 2.1 MPa, UTS of 296 ± 5.2 MPa, with an elongation of 4.9 ± 0.68%. The SLMed NZ30K Mg alloy with a bimodal-grained structure consisting of fine equiaxed grains and coarser columnar grains has better elongation and a yield drop phenomenon.

Laser powder bed fusion of cemented carbides by developing a new type of Co coated WC composite powder

The study addresses the issue of the absence of high-quality WC-Co composite powder for laser powder bed fusion and assessing the printability of the new powder in terms of densification, microstructure and mechanical properties. The fluidized bed chemical vapor deposition (FBCVD) combined with electroless plating process was adopted to make core-shell structured WC-Co composite powder. The excellent uniformity of Co and good powder flowability were achieved by tuning the size and distribution of Co catalyst during FBCVD and regulating their relationship to the subsequent electroless plating behavior. The newly developed composite powder exhibited excellent printability. The densification mechanism was largely dependent on the laser energy density. The liquid formed by the melting of Co was responsible for the densification at low laser energy density, while the Co-W-C ternary liquid resulted from the dissolution of WC into Co melt dominated the densification at high laser energy density. Microstructurally, the morphology and size of WC grains were insignificantly changed because of its non-participation with liquid formation at low laser energy density. The rectangular/triangular WC grain morphology similar to the traditional sintered was formed at high laser energy density because of the precipitation of WC from the Co-W-C ternary liquid. Affected by the different liquid formation processes, increasing laser energy density increased the relative density, promoted the W 2 C phase formation and decreased WC grain size, which remarkably improved the hardness and tribological properties of the printed cemented carbide.

Effects of laser remelting on microstructural characteristics of Ni-WC composite coatings produced by laser hot wire cladding

The work presented in this paper focused on the effects of laser remelting with varied laser energy densities from 800 J/mm to 1400 J/mm on the phase transformation and microstructural characteristics in the tungsten carbide nickel-based matrix composite (Ni-WC) tracks obtained by laser hot wire cladding. The microstructures of the as-deposited tracks fabricated by laser hot-wire deposition were mainly composed of spherical residual carbons, eutectic Ni/Ni3B, retained WC/W2C particles and the granular in situ W2C particles formed in the interdendritic regions of primary (Ni,Fe) dendrites. When the low laser energy densities of 800 J/mm and 1000 J/mm were applied, the dispersive W2C particles in the as-deposited tracks were dissolved, and the WC particles were successfully in situ synthesized and uniformly distributed in the remelted tracks. When the laser energy density was increased to 1200 J/mm, the feather-like M6C(Ni2W4C) carbides homogeneously precipitated in the remelted tracks. When the laser energy density was further increased to 1400 J/mm, the dissolution degree of the ex-situ WC/W2C particles was significantly increased, resulting in a remelted track with a relative low volume fraction of retained ceramic particles and a high dilution ratio of the base metal. In addition, the dendritic M6C(Fe3W3C) carbides were precipitated around the retained particles, and the eutectic M6C(Fe3W3C) carbides in the herringbone morphology were uniformly distributed in the matrix of the track remelted by a high laser energy density of 1400 J/mm. These experimental results confirmed that the homogenization of the in situ synthesized reinforcements in the tracks could be well realized by laser hot wire cladding and the subsequent laser remelting. The coatings remelted by the low energy densities (800 J/mm and 1000 J/mm) possessed higher wear resistance than that of the as-deposited coating, but the wear performance of the remelted coatings was decreased when the laser energy densities were increased to 1200 J/mm and 1400 J/mm.

Detection model of the plasma spectrum based on the polarization recognition rate induced by a low energy density laser.

Laser-induced breakdown spectroscopy (LIBS) is a fast recognition method for heavy-metal detection. The recognition rate of the characteristic spectrum is related to the laser-induced energy. In order to analyze the polarization characteristics of plasma and the improvement mechanism of recognition rate under the condition of low energy density, a detection model of polarization recognition rate was established by exploring the intensity formula of discrete spectral data. At the Brewster angle, the LIBS and discrete spectral data of Cd and Cu elements induced by five energies were measured in combination with the polarization spectrometric path. According to the model of polarization recognition rate, the optimization effect of the polarization separation approach on the plasma spectra of heavy-metal elements has been clarified, and the recognition rate of discrete spectra induced by low energy density has been improved. This study shows that the increase of laser energy is helpful to the recognition of characteristic spectral lines. Under the same energy induction, the polarization recognition rate of heavy-metal elements is larger, and this characteristic is more obvious under low energy density. This model not only improves the recognition rate of the plasma spectrum but also greatly reduces the requirement of laser energy and the damage of the medium surface, which is a more effective nondestructive testing technology.

