Investigation of the effect of diffusion bonding parameters on microstructure and mechanical properties of 7075 aluminium alloy

In the present study, diffusion bonding of aluminium alloy (AA7075) sheet materials which are used especially in the automobile and aerospace industry has been investigated at temperatures of 425 and 450 °C and pressures of 2 and 3 MPa for 180 min in argon atmosphere. The microstructural and mechanical properties of bonding have been characterized with different welding parameters such as bonding temperature and pressure. The microstructure was characterized by light optical microscope, scanning electron microscope and energy dispersive spectroscopy, while the mechanical properties were determined by tensile-shear tests and microhardness tests. The results obtained are discussed from both the microstructural and mechanical points of view. It was observed in the microstructural investigations that the interfacial oxide layer decreased with increasing of the bonding temperature and pressure. The maximum shear strength was found to be 131 MPa for the Al 7075 sample bonded at 450 °C and 3 MPa for 180 min. It is shown that in certain extent, the bonding temperature and bonding pressure have great effect on the joint shear strength. With the increasing of bonding temperature and pressure, the shear strength of the joints increases due to diffusion of atoms in the interface. The strength achieved after bonding were dependent on interface grain boundary migration and on grain growth during the bonding process. The maximum hardness value of the Al 7075 sample bonded at 450 °C, 3 MPa for 180 min is 92.5 HV_0.2. Increasing hardness with increasing temperature can be attributed to the formation of metallic bond at high temperatures and pressures.     

Tribological Behavior of WC-12Co Air Plasma–Sprayed Coating at Elevated Temperatures

The friction and wear performance of WC-12Co air plasma–sprayed (APS) coating at temperatures of 25–650°C under loads of 8 and 28 N in at atmospheric environment have been studied by a ball-on-disc tribometer. The effect of temperature and load on the tribological behavior of WC-Co coating was investigated. The results show that under a load of 8 N, the wear volume of the coating increases at 250°C due to the coating splat delamination and then it gradually decreases at 350–500°C. The friction could promote the formation of double oxide (CoWO₄), which is beneficial to reduce friction and wear. At higher temperatures, the wear volume increases again due to the removal of oxides. Under a load of 28 N, the wear volume of the coating increases enormously at 250°C due to the serious splat delamination. At 350°C, the load promotes double oxide formation, resulting in an early decrease in the coefficient of friction and a rapid reduction in wear volume. Although the wear volume decreases at 350–500°C, it is 10-fold higher than that under a load of 8 N. Above 500°C, the differences of the wear volumes of coatings under the two loads become less obvious, and similar trends also appear for the coefficients of friction. The synergistic effect between the load and temperature on the friction and wear mechanism of WC-12Co APS coating is discussed.     

Evaluation of thermal debinding of injection-molded boron carbide in an ambient atmosphere

Characterization of thermal debinding of boron carbide green samples in an ambient atmosphere with the temperature range of 350 to 500 °C was investigated. Binders are easier to remove in the ambient atmosphere because of the continuous oxygen supply. XRD results show that the diffraction peak of B₂O₃ appears in debound samples above 350 °C. The diameter and mass of the samples display no significant changes when the debinding temperature is below 400 °C. Moreover, glassy phases are not observed through the analysis on fracture surfaces by scanning electron microscope and debound samples have enough strength for the handling. The linear shrinkage and mass growth of debound samples improve markedly with increasing debinding temperature. Glassy phases are observed when debinding temperature is above 500 °C. Furthermore, the linear shrinkages of these sintered samples debound in the ambient atmosphere below 400 °C are about 18 % which are consistent with the sintered samples debound beforehand in Ar. Therefore, thermal debinding in an ambient atmosphere under suitable temperature (400 °C) is feasible for powder injection-molded B₄C material.     

