Thermal Defects Research Articles

Thermal defects generation mechanism in laser precision dressing of metal-bonded micro-groove diamond wheel and their influence on grinding silicon wafer

The metal-bonded diamond grinding wheel demonstrates superior grinding capabilities and has been frequently employed to chamfer the edges of semiconductor wafers by meticulously processing micro-groove on the surface. However, the high hardness of diamond wheel poses challenges to the micro-groove processing task. In response to this issue, this paper adopts the deflection laser method to dress the micro-groove of the wheel using a fiber laser. In this paper, the influence of the deflection angle on contour accuracy was studied. The formation mechanism of the recast layer and spatter layer in the laser dressing process was demonstrated, and the influence of surface defects of the wheel on grinding performance was analyzed. The results demonstrated that the laser deflection dressing method had effectively solved the laser energy dispersion problem, improved the micro-groove bevel profile, and arc profile dressing accuracy, the maximum angle, arc radius error values were 0.189°, 0.008 mm. The application of low-power sharpening techniques successfully removed thermal defects, such as hot cracks, recast and spatter layers, which were typically produced during high-power shaping, and this refinement improved the grinding capabilities of the wheel. The dimensions of the silicon wafers after chamfering met the requirements and the surface morphology was found excellent with no cracks.

Influence of growth temperature on the structural and optical characteristics of PbZnS thin films

Ternary thin films have garnered significant attention due to their adjustable band gap characteristics, making them suitable for many applications. In this research, we examined the structural and optical characteristics of PbZnS thin films fabricated onto glass substrates employing the spray pyrolysis techniuqe. The films were fabricated at different temperatures ranging from 300 to 400°C. X-ray diffraction (XRD) analysis indicated that all films displayed a cubic structure with a preferred orientation along the (200) plane. From the analysis of XRD peaks, it was observed that the crystal structure initially improved with increasing temperature but then began to degrade. These films demonstrated strong absorption in the range of 350–1600 nm. The optical bandgap (Eg) values, calculated based on the relationship between the absorption coefficient and photon energy, showed slight variations around 2 eV. Notably, the band gap decreased with growth temperatures up to 350°C, then increased at higher temperatures. These fluctuations are attributed to factors such as thermal expansion, strain, defects, surface/interface effects, and changes in doping or composition.

Combined hybrid machining of laser ablation-drilling small holes in Cf/SiC composites

Carbon fiber-reinforced silicon carbide composite material (Cf/SiC) serves as a typical advanced material for fabricating hot-end components of aircraft engines. Its unique properties, including high hardness and anisotropy, pose significant challenges in processing. Nevertheless, a crucial structural element of hot-end components, specifically the cooling and heat dissipation holes, necessitates hole machining within the Cf/SiC composites. The intricate nature of machining this material often leads to compromised surface quality and severe wear on cutting tools during the hole machining process. This study delves into the feasibility of combined hybrid machining of laser ablation-drilling(L-DCM) for fabricating small holes in woven Cf/SiC composites, along with elucidating the underlying material removal mechanisms. The findings reveal approach significantly enhances the cutting performance of hard alloys tools when dealing with ceramic matrix composites. Notably, the wear mechanisms of the cutting tools primarily involve abrasive wear and adhesive wear. The primary processing defects observed in the small holes include fiber pullout at the entrance and exit, as well as material delamination at the exit, which are primarily attributed to debonding at the layer interface and brittle fracture of the matrix. The subsequent drilling process, conducted based on laser drilling, effectively eliminates thermal defects such as the heat-affected zone and recast layer, as well as morphological defects like taper. Additionally, this approach mitigates the tool wear process, minimizes the impact of compression effects on material removal, and enhances the shear action of the tool. Consequently, the layer interface remains securely bonded and intact after processing, thereby preserving the material's strength.

Influence of annealing on microstructures and mechanical properties of laser powder bed fusion and wire arc directed energy deposition additively manufactured 316L

