Deep Dimples Research Articles

The Evaluation of Quenching Temperature Effect on Microstructural and Mechanical Properties of Advanced High Strength Low Carbon Steel After Quenching Partitioning Treatment

The influence of quenching temperature on microstructural and mechanical properties of low alloy steel of the following chemical composition: 0.26 C, 1.70 Mn, 1.42 Si, 1.10 Cr, 1.10 Ni, 0.94 Cu, 0.24 Mo, 0.1 V, Bal. Fe (Wt.%) was investigated after applying a quenching-partitioning (Q-P) treatment. The steel samples were isothermally quenched at 260, 280, and 300 °C, from the austenitizing temperature and then Q-P treated at 340 °C. After the Q-P treatment, the steel showed a multiphase microstructure containing bainite, martensite, and retained austenite. It was determined that the tensile strength and Charpy impact energy increased with a decrease in quenching temperature to 1415 MPa and 43 J, respectively. This effect was attributed to an increase in the volume fraction of austenite/martensite micro blocks that introduces a hard phase mixture strengthening factor and the presence of tempered martensite, which is strengthened by fine particle dispersion and moreover, a decrease in thickness of the bainitic-ferrite subunits that refine the microstructure. The fractographic examination of the Charpy tested specimens showed that the sample quenched at 260 °C contained finer and deeper dimples, which indicates that more energy was spent on the nucleation and growth of ductile fracture microvoids, thus increasing the toughness.

Experimental Investigation of Pressure Drop Performance of Smooth and Dimpled Single Plate-Fin Heat Exchangers

Passive heat exchangers (HXs) form an inseparable part of the manufacturing industry as they provide high-efficiency cooling at minimal overhead costs. Along with the aspects of high thermal cooling, it is essential to monitor pressure loss while using plate-fin HXs because pressure loss can introduce additional power costs to a system. In this paper, an experimental study was conducted to look at the effects of dimples on the pressure drop characteristics of single plate-fin heat exchangers. To enable this, different configurations of National Advisory Committee for Aeronautics (NACA) fins with smooth surfaces and 2 mm-diameter dimples, 4 mm-diameter dimples and 6 mm-diameter dimples were designed and 3D printed using fused deposition modelling (FDM) of ABS plastic. The depth to diameter ratio for these dimples was kept constant at 0.3 with varied diameters and depths. These were then tested using a subsonic wind tunnel comprised of inlet and outlet pressure taps as well as a hot wire velocimeter. Measurements were taken for pressure differences as well as average velocity. These were then used to calculate friction factor values and to compare the smooth fin to the dimpled fins in relation to their relative pressure drop performance. It was observed that for lower velocities the 4 mm dimples provided minimum pressure drop, with a difference of 58% when compared to smooth fins. At higher velocities, 6 mm dimples increased the pressure drop by approximately 34% when compared to smooth fins. It can also be concluded from the observed data in this study that shallower dimples produce lower pressure drops compared to deeper dimples when the depth to diameter ratio is kept constant. Accordingly, deeper dimples are more effective in providing drag reduction at lower velocities, whereas shallower dimples are more effective for drag reduction at higher velocities.

Numerical Simulation of a Periodic Quasi-Switching Mode of Flow around a Conical Dimple with a Slope Angle of 10 Degrees on the Wall of a Narrow Channel Using URANS

The applicability of the solution of the unsteady Reynolds-averaged Navier–Stokes equations (URANS) for the numerical simulation of the periodic quasi-switching regime of vortex generation and heat transfer in a deep conical dimple with a slope angle of 10∘ on the wall of a narrow channel is substantiated. To calculate the turbulent regime, the model of shear stress transfer by Menter 2003, modified taking into account the influence of the curvature of streamlines within the framework of the Rodi-Leshziner-Isaev approach, is used. At Reynolds number Re=104, the oscillation period of the transverse Rz and longitudinal forces Rx, as well as the total heat transfer Numm to the control section of the heated channel wall with a dimple, is set equal to 60, which corresponds to the Strouhal number St=0.0167. Computer visualization of swirling jet-vortex flows demonstrates focus-type sources on the side faces of the dimple. In the self-oscillating mode, a two-cell vortex system is formed with different intensities at the oscillation period Rz. Periodic changes in friction, Nusselt numbers and temperature are recorded in the longitudinal and transverse median sections of the dimple and reflect the oscillations of the vortex structure from left to right and from right to left. The formation of a fan jet is shown, which oscillates relative to the plane of longitudinal symmetry, causing a redistribution of power and thermal loads.

