Retrogression And Re-aging Research Articles

Healing the high-temperature-retrogression-caused wide precipitation-free zones in Al-Zn-Mg-Cu alloy via strain-aging induced precipitates

Retrogression and re-aging (RRA) treatments have been demonstrated to effectively increase the corrosion resistance of high-strength Al-Zn-Mg-Cu alloys. However, RRA-treated Al-Zn-Mg-Cu alloys still suffer from poor ductility and intergranular corrosion because the wide precipitation-free zones (PFZs) caused by high-temperature retrogression treatment cannot be eliminated during the following re-aging process. Herein, inspired by the fact that retrogression dissolves part of the existing precipitates to obtain supersaturation in the matrix and dislocations as heterogeneous nucleation sites for precipitation are more prone to accumulate into the soft PFZs, a strategy of introducing pre-deformation after retrogression to trigger the secondary precipitation in the retrogression-caused PFZs during the subsequent re-aging is developed to heal this inherent weakness. Furthermore, the dislocations aggregation around the precipitated phases within grains accelerate the transformation of their morphology from platelet-like to circle-like after re-aging. In particular, the precipitates in the dislocation-dense band structures become coarser than those in the out-of-band regions. The synergy of the secondary precipitation at retrogression-caused PFZs and the circle-like and band-distributed precipitates within grains weakens the strength and electrochemical difference between the grain interior and the grain boundary. Compared with the traditional RRA treatment, this innovative technique further enhances the corrosion resistance and ductility of the fully recrystallized Al-8.99Zn-2.12Mg-1.98Cu-0.092Hf-0.12Zr alloy without sacrificing strength. This finding offers practical and straightforward implications for overcoming the current limitation in RRA-treated Al-Zn-Mg-Cu alloys and has the potential to expand to other aging-hardening alloys.

Effects of Retrogression and Re-aging (RRA) Processes on Corrosion Properties in AA 7020 Aluminium Alloy

Effects of Retrogression and Re-aging (RRA) Processes on Corrosion Properties in AA 7020 Aluminium Alloy

The effect of three types of heat treatment on the hardness and corrosion resistance of Al 2014 alloy

AbstractAl 2014 is a high‐strength aluminum alloy widely used in the aerospace and automotive industries for its mechanical strength. This research examines the effects of three heat treatment processes—retrogression and re‐aging (RRA), T6 standard aging, and a modified RRA with high‐temperature pre‐aging—on the hardness and corrosion resistance of Al 2014. Polarization studies using potentiodynamic electrochemical methods in a 3.5 wt% sodium chloride solution were conducted to evaluate the corrosion resistance. The results showed that heat treatment, which causes precipitation hardening, shifted the corrosion potential (E) toward a more noble direction. The formation of Al2Cu precipitates is associated with improved corrosion resistance. Additionally, samples treated with the T6 process exhibited a higher corrosion current density compared to untreated Al 2014 alloy samples, indicating superior corrosion resistance. Analysis of corroded surfaces using scanning electron microscopy (SEM) with energy‐dispersive X‐ray spectroscopy (EDS) showed evidence of general and pitting corrosion, with distinct patterns among the three heat treatment processes. A comparison of the results revealed that the T6 standard aging process provided the best combination of hardness and corrosion resistance. This was likely due to the formation of stable precipitates during aging. The results also indicated that the RRA process showed good performance, suggesting it is a viable alternative when a balance between hardness and toughness is required. The modified RRA with high‐temperature pre‐aging resulted in lower performance, likely due to overaging, which reduced hardness and corrosion resistance. These findings highlight the significance of heat treatment in improving the corrosion resistance of Al 2014 alloy. This suggests that particular processes can enhance the alloy's durability in corrosive environments, potentially leading to a longer lifespan and reduced maintenance costs. The T6 standard aging process offers the best balance of enhanced hardness and corrosion resistance for Al 2014 alloy, making it ideal for extending lifespan and reducing maintenance costs in corrosive environments. Careful selection of heat treatment is crucial based on specific alloy performance requirements.

