Comparison of Stellite coatings on low carbon steel produced by CGS and HVOF spraying

Stellite alloys are of great interest in industries due to a unique combination of high temperature mechanical strength, outstanding wear and corrosion resistance. Different thermal spraying processes are used for deposition of stellite alloys on industrial components. However, the investigations on the structure-property relationship of these alloys produced via different deposition process are limited. This study focuses on the microstructure, oxidation, and tribo-mechanical properties of Stellite 21 deposited by cold gas spraying (CGS) and high velocity oxy-fuel (HVOF) process on a low carbon steel substrate. The coating cross- section was characterized by SEM and optical microscopy. The coatings were further characterised by using nanoindentation, adhesion, and ball-on-disk wear tests. Moreover, XRD tests were run on the powder and the coatings to reveal possible phase transformation during spraying, as well as during wear and oxidation tests. The results showed no phase transformation in the as-sprayed CGS coating, besides higher values of porosity and oxide phase in the HVOF coating. However, an fcc-to-hcp phase transformation occurs at the surface layer of both types of coating during the ball-on-disk wear test. The presence of continuous oxide networks in HVOF coatings leads to delamination during the wear test. Overall, the CGS Stellite 21 coatings exhibit better performance than HVOF coatings in wear and oxidation tests

Pre- and post-treatment of α-Tocopherol on cognitive, synaptic plasticity, and mitochondrial disorders of the hippocampus in icv-streptozotocin-induced sporadic Alzheimer’s-like disease in male Wistar rat

ObjectiveMost dementia cases in the elderly are caused by Alzheimer’s disease (AD), a complex, progressive neurological disease. Intracerebroventricular (ICV) administration of streptozotocin (STZ) in rat’s results in aberrant brain insulin signaling, oxidative stress, and mitochondrial dysfunction that impair cognition change neural plasticity, and eventually lead to neuronal death. The current study aims to define the neuroprotective action of alpha-tocopherol in enhancing mitochondrial function and the function of synapses in memory-impaired rats brought on by icv-STZ.MethodsMale Wistar rats were pre-treated with (α-Tocopherol 150 mg/kg) orally once daily for 7 days before and 14 days after being bilaterally injected with icv-STZ (3 mg/kg), while sham group rats received the same volume of STZ solvent. After 2 weeks of icv-STZ infusion, rats were tested for cognitive performance using a behaviors test and then were prepared electrophysiology recordings or sacrificed for biochemical and histopathological assays.ResultsThe cognitive impairment was significantly minimized in the behavioral paradigms for those who had taken α-Tocopherol. In the hippocampus of icv-STZ rat brains, α-Tocopherol ocopherol effectively prevented the loss of glutathione levels and superoxide dismutase enzyme activity, lowered mitochondrial ROS and mitochondrial membrane potential, and also brought about a decrease in Aβ aggregation and neuronal death.ConclusionOur findings demonstrated that by lowering neurobehavioral impairments caused by icv-STZ, oxidative stress, and mitochondrial dysfunction, α-Tocopherol enhanced intracellular calcium homeostasis and corrected neurodegenerative defects in the brain. These findings examine the available approach for delaying AD connected to mitochondrial malfunction and plasticity issues

The investigation of deformation, recovery, recrystallization and precipitation in austenitic HSLA steel analogue alloys

In high strength low alloy steels, the deformation, restoration and precipitation effects which occur during finish rolling in the austenitic condition are difficult to study because of transformation to ferrite and/or martensite on cooling. In the current research program, a series of Fe-31Ni-0.15C analogue alloys (with and without 0.02%Nb), which remain austenitic on cooling to room temperature, were prepared to allow a more detailed study of phenomena occurring during simulated finish rolling. The kinetics of static softening and precipitation were studied using uniaxial compression testing, hardness testing, optical microscopy, and scanning and transmission electron microscopy. Specimens were deformed in uniaxial compression at a constant strain rate of 0.7sec-1 to strains of 0.25, 0.5 and 0.9 in the temperature range of 850-1000°C, followed by holding at the deformation temperature before quenching. Retardation of static recrystallization was produced by either niobium in solution and/or strain-induced precipitates of NbC . Static recrystallization in the niobium steel was retarded when niobium was in solution, due to solute drag, by approximately an order of magnitude compared to the niobium-free steel. However, much stronger retardation in the niobium steel was observed when strain-induced precipitates were formed, due to the interaction between dislocations and strain-induced precipitates. An increase of strain and/or temperature accelerated the rate of recrystallization. Deformation bands and distorted recrystallization twin boundaries were observed in as-deformed structures for all strains. Although grain boundaries were the most potent sites for recrystallization, nucleation at twin boundaries was also observed to occur at a lower rate, after deformation to strains of 0.5 and 0.9. During the early stages of recrystallization, nuclei formed preferentially at serrated boundaries produced by thermally induced boundary migration. The experimental evidence indicated that serrations reached stable sizes and did not progress into recrystallization by strain-induced grain boundaries motion. Rather, recrystallization appeared to proceed from heavily deformed pocket between the serrations by subgrain coalescence. The formation of bands of recrystallized grains in zones around boundaries at high strains indicated the operation of subgrain growth and/or subgrain coalescence mechanisms. Nucleated grains tended to grow into only one side of the two deformed grains. A heavily twinned structure was observed in growing recrystallized grains and after recrystallization, consistent with the low stacking fault energy of these steels. Recrystallized grain size decreased markedly with increasing strain and only slightly with decreasing temperature. The recrystallized grain size data were found to be closely predicted by the equations proposed for statically recrystallized austenite in C-Mn and Nb-microalloyed steels. In addition, existing equations for predicting the recrystallization times were found to be consistent with the experimental kinetics data obtained in the present work. It was observed that the dislocations, subgrain boundaries, and grain boundaries were the preferential sites for strain-induced NbC precipitation. Very small precipitates ( By using an analogue alloy, this work provides the first direct observations of NbC precipitation behaviour in deformed austenite under simulated hot working conditions and confirms, the interactions occurring during finish rolling of low carbon niobium steels, which have been previously inferred using indirect experimental techniques

