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Sol–gel synthesized ceramic α-Al2O3 nanoparticles were mixed with commercially pure Al powder from 0 to 2.5 wt% in composition to prepare nanocomposites. Mixing of the powders was carried out by using two different techniques viz. normal wet-mixing and ultrasonication. Among these two techniques, ultrasonication was found to be more effective method to achieve uniform distribution of nanoparticles. After uniform mixing, the samples were prepared through powder metallurgy route. Hardness, compression and bending test were conducted to investigate the effect of α-Al2O3 nanoparticles on Al. The microstructure of the nanocomposites revealed that the nanoparticles impede the grain growth of Al particles during the time of sintering, which resulted in the grain refinement. Moreover, the strength of the nanocomposite was improved by the subsequent incorporation of nanoparticles. However, beyond 2wt% of nanoparticles content, the bending strength and hardness were reduced considerably. Maximum improvement in the values of compressive and bending strength was found as 64% and 50%, respectively. [ABSTRACT FROM AUTHOR]

Effects of different pressures on the isothermal-solidification process of pure Al were studied by molecular dynamics (MD) simulation using the embedded-atom method (EAM). Al was first subjected to a rapid-cooling process, and then it was annealed under different pressures conditions. Mean first-passage times (MFPT) method, Johnson-Mehl-Avrami (JMA) law, and X-ray diffraction (XRD) simulation analysis method were used to qualify the solidification- kinetic processing. Nucleation rate, critical-nucleus size, Avrami exponent, growth exponent, and crystallite size were calculated. Results show that the nucleation rate increases as the pressure increases. The change of critical-nucleation size is not obvious as the pressure increases. With the pressure increasing, growth exponent decreases, indicative of decreased grain-growth rate. It was also found that with the pressure increasing, the Avrami exponent decreases, indicating that the increased pressure has an effect on growth modes during solidification, which changes from three-dimensional growth to one-dimensional growth. Results of XRD simulation shows that with pressure increasing, crystallite size decreases. [ABSTRACT FROM AUTHOR]

Fine, uniform and equiaxed grain structure is one of the most striking features of high-quality Al targets. In general, the imposition of magnetic stirring on the liquid Al during the solidification process is an effective method to promote grain refinement. In the present study, a self–made rotating system of permanent magnets is reported to stir liquid Al during solidification. The temperature change and grain structure of pure Al subjected to a permanent magnet stirring (PMS) are analyzed. The experimental results reveal that the forced convection driven by PMS can lead to a more homogeneous temperature field in the melt. Meanwhile, PMS can greatly refine the grain structure of pure Al and eliminate cavities, but it may induce more gas holes in the ingots. Moreover, a fresh intermittent permanent magnet stirring (I-PMS) approach has been proposed and successfully applied to realize grain refinement of pure Al. Some casting problems can be perfectly solved using suitable processing parameters. Through some valuable experimental works, systematic researches on grain refinement of pure Al under PMS are performed to better understand its refining mechanisms. Therefore, this article further enhances our understanding of related fields and fills the research blank.

• Aluminium matrix composites with varying weight percentages (5–30%) of hollow glass microsphere (HGM) were fabricated via the stir casting method. • Nanoindentation and fatigue crack growth rate (FCGR) testing of the aluminium matrix composites with M were carried out and compared with pure aluminium cast specimens. • The addition of HGM in the aluminium matrix resulted in higher hardness and lower indentation modulus compared to pure aluminium cast specimens. • Aluminium matrix composite reinforced with HGM exhibited extended fatigue life compared to pure aluminium cast specimens, particularly at low stress ratios (0.3). This study investigates nanoindentation and fatigue crack growth rates of pure aluminium and aluminium hollow glass microsphere metal matrix composite (Al + HGM MMCs) cast plates. Using the stir casting method, pure aluminium, and Al + HGM MMCs were fabricated with hollow glass microsphere (HGM) additions ranging from 5 % to 30 % by weight. Nanoindentation techniques were utilized to assess fundamental mechanical properties such as hardness and elastic modulus. This study primarily focuses on analyzing fatigue crack growth behaviour within the linear region of da/dN vs. ΔK graphs for these stir-cast plates. Chevron-notch CT specimens were prepared following ASTM E-647 standards, and the constant amplitude increasing ΔK method was employed to generate Paris curves. Furthermore, the research investigated the influence of stress ratios (R=0.1, 0.2, and 0.3) on the fatigue crack growth rate in both Pure Al and Al + HGM MMCs. The study also determined the threshold and critical stress intensity factor ranges (ΔK th and ΔK c) for these plates. Additionally, Paris constants (C, m) were calculated to characterize the fatigue behaviour of the cast plates. X-ray diffraction analysis was conducted to reveal dislocation densities, crystalline sizes, and micro-strain responses of the fatigue-fractured specimens. Moreover, SEM fractography analysis provided insights into the fracture behaviour and crack branching observed in both pure aluminium and Al + HGM MMCs plates. [ABSTRACT FROM AUTHOR]