Deformation behavior and microstructure evolution of wrought magnesium alloys

There are many researches on the deformation behavior of wrought magnesium alloys, such as AZ31, AZ80, AZ91, and ZK60 magnesium alloys at different temperatures and strain rates, but few of them focuses on the deformation behavior of AZ41M and ZK60M alloys, especially under the twin-roll casting (TRC) state. Meanwhile, the existing researches only focus on the grain refinement law of the magnesium alloys under deformation conditions, the deformation mechanism has not been revealed yet. The hot compression behavior of AZ41M and ZK60M magnesium alloys under the temperature and strain rate ranges of 250-400 ℃ and 0.001-1 s-1 are studied by thermal simulation methods using Gleeble 1500 machine and virtual simulation using finite element analysis software. Simulation results show that sine hyperbolic law is the most suitable flow stress model for wider deformation conditions. The most reasonable selected deformation conditions of ZK60M alloy is 350 oC/0.1 s-1 for TRC and 350 oC/1 s-1 for conventional casting (CC), while AZ41M alloy is 300 oC/0.01 s-1 for TRC and 350 oC/0.1 s-1 for CC. Deformation behavior and dynamic recrystallization (DRX) mechanism of them are analyzed at the same deformation conditions. The microstructures of AZ41M and ZK60M alloys are observed at different deformed conditions by optical microscopy (OM) and electron back scatter diffraction (EBSD) and it reveals the flow behavior and deformation mechanism of them. Working harden and work soften contribute to the activation of basal, non-basal slip systems which promote DRX. The proposed research reveals the deformation behavior and mechanism of the AZ41M and ZK 60M magnesium alloys and concludes their optimized deformation parameters and processes and provides a theory basis for their manufacturing and application.     

The influence of heat treatment and hot deformation conditions on γ′ precipitate dissolution of Nimonic 115 superalloy

In precipitation hardenable materials, it is desirable to determine the precipitate dissolution temperature for homogenizing the microstructure by controlling the size and distribution of the precipitates. In this research, the influence of various heat treatment and hot deformation conditions on the kinetics of γ′ dissolution and its morphological evolution in Nimonic 115 was studied. In addition, hot deformation behavior of the material was investigated using hot compression experiments at varying temperature (between 1,050°C and 1,175°C) and strain rates (between 0.01 and 1 s^?1) up to a true strain of 0.8. The values obtained for the solvus temperature of γ′ precipitates by two methods are all in agreement indicating this temperature at approximately 1,165?±?5°C. Through examination of the influence of temperature and strain rate on the hot deformation behavior, it was determined that the experimental flow stress observations could be effectively related to the processing parameters using an Arrhenius relationship. The results indicate that dynamic recrystallization is the main softening mechanism during the high temperature deformation of Nimonic 115, and it can be effectively promoted by increasing the deformation temperature. By deformation at temperatures higher than 1,125°C, a completely recrystallized microstructure is obtained.     

Effective predictions of ultimate tensile strength,peak temperature and grain size of friction stir welded AA2024 alloy joints

A series of welds were made by friction stir welding (FSW) under different welding and rotation speeds. A 2D ultimate tensile strength (UTS) map was developed based on various experimental data to predict the UTS of friction stir welded AA2024 alloy joints. The accuracy of the UTS map was evaluated by comparing the estimated UTS with the corresponding experimental results from the FSW of the same material available in the open literature. Analytical models were developed to estimate the peak temperature and grain size in the nugget zone. The predicted optimal peak temperature and welding and rotation speeds for AA2024 were within the windows of 400–465 °C, 175–350 mm/min and 800–1,200 rpm, respectively, under which the joint tensile strength could be higher than 458 MPa (about 94.6 % of the base metal) and the estimated average grain sizes in the nugget zone were about 2–3.9 μm.     

Dry-Blasting of α- and κ-Al2O3 CVD Hard Coatings: Friction Behaviour and Thermal Stress Relaxation

Post-deposition blasting treatments of cemented carbide cutting tools coated by chemical vapour deposition have gained increasing interest. Within the present work, α- and κ-Al₂O₃ coatings dry-blasted using different pressures as well as globular and edged blasting materials were investigated. A higher pressure resulted in an increase in the compressive residual stresses, as determined by X-ray diffraction. Both blasting materials had a minor influence on the roughness of α-Al₂O₃, while the edged material caused a significant roughness increase for κ-Al₂O₃. For α-Al₂O₃ dry-blasted using the globular material, complete stress relaxation could be observed after annealing at 500 °C, whereas for α-Al₂O₃ dry-blasted using the edged material as well as for the κ-Al₂O₃, an annealing temperature of 900 °C was necessary for stress relaxation. The friction coefficient of the dry-blasted samples first increases with increasing temperature, due to increasing plastic deformation of the blasting material transferred onto the coating surface, until it decreases again above 700 °C, due to softening of the transferred material.     