Effect of cold spray processing parameters on the microstructure, wear, and corrosion behavior of Cu and Cu–Al2O3 coatings deposited on AZ31 alloy substrate

Cold spray coatings represent a newly emerging and recently implemented approach to enhance the poor wear and corrosion resistance of AZ31 magnesium alloy. In this study, the authors applied pure Cu and Cu-50 wt% Al2O3 composite coatings on an AZ31B substrate using the cold spray deposition method and investigated the effects of gas pressure (1, 2, and 3 MPa) and stand-off distance (1, 2, and 3 cm) on their microstructure characteristics. An increase in gas pressure from 1 MPa to 3 MPa resulted in a decrease in porosity, ranging from 33 % to 38 %, across varying stand-off distances. Increasing the stand-off distance from 1 to 3 cm resulted in a nearly four-fold rise in porosity for 2 and 3 MPa pressures and about 1.5 times for 1 MPa. The porosity increased with higher pressure due to the fragmentation of Al2O3 particles but decreased with greater spraying distance due to reduced Al2O3 retention. Additionally, the incorporation of Al2O3 particles into Cu coatings led to a significant improvement in sliding wear resistance, by up to 50 %, compared to the bare substrate. Abrasive wear and delamination were identified as the dominant wear mechanisms for the composite coatings based on friction coefficient values and micromorphology of wear tracks. Electrochemical results indicated a significant increase in the corrosion resistance of the Cu coating compared to both the bare Mg substrate and Cu–Al2O3 coating, attributed to improved resistance to galvanic corrosion

Submerged Friction-Stir Welding (SFSW) Underwater and Under Liquid Nitrogen: An Improved Method to Join Al Alloys to Mg Alloys

Submerged friction-stir welding (SFSW) underwater and under liquid nitrogen is demonstrated as an alternative and improved method for creating fine-grained welds in dissimilar metals. Plates of AZ31 (Mg alloy) and AA5083 H34 were joined by friction-stir welding in three different environments, i.e., in air, water, and liquid nitrogen at 400 rpm and 50 mm/min. The temperature profile, microstructure, scanning electron microscopy (SEM)-energy-dispersive spectroscopy (EDS) analysis, X-ray diffraction (XRD), hardness, and tensile testing results were evaluated. In the stir zone of an air-welded specimen, formation of brittle intermetallic compounds of Al3Mg2, Al12Mg17, and Al2Mg3 contributed to cracking in the weld nugget. These phases were formed because of constitutional liquation. Friction-stir welding underwater and under liquid nitrogen significantly suppresses the formation of intermetallic compounds because of the lower peak temperature. Furthermore, the temperature profiles plotted during this investigation indicate that the largest amount of a dagger T is generated by the weld under liquid nitrogen, which is performed at the lowest temperature. It is shown that in low-temperature FSW, the flow stress is higher, plastic contribution increases, and so adiabatic heating, a result of high strain and high strain-rate deformation, drives the recrystallization process beside frictional heat

The effects of friction stir welding on microstructure and formability of 7075-T6 sheet

The present research aims to investigate the effects of friction stir welding (FSW) on the microstructure and mechanical properties of 7075-T6 aluminum alloy thin sheets. The welding has been performed perpendicular to the rolling direction of the sheets at three rotational speeds of 600, 1000, and 1600 rpm. From mechanical and microstructural points of view, a suitable welding condition has been recognized at 1000 rpm and welding speed of 50 mm/min. It is observed that with increasing the welding rotational speed more precipitates of the base metal are being dissolved in the metallic matrix and their distribution is more uniform. This phenomenon is recognized as the main mechanism of mechanical properties decline at 1600 rpm. Surprisingly 15,000 h after the welding process, the natural aging could not recover the precipitations within the welding zone of the specimen welded by 1600 rpm rotational speed. The effects of FSW on the three-dimensional forming behavior of the joints have been examined by the hemispherical punch stretching (HPS) method. Forming limit diagrams (FLD) of the weldments and the starting material have been measured and compared to each other. The formability examinations have demonstrated a 40% decline in the forming limit of the welded specimens in comparison to the base metal. The main reasons for the smaller formability of the welded specimens are comprehensively discussed