Generally, it is believed that inside the material the smaller grain size is, the higher yield strength is. In addition to this effect, grain refinement method also ensures that the toughness of the material is not reduced. However, it is found that the relationship between the grain size distribution and mechanical properties is contradiction with this law after the properties have been studied in the transverse direction of a large cross-section 7050 aluminum alloy profile. That is, the impact energy and yield strength in the center with a large grain size is higher than those at the edge with the smaller grain size in the thickest section of the profile. Besides that, during the establishment of the yield strength model in over-aging 7050 aluminum alloy, there are two models for the grain boundary strengthening which are Nes model and Hall-Petch model, so the choice from these model is found to affect the final results of the yield strength model. In order to study and understand the reasons for this phenomenon, the difference of mechanical properties distribution in the cross-section of 7050 aluminum extrusion profile has been investigated by impact test and tensile test at normal temperature, meanwhile, the microstructures have been analyzed by OM, EBSD and TEM. The results show that lots of the harder deformation textures, i.e., copper texture in the core of the profile lead to higher yield strength in the core with grain size of 12 mm than that in the edge with grain size of 6 mm. The Taylor factor could be calculated after the solution strengthening by alloying elements, grain boundary strengthening by the sub-grain and the yield stress of the alloy, at last, it reaches to 3.925 in the core, while that is just 2.257 in the edge. Compared with Nes model, the Hall-Petch model is much preferable to the calculation of grain boundary strengthening in yield stress of 7050 aluminum alloys after solid solution treatment. It is established that there is a linear relationship between impact energy and grain size of three over-aging specimens.
Al alloy 2219 (AA2219) is widely used in the aerospace industry, and friction stir welding (FSW) is an ideal method to join it. The ultimate tensile strength of an FSW AA2219-T8 joint can be as high as 344 MPa which is significantly higher than that welded by other methods such as gas tungsten arc welding. However, the thermo-mechanically affected zone (TMAZ) in the FSW joints of AA2219-T6/T8 is a weakness zone of mechanical property and is susceptible to stress corrosion cracking (SCC), but the reasons are not been well understood. In this work, the mechanical and electrochemical properties of different zones in AA2219-T8 joints obtained by the FSW method were studied. The welding thermal cycles during welding were measured using an array of type K thermocouples. During the tensile process of the joints, digital image correlation (DIC) technique and high speed video technique were employed to investigate the deformational behavior and fracture pathway of the TMAZ, respectively. A microcell method was used to study the micro-electrochemical characteristics of the joints with and without stress. The results showed that the minimum strength located at a position where the weighted strengthening effects of both thermal cycles and stir action were the weakest. The DIC results revealed that the deformation concentrated mainly in the TMAZ during the tensile tests. However, due to the different restraints from the nugget zone (NZ) led to a large strain in the root side than that in the crown side. This made the root side susceptible to cracks initiation. In situ tensile testing indicated that cracks occurred only in the TMAZ at 190 MPa, indicating that the protective surface films in the TMAZ were more prone to crack than those in other zones of the joint. This led the TMAZ to be the weakest zone to pitting corrosion in an aggressive environment. Once pits generate in the TMAZ, the local stress will concentrate near the tip of the pitting, resulting in failure.
Chaotic advection could be strengthened mix, mass transfer and heat transfer in the viscous fluid, the melt flow forming was affected by electromagnetic field in the metal solidification process, so were the macro and microstructure of materials. So it was necessary to study chaotic characteristics of semisolid Al alloy in the electromagnetic field. The characterization of chaotic convention and morphology of primary phase were mainly researched in semisolid A356 alloy under the electromagnetic field. The trajectories of the particle in semisolid A356 alloy melt was simulated by the computational fluid dynamics software Fluent, the Kolmogorov entropy and fractal dimension of the flow trajectories of semisolid A356 alloy melt were judged and analyzed. The results showed that chaotic advection may happen in semisolid alloy melt under electromagnetic field. Combined with experiment, the results of process parameters and different current frequencies were compared. The results showed that the primary phase with 59.86 μm in average equal-area circle diameter, 0.71 in average shape factor, 6829.5 nat/s of Kolmogorov entropy and 2.2439 of fractal dimension can be obtained in semisolid A356 alloy with pouring 650 ℃, stirring for 15 s at 30 Hz, and holding at 590 ℃ for 10 min. Meanwhile, the morphology of primary phase can be observed with the best parameters.
In order to obtain the fine, round and uniform distribution primary α phase in semisolid A356 alloy, the different amount of La was added into the alloy melt, and the melt was poured at 650 ℃ and slightly electromagnetically stirred under the condition of 30 Hz and 15 s, then, it was isothermally held at 590 ℃ for 10 min. The microstructure of the samples was observed by OM and SEM. The influences of La and electromagnetic stirring on morphology of primary α phase in semisolid A356 alloy were studied, and the symbolization of the characteristics of morphology of primary α phase by the fractal dimension was discussed in this work. The results showed that the morphology of primary α phase in semisolid A356 alloy was effectively improved by the suitable addition of La, no matter whether the semisolid slurry of A356-La alloy was prepared by electromagnetic stirring or not, the morphology of primary α phase showed better at first and then worse as the amounts of La increases, and the morphology and grain size of primary α phase reach the optimal state when the content of La was 0.4% (mass fraction). At the same time, the average equal-area circle diameter of the morphology of primary phase in semisolid A356-La alloy by electromagnetic stirring was finer than that without stirring, on the other hand, the shape factor was bigger than that without stirring. It implies that the primary α phase in semisolid A356-La alloy by electromagnetic stirring was smaller and more rounded than that without stirring, that is, the morphology of primary α phase in semisolid A356-La alloy by electromagnetic stirring was better than that without stirring. In addition, the real microstructure has fractal characteristics, and it was feasible to describe and analyze the change regularity and even the formation mechanism of the morphology of primary α phase in semisolid aluminum alloy by the principle of fractal geometry. The morphology of primary α phase in semisolid A356 alloy by the different process parameters had different fractal dimension. The fractal dimension of the semisolid primary α phase gradually became smaller with its morphology changed from dendritic-like to particle-like or globular-like.
