Large Number Of Dimples Research Articles

Effect of Al on the Microstructure and Mechanical Properties of Mg-6Zn Alloy

In this work, the effect of trace Al on the microstructure and mechanical properties of Mg-6Zn alloy was studied. The results show that the as-cast ZM60 alloy consists of α-Mg phase and Mg51Zn20 phase. After the addition of Al, fine Al8Mn5 phase increases in the microstructure of the experimental alloy, and the number of the second phase increases. Dynamic recrystallization occurred in all the experimental alloys during extrusion deformation, the grain size of Al alloy is significantly smaller than that of ZM60 alloy, and the average grain size of ZAM610 alloy is the smallest, which is 5.6µm. ZAM610 alloy has the best mechanical properties, with tensile strength, yield strength, elongation after fracture, U-notch impact work and non-notch impact work of 302.7MPa, 216MPa, 19.7%, 10.0 J/cm2 and 56.0 J/cm2, respectively. The fracture of the impact sample shows that there are many tearing edges in the ZM60 alloy, which shows the characteristics of quasi-cleavage fracture. The ZAM600 and ZAM610 alloys have a large number of dimples, which are characterized by dimple fracture. The improved properties of magnesium alloys after microalloying can be attributed to the formation of fine Al8Mn5 phase, which has good Zener blocking effect, refines the dynamic recrystallized grains, and improves the strength and impact toughness of the alloy.

Hot tensile deformation behavior and microstructure evolution of Mg–Mn–Ce alloy

Magnesium-manganese series alloys, as materials with very low-density value, have excellent mechanical properties and corrosion resistance, and have broad prospects in automotive, aerospace, biomedical and other fields. In this work, the thermal deformation behavior and microstructural evolution of Mg–Mn–Ce alloys were investigated through uniaxial hot stretching experiments on Mg–Mn–Ce alloy plates under different deformation conditions, with the main focus on the plastic deformation mechanism of Mg–Mn–Ce alloys under different forming conditions. The unified viscoplastic constitutive equation for coupled damage is developed and compared with different types of constitutive equation models. Remarkably, the decrease in peak stress with increasing temperature or decreasing strain rate in Mg–Mn–Ce alloys is due to the fact that the internal slip system of the material is more easily activated with increasing temperature. As the temperature increases, the dislocations within the material decrease, which is due to the fact that the temperature increase makes the DRX action progressively stronger than the dislocation action, and the DRX leads to a significant coarsening of the grains at 573 K. In addition, the fracture behavior of the alloy under different deformation conditions has also been investigated, and the fracture pattern of Mg–Mn–Ce alloys transitions from pronounced cleavage fracture to the appearance of dimples and finally to ductile fracture with a large number of dimples as the temperature increases.

Microstructure Characterization and Mechanical Properties of Laser Welded Super High Strength QP980 Automotive Steel

The Ultrahigh strength Q&P automotive steel, i.e. QP980, has a broad application prospect in lightweight due to its high strength and good plasticity. In this study, the range of heat input (30 ~ 40 J/mm) was selected by controlling laser power in laser welding of QP980 in order to investigate the microstructure and properties of welded joint. At the heat input of 30 ~ 40 J/mm, the joint of QP980 had acceptable penetration. The weld widths were 417.93 mm, 582.02 mm and 521.56 mm, respectively. The macroscopic morphology of the joint is hourglass type. The microhardness of the welded joint hardening zone is higher than that of the base metal, and the maximum value is 519 HV0.5. When the heat input is 35 J/mm, the tensile strength of the welded joint is 1109 MPa. The maximum joint factor is 91.88 %. The fracture is close to the base metal. A large number of dimples are observed on the fracture surface, implying as ductile fracture. Based on the EBSD results, the proportion of low angle grain boundary was consistent with mechanical properties. A large number of deformation twins are formed in the 35 J/mm sample through deformation, which has a great contribution to the strength of the weld.

Microstructure and Mechanical Properties of TC4 Titanium Alloy at the Temperature of 77K

