The plastic deformation showing instability has been a subject receiving considerable attention for centuries due to its importance in many industrial processes. For Al alloys, the major instability is the Portevin-Le Chatelier (PLC) effect that appears within a certain region of strain, strain rate and temperature. It manifests itself on the stress — strain curve as serrations associating with the rapid accumulation of plastic deformation within inclined slip bands. The PLC effect has severe practical consequences, which damages the surface quality after the sheet metal forming process and threatens the tensile ductility. Therefore, it is crucial to investigate the fundamental mechanisms underlying the PLC effect and in particular to investigate how it can be tempered by tailoring the material microstructure. In this paper, we review the common interpretations of the PLC effect and summarize the experimental results of the effects of the precipitation and the grain refinement, two conventional strengthening methodologies in Al alloys, on the serrated plastic flow. The effectiveness of solute atom clusters in suppressing the PLC effect is emphasized.

TiC nanoparticles reinforced 2219 aluminum matrix composites were successfully prepared by ultrasonic casting, followed by forging and T6 heat treatment. The friction and wear properties of the disc-to-column were studied under four separate normal values of 5, 10, 20 and 30 N. The increasing hardness value of the nanocomposite may be attributed to the large amount of TiC (i.e., 1.3 wt.% and 1.7 wt.%) introduced to the composites. The friction coefficient of the nanocomposite decreased with the increase of TiC nanoparticles (0–1.7 wt.%) under the same load. But the wear resistance of the TiC/AA2219 nanocomposite increased by 30%–90% as compared to the 2219 matrix alloy. And it decreased with the increasing load. The composite with 0.9 wt.% TiC produced the best results in terms of friction and wear because of its relatively higher hardness and perfect ability to retain a transfer layer of a comparatively larger thickness. On the wear surface, some Al₂O₃ particles were found which aided in the development of protective shear regions and improved the wear resistance. The wear mechanism for the TiC/AA2219 nanocomposite was a combination of adhesive and oxidative wear, with the composites containing hard TiC nanoparticles being mainly abrasive.

In the present work, scandium elements with a series of contents (0.06 wt.%, 0.10 wt.%, 0.14 wt.%, 0.17 wt.%, 0.20 wt.% and 0.25 wt.%) were added in a high Zn-containing Al-Zn-Mg-Cu-Zr alloy and the corresponding as-cast microstructure characteristics including grains and phases were thoroughly investigated. The results indicated that fine grain boundaries existed in these alloys and fine MgZn₂ phases discontinuously distributed on them. Besides, AlZnMgCu eutectic phases and Sc, Zr-containing phases with flocculent morphology were observed. As scandium contents vary from 0.06 wt.% to 0.17 wt.%, the average grain size continuously decreased and its equiaxial characteristics were strengthened. Meanwhile, the content of AlZnMgCu eutectic phase showed a decrease trend. When scandium contents were 0.20 wt.% and 0.25 wt.%, no further enhancement on grain refinement was observed, so as to the reduction of AlZnMgCu eutectic phase content. Besides, Sc, Zr-containing phases with blocky morphology were observed and the alloy with a scandium content of 0.25 wt.% possessed a larger amount of blocky Sc, Zr-containing phase than the alloy with a scandium content of 0.20 wt.%. Grain refinement and reduction of AlZnMgCu eutectic phase content associated with scandium addition were discussed.

The solidification microstructure and mechanical properties of hypoeutectic Al-10Mg₂Si cast alloys with different Cu contents and 0.45 wt.% Er were investigated using an optical microscope, scanning electron microscope, and an electronic universal testing machine. Results showed that Cu and Er could significantly reduce the grain size of the eutectic Mg₂Si phase from 15.4 µm for the base alloy to 5.8 µm for the alloy with 1.5 wt.% Cu and 0.45 wt.% Er. Meanwhile, the needle-like β-Al₅FeSi phase was modified to fine sized irregular shaped Cu, Er and Fe-rich particles with the additions of Cu and Er. The strength and ductility of the Al-10Mg₂Si as-cast alloys were simultaneously improved by the additions of Cu and Er, and the ultimate tensile strength, yield strength and elongation increase from 223 MPa, 136 MPa and 2.1% to 337 MPa, 169 MPa and 4.7%, respectively. The heterogeneous nucleation of Mg₂Si on AlP was avoided by forming Cu, Er and P-containing phases, due to the additions of Cu and Er. Moreover, the Cu and Er atomic clusters and their intermetallic segregated on the surface of the eutectic Mg₂Si and inhibited the growth of the eutectic Mg₂Si, which were responsible for the modification of the eutectic Mg₂Si.