The annealing response of 316L stainless steel manufactured with both laser powder bed fusion (PBF-LB) and wire-arc directed energy deposition (DED-Arc) was investigated in the context of microstructural evolution and mechanical properties. Because additive manufacturing (AM) comprises several technologies that differ in heat source, feedstock, and scan strategy, among other build variables, the final products can experience a range of thermal histories and defect (dislocation) populations. These unique thermal histories can subsequently influence as-built properties and annealing response of AM materials, most notably those manufactured with different AM deposition processes. The PBF-LB process (with higher cooling rates) yielded finer austenite microstructures having more lattice strain, higher dislocation densities, and higher yield strength values than the DED-Arc 316L process (with lower cooling rates). Electron backscattered diffraction (EBSD) data of the kernel average misorientation (KAM), boundary density, and geometrically necessary dislocation (GND) density support the differences in lattice strain. As a result, the PBF-LB 316L exhibited more recovery and recrystallization after annealing at elevated temperatures above 873 K based on changes to yield strength, work hardening behavior, and microstructure evolution. The DED-Arc 316L exhibited a δ-ferrite/austenite microstructure with microsegregation that influenced deformation mechanisms active after annealing. Tensile data, deformed microstructures and misorientation histograms from EBSD showed a decreasing amount of deformation twinning when comparing the as-built condition to annealed conditions (up to 1473 K/1h) of PBF-LB 316L. The opposite trend was noted in DED-Arc 316L. The behavior was interpreted to be due to the differences in chemical segregation during solidification and the effects of heat treatment on chemical homogenization and local stacking fault energy.

Evaluation of aerial thermography for measuring the thermal transmittance (U-value) of a building façade

In efforts to mitigate greenhouse gas emissions in the construction sector, policies promoting energy efficiency improvements in existing buildings are being implemented. Accurately assessing the impact of energy retrofit strategies requires precise determination of thermal envelope performance through energy audits. Aerial thermography, employing unmanned aerial vehicles (UAVs) equipped with infrared thermal cameras (IRT), is an underused method overcoming limitations in energy audits of large buildings or complexes. This approach offers an elevated perspective, providing an overview of the building’s thermal patterns, aiding in the identification of thermal envelope defects. By measuring temperature distribution, even in inaccessible envelope areas, the severity of identified defects can be estimated. Beyond detecting thermal bridges, air leaks, and moisture, aerial thermography is valid for estimating thermal transmittance (U-value) of enclosures. The aim of this research is to evaluate this method to calculate the U-value of the façade of an university building located in a Subtropical Mediterranean climate, during a typical full day in different seasons of the year. Concurrently with aerial thermography, the thermometric method (THM) was applied to compare results and refine the operational conditions during U-value measurement using different statistical indices and uncertainty analysis. The results demonstrated the validation of airborne thermography as a method for U-value calculation under specific meteorological and operational conditions, overcoming the common physical limitations of traditional building audit methods.

Ultrasonic Rough Crack Characterization Using Time-of-Flight Diffraction With Self-Attention Neural Network.

Time-of-flight diffraction (ToFD) is a widely used ultrasonic nondestructive evaluation (NDE) method for locating and characterizing rough defects, with high accuracy in sizing smooth cracks. However, naturally grown defects often have irregular surfaces, complicating the received tip diffraction waves and affecting the accuracy of defect characterization. This article proposes a self-attention (SA) deep learning method to interpret the ToFD A-scan signals for sizing rough defects. A high-fidelity finite-element (FE) simulation software Pogo is used to generate the synthetic datasets for training and testing the deep learning model. Besides, the transfer learning (TL) method is used to fine-tune the deep learning model trained by the Gaussian rough defects to boost the performance of characterizing realistic thermal fatigue rough defects. An ultrasonic experiment using 2-D rough crack samples made by additive manufacturing is conducted to validate the performance of the developed deep learning model. To demonstrate the accuracy of the proposed method, the crack characterization results are compared with those obtained using the conventional Hilbert peak-to-peak sizing method. The results indicate that the deep learning method achieves significantly reduced uncertainty and error in rough defect characterization, in comparison with traditional sizing approaches used in ToFD measurements.

Making Patterned Single Defects in MoS2 Thermally with the MoS2/Au Moiré Interface.

Normally, it is hard to regulate thermal defects precisely in their host lattice due to the stochastic nature of thermal activation. Here, we demonstrate a thermal annealing way to create patterned single sulfur vacancy (VS) defects in monolayer molybdenum disulfide (MoS2) with about 2 nm separations at subnanometer accuracy. Theoretically, we reveal that the S-Au interface coupling reduces the energy barriers in forming VS defects and that explains the overwhelming formation of interface VS defects. We also discover a phonon regulation mechanism by the moiré interface that effectively condenses the Γ-point out-of-plane acoustic phonons of monolayer MoS2 to its TOP moiré sites, which has been proposed to trigger moiré-patterned thermal VS formation. The high-throughput nanoscale patterned defects presented here may contribute to building scalable defect-based quantum systems.

Physical mechanism of gettering of impurity Ni atom clusters in Si lattice

This article presents the gettering mechanism and the physical model of impurity Ni atom clusters in the Si crystal lattice. The study finds out that the formed Ni atom clusters lead to gettering various rapidly diffusing impurities, both present in the Si lattice and introduced, as well as oxygen atoms, by stimulating generation of recombination centers of thermal and radiation defects.