Effects of different pressures on the isothermal-solidification process of pure Al were studied by molecular dynamics (MD) simulation using the embedded-atom method (EAM). Al was first subjected to a rapid-cooling process, and then it was annealed under different pressures conditions. Mean first-passage times (MFPT) method, Johnson-Mehl-Avrami (JMA) law, and X-ray diffraction (XRD) simulation analysis method were used to qualify the solidification- kinetic processing. Nucleation rate, critical-nucleus size, Avrami exponent, growth exponent, and crystallite size were calculated. Results show that the nucleation rate increases as the pressure increases. The change of critical-nucleation size is not obvious as the pressure increases. With the pressure increasing, growth exponent decreases, indicative of decreased grain-growth rate. It was also found that with the pressure increasing, the Avrami exponent decreases, indicating that the increased pressure has an effect on growth modes during solidification, which changes from three-dimensional growth to one-dimensional growth. Results of XRD simulation shows that with pressure increasing, crystallite size decreases.

In this paper, the effect of grain sizes on mechanical properties and microstructure evolution of pure Al sheet at cryogenic temperatures were studied by uniaxial tensile tests and microstructure observations. The results show a great exciting strain hardening ability for the sheet with ultra-fine grain (UFG) at cryogenic temperature, and the cooperative increasing in strength and ductility of pure Al at cryogenic temperatures become more and more prominent with the decrease of grain size. With the decrease of grain size, the increase of yield strength by decreasing temperature becomes more dramatic. The mechanism of increase in yield strength at cryogenic temperatures is attributed to the contribution of three temperature-dependent factors: grain boundary strengthening, lattice friction resistance and dislocation strengthening, in which grain boundary strengthening plays a dominant role. The grain size dependence of the strain hardening ability increased with decreasing temperature is mainly related to the inhibition of cross-slip in the grain interiors and the dislocation annihilation at grain boundaries, so that dislocation can effectively multiply and accumulate, which is totally different from the strain softening and poor plasticity of UFG pure Al at room temperature (RT) caused by the enhanced dislocation annihilation. As a result, the trade-off relationship between the strength and plasticity caused by different grain sizes is significantly weakened at cryogenic temperatures, which provides a very potential method for the complex parts forming of UFG sheets and mechanical properties improvement. • The increase of yield strength with decreasing temperature is more significant with decreasing grain size. • The increase of yield strength at cryogenic temperatures is mainly attributed to grain boundary strengthening. • The enhancements in uniform elongation at cryogenic temperatures become more prominent with the decrease of grain size. • With the decrease of temperature, the UFG pure Al gradually transitions from strain softening to strain hardening. • The trade-off relationship between the strength and plasticity is significantly weakened at cryogenic temperatures. [ABSTRACT FROM AUTHOR]

The paper presents the microstructure and mechanical property of pure aluminum (Al) fabricated by multi-pass caliber rolling at room temperature. The finite element modeling (FEM) simulation was performed to explore the changes in rolling force, effective stress and strain, and temperature under various rolling passes. As the number of rolling passes increased, the overall temperature, effective stress, and strain gradually increased, while the maximum rolling force decreased. In addition, due to the dynamic recrystallization (DRX), the average grain size reduced from 1 mm to 14 µm with the increase in rolling passes. The dislocation density increased and it gradually evolved into the high-angle grain boundaries (HAGBs). Moreover, the initial cubic texture rotated to the brass component and finally changed to a mixture of Cube and Brass types. The highest tensile yield strength (TYS), ultimate tensile strength (UTS) and elongation (El.) of caliber rolled pure Al (116 MPa, 135 MPa, and 17%, respectively) can be achieved after 13 rolling passes, which mainly attributed to grain refinement.

