Local Laser Heating Research Articles

Influence of Mg Content on Microstructure Coarsening, Molten Pool Size, and Hardness of Laser Remelted Al(-x)-Mg-Sc Alloys.

Investigations leading to high-quality surfaces and optimized properties in laser remelted Al-Mg-Sc alloys remain scarce. Laser surface remelting (LSR) has been used for the final treatment of these alloys processed through additive manufacturing. However, the direct microstructure responses of the treated cast surfaces have not yet been investigated. In the present research, Al-3, 5, and 10 wt %-Mg-0.1 wt %-Sc alloys plates were processed using LSR to study the effects of local melting and rapid solidification. The morphology of the α-Al phase, microstructure coarsening, and hardness were mapped from the bottom to the top of the molten pools, varying with local Mg content and laser heat input (2.5 J/mm, 5 J/mm, and 10 J/mm). This study aimed to create a comprehensive map of the microstructures, hardness, and molten pool sizes under various conditions. The findings may help to optimize these alloys through understanding laser processing parameters. Methods used included CALPHAD computations, optical microscopy, SEM, EDS, image analysis, hardness tests, and heat flow models. The results obtained showed α-Al cell growth with bands in all alloys with hardness changes correlating with cell spacing and heat input. Higher Mg content resulted in more refined cells and a higher fraction of bands. Increased Mg content decreased the thermal diffusivity and enthalpy of melting, enlarging the molten pool size. Hardness increased with decreasing heat input and higher Mg content in the tested alloys, especially in the Al-10 wt %-Mg-0.1 wt %-Sc alloy as the heat input was varied.

Mechanical and microstructural behavior of Inconel 625 deposits by high-pressure cold spray and laser assisted cold spray

Thick deposits of Inconel 625 (IN625) by high-pressure cold spray (HPCS) have been described as cold spray additive manufacturing (CSAM). However, given the solid solution and numerous precipitation phases of the IN625 superalloy, these deposits can exhibit low ductility and high residual stresses due to severe deformation of the particles upon impact against the substrate. To counteract this phenomenon, laser-assisted cold spray (LACS) was used here to soften the material using two laser surface temperatures (650 and 900 °C), and to elucidate their effect on the microstructure and mechanical properties, specifically elastic modulus, hardness, and stress relaxation behavior of the deposits. For comparison, IN625 was also deposited by HPCS. The microstructure of the deposits was characterized by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Mechanical properties were determined by nanoindentation. The HPCS and LACS 650 °C deposits showed an interdendritic network of intermetallics, a feature also present in the powder feedstock material. However, the deposit produced with LACS at 900 °C showed a microstructure suggesting a dissolution of this network, a critical surface laser temperature for inducing an evolution on the microstructure in LACS processes. The EBSD maps show strong grain deformation in the material deposited using HPCS, whereas the sample made with LACS at 900 °C showed significant recrystallization and grain growth (3.5 μm in average). Mechanical properties, including stress relaxation phenomena, were measured using nanoindentation. The hardness of the LACS 900 °C sample was found to be 7.2 GPa, which is a decrease from the 8.9 GPa found in the HPCS sample and 9.1 GPa found in the LACS 650 sample. This decrease in hardness can be attributed to the recrystallized microstructure due to the localized laser heating. In contrast, the elastic modulus remained practically the same for all three samples, around 265 GPa.

Investigation of Two Laser Heat Treatment Strategies for Local Softening of a Sheet in Age-Hardening Aluminum Alloy by Means of Physical Simulation

AbstractCar manufacturers increasingly aid high-strength aluminum alloys for their advantageous weight-to-strength ratio, but their limited formability poses challenges in plastic deformation processes. Tailored heat-treated blanks (THTBs) are a propitious approach to improve formability. Surface laser treatment is the predominant technology for obtaining THTB. To design this process quickly and accurately, without material waste, the use of physical simulation is increasingly promising. It allows replicating the process on a lab-scale and studying posttreatment mechanical and metallurgical properties. By adopting Gleeble® physical simulator, this study investigates the softening effects of local surface laser heat treatment on a EN AW 6082 T6 aluminum alloy blank. Two laser movement strategies—single linear path and multiple rectangular paths—were investigated at two treatment speeds for each. A finite element (FE) model was developed for simulating the process under all explored conditions. FE-derived thermal cycles were reproduced by means of physical simulation. After physical tests, alloy mechanical properties were evaluated. Results show that these properties depend on both the peak temperature thermal cycle and the interaction time between the laser source and the material surface. The comparison between the two strategies revealed that the multiple rectangular paths strategy allows to achieve a wider softened area at comparable interaction times.