Titanium alloy has the advantages of low thermal conductivity, a small expansion coefficient and being non-magnetic, making it an ideal low-temperature structural material. In this paper, the typical TC4 titanium alloy in industrial titanium alloy is selected as the research object. The microstructure deformation law and mechanical behavior of TC4 titanium alloy at liquid nitrogen temperature are mainly investigated, and compared with the microstructure and properties at room temperature. The macroscopic and microscopic deformation mechanism of the simultaneous increase in elongation and hardening index of titanium alloy at low temperature is revealed, which provides a basic basis for the low-temperature deformation mechanism and strengthening and toughening design of titanium alloy. Based on the uniaxial tensile tests at room temperature (298 K) and low temperature (77 K), the effects of low temperature on the yield strength, elongation, tensile strength and work hardening curve of titanium alloy were compared and analyzed. The strength/plasticity synergistic improvement of TC4 titanium alloy under low-temperature deformation was found. At low temperature, the yield strength, tensile strength and elongation of TC4 titanium alloy are improved compared with room temperature. The tensile strength increases from 847.93 MPa at 298 K to 1318.70 MPa at 77 K, and the elongation increases from 21.8% at 298 K to 24.9% at 77 K. The grain morphology, grain orientation, dislocation density and fracture morphology of titanium alloy under room temperature and low-temperature tensile conditions were studied by SEM and EBSD. The results of fracture morphology characterization at room temperature and low temperature show that TC4 titanium alloy exhibits ductile fracture characteristics and a large number of dimples are formed on the fracture surface. The dimple depth at low temperature is shallower than that at room temperature and the overall surface is more flat. Compared with room temperature deformation, the deformation process of TC4 titanium alloy in a low-temperature environment produces stronger dislocation pile-up and forms a large number of twins, but the grain rotation is more significant, which effectively alleviates the stress concentration and delays the initiation and propagation of cracks at grain boundaries.

Hot tensile deformation behavior and microstructure evolution of 7075 aluminum alloy sheet

The hot tensile deformation behavior of 7075 aluminum alloy sheet was studied using Gleeble-3800 thermal simulation machine at deformation temperatures of 400–475 °C and strain rates of 0.001∼1s−1. Microstructure evolution and fracture morphology were observed by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The hyperbolic sine model is used to establish the constitutive equation of the relationship between peak stress and Z parameter. The results show that the hyperbolic sine Arrhenius constitutive equation can well characterize the flow behavior of the studied alloy, and the hot deformation activation energy is 132.52 kJ/mol. The average relative error between the predicted and experimental values is only 4.58%, and r2 is as high as 0.988, indicating that there is good consistency between the experimental value and the predicted value. When the strain rate keeps unchanged, the elongation of the studied alloy shows an upward trend in the lower deformation temperature range (400–425 °C). When the deformation temperature continued to rise to 475 °C, the elongation of the studied alloy decreased sharply. Based on the analysis of fracture morphology, a large number of dimples can be observed at 450 °C/0.001s−1, which is a typical ductile fracture. With increasing of deformation temperatures, the fracture mechanism gradually changes from ductile fracture to brittle fracture. Under the condition of 425 °C/0.01s−1, there are a lot of dislocations in the alloy, and the dislocation walls are formed. With the increase of deformation temperatures or the decrease of strain rates, the dislocation density gradually decreases and dynamic recrystallization occurs. The softening mechanism of the studied alloy changes from dynamic recovery to dynamic recrystallization.

Oscillating Laser-Arc Hybrid Additive Manufacturing of AZ31 Magnesium Alloy

To overcome the instability of the traditional magnesium alloy additive process, the research on oscillating laser-arc hybrid additive manufacturing (O-LHAM) for AZ31 Mg alloy was developed first, and it focused on the effect of beam oscillation on process characteristics, such as macroscopic morphology, porosity defects, microstructure, and mechanical properties of thin-walled components. The increasing oscillation frequency was effective in suppressing the defects, including lack of fusion, wavy hump, internal porosity, etc. Compared with the case without oscillation, the average grain size of O-LHAM samples was refined from 22–32 μm to 18–20 μm at 300 Hz, while the percentage of Al8Mn5 and Mg17Al12 precipitation increased from 1.42–1.61% to 2.55–3.32%. For this reason, a stirring laminar flow was induced using the oscillating laser in the melt pool, with the ability to disrupt the grain structure and provide more nucleation sites, which is beneficial in reducing the average grain size and promoting the precipitation of the precipitated phase. Meanwhile, for the component without pores, the ultimate tensile strength is 205 MPa, slightly less than the base, but the elongation is 20.7%, twice that of the base. The tensile fracture is characterized by a large number of dimples and some ductile tearing ridges, demonstrating good ductility, which is associated with the grain refinement and precipitation strengthening induced using the oscillating laser. The results indicated that O-LHAM would be an effective method for manufacturing Mg alloy fast and well.