Owing to excellent mechanical properties, Al alloys are widely used in aerospace, automotive and civil industry. In order to optimize the properties and performance of the currently used Al alloys and/or even design novel Al alloys, the quantitative description of the microstructure during alloys preparation is the key. In recent years, the phase-field simulation coupling with the CALPHAD thermodynamic and atomic mobility databases has become an effective way to quantitatively simulate the microstructure evolution. So far, the accurate thermodynamic database for Al alloys has been established. However, it is not the case for atomic mobility database for Al alloys. The major obstacle lies in the lack of reliable diffusion coefficients in ternary and higher-order Al alloys, and thus there is an urgent need to remedy this situation. In this work, several semi-infinite and finite (thin film) single-phase solid-state diffusion couples in bcc Al-Fe-Mn and fcc Al-Cu-Ni alloys were first prepared. The concentration profiles for all the diffusion couples were then measured by means of EPMA. After that, the pragmatic numerical inverse method, which has been recently developed for high-throughput determination of the interdiffusion coeffi cients in ternary system and validated in several systems, was employed to compute the composition-dependent interdiffusivities in the corresponding systems at 1273 K. In order to eliminate the possibility that different interdiffusivities at the same composition would be obtained from different sets of diffusion couples, only one set of adjustable parameters was used for one system. All the obtained interdiffusivities satisfy the thermodynamic constrains. On the basis of the determined interdiffusivities as well as Fick's second law, all the experimental concentration profiles were reproduced nicely via numerical simulation, which verifies the reliability of the determined interdiffusivities. The further analysis indicates that the pragmatic numerical inverse method can not only realize the determination of reliable composition-dependent interdiffusion coefficients in ternary diffusion couples, but also cover the cases which cannot be dealt with by the traditional Matano-Kirkaldy method, such as the diffusion couples without intersection along their diffusion paths, and the finite (thin film) diffusion couples. In addition, the comparison between the interdiffusivities from semi-infinite diffusion couples and those from finite (thin film) diffusion couples was made, and the probable reason for their difference was also pointed out. All the presently obtained interdiffusivities in bcc Al-Fe-Mn and fcc Al-Cu-Ni alloys will be utilized to develop the accurate atomic mobility databases in ternary Al-Fe-Mn and Al-Cu-Ni systems in the next step.
The Al-Zn-Mg-Cu series alloys has been widely used in the aircraft, automotive, ship-building and nuclear industries for the advantages of excellent combination of low density, high strength to weight ratio, good toughness and high corrosion resistance, etc.. Most of the researchers focused on alloying and heat treatment at aging temperature, however, rare work had paid attentions on the deformation process, and the microstructure evolution and mechanical properties has not been analyzed completely. Grain refinement can not only improve the strength and hardness, but also the plasticity and toughness of the alloy. Thermo-mechanical treatment is an efficient and economical treatment for obtain grain refinement by a combination of the deformation and heat treatment. In the present work, an improved thermo-mechanical processing, double step hot rolling (DHR), including low temperature pre-deformation, intermediate short-term annealing and final hot rolling has been proposed, aiming to investigate the microstructural evolution, strain induced precipitation and grain refinement mechanism of the alloys during the DHR process. A 7055 aluminum plate has also been manufactured by the conventional hot rolling (CHR) route. The corresponding microstructure evolution and mechanical properties were investigated by OM, XRD, TEM, SEM, EBSD and tensile test. The results reveal that the grain refinement is mainly preceded via dislocation rearrangement and low angle grain boundaries migration, which in turn leads to the pinning effects of strain induced precipitates. Low temperature pre-deformation can accelerate the formation and spheroidization of fine precipitates. The pre-deformation makes influence on the morphology and average size of precipitates without changing their area fraction. The precipitates generated by the pre-deformation can exert significant drag force to the migration of the grain boundaries and dislocation movements, which subsequently promotes the formation of dislocation cells. Although some smaller particles have been dissolved into the matrix during intermediate annealing treatment, some particles are still fine and can pin the dislocation boundaries. At the same time, the activated dislocation boundaries rearranged to form polygon sub-grains. Grains are further elongated after the final hot rolling. The low angle grain boundaries (like sub-grain boundaries) into high angle grain boundaries transition will be accelerated if the motion of boundaries is impeded by the particles. And the new small grains formed near the original grain boundaries can finally cause the fine-grained structures. The results indicate that the optimum thermo-mechanical treatment of 7055 aluminum alloy may be solid heat treatment+pre-deformation (300 ℃, 20%)+intermediate annealing (430 ℃, 5 min)+hot deformation (400 ℃, 60%). The elongation of the alloy produced by the proposed process can increase by 25% without strength loss comparing with that of conventional hot rolling. And the present DHR process is supposed to be a good alternative manufacturing process for the aluminum alloys to obtain fine grain structured heat-treatable sheets.