Effect of silicon carbide volume fraction on the hot workability of 7075 aluminium-based metal–matrix composites

The hot deformation behaviour and microstructure evolution of stir cast 7075Al alloy and 7075Al alloy with 10, 15 and 20 % volume fraction of 20 μm SiCp composites have been studied by using the processing maps. The compression tests were conducted on both alloys and composites in the temperature range of 300–500 °C and the strain rate range of 0.001–1.0 s^?1 to establish the processing map. The dynamic recrystallization and instability zones were identified and validated through micrographs. The composites showed higher flow stress, efficiency and lower instability regimes than alloy. The 15 % volume fraction of SiCp composites achieved better hot workability due to grain refinement, hardening and strengthening of the material.     

Thermorheological behaviour of a lithium lubricating grease

Thermal-induced changes in the viscous and viscoelastic responses of lubricating greases have been investigated through different rheological techniques in a temperature range of 0–175 °C. Small-amplitude oscillatory shear and viscous flow measurements were carried out on a model conventional lithium lubricating grease prepared by inducing the in situ saponification reaction between 12-hydroxystearic acid and hydrated lithium hydroxide. The linear viscoelasticity functions dramatically decrease above 110 °C, but not below this critical temperature, which determines the maximum recommended operating temperature in relation to its durability and resistance under working conditions. Two different regions, below and above this critical temperature, in the plateau modulus versus temperature plot have been detected. From this thermal dependence, a much larger thermal susceptibility of the lubricating grease at temperatures above 110 °C is apparent. The thermo-mechanical reversibility of this material has been studied by applying different combined stress–temperature protocols. Regarding the viscous flow, a minimum in the shear stress versus shear rate plots appeared at temperatures above 60 °C, more pronounced as temperature increases, resulting from material instabilities. The experimental results obtained have been explained on the basis of the thermo-mechanical degradation of the lubricating grease microstructure.     

Structure and microstructure evolution of Al–Mg–Si alloy processed by equal-channel angular pressing

An ultrafine grained Al–Mg–Si alloy was prepared by severe plastic deformation using the equal-channel angular pressing (ECAP) method. Samples were ECAPed through a die with an inner angle of Φ = 90° and outer arc of curvature of ψ = 37° from 1 to 12 ECAP passes at room temperature following route B_c. To analyze the evolution of the microstructure at increasing ECAP passes, X-ray diffraction and electron backscatter diffraction analyses were carried out. The results revealed two distinct processing regimes, namely (i) from 1 to 5 passes, the microstructure evolved from elongated grains and sub-grains to a rather equiaxed array of ultrafine grains and (ii) from 5 to 12 passes where no change in the morphology and average grain size was noticed. In the overall behavior, the boundary misorientation angle and the fraction of high-angle boundaries increase rapidly up to 5 passes and at a lower rate from 5 to 12 passes. The crystallite size decreased down to about 45 nm with the increase in deformation. The influence of deformation on precipitate evolution in the Al–Mg–Si alloy was also studied by differential scanning calorimetry. A significant decrease in the peak temperature associated to the 50% of recrystallization was observed at increasing ECAP passes.     

Effect of temperature on the fracture toughness of A516 Gr70 steel

Fracture toughness JIC and KIC tests were performed on A516 Gr70 carbon steel plate at the temperature ranging from −160°C to 600°C, and test results were analyzed according to ASTM E 813 and ASTM E 399. Unloading compliance J-integral tests were performed on ITCT specimens. The relation between the J_IC value and the test temperature was obtained. It was concluded that the temperature ranging from −15°C to 600°C is the upper shelf region of ductile-brittle transition temperature, and in this temperature range, fracture toughness J_IC values decreased with increasing temperature. The ductile brittle transition temperature of the material may be around −30°C. In the region near −30°C, the tendency of J_IC to decrease with decreasing temperature was significant.     

Studies on wear behavior of nano-intermetallic reinforced Al-base amorphous/nanocrystalline matrix in situ composite

Resistance to wear is an important factor in design and selection of structural components in relative motion against a mating surface. The present work deals with studies on fretting wear behavior of in situ nano-Al₃Ti reinforced Al–Ti–Si amorphous/nanocrystalline matrix composite, processed by high pressure (8 GPa) sintering at room temperature, 350, 400 or 450 °C. The wear experiments were carried out in gross slip fretting regime to investigate the performance of this composite against Al₂O₃ at ambient temperature (22–25 °C) and humidity (50–55%). The highest resistance to fretting wear has been observed in the composites sintered at 400 °C. The fretting wear involves oxidation of Al₃Ti particles in the composite. A continuous, smooth and protective tribolayer is formed on the worn surface of the composite sintered at 400 °C, while fragmentation and spallation leads to a rougher surface and greater wear in the composite sintered at 450 °C.     