Influence of heat treatment on strength and toughness of laser cladding iron-based coatings

Fe314 coatings were prepared on EA4T steel by laser cladding method, and were respectively subjected to 550 ℃ × 2 h Two kinds of heat treatment: annealing and annealing (1150℃×1h + 980℃×1h + 650℃×2h) after normalizing twice. The microstructure of the precipitated phase after heat treatment, the impact toughness and microhardness of the coating reveal the mechanism of heat treatment to improve the strength and toughness of the cladding layer. The results show that the Fe314 cladding layer had good plasticity and good room temperature strength after adopting the two-stage heat treatment process. The average microhardness was 228HV and the impact toughness was 64 J. The impact toughness of the cladding layer was related to the morphology and grain size of the precipitates. A large number of dimples can be observed on the impact fracture morphology, indicating that the fracture mechanism of the alloy was mainly ductile fracture. Comprehensive comparison, the comprehensive properties of strength and toughness were better at 1150℃×1h and 980℃×1h followed by 650℃×2h heat treatment.

Analysis of Failure Causes of 0Cr19Ni9 Blade Crack

In this paper, fracture morphology observation, microstructure analysis, standard corrosion test, and other experimental methods are used to analyze the causes of corrosion and cracking of steam turbine blades (0Cr19Ni9) produced by a certain company. The results showed that cracks appeared during the operation of the blade. A large number of dimples were observed under the scanning electron microscope, almost all of which were shear dimples; at the same time, fatigue bands were also distributed in them. The internal cause of the cracks is the long-term service, which causes the carbon and the alloy to undergo a diffusion reaction to form carbon-rich areas and carbides. They are thermally corroded in the service environment, and the carbon-rich zone is reduced, which reduces the strength of the zone; carbides accumulate at the grain boundary, causing groove-shaped defects at the grain boundary to form a cutting effect. It is easy to form a source of fatigue cracks. The external cause is that the material is subjected to transverse stress for a long time during operation, which leads to the generation of cracks. This article analyzes the failure causes of the blade and proposes related solutions, which have important reference value for production practice.

A study on tensile flow and strain hardening behaviour of β-Titanium at elevated temperatures

In this paper, the tensile flow and work hardening behaviour of Beta titanium alloy was examined over a wide temperature range of 28 to 500°C. The work hardening of the material was modelled using different empirical relations such as Swift, Voce and Ludwigsen and the prediction accuracy of these models were compared with different statistical parameters. The hardening rate of the material was analysed with K-M analysis. The fracture behaviour of the tensile samples was studied using SEM results. The yield strength and ultimate tensile strength of the material was found to be decreased from room temperature to 200°C followed by a gradual increase with increase in temperature. Uniformly distributed serrations were observed in the entire plastic region of the flow curve in the temperature range of 200– 400°C. This serrated behaviour could be due to the manifestation of dynamic strain aging in this temperature range. The Voce hardening model was identified as a best hardening model to describe the material hardening. SEM results revealed the ductile mode of fracture at all temperature with large number of dimples in fractographs.

Effect of on-Line Annealing Temperature on Mechanical Properties, Microstructure, Fracture Structure and Superelasticity of Ti49.1Ni50.9 Alloy

Ti-50.9Ni (at%) shape memory alloy (SMA) ingot was prepared by the vacuum induction furnace, and was produced into Φ2.0 mm wire successively by forging, rolling and hot drawing. After complete annealing of 800 °C×5 min and cold drawing with 30% deformation, on-line annealing with different temperatures (500 °C×5 min, 600 °C×5 min, 700 °C×5 min) were employed to TiNi wires. Both tensile tests and fivecycles loading-unloading tensile tests with 7% strain were applied to study the mechanical properties, and superelasticity (SE) of TiNi wires. In addition, microstructure and fracture characteristics were investigated by OM, SEM and EDS. The results showed that the tensile strength of 500 °C×5 min annealing reached 1431 MPa, the elongation of 700 °C×5 min annealing reached 129%. The superelasticity behavior of 500 °C×5 min showed excellent performance in five-cycles loading-unloading tensile tests, and the transformation platform peak stress with >7% strain showed a small attenuation (about 4 MPa ), and cumulative residual strain was only 0.2%. In addition, fracture characteristics for all TiNi wires were all plastic fractures, there were a large number of dimples in fractures.

Friction spot joining of aluminum AA5052 and short glass fiber-reinforced PPS: Effects of material properties and process parameters on joint structure and strength

A simplified friction spot joining (FSpJ) technique was developed to join AA5052 and short glass fiber-reinforced polyphenylene sulfide (GF-PPS) sheets. A full factorial design of experiments and analysis of variance were carried out to understand the influence of surface condition and process parameters on the joining process and mechanical strength of such joints. A maximum peak load as high as 4426.5 N was obtained. Examination of microstructures and fracture surfaces of the joints revealed important information about FSpJ joints. A microscopic serrated bonding interface at the faying interface was formed at the center of the bond, which renders a desirable bonding, together with the macroscopic interlocking produced by the interpenetration of aluminum and GF-PPS. Examination of fractured joints shows that there are two distinctive areas on a faying interface: a plastic deformation zone and a transition zone. A large number of dimples and glass fibers around these dimples were observed in fractured joints made using treated Al and GF-PPS around the center of the faying interface, which is beneficial to the bonding strength.