Direct friction welding between type 7075-T6 aluminum alloy (AA7075) and low-carbon steel (LCS) is extremely difficult, since AA7075 flash has cracks that reach the weld interface during the welding process. In this study, to obtain a joint with no cracks in the flash, AA7075 and LCS were simultaneously friction welded by using pure Al (CP-Al) as an insert metal. The resulting joint had no cracks in the AA7075 flash. When joints were made at a friction pressure of 36 MPa, the joint strength increased with the applied friction time; a friction time of 6.5 s or longer provided approximately 40% of the tensile strength of the LCS base metal. Moreover, the joint strength for a friction time of 6.5 s increased with increasing forge pressure; a forge pressure of 450 MPa provided approximately 71% of the tensile strength of the base metal. That is, the joint strength was higher than the yield strength of the LCS base metal. This joint did not have any not-joined regions or any intermetallic compound layer at both weld interfaces. These results indicate that a good joint between AA7075 and LCS can be easily made by simultaneous friction welding with CP-Al as an insert metal. [ABSTRACT FROM AUTHOR]

Mg alloys have been gaining attention as the lightest metal material utilized for improvement of fuel efficiency of automobiles. However, their disadvantages, i.e., poor corrosion resistance and poor plastic deformability limit their application fields. The authors have fabricated Al-coated Mg alloy sheets by newly-developed hot extrusion process followed by hot rolling process. In the authors' previous study, it was demonstrated that the sheet exhibited superplasticity without peeling-off of the Al coating layer. However, the mechanism of superplastic deformation of the Al-coated Mg alloy sheet has not been clarified yet. Especially, the reason why the pure Al coating was able to be superplastically deformed was not been clearly understood yet. In the present study, therefore, the deformation mechanisms of both the Mg alloy substrate and the Al coating layer have been closely investigated by observing the change of a micro-grid structure drawn on the surface of the sheet based on an electron beam lithography technique, and by carrying out electron backscatter diffraction analysis of the crystal orientations. From the micro-grid observation, it has been shown that the dominant deformation mechanism of the Mg alloy substrate is grain boundary sliding, while that of the Al coating layer is intragranular dislocation slip. Additionally, from the crystal orientation investigation, it has been demonstrated that the texture of the Mg alloy substrate is randomized during the superplastic deformation, which is indicative of the occurrence of grain boundary sliding. On the other hand, in the Mg alloy substrate near the Al/Mg interface, it has been observed that twinning occurs to accommodate shear stress generated by the deformation limit of the Mg alloy by the Al coating layer. • The Mg alloy substrate underwent dynamic recrystallization after 700% elongation. • Significant grain coarsening occurred in the Al coating layer. • Grain boundary sliding occurred in the superplastic Mg alloy substrate. • Dislocation slip inside grains was dominant in the Al coating layer. • In the Mg alloy near the Al/Mg interface, twinning occurred inside the grains. [ABSTRACT FROM AUTHOR]

Severe plastic deformation (SPD) techniques are known to promote exceptional mechanical properties due to their ability to induce significant grain and cell size refinement. Cell and grain refinement are driven by continuous newly introduced dislocations and their evolution can be followed at the earliest stages of plastic deformation. Pure metals are the most appropriate to study the early deformation processes as they can only strengthen by dislocation rearrangement and cell-to-grain evolution. However, pure metals harden also depend on texture evolution and on the metal stacking fault energy (SFE). Low SFE metals (i.e., copper) strengthen by plastic deformation not only by dislocation rearrangements but also by twinning formation within the grains. While, high SFE metals, (i.e., aluminium) strengthen predominantly by dislocation accumulation and rearrangement with plastic strain. Thence, in the present study, the early stages of plastic deformation were characterized by transmission electron microscopy on pure low SFE Oxygen-Free High Conductivity (OFHC) 99.99% pure Cu and on a high SFE 6N-Al. To induce an almost continuous rise from very-low to low plastic deformation, the two pure metals were subjected to high-pressure torsion (HPT). The resulting strengthening mechanisms were modelled by microstructure quantitative analyses carried out on TEM and then validated through nanoindentation measurements.