Carbon nanotube-induced localized laser heating toward simultaneously enhanced laydown efficiency and fracture toughness of in-situ consolidated glass fiber thermoplastic composites

Thermoplastic composites suffer from weak inter-tape bonding during high-rate laser-assisted automated fiber placement (L-AFP). Here, we propose a strategy to simultaneously enhance the laser heating efficiency and fracture toughness of glass fiber-reinforced polypropylene prepreg tape for L-AFP process by depositing carbon nanotubes (CNTs) on the tape surface by a low-cost bath coating process. CNTs function as efficient light absorbing and photothermal coating to promote molecular inter-diffusion and interfacial healing between successive tapes, as well as toughen the weak interface by CNT pull-out and rupture. A 56 % increase in the near-infrared laser absorption of tapes results in a 946 % improvement in the laydown rate. CNTs also remarkably improve transverse temperature uniformity. Furthermore, double cantilever beam and pants tearing tests show that 0.5 wt% CNTs improve the mode I interlaminar fracture toughness and mode III fracture strength by 17 % and 52 %, respectively, attributed to the enhanced molecular inter-diffusion and CNT bridging effect.

Enhancement of the Microstructure and Fatigue Crack Growth Performance of Additive Manufactured Titanium Alloy Parts by Laser-Assisted Ultrasonic Vibration Processing

Post-processing techniques can efficiently improve the surface quality, address microstructural defects, and optimize mechanical properties in additively manufactured parts. Surface severe plastic deformation processes such as ultrasonic nanocrystal surface modification (UNSM) integrated with localized laser heating were explored to enhance the surface properties, microstructure as well as the fatigue crack growth properties (FCG) in both directions of built. We successfully induced greater plasticity flow and achieved beneficial refinement of the surface grain structure by precisely controlling the heat and impact energies during surface treatment process. The LA-UNSM process, with the parameters utilized in this study considerably decreased the FCG rates of treated samples, when compared to samples without surface treatment. Improved fatigue crack growth properties along vertical and horizontal orientations after the post-process treatment were attributed to the induced-microstructural changes, improved surface quality, induced compressive residual stresses through gradient structure deformation layer that was prepared on the surface of the material. The fractographic analysis revealed that the cracks mostly originated from the pores in the as-produced state. This observation shows a clear correlation between the surface treatment performed and a substantial improvement in fatigue crack growth resistance.

Local laser heating effects in diamond probed by photoluminescence of SiV− centers at low temperatures

The accurate measurement of thermal conductivity of diamond below 10 K has always been a challenge, mainly due to significant error in temperature sensing using the thermocouple method. Diamond is generally considered to have high thermal conductivity, so little attention has been paid to the laser heating effects. Here, we observed the dynamic redshift and broadening of zero phonon line of silicon-vacancy (SiV−) centers at 4 K. Utilizing the intrinsic temperature response of the fine structure spectra of SiV− as a probe, we confirmed that laser heating effect appears and the temperature rising results from high defect concentration. By simulating the thermal diffusion process, we have estimated the thermal conductivity of around 1 W/(m K), which is a two-order magnitude lower than that of single-crystal diamond. Our results provide a feasible scheme for all-optical non-contact temperature sensing and help to solve the problem of accurate measurement of thermal conductivity at cryogenic temperatures.

Intensifying the particle deposition from a colloidal solution for the purpose of liquid purification: Comparison of deposition mechanisms and rates

The deposition of particles from a layer of colloidal solution (water and microparticles) has been experimentally studied in the presence of laser heating. Instantaneous temperature and velocity fields are measured using a thermal imager and the Particle Image Velocimetry and the Particle Tracking Velocimetry methods. The unstable behavior of the velocity field in the presence of heating and a free liquid surface are assumed to be determined by the Bond number (Bo). Of interest is also an additional mechanism for the instability formation when the Peclet number (Pe) exceeds the critical value and when the ratio of characteristic velocities VT/Vavr > 1, where VT is the heat front velocity at the initial moment of laser heating, and Vavr is the average convection velocity. A new mechanism of particle deposition due to the use of local laser heating is proposed for the first time. The control over local heating and local deposition increases the particle deposition rate by 2–3 orders compared with that without convection. The particle deposition rate and the deposit area increase with increasing height of the liquid layer, reach maximum values at a critical height, and then decrease sharply with increasing height. The dependence for determining the area and mass of the deposit is proposed to take into account the deposition time, the height of the liquid layer and the rate of convection. The resulting particle deposition method and calculation method may be used in technologies related to the purification of liquid from solid impurities in small volumes, as well as in problems where deposition in a given direction is required to form an ordered nanocoating.