Effect of Isothermal Annealing on the Microstructure and Impact Properties of TC10 Titanium Alloy

The microstructure and impact properties of TC10 titanium alloy bar after isothermal annealing are studied by metallographic microscope, SEM and impact properties test. The results show that there were two forms of a phases in the original microstructure of TC10 titanium alloy forging bar, one was primary equiathetic a phases, the other was secondary a phases. After the alloy was annealed with isothermal temperature, the content of the equiaxed a phases in the metallographic structure decreased with the increased of temperature, and disappeared after reaching the transformation point. While the number of platelet a in the structure increased with the increased of temperature, and the size increased. The impact toughness of the alloy shows a trend that first increases and then decreases with the increase of heating temperature. When the temperature exceeded the transformation point, the impact toughness decreases significantly. With the increase of heating temperature, the fracture morphology of the alloy mainly changes from a large number of relatively deep dimples to a few relatively shallow dimples, and dissociation steps appear. After the whole investigation, it can be concluded that the best heat treatment system in this experiment was 920°C×1.5h/FC→800°C×1.5h/AC+560°C×4h/AC, and the maximum impact toughness was 49.5 J/cm2.

Effect of FSW process parameters on mechanical properties and microstructure of dissimilar welded joints of AA2024 and AA6082

The aluminum alloys AA2024 and AA6082 are the most common alloys used in the storage tank, pipelines, maritime frames, automobile and aerospace industries due to their high corrosion resistance and toughness.In the present work, these dissimilar aluminum alloys were successfully fabricated via friction stir welding (FSW) by varying input parameters (rotational tool speed (TRS) and welding speed). The great mixing of AA2024 and AA6082 was observed at a high rotational speed. The higher TRS enhanced the mechanical properties of the stir zone (SZ). The maximum ultimate tensile strength (UTS) of dissimilar alloys was observed 218.1 MPa at TRS of 1400 rpm with a welding speed of 30 mm/min caused by strain-free fine grains during the dynamic recrystallization (DRX) mechanism. The maximum hardness (93 HV) at SZ was observed at a TRS of 1200 rpm with a 30 mm/min welding speed. The fractured FSWed region at 1200 rpm shows the big and deep dimples, while equiaxed, fine dimples were observed at TRS of 1400 rpm.

Correlation Between Microstructure and Fracture Behavior in Thick HARDOX 450 Wear-Resistant Steel With TiN Inclusions

This work investigates the correlation between TiN inclusions and microstructural properties of HARDOX 450 steel using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscattered diffractometer (EBSD) methods. Some amount of microsized TiN inclusions were formed in the temperature range of the solid–liquid zone; however, they exhibited fracture features of deep dimples rather than a cleavage plane, which is closely related to the ability of the microstructure to arrest cracks. Upon tensile loading, a single microcrack first appeared inside the microsized TiN inclusions, and then, multiple microcracks formed, parts of which widened in the direction of tensile stress. A schematic mechanism map was plotted to reveal the propagation behavior and fracture mechanism of the microcracks in the TiN inclusions.

Study on Fatigue Crack Propagation and Fracture Characterization of 7050-T7451 Friction Stir Welded Joints

The fatigue crack propagation (FCP) behavior of FSW welded 7050-T7451 joints was studied using compact tensile specimens. Based on Paris law and three-stage theory of FCP, the FCP rate of joint was obtained by data fitting and calculation. The fracture morphologies at different stages of FCP were characterized by metallography and scanning electron microscopy (SEM). Results showed that the FCP rate in each region for FSW joint was as follows: heat affected zone (HAZ) > weld nugget zone (WNZ) > base metal (BM). In the first stage of FCP, the fracture morphologies of BM and HAZ were mainly tearing ridges and river pattern. There were a small number of dimples on the fracture of BM, but no dimples on the fracture of HAZ. In the second stage of FCP, the fatigue striations of the BM were denser and finer than those in the HAZ, and have dimple fracture characteristics. In the third stage of FCP, dimples still existed in the fracture surface of the BM, and the fatigue striations disappeared. The HAZ was dominated by lamellar tearing and cleavage fracture without dimples. During the three stages of FCP, the fracture morphology of the WNZ in the first and second stages was fine dimple fracture with significant equiaxed deep dimple morphology. In the third stage, the fracture dimple in the WNZ changed from equiaxed deep dimples to tearing dimples.