In the present investigation, the static electrochemical corrosion behavior of nano (Al2O3)P and nano (SiC)P based aluminum in Fao water was compared. The nanocomposites were fabrication using liquid metallurgy technique. The effect of nanoparticulates weight percentage (5%, 15% and 25%) on the corrosion was studied. The results showed that the Al reinforced with nano (Al2O3) composites exhibited lower corrosion rates that the Al reinforced with nano (SiC)Pcomposites.The corrosion rate was found to be increased by increasing of the weight percentage of the nanoparticles more than 5% nano (Al2O3)P composites exhibited the highest corrosion resistance among all the investigated nanocomposites.

The obtainable minimum structural size and microstructural characteristics of pure Al processed by three different deformation techniques, i.e., cold rolling (CR), dynamic plastic deformation (DPD) and surface mechanical grinding treatment (SMGT), have been investigated by means of transmission electron microscope and orientation mapping. A systematic evaluation of the effects of deformation parameters on structural refinement was performed, mainly focusing on the microstructures at high strains. In contrast to the submicro-scale laminated structures from CR at ambient temperature, which are mainly composed of high angle grain boundaries, deformation at low temperature and high strain rate facilitates formation of high density low angle grain boundaries (LAGBs) in DPD and SMGT samples. With a combination of low temperature, high strain rate and large strain gradient, nanolaminated (NL) structures with a size of only ∼68 nm and a large fraction of LAGBs (∼65%) was fabricated by means of SMGT. NL Al exhibits a record high Vickers hardness of 780 MPa, which has its origin in the capability of accommodating high density dislocations in the form of LAGBs. This study provides an alternative approach to design nanostructures below the traditional refinement limit by generating more LAGBs. [ABSTRACT FROM AUTHOR]

Through X-ray phase contrast method, solid-liquid interface, keyhole, and porosity in laser welding of pure Al could be clearly observed, and the shapes could be quantitatively evaluated. Almost no molten metal occurred at the keyhole bottom, and the temperature gradient and cooling rate there were estimated to be enormously large as compared to other locations in the weld pool. The flow of molten metal around the keyhole bottom was about twice as fast as in the upper part of the weld pool, suggesting that the amount of heat transport around the bottom was larger than in the upper part. Porosity formation is determined by keyhole behavior. In the case of a stable keyhole, obtained with a fan to remove the laser-induced plume, the porosity rate was around 6%, and the gas in the voids consisted entirely of hydrogen. In the case of an unstable keyhole, obtained without fan usage, the porosity rate was around 18%, and the gas included a component of air (nitrogen) in addition to hydrogen. [ABSTRACT FROM AUTHOR]

The recent development of the cold spray technology has made possible the deposition of low porosity and oxide-free coatings with good adhesion and with almost no change in the microstructure of the coated parts. This focuses on the use of low-pressure cold spray process to repair damaged Al-based aircraft skin, aiming at obtaining dense coatings with strong adhesion to the Al2024-T3 alloy. In order to prove the feasibility of using of the cold spray process as a repair process for aircraft skin, series of characterisation/tests including microstructures, microhardness, adhesion strength, three-point bending, surface finish, fatigue test, and corrosion resistance were performed. The obtained results revealed that the low-pressure cold spray process is a suitable for the repair of aircraft skin. [ABSTRACT FROM AUTHOR]

Abstract: Commercially pure aluminum and aluminum blended with 50vol.% and 75vol.% of the intermetallic Mg17Al12 compound feedstock powders were successfully deposited onto as cast AZ91D magnesium substrate using a cold spray process that was employed at low working gas pressure and temperature. The microstructure, mechanical performance and corrosion resistance properties of the Al coatings with and without reinforcement particles were investigated and discussed. The results show that under the same deposition conditions, the coating porosity level can be significantly decreased by adding hard particles into the pure aluminum feedstock. Although, only a small fraction (less than 10vol.%) of the Mg17Al12 particles was retained in the coatings, the hardness, was increased from 47±5HV0.1 to 58±3HV0.1, while the bonding strength was 2 to 3 times higher than the pure Al coating. The spherical Mg17Al12 particles play a role similar to that of the hard ceramic reinforcement particles in the metal matrix composite (MMC) coatings. Electrochemical corrosion experiments also demonstrated that the composite coatings have similar electrochemical characteristics to the bulk Al, and they can provide suitable protection for the as cast AZ91D magnesium material. [Copyright &y& Elsevier]