Numerical Analysis with Experimental Validation of Single-Phase Fluid Flow in a Dimple Pattern Heat Exchanger Channel

A plate heat exchanger with a dimple pattern heat plate has a large number of dimples. The shape of dimples defines the characteristics of the plate heat exchanger. Although such heat exchangers have become increasingly popular due to their beneficial characteristics, knowledge of the flow characteristics in such kind of channel is poor. A good knowledge of the flow conditions inside of such channel is crucial for the successful and efficient development of new products. In this paper single-phase water flow in dimple pattern plate heat exchanger was investigated with application of computational fluid dynamics and laboratory experiments. Numerical analysis was performed with two turbulence models, Realizable - with enhanced wall treatment function and - SST. The first predicts a slightly smaller pressure drop and the second slightly larger compared to the results of laboratory measurements. Our research found that despite the relatively low velocity of the fluid, turbulent flow occurs in the channel due to its shape. We also found that there are two different flow regimes in the micro plate heat exchanger channel. The first regime is the regime that dominates the heat transfer, and the second is the regime where a recirculation zone appears behind the brazing point, which reduces the surface for heat transfer. The size of the second regime does not change significantly with the velocity of the fluid in the volume considered.

Effect of controlled rolling temperature on microstructure and properties of Mg-9Li-1Zn alloy sheet

Abstract In this research, the rolling of Mg-9Li-1Zn alloy at different temperatures was simulated, and it was found that the temperature control had an effect on the grain size and temperature field of the sheet. Combined with the experimental analysis, the results showed that the alloy with proper rolling temperature control had fine microstructure and plasticity. Under the conditions of 300 °C for the first pass and 275 °C for the second pass, the β-Li phase was recrystallized dynamically and most of the α-Mg phase was spheroidized. The tensile strength was 233 MPa, and a large number of dimples were produced on the tensile fracture, which was ductile fracture with an elongation of 45%. In addition, according to Gibbs-Thomson theorem and metallographic analysis, the mechanism of α-Mg spheroidization was explained.

Fracture toughness of friction stir welded TA5 titanium alloy joint

Fracture toughness of friction stir welded titanium alloy joints was investigated using Compact tension (CT) toughness specimens. The fracture toughness slightly decreases to about 90% of the fracture toughness of base metal. The fracture surface was mainly ductile fracture mechanism with a large number of dimples. The crack propagation path was straight and mixed with intergranular and transgranular fracture. The stir zone of joint has a large number of fine ɑ grains with some dislocations. For texture, the stir zone has hard orientation {0002} <112¯0>, the directions of cylinder and plate thickness are parallel. The reduction of fracture toughness in the stir zone is mainly related to microstructure and texture.

Fracture toughness of TA31 titanium alloy joints welded by electron beam welding under constrained condition

Electron beam welding (EBW) of titanium alloy was carried out by simulating the constraint conditions in actual welding. The fracture toughness of welded joints was studied. It is found that the fracture toughness decreases a lot, only 70% of the fracture toughness of base metal. The fracture surface was mainly ductile fracture mechanism with a large number of dimples by SEM. It was found that the crack propagation path was straight and mixed mode of intergranular and transgranular fracture by OM. OM and TEM observations show that the weld is mainly composed of fine and disordered acicular martensite with a large number of dislocations and twins. The grain size of constrained weld is finer and more disorderly than that of non-constrained weld, and there is no obvious grain orientation observed by EBSD. The crack propagation behavior of constrained weld was analyzed by the model.

Effects of laser processing parameters on microstructure and mechanical properties of additively manufactured AlSi10Mg alloys reinforced by TiC

AlSi10Mg alloys reinforced by TiC particles were fabricated on 5052 aluminum alloys by laser additive manufacturing. The surface morphology, microstructure, microhardness, and tensile properties of the alloys fabricated with different processing parameters were investigated by optical microscope (OM), scanning electron microscope (SEM), Vickers hardness tester, and high-precision electronic universal testing machine. The results show that appropriate laser energy input leads to uniform distribution of TiC particles in the alloy. Fine needle-like TiC particles are precipitated from the grain boundary, which is beneficial to the improvement of mechanical properties of the alloy. The optimum process parameters are found to be 2500–3500 W for laser power, 600 mm/min for scanning velocity, and 2.0 mm for hatch spacing. For lower or higher laser energy input, aggregations are formed in the coatings. At the optimum processing parameters, the maximum microhardness and tensile strength of the TiC/AlSi10Mg alloys are much higher than those of others, which are 139.1 HV0.05 and 278.8 MPa, respectively. A large number of dimples have been found on the fracture surface of the alloys, which indicates the fracture mechanism of the alloy is ductile fracture.