Local laser heat treatment of AlSi10Mg as-built parts produced by Laser Powder Bed Fusion

Today, complex structural components for lightweight applications are frequently manufactured by laser powder bed fusion (PBF-LB), often using aluminum alloys such as AlSi10Mg. However, the application of cyclic load cases can be challenging as PBF-LB produced AlSi10Mg parts typically have low ductility and corresponding brittle failure behavior in the as-built condition.Therefore, this paper presents investigations on the feasibility of a laser heat treatment of PBF-LB produced AlSi10Mg parts to locally increase the ductility and decrease the hardness in critical areas. Potential heat treatment process parameters were derived theoretically based on the temperature fields in the material calculated assuming three-dimensional heat conduction and a moving heat source. PBF-LB produced specimens were then laser heat treated at varying laser power and scan speed. Hardness measurements on metallographic cross sections showed hardness reductions of over 35 % without inducing hydrogen pore growth.

Sensitive Detection of Biomolecular Adsorption by a Low-Density Surfactant Layer Using Sum-Frequency Vibrational Spectroscopy.

Small molecules or proteins interact with a biomembrane in various ways for molecular recognition, structure stabilization, and transmembrane signaling. In this study, 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP), having a choline group, was used to investigate this interaction by using sum-frequency vibrational spectroscopy. The sum-frequency spectrum characteristic of a neat monolayer changed to that of a bare air/water interface at a larger molecular area of the DPTAP molecules due to local laser heating. Upon introduction of the aromatic molecules in the subphase at around 120 Å2 per molecule, the sum-frequency signal suddenly reappeared due to molecular adhesion, and this was utilized to probe the adsorption of the aromatic ring molecules in the water subphase to the choline headgroup of the DPTAP by cation-π interaction. The onset concentrations of this sum-frequency signal change allowed a comparison of the relative interaction strengths between different aromatic molecules. A zwitterionic surfactant molecule (DPPC) was found to interact weakly compared to the cationic DPTAP molecule.

Automated crack detection on metallic materials with flying-spot thermography using deep learning and progressive training

ABSTRACT In non-destructive testing for metallic materials, ‘Flying-spot’ thermography allows the detection of cracks thanks to the scanning of samples by a local laser heat source observed in the infrared spectrum. However, distinguishing a crack from other surface structures such as air ducts or non-planar shapes on the material surface can be challenging in an automation perspective. To address this, we propose to use deep learning techniques, which can exploit contextual information but require a significant amount of labelled data. This study presents a training method based on curriculum learning and recent denoising diffusion models to generate synthetic images. The protocol progressively increases the complexity of training images, using successively simulated data from a multi-physics finite-element software, synthetically generated data with diffusion process, and finally real data. Several detection scores are measured for various machine learning and deep learning architectures, demonstrating the benefits of the proposed approach for regular application cases and degraded experimental conditions, consisting of limited thermal enlightenment recordings.

Parametric study of local laser heat treatment technology on multi forming of advanced-high strength steel (AHSS) part with complex shape

The effectiveness of local laser heat treatment technology to enhance the in situ formability of steels and aluminum alloys has already been widely acknowledged for the one-step forming of components with simple shape geometries. The present study demonstrates that this technology is also able to significantly improve the formability of a complex shaped multi-forming industrial part. An industrial grade advanced-high strength Dual-Phase DP1000 steel is used to analyze the multi-forming of a complex part to determine the most appropriate local laser heat treatment parameters and optimize in situ softening by correlating yield strength, ultimate tensile strength, elongation at fracture, strain hardening exponent and instantaneous strain hardening with local temperature dynamics during the laser treatment. Additionally, numerical simulation analysis using Autoform software is carried out to validate the selected heat affected zone and the in situ softening, ensuring that they are appropriate for improving the formability of the industrial part. These findings are then expanded to study the experimental forming of five in situ laser heat treated models, followed by comparative analysis with a benchmark. This study provides an insight and fundamental guidelines to perform in situ laser heat treatment on complex industrial parts leading to the production of the industrial multi-formed component with optimized formability.