Microstructure and Mechanical Characteristics of Dissimilar TIG Welded 9% Cr Heat-Resistant Steels Joints

Dissimilar 9% Cr heat-resistant steels (G115 and CB2) with good creep properties for ultra-supercritical steam turbines were butt-joined by tungsten inert gas welding. The microstructure of welded metal (WM) was quenched martensite without carbide precipitates and lath packets existed inside prior austenite grains (PAGs), which leaded to higher hardness of WM. Partially melted zone at G115 side was composed of untempered martensite within equiaxed PAGs. The lowest hardness occurred in both G115 and CB2 steels which was attributed to tempered martensite with many M23C6 precipitates. The heat-affected zone consisted of three sub-grains and their microstructure was detailly analyzed in current work. As current increased from 130 to 150 A, both the tensile strength at room temperature and 650 ℃ increased while strength had no obvious change with further increasing current. The values of 673 MPa and 309 MPa corresponded to the tensile stress with 150 A at room temperature and 650 ℃, respectively. The fracture mode of joints at room temperature was cleavage and ductile failure at 130 and 150 A, respectively. The high-temperature fracture surface at 150 A was composed of deep and fine dimples.

Effect of grain size refinement on microstructure and mechanical properties of AZ31 magnesium alloy

In this paper, microstructure and mechanical properties of ultra-grain size and coarse-grain size bulk AZ31 magnesium alloy were evaluated using tensile testing. The ultra-grain size (200nm) bulk AZ31 magnesium alloy showed well mechanical properties with stronger tensile strength (315MPa), typical ductile fracture surface and deep dimples, which can be attributed to grain size refinement of bulk AZ31 magnesium alloy.

The Weld Microstructure and Mechanical Properties of the Alloy 52 and Its Variants with Applied Electromagnetic Stirring during Welding

This study investigated the impact of electromagnetic stirring (EMS) on nickel-base alloy welds prepared with the gas tungsten arc welding process. Alloy 52 and its variants, Alloy 52M and Alloy 52MSS, were carefully evaluated with their weld microstructure and mechanical properties. The results showed that the welds exhibited a typical microstructure of dendrites, and that the dendrites could be refined by electromagnetic stirring. Meanwhile, with an application of EMS, the precipitates became smaller and more evenly distributed in the inter-dendritic areas. Ti(N,C)s, Nb/(Nb,Si)Cs, and large-scale Laves phase with (Nb,Mo,Ti)Cs were the precipitates present in the Alloy 52, Alloy 52M, and Alloy 52MSS welds, respectively. With the refined microstructure, both Alloy 52 and Alloy 52M welds were observed to have an increase in their tensile strength, with a decrease in their elongations. Comparatively, for the Alloy 52MSS weld, the tensile strength was enhanced along with a slight increase in elongation. Deep and dense dimples were a dominant feature of low-Nb-additions welds, and dendrite-like features were found prevalent among the Alloy 52MSS welds. With EMS, the dimples of Alloy 52 welds and the dendrite-like features of Alloy 52MSS welds became finer, while the dimples of Alloy 52M welds grew coarser.

Hot ductility behavior of a Fe-0.3C-9Mn-2Al medium Mn steel

The hot ductility of a Fe-0.3C-9Mn-2Al medium Mn steel was investigated using a Gleeble3800 thermo-mechanical simulator. Hot tensile tests were conducted at different temperatures (600–1300°C) under a constant strain rate of 4 × 10−3 s−1. The fracture behavior and mechanism of hot ductility evolution were discussed. Results showed that the hot ductility decreased as the temperature was decreased from 1000°C. The reduction of area (RA) decreased rapidly in the specimens tested below 700°C, whereas that in the specimen tested at 650°C was lower than 65%. Mixed brittle-ductile fracture feature is reflected by the coexistence of cleavage step, intergranular facet, and dimple at the surface. The fracture belonged to ductile failure in the specimens tested between 720–1000°C. Large and deep dimples could delay crack propagation. The change in average width of the dimples was in positive proportion with the change in RA. The wide austenite-ferrite inter-critical temperature range was crucial for the hot ductility of medium Mn steel. The formation of ferrite film on austenite grain boundaries led to strain concentration. Yield point elongation occurred at the austenite-ferrite intercritical temperature range during the hot tensile test.