Comprehensive characterization of nitrogen-related defect states in β-Ga2O3 using quantitative optical and thermal defect spectroscopy methods

This study provides a comprehensive analysis of the dominant deep acceptor level in nitrogen-doped beta-phase gallium oxide (β-Ga2O3), elucidating and reconciling the hole emission features observed in deep-level optical spectroscopy (DLOS). The unique behavior of this defect, coupled with its small optical cross section, complicates trap concentration analysis using DLOS, which is essential for defect characterization in β-Ga2O3. A complex feature arises in DLOS results due to simultaneous electron emission to the conduction band and hole emission to the valence band from the same defect state, indicating the formation of two distinct atomic configurations and suggesting metastable defect characteristics. This study discusses the implications of this behavior on DLOS analysis and employs advanced spectroscopy techniques such as double-beam DLOS and optical isothermal measurements to address these complications. The double-beam DLOS method reveals a distinct hole emission process at EV+1.3 eV previously obscured in conventional DLOS. Optical isothermal measurements further characterize this energy level, appearing only in N-doped β-Ga2O3. This enables an estimate of the β-Ga2O3 hole effective mass by analyzing temperature-dependent carrier emission rates. This work highlights the impact of partial trap-filling behavior on DLOS analysis and identifies the presence of hole trapping and emission in β-Ga2O3. Although N-doping is ideal for creating semi-insulating material through the efficient compensation of free electrons, this study also reveals a significant hole emission and migration process within the weak electric fields of the Schottky diode depletion region.

Modelling and Evalaution of the Bidirectional Surge Current Robustness of Si(-IGBT and -Diode), SiC(-MOSFETs and -JFET) and GaN(-HEMTs) Devices

This paper will evaluate the surge current robustness of different devices in relation to the active short circuit (ASC). For the purposes of this study, a Si IGBT and its diode, two SiC MOSFETs with different voltage ratings, a SiC JFET, and three GaN HEMTs will be compared. For the GaN devices, a eMode, a dMode, and a cascode device are employed. With the exception of the Si diode, all devices exhibited a current saturation effect. This saturation will result in significant losses and, ultimately, a thermal defect. For all devices, a safe operating area (SOA) criterion is established. For the SiC and GaN devices, the saturation voltage can be employed to define the safe operating area (SOA) criterion. In this context, two on-state resistance models will be defined for these devices. One is solely temperature-dependent, while the other also considers current saturation. Consequently, the saturation voltage and the on-resistance model represent a straightforward methodology for evaluating the ASC robustness of the devices. For all devices, a recommendation for a loss model and SOA criterion will be provided. Finally, the surge current robustness of all devices is compared. The Si, SiC and GaN devices exhibit comparable high surge current robustness in the application, with the exception of the GaN eMode, which is susceptible to strong current saturation.

Optimization of nanosecond laser drilling strategy on CFRP hole quality

The drilling and cutting of carbon fiber-reinforced epoxy resin matrix composite (CFRP) structural parts is a prerequisite for one-off moulding and assembly connections. However, the thermal ablation effect observed during nanosecond laser hole-making of CFRP results in significant accuracy errors and thermal damage defects in the quality of the holes obtained from the process. To enhance the quality of laser-drilling CFRP holes, a spiral drilling path was employed in this work. The influence of diverse drilling methodologies, encompassing the trajectory of the laser beam, the spacing between scans, and the direction of the suction system's pumping, on the quality of the holes was examined. The impact of these techniques on the precision and integrity of the holes was assessed in terms of their dimensions, the quality factor, the width of the heat-affected zone (HAZ), and the prevalence of microscopic defects. The results demonstrated that when the drilling strategy involves moving the laser beam from the outside to the inside (Scheme I), a scanning spacing of 20 μm, and backward pumping, the optimal micro-hole accuracy and surface morphology, as well as minimal thermal damage defects can be achieved. This study provides a reference for further optimization of the nanosecond laser drilling process.

Impact of applied loads on wear mechanisms in H13 steel at various preheating temperatures during laser powder bed fusion additive manufacturing

In laser powder bed fusion (LPBF), processed H13 tooling has a broad application prospect in the mold and die industry. In practice, however, it is often impossible to obtain hot wear resistance and compressive residual stresses which constrain its development, affecting hence the wear performance and service life. Under these conditions, substrate preheating is an effective way to reduce thermal stress and defects of H13 for tooling applications. This research paper emphasizes the main characteristics of preheating temperature and its chief induced properties on microstructure and wear behavior of LPBF-processed H13 steel. The elevated preheating temperature altered the microstructure, increasing hardness and wear resistance. Under low applied loads, better wear resistance was attributed to high hardness and tribo-oxide layer formation. Whereas under high applied loads, it was dominated by the increased hardness due to strain hardening.

