Maximum Thermal Gradient Research Articles

Achieving a Columnar-to-Equiaxed Transition Through Dendrite Twinning in High Deposition Rate Additively Manufactured Titanium Alloys

AbstractThe coarse β-grain structures typically found in titanium alloys like Ti–6Al–4V (wt pct, Ti64) and Ti–6Al–2Sn–4Zr–2Mo–0.1Si (Ti6242), produced by high deposition rate additive manufacturing (AM) processes, are detrimental to mechanical performance. Certain modified processing conditions have been shown to lead to a more refined grain structure, which has generally been attributed to a change in the solidification conditions with respect to the experimental Hunt diagram proposed by Semiatin and Kobryn. It is shown that with Wire Arc AM (WAAM) increasing the wire feed speed (WFS) is effective in promoting a columnar-equiaxed transition (CET). Conversely, estimates of the dendrite-tip undercooling using the KGT model suggest that this will be too small for free nucleation without the addition of artificial nucleants, due to the very low solute partitioning in Ti alloys. It is also shown that it is difficult to promote a CET with plasma transferred arc WAAM as computational fluid dynamics (CFD) melt-pool simulations indicate that the solidification parameters remain within the columnar region on the Semiatin-Kobryn Hunt map, within the constraints of a stable process. However, a high fraction of twin boundaries was observed in the refined β-grain structures seen at high WFS. This has been attributed to departure of $$\left\langle {001} \right\rangle _{\beta }$$ 001 β alignment from the direction of maximum thermal gradient, caused by the curvature of the fusion boundary, stimulating dendrite twinning during solidification. In addition, it is shown that increasing the WFS leads to a change in melt-pool geometry and a reduction of remelt depth, which promoted dendrite twinning and grain refinement.

Predicting Maximum Effective Temperatures and Thermal Gradients for Steel I-Girder in Canadian Climate Regions

The constant fluctuation of thermal loads on steel members, especially during construction, causes non-uniform distributions of temperatures, resulting in possible constructional and structural defects leading to unfavorable thermally induced responses and potential safety risks. The Canadian Highway Bridge Design Code (CHBDC) provides one thermal gradient variation profile without accounting for the differences in the geometrical parameters of the steel members and the variations in the climate regions of Canada. Therefore, in this study, three-dimensional finite element (FE) thermal simulations were conducted to investigate the maximum effective temperatures and positive vertical thermal gradients for different Canadian climate regions. Parametric studies were performed to conduct the FE thermal analysis using the thermal model validated in ANSYS. The comprehensive study results showed that Canada could be divided into two main zones for vertical thermal gradient calculations, meaning that one stationary thermal gradient profile cannot be applicable to all climate regions of Canada, as recommended by the CHBDC. Based on the FE thermal analysis results, empirical formulas as a function of the significant parameters were proposed to predict the maximum effective temperature and thermal gradient variations of the steel I-girder. The predicted maximum effective temperature and thermal gradient variation values were found to be highly correlated with the FE values with coefficients of determination R2 of approximately 0.97 and 0.98, respectively.

Coupling of 3D thermal with 1D thermohydraulic model for single-phase flows of direct steam generation in parabolic-trough solar collectors

Estimating 3D temperature distribution around the heat collector element (HCE) is an essential matter for the safety and efficiency of parabolic-trough solar collectors (PTC) due to non-uniform heat flux distribution (NUHFD). Particularly in 2nd and 3rd generation PTC systems, such as direct steam generation (DSG), because of the higher thermal stress around the absorber. 3D HCE thermal models linked to a 1D approach of fluid flow appear as the fastest alternative to study this phenomenon. However, there are still challenges such as the thermal coupling between HCE and heat transfer fluid (HTF) for accurately estimating the receiver thermal field. In this work, the coupling is studied in depth for single-phase flows of DSG in PTC, with a fast and realistic 3D HCE − 1D HTF model, which is validated with experimental data of the Direct Steam Solar (DISS) facility for heat transfer and absorber thermal field variables. Firstly, the inability of standard heat transfer coefficient (HTC) correlations to predict absorber thermal field is constated, being deviations greater than −36.1% for absorber cross-sections (CS) maximum thermal gradients. Assuming underestimated and constant thermophysical properties of the absorber, as former models did, underestimations of more than −29.6% are reported. Besides, the heat transfer variables are overestimated by around 14.3–15.9% for temperatures above 600 K. Secondly, a new correction factor (CF) for standard HTC is proposed involving the NUHFD. Considering this CF and the thermal dependence of the absorber thermophysical properties, deviations fall below 7.5% for heat transfer variables, whilst the absorber CS maximum thermal gradients are under 6.0%. This approach also improves the results reported by former models around 4.3–7.3% for heat transfer variables, without underestimating absorber CS maximum thermal gradients. These deviations are between 4.9–19.1% guaranteeing the safety of the absorber when the model is used during the design stage.