2D X-ray diffraction method for evaluating the local crystallization of Fe-based amorphous alloy ribbon induced by ultrashort pulsed laser local heating

Fe-based amorphous alloys, which exhibit excellent soft magnetic properties, are difficult to machine, owing to their high strength, toughness, and hardness resulting from their unique structures. The authors proposed a novel blanking method that improves the machinability by crystallizing only the local regions involved in machining, thereby reducing the local strength and toughness. Because the micro-region mechanical properties of amorphous alloys vary greatly depending on the precipitated crystal species and volume fraction, it is necessary to nondestructively investigate the type of microstructure achieved by local heat treatment to determine the appropriate blanking conditions. However, an appropriate method for investigating crystals precipitated locally in an amorphous matrix is yet to be established. In this study, we used the 2D X-ray diffraction method to investigate the local crystallization of an Fe-based amorphous alloy ribbon after local heat treatment using an ultrashort pulsed laser. Furthermore, the 2D method was used to investigate the residual stresses in the fully crystallized amorphous alloy ribbons, and the results were compared using the sin2(ψ) method. The 2D method detected the slightly precipitated crystals in a local area with a radius of about 40 µm in the amorphous matrix. The integrated intensity of the diffraction peaks can be used to predict the internal structure after local heat treatment. Furthermore, the 2D method could measure residual stress with smaller error than the sin2(ψ) method. The core-shell crystals precipitated by heat treatment exhibited a compressive residual stress of −82 MPa to −67 MPa in the core Fe3B crystals and tensile residual stress of 39–60 MPa in the shell α-Fe crystals, approximately. The residual stress values of the precipitated crystals depended on the direction of rotation of the quenching roll used in the production of the amorphous alloy ribbons.

Temperature dependence of resistivity of carbon micro/nanostructures: Microscale spatial distribution with mixed metallic and semiconductive behaviors

The temperature coefficient of resistivity (θT) of carbon-based materials is a critical property that directly determines their electrical response upon thermal impulses. It could have metal- (positive) or semiconductor-like (negative) behavior, depending on the combined temperature dependence of electron density and electron scattering. Its distribution in space is very difficult to measure and is rarely studied. Here, for the first time, we report that carbon-based micro/nanoscale structures have a strong non-uniform spatial distribution of θT. This distribution is probed by measuring the transient electro-thermal response of the material under extremely localized step laser heating and scanning, which magnifies the local θT effect in the measured transient voltage evolution. For carbon microfibers (CMFs), after electrical current annealing, θT varies from negative to positive from the sample end to the center with a magnitude change of >130% over <1 mm. This θT sign change is confirmed by directly testing smaller segments from different regions of an annealed CMF. For micro-thick carbon nanotube bundles, θT is found to have a relative change of >125% within a length of ∼2 mm, uncovering strong metallic to semiconductive behavior change in space. Our θT scanning technique can be readily extended to nm-thick samples with μm scanning resolution to explore the distribution of θT and provide a deep insight into the local electron conduction.

Identification of overloads on splined shafts by means of eddy current testing technology

For the development of resource-efficient machines, the availability of high-quality information about the actual loads that occur is a key prerequisite. By continuously monitoring the loads on the component, critical events can be detected, and thus component failure prevented. Future product generations can also be designed in a resource-saving way depending on the loads that actually occur. The information is most useful when it is obtained directly from the highly loaded areas of the component. In this study, a concept is presented that enables in-situ detection of mechanical overloads on splined shafts by eddy current techniques. Splined shaft connections are among the most heavily stressed machine elements in the drive train and are usually located centrally in the powertrain. To monitor mechanical overloads, a material-integrated sensor will be developed. The structural change that takes place in the sensor as a result of overloads can be monitored with the help of eddy current testing technology. To ensure permanent monitoring of the sensor area, a compact eddy current testing system has to be integrated into the component. This consists of a printed circuit board coil, an evaluation unit, a data transmission module and an energy-harvesting module. The measurement principle for the stainless steels employed is based on the microstructural transformation from metastable austenite to martensite when the material is stressed beyond a threshold value. The threshold at which the structural transformation occurs can individually be adjusted in the sensor area by a local laser heat treatment.

