Thermal Distortion Research Articles

Powder Bed Monitoring Using Semantic Image Segmentation to Detect Failures during 3D Metal Printing.

Monitoring the metal Additive Manufacturing (AM) process is an important task within the scope of quality assurance. This article presents a method to gain insights into process quality by comparing the actual and target layers. Images of the powder bed were captured and segmented using an Xception-style neural network to predict the powder and part areas. The segmentation result of every layer is compared to the reference layer regarding the area, centroids, and normalized area difference of each part. To evaluate the method, a print job with three parts was chosen where one of them broke off and another one had thermal deformations. The calculated metrics are useful for detecting if a part is damaged or for identifying thermal distortions. The method introduced by this work can be used to monitor the metal AM process for quality assurance. Due to the limited camera resolutions and inconsistent lighting conditions, the approach has some limitations, which are discussed at the end.

Fast distortion prediction in directed energy deposition using inversely-identified inherent strains

Large thermal distortions due to the cyclic rapid melting and solidification mechanism in metal additive manufacturing (MAM) affect part manufacturing precision. The so-called inherent strain method is one of the most computationally efficient methods to predict distortion, but the current inherent strain methods extract inherent strains through the full-scale detailed thermo-mechanical model (TMM) with a long computation time. This work proposes an inverse parameter identification method to fast identify inherent strains based on the measured distortion results from a two-track-two-layer workpiece. The identified inherent strains are employed in a static mechanical analysis to efficiently predict distortion in MAM. To verify the proposed method, a multi-track-multi-layer workpiece and a square-shaped workpiece deposited by the directed energy deposition process are studied. The simulated distortion results demonstrate high simulation accuracy by comparing with the experimental results. In addition, comparisons with the TMM and the modified inherent strain method indicate that the inversely-identified inherent strains can improve the distortion simulation accuracy and reduce the simulation time, which is practical to be applied in industrial applications.

Timepix3: Compensation of Thermal Distortion of Energy Measurement.

The Timepix3 is a hybrid pixellated radiation detector consisting of a 256 px × 256 px radiation-sensitive matrix. Research has shown that it is susceptible to energy spectrum distortion due to temperature variations. This can lead to a relative measurement error of up to 35% in the tested temperature range of 10 °C to 70 °C. To overcome this issue, this study proposes a complex compensation method to reduce the error to less than 1%. The compensation method was tested with different radiation sources, focusing on energy peaks up to 100 keV. The results of the study showed that a general model for temperature distortion compensation could be established, where the error in the X-ray fluorescence spectrum of Lead (74.97 keV) was reduced from 22% to less than 2% for 60 °C after the correction was applied. The validity of the model was also verified at temperatures below 0 °C, where the relative measurement error for the Tin peak (25.27 keV) was reduced from 11.4% to 2.1% at -40 °C. The results of this study demonstrate the effectiveness of the proposed compensation method and models in significantly improving the accuracy of energy measurements. This has implications for various fields of research and industry that require accurate radiation energy measurements and cannot afford to use power for cooling or temperature stabilisation of the detector.

Thermal Intra-Layer Interaction of Discretized Fractal Exposure Strategies in Non-Isothermal Powder Bed Fusion of Polypropylene

Additive manufacturing of material systems sensitive to heat degradation represents an essential prerequisite for the integration of novel functionalized material systems in medical applications, such as the hybrid processing of high-performance thermoplastics and gelling polymers. For enabling an inherent process stability under non-isothermal conditions at reduced ambient temperatures in laser-based additive manufacturing, maintaining a homogeneous layer formation is of vital significance. To minimize crystallization-induced deflections of formed layers while avoiding support structures, the temporal and spatial discretization of the melting process is combined with the subsequent quenching of the polymer melt due to thermal conduction. Based on implementing superposed, phase-shifted fractal curves as the underlying exposure structure, the locally limited temporal and spatial discretization of the exposure process promotes a mesoscale compensation of crystallization shrinkage and thermal distortion, enabling the essential homogeneous layer formation. For improving the understanding of local parameter-dependent thermal intra-layer interactions under non-isothermal processing conditions, geometric boundary conditions of distinct exposure vectors and the underlying laser power are varied. Applying polypropylene as a model material, a significant influence of the spatial distance of fractal exposure structures on the thermal superposition of distinct exposure vectors can be derived, implicitly influencing temporal and temperature-dependent characteristics of the material crystallization and the emerging thermal material exposure. Furthermore, the formation of sub-focus structures can be observed, contributing to the spatial discretization of the layer formation, representing a decisive factor that influences the structure formation and mesoscopic part properties in non-isothermal powder bed fusion of polymers. Consequently, the presented approach represents a foundation for the support-free, accelerated non-isothermal additive manufacturing of both polymers and metals, demonstrating a novel methodology for the mesoscale compensation of thermal shrinkage.