Laser-assisted Belt Grinding with Creep Feed for Enhanced Surface Integrity and Abrasive wear Reduction

Creep feed belt grinding (CFBG) is a vital machining technique used for intricate curved parts like aero-engine blades and flaps. CFBG effectively removes material by using larger depths of cut and lower feed rates, but high loads can lead to severe belt wear and thermal damage defects in the process. Laser-assisted processing technology can mitigate tool wear and enhance machining quality. This research compared and analyzed belt wear behavior and its impact on the surface integrity of titanium alloys in laser-assisted creep feed belt grinding (LCFBG) and CFBG processes. The following conclusions were drawn: The local thermal effect of the laser softened the material, reduced the grinding force. The wear pattern of the abrasive grains changed from abrasive fracture and stripping to steady abrasive wear. The effective life of the belt increased by about 33 %. In addition, due to reduction in grinding force, the thermo-mechanical coupling effect caused by material removal is reduced, and the grinding temperature is lowered. As a result of the reduction in grinding force and temperature, the mechanical and thermal stresses are reduced, the oxidation reaction on the surface of the titanium alloy is weakened, and the residual stress is lowered, leading to an improvement in surface quality.

Visual localization and quantitative detection method for thermal defects in cable terminals of high-speed trains based on a temperature derivative

Abstract Cable terminal defect detection plays an important role in ensuring the safe and stable operation of high-speed trains (HST). In this paper, a numerical model of the electromagnetic thermal field of overheating defects of cable terminal shielding grids and a method of detecting internal defects of cable terminals - even-order temperature derivative (EOTD) is proposed for quantitative detection of internal defective structures of vehicle-mounted cable terminals. Firstly, a numerical model of the electromagnetic thermal field of cable terminals under the condition of leakage current is constructed, through which the temperature field distribution characteristics of different defective structures are analyzed. Then, the intuitive location of the defective region is obtained by investigating the derivative characteristics of the temperature image, and the depth and intensity of the defects are quantitatively assessed by using the main side peak (MSP) distances and the MSP values extracted from the derivative curves. Finally, the simulation and experimental results achieve the identification of defect structures and the quantitative detection of defect depth and intensity, proving the effectiveness and accuracy of the proposed method.

Laser and mechanical micro-drilling for aerospace applications

Traditional laser percussion drilling encounters challenges related to poor geometry and thermal defects. At the same time, mechanical micro-drilling while generating high-quality holes, faces issues such as premature drill breakage and difficulty drilling at acute angles. This review paper introduces a novel approach to micro-drilling Inconel 718 alloy sheets at acute angles by combining sequential laser and mechanical drilling. The study showcases the feasibility and fundamental characteristics of this innovative technique. Results indicate that the sequential laser-mechanical micro-drilling approach mitigates the drawbacks associated with laser-drilled holes, leading to reduced burr size, decreased machining time, and increased tool life when compared to conventional mechanical drilling methods. This advancement holds promise for enhancing precision and efficiency in aerospace applications, paving the way for improved manufacturing processes in the aerospace industry.

Evaluation and prediction of thermal defects in SLM-manufactured tibial components using FEM-based deep learning and statistic methods

Evaluation and prediction of thermal defects in SLM-manufactured tibial components using FEM-based deep learning and statistic methods

Wire arc additive manufacturing of a high-strength low-alloy steel part: environmental impacts, costs, and mechanical properties

Additive manufacturing (AM) technologies have demonstrated a promising material efficiency potential in comparison to traditional material removal processes. A new directed energy deposition (DED) category AM process called wire arc additive manufacturing (WAAM) is evolving due to its benefits which include faster build rates, capacity to build large volumes, and inexpensive feedstock materials and machine tools compared to more technologically mature powder-based AM technologies. However, WAAM products present challenges like poor surface finish and lower dimensional accuracy compared to powder-based processes or machined parts, prevalence of thermal distortions, residual stresses, and defects like porosity, cracks, and humping, often requiring post-processing operations like finish machining and heat treatment. These post-processing operations add to the production cost and environmental footprint of WAAM-built parts. Therefore, considering the opportunities and challenges presented by WAAM, this paper analyses the environmental impact, production costs, and mechanical properties of WAAM parts and compares them with those achieved by laser powder bed fusion (LPBF) and traditional computer numerical control (CNC) milling. A high-strength low-alloy steel (ER70S) mechanical part with medium complexity was fabricated using WAAM. Based on the data collected during this experiment, environmental impact and cost models were built using life cycle assessment and life cycle costing methodologies. WAAM was observed to be the most environmentally friendly option due to its superior material efficacy than CNC milling and has a better energy efficiency than LPBF. Also, WAAM was the most cost-friendly option when adopted in batch production for batch sizes above 3. The environmental and cost potential of WAAM is amplified when used for manufacturing large products, resulting in significant material, emission, and cost savings. The fabricated WAAM part demonstrated good mechanical properties comparable to that of cast/forged material. The methodology and experimental data presented in this study can be used to calculate environmental impacts and costs for other products and can be helpful to manufacturers in selecting the most ecofriendly and cost-efficient manufacturing process.