Numerical modelling of the process chain for aluminium Tailored Heat-Treated Profiles

Lightweight construction in modern car design leads to an increased usage of various aluminium semi-finished products. Besides sheet material, aluminium extrusion profiles are frequently used due to their high stiffness and variety of possible cross-sections. However, similar to sheet material, aluminium profiles exhibit limited formability in comparison to mild steel materials. One possibility to increase the forming limits of precipitation hardened aluminium alloys is the so-called Tailored Heat Treatment technology. By a local short-term heat treatment, the material is softened and the material flow can be controlled to reduce stresses in critical forming zones. The purposeful definition of the heat treatment zones is mandatory to improve the forming results. Therefore, numerical methods are necessary. In this investigation, a numerical process chain is presented. It combines the thermo-mechanical simulation of a local laser heat treatment with a subsequent bending process of the heat-treated profile using the alloy EN AW-6082. The temperature distribution, mechanical properties, and finally, the bending result of the numerical model are validated by experimental tests.

Excellent mechanical properties of a ZrTi-based alloy via laser surface remelting

In recent years, more and more attention has been paid to the research and development of high strength Zirconium-Titanium (ZrTi) based alloy system. However, like almost all metallic materials, increasing the strength of ZrTi alloy leads to a significant decrease in plasticity. In this paper, high strength and toughness ZrTi alloy with gradient structure was prepared by laser surface remelting. The metastable β phase with high dislocation density was obtained by laser local heating and rapid cooling, and a large amount of acicular martensite structure was formed in the heat affected zone, combined with the dual phase of the matrix, a gradient structure was formed from the surface to the matrix. The tensile strength was increased to 1550 MPa and the tensile elongation was kept above 10%. The composite deformation mechanisms of dislocation slip, twinning and stress-induced martensitic transformation of gradient structure have also been discussed.

Improving fatigue life of additively repaired Ti-6Al-4V subjected to laser-assisted ultrasonic nanocrystal surface modification

Ultrasonic nanocrystal surface modification (UNSM) combined with localized laser heating (LA) were implemented to enhance the surface fatigue life of additively repaired Ti-6Al-4V alloy parts in this study. The effect of induced surface severe plastic deformation on microstructure development and grain refinement is analyzed. The results showed that surface metallurgical defects, hardness, residual stress, and roughness were significantly improved after LA-UNSM treatment. As a result of the improved surface textures and higher grain refinement, the fatigue life of the LA-UNSM treated samples was an order of magnitude longer. Surface free of pores and displaying a healthy topography provides evidence.

Controlling the Modes of Spin Wave Propagation in an Yttrium Iron Garnet Waveguide by Local Laser Heating

Controlling the Modes of Spin Wave Propagation in an Yttrium Iron Garnet Waveguide by Local Laser Heating

Control of spin wave propagation modes in the yttrium iron garnet waveguide by means of local laser heating

The results of micromagnetic modeling of the magnetic structure on the surface of a film of yttrium iron garnet (YIG) are presented, where a region of altered saturation magnetization was created by focused laser radiation. Based on the constructed amplitude-frequency characteristics of signal transmission through a magnon structure with a heating region, the possibility of implementing spin-wave signal filtering modes with a change in the diameter of the heated region on the YIG surface is shown.

Direct formation of carbon nanotube wiring with controlled electrical resistance on plastic films

We have developed a simple method to fabricate multi-walled carbon nanotube (MWNT) wiring on a plastic film at room temperature under atmosphere pressure. By irradiating a MWNT thin film coated on a polypropylene (PP) film with a laser, a conductive wiring made of a composite of MWNT and PP can be directly fabricated on the PP film. The resistance of MWNT wiring fabricated using this method were ranging from 0.789 to 114 kΩ/cm. By changing the scanning speed of laser, we could fabricate various regions with different resistances per unit length even within a single wiring. The formation mechanism of the MWNT wiring with tunable resistance was discussed from both experimental results, such as microscopic structural observation using cross-sectional scanning electron microscopy and microscopic Raman imaging, and simulation results, such as heat conduction in the film during local laser heating. The results suggest that the MWNT wiring was formed by PP diffusion in MWNT at high temperature. We also demonstrated that excess MWNTs that were not used for wiring could be recovered and used to fabricate new wirings. This method could be utilized to realize all-carbon devices such as light-weight flexible sensors, energy conversion devices, and energy storage devices.