Transformation of Fe-rich intermetallic phases and eutectic Si morphology in Al–Si–Cu–Fe cast alloy with solution treatment

The transformation of Al5FeSi intermetallic phase and eutectic Si morphology were studied with different solution treatment time at 510 °C. The results show that solution treatment can change the morphology of the acicular Fe phase and the eutectic silicon. As the solution time increases, the acicular Fe-rich phase and eutectic silicon gradually spheroidize, and the Fe-rich phase changes from Al5FeSi to Al8Fe2Si phase. When the solid solution time is 10 h, the ultimate tensile strength and elongation are 275.6 MPa and 3.5%, respectively. The fracture mode changes from quasi-cleavage fracture to ductile fracture after solution treatment, and a large number of dimples appear in the fracture morphology.

Fatigue Lifetime of Laser-MIG Hybrid Welded Joint of 7075-T6 Aluminum Alloy by in-situ Observation

The fatigue lifetime of the laser-MIG hybrid welded joint of 7075-T6 aluminum alloy was studied by in-situ observation. The results show that there exists a strong relationship between the fatigue crack initiation lifetime and fatigue failure lifetime. For the base metal specimens the ratio of fatigue crack initiation lifetime and fatigue failure lifetime is about 64.5%, while for the welded joint specimens it is about 20.2%. The observation of fatigue fracture surface indicates that there are a large number of dimples in the base metal specimens, while lots of gas porosities appear in the welded joint specimens instead. These gas porosities are regarded to be the reason of lower fatigue crack initiation lifetime.

Numerical and Experimental Analyses of the Dynamic Characteristics of Journal Bearings With Square Dimples

Numerous previous numerical studies have investigated the effect of surface texturing upon the static characteristics of journal bearings, including their load-carrying capacity and friction torque. In general, the dynamic characteristics of journal bearings are also important, since they are essential factors in predicting the vibration behavior of actual rotors supported by journal bearings. However, the effects of surface texture upon these dynamic characteristics have not been investigated through either numerical or experimental analysis. Thus, in the present study, such analyses were conducted to investigate the dynamic characteristics of textured journal bearings, such as their dynamic coefficients of oil film and the stability-threshold shaft speed supported by the bearings. Numerical analysis was done using a model that included inertial effects and energy loss; this model agreed well with experimental results concerning static characteristics from our previous study. Dynamic testing based on a sinusoidal-excitation method was also performed using textured journal bearings with uniform square dimples to verify the numerical results, which agreed qualitatively with those of experiment, confirming the validity of the numerical analysis. These results suggest that under the same operating conditions, the main effect of texturing upon the dynamic coefficients is to yield the cross-coupled stiffness coefficients with lower absolute values than the conventional ones with a smooth surface. The linear stability-threshold shaft speed of the rotor supported by the textured journal bearings became higher than that of a smooth bearing, mainly due to the reduction of cross-coupled stiffness coefficients. This tendency became more pronounced for high Reynolds number operating conditions and textured bearings with a large number of dimples.

Static Characteristics of Journal Bearings With Square Dimples

Surface texturing is a technique for improving frictional and hydrodynamic performances of journal bearings because microtextures can serve as reservoirs for oil or traps for debris and may also generate hydrodynamic pressure. Over the past two decades, many researchers have experimentally demonstrated that texturing of various tribological elements can reduce friction force and wear, contributing to improvement of lubrication performance. Some numerical studies have examined the hydrodynamic lubrication conditions and reported that surface texturing affects the static characteristics of journal bearings, such as their load carrying capacity and friction torque. However, the validity of these numerical models has not been confirmed because of a lack of experimental studies. This study proposes a numerical model that includes both inertial effects and energy loss at the edges of dimples on the surface of a journal bearing in order to investigate the bearing's static characteristics. Experimental verification of journal bearings is also conducted with a uniform square-dimple pattern on their full-bearing surface. The results obtained by the model agree well with those of experiment, confirming the model's validity. These results show that under the same operating conditions, textured bearings yield a higher eccentricity ratio and lower attitude angle than the conventional ones with a smooth surface. This tendency becomes more marked for high Reynolds number operating conditions and for textured bearings with a large number of dimples.