In-situ tailoring microstructures to promote strength-ductility synergy in laser powder bed fusion of NiCoCr medium-entropy alloy

Laser powder bed fusion (LPBF) has received widespread attention owing to its digital, flexible, and controllable fabrication process, which opens new possibilities for the direct and fast production of metal components with extremely complex geometries and good performance. Generally, post-treatments are conducted to further regulate the microstructure and performance of as-deposited geometrically-complex metal components. In this work, instead of applying post-treatments, we demonstrate a novel in-situ remelting strategy to effectively enhance the strength without sacrificing the ductility of LPBF-fabricated NiCoCr medium-entropy alloy by in-process tailoring microstructures. We find that in-situ remelting processing not only changes the melt pool geometry in favor of optimizing densification and developing unique crystallographic lamellar microstructures, but also promotes inherent thermal distortions and heat treatments during manufacturing resulting in increased dislocation density and grain refinement in the LPBF-remelted specimen. The present work paves a new way in additively manufacturing metal materials to tailor microstructures for enhanced mechanical performance without additional treatments.

Near-zero thermal mismatch flexure metastructure with high-resonant frequency

Heterogeneous mechanical metastructures, especially fabricated by additive manufacturing techniques, have attracted attention in lightweight multifunctional material engineering. Near-zero thermal expansion materials play an important role in precision equipment. However, undesired thermal mismatch flexure distortion occurs when the precision equipment is mounted on the main load-bearing structures of spacecraft and the environmental temperature changes. The present study presents a near-zero thermal mismatch flexure metastructure, including the design strategy inspired by plants with axisymmetric distributing anisotropic microstructures, the thermal deformation constitutive relation derived by the flexibility method, and the fabrication of this metastructure by selective laser melting. Thermal deformation experiments demonstrate that the coefficient of thermal flexure reduces by 92.4% compared with traditional bi-material plates connected by screws. Mechanical vibration experiments demonstrate that the fundamental resonant frequency of this metastructure can be as high as 956 Hz, which is 5.8 times larger than the common requirements of functional components for spacecraft to avoid mechanical resonance with the main structure during launch. The excellent thermal mismatch shape stability and high-resonant frequency properties make it a potential candidate for thermal geometric stable structures working in variable thermal and mechanical environments.

Effect of Water and Aerosols Absorption on Laser Beam Propagation in Moist Atmosphere at Eye-Safe Wavelength of 1.57 μm

To realize optical wireless power transmission, atmospheric propagation of eye-safe wavelength (1.57μm) laser beams was theoretically investigated. Laser beams are affected by the presence of water vapor and aerosols which absorb and scatter the laser energy. The scattering coefficients of water molecules and aerosols were estimated to be about 6.3 × 10^-7 and 5.6 × 10^-5 m^-1, respectively, at wavelength (λ₀) of 1.57μm. Furthermore, the absorption coefficients of moist air at 30% relative humidity and aerosols were estimated to be about 6.16 × 10^-3 and 2.52 × 10^-5 m^-1, respectively, at λ₀ = 1.57μm. Then simulation of laser beam propagation in the moist atmosphere at λ₀ = 1.57μm was performed using these coefficients. Under the condition of no wind, the beam intensity decreases rapidly with increasing the length <i>z</i> and the rate of decrease slows down as the beam radius (ω) increases. When <i>z</i>_h is defined as the <i>z</i> where the normalized intensity is halved, the <i>z</i>_h (= 25 m) at ω = 20 mm when input power <I>P</I> = 10 W is about three times longer than that (= 8 m) when <I>P</I> = 100 W. This result indicates that the thermal distortion of laser beams due to accumulated heat around the <i>z</i> axis becomes more conspicuous as the optical power increases. The effect of this thermal beam distortion can be weakened when the laser beam is subject to crosswinds. Under the condition of gentle uniform wind with wind velocity <i>v</i> = 5 m/s, propagation of laser beams with ω = 20 mm was studied when <I>P</I> = 100 W. The <i>z</i>_h (= 105 m) when <i>v</i> = 5 m/s is about 13 times longer than that (= 8 m) when <i>v</i> = 0 m/s. Thus, under conditions of <i>v</i> = 5 m/s and 30% relative humidity, laser beams with <I>P</I> = 100 W and ω = 20 mm can propagate over 100 m without damaging the initial beam shape at λ₀ = 1.57μm.