Influence of printing parameters on the thermal properties of 3D-printed construction structures

3D printing in the construction sector is a promising, sustainable approach with the potential to build energy-efficient buildings. However, there is a lack of studies on the thermal properties of 3D-printed structures, which are fundamental to their energy efficiency. This study investigates the impact of printing parameters on thermal conductivity and defect development in 3D-printed concrete structures. Heat flow meter, scanning electron microscope and infrared imaging techniques were employed to assess the thermal properties and microstructure of 3D-printed samples. The results demonstrate that printing parameters significantly influence the porosity, void formation, and microstructural characteristics of 3D-printed concrete, ultimately affecting thermal properties. An increase in extrusion rate leads to higher thermal conductivity, whereas an increase in speed and standoff distance reduces thermal conductivity. Excessive spacing between printed lines can induce voids and gaps, contributing to lower thermal conductivity, while minimal spacing can promote higher porosity and larger pores. Infrared imaging revealed nonuniform thermal distribution in 3D-printed structures caused by the layered structure and voids in interlayer gaps. This study provides valuable insights for designing and constructing energy-efficient 3D-printed buildings, contributing to the development of sustainable building practices and the advancement of additive manufacturing in construction.

Effects of joint tolerances on thermal bridging in precast concrete shear walls: Field tests and numerical simulations

Prefabricated buildings are gaining momentum due to their ecological benefits, speedy construction, and precision. However, unlike conventional cast-in-place structures, precast concrete shear walls are more susceptible to thermal and quality defects due to inherent tolerances. Four precast concrete shear wall connections prone to thermal bridges were identified through case studies of factories and real projects. Steady-state and dynamic modeling was conducted using determinant and stochastic matrices to assess the impact of joint tolerances on thermal distribution. The effects of joint tolerance magnitude and insulation material properties on linear thermal transmittance were investigated, complemented by a polynomial-based sensitivity analysis. A versatile prediction model was developed, integrating physical modeling and data-driven approaches to quickly quantify the thermal bridges of precast concrete shear wall connections. The results revealed a significant positive correlation between thermal bridges and joint tolerances, leading to extensive low-temperature regions within precast concrete shear walls. In dynamic modeling, joint tolerances contributed to additional heat dissipation, with a peak heat flux of 20.07 W/m. The sensitivity analysis demonstrated that joint tolerances greatly influence the correlation between linear thermal transmittance and insulation layer properties. Leveraging Categorical Boosting and Extreme Gradient Boosting, the prediction of linear thermal transmittance for precast concrete shear wall connections exhibited exceptional accuracy while significantly reducing computational costs. This research aids designers and policymakers by providing rapid and precise linear thermal transmittance calculations, offering valuable guidance for manufacturing prefabricated components to ensure enhanced energy efficiency and sustainability.

Influence of electrochemical machining on shape memory and surface characteristics of nickel-titanium-cobalt shape memory alloy: An experimental study

ABSTRACT Electro-chemical machining (ECM) can safely be categorized as being a unique unconventional machining technique, in which the role of thermal defects are not involved. The NiTiCo shape memory alloy has high yield strength coupled with good resistance to aggressive aqueous environments, which makes it a potential candidate for use in several bio-medical applications. This research concentrates on electro-chemical machining of a NiTiCo alloy. Taguchi’s Design of Experiment (DOE) containing three-parameters and three-factors was used to conduct the experimental study. The influence of concentration of the sodium chloride electrolyte, voltage, and frequency on output characteristics, such as (i) material removal rate (MRR), (ii) surface roughness (SR), and (iii) over-cut (OC), was analyzed. Experimental outputs revealed the voltage (45.29%) to be the most influential among the three controlling parameter. Grey relational analysis (GRA) was utilized to discover the ideal parametric relation and optimize machining characteristics.