Polishing soft metals using hard abrasives such as diamond, alumina, and silica can easily damage the worn surface by deep scratches and by large material removal due to cutting wear mechanism. An abrasive material with appropriate hardness, hardness/elasticity ratio, and low friction is then highly desirable, which would avoid intense abrasion while at the same time minimize scratching on soft metals. Quasicrystals are characterized by low friction and high hardness/elasticity ratio, making them potentially suitable for use as abrasives for soft metals. It has been pointed out by the authors that AlCuFe quasicrystal abrasive shows a particular smearing dominant wear mechanism and can be used as a special abrasive for flattening soft metals. In this work, the Al62Cu25.5Fe12.5 quasicrystal abrasive was chosen, to compare with conventional hard abrasives such as diamond, alumina and silica, to wear against copper, 2024 aluminum alloy and 304 stainless steel. The surface topography, nano-indentation hardness, smearing coefficient, mass loss and electrochemical impedance were measured and the results indicate that the surface flattening is influenced by the smearing coefficient, a parameter developed to assess the degree of smearing-type wearing. A larger smearing coefficient leads to a more flatten surface at the least expense of mass loss. It is specially noticed that the characteristic smearing mechanism of quasicrystal abrasive produces an obvious surface hardening effect, with the nano-hardness of 304 stainless steel being increased by about 0.3 GPa. The corrosion resistance of the Al alloy is also enhanced due to the formation of a thick and dense passive film.
Aluminum alloy and steel thin sheets have been mostly used in the automotive industry to get a lightweight car body. Nowadays several studies are focused on the joining of aluminum alloy to steel by new welding methods especially by laser welding. In this work dual-beam fiber laser keyhole welding was introduced to joining of 1.5 mm-thick aluminum alloys to 1.8 mm-thick 304 stainless steels in an overlap joint configure. The influences of different laser focusing positions on the weld appearance, interface microstructures and tensile mechanical resistance of the welded joints were studied. As a result, the good weld appearance of the aluminum alloy to stainless steel joints were obtained by dual-beam fiber laser keyhole welding process without any filler materials. The thickness of the intermetallic compound layer of the joint interface is comparatively thin when the laser beam with low energy is focusing on the front. The nano-hardness testing results show that the average hardness of intermetallic compound layer is 9.61 GPa, which is significantly higher than that of the parent stainless steel of 4.12 GPa and aluminum alloy of 1.09 GPa. The fracture of the welded joints occurs on the aluminum alloy/stainless steel interface layer. The highest mechanical resistance of 131 N/mm can be obtained by the low energy laser beam focused on the front.
Friction stir welding (FSW) is a new solid-state joining method which offers several advantages compared with conventional welding methods, including better mechanical properties, lower residual stress and reduced occurrence of defects. It has already been used for joining Al alloys in the aerospace and automotive industries. In spite of the advantages, FSW also has drawbacks, such as the risk of root flaws in single-side welds. Using a bobbin tool instead is a promising way to solve this problem since the root region is avoided. Compared with standard (single-side) FSW techniques, the bobbin tool FSW has an extra shoulder attached to the tip of the probe, namely the lower shoulder. This setup makes BTFSW capable of joining closed profiles like hollow extrusions. Furthermore, root flaws, such as lack of penetration, which occasionally occurred in standard FSWtechiques, can be completely avoided. In this work, 6061-T6 aluminum alloy was welded by using bobbin tool friction stir weld (BTFSW). The influence of BTFSW on the microstructure development and hardness distribution in the weldment has been investigated. The corrosion behaviors of the base metal and weld nugget in 3.5%NaCl (mass fraction) solution were investigated using SEM, XRD and electrochemical measurements. The results showed that the weld surface of 6061-T6 welded by BTFSW is of good quality. No welding defect was detected in the joints. Three microstructural zones, i.e., nugget zone, thermo-mechanically affected zone, and heat affected zone were discernible. The microstructural analysis indicates that the weld nugget region exhibited fine and equiaxed grain structure with an average grain size of ~8 μm, indicating the occurrence of dynamic recrystallization due to severe plastic deformation and thermal exposure. The thermo-mechanically affected zone underwent plastic deformation and recrystallization occured in this zone due to deformation strain and thermal input. The low hardness zone, determined by constructing the hardness distribution profile on cross-section of joint, located at thermo-mechanically affected zone of advancing side. Although 6061-T6 alloys are readily weldable, they suffered from severe softening in the heat affected zone because of the dissolution of Mg2Si precipitates during the weld thermal cycle. BTFSW can improve the corrosion resistance of 6061-T6 aluminum alloy in 3.5%NaCl solution. The corrosion behavior results showed that both anodic dissolution and pitting were observed after the immersion test due to the inhomogeneous microstructure of 6061-T6 aluminum alloy. The corrosion products mainly composed of Al(OH)3 and Al2O3. Furthermore, the corrosion process and mechanism were also discussed.
The prospect of joining titanium and aluminum components into structures is desirable for a wide range of aerospace and automobile industry applications. One of the problems related with the joining processes for dissimilar metals such as Ti and Al is the formation of residual stress in the bonded joint, which has significant effect on the joint mechanical properties. In this work, joining of a titanium alloy to an aluminum alloy by ultrasonic assisted brazing using a Zn-Al filler metal was investigated. The microstructures of the titanium/aluminum brazed joints were determined by OM, SEM and TEM. The local tensile deformation characteristics of the brazed joints were also examined using the digital image correlation (DIC) methodology by mapping the local strain distribution during in situ tensile tests. The results showed that the Ti7Al5Si12 phase and the TiAl3 phase were formed at the titanium/brazing seam interface. The brazing seam was primarily composed of a Zn-rich phase and a Zn-24.14%Al (mass fraction) eutectoid structure. At the aluminum/brazing seam interface, no interfacial reaction layer was observed and the primary phase Zn-Al dendrites nucleated at the aluminum base metal and grew into the inside of the bonding region. A diffusion layer was formed in the aluminum base metal. It was found that the tensile deformation of the brazed joints was highly heterogeneous, which led to the deflection of the crack during propagating in the joint. The fracture initiated at the Zn-rich phases, where contained the highest stress concentration due to their low elastic modulus, and propagated in the Zn-rich phases or through the interface between Zn-rich phase and Zn-Al eutectoid structure.