Fabrication of high-performance CoCrNi medium entropy alloy by laser powder bed fusion: The effect of grain boundary segregation

A CoCrNi composite material with 3 wt% TiC was successfully fabricated by laser powder bed fusion (LPBF). With increasing content of TiC particles, C gradually segregated along the grain boundaries (GBs). The segregation of C improved the grain coalescence, intensified the attraction of the columnar grains, and increased the consumption of the liquid film. Furthermore, the segregation of C led to the formation of Cr23C6 and core–shell structure (TiO2/TiC) nanoprecipitates along the GBs. These precipitates reduced the mobility of the boundaries and restricted the grain growth along the direction of the maximum thermal gradient. Reinforcing the CoCrNi microstructure with 3 wt% TiC yielded optimal results, as the resultant composite microstructure was free of hot cracks, consisted of small grains, and exhibited a more random texture. The high density of wide stacking faults and Lomer–Cottrell locks hindered cross-slip during further deformation at ambient temperature, and twins dominated at elevated temperatures. These nanoprecipitates hindered dislocation propagation at all temperatures. Therefore, the synergistic strengthening effects from the different types of dislocations and nanoprecipitates endowed the CCN-3M alloy with excellent mechanical properties. Ultimate tensile strengths of approximately 1.34 and 0.88 GPa, yield strengths of approximately 1.02 and 0.75 GPa, and elongations of approximately 15.6% and 25.3% were observed at ambient and elevated temperatures (700 °C), respectively. These results indicate that GB segregation is an effective method for suppressing the evolution of hot cracks, thus expanding the scope of LPBF-fabricated components for industrial applications.

Computational analysis of passive strategies to reduce thermal stresses in vertical tubular solar receivers for safety direct steam generation

When exposed to high non-uniform heat fluxes on the receiving surface, significant temperature gradients develop in the tubes, producing thermal stress and deformations in the receivers. This paper reports a detailed investigation of the thermal and structural performance of a solar tower system that operates with direct steam generation tubes when the fins are added to the internal surface and thickness varies. Six tubes were evaluated, two without fins varying the wall thickness (5 mm and 3 mm) and four with fins under different configurations. The thermal system corresponds to a representative tube of an external tubular receiver of a central tower system. The thermal receiver operates with direct steam generation under a non-uniform concentrated solar flux with a maximum value of 0.93 MW/m2. The best configuration of the tube was with fins on the frontal surface of the tube, maintaining a receiving wall thickness of 3 mm and an adiabatic wall thickness of 5 mm. It could reduce maximum thermal gradients and temperature by 31% and 14%, respectively, maximum thermal stresses by 38%, and maintain vapor mass flow at the output (276 kg/h).