A Bayesian regularization network approach to thermal distortion control in 3D printing

A Bayesian regularization network approach to thermal distortion control in 3D printing

INVESTIGATION OF THE SURFACE QUALITY OF AA6082-ZrO2-Gr MMCs USING ABRASIVE WATERJET MACHINING

Aluminium metal matrix composites have wide applications in the aerospace and automotive industries due to their excellent physical properties like hardness, tensile strength, etc. The reinforcement of ZrO2 hard ceramic particles and soft solid lubricant graphite on AA6082 produces a high-strength composite. This research aims to fabricate AA6082-ZrO2-Gr MMCs with different compositions and evaluate the abrasive-waterjet-machining (AWJM) parameters in the machining of the fabricated composites, which eliminates the thermal distortion and damage to the work material. The experiments were conducted by varying the dominant process parameters such as water pressure, traverse speed, abrasive flow rate and mesh size. The kerf taper angle was affected by the traverse speed and water pressure, while the surface roughness was affected by the abrasive flow rate, mesh size and water pressure. The abrasive mesh size of 120 provided the best surface finish.

Wavelength selection for spectrally combined laser beams propagating in free space and the atmosphere

Wavelength selection has a major impact on spectral beam combining (SBC) of laser beams that propagate in free space and in the atmosphere. We derive analytical expressions for the intensity and four characteristic quality parameters of free-space SBC beams. We show that the peak intensity, the mean-square beam width, and the M 2 factor of an SBC beam at the target are all larger than those of one single laser beam of the same power and average wavelength. The peak intensity can be made higher by increasing the wavelength spacing but reducing the mean wavelength. However, when an SBC laser beam propagates in the atmosphere, the beam quality depends on the absorption coefficient of the multi-wavelength field due to thermal blooming. A thermal distortion number is introduced for an SBC beam and its validity is confirmed. We show that the power proportion of a high-absorption element within an SBC beam should be as low as possible to reduce the thermal blooming effect.

Research on thermal stability of monolithic near-infrared Doppler asymmetric spatial heterodyne interferometer

A Doppler asymmetric spatial heterodyne (DASH) interferometer was designed to measure atmospheric winds at a height of 60 to 80 km by observing the airglow emission line of molecular oxygen at 867 nm. The designed monolithic DASH interferometer exhibited decent thermal stability. The phase thermal drift of the fabricated interferometer obtained from thermal performance measurements was 0.376 rad / ° C. To accurately model and minimize the thermal drift performance of an interferometer in the design phase, it is necessary to include the influence of thermal distortion of the monolithic interferometer components. Therefore, an optical–structural–thermal integrated analysis method based on Zernike polynomials was proposed to accurately calculate the phase thermal drift of the interferometer. The optical model modified by the finite-element method calculated the phase thermal drift to be 0.420 rad / ° C, which agreed with the experimental result within 11.7%. This analysis method can accurately calculate and optimize thermal stability during the design of a DASH interferometer.

Feasibility study of shell element-based elastic FE approach for welding-induced thermal distortion prediction in HDPE welded structures

High-density polyethylene (HDPE) is considered an eco-friendly material for boat construction worldwide. However, managing thermal distortion in HDPE welding is challenging, impacting productivity. Traditional steel shipbuilding has established methods to predict welding-induced thermal distortion, but HDPE lacks comprehensive studies and standards. This research explores applying the elastic Finite Element (FE) approach, commonly used in steel structures, to HDPE welding. The elastic FE approach simplifies complex welding simulations, enabling its use in large structures like ship hulls. Our research assesses whether HDPE welded specimens exhibit similar distortion patterns to conventional welded structures and whether consistent parameters yield similar thermal distortion. Alignment between our FE analysis, based on specimen data, and experimental results validates the feasibility of using the elastic FE approach to predict HDPE thermal distortion. This study suggests it as a practical method to enhance HDPE boat manufacturing productivity.