Abstract: The pitting corrosion of Al, and two Al–Cu alloys, namely (Al–2.5% Cu) and (Al–7.0% Cu) was investigated in gluconic acid (HG) solution through linear polarization and cyclic voltammetry techniques complemented with ex situ EDX and SEM examinations of the electrode surface. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) method of chemical analysis was used to study the effect of alloyed Cu on the rate of the uniform corrosion of Al in these solutions. Results obtained from ICP were compared with those obtained from polarization measurements. For the three Al samples, the anodic responses did not involve active/passive transition due to spontaneous passivation. Addition of HG induced pitting (confirmed from SEM) within the passive oxide film due to the aggressive attack of gluconate (G−) anions. Relationships between pitting potential (E pit), HG concentration, temperature, pH and potential scan rate were established. Alloyed Cu was found to enhance uniform corrosion, while it suppressed pitting attack. Local acidification model is employed to explain passivity breakdown induced by pitting corrosion as a result of the aggressive attack of G− anions. [Copyright &y& Elsevier]

Abstract: Pure Al (99.99%) was processed using high-pressure torsion (HPT), and Vickers microhardness was measured along the diameter of the disk samples. When all hardness values were plotted as a function of equivalent strain, they fell on a single line having three distinctive regions. The hardness increases with the strain in the first region and, after taking a maximum at an equivalent strain of ∼2, the hardness decreases with further straining in the second region. The third region appears at an equivalent strain of ∼6 or higher as a steady state where the hardness remains unchanged. Electron backscatter diffraction analysis and transmission electron microscopy were conducted in the corresponding regions and microstructural evolution with straining was examined. Grain refining mechanism using HPT was discussed based on the change in the hardness and microstructures. [Copyright &y& Elsevier]

The paper presents the microstructure and mechanical property of pure aluminum (Al) fabricated by multi-pass caliber rolling at room temperature. The finite element modeling (FEM) simulation was performed to explore the changes in rolling force, effective stress and strain, and temperature under various rolling passes. As the number of rolling passes increased, the overall temperature, effective stress, and strain gradually increased, while the maximum rolling force decreased. In addition, due to the dynamic recrystallization (DRX), the average grain size reduced from 1 mm to 14 µm with the increase in rolling passes. The dislocation density increased and it gradually evolved into the high-angle grain boundaries (HAGBs). Moreover, the initial cubic texture rotated to the brass component and finally changed to a mixture of Cube and Brass types. The highest tensile yield strength (TYS), ultimate tensile strength (UTS) and elongation (El.) of caliber rolled pure Al (116 MPa, 135 MPa, and 17%, respectively) can be achieved after 13 rolling passes, which mainly attributed to grain refinement. [ABSTRACT FROM AUTHOR]

Experiments were conducted to investigate the ultra-fine microstructures on the surface layer of pure Al by Nd:YAG pulsed laser irradiation. When lower laser power densities were employed, nano-crystalline films with a grain size of 50-120 nm were formed on the pure Al surface. When larger laser power densities were employed, the films with small spherical grains of 150-300 nm were formed. The formation process of nano-crystalline films on the pure Al surface was explained by an increase in the laser power density of the solidification process following the one-dimensional numerical simulation of the thermal process. The finer nano-structures were significant features, which were also confirmed by TEM. XRD showed that nano-structured films had almost the same phase compositions with the original pure Al surface. It was found that the micro-hardness values of all these Al nano-films were higher than that of pure Al without laser treatment. [ABSTRACT FROM AUTHOR]