Effect of Retrogression with Different Cooling Ways on the Microstructure and Properties of T'/η' Strengthened Al-Zn-Mg-Cu Alloys.

Retrogression and re-aging (RRA) treatment has been proven to effectively overcome the trade-off between strength and corrosion resistance. Current research focuses on the heating rate, temperature, and holding time of retrogression treatment while ignoring the retrogression cooling ways. In this paper, the effects of RRA treatment with different retrogression cooling ways on the microstructure and properties of newly developed T'/η' strengthened Al-Zn-Mg-Cu alloys were investigated by performing tests on mechanical properties, intergranular corrosion (IGC) resistance, and electrochemical corrosion behavior. The results show that the mechanical properties of samples subject to RRA treatment with water-quenching retrogression (ultimate tensile strength, yield strength, and elongation of 419.2 MPa, 370.2 MPa, and 15.9, respectively) are better than those of air-cooled and furnace-cooled samples. The corrosion resistance of water-quenching (IGC depth of 162.2 μm, corrosion current density of 0.833 × 10-5 A/cm2) and furnace-cooled samples (IGC depth of 123.7 μm, corrosion current density of 0.712 × 10-5 A/cm2) is better than that of air-cooled samples. Microstructure characterization reveals that the effect of the retrogression cooling rate on mechanical properties is related to the size of T'/η' precipitates with grains as well as the proportion of T' and η', while the difference in corrosion resistance depends on the continuity of grain boundary precipitates (GBPs). With mechanical properties, corrosion resistance, and time cost taken into consideration, it is appropriate to select water quenching for retrogression. These findings offer valuable insights for further design to achieve superior performance in various applications.

Examining The Phase Formation of Aging and Shallow Cryogenic Process Applied to Aluminum Alloys with Thermal Analysis

In this study, cryogenic treatment was applied to the 7*** series alloy, which is one of the aluminum alloys frequently used in the aviation and space industry, after the retrogression and re-aging process, and the phase formations were examined by thermal analysis. First of all, solution heat treatment was applied at 480 °C for 2 hours and water was given. After quenching, artificial aging heat treatment was applied at 120 °C for 24 hours. To start the RRA (retrogression and re-aging) heat treatment, after artificial aging, retrogression was performed at 200 °C for 10 minutes and quenched. Then, re-aging was performed at 120 °C for 24 hours and the aging process was completed. After the RRA heat treatment, cryogenic treatment was applied for 2 hours at -40 °C, -80 °C respectively. The heat treated samples were analyzed with a differential thermal analyzer and the transformations of GP, η′ and η phases were found. Since the η′ phase is known as the strength-increasing phase in the structure, the activation energies of each sample were calculated using the Augis-Bennet and Kissinger equations. The results showed that the activation energy of the sample treated with -40 cryogenic treatment was 50% less than the sample without cryogenic treatment. This situation proved with the Arrhenius equation that the formation of the η′ phase would be easier.

Effects of Aging Treatments on the Age Hardening Behavior and Microstructures in an Al-Mg-Si-Cu Alloy