In recent years, the surface nanocrystallization (SNC) technology has received extensive attentions in the field of metal material. The shot peening and surface mechanical rolling processing technology can form the gradient nanostructured (GNS) layer on the surface of metal. The material surface roughness is large generally. Therefore, the problem how to form the thick, smooth, flawless GNS layer is need to solve urgently. By means of the hybrid surface nanocrystallization (HSNC) method of both supersonic fine particles bombarding (SFPB) and surface mechanical rolling treatment (SMRT), a gradient nanostructured surface layer was formed on 2A14 aluminum alloy plate. The electrochemical corrosion behavior of the HSNC sample at the air of room temperature and low temperature liquid nitrogen was compared with that of the original sample in aqueous solution of 3.5%NaCl. The results showed that grain size increases from about 30 nm at the surface layer gradually to coarse grain size of the matrix when the sample was processed by HSNC. The total thickness of the plastic deformation layer is about 130 μm. The surface roughness Ra is about 0.6 μm with the surface microcrack disappeared. Compared to the original sample, the pitting corrosion resistance of the SFPB samples was not improved and the pitting corrosion resistance of the HSNC samples was improved. The self-corrosion potential and pitting corrosion potential increase respectively from -1.01228 and -0.29666 V in the original sample to -0.67445 and 0.026760 V at the air room temperature of the HSNC sample. The pitting corrosion resistance of the HSNC sample at the air of room temperature was the biggest. The analysis showed that the surface GNS grain, significant increase of the nanocrystal boundaries, the introduction of compressive residual stress and the decrease of surface roughness were beneficial to improve the pitting corrosion resistance.
Al-Cu-Li-X alloy has attractive applications in the aerospace and aeronautic industry due to its excellent combination of mechanical properties and corrosion resisting properties. However, the microstructural complexity, especially the type and distribution of precipitates have effects on the properties. Besides changing the chemical compositions of alloy, the preaged stretch after quenched and aging process is the main method to control the type and distribution of precipitates. In this work, the effect of preaged stretch after quenched on tensile properties, corrosion property, aging response and microstructure of a naturally aged Al-Li alloy were studied by DSC, TEM, tensile test and corrosion test. The results show that the yield strength increases gradually at the stretch range from 0 to 6%, ultimate tensile strength decreases with the increase of stretch from 0 to 3%, and then become stable when stretch over 3%. Accompany with the increasing stretch, the corrosion type of alloy changes from intergranular corrosion to point corrosion. The T34 state of alloy has the least corrosion depth, about 0.03 mm. The aging response character of alloy is changed by preaged stretch. Compared with T4 state, the endothermic peak near 100 ℃ move to higher temperature, and two exothermic peaks near 180 ℃ and 260 ℃ move to lower temperature in T3x state, respectively. Preaged stretch after quenched restrains the precipitation of δ phase in grain and grain boundary with increasing the density of dislocation in grains. The variations of δ phase and dislocation affect the tensile and corrosion properties of alloy.
7050 aluminum alloy is an important structural material widely used in aerospace industry with a high quench sensitivity. In this work, the time-temperature-transformation (TTT) curves of 7050 aluminum alloy were determined by interrupted quench method. The microstructure evolution and phase transformation kinetics during solid solution, isothermal quenching, ageing treatment were studied by TEM and JMA equation．The results show that the nose temperature of TTT curves is about 330 ℃, with the quench sensitivity range of 300~380 ℃. The quenching sensitivity of high temperature range of 400~450 ℃ is lower than that of low temperature range of 210~270 ℃. The laminar equilibrium η phases characterized with nucleus of Al3Zr particles and several needle-shaped S phases are the main precipitations of the supersaturated solid solution decomposes during isothermal holding process. With the increase of holding time, the volume fraction of precipitated phases rises quickly, which coarsen the grain boundary continuously and broaden precipitation free zone (PFZ). While holding far away from the nose temperature, the speed of precipitation slow down and the degree of continuity and coarsening of grain boundary decrease. The values of n in JMA equation vary from 0.50 to 0.65, indicating that the precipitations are mainly laminar precipitated phases and supplementarily needle-shaped phases.
Automobile lightweight can effectively save fuel consumption and reduce CO2 emissions. Aluminum and its alloys are desirable for the automotive industry due to their excellent high-strength to weight ratio. However, due to the introduction of the welding seam, it has brought new changes to the forming process, especially to the forming limit. To establish a reasonable forming limit curve (FLC) analysis method of friction stir welding (FSW) aluminum alloy blank, a new theoretical model was proposed based on the new second order function constitutive model. The main idea is using the differences in mechanical property between the welding and heat affected zone substitution for the hypothesis of geometry groove in the classic M-K theoretical model. The new second order function constitutive model was applied to M-K theoretical model. Eventually, a new FLC theoretical model for FSW aluminum alloy blank was established. Such theoretical model also overcomes the low strain hardening exponent of aluminum alloy material, which leads to a poor regression accuracy by power-exponent function model. The forming limit test for FSW aluminum alloy blank was performed, and the real-time strain was measured by three-dimensional digital speckle strain measurement system (XJTUDIC). Finally, the results of experiments and the theoretical analysis are compared. Compared with the traditional power law, the regression result of the new second order function constitutive model on the stress-strain curve no matter in the initial yield stage or in late deformation stage has a good fitting precision. The maximum fitting error of the power law on the stress-strain curve is more than 12%, but the fitting error of the new second order function constitutive model is less than 1%. The theoretical prediction based on the new second order function constitutive model is significantly better than the theoretical predictions based on power law in predicting the forming limit of FSW aluminum alloy blank. The prediction error of the first principal strain based on the new second order function constitutive model is less than 0.01. While the maximum prediction error of the first principal strain based on the power law is 0.14.