The effect of electrolyte current density on the electrochemical machining S-03 material

Selecting an appropriate electrolyte is very important for high-efficiency electrochemical machining novel S-03 special stainless steel aerospace component. A series of experiments were conducted with NaCl, NaNO₃, and their admixture solutions. This research focused on the relationship between current efficiency and current density. The current density effects on surface roughness, machining velocity, and grain boundary corrosion were analyzed. The results showed that: the current efficiency in NaCl electrolyte was 100 % with different concentrations. Under the conditions of 24 V voltage, 30 °C electrolyte, and 0.8 MPa electrolyte pressure, the 10 % NaCl electrolyte can obtain 3.6 mm/min cathode feed speed; the surface roughness is Ra 0.08 μm; and the material removal rate is 411.4 mm³/min. Comparing forward flow to forward flow with added backpressure, we found that: the surface roughness value decreased sharply at 3.6 mm/min in NaCl electrolyte.     

Tribological properties of nanostructured DLC coatings deposited by C60 ion beam

The frictional and wear characteristics of nanostructured DLC films were investigated. The coatings were deposited on silicon substrates by irradiation of a mass-separated C₆₀ ion beam with 5 keV of energy and a deposition temperature ranging from 100 to 450 °C. The effects of deposition temperature on the surface morphology, nano-structure, mechanical properties and tribological characteristics of the coatings were assessed. Results showed that deposition temperature strongly affects the nanostructure and surface morphology of the coatings. Coatings deposited at temperatures exceeding 350–400 °C exhibited an increase in surface roughness as well as compressive stress due to the formation of graphite, which led to a significant increase in the friction coefficient and wear rate. Coatings deposited at 300 °C showed the best tribological properties.     

Porous TiO2 nanowire microsphere constructed by spray drying and its electrochemical lithium storage properties

Porous TiO₂ nanowire microspheres with greatly decreasing agglomeration were successfully prepared by spray drying of hydrothermal reaction suspension, followed by calcination at 350°C. The as‐obtained nanowire microspheres with TiO₂‐B structure reach an initial discharge capacity 210 mAh g^?1 with an irreversible capacity 25 mAh g^?1 at a current density of 20 mA g^?1. For the 450°C‐calcined one with anatase TiO₂ crystal structure, the initial discharge capacity is 245 mAh g^?1 but with a much higher irreversible capacity of 80 mAh g^?1. The hierarchical porous structure in the 350°C‐calcined TiO₂ nanowire microspheres collapsed at 450°C, annihilating the main benefit of nanostructuring. Microsc. Res. Tech. 77:170–175, 2014. © 2013 Wiley Periodicals, Inc.     

Nanoscale Friction Measurements Up to 750 °C

A new experimental capability for elevated temperature nanoscale friction measurement is described. Its stability and resolution were demonstrated in two case studies up to 750 °C. A stainless steel probe was used to study friction between steel and glass at 25, 200 and 400 °C. Friction forces were calibrated at temperature. The friction coefficient increased between 25 and 200 °C, but stick–slip was dominant at 400 °C due to chemical interaction between the stainless steel probe and the glass. This was verified by scanning Energy Dispersive X-ray Spectroscopy analysis. A WC–Co probe was used to study friction on a range of TiN-based and Cr₅₄Al₂₀C₂₆ (so named MAX-phase composition) coatings at 25, 400 and 750 °C. A maximum in friction coefficient was observed at 400 °C. The decrease in friction at 750 °C was associated with the formation of lubricating surface oxides and oxidation-associated surface roughening.     