Process Parameter Optimization on Selective Laser Melting of CuSn10 Powders Based on Response Surface Methodology

Abstract A shaping test of CuSn10 powders was carried out by using the selective laser melting (SLM) technology to study influences of process parameters of SLM on compactness of specimens. Meanwhile, process parameters were optimized based on the response surface methodology. Meanwhile, phase composition and microstructure of formed part were analyzed by XRD and OM. Results demonstrated that as the energy density of laser body increases from 146.7 J·mm-3 to 181.3 J·mm-3, pores of samples decrease and the compactness increases from 98.97% to about 99.7%. Subsequently, the compactness begins to decrease with the continuous increase of laser energy density. The incomplete melting of powder under low energy density and gasification of powders under high energy density are major factors that influence the compactness. Over low or high laser energy density is disadvantageous for the improvement of compactness. A mathematical model of interaction between three factors (laser power, laser scanning speed and scanning space) and response value (compactness) was built up by using the BBD principle design test of response surface methodology. The degree of fitting (R2) and correction coefficient (R2 adj) of the model were 0.9952 and 0.9866, which agree well with the fitting degree of experimental tests. Among three factors, influences of laser power and scanning speed on compactness are equivalent and more significant than scanning space. The optimal process parameters which were predicted by the model were laser power=331.471 W, scanning speed=648.045 mm·s-1, scanning space=69.9302 μm. Under this circumstance, the compactness of CuSn10 powders after SLM is 99.75%. According to experimental verification, the practical value of compactness is 99.74%, showing a very small relative error. This proves that the BBD model is reliable. The shaped part is mainly composed of α-Cu phase and metastable phase β’-Cu13.7Sn. Compared with the original powders, the content of metastable phase β’ increases significantly, while the δ-Cu41Sn11 phase disappears.

Ti6Al4V matrix composites fabricated by laser powder bed fusion in dilute nitrogen

In situ synthesised TiN reinforced Ti6Al4V matrix composites were fabricated via laser powder bed fusion (L-PBF) in an atmosphere of 10vol.-% N2 and 90vol.-% Ar based on novel gas–liquid reaction between N2 and Ti6Al4V melt pool. With lower laser energy density (LED), the microstructures of the composites mainly consisted of acicular α′ martensites, while the martensites were shortened and coarsened with increasing LED. The X-ray diffraction analysis demonstrated that the interstitial solid solution of N was formed in the Ti lattice. High-resolution transmission electron microscopy analysis confirmed the generation of TiN. The microhardness, yield strength, ultimate strength and plasticity of the composites initially increased and then decreased with increasing LED. The strengthening mechanisms of the composites were elucidated.

Elucidation of dross formation in laser powder bed fusion at down-facing surfaces: Phenomenon-oriented multiphysics simulation and experimental validation

Dross formation is a phenomenon that is observed while printing metallic components using Laser Powder Bed Fusion ( L -PBF) and occurring primarily at down-facing surfaces that are unsupported and suffer inadequate heat removal. Naturally, dross formation causes dimensional inaccuracy, high surface roughness and also adversely affects the mechanical properties of printed components. Through simulation and experimentation, this study fundamentally elucidates the driving phenomenon behind dross formation. The simulation results, in terms of the degree of generated dross domain, well agree with the ones observed in the printed samples and the behaviour of the melt pool while moving from bulk material to the powder domain is clearly depicted in this study. The simulations show that due to the low thermal conductivity of loose powder and the inability to conduct heat away, the quasi steady state melt pool collapses while entering the powder domain and transitions to a keyhole-like melt mode which causes a pronounced drilling effect. This causes excessive melting known as dross that is seen both in the simulation and the experimental parts. This work also shows through simulation and experimentation the reasoning behind the production of larger and smaller dross domains while printing with high and low laser energy densities respectively. Additionally, through SEM imagery this study also explains the observed deep internal grooves and near-surface porosity that are present within this dross domain which can further affect mechanical properties such as density, fatigue strength etc.