Al alloy 2219 (AA2219) exhibits excellent mechanical properties in a wide temperature range from -250 ℃ to 250 °C, indicating great potential for application in aerospace structures. Compared to fusion welding, friction stir welding (FSW) could significantly improve mechanical properties of the AA2219 joints. Since invented by the welding institute (TWI) of UK in 1991, FSW has been treated as a solid-state joining technique by the commercial companies, which has been in an agreement in most scientific researchers. However, recently a controversy that has been raised over the viewpoint that FSW is a strict solid-state process, and some observations of liquation have been reported, especially in the stir zone of friction stir spot welding (FSSW) joint. However, the phenomenon of liquation in FSW AA2219 joints has not been reported previously. Therefore, the aim of this work is to reveal the evidence of local liquation during FSW AA2219-T8 and its effect on mechanical properties of the joints. In this work, AA2219-T8 plates (8 mm thick) were friction stir welded at a welding speed of 180 mm/min and a rotation speed of 800 r/min using a welding tool with threaded pin. Heat treatment and thermal simulation experiments were carried out to contrast the characteristics of the local liquation regions. A Vickers microhardness testing machine and an in situ SEM imaging tensile test facility were employed to study the effect of local liquation on mechanical properties of the joints. The results showed that the microstructures in the local liquation regions were divorced eutectic, and its formation was related to the coupled thermal-mechanical interaction during the FSW process. In the FSW process, the local high temperature led to constitutional liquation. During the cooling period, the semisolid mixtures decomposed into α(Al) matrices and θ (Al2Cu) particles under stir and material flow actions. The liquation regions had a lower value of hardness than the normal regions in the nugget zone (NZ), making the liquidation region susceptible to cracks initiation and decreasing the ultimate tensile strength and elongation for a local liquation region contained NZ sample. However, the negative effect of local liquation regions on the mechanical properties of the FSW AA2219-T8 joint was less than that of the thermo-mechanically affected zone (TMAZ), since the local liquation regions were only localized and tiny fractions being in the NZ, whereas the TMAZ was whole softened.
Notable properties of aluminum alloy such as high strength-to-weight ratio, easy to be recycled and good welding properties lead to a wide range of applications in marine industry. However, in addition to many advantages, there are also a lot obvious shortcomings in tribological properties. Especially, the passive state film of aluminum alloy could be destroyed by the Cl- in seawater and harsh marine environment, which can erode into the defects and then aggravate the friction behavior, and limit the use of aluminum alloy in the field of marine engineering. In recent years, the super-hydrophobic surfaces are gaining a wide application prospects in the field of marine engineering due to their properties of drag reduction, anti-adhesion and anti-corrosion abilities. In order to improve the tribological properties of aluminum alloy, the amphiphobic aluminum alloy surface is constructed through building dimple of cone frustum texture with depths of 15 and 30 μm on the surface of 5083 warship aluminum alloy by laser processing and changing the surface wettability by coating the nano-SiO2 powders and low surface energy modification. And the tribological performance was examined by high speed reciprocating friction test machine (HSR-2M) in the water/seawater/oil lubrication respectively. The test results show that the surface with dimple depth of 30 μm has stronger amphiphobic performance and tribological performance than that of 15 μm. Compared with the simple texture surface, the amphiphobic surface with both texture and chemical composition can improve the tribological performance significantly. The friction coefficient and the wear loss of amphiphobic surface are minimal in oil. The friction coefficient of amphiphobic surface in seawater is smaller than that in water while the wear loss of the former is bigger. The simulation results showed that the carrying capacity of the lubricating film increases first and then decreases as the increment of the dimple depth. The carrying capacity of the lubricating film is the biggest when the depth of cone frustum was 75 μm. It can be concluded that the amphiphobic surface can significantly improve the tribological properties of aluminum alloy in different lubrications.
In order to improve the inoxidizability of TC4 alloy at high temperatures, hot dip aluminizing process is an efficient and economical way for industrial application. In this process, the wetting of TC4 alloy by molten Al alloy is the main factor which determined the coating quality. In this work, wetting of TC4 alloys by two industrial grade Al alloys (i.e., 6061 Al and 4043 Al alloys) were studied by using the modified sessile drop method at 600~700 ℃ under high vacuum. The results show that Al/Ti system is a typical reactive wetting, and the spreading dynamics can be described by reaction product control model, further the whole wetting behavior can be divided into two stages: the first stage for the nonlinear spreading and the second stage for the linear spreading. The small amount of alloying element Si in the Al alloys can cause significantly segregation at liquid/solid interface and formation of the Si-rich phase (Ti7Al5Si12). Ti7Al5Si12 decomposition is responsible for the nonlinear spreading, and Ti7Al5Si12 decomposition and Al3Ti formation are together responsible for the linear spreading. The formation of precursor film accompanies with the good final wettability.