Wear Behaviour of In Situ Al/TiB<Subscript>2</Subscript> Composite: Influence of the Microstructural Instability

In this present work, the in situ Al (A380)/5 wt%TiB₂ composites were fabricated through salt–melt reaction using halide salts such as potassium hexafluorotitanate (K₂TiF₆) and potassium tetra fluoroborate (KBF₄) salts as precursors. The composites were produced at four different melt temperatures (700, 750, 800, 850 °C). The formation of particle was confirmed from XRD results. The wear behaviour of Al/5 wt% TiB₂ composite was investigated by varying the wear test parameters such as sliding temperature (25, 100, 150, 200 °C), applied load (10, 20, 30, 40 N), sliding velocity (0.4, 0.7, 1, 1.3 m/s). The microstructure of Al/5 wt% TiB₂ composite was correlated with the wear characteristics of the composites. The wear resistance of Al/5 wt% TiB₂ composite was significantly improved due to the presence of TiB₂ particle in Al matrix material. The composite produced at melt temperature 800 °C showed a higher wear resistance at applied load: 10 N, sliding temperature: 25 °C and sliding velocity: 0.7 m/s. The wear mechanism for each of the tested condition was identified from the worn surfaces using scanning electron microscopy (SEM). ANOVA test was carried out to find out significant factor for the wear resistance of composite. The checking of adequacy of experimental value for the wear behaviour of composite for different testing condition was analysed by residual plots using statistical software.     

Tribological Properties of New MoS2 Nanoparticles Prepared by Seed-Assisted Solution Technique

Seed-assisted solution synthesis of hollow IF-MoS₂ nanoparticles allows independent control of particles size and MoS₂ slabs crystallinity. Variations of the reaction mixture composition influence the particle size in the range 50–150 nm. As demonstrated by Rietvelt refinement of the X-ray diffraction patterns, the sulfide crystallinity depends only on the post-treatment temperature (350–750 °C) and not on the particle size. The tribological properties of new MoS₂ nanoparticles prepared by seed-assisted solution technique were investigated and showed a strong decrease in the friction coefficient and wear compared with base oil. Small particles of 50–60-nm size showed the best results. The particle size above 100 nm is deleterious for the lubrication properties since it hinders particles penetration into the contact zone. MoS₂ slabs crystallinity had lesser influence on the lubrication efficiency. However, less-crystallized samples treated at 350 °C showed better lubrication, apparently because of easier exfoliation of the individual MoS₂ slabs, leading to more efficient formation of tribofilm.     

Multi-filed coupling numerical simulation and experimental investigation in electromagnetic riveting

In this work, electromagnetic riveting (EMR) of aluminum alloy rivet was investigated by numerical simulation and experiments. The numerical simulation was carried out by means of ANSYS and LS-DYNA software. The SPHB (Split Hopkinson Pressure Bar) test was performed for 2A10 aluminum alloy rivets, and Johnson-Cook material model was used to describe it. The sequential electromagnetic-thermal-mechanical coupling model was established to analyze magnetic pressures, adiabatic temperature rise, and deformation process. Experiments and microstructure observation were performed to verify the proposed model. The formation of adiabatic shear band and the effective strain distribution were simulated, and the maximum temperature rise was up to 252 °C. The dynamic recrystallization was observed by optical microscopy observation under a discharging voltage of 2.0 kV. The simulation result of rivet heading accorded with experiments.     

Springback evaluation in hot v-bending of Ti-6Al-4V alloy sheets

In this paper, v-bending of Ti-6Al-4V alloy sheet was conducted from room temperature to 850 °C at a fixed velocity of 0.1 mm/s. Punches with punch radii of 1, 2, 4, and 6 mm, as well as several holding times were used. V-bending and springback behaviors were numerically analyzed with an isotropic hardening model that considered rate-dependent effects. Using a punch radius of 1 mm always leads to negative springback in the temperature range of 550–750 °C. This behavior occurs because an arc formed in the transition side near the end of bending and flattened at the end of bending, leading to an internal bending moment which causes specimen to bow inward after unloading. At a punch radius of 2 mm, positive springback occurs at 300–650 °C, while negative springback occurs at 700–750 °C. At punch radii of 4 and 6 mm, positive springback occurs at 600–750 °C, and the angle decreases as temperature increases. At 850 °C, negative springback occurs at a punch radius of 4 mm due to the decrease in yield strength. At a punch radius of 1 mm, cracking occurs at room temperature and 500 °C, while at 2 mm, it occurs only at room temperature. This discrepancy is ascribed to the greater plastic deformation caused by the smaller punch. As holding time increases, the shape of the deformed specimen more closely matches the desired shape.