Study of microstructure and compressive strength of a novel Ti-0.75Al-9.5V alloys manufactured by laser deposition melting

A novel Ti-0.75Al-9.5 V alloy is designed based on the typical Ti-6Al-4V alloy and prepared by laser melting deposition in this study. The microstructure, texture, and properties of the different scanning and building planes of the LMDed samples under a serpentine reciprocating scanning strategy are characterized and tested. It is found that the more rapid cooling rate on the building planes leads to a partial morphology transformation of the α phase from the original rod-like shape to a needle-like shape. Furthermore, different maximum thermal stress gradients result in the formation of (110)β texture and (001)β texture on different building planes while The (110)β // (0001)α crystallographic orientation relation leads to the formation of the (21¯1¯0)α texture on both two building planes during solid-phase transformation. The texture evolution on two different building planes also affects corresponding surface slip and mechanical properties. The (001)β texture leads to parallel surface slip tracks on its building plane. The strong (110)β and (21¯1¯0)α textures increase the Vickers hardness from 348.6 ± 2.6 to 371.5 ± 2.7 HV and the compressive yield strength from 1188.0 ± 1.6 to 1276.5 ± 7.3 MPa but suppress the plastic strain from 16.7 ± 0.9% to 12.6 ± 1.0%. All the samples have their compressive fractural strength nearing 1730 MPa, which is higher than that of the reported LMDed Ti-6Al-4V alloy (1690 MPa).

Temperature curling and gradient of roller-compacted concrete composite pavements

Rational design requires a realistic evaluation of the effect of curling stress on the long-term performance of roller-compacted concrete composite pavement (RCCCP). The primary purpose of this study is to quantify the thermal behavior of RCCP, as well as the effects of the aggregate type and the hot mix asphalt (HMA) overlay on it. The research problem concerns thermal gradient and the curling caused by thermal gradient, which can be destructive, particularly with wheel loads and in the corners of the slab. The study also aims to find a method for reducing the amount of thermal gradient and curling in roller-compacted concrete pavement (RCCP). In this regard, the current study evaluated the effect of aggregate type and HMA overlay on the top of RCCP. This study involves field instrumentation of newly constructed RCCP using temperature and deflection sensors. The most significant findings of this research are as follows: 1) The use of silica aggregates in RCCP results in a 10% increase in thermal gradient throughout the depth compared to the application of lime aggregates. 2) HMA overlay on RCCP diminishes the maximum thermal gradient by up to 20%. 3) A second-order power function describes the temperature gradient of RCCP more accurately. 4) It is suggested, based on the thermal gradient trend, that concrete pavements be designed as a two-layer system. 5) The curling of RCCP made with silica aggregate is roughly-two to two-and-a-half times that of RCCP made with lime aggregate. In addition, implementing an HMA overlay can increase temperature curling by 30 to 50 percent.

Thermally induced fracturing in hot dry rock environments - Laboratory studies

In enhanced geothermal system (EGS) wells, where strong thermal gradients exist, hydraulic fracturing is the apparent driver for creating permeability in the reservoirs. Thermal effects, however, have been observed to accompany hydraulic loading and may further enhance the permeability of the reservoir. The mechanism of thermal fracturing in downhole conditions, however, is not well understood, limiting the efficient engineering of thermal effects on EGS stimulation. This laboratory study examines the behavior of thermal shock stimulation and temperature propagation by flowing room-temperature water through a borehole of a hot specimen block without confining stresses or borehole hydraulic loading. This condition isolates the effect of thermal shock from hydraulic pressure and confining stress. We monitored temperatures across the inside of the specimens and assessed fractures by visual inspection, bubble leakage, pressure decay, and acoustics. Thermal loading resulted in an enhancement of permeability by induced macro/microcracks. The profiles of borehole pressure decay obtained before and after each stage of stimulation show increased permeability of the treated specimens. The acoustic measurements indicate the extent of fractures, and the bubble leakage tests on the specimen surface visually indicate localized permeation paths created by newly created cracks. The maximum thermal gradient was achieved near the borehole surface at the start of the water flow. As the flow continued, the magnitude of the thermal gradient near the borehole decreased while that of the thermal gradient away from the borehole increased. Thermally driven fractures, initiated from the borehole walls, propagated perpendicular to the borehole surfaces. These “seed” fractures created during thermal stimulation, although they might be suppressed under very high stress, help reduce the levels of breakdown pressure in concurrent or subsequent pressure-based fracturing methods.

Single-Mode Electromagnetic Resonance Rewarming for the Cryopreservation of Samples with Large Volumes: A Numerical and Experimental Study.