Variable polarity plasma arc (VPPA) is a kind of source to provide heat and force at welding process. It can remove the oxide layer with high melting point on the surface of base metal using the cleaning action of cathode spots (the special property of VPPA). So variable polarity plasma arc welding (VPPAW) is a very suitable method to join aluminum alloys which always have extremely tenacious surface oxides. It is great significant to understand clearly the physical characteristics of VPPA for predicting welding defects and making the welding process stable. Therefore, modeling and simulating VPPA are necessary and helpful to understand welding process theory and promote its application further. In this work, a three dimensional transient calculated model of VPPA was established. To describe the electrical characteristics of VPPA at different polarities, a sequential electric conducting model was proposed. With finite difference method, the temperature field, fluid flow and current density of VPPA were solved out. And the distribution of plasma arc pressure on the anode surface, as well as its evolution process as the time going on were analyzed. Arc pressure was measured experimentally to verify the calculated model. The results show that the arc temperature field of electrode negative (EN) is more compressed than that of electrode positive (EP). The range of high temperature at EN is a little larger. Arc pressure and current density of EN at central area are both higher than EP. Nonetheless, the magnitude of these values begins to reverse at a certain distance to center in radial direction. Moreover, the arc pressure rapidly responses to welding current. Pressure at EP is about 20% lower than that of EN. The pressure reduces to the lowest value when the current pass through 0. After that, while the current reaches to normal value, the pressure will immediately impact to a larger value, then quickly recover to an average value. Otherwise, to compare the experimental results with calculated results of arc images and arc pressure, they are in good agreement with each other.
2A14 aluminum alloy is the important raw materials of aerospace, which belongs to the heat treatment aluminum alloy. Friction stir welding (FSW) can weld aluminum alloy with high quality, and can avoid the pores and cracks of fusion welding effectively. In order to obtain better mechanical properties of FSW joints, the surface nanocrystallization method is introduced into FSW technology. By means of the hybrid surface nanocrystallization (HSNC) method of both supersonic fine particles bombarding (SFPB) and surface mechanical rolling treatment (SMRT), a smooth gradient nanostructured (GNS) layer was formed on the surface of 2A14 aluminum alloy before FSW. The FSW joints microstructure and fracture morphology of the original and HSNC specimens were researched by OM, SEM and TEM. The results showed that nanostructure layer zone (NLZ) was formed when GNS with shape similar to the "S" line was distributed in the thermal-mechanical affected zone (TMAZ) and the nugget zone (NZ) of the HSNC specimen. The lowest micro-hardness and fracture position of the original specimen occurred on the TMAZ of advancing side (AS). The lowest micro-hardness and fracture position of the HSNC specimen occurred on the NZ. The tensile strength of HSNC specimen was 6.4% higher than the original sample. The elongation of HSNC specimen was 14.1% more than the original specimen. The fracture mode of both specimens was toughness fracture. The fracture morphology of the HSNC was isometric dimple when the fracture morphology of original specimen were non-isometric dimple and avulsion dimple. Analysis showed that the NLZ of the FSW joints was beneficial to improving the strength and the plastic deformation capability simultaneously.
Graphite flakes reinforced Al matrix composites (Gf /Al) with low density, good machining property and high thermal conductivity are considered an excellent heat sink materials used in electronic industry. When the composites are manufactured by liquid method such as liquid infiltration, it is easy to achieve a high thermal conductivity composite. However, the Al4C3 phase would be formed in the composite, which will decrease the corrosion properties of the composites. The powder metallurgy technique could avoid the formation of the Al4C3 phase. In this work, three seized graphite flakes (150, 300, 500 μm) were used to investigate the effect of the graphite flake size on the strength and thermal conductivity of Gf/Al alloy composites. The 50%Gf /Al alloy (volume fraction) composites were fabricated by the powder metallurgy technique. The density of all the three Gf /Al alloy composites were similar to the theoretical density. The graphite flakes had a well bonding with Al alloy matrix without cracks and pores. The (001)Gf basal plane of the graphite flakes were almost parallel to the circular plane (xy plane) of the composites ingot. However, for the small graphite flakes, their (001)Gf basal plane was not well parallel to the xy plane of the composite ingot due to the powder metallurgy process. For the large graphite flakes, they exhibited a good orientation in the xy plane of the composite ingot. The strength of the Gf /Al alloy composites decreased with the increase of the graphite flake size. For the 150 μm graphite flake, the bending strength of the Gf /Al alloy composite was 82 MPa. However, for the 500 μm graphite flake, the bending strength of the composite decreased to 39 MPa. Due to the low strength between the layers of the graphite flake, the cracks were prone to expand in the graphite flake. As the size of the graphite flake increased, this phenomenon became more obviously. It is easy to observe that the graphite flakes peeled off on the fracture surfaces. When the size of the graphite flake increased from 150 μm to 500 μm, the thermal conductivity increased by 63%. The highest thermal conductivity was 604 W/(mK). The interfacial thermal conductance (hc) of the composites were calculated by the Maxwell-Garnett type effective medium approximation model. The hc of 300 and 500 μm graphite flake Gf /Al alloy composites were slightly lower than the theoretical value (calculated by the acoustic mismatch model). However, the hc of the 150 μm graphite flake Gf /Al alloy composite was lower than that of the theoretical value. Besides the size of the graphite flakes, the shape, distribution and defect of the graphite flakes also influenced the thermal conductivity of the composites.