During the process of cross wedge rolling of aluminum alloy hollow shaft, the evolution of its microstructure has an important influence on the mechanical properties of the rolled piece. In order to obtain the microstructure evolution law of aluminum alloy hollow shaft in cross wedge rolling without mandrel, a finite element model is constructed through the finite element software Deform-3D. The influences of rolling temperature, sectional shrinkage, spreading angle and forming angle on the average grain size of rolled piece are studied by numerical simulation of microstructure evolution. The cellular automata method reveals the inherent relationship between the process parameters and the evolution of the microstructure, and provides a reference for optimizing the rolling process parameters of aluminum alloy hollow shafts and improving the forming quality. The results show that the average grain size of the rolled piece increases with the increase of the rolling temperature, decreases with the increase of the sectional shrinkage, and decreases first and then increases with the increase of the spreading angle, and changes little with the increase of the forming angle.

Effects of interrupted ageing (T6I6) and asymmetric rolling on microstructures, mechanical properties, and intergranular corrosion (IGC) behaviors of Al-Mg-Si-Zn alloy were investigated. Results showed that the T6 alloy has the lowest strength and the worst IGC resistance, while the T6I6 alloy has higher strength and better IGC resistance. What’s more, the alloy treated by pre-rolling deformation has higher strength and better IGC resistance; and the alloy treated by the pre-asymmetry rolling achieves the highest strength, the best IGC resistance and lower elongation. The mechanical properties depend on microstructures such as the grain size, texture, dislocation density and precipitation, the grain boundary misorientation and grain boundary microstructure are responsible for the IGC resistance.

While pre-deformation is often conducted before aging treatment to increase the strength and microhardness of 2195 Al-Li alloy, it often increases the fatigue crack growth (FCG) rate and thus reduces the fatigue life of the alloy. To determine the effects and causes of pre-deformation and heat treatment on the mechanical properties and FCG rate of 2195 Al-Li alloy, and to provide a suitable calculation model for the FCG rate under different pre-deformation conditions, 2195 Al-Li alloy specimens with different degrees of pre-rolling (0, 3%, 6%, and 9%) were investigated. The experimental results indicate that with the increase of pre-rolling, the density of the T₁ phase and the uniformity of the S′ distribution and the microhardness, tensile strength, and yield strength of the alloy increase and at the same time the FCG rate increases, and thus the fatigue life is reduced. It was also found that the normalized stress intensity factor of elastic modulus (E) can be applied to correlate the FCG rate of pre-rolled 2195 Al-Li alloy with constant C and K parameters.

In order to prolong the service life of aircraft skin made from AA2524, the effects of laser shock peening (LSP) on fatigue crack growth (FCG) rate and fracture toughness (K_c) of AA2524 were investigated. Multiple LSP treatment was performed on compact tension (CT) specimen from single side and double sides. The surface integrity was measured with Vickers hardness tester, X-ray diffractometer and confocal laser scanning microscope, respectively. FCG rate test and fracture toughness test under plane stress were carried out after LSP treatment. The microstructure features of cross-sections were observed with scanning electron microscope. The results showed that the micro-hardness and residual stress of CT specimens were increased dramatically after LSP treatment. Compared to the base metal (BM), the fatigue life was prolonged by 2.4 times and fracture toughness was increased by 22% after multiple LSP.

Hypereutectic Al-40 wt.% Si alloys were fabricated by the combination of gas atomization and spark plasma sintering (SPS) technology. The effects of holding time (15–60 min) on phase composition, microstructure, density, mechanical properties of Al-Si alloys were investigated by XRD, SEM, a hydrostatic balance, an automatic micro hardness tester and a universal tensile testing machine. The results showed that homogenous distribution of ultrafine primary Si and high density of alloys can be obtained at holding time of 30 min. Compared with primary Si (3.7 µm) fabricated by gas atomization, the average size increased from 5.17 to 7.72 µm with the increase of holding time during SPS process. Overall, the relative density, maximum tensile strength and Vickers hardness of 94.9%, 205 MPa and HV_0.2 196.86 were achieved at holding time of 30 min, respectively. In addition, all the diffraction peaks were corresponded to α-Al or β-Si and no other phase can be detected. Finally, the densification process of SPS was also discussed.