MACHINE LEARNING MODELS IN PULSE TIG WELDING OF TITANIUM ALLOY

Titanium is one of the most important non-ferrous metals, which finds extensive application in the aerospace industry, because of its lightweight, excellent corrosion resistance, high strength level. Pulsed current tungsten–inert-gas welding is widely used in manufacturing important structures and products because it has quite a few advantages and practical benefits. With this process, it is easy and convenient to control heat input into parent materials and weld formation, to improve arc stability, to reduce thermal distortion and to achieve fine grains in the weld zone. Important process parameters from the previous investigation are identified and an experimental plan is derived to perform the experiments. Machine learning is a beneficial method to solve problems faced in welding processes. It can improve the welding process effectiveness as well as the process of quality monitoring. Machine learning models developed will optimize the parameters to give better mechanical properties.

Faulty Residential Circuit Breakers—A Persistent Fire Safety Problem

Circuit breakers for residential branch circuits must trip at or below 135% of rated current. A breaker that fails that requirement is defective. Samples of two brands, purchased from retail sources, are tested for that basic calibration. Both brands were tested 4 years ago. Previous samples of one brand were 50% defective, and new samples manufactured in 2021 are 28% defective. The second brand, previously defect-free, is again defect-free. The test results, past and present, imply that some manufacturers are calibrating breakers to trip too close to the allowable upper current limit, and are checking calibration by testing at higher current. The standard calibration test at 200% of rated current is shown to be incapable of indicating whether or not a breaker will trip properly, as required by the applicable standard, at 135% of rated current. A third brand tested came on the market recently. Its thermal-magnetic breakers trip correctly, but the brand's hydraulic-magnetic breakers are erratic, with 38% of the samples malfunctioning. The malfunctions are attributed to thermal distortion that causes mechanical binding of the triggering mechanism. Some breaker brands with a high defect rate have been in the distribution chain for many years and are permanently installed in homes. The increased risk of fire and injury for the occupants of these dwellings is significant. The long-standing history of this problem and the fire safety consequences are discussed.

"A NUMERICAL SOLUTION FOR THE ELASTIC SLIDING CONTACT WITH FRICTIONAL HEATING "

A thermoelastic contact model for non-conforming smooth or rough surfaces is proposed in this paper, based on a method for thermoelastic displacement computation. A classical contact model for the contact of rough surfaces is enhanced by integrating the thermal component of displacement. In the sliding contact model, heat is generated by friction at the contact surface, at a rate proportional to the sliding velocity, the frictional coefficient and the contact pressure. The latter, in its turn, is influenced by the initial clearance, the rigid body approach and the normal displacement, which comprises contributions from (1) the elastic displacement due to pressure, (2) the elastic displacement due to the shear tractions, assumed proportional to pressure according to Coulomb law of friction, and (3) the thermoelastic displacement due to the liberated heat, also proportional to pressure. These dependencies suggest that the geometric contact equation can be reduced to an equation in pressure, whose solution is obtained numerically based on an iterative search of the contact area and of the pressure distribution as in the case of isothermal contacts. A spherical contact is simulated with the newly advanced computer program, by including the thermoelastic effects. It is found that the contact area predicted by the isothermal contact model is larger than for the thermoelastic contact, and the pressure in the central contact region is underestimated when the thermal distortion is neglected. The advanced method is expected to provide base for the study of transient contact processes with consideration of thermal parameters.

Addressing Thermal Distortion in Additive Manufacturing of Topology Optimized Structures Through Reverse Shape Morphing

Design of light weight structures is an important aspect in the aircraft industry, since minimizing the weightof components improves the overall aircraft performance. However, conventional manufacturing methods work on standard geometries and shapes, and often lead to overdesigning of parts. Additive Manufacturing (AM) overcomes these issues by allowing more design freedom. The present study focuses on two aspects of AM: (1) part consolidation through topology optimization, and (2) addressing thermal distortion through reverse shape morphing. An assembly of two load bearing brackets is first amalgamated into a single Topology Optimized (TO) part, which satisfies the displacement and stress requirements of the original design. After a series of optimization iterations, the final TO part (278 g) weighs 69 % lesser than the original assembled design (909 g), still meeting the design constraints. The TO part thus eliminates the need of fasteners to join both the brackets, thereby, making the design simpler yet effective. Moreover, a homogeneous stress distribution in the optimized part allows for efficient material utilization. In order to overcome thermal distortion that results during the AM process, the shape of the TO part is transformed in a sense opposite to the distortions produced. This is achieved through reverse shape morphing technique, that reduces thermal distortions in the printed part to sub-micron levels, and the morphed TO part conforms to the requirements meeting the design constraints. Therefore, the implementation of topology optimization along with reverse shape morphing makes the design simple and efficient having reduced distortion. This is achieved without any need of modifications in the manufacturing system or equipment, and such a strategy can be replicated and implemented at industrial scale as well.