In this study, we investigated the effects of modified aging treatments on the microstructures and hardness in a commercial 6016 Al alloy through hardness tests and transmission electron microscopy (TEM) observations. The results demonstrate that many fine needle-like β″ phases contribute to the high hardness of peak-aged (T6) alloys. Over-aging treatments lead to the precipitation of lath-like β′, β″/disordered, or B′/disordered composite phases. Moderate over-aging treatment results in the coarsening of grain boundary precipitates (GBPs) and widening of the precipitate-free zone (PFZ), while heavy over-aging treatment triggers the re-precipitation of Cu-containing GBPs and increases the number density of GBPs. A retrogression and re-aging (RRA) treatment precipitates β″, lath-like β′, and disordered phases, while a two-step aging (T78) treatment precipitates β″, B′, and disordered phases. Both the T78 and the RRA treatments lead to the coarsening of GBPs and the widening of PFZs. The decreased hardness during over-aging treatments is attributed to a combination of coarsening intragranular precipitates and/or wider PFZs. The T78 and RRA tempers achieve 95.5% and 94% of the hardness values of the T6 treatment, respectively. The hardness values of the RRA and T78 treated alloys are related to the finer nano-sized precipitates formed during the high temperature process. These precipitates can compensate for the loss of hardness caused by the increase in the widths of the PFZs and the coarsening of the matrix precipitates. The relationship between the hardness and microstructures such as PFZs and precipitates in the matrix during various heat treatments is elucidated.

Enhancing properties of Al-Zn-Mg-Cu alloy through microalloying and heat treatment

This paper aims to understand the influence of microalloying constituents (Ti, Zr, Sr and Sc), and three heat treatment conditions i.e., T6, retrogression and reaging (RRA) and high temperature aging (HTA) on mechanical and thermal characteristics of modified Al-Zn-Mg-Cu-X alloy using microstructural analyses consist of precipitation characteristics, phase fraction and phase morphology. Addition of 0.25 wt % Sc significantly refines the grain structure, reducing the grain size from 62 μm to 34 μm and increasing Al3Zr, Al3(Zr, Ti), and Al3(Sc, Zr) precipitate fraction. After T6 aging, the alloy exhibits primary precipitates of fine η and T phases, accompanied by secondary intermetallics as well as S phases. RRA treatment led to a slightly larger fraction of coarse Zn and Cu-rich phases, while HTA results in larger η, T, and S phases due to high-temperature aging. The addition of Sc reduces the cooling rate by raising the transition temperature to 532 °C with an enthalpy of 22.1 J/g. T6 treatment shows dissolution of most η phases within the matrix, although RRA and HTA exhibit higher enthalpy due to a greater fraction of η phases. The tensile strength of Al-Zn-Mg-Cu-X alloy follows the following sequence: T6 > RRA > HTA > as-cast condition, regardless of micro-alloy additions. Under T6 conditions, Sc added alloy achieves 30 % higher strength, i.e., 575 MPa, than base alloy due to fine dispersoids. RRA treatment enhances elongation without compromising mechanical properties, resulting in a shift from transgranular brittle fracture to intergranular-dominated mix-mode fracture, with the presence of secondary cracking.

Effect of post-heat treatment on the microstructure, mechanical, and corrosion properties of laser powder bed fused Al–Zn–Mg–Cu–Si–Zr–Er alloys

Three different post-heat treatments, including direct aging (DA), solution heat treatment + precipitate hardening (T6), and retrogression and re-ageing (RRA), were carried out on a Si–Zr–Er modified Al–Zn–Mg–Cu alloy, which is manufactured by laser powder bed fusion (LPBF). The results show that direct aging maintains the cellular structure of the as-printed (AP) alloy, while T6 and RRA promoted the damage and decomposition of the cellular structure. The size of the primary Al3(Er,Zr) phase increased after heat treatment, and secondary Al3(Er,Zr) phases were uniformly distributed in the matrix with a size of 5–20 nm. The Mg2Si phase in the DA alloy was a long strip pattern, but uniformly distributed in the T6 and RRA alloy. The ultimate tensile strength of the AP, DA, T6, and RRA alloys reached 468, 474, 389, and 354 MPa, respectively. The size, shape, and distribution of the Al3(Er,Zr) and Mg2Si phases have a significant impact on their corrosion resistance, with the latter having a stronger effect. To sum up, the strength of the DA alloy is increased, but its corrosion performance is decreased with heat treatment. On the other hand, the strength of the T6 and RRA alloys is somewhat sacrificed, but their plasticity and corrosion resistance are significantly enhanced.