The extruded plate of powder metallurgy AA2024 aluminum alloy was successfully solid-state joined by friction stir welding (FSW) to demonstrate potential applications in the aerospace and automotive industries. For determining the optimal processing parameters of FSW, the microstructure, mechanical properties, and fracture behavior of FSW joints were evaluated. When the processing parameters were optimized with 2000 r/min rotation speed and 100 mm/min traverse speed, high quality welds were achieved. The ultimate tensile strength yield strength and elongation of the joint can reach 415 MPa (85% of the base metal strength), 282 MPa, and 9.5%, respectively. The hardness of the joint gradually decreased from the alloy matrix to the heat-affected zone. The lowest strength and hardness appeared near the heat-affected zone because of the over-aging caused by heat flow from repeated stirring during FSW. The average grain size of the stir zone (2.15 µm) was smaller than that of the base metal (4.43 µm) and the heat-affected zone (5.03 µm), whose grains had <110> preferred orientation.

In this study, a novel punch toolset was developed to investigate the hot stamping of AA6082-T4 sheet. The effect of the process parameters, including forming temperature, punching velocity, friction coefficient, and blank holder force (BHF) on formability was quantified using Taguchi design, analysis of variance (ANOVA) and mathematical statistics. The finite element (FE) model has been established in software Pamstamp for simulation and analysis of their effects on the minimum thickness and thickness variation of the hot-stamped component. The major factors influencing the minimum thickness of the hot-stamped part has been found to be BHF and friction coefficient with influence significance of 35.3% and 34.88%, respectively. Additionally, punch velocity and BHF affect the thickness deviation significantly with influence significance of 40.43% and 35.42%, respectively. Furthermore, a serious thinning occurs on the punch corner region of the hot-stamped cup when the BHF is larger than 2.4 kN. The thickness deviation of the hot-formed cup has been found to be firstly decreased and then increased with the increase of punch velocity. Low friction coefficient between punch and blank led to crack at bottom centre of the cup. Moreover, different type, phenomenon and mechanism of defects occurring during hot stamping process, such as crack and wrinkling, were discussed. The crack mode was dimple-dominated ductile fracture, which was induced by micro-void nucleation, growth and coalescence.

In this paper, 3 mm 6061 aluminum alloy sheets were welded by laser MIG hybrid welding. Based on the experiment, the best welding parameters were determined to ensure the penetration welding. The detailed microstructure, tensile and fatigue fracture morphology and surface fatigue damage of the welded joints were analyzed by optical microscope (OM), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The results show that there are two main kinds of precipitates, one is the long Si rich precipitates at the grain boundaries, the other is the intragranular Cu rich precipitates. The tensile test results show that the tensile strength of the joint is 224 MPa, which is only 70.2% of the base metal. Through the analysis of tensile fracture, there are great differences in the formation of tensile dimple. In the tensile-tensile fatigue test with a stress rate of 0.1, the conditional fatigue limits of base metal and welded joint are 101.9 MPa and 54.4 MPa, respectively. By comparing the fatigue fracture of the welded joints under different stress amplitudes, it was found that the main factor leading to the fracture of the joint is porosity. Through further analysis of the pore defects, it was found that there are transgranular and intergranular propagation ways of microcracks in the pores, and the mixed propagation way was also found.

Micro porosity in aluminum alloys may contribute to fatigue life degradation, which can largely limit the application of alloys. Therefore, the fatigue life of a commercial 7050-T7451 thick plate and an experimental plate with different porosities was compared in this study. The X-ray computed tomography (XCT) was utilized to characterize the size, number density and spatial distribution of porosity inside various samples, and the fracture surface of fatigued specimens was compared by using scanning electron microscope (SEM). The results showed that the fatigue cracks prefer to initiate from constituent particles in the commercial alloy. Whereas the micro porosity is the predominant site for crack nucleation and subsequent failure in the experimental one. The presence of micro porosity in experimental 7050-T7451 thick plate may reduce the fatigue life by an order of magnitude or more compared with the defect-free alloy. The pores close to sample surface are the main fatigue crack initiation site, among which larger and deeper pore leads to a shorter fatigue life. The crack initiation is also affected by the pore geometry and direction. Besides, the overall porosity inside the bulk can affect the crack propagation during fatigue tests.