AA 7055 aluminum alloy is a newly advanced Al-Zn-Mg-Cu alloy. It has been wide applied in aviation and aerospace field due to its attractive combined properties, such as high strength, high fracture toughness, good resistance to the growth of fatigue cracks and good stress corrosion resistance, and so on. It is generally believed that the optimum ageing precipitates are responsible for these good properties. However, the detailed information, such as size and its distribution, volume fraction, and morphology of precipitate in this alloy is still not clear. Although TEM is used to determine these information, the results are mostly qualitative. Small angle X-ray scattering (SAXS) provides a direct technique to determine the size, morphology and volume fraction of nano-scale particles and the sampling size is much larger than that in TEM. In this work, the evolution of the precipitates during ageing at 120 and 160 ℃ in AA 7055 aluminum alloy were investigated systematically and quantitatively by SAXS technique. The results show that, when ageing at 120 ℃, the average radius of the precipitates increases with increasing the ageing time. After ageing for 5 h and later, the average radius of the precipitates is 3.3 nm, and its distribution almost keeps stably. The volume fraction of the precipitates is also increased with increasing the ageing time. When ageing from 5 h to 60 h, the volume fraction increases from 2.4% to 5.2%. When ageing at 160 ℃, however, the average radius of the precipitates increases from 3.1 nm to 11.7 nm with increasing the ageing time from 0.5 h to 72 h. The volume fraction of the precipitates increases from 1.4% to 5.4% with increasing the ageing time from 0.5 h to 16 h. After ageing for 16 h and later, the volume fraction of the precipitates keeps stably. Both ageing at 120 and 160 ℃, the morphology of the precipitates is similar to a flat ellipsoid with an axis ratio between 0.2 and 0.3. Based on these quantitative results of the precipitates, the strength models during ageing will be built possibility.
The growing interest for a wide range of usable Al alloy parts with complex shape in industrial field makes selective laser melting (SLM) stand out as a new technology for rapid prototyping manufacturing. The objective of this work is to investigate AlSi10Mg cast aluminum alloy manufacturing by SLM. The investigation involved the influence of process parameters on the relative density and the influence of heat treatment on the microstructure and mechanical properties. High density and performance were achieved. The results show that the tensile strength of the SLMed AlSi10Mg is much higher than that of press wrought AlSi10Mg, but the elongation is as almost same as that of the press wrought AlSi10Mg. The heat treatment has a significant effect on the mechanical properties and microstructure of SLMed AlSi10Mg parts. The mechanical properties changes with the annealing temperature. Compared with the mechanical properties without annealing process, the tensile strength decreases from 507~518 MPa to 378~406 MPa and the elongation increases from 3.0%~3.5% to 6.5%~9.0% when the annealing temperature is 300 ℃ and the soap time is 2 h because of the changes in the morphology and distribution of the Si.
The high strength or flow stress as well as low plastic deformability of 7000 series Al alloys makes it difficult to improve their microstructures and mechanical properties by cold processing, and many advanced alloying methods and processing technologies are continually developed for higher mechanical properties and acceptable elongation. In this work, the cryogenic deformation (rolling) was applied to process high-strength 7050 Al alloys, and its effects on the microstructures and mechanical properties were studied. The results showed that after the pre-cooling with liquid nitrogen, the quenched 7050 Al alloy can obtain much higher rolling reduction, similar to that under warm or hot rolling, and a great number of substructures and high-density dislocations were formed which greatly increased the strength. The higher cryogenic deformability would be mainly related with the higher work-hardening ability at low temperature, while the strength enhancement would be largely attributed to the solution strengthening and dislocation strengthening. The cryogenic deformation can obviously stimulate the ageing process of the quenched 7050 Al alloy, but the direct ageing of the cryogenic-rolling 7050 Al alloy can assure higher strength and acceptable elongation, which would be greatly attributed to the precipitation strengthening and dislocation strengthening, while the recovery and ageing-induced precipitates help improving the tensile elongation. During room-temperature rolling, the formation of GP zones and η′ phases caused by the heats transformed from the deformation as well as their interaction with dislocations leads to the appearance of amounts of shear bands (instability areas), which will easily cause the cracking or edge-cracking of the rolling sheets. However, the cryogenic rolling with distinctly impeding the solute diffusion can result in the suppression of precipitation of the strengthening phases so as to decrease the occurrence of the shear instability areas, and uniform and stable plastic deformation or good work-hardening as well as high-quality rolling sheets are obtained. The excellent plastic deformability of high-strength Al alloys at cryogenic temperatures could be suggested as an effective way to improve the processing of high-strength Al alloys.
Due to the increasing demands for lightweight parts in various fields, such as bicycle, automotive, aircraft and aerospace industries, hydroforming processes have become popular in recent years. Since tubular materials during tube hydroforming are under a bi-axial even tri-axial stress state, which is different from that in the tensile test, it is necessary to test the mechanical properties of the material under bi-axial stress state. Tube bulging test is an advanced method for characterizing the mechanical properties of tubular materials under bi-axial stress state. But there are excessive physical quantities in the theoretical model of tube bulging test for testing the mechanical properties of tubes under bi-axial stress state which are difficult to be obtained during the experiment. In order to solve the problems, a method for directly testing the mechanical properties of tubes under bi-axial stress state was proposed in this work, which will be referred to as "one point method". Because of circular model is characterized by a dominant function expression, theoretical models of both the pole axial curvature radius and the pole thickness during bulging test are derived under supposing the geometrical models for bulging zone as circular. Thus, the mechanical properties of tubes under bi-axial stress state can be obtained only through measuring the bulging height at the pole point during the bulging test, which laid the foundation for the establishment of a simple and reliable method for testing the mechanical properties of the tube online. Based on the above proposed method, the extruded aluminum alloy tubes AA6061 were tested. The results showed that both the pole axial curvature radius and the pole thickness during bulging test can be expressed as display functions pertaining to the bulging height at the pole point. For the theoretical model of the pole axial curvature radius, as the bulging rate increases, the prediction accuracy increases at beginning, and decreases at the end when using circular as the theoretical geometrical models for bulging zone. The prediction accuracy is the highest as the bulging rate is about 13%, the prediction accuracy decreases after the bulging rate is more than 20%. Fortunately, the overall prediction error is small. The maximum error does not exceed ±0.9%. The prediction accuracy of the pole thickness using the theoretical model is almost unaffected by the specimen geometry. When the ratios of length to diameter and diameter to thickness change, the difference is very small, the prediction error is not more than 0.8%. This is very helpful to ensure the accuracy of mechanical testing under bi-axial loading conditions. Using the "one point method", the stress and strain components along the circumferential and axial directions can be simultaneously measured, this laid the foundation for further analysis of the anisotropic property impacting on the flow and subsequent yield under complex stress state.
