Hardness Distribution Research Articles

A practical approach to fabrication of nano-Al2O3 reinforced MMC coatings by cold spray: Characterization of nanomechanical and tribological performance

Cold spray is a crucial solid-state deposition technique involving high-speed spraying of different-sized particles onto a base material. In this study, Ni-based powder reinforced with different proportions of nano-Al2O3 particles was sprayed onto Al6061 alloy by cold spray to improve the coating performance. Nanoparticles are very difficult to deposition as they are affected by the aerodynamic forces generated during cold spraying. With a practical approach, nanoparticles were agglomerated with micron-sized feedstock powders, and nanoparticles in the sprayed powder acted as larger particles and successfully defeated the aerodynamic force. The effects of adding nano-particles on the microstructural, nanomechanical, and tribological properties were investigated. Microstructural analyses revealed that MMC coating layers comprised ceramic particles and metal matrix (Ni and Zn). Due to the high spraying speed, the nano-particles were spread in the relatively ductile matrix formed by Ni and Zn. The micro- and nanohardness values were increased with nanoparticle volume by two mechanisms. In the first mechanism, the hard particle volume increased with the addition of nano powders, and in the second mechanism, the plastic deformation of the matrix increased due to the nano-particles. The sample without nanoparticle reinforcement exhibited plastic deformation during the initial stage of the nanoscratch test. The dry sliding wear test results showed that the nano-Al2O3-reinforced spraying powders increased the wear resistance approximately 1.9 times. According to FESEM and EDS examinations of worn surfaces and WC counter-bodies, nano-particles reduced the transferred layers caused by adhesion to the abrasive counter-body. In addition, the nano-particles were provided a more homogeneous hard particle distribution and improved wear performance.

Microstructure and Hardness Characteristics of Swing-Arc SAW Hardfacing Layers.

Hot-rolled backup rolls are widely used in steel rolling and usually need to be repaired by arc hardfacing after becoming worn. However, a corrugated-groove defect commonly occurs on the roll surface due to the uneven hardness distribution in the hardfacing layers, affecting the proper usage of the roll. Accordingly, a new swing-arc submerged arc welding (SA-SAW) process is proposed to attempt to solve this drawback. The microstructure and hardness are then investigated experimentally for both SAW and SA-SAW hardfacing layers. It is revealed that a self-tempering effect occurs in the welding pass bottom and the welding pass side neighboring the former pass for both processes, refining the grain in the two areas. In all the zones, including the self-tempering zone (STZ), heat-affected zone (HAZ), and not-heat-affected zone in the welding pass, both SAW and SA-SAW passes crystallize in a type of columnar grain, where the grains are the finest in STZ and the coarsest in HAZ. In addition, the arc swing improves the microstructure homogeneity of the hardfacing layers by obviously lowering the tempering degree in HAZ while promoting the even distribution of the arc heat. Accordingly, the hardness of the SA-SAW bead overall increases and distributes more uniformly with a maximum difference of < 80 HV0.5 along the horizontal direction of the bead. This hardness difference in SA-SAW is accordingly decreased by ~38.5% compared to that of the SAW bead, further indicating the practicability of the new process.

Hardness Distribution and Growth Behavior of Micro-Arc Oxide Ceramic Film with Positive and Negative Pulse Coordination.

Micro-arc oxidation (MAO) is a promising technology for enhancing the wear resistance of engine cylinders by growing a high hardness alumina ceramic film on the surface of light aluminum engine cylinders. However, the positive and negative pulse coordination, voltage characteristic signal, hardness distribution characteristics of the ceramic film, and their internal mechanism during the growth process are still unclear. This paper investigates the synergistic effect mechanism of cathodic and anodic current on the growth behaviour of alumina, dynamic voltage signal, and hardness distribution of micro-arc oxidation film. Ceramic film samples were fabricated under various conditions, including current densities of 10, 12, 14, and 16 A/dm2, and current density ratios of cathode and anode of 1.1, 1.2, and 1.3, respectively. Based on the observed characteristics of the process voltage curve and the spark signal changes, the growth of the ceramic film can be divided into five stages. The influence of positive and negative current density parameters on the segmented growth process of the ceramic film is mainly reflected in the transition time, voltage variation rate, and the voltage value of different growth stages. Enhancing the cathode pulse effect or increasing the current density level can effectively shorten the transition time and accelerate the voltage drop rate. The microhardness of the ceramic film cross-section presents a discontinuous soft-hard-soft regional distribution. Multiple thermal cycles lead to a gradient differentiation of the Al2O3 crystal phase transition ratio along the thickness direction of the layer. The layer grown on the outer surface of the initial substrate exhibits the highest hardness, with a small gradient change in hardness, forming a high hardness zone approximately 20-30 μm wide. This high hardness zone extends to both sides, with hardness decreasing rapidly.