The precipitates and properties evolution behaviors of AlZnMgCu alloy during the retrogression process with slow heating

In order to develop the retrogression and re-aging (RRA) effect on the performance of AlZnMgCu alloy products with large scale, the hardness, electrical conductivity, tensile properties, and exfoliation corrosion resistance of the Al7.7Zn2.1Mg2.0Cu alloy were investigated at two pre-aging temperatures and a 3 °C/min retrogression heating rate. Transmission electron microscopy (TEM) was used to investigate the evolutions of precipitates at different RRA routes. Results revealed that the standard compliant performances could also achieved by setting the pre-aging as the under-aged state when the slow retrogression heating rate existed. Continuous precipitation occurred at the early stage of heating retrogression. The dissolution behavior of the GP zone with the radius lower than the critical value had finished in heating retrogression. The η′ phase re-precipitated firstly in the isothermal retrogression, then grew and coarsened with the retrogression time increase. During the whole retrogression including heating and isothermal stages, the hardness undergone an “up-down-up-down” transversion. After RRA-treated at (105 °C, 24 h + (heating to 190 °C at 3 °C/min)190 °C, 50 min +120 °C, 24 h), excellent corrosion resistance achieved with only a 5.5% sacrifice of tensile strength when compared to peak aged state. Due to the slow temperature-rising process in retrogression for the large section products, the optimized RRA schedule is more practical for industrial applications.

Fatigue behavior of notched and unnotched 7075-T6 aluminum alloy subjected to retrogression and re-aging (RRA) heat treatment and plasma nitriding

Aluminum alloys are widely used in aircraft components, which present the fatigue process as one of the main failure modes. The aeronautical industry has been searching for alternatives to increase the service life and safety of structural components. To address this issue, this work subjected the 7075-T6 alloy to retrogression and re-aging (RRA) heat treatment and plasma nitriding in order to obtain an improvement in the corrosion resistance. The objective was then to evaluate whether such treatments affected the fatigue behavior. Fatigue tests were carried out on the material with a smooth surface and with a notch (theoretical stress concentration factor of 5.7). A maximum likelihood estimation method was used to estimate the S-N curve parameters. The fatigue behavior without notch for the T6 and RRA treatments was similar, which is expected because the resulting microstructure of the RRA condition is composed of a homogeneous distribution of very fine η’ precipitates in the aluminum matrix, similar to the T6 condition. The worst fatigue behavior without notch was found for the material underplasma nitriding for two main reasons: the softening of the microstructure after processing and the substantial increase of the surface roughness. Regarding the notched material, the fatigue performance was reduced to a similar level regardless of the condition.

Improving the stress corrosion resistance of 7075 matrix composites through combination of nanoparticle incorporation and retrogression re-aging treatment

7xxx aluminum matrix composites reinforced with ceramic nanoparticles are expected to possess advantageous comprehensive performance. However, further applications of these composites remain in doubt because it is likely that introducing these extrinsic phases could impair the resistance to stress corrosion crack (SCC). In this work, we used 7075 as the matrix, and TiB2 nanoparticles as the reinforcement, to study the influences of the retrogression and re-aging (RRA) treatment and nanoparticle incorporation on the SCC behavior of 7075 aluminum alloy. The intergranular corrosion test (IGCT) and slow strain rate test (SSRT) results show that both TiB2 and RRA are conducive to the improvements in the SCC resistance. The positive effects of RRA can be two-fold: (i) the lattice distortion is alleviated during the RRA temper; (ii) the coarse and discrete grain boundary precipitates (GBPs). The accumulation of active solute elements, however, also occurs at the grain boundaries, leading to severe intergranular corrosion. Surprisingly, some coarse TiB2 particles larger than the thickness of particle-free zones (PFZs) could obstruct the mass transportation, reducing the susceptibility to intergranular corrosion. In conclusion, the strength and SCC resistance of 7075 alloy can be concurrently improved by the combination of particle incorporation and RAA treatment.