Al-7Si-Mg alloy castings have extensive applications in automotive industries, and the tensile properties of these alloys including yield strength, ultimate tensile strength and elongation are commonly used to judge their mechanical properties. In this work, the modified precipitation kinetics model, yield strength model and strain hardening model have been proposed to predict the tensile properties of Al-7Si-Mg alloys. The precipitation kinetics model can be used to predict the precipitate microstructure parameters including the precipitate density, size, size distribution, volume fraction, and composition and so on in these alloys, combining which with the strength model, their yield strengths can be obtained. The strain hardening model can be applied to simulate the stress-strain curves during tensile process, and the ultimate tensile strengths and elongations can be obtained by combining this model with the experimental data fitted with the expression (σUTS-σY)=mσY+n+f (Tss). First, the evolution of precipitate microstructure parameters and yield strengths as a function of ageing time were simulated, and then their comparisons with the experimental results were performed. The possible reasons resulting in the deviations between simulated and experimental yield strengths were analyzed. The stress-strain curves during tensile process of Al-7Si-0.36Mg alloy were simulated using strain hardening model, and the influences of ageing treatment and as-cast microstructure refining scale on the parameters of dislocation storage rate, dynamic recovery rate and the stress-strain curves were analyzed. Then, the ultimate tensile strengths and elongations of Al-7Si-0.4Mg alloy aged at different temperatures were predicted which are in better agreement with the experimental results, and the influence of secondary dendrite arm spacing on tensile properties was also analyzed. Finally, the limitation of present model and the factors influencing the prediction precision of tensile properties were outlined.
Effective grain size prediction for aluminum alloy die castings is of great significance to the rational formulation of die casting process parameters and to the improvement of casting mechanical properties. The traditional grain size prediction method cannot give consideration to both the efficiency and accuracy because of its inherent defects. To improve the efficiency and accuracy of predicting grain size for aluminum alloy die castings, this study proposes a prediction method that is based on the genetic algorithm-extreme learning machine (GA-ELM) model. ELM has the characteristics of few parameter settings, fast learning and good generalization performance, but the algorithm randomly generates the initial input layer weight matrix and the hidden layer threshold matrix, which greatly affects the prediction result. By exploiting GA's excellent global optimization ability, the optimal initial input layer weight matrix and the hidden layer threshold matrix for ELM can be found. The establishment of GA-ELM model can considerably improve the prediction accuracy of ELM model. This study uses grain size as the output parameters and relevant die casting process parameters as the input parameters. The castings produced under different die-casting process parameters are obtained experimentally, and the microstructures of specified sections of key casting positions are analyzed and measured to obtain the average grain size of the sec tions, i.e. the output parameters. The GA-ELM model is trained and tested using these data. To verify the superiority of the GA-ELM model in grain size prediction, this study compares the prediction results of GA-ELM model with the GA-BP neural network model and the original ELM model, and eventually verifies the reliability of GA-ELM model prediction results through metallographic structure measurement experiment. The results show that the GA-ELM model has higher prediction accuracy than the GA-BP neural network model and the original ELM model. Besides, its prediction efficiency is higher than the GA-BP model, while is lower than the original ELM model. With fairly high prediction accuracy and efficiency, the GA-ELM model can meet the actual engineering requirements. Furthermore, its prediction reliability and excellent prediction effect are verified by the results of metallographic structure measurement experiment.
Aluminium alloys were widely applied in rail transit, ships and aerospace owing to their unique properties, such as low density, high strength and stiffness, outstanding corrosion resistance and low temperature performance. As a type of structure material, aluminium alloy joining was inevitable. However, these alloys were often considered very difficult to weld using traditional fusion welding technique since the welding seams were often accompanied with metallurgical defects, large deformation and stress. Friction stir welding (FSW), an innovative solid-state welding technology invented at the welding institute (TWI), was seen by designers as an effective joining methods in welding aluminium alloys due to low heat input, small stress-strain and environment friendly. In this work, 0.8 mm thick plate of 6061-T6 aluminium alloy was successfully welded by use of high rotational speed fiction stir welding technology. The microstructure and mechanical property of the butt joints prepared by high rotational speed friction stir welding were analysed in detail. The results show that the well surface topography and excellent bonding interface existed in the nugget zone (NZ) were observed. Both of the microhardness of the weld seam was lower than that of the substrate. The lowest microhardness of the butt joints located between the thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). Compared with the conventional rotational speed, the number of β-Mg2Si, Al2CuMg and Al8Fe2Si precipitated phases existed in the NZ was more, which made the microhardness in the NZ improved significantly. The rod-shaped precipitates (Mg2Si) have the greatest influence on the microhardness. The excellent mechanical properties were obtained at the rotational speed of 8000 r/min and welding speed of 1500 mm/min. The maximum tensile strength was 301.8 MPa, which was 85.8% of the as-received 6061-T6 (351.7 MPa). And the toughness-brittleness fracture mode appeared.