Effect of V and Mn addition on the HAZ softening and tensile properties of friction stir welded martensitic steel

This study explores the suppression of heat-affected zone (HAZ) softening in friction stir welding (FSW) joints formed above the A3 temperature of martensitic steels by leveraging short-time secondary hardening induced by vanadium (V) addition. The HAZ microstructure was examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atom probe tomography (APT). Mechanical properties were evaluated through Vickers hardness (HV) measurements and tensile tests. Analysis of hardness distribution indicates effective HAZ softening suppression with V addition, attributed to the precipitation of fine and high-density V carbides within the HAZ. The inhibition of HAZ softening was correlated not only with V content but also with the A1 temperature of the steels, regulated by the manganese (Mn) content. However, at 1 mass% V content, intergranular brittle fracture occurred within the HAZ due to continuously distributed fine carbides and Mn segregation along the prior austenite boundary.

Faulty machining and micro-segregation assisted fatigue failure of cylinder head stud of a diesel generator

A detailed failure analysis of cylinder head stud bolt of a four-stroke diesel generator has been presented. The study included optical emission analysis, tensile testing, hardness testing, optical microscopy, SEM, and EDX analyses. The investigation showed that the main mechanism of failure was corrosion-fatigue. Fatigue initiation was assisted by pitting corrosion induced by faulty machining, moisture, and presence of tempered martensite and non-metallic inclusions at the surface. Propagation of the fatigue crack was encouraged by localized corrosion within the tempered martensite, non-metallic inclusions, and Mn/Cr/Mo/Si micro-segregation. Both excessive C and Mn/Cr/Mo/Si micro-segregation caused the formation of mixed microstructure of tempered martensite and fresh martensite with retained austenite. The non-uniform microstructure led to non-uniform hardness distribution and caused localized deformation during cyclic loading and promoted fatigue crack propagation.

Revolutionizing hardness via nanoparticle flux in welding of high-hardness armor steel

Given the direct correlation between the hardness of high-hardness armor (HHA) steel and its ballistic performance, mitigating softening in the heat-affected zone (HAZ) becomes essential. In this study, welding was performed using a nanoparticle flux composed of Tungsten Carbide (WC), Titanium Carbide (TiC), Silicon Carbide (SiC) nanoparticles, and ethanol. Subsequently, the shape and hardness distribution of each weld bead were comprehensively analyzed. The analysis revealed an increase in penetration depth in the order of 0.37 mm for WC 8 %, 0.31 mm for SiC 8 %, and 0.12 mm for TiC 2 %. Furthermore, the hardness evaluation results demonstrated a significant improvement at the fusion line (FL) when nanoparticle flux was utilized. Specifically, hardness values of 654.0 HV and 590.4 HV were observed at 8 % WC and 8 % SiC, respectively, representing increases of 16.6 % and 5.28 % compared to the absence of nanoparticle flux. Additionally, further investigations were conducted to elucidate the mechanism behind the improvement in hardness. This experimental evidence confirms the effectiveness of incorporating WC 8 % and SiC 8 % nanoparticles into the welding process of ARMOX 500 T in preventing hardness degradation due to softening.