Rapid and uniform rewarming has been proved to be beneficial, and sometimes indispensable for the survival of cryopreserved biomaterials, inhibiting ice-recrystallization-devitrification and thermal stress-induced fracture (especially in large samples). To date, the convective water bath remains the gold standard rewarming method for small samples in the clinical settings, but it failed in the large samples (e.g., cryopreserved tissues and organs) due to damage caused by the slow and nonuniform heating. A single-mode electromagnetic resonance (SMER) system was developed to achieve ultrafast and uniform rewarming for large samples. In this study, we investigated the heating effects of the SMER system and compared the heating performance with water bath and air warming. A numerical model was established to further analyze the temperature change and distribution at different time points during the rewarming process. Overall, the SMER system achieved rapid heating at 331.63 ± 8.59°C min-1 while limiting the maximum thermal gradient to <9°C min-1, significantly better than the other two warming methods. The experimental results were highly consistent, indicating SMER is a promising rewarming technology for the successful cryopreservation of large biosamples.

Curie-Point Depths, Geothermal Gradients and Sub-Surface Heat Flow Estimation from Spectral Analysis of High-Resolution Aeromagnetic Data over Gongola Basin and Its Environs, Northeastern Nigeria

Geothermal energy resources have been established globally to be among the sustainable and environmentally harmless means of energy generation. Curie-point depths (CPD), temperature gradients, and heat flow data over the study area were computed using a spectral analysis method in order to have a preliminary view of the geothermal implications (prospect) of the current area. Aeromagnetic data acquired by the Nigerian Geological Survey Agency (NGSA) in the year 2009 was used for the research. The results showed the minimum, maximum, and the average CPD values as 8.18 km, 31.48 km, and 13.0 km, respectively. The minimum, maximum and average thermal gradients obtained were 18.42 °C/km, 70.91 °C/km, and 50.2 °C/km, respectively. The heat flow data obtained ranged from 33.15 mW/m2 to 177.28 mW/m2, with an average value of 110.65 mW/m2. Locations depicting shallow CPDs anomalies (Alkaleri, Darazo, Dukku, Misau, Wuyo, Deba, and Tula), also showed conformity with high heat flow areas. As such, they are regarded as areas of promising geothermal prospects and are recommended for further detailed investigation. Locations depicting a high magnetic susceptibility contrast from a generated analytic signal map, as well as high temperature gradients, high heat flow, and shallow CPDs are attributed to crustal thinning along the sedimentary basin and magmatic intrusions along basement areas, respectively. The magnetic depth to the basement calculated for the study area using the source parameter imaging (SPI) method ranges from 0.610 km to 3.055 km. The present study has provided an insight on preliminary information, regarding new areas of possible geothermal prospects for further detailed investigation.

Technical note: Turbulence measurements from a light autonomous underwater vehicle

Abstract. A self-contained turbulence instrument from Rockland Scientific was installed on a light autonomous underwater vehicle (AUV) from OceanScan Marine Systems and Technology Lda. We report on the data quality and discuss limitations of dissipation estimated from two shear probes during a deployment in the Barents Sea in February 2021. The AUV mission lasted for 5 h, operating at a typical horizontal speed of 1.1 m s−1. The AUV was programmed to find and cross the maximum along-path thermal gradient at 10, 20 and 30 m depths along 4 km transects. Although the AUV vibrations contaminate the shear probe records, the noise is mitigated by removing vibration-induced components from shear spectra using the accelerometer signal measured in multiple directions. Dissipation rate estimates in the observed transects varied in the range 1×10-8 and 6×10-6 W kg−1, with the values from the two orthogonal probes typically in agreement to within a factor of 2. Dissipation estimates from the AUV show good agreement with nearby vertical microstructure profiles obtained from the ship during the transects, indicating that the turbulence measurements from the AUV are reliable for this relatively turbulent environment. However, the lowest reliable dissipation rates are limited to 5×10-8 W kg−1, making this setup unfit for use in quiescent environments.