Abstract: The influence of various concentrations of NaClO4, as a pitting corrosion agent, on the corrosion behaviour of pure Al, and two Al–Cu alloys, namely (Al+2.5wt% Cu) and (Al+7wt% Cu) alloys in 1.0M Na2SO4 solution was investigated by potentiodynamic polarization and potentiostatic techniques at 25°C. Measurements were conducted under the influence of various experimental conditions, complemented by ex situ energy dispersive X-ray (EDX) and scanning electron microscopy (SEM) examinations of the electrode surface. In free perchlorate sulphate solutions, for the three Al samples, the anodic polarization exhibits an active/passive transition. The active dissolution region involves an anodic peak (peak A) which is assigned to the formation of Al2O3 passive film on the electrode surface. The passive region extends up to 1500mV with almost constant current density (j pass) without exhibiting a critical breakdown potential or showing any evidence of pitting attack. For the three Al samples, addition of ClO4 − ions to the sulphate solution stimulates their active anodic dissolution and tends to induce pitting corrosion within the oxide passive region. Pitting corrosion was confirmed by SEM examination of the electrode surface. The pitting potential decreases with increasing ClO4 − ion concentration indicating a decrease in pitting corrosion resistance. The susceptibility of the three Al samples towards pitting corrosion decreases in the order: Al>(Al+2.5wt% Cu) alloy>(Al+7wt% Cu) alloy. Potentiostatic measurements showed that the rate of pitting initiation increases with increasing ClO4 − ion concentration and applied step anodic potential, while it decreases with increasing %Cu in the Al samples. The inhibitive effect of SO4 2− ions was also discussed. [Copyright &y& Elsevier]

A facility is developed for high-pressure torsion (HPT) using ring samples and the results are compared with those of conventional disk HPT samples. With pure Al (99.99%), both types of HPT are consistent. When hardness values are plotted against equivalent strain, they fall on a single line having a maximum at a strain of ∼2 followed by a steady-state at a strain of ∼6 or more where hardness remains constant with straining. [Copyright &y& Elsevier]

This paper describes the use of the potentiodynamic polarization and electrochemical impedance spectroscopy technique (EIS) in order to study the corrosion inhibition process of pure Al, (Al + 6%Cu) and (Al + 6%Si) alloys in 1.0 M HCl solution at the open circuit potential (OCP) in the temperature range 10–60 °C. Dodecyl phenol ethoxidate as a non-ionic surfactant (NS) inhibitor has been examined. The Nyquist diagrams consisted of a capacitive semicircle at high frequencies followed by a well defined inductive loop at low frequency values. The impedance measurements were interpreted according to a suitable equivalent circuit. The results obtained showed that the addition of the surfactant inhibits the hydrochloric acid corrosion of the three Al samples. The inhibition occurs through adsorption of the surfactant on the metal surface without modifying the mechanism of corrosion process. Potentiodynamic polarization measurements showed that the surfactant acts predominately as anodic inhibitor. The inhibition efficiency increases with an increase in the surfactant concentration, but decreases with an increase in temperature. Maximum inhibition is observed around its critical micelle concentration (CMC). The inhibition efficiency for the three Al samples decreases in the order: (Al + 6%Si) > (Al + 6%Cu) > Al. Kinetic-model and Frumkin adsorption isotherm fit well the experimental data. Thermodynamic functions for both dissolution and adsorption processes were determined. [Copyright &y& Elsevier]

Severe plastic deformation (SPD) techniques are known to promote exceptional mechanical properties due to their ability to induce significant grain and cell size refinement. Cell and grain refinement are driven by continuous newly introduced dislocations and their evolution can be followed at the earliest stages of plastic deformation. Pure metals are the most appropriate to study the early deformation processes as they can only strengthen by dislocation rearrangement and cell-to-grain evolution. However, pure metals harden also depend on texture evolution and on the metal stacking fault energy (SFE). Low SFE metals (i.e., copper) strengthen by plastic deformation not only by dislocation rearrangements but also by twinning formation within the grains. While, high SFE metals, (i.e., aluminium) strengthen predominantly by dislocation accumulation and rearrangement with plastic strain. Thence, in the present study, the early stages of plastic deformation were characterized by transmission electron microscopy on pure low SFE Oxygen-Free High Conductivity (OFHC) 99.99% pure Cu and on a high SFE 6N-Al. To induce an almost continuous rise from very-low to low plastic deformation, the two pure metals were subjected to high-pressure torsion (HPT). The resulting strengthening mechanisms were modelled by microstructure quantitative analyses carried out on TEM and then validated through nanoindentation measurements. [ABSTRACT FROM AUTHOR]