Transverse energy-energy correlators in the color-glass condensate at the electron-ion collider

We investigate the transverse energy-energy correlators (TEEC) in the small-x regime at the upcoming Electron-Ion Collider (EIC). Focusing on the back-to-back production of electron-hadron pairs in both ep and eA collisions, we establish a factorization formula given in terms of the hard function, quark distributions, soft functions, and TEEC jet functions, where the gluon saturation effect is incorporated. Numerical results for TEEC in both ep and eA collisions are presented, together with the nuclear modification factor RA. Our analysis reveals that TEEC observables in deep inelastic scattering provide a valuable approach for probing gluon saturation phenomena. Our findings underscore the significance of measuring TEEC at the EIC, emphasizing its efficacy in advancing our understanding of gluon saturation and nuclear modifications in high-energy collisions. Published by the American Physical Society 2024

Effect of Strain Hardening on Burr Control in Drilling of Austenitic Stainless Steel

Austenitic stainless steel has been widely used in various industries, such as aerospace, medical, and hydrogen energy, due to its high strength over a wide range of temperatures, corrosion resistance, and biocompatibility. However, stainless steel is a difficult-to-cut metal because its ductility and low thermal conductivity induce a strain hardening with significant plastic deformation at high temperatures. Burr formed at the back side of a plate is a critical issue which deteriorates the surface quality, especially in drilling. Burr removal operation, therefore, should be done in the machine shop. This study discusses the effect of strain hardening of austenitic stainless steel, SUS 316L, on burr formation. Hardness and cutting tests were conducted to compare the strain hardening effect for three types of workpieces: as-received, pre-machined, and tensile treated specimens. In the employed specimens, the tensile treated specimen is harder than the as-received specimen. Those specimens have uniform hardness in the depth direction from surfaces. Pre-machined specimen, in which the back side of the plate was finished by face milling, has a distribution of hardness in the depth direction from a surface. The highest hardness appears in the subsurface of the pre-machined specimen. The cutting forces in the steady processes, in which the entire edges remove material, were nearly the same as the tested specimens each other. However, remarkable differences were confirmed in the chip thickness and burr formation. The higher strain hardening of the tensile treated specimen is effective to suppress burr formation. The cutting characteristics are then identified to associate burr control with the shear plane model of orthogonal cutting using an energy-based force model. The shear stresses, shear angles, and friction angles of the tensile treated and as-received specimens are compared to discuss the effect of strain hardening on reduction of burr formation.

EBSD-based image quality analysis of in-situ tempered martensitic steel generated by L-PBF

Microstructures resulting from additive manufacturing technologies, such as laser powder bed fusion (L-PBF), can exhibit both intended and unintended gradation. Intended in form of functionally graded materials (FGM) providing improved material performance. Un-intended in form of part specific local changes due to variations in thermal boundary conditions resulting from the geometry at hand (e.g. slender lattices or downskin areas). To characterize, avoid and optimize such gradations, vast spacial measurements are necessary. This work investigates if electron backscatter diffraction (EBSD) can provide such information, to characterize the microstructural gradation of low alloy steel. Central to this work is the analysis of the image quality (IQ), since the tempering of martensitic steel microstructures results in a reduction of dislocation density, changes in carbide precipitation as well as crystal structure and thus varying IQ. Based on the multi-peak IQ approach and information about the thermal history, the microstructure is considered as purely martensitic. The analysis of single laser track experiments reveals that the impact of cooling rates as well as in-situ tempering effects can be measured, which present the two major effects regarding microstructural formation in L-PBF. Experiments on in-situ and conventionally heat-treated samples show correlating trends regarding tempering states, hardness and IQ distribution. Vast measurements over a sample size of 10 × 10 mm2 demonstrate the possibility to analyze FGM. Furthermore, micro-gradations are observed within each laser track within the material. Consistent trends and correlations indicate that microstructural changes dominate changes in IQ values, while the impact of EBSD parameter and sample preparation appears to be minor. The results promote the suitability of IQ analysis for the spatial characterization of low alloy steel generated by L-PBF.

A hardness-based constitutive model for numerical simulation of blind rivet with gradient hardness distribution