Novel Wear-Resistant Mechanism Induced by MUPZs via RRA Process in Microalloyed High Manganese Steel

Microalloying and heat treatment have been regarded as an efficient way to get higher wear resistance in high manganese steel, and multiscale precipitates can be obtained randomly by the aging process; however, most of the previous work on heat treatment was more concerned with peak aging time and not the synergistic mechanism of different sized precipitates. Here, we propose a novel wear-resistant mechanism by multiscale precipitates regulated via a retrogression and re-aging (RRA) process. Micron, submicron, and nano precipitates are obtained by the RRA process and jointly form micro-scale ultrafine precipitation zones (MUPZs), which can protect the matrix surface and reduce the abrasive embedded probability, thus ameliorating the micro-cutting and micro-plowing mechanisms. This novel wear-resistant mechanism induced by MUPZs shows better effect under high impact energy due to sufficient work hardening caused by the interaction between dislocations and multi-scale precipitates in MUPZs. This work was investigated using SEM, EDS, and TEM, combined with mechanical properties and impact abrasive wear tests.

Microstructures and properties of wire-arc additively manufactured ultra-high strength aluminum alloy under different heat treatments

The microstructures, mechanical properties and corrosion behaviors of the ultra-high strength Al–Zn–Mg–Cu-Sc aluminum alloy fabricated by wire-arc additive manufacturing process using a self-prepared 7B55-Sc filler wire were systematically investigated under different heat treatments. The results showed that the microstructures of the as-deposited, T6, T73, and retrogression and re-aging (RRA) heat treatments were all composed of fine equiaxed grains with a size of about 6.0 μm. The grain boundary precipitates (GBPs) of the as-deposited sample were very coarse and continuously distributed along the grain boundaries, and the intragranular precipitates (IGPs) mainly consisted of a small amount of ηMg(Zn,Cu,Al)2 and η′ phases. After T6 heat treatment, the GBPs became much finer, but still continuously distributed along the grain boundaries. The IGPs mainly consisted of fine GP zones and η′ phases. After RRA heat treatment, the GBPs became coarser and discontinuously distributed along the grain boundaries. The IGPs were composed of η′ phases and ηMg(Zn,Cu,Al)2 phases. After T73 heat treatment, the GBPs became much coarser and sparsely distributed along the grain boundaries. The IGPs mainly consisted of coarser ηMg(Zn,Cu,Al)2 phases. The T6 heat-treated sample achieved the highest tensile strength of 618 MPa. While the strength of T73 heat-treated sample was sacrificed by up to 17% compared with the T6 heat-treated sample, but the corrosion resistance was the best of all heat-treated conditions. After RRA heat treatment, the strength was about 10% lower than the strength of the T6 condition, but the corrosion resistance was better than that of the T6 heat-treated sample.

Effect of retrogression and reaging (RRA) on pitting and stress corrosion cracking (SCC) resistance of stir zone of high strength AA7075-T651 alloy joined by friction stir welding

The main purpose of this study is to investigate the influence of retrogression and re-aging post weld heat treatment (RRA-PWHT) on potentiodynamic corrosion (PC) and stress corrosion cracking resistance (SCC) of stir zone (SZ) of AA7075-T651 alloy butt joints developed using friction stir welding (FSW). FSW was employed to overcome the problems in fusion welding of AA7075-T651 alloy such as solidification cracking, heat affected zone (HAZ) softening, grain coarsening, dissolution of strengthening precipitates and inferior mechanical performance. The SZ of joints were subjected to potentiodynamic corrosion test in 3.56 wt% NaCl solution. The circumferential notch tensile (CNT) specimens were subjected to SCC test at various conditions of axial stress level in 3.56 wt% NaCl solution. The microstructural features of butt joints SZ were characterized using optical (OM), scanning electron (SEM) and transmission electron microscope (TEM). The results disclosed that RRA-PWHT joints exhibited greater PC and SCC resistance compared to as-welded joints. The threshold stress level for the failure of CNT specimens of parent metal, as-welded joints and RRA-PWHT joints was 242 MPa, 175 MPa and 198 MPa, respectively. The threshold stress level of RRA-PWHT joints and as-welded joints is 81.81% and 72.31% of parent metal threshold stress level. It is correlated to increase in vol% and coarsening of grain boundary precipitates with enrichment of copper at the grain boundaries.