Numerical Simulation of Heat Removal from a Window Slab Partition of a Radiative Coil Coating Oven

In this work, fluid flow and heat transfer performance of a radiative coil coating oven is numerically investigated. In the coil coating oven concept under consideration, porous radiant burners provide the required energy to evaporate the volatile species (solvents) from the applied coating and to promote curing reactions. To avoid the mixing between burners flue gas (with a non-negligible oxygen content) and evaporated (combustible) solvents in the oven (which could lead to a catastrophic oven failure), a semi-transparent window in between both atmospheres is applied. To ensure the window thermal stability during the oven operation, window cooling by wall jets is considered. Different turbulence models were compared against available wall jet heat transfer correlations to select the most suitable for three-dimensional (3D) numerical simulations. Convective heat transfer correlations purposefully developed were embedded in a one-dimensional (1D) window energy model for fast performance characterization, analysing the most influencing parameters—window radiative properties, thickness, inlet temperature and velocity of wall jets, and cooling strategy. The 1D window thermal performance is compared with literature and 3D results considering the full coil coating oven, providing satisfactory confidence on the developed strategy. The 1D model is used for an optimisation study to find the minimum energy consumption while ensuring the safety requirements (maximum window temperature and thermal gradient) are met.

Microstructural evolution and mechanical characterization of wire arc additively manufactured 2Cr13 thin-wall part

Abstract 2Cr13 thin-wall part was additively deposited by robotic cold metal transfer (CMT) technology, and two traditionally manufactured counterparts in annealing (as-Aed) and quenching & tempering (as-QTed) conditions were adopted as comparison. The results show that only a strong texturing corresponding to α-Fe phase was detected for the wire-arc additively manufactured (WAAM) part, indicating an austenite-free structure. As-deposited microstructure was consisted of martensite laths and ferrite matrix, along with irregular δ-ferrite precipitation. The martensitic growth direction was non-oriented in the X–Y plane, but primarily parallel to the depositing direction in the X-Z and Y-Z planes along the maximum thermal gradient. Both nanohardness and ultimate tensile strength (UTS) for each WAAM sample were enhanced when compared with the as-Aed BM, while poorer than that of the as-QTed BM. Such mechanical evolution was a result of the intrinsic microstructural features, whereas the similar elastic modulus properties were mainly attributed to the similar 2Cr13 atomic bonding. A ductile tensile fracture behavior was dominant in the X-Z plane, while a mixed mode of ductile and brittle fracture occurred in the X–Y and Y-Z planes. The findings above reveal an isotropy in mechanical properties despite a slightly microstructural discrepancy in different planes for the as-deposited 2Cr13 WAAM part.

Generation of stable thermal gradient by solar energy harvesting in porous cobalt oxide based nanofluid

Tailored nanoforms of cobalt oxide were synthesized to prepare water based nanofluid system for developing stable and significantly large thermal gradient within nanofluid column. The decisive impact of morphological growth on both optical and thermal properties of the dispersed nanomaterial has been elaborately studied. Morphological evolution from rod to flower induces significant enhancement in optical absorption within VIS-NIR region whereas, the thermal conducting property of flower nanoform is found inferior to rod. The morphological growth implements 48% enhancement in thermal gradient for ultralow concentration around 0.005 wt% at sunlit condition. The highly porous flower nanoform exhibiting an enhanced optical absorption and improved dispersion stability but inferior thermal conducting property leads to the formation of thermal gradient ~43 °C stable for >3 h at a filler fraction of 0.011 wt% in sunlit condition (800 W/m2). The optimum concentrations for maximum thermal gradient for each system have been investigated. Due to the significant heat localization, proposed nanofluid system is promising in field of solar driven evaporation. Employing the stable and significantly large thermal gradient within the nanofluid column the possibility of thermo-emf generation with known thermoelectric materials has also been proposed.