The effect of T6I6 treatment on the dynamic mechanical and microstructure behaviour of Al-Si-Mg-Cu cast alloy was investigated using split Hopkinson pressure bar (SHPB), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). Besides, the impact resistances of T6I6 and T6 motor shells of new energy vehicles made of Al-Si-Mg-Cu cast alloy were compared using a trolley crash test. The results indicated that the main strengthening-phases of the T6 peak-aged and T6I6 peak-aged alloy were GP zone and β″ precipitates. T6I6 treatment can increase the density and size of β″ precipitates in peak-aged alloy and enhance both its tensile strength (σ_b) and elongation (δ). The dynamic toughness values of T6I6 samples are 50.34 MJ/m³ at 2000 s⁻¹ and 177.34 MJ/m³ at 5000 s⁻¹ which are 20% and 12% higher than those of T6 samples, respectively. Compared with a T6 shell, the overall deformation of T6I6 shell is more uniform during the crash test. At an impact momentum of 3.5×10⁴ kg·m/s, the T6I6 shell breaks down at 0.38 s which is 0.10 s later than the T6 shell.

The effects of solution treatment temperature and holding time on the microstructure and mechanical properties of extruded Al-6.02 wt.%Zn-1.94 wt.%Mg alloy were investigated by differential scanning calorimetry (DSC), optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and tensile test. The results showed that the optimum solution treatment process for the alloy was 470 °C, 2 h. The tensile strength, yield strength, and elongation of the samples after the aging treatment at 120 ° C for 24 h were 486 MPa, 431 MPa, and 14.8%, respectively. The alloy produced more copious recrystallization with the augment of solution temperature and the extension of holding time. While the second phase of η (MgZn₂), and T (AlZnMgCu) in the matrix was not fully re-dissolved under the treatment condition of lower temperature or shorter holding time. Interestingly, the Zr aggregation was observed in the samples treated at 510 ° C for 2 h, which led to the growth of the second phase particles and the increase of their area fraction.

In the industrial production, the dynamic cooling pre-aging treatment was employed to replace the isothermal pre-aging during the continuous heat treatment production of Al-Mg-Si alloy automotive sheets. The effects of dynamic cooling pre-aging treatment on mechanical properties and paint-bake hardening behavior of an Al-Mg-Si alloy sheet are proposed in this study. The scanning electron microscopy, transmission electron microscopy, tensile test, Vickers hardness test, and differential scanning calorimetry were conducted for the purpose. It was found that the dynamic cooling pre-aging treatment at low temperature region led to the decreasing of cluster II, resulting in the deterioration of the ability of the paint-bake hardening. With the increase of the cooling pre-aging temperature, the increasing of cluster II effectively improved the stability against natural aging and the paint-bake hardening ability. The optimized dynamic cooling pre-aging temperature was ∼150 ° C. In this condition, the hardness increase of the alloy sheet with pre-aging treatment is low during storage at room temperature. The high yield strength increment is obtained after paint-bake hardening.

In order to improve the through-thickness homogeneity and properties of aviation aluminum alloy thick plate. The effect of heating-cooling retrogression and re-ageing on the performance of Al-8Zn-2Mg-2Cu alloy thick plate was investigated by hardness tests, electrical conductivity tests and transmission electron microscopy (TEM) observation. Results revealed that, during retrogression heating, the fine pre-precipitates in surface layer dissolve more and the undissolved η′or η phases are more coarsened than that of center layer. During slow cooling after retrogression, precipitates continue coarsening but with a lower rate and the secondary precipitation occurs in both layers. Finer precipitates resulting from the secondary precipitation are more in surface. However, the coarsening and secondary precipitation behaviors are restrained in both layers under quick cooling condition. The electrical conductivity and through-thickness homogeneity of precipitates increases while the hardness decreases with cooling rate decreasing. After the optimized non-isothermal retrogression and re-ageing (NRRA) including air-cooling retrogression, the through-thickness homogeneity which is evaluated by integrated retrogression effects has been improved to 94%. The tensile strength, fracture toughness and exfoliation corrosion grade of Al-8Zn-2Mg-2Cu alloy plate is 619 MPa, 24.7 MPa·m^1/2 and EB, respectively, which indicates that the non-isothermal retrogression and re-aging (NRRA) could improve the mechanical properties and corrosion resistance with higher through-thickness homogeneity.