A286 superalloy blind rivets find extensive applications in the structural connections of aerospace equipment owing to their exceptional strength and heat resistance. However, the gradient hardness distribution within the rivet sleeves complicates achieving precise numerical simulation results with traditional constitutive models. This study aims to establish a constitutive model suitable for materials exhibiting gradient hardness to support the development of A286 superalloy blind rivets. Initially, quasi-static compression experiments were conducted on A286 superalloy samples at different hardness levels (160–324 HV), strain rates (0.01–10 s⁻¹), and temperatures (25–600°C). Subsequently, the influence of material microstructural evolution on flow stress during compression was investigated using electron backscatter diffraction technique. Furthermore, a hardness-based Johnson-Cook (J-C) constitutive model was developed by introducing hardness and offset compensation onto the original J-C model and compared with its counterpart. Finally, the hardness-based model was employed to simulate the bulge compression forming of A286 blind rivets and validated through experimental trials. The findings reveal that dislocation strengthening predominantly contributes to the increased material flow stress with escalating strain rates and sample hardness. Compared to the original J-C model, the stress prediction error of the hardness-based model reduced from 26.8 % to 2.9 %, effectively depicting the variation in material flow stress under different hardness values. The simulated values of bulge morphology, dimensions, and load closely align with experimental data, affirming the efficacy of the hardness-based model in numerically simulating blind rivets with gradient hardness distribution.

Online Streaming: A Paradigm Shift for Nollywood Movie Distribution

The Nigerian movie distribution and exhibition business has undergone major transformation in the last few years due to technological innovation. The study is about Nollywood paradigm shift from hard copy movie distribution to online streaming services. The aim is to ascertain why producers are moving to soft screening through online pay-to-view platforms. The objectives were to find out if this new trend is financially beneficial to the producers and whether it will likely continue in the future. The study adopted qualitative methods to interrogate existing data on some of the most popular Nollywood films released online by Netflix. This is based on a snap-shot of Netflix’s popularity chart in the month of October, 2020 and news of cinema theatre films release around the same period. The findings are that the online distribution deals must have been profitable for the producers, judging by repeated use by some of them and others enlisting in the business. The expanding scope of Nollywood producers using these online streaming platforms for distributions of their work suggests the likelihood of a continuing process. The study concludes that this is a mutually beneficial relationship between local producers and online business global entities. It, therefore, recommends that Nollywood producers avoid signing exclusive deals with online distributors only. This will allow home-grown cinema theatres to be financially rewarding to prevent the nation’s cinema theatre chains from collapsing due to redundancy.

Failure Analysis of Large-Size Drilling Tools in the Oil and Gas Industry

Abstract The safety problem of large-size drilling tools in large-size boreholes has become increasingly prominent with the exploration and development of deep and ultradeep wells. This study analyzes the causes of large-size drilling tool failures from the engineering point of view via statistical analysis, experimental material test, and vibration and bending analyses. Results show that the violent downhole vibration changes the drilling tool's mechanical properties. These changes result in an uneven distribution of hardness and reduced impact work, finally leading to the initiation of fatigue cracks at stress concentration points. Drilling tool bending is closely related to drilling parameters and bottom hole assembly (BHA) configuration. Unreasonable BHA configuration and drilling parameters increase BHA bending and accelerate fatigue failure. Once a crack is generated, the corrosive ions in water-based drilling fluids invade the microcrack, causing the corrosion of the drilling tool material. As a result, the strength is reduced, and the fracture is aggravated. Therefore, measures for preventing the failure of large-size drilling tools are proposed. We hope that the results of this work can provide useful guidance for drilling engineers.

A Study of {10-12} Twinning Activity Associated with Stress State in Mg-3Al-1Zn Alloy during Compression

An eight-sided prism sample, obtained from a hot-rolled AZ31 magnesium alloy sheet, was compressed at room temperature along the transverse direction to investigate the influence of local strain on twinning behavior using electron backscatter diffraction (EBSD) measurements, hardness distribution, and metallographic observations. The octagonal surface of the sample was divided into distinct regions based on hardness distribution and metallographic observations. Combined analysis of the Schmid factor (SF) and the strain compatibility factor (m’) was employed to study twin variant selection. Basal on SF ratio distribution, the Schmid factor criterion, can predict over 75% of observed twin variants in regions A and D (normal stress samples). In contrast, 64% of twin variant selection behavior in region C (shear stress sample) can be effectively explained using a pure shear model. Twin variants with high strain compatibility factors may prefer activation to reduce stress concentration. The strain compatibility factor is more appropriate than the Schmid factor for analyzing the effect of local strain on the selection behavior of twin variants.