In-situ monitoring and modelling of distortion in multi-layer laser cladding of Stellite 6: Parametric and numerical approach

Laser cladding is a direct metal deposition technique used for repair and rebuilding of worn-out components. In this process, the component is subjected to high heating and cooling cycles, which generate residual stresses and distortion affecting the part quality. The present work is to investigate and evaluate distortions generated during deposition using in-situ measuring method and also with numerical modelling. The role of different support conditions (simply, cantilever, and fixed) and process parameters (laser power, scan speed) on distortion are examined. A numerical model in COMSOL Multiphysics software was developed to generate thermal history and distortion in multi-layer cladding of Stellite 6. The temperature data shows large thermal gradients development along the thickness; allowing material to expand and contract at different rates, leading to substrate distortion. This was evidence in laser displacement sensor data and simulation. The results showed simple and fixed supports develop 40 - 50% and 60 - 70% less distortion than cantilever. Additionally, when the scan speed is decreased from 20 mm/s to 12 mm/s, distortion is lowered by 15 - 40%. An increase in laser power from 2400 W to 3400 W showed distortions to be 30 - 60% higher; while the second layer depositions have given 15 - 25% lower deflection. The simulation results are in good agreement with experimental values and reveals that support conditions are significant on distortion. Conclusion:- Laser cladding with simple and fixed supports developed 40 % - 50 % and 60 % - 70 % lower distortions than cantilever support; and increases with increase in laser power and decrease in scan speed.

Calculation and Prediction of the Thermal Distortion Temperature of Polyamide-6/Organoclay Nanocomposites

Calculation and Prediction of the Thermal Distortion Temperature of Polyamide-6/Organoclay Nanocomposites

Computational investigation of the transient cyclic thermal distortion of funnel shaped mould in thin slab continuous casting process

The response of the thin slab continuous casting mould subjected to thermal and mechanical interactions needs to be studied, since they alter the shape of the mould. A 3D thermomechanical model is developed using non-linear kinematic and isotropic plasticity models along with the creep model to calculate the transient cyclic distortion, residual stress, and elastic–plastic-creep strains in an assembly of funnel-shaped copper mould and waterbox, as the mould undergoes multiple heating cycles in its lifetime. Most of the funnel transition region of the wide face is accumulated with higher temperatures and hence, displacements and strain distributions are maximum in that region. Distortion of the narrow face is majorly affected by the mechanical boundary condition. It is observed that excluding the time dependent creep model highly overestimated the mechanical behaviour of the mould. It is also observed that there is a cyclic variation of distortion of the mould towards the liquid metal side during the casting process and away from the liquid metal side during the cooling process which causes the development of residual stresses inside the mould. This phenomenon further leads to reduction in the magnitude of distortion during subsequent heating cycles of casting operation.

The influence of printing accuracy on the performance of additively manufactured AlSi10Mg phononic crystals

The printing accuracy is one of the key factors affecting the final additively manufactured structures. As metamaterials for wave controlling, the printing accuracy caused in additive manufacturing can affect the band gap structures and the wave transmissions in phononic crystals (PnCs). Selective laser melting (SLM) was used to fabricate the designed PnCs. AlSi10Mg was used as the material for the fabrication. The distorted geometry of the additively manufactured PnCs by selective laser melting (SLM) was experimentally measured and then was re-built as finite element model. The band gap structures and the wave transmissions were calculated based on finite element method for the comparison of the structural performances of the theoretically designed and fabricated PnCs. Result indicated that the fabricated PnCs were thermally distorted from 0.298 mm to 1.664 mm depending on the designed geometries. The thermal distortion can lead to a deviation of central frequency in the range of −6.2%∼3.7% in comparison with the theoretical design. Compared with the theoretical design, the band gap widths of the distorted PnCs vary from −7.4% to 5.6% due to the frequencies change of the eigenmodes at the upper and lower band edges which are directly affected by the thermal distortion. The further analysis shows that the change of the band structures of the distorted PnCs is due to the increase of the inertia moment of the distorted PnCs caused by the printing accuracy.