Monocrystalline fiber growth technique: New critical radius considerations

Laser-heated pedestal growth (LHPG) is an advantageous technique for crystalline fibers pulling due to its fast growth rate, related to the high axial thermal gradient. However, internal cracks can occur if the radius of the fiber is greater than the so-called thermal critical radius. For this reason, it is important to accurately determine the maximum axial thermal gradient to obtain crystalline fibers free of cracks. Herein, we improved the current method for critical radius determination by considering that the maximum temperature gradient is not present in the liquid/solid interface, but occurs directly on fiber, at a certain distance Δ from the liquid/fiber interface. In order to evaluate such improvement, cracks-free fibers of SrTiO3 (Melting Point = 2353 K) and Al2O3 (2345 K) were grown with this new criterion.

Dynamical evolution and thermal history of asteroids (3200) Phaethon and (155140) 2005 UD

The near-Earth objects (NEOs) (3200) Phaethon and (155140) 2005 UD are thought to share a common origin, with the former exhibiting dust activity at perihelion that is thought to directly supply the Geminid meteor stream. Both of these objects currently have very small perihelion distances (0.140au and 0.163au for Phaethon and 2005 UD, respectively), which results in them having perihelion temperatures around 1000K. A comparison between NEO population models to discovery statistics suggests that low-perihelion objects are destroyed over time by a, possibly temperature-dependent, mechanism that is efficient at heliocentric distances less than 0.3au. By implication, the current activity from Phaethon is linked to the destruction mechanism of NEOs close to the Sun.We model the past thermal characteristics of Phaethon and 2005 UD using a combination of a thermophysical model (TPM) and orbital integrations of each object. Temperature characteristics such as maximum daily temperature, maximum thermal gradient, and temperature at different depths are extracted from the model, which is run for a predefined set of semi-major axis and eccentricity values. Next, dynamical integrations of orbital clones of Phaethon and 2005 UD are used to estimate the past orbital elements of each object. These dynamical results are then combined with the temperature characteristics to model the past evolution of thermal characteristics such as maximum (and minimum) surface temperature and thermal gradient. The orbital histories of Phaethon and 2005 UD are characterized by cyclic changes in e, resulting in perihelia values periodically shifting between present-day values and 0.3au. Currently, Phaethon is experiencing relatively large degrees of heating when compared to the recent 20,000yr. We find that the subsurface temperatures are too large over this timescale for water ice to be stable, unless actively supplied somehow. The near-surface thermal gradients strongly suggest that thermal fracturing may be very effective at breaking down and ejecting dust particles. Observations by the DESTINY+ flyby mission will provide important constraints on the mechanics of dust-loss from Phaethon and, potentially, reveal signs of activity from 2005 UD.In addition to simulating the recent dynamical evolution of these objects, we use orbital integrations that start from the Main Belt to assess their early dynamical evolution (origin and delivery mechanism). We find that dwarf planet (2) Pallas is unlikely to be the parent body for Phaethon and 2005 UD, and it is more likely that the source is in the inner part of the asteroid belt in the families of, e.g., (329) Svea or (142) Polana.

Investigation of the Film-Cooling Performance of 2.5D Braided Ceramic Matrix Composite Plates with Preformed Hole

The film-cooling performance of a 2.5D braided ceramic matrix composite (CMC) plate with preformed holes was numerically studied. Four numerical models containing braided structures were established: one model with film-cooling holes preformed through fiber extrusion deformation (EP-Hole), one model with film-cooling holes directly woven through fibers (WP-Hole), and two models with directly drilled holes (DP-Hole1,2). Besides, the influence of the ratio between the equivalent thermal conductivities on the axial and radial directions of fiber Kr was investigated. The results show that the preformed holes have better performance in controlling the thermal gradient with the increase of Kr. The maximum thermal gradient around the DP-Hole is significantly higher than that of the WP-Hole and EP-Hole, and the maximum relative variation reaches 123.3%. With Kr increasing from 3.32 to 13.05, the overall cooling effectiveness on the hot-side wall decreases for all models, by about 10%. Compared with the traditional drill method, the new preformed film-cooling hole studied in this paper can reduce the temperature and the thermal gradient in the region around the holes.