Densification, hydrogen retention, microstructure, and mechanical properties of ε-ZrH2-x monolith fabricated by high-pressure spark plasma sintering

The sintering of zirconium hydride (ZrH2-x) without hydrogen atmosphere faces the challenge of thermal dehydrogenation problem, which is overcome by the high-pressure spark plasma sintering (HPSPS) technique in this study. The application of high pressure remarkably reduces the initial densification temperature and effectively restrains the thermal dehydrogenation of ZrH2-x in SPS. Under 500 MPa loading, ε-ZrH1.72 monolith with high densification (96.62 %) and high hydrogen density (6.04 × 10 22 atoms H/cm3) is sintered at a relatively low temperature of 700 °C for only 5 min. Meanwhile, the grain growth of ZrH2-x is very limited. The Young’s modulus and hardness of the sintered ε-ZrH1.72 are 58.68 GPa and 2.61 GPa respectively. Compared with the directly hydrided ε-ZrH2-x, the sintered ε-ZrH1.72 has a more uniform hardness distribution and improved fracture toughness of 3.19 MPa m1/2. This study provides a valuable reference for the sintering of other easily decomposable materials.

Experimental study on mechanical behaviour of friction stir welded aluminium alloy butt joints

The purpose of this paper is to understand the softening extent and stress-strain behavior of the normal-strength 6061-T6, 6013-T6, and high-strength 7075-T6 aluminium alloy butt joints using the novel friction stir welding (FSW) technique. A total of 107 monotonic tensile coupons and 33 Vickers hardness coupons were prepared from 8- or 12-mm thick welded plates using various tool rotation speed, welding transverse speed and tool configurations. The surface morphology, stress-strain curves and hardness distribution laws after welding were evaluated. The W-shaped hardness profiles were clearly observed, with the fracture location positively correlated at the lowest point of Vickers hardness near the edge of the weld toe. The strength reduction and softening extent of extruded aluminium alloys in T6 temper induced by FSW were superior to those of aluminium alloys using fusing welding process outlined in the Chinese and European standards, indicating the effectiveness and applicability of the FSW technique in the formation of components and connections for aluminium alloy structures. Additionally, the current widely used constitutive models generally yielded under-estimations on the strength under large strains. The developed three-stage Ramberg-Osgood model, using seven key input parameters, significantly improved the accuracy and consistent predictions in the full-range stress-strain curve of FSW aluminium alloys. Furthermore, the suggested model could still achieve a great balance between accuracy and practicality when using just three common available parameters with the employment of predictive expressions on other four parameters.

Effect of Case Depth and Hardness Distribution on the Rolling Contact Fatigue Performance of G20CrNi2MoA Carburized Steel

This study aims to investigate the effects of the case depth and hardness distribution of carburizing on the rolling contact fatigue (RCF) properties of G20CrNi2MoA steel under deeper carburizing conditions. Four groups of specimens of different case depths are prepared and subjected to RCF tests. A 2D finite element model based on continuum damage mechanics is developed to further analyze these two factors. As the depth of the carburized case increases, the RCF life first increases and then decreases. The optimum case depth is approximately one‐quarter of the thickness of the specimen. A gentler hardness distribution due to a change in the diffusion process parameters ensures a minimal increase (8%) in the RCF performance. The proper case depth has a more notable effect on the RCF life compared with those of the gentle or steep hardness distribution. It is expected that the results can help to promote a better understanding of the heat treatment process of carburized bearings and ensure more reliable carburized bearings are produced.

Micro-macro properties of stainless-clad bimetallic steel welded connections with different configurations

This paper aims to investigate the microstructure and mechanical properties of S31603 + Q355B stainless-clad (SC) bimetallic steel welded connections with different welding configurations. A comprehensive research approach was employed, including microstructure analysis, static tensile test, impact test, bending test, Vickers hardness distribution measurement, and analysis of welding economy and efficiency. Four types of welded connections were examined to understand their performance variations arising from different welding configurations. Based on the findings, a recommended welding configuration suitable for structural engineering applications was proposed. Research outcomes showed that the decarburization and carbon accumulation zones were observed at the fusion interface during the welding of dissimilar metals. It should be noted that martensite was formed in the substrate, or the base metal, due to diffusion of alloying elements, with a more pronounced effect observed with higher welding heat inputs. In order to minimize the fusion area of dissimilar metals, it is advisable to avoid using a single welding consumable made of stainless steel as the mechanical properties of the welded connection were not significantly influenced by the use of stainless steel welding consumables with higher Nickle (Ni) and Chromium (Cr) alloying content in the transition region. Consequently, the same welding consumable should be employed in the transition region to simplify the welding procedure and to enhance the welding efficiency.