Effects of pre-strain and aging treatments on the mechanical property and corrosion resistance of the spray formed ultra-high strength Al-Zn-Mg-Cu alloy

Mechanical property and corrosion resistance are essential performances in ultra-high strength Al-Zn-Mg-Cu alloys for the aerospace and weapon industries. In this study, the pre-strain and aging treatments were used to enhance such performances of the spray formed ultra-high strength Al-Zn-Mg-Cu alloy. After 2% pre-strain with peak aging (PA) and retrogression and re-aging (RRA) treatments, the ultimate tensile strength, yield strength, and elongation can reach to 774 MPa, 735 MPa, 10.7% and 774 MPa, 747 MPa, 9.4%, respectively. The coarser precipitates of the samples with 4% pre-strain can lead to the slight lower strength compared with the samples with 2% pre-strain. The proper combination of pre-strain and aging treatments can also effectively enhance the stress corrosion resistance of the studied alloy by the following reasons. First, the higher strain concentration degree and dislocation density introduced by pre-strain treatments have high internal energy and provide more stored energy for the growth of the passive film. Second, the spacing of grain boundary precipitates under double aging (DA) and RRA states is larger than that under PA state. The width of uniform precipitation free zone (PFZ) with 2% pre-strain is larger than that with 4% pre-strain. Third, after pre-strain treatments, the cracks perpendicular to the deformation direction can be produced in Al 7 Cu 2 Fe phase, which further breaks up such particles. According to the results of Kelvin probe microscopy, the Al 7 Cu 2 Fe phase with smaller dimension can obtain the lower difference of potential to Al matrix. • The strength and corrosion resistance of the alloy were enhanced by pre-strain. • The pre-strain produced cracks in Al 7 Cu 2 Fe particles and further broke up them. • The pre-strain improved the strain concentration degree and dislocation density. • The effect of precipitation free zone and precipitate is better under 2% strain. • The improvement mechanism of pre-strain treatment was revealed.

Investigation on strength, toughness and microstructure of cryogenically-deformed 7A85 aluminum alloy under various aging tempers

The strength, fracture toughness, electrical conductivity, and microstructure evolution of cryogenically-deformed 7A85 aluminum alloy at different aging temperatures and aging times of single-step (T6), double (T74), and retrogression and re-aging (RRA) treatments were investigated. The results showed that the high-strength samples under T6 had low electrical conductivity and fracture toughness, which were significantly improved under T74 and RRA. Upon extending the aging time, the strength and intergranular fracture proportion increased and the fracture toughness decreased under T6, while the strength decreased and the transgranular fracture proportion and fracture toughness increased under T74 and RRA. The electrical conductivities were improved under T6, T74, and RRA by prolonging the aging time. The recommended single-step, double, and RRA aging tempers for cryogenically-deformed 7A85 aluminum alloy were T624 (120 °C/24 h), T742 (120 °C/6 h + 170 °C/2 h), and RRA0.5 (120 °C/24 h + 180 °C/0.5 h + 120 °C/24 h), respectively. By comprehensively considering the conductivity, fracture toughness, and mechanical properties, the optimal aging temper for the cryogenically-deformed 7A85 aluminum alloy was RRA0.5. Compared with T624, the fracture toughness of T742 and RRA0.5 increased by 38.7% with a certain strength loss and by 10.4