Indoor Room Air Conditioning Using External Enclosure

This paper presents a new air conditioning system using an external air chamber enclosure. The system is based on evaporative refrigeration technique humidifying hot air from the environment by means of a new design humidifier. Temperature at the air chamber is set up according to the required comfort conditions. The air chamber plays the role of a heat collector from indoor room radiative energy. This new system avoids thermal gradient inside the room improving the comfort because of a more homogeneous temperature distribution. Maximum thermal amplitude at the air chamber and indoor room is of 2°C. Theoretical analysis has been developed to determine the refrigeration energy consumption matching values with literature data. Experimental tests have shown a good correlation with theoretical prediction, within accuracy of 7.8%, close to the 8.2 % uncertainty of experimental measurements. A significant reduction in energy consumption has been achieved with COP values up to 4.8 for 31.5°C of ambient temperature. COP increases linearly with ambient temperature and enhances efficiency of air conditioning compression units above 31.5°C, although shows poorer performance below 29°C, what makes the new system especially suitable for hot climates. The result of the evaporative refrigeration at the air chamber has permitted the reduction of daily thermal amplitude from 17°C at the outside, to only 2°C, at the inside. COP of the new system moves from 1.1 to 4.8 within ambient temperature limits, 25.5°C to 31.5°C. Maximum amplitude of temperature at the indoor room has been of 2°C, with a maximum thermal gradient that has not exceeded 0.5°C/m.

Rapid Solidification Microstructure and Carbide Precipitation Behavior in Electron Beam Melted High-Speed Steel

The solidified microstructure and carbide precipitation behavior in an S390 high-speed steel processed by electron beam melting (EBM) have been fully characterized. The as-EBM microstructure consists of discontinuous network of very fine primary carbides dispersed in auto-tempered martensite matrix together with a limited amount of retained austenite. The carbide network consists of M2C/M6C and MC carbides. Both the columnar and near-equiaxed grain structures were found in as-EBM microstructure and the presence of inter-dendritic eutectic carbides assisted in revealing the dendritic solidification nature. The top-layer microstructure observation confirmed that the columnar dendritic structured grains were located adjacent to the micro-melt pool boundary, indicating an epitaxial growth with the average growth direction parallel to the maximum thermal gradient. At the center of the micro-melt pool, the near-equiaxed grains were developed by dendritic growth parallel to the beam traveling direction. The carbide decomposition was revealed by scanning transmission electron microscopy and confirmed by transmission Kikuchi diffraction. The MC carbides (rich in V followed by W) nucleated at the interface between M2C (W, Fe, Mo, and Co in the order of significance) and the matrix and then grew from the outside inward, but their nucleation might occur from the M2C carbide itself. The thermal effect induced by the adjacent scan lines seems to trigger a solid-state phase transformation of MC → M2C + γ-Fe. The elemental migration was theoretically calculated and compared with the experimental results. The high hardness of ~ 65 HRC and good transverse rupture strength of ~ 2500 MPa in as-EBM S390 means that EBM processing can be used to fabricate highly alloyed tool steels. With the help of the post-processing heat treatment, the best Rockwell hardness of 73.1±0.2 HRC and transverse rupture strength of 3012±34 MPa can be obtained.

Effect of Local Heating on the Mechanical Characteristics of Repaired Automotive Panels

Flame straightening is a technology process used to eliminate deformations. This method relies on local heating of the material to correct geometry or damaged parts. In the local automobile services its main use is for repairs of less critical deformed components. The maximum temperature and thermal gradient, heating time, cooling rate and number of heating cycles affect the mechanical properties since local heating can alter material microstructure. The aim of this research was to determine the mechanical characteristics of thin steel plates repaired by local heating associated with plastic deformation (similar to hot working) and cold straightening (similar to local cold working) for automotive side and door panels made of structural steel. Thin sheet plates, 0.9mm thickness, were deformed by impact and repaired by local heating using the flame and induction heating then plastically deformed while hot as well as straightened without heating. The heat repaired samples were studied by light microscopy to determine microstructure change and samples were tensile tested to determine their mechanical characteristics. Local excessive grain growth generates anisotropy, the assembly behaves as a composite material with regions that show significant plastic deformations while others little or no deformations at al. Without procedures adjusted to each material repairs involving heating are to be avoided, cold working should be employed when replacement is not possible.