In the present investigation, the relation of pre-ageing temperature and pre-ageing time to mechanical properties was studied, and a model was established to predict the mechanical properties of AA6005 Al alloy. Compared with the experimental results, the deviation of the proposed model was limited to 8.1%, which showed reasonable accuracy of forecasting. It was found that the performance of AA6005 alloy was better at higher pre-ageing temperature with shorter pre-ageing time than that at T6 temper. The microstructure of the alloy was observed by transmission electron microscopy, and the results showed that high dislocation density and precipitate density existed at 160 °C and 200 °C pre-ageing, which were in good agreement with the model.

A study was conducted to better understand how different parameters, namely, regression aging time and regression aging temperature, affect the creep aging properties, i.e., the creep deformation and performance of Al-Zn-Mg-Cu alloy during regressive reaging. The corresponding creep strain and mechanical properties of samples were studied by conducting creep tests and uniaxial tensile tests. The electrical conductivity was measured using an eddy-current conductivity meter. The microstructures were observed by transmission electron microscopy (TEM). With the increase in regression aging time, the steady creep strain first increased and then decreased, and reached the maximum at 45 min. The steady creep strain increased with the increase in regression aging temperature, and reached the maximum at 200 °C. The level of steady creep strain was determined by precipitation and dislocation recovery. Creep aging strengthens 7B50-RRA treated with regression aging time at 190 °C for 10 min, and the difference in the mechanical properties of alloy becomes smaller. The diffusion of solute atoms reduces the scattering of electrons, leading to a significant improvement in electrical conductivity and stress corrosion cracking (SCC) resistance after creep aging. The findings of this study could help in the application of creep aging forming (CAF) technology in Al-Zn-Mg-Cu alloy under RRA treatment.

The Al-3.40Mg-1.08Sc alloy plates were manufactured by selective laser melting (SLM) at platform temperatures of 35 °C and 200 °C, respectively, and the corrosion performance of them was studied along height direction. The results show that the corrosion resistance of the alloy plate built at platform temperature of 35 °C along height direction is basically the same due to a uniform microstructure; While the corrosion resistance of the alloy plate built at platform temperature of 200 °C along height direction is different. The evolution of microstructure and the distribution of secondary phases are investigated, and the results show that the Cu-rich phases in alloy play a key role on corrosion performance. At higher platform temperature, the cooling rate is relative slow and a certain degree of in situ ageing leads to the significantly different distribution of Cu-rich phases along grain boundary. Specimens built at the platform temperature of 200 °C are inclined to locate at the crossed grain boundary, rather than continuous segregation of Cu-rich phases along grain boundary that is built at platform temperature of 35 °C. Therefore, the corrosion resistance of Al-3.40Mg-1.08Sc alloy plate manufactured at platform temperature of 200 °C is higher, and presents a gradually decreasing trend along height direction.

The corrosion fatigue fracture mechanism of friction stir welding (FSW) joints of 7075 aluminium alloy in 3.5% NaCl solution is investigated. The corrosion fatigue crack source originates from the junction of nugget zone (NZ) and thermo-mechanical affected zone (TMAZ). Multiple crack sources are developed at the same time, and they merge into large cracks along the boundary line of NZ and TMAZ during the propagation stage. Furthermore, a mutual reinforcement coupling always exists between corrosion and cyclic loading during the initiation and propagation of corrosion fatigue crack. It is necessary to consider the effect of welding residual stress for understanding the mechanism of corrosion fatigue fracture of FSW joints.

The influence of different ageing processes on the microstructure, corrosion behaviors and mechanical properties of extruded Al−5.6Zn−1.6Mg−0.05Zr (wt.%) alloy was studied in this work. The changes of morphology, size and distribution of MgZn₂ precipitate with ageing temperature and time were revealed by optical and electron microscopy. Intergranular corrosion (IGC) and exfoliation corrosion (EXCO) tests were carried out to assess the changes in corrosion susceptibility of the tempered alloy, and some white spots on the surface of the sample aged for longer time were found to be precursors of pits. Electrochemical cyclic polarization test depicted the corrosion behavior under different tempers. Ageing influences on the mechanical behaviors of the alloy were revealed by evaluating its microhardness and tensile strength. The microscopic features of the strengthening phases determined by the ageing procedure directly affect the corrosion resistance and mechanical properties of the alloy.