In-situ manipulating microstructure of CoCrMo alloy using selective laser melting with a novel scanning strategy

Three types of distinct dense microstructure, namely narrow columnar grains, nearly equiaxed refined grains, and duplex ε + γ microstructure, can be in-situ fabricated using a novel laser scanning strategy. The strategy includes a melting scanning with the laser energy density of 2∼26 J/mm3 to refine the microstructure and multiple non-melting scanning with an extremely low laser energy density of 0.9∼1.8 J/mm3 to produce a large amount of ε-martensite in the γ austenite matrix. As a result, an exceptionally high tensile strength of 1,503 MPa, outstanding ductility of 18%, and improved hardness of 502 HV can be achieved through additional scanning with appropriate energy density. This work demonstrates that the microstructure of selected laser melted alloy can be in-situ manipulated to tune mechanical properties.

Detection model of copper based on polarization degree induced by low-energy density laser.

In order to analyze the mechanism of plasma polarization characteristics and the improvement effect of spectral signal-to-back ratio, the intensity formulas of continuous spectrum and discrete spectrum were derived by exploring the path of the radiation spectrum. At the Brewster angle, the model of polarization degree was established based on the measured spectral data to identify the radiation intensity of plasma. The experimental results showed that the polarization characteristics of the background and discrete spectrum were both observed in the plasma spectrum of a copper element, and there were obvious differences in polarization degree and vibration direction. Moreover, cadmium and chromium were used to verify the detection model. It was found that the characteristic signals of the polarization spectrum were more than the effective peaks in laser-induced breakdown spectroscopy, and the variation trend was relatively gentle. The model retained the effective information in the continuum spectrum and fully explored the basic polarization mechanism of plasma. The measured data were not only convenient to observe the characteristic signal peaks of elements, but also greatly improved the recognition effect. This method could extract effective information of illumination plasma under the condition of low incident light intensity and reduce the damage of medium surface, which is a more effective nondestructive testing technology.

Laser remelting of WC-CoCr surface coated by HVOF: Effect on the tribological properties and energy efficiency

In this article, the tribological behavior and energy efficiency of surfaces coated with WC-CoCr/HVOF were evaluated after a laser remelting process, using low and high laser energy densities, respectively, 33.3 and 150 J/mm2. The purpose of laser remelting was to adequately modify the microstructure of the coatings and promote better performance during surface sliding. Therefore, the microstructure, phase composition and microhardness of the coatings were investigated, and heat effect on the substrate. During dry and lubricated tribological tests, friction coefficient (COF), wear, and surface roughness also were evaluated. In the dry tests, friction, wear and heat dissipation energies were obtained, which were then correlated with the energy consumed by the tribometer. A HEPR-type biodegradable oil was used in the lubricated tests. The proper formation of W2C and Co3W3C, obtained for the lower energy density, increased the hardness, without weakening the material, while the CrC phase acted as an anti-wear barrier. Whereas for the high energy density, thermal decomposition produced fragile phases, which were easily removed from the matrix during the wear test. The sample tested in as-sprayed condition had a high adhesion and friction compared to the remelted samples. On the remelted surfaces, greater friction stability was obtained. The lower dry friction was not linked to the lower power consumption, a result which was then attributed to the higher heat dissipation from the surface during the tests. Finally, this study proposes a methodology for quantifying the efficiency of sliding surfaces and points to a sustainable solution for tribology.

Ni-loss compensation and thermomechanical property recovery of 3D printed NiTi alloys by pre-coating Ni on NiTi powder

The approach of pre-coating Ni on NiTi powder was adopted as an effective route to address the thermomechanical property degradation of NiTi alloy caused by Ni-loss during the printing process of laser powder bed fusion. The electroless plating process of Ni on NiTi powder was initiated with the removal of the outermost TiO 2 layer by alkaline solution and the exposure of Ni catalytic nucleation site. The newly exposed Ni was an active catalyst for coating Ni and eventually triggered the deposition of Ni on NiTi powder via self-catalytic reaction. The intrinsic homogeneity of Ni, as a major alloying element in NiTi powder, ensured high uniformity of Ni-coating on the surface of the powder and good sphericity, necessary for good printability. Furthermore, Ni-coating increased laser absorbability, and a higher content of Ni compared to the theoretical value was thereby required to compensate for the Ni-loss. Simultaneously, densification was enhanced even at lower laser energy density on using Ni-coated powder. During the printing process, ultrafine Ni-coating was rapidly homogenized such that the thermomechanical property was completely recovered. As a consequence, it is underscored that Ni-loss can be addressed by compensating Ni on the original powder prior to the printing process, providing a new pathway toward printing of high-performance shape memory alloys.