Effect of welding speed on mechanical properties of dissimilar friction stir lap-joint welding between AA6061 and C1100 copper

Nowadays, renewable energy such as wind and solar energy can be employed for marine application to enhance clean and green technology. So, using battery is necessary to apply power for ship activities. To contribute sustainable development, optimizing structure in electric transmission such as bimetal connector Cu-Al is necessary to improve joint quality. This study focused the influence of welding parameters on the quality of the lap-joint between AA6061 aluminum alloy and C1100 copper. Experimental results indicate that the welding interface was sensitive to welding speed. Some welding defect was found in the joint such as tunnel and hook defects. The highest tensile strength produced by the welding speed of 200 mm/min was approximately 1100 N. In all experimental regimes, the weld zone structure, hardness distribution, tensile strength, fracture location, and local deformation were analyzed and discussed in detail. Keywords: Friction stir welding, microstructure, mechanical properties, hardness

Tribology, corrosion, and tribocorrosion performance of aged lightweight steels: Effects of oxide film and carbide

The study investigated the tribology, corrosion, and tribocorrosion performance of aged Fe–30Mn–8Al–1.2 C–0.1Ti–(2Cr/Mo) steel, with a focus on oxide film and carbide. The precipitates changed from κ carbide to Cr7C3/MoC with the addition of Cr/Mo. The inhomogeneous precipitation of Cr7C3/MoC created zones of varying hardness, one being hard with strong strengthening and the other soft with weak strengthening. The inhomogeneous hardness distribution deteriorates the tribology performance of the steel. By minimizing galvanic coupling corrosion and improving the protectiveness of the oxide film, the addition of Cr/Mo can improve the corrosion and tribocorrosion resistance of lightweight steel.

Investigating the Dynamic Mechanical Properties and Strengthening Mechanisms of Ti-6Al-4V Alloy by Using the Ultrasonic Surface Rolling Process.

The demand for titanium alloy has been increasing in various industries, including aerospace, marine, and biomedical fields, as they fulfilled the need for lightweight, high-strength, and corrosion-resistant material for modern manufacturing. However, titanium alloy has relatively low hardness, poor wear performance, and fatigue properties, which limits its popularization and application. These disadvantages could be efficiently overcome by surface strengthening technology, such as the ultrasonic surface rolling process (USRP). In this study, the true thermo-mechanical deformation behavior of Ti-6Al-4V was obtained by dynamic mechanical experiment using a Hopkinson pressure bar. Moreover, USRP was applied on the Ti-6Al-4V workpiece with different parameters of static forces to investigate the evolution in surface morphology, surface roughness, microstructure, hardness, residual stress, and fatigue performance. The strain rate and temperature during the USRP of Ti-6Al-4V under the corresponding conditions were about 3000 s-1 and 200 °C, respectively, which were derived from the numerical simulation. The correlation between the true thermo-mechanical behavior of Ti-6Al-4V alloy and the USRP parameters of the Ti-6Al-4V workpiece was established, which could provide a theoretical contribution to the optimization of the USRP parameters. After USRP, the cross-sectional hardness distribution of the workpiece was shown to initially rise, followed by a subsequent decrease, ultimately to matrix hardness. The cross-sectional residual compressive stress distribution of the workpiece showed a tendency to initially reduce, then increase, and finally decrease to zero. The fatigue performance of the workpiece was greatly enhanced after USRP due to the effect of grain refinement, work hardening, and beneficial residual compressive stress, thereby inhibiting the propagation of the fatigue crack. However, it could be noted that the excessive static force parameter of USRP could induce the decline in surface finish and compressive residual stress of the workpiece, which eliminated the beneficial effect of the USRP treatment. This indicated that the choice of the optimal USRP parameters was highly crucial. This work would be conducive to achieving high-efficiency and low-damage USRP machining, which could be used to effectively guide the development of high-end equipment manufacturing.