Dynamic strain aging behavior of accident tolerance fuel cladding FeCrAl-based alloy for advanced nuclear energy

The Fe-13Cr-4Al alloy has become a promising candidate material for accident tolerance fuel (ATF) cladding of light-water reactors (LWRs) due to its excellent oxidation resistance to high-temperature water vapor. However, the tensile deformation behavior of the Fe-13Cr-4Al alloy under different strain rates at different temperatures is still unclear. In the present study, the tensile behavior and deformed microstructure of the Fe-13Cr-4Al alloy were investigated at strain rates from 5 × 10–4 to 1 × 10–2 s−1 in the temperature range from RT to 600 °C. Serrations were observed in the tensile engineering stress–strain curves of the intermediate temperature ranging from 300 to 450 °C at all the three strain rates, indicating the occurrence of dynamic strain aging (DSA). As the strain rate increased, the temperature range where serrated plastic flow occurred shifted to the high temperature regime. Serrated plastic flow occurred after a certain critical plastic strain, and the critical plastic strain decreased with the increase in temperature and increased with the increase in strain rate. In the DSA regime of the Fe-13Cr-4Al alloy, the plateau in the yield strength, ductility minima, negative strain rate sensitivity, the peaks of strain hardening exponent and work hardening rate were observed, which were the typical manifestations of the DSA. The activation energies of serrated plastic flow evaluated by three different methods were 157 ± 35, 92 ± 10 and 93 ± 10 kJ/mol, respectively. According to the values of activation energy, the controlling mechanism responsible for the DSA of the Fe-13Cr-4Al alloy was found to be the interaction between substitutional aluminum atoms and dislocations. The fracture surfaces of the specimens tested at 400 °C under three strain rates showed the mixed fracture mode, which contained dimples and cleavage facets. The tensile samples at 600 °C showed a completely ductile fracture mode with large and deep dimples. A large number of dislocation tangles in the microstructure of the specimens tested at 400 °C under all the strain rates were observed, with a large amount of Fe2Nb Laves particles along the grain boundaries and in the matrix. Dislocation pile-up was also observed around the grain boundaries and second-phase particles. With the increase in the strain rate, the number of subgrains increased, and a clear dislocation cell structure was also observed.

Fabrication of Al/Cu Composite Reinforced with AISI 304 Stainless Steel Wires Through Accumulative Roll Bonding and Evaluation of Their Mechanical Properties

In the present study, a multilayer Al/Cu composite reinforced with AISI 304 stainless steel wires was fabricated through an accumulative roll bonding process. The microstructure and bonding integrity of different layers were investigated using optical and scanning electron microscopy. Mechanical properties of the produced composite samples were evaluated by tensile testing and micro-hardness measurements. Results showed that after six cycles of ARB processing, a multilayer Al/Cu composite with uniform distribution of Cu and AISI304 wires in an Al matrix is produced. Also, the results of tensile testing indicate that both the strength and elongation of the fabricated composite decrease with increasing the ARB cycle. Scanning electron microscopy analysis of the fractured surfaces of tensile specimens showed a combination of ductile rupture with deep dimple forming and cleavage fracture at different composite layers.

Impacts of Warm Equal‐Channel Angular Pressing on Microstructure and Mechanical Properties of Granular Pearlitic Steel

Equal‐channel angular pressing (ECAP) of granular pearlite high‐carbon steel at 650 °C via the Bc route is thoroughly investigated. Together with microstructural evolution investigated through scanning and transmission electron microscopies, microtensile and microhardness testing are conducted for their mechanical properties. After four passes of warm deformation, the formed ultra‐microduplex structure is found to contain both ferrite grains of size ≈0.45 μm and cementite particles with the diameter of ≈0.3 μm. The corresponding microhardness and tensile strength are observed to increase first, followed by a decrement with the number of deformations passes. Meanwhile, yield strength and yield ratio increase with the ECAP passes, along with a slight decrease in elongation. The fracture morphology also changes from many deep dimples before ECAP to many small dimples after ECAP application, denoting a typical ductile fracture of the pearlitic steel.

High-temperature tensile characteristics and constitutive models of ultrahigh strength steel

Abstract In this investigation, the isothermal tensile experiments over wide ranges of deformation parameters (strain rate and tensile temperature) are conducted for studying the high-temperature tensile behaviors of an ultrahigh strength steel. The influences of deformation parameter on high-temperature tensile behaviors, fracture characteristics and deformation mechanisms are analyzed. Moreover, Arrhenius-type phenomenological (AP) model developed by the regression method or the Nelder-Mead (NM) simplex method, and the artificial-neural-network (ANN) model developed by combining genetic algorithm (GA) and back propagation learning algorithm (BP) are proposed, respectively. The results show that the high-temperature tensile behavior of the studied steel exhibits the typical work hardening and dynamic recovery characteristics. The necking capability increases with the strain rate decreasing and tensile temperature increasing. However, the large deep dimples dramatically deteriorate the loading capability during the localized necking, leading to the poor elongation to fracture at low strain rate. Both for the modeling and verifying data, the AP model developed by the NM simplex method shows the relatively high relative coefficient (higher than 0.9963), low average absolute relative error (lower than 1.6692%) and narrow error band (controlled in ±6.8MPa), compared with the AP model developed by the regression method and the GA-BP ANN model.

Multiphase microstructure formation and its effect on fracture behavior of medium carbon high silicon high strength steel

This work investigated the evolution of multiphase microstructure and impact fracture behavior of medium carbon high silicon high strength steel subjected to the austempering treatment at 240, 360, and 400 ℃. The results show that martensite, bainite, and retained austenite (RA) are the main microstructural phases. The austempering treatments at 360 and 400 ℃ caused the formation of carbon-poor ferrite in the matrix, and the transformation of ultrafine bainite into coarse lath bainite and granular bainite, respectively. Thick filmy RA was distributed between bainite laths. The polygonal martensite-austenite islands and blocky RA formed along the grain boundaries. The average carbon concentration in the matrix decreased with the temperature increase, while the impact toughness initially increased and then dropped with temperature. The quasi-cleavage brittle fracture dominated the impact fracture mechanism of the sample austempered at 240 ℃ by forming tearing surfaces and tearing steps. The microcracks disappeared in the RA on the prior austenite grain boundaries. On the other side, the fracture surface of the sample austempered at 360 ℃ exhibited ductile fracture with deep dimples and brittle fracture with cleavage river patterns. The polygonal martensite-austenite islands or blocky RA constrained the microcracks. After austempered at 400 ℃, the brittle fracture was dominant, showing river patterns, and the microcracks propagated through the granular bainite without any resistance.

Nasal or Temporal Internal Limiting Membrane Flap Assisted by Sub-Perfluorocarbon Viscoelastic Injection for Macular Hole Repair

To compare the outcomes between using a nasal and a temporal inverted internal limiting membrane (ILM) flap both assisted by a novel technique in repairing a full-thickness macular hole (FTMH). Retrospective interventional case series. Thirty-nine eyes from 39 patients with a FTMH <600μm were included from a single institution. All patients underwent vitrectomy using a semicircular single-layered ILM inverted flap assisted by a sub-perfluorocarbon liquid injection of ophthalmic viscoelastic device (OVD) technique. Best-corrected visual acuity (BCVA) and spectral domain optical coherence tomography were used to compare outcomes between nasal (n = 19) and temporal (n = 20) groups. At 6months postoperatively, all FTMHs closed and BCVA were significantly improved. Overall, 36 eyes (92%) achieved U-shaped closure, and ellipsoid zone restoration was noted in 24 eyes (62%). An ILM flap was present in 29 eyes (74%) and 86% remained single-layered. There were significantly more deep inner retinal dimples in the temporal group (35%) compared with 5% in the nasal group (P= .04), but these were unrelated to BCVA. Significant retinal thinning in the temporal outer sub-field was noted in the temporal group and was negatively correlated with BCVA (rho [ρ]:- .53; P= .03). No significant postoperative retinal displacement was noted in either group. The technique of using sub-perfluorocarbon liquid injection of OVD secured single-layered flaps intraoperatively and postoperatively. Both the nasal and temporal inverted ILM flaps repaired FTMH and improved visual acuity. However, both temporal macular thinning and deep inner retinal dimples were significantly greater in the temporal group.

EFFECT OF TIO2 PARTICLES ON THE MECHANICAL AND MICROSTRUCTURAL EVOLUTION OF HYBRID ALUMINUM-BASED COMPOSITES FABRICATED BY WARB

In this study, the effect of powder particle contents on the mechanical properties of aluminum-based hybrid composites produced by warm accumulative roll bonding (WARB) process was investigated. Three hybrid composite groups with fully annealed AA1060 strips, 1[Formula: see text]wt.% of MnO2 and 0, 7.5 and 15[Formula: see text]wt.% of TiO2 were first roll-bonded and then the ARB process continued up to eight rolling cycles. In the WARB process, the samples were preheated at 300∘C for 5[Formula: see text]min before each rolling cycle. It was realized that increasing the number of ARB cycles with 0 and 7.5[Formula: see text]wt.% of TiO2 particles improved the uniformity of TiO2 particles distribution in the aluminum matrix. Also, in samples with high contents of TiO2 particles (15%), TiO2 clusters converted into small clusters with a better distribution through the aluminum matrix. Also, by increasing the number of cycles and TiO2 wt.% up to 15%, the tensile strength, elongation, tensile toughness and average Vickers microhardness of hybrid composites were increased first and decreased then. Moreover, the effects of ARB cycles and TiO2[Formula: see text]wt.% on the fracture surface of hybrid composites have been studied by scanning electron microscopy (SEM). It was found that all the samples fabricated with one and two ARB cycles exhibited a typical ductile fracture containing deep dimples, whereas by increasing the TiO2[Formula: see text]wt.% and ARB cycles, a combination of ductile and shear ductile fracture have been detected in the fracture behavior.

On the High-Temperature Flow Response of Friction Stir Processed Magnesium Metal Matrix Composites

AbstractIn the current work, multi-pass friction stir processing (FSP) was utilized to fabricate samples of fine-grained aluminum–zinc (AZ) magnesium alloy and its metal matrix composite (MMC). The microstructure and high-temperature tensile behavior of friction stir processed (FSPed) AZ31 and AZ31/SiC MMC at various strain rates in the range of 10−2 to 10−4 s−1 were investigated, and the fracture mechanisms of each condition were analyzed. The results verified that MMC samples exhibited a remarkable enhancement in microhardness. The evolution of inclined basal texture was observed after processing for both FSPed and MMC samples. The ambient temperature stress–strain response revealed that the formability of AZ31 has improved after friction stir processing, whereas high-temperature flow curves were discernibly sensitive to strain rate. Equiaxed deep dimples were detected on the fracture surfaces of FSPed samples, but decreased strain rate led to an increase in the number of dimples as attributed to the recrystallization of new grains.

Damage evolution and spall failure in copper under complex shockwave loading conditions

The damage evolution and spall behavior of copper under complex shockwave loading conditions were investigated using plate impact experiments with conical targets. Sweeping tensile waves were generated by the interaction of the released waves that were reflected from the free surfaces of the impactor and the cone surface. From the free-surface velocity profiles measured by multi-channel velocimetry, the classic pull-back spall signals were observed in incipient and complete spallation experiments. The spall strength estimated from the pull-back velocity strongly depended on the loading path and the loading wave profile. Post-experiment analysis based on the soft-recovery technique revealed that the damage distributions were very different from the bottom to the top of the conical target, but the corresponding free-surface velocity data measured at different locations suggested that similar responses occurred, which indicated that the spall strength was the critical threshold stress of micro-void nucleation or early growth. The fractography analysis of the fracture surfaces showed that metal micro-spheres were scattered in deep dimples, which indicated that the increase in temperature due to local severe plastic deformation around the voids was important. With the same set of model parameters, the plate impact spallation experiments with plane and conical targets were simulated using a critical damage evolution model. A good agreement was obtained between the simulations and experiments, which demonstrated the model capabilities for predicting the spall responses of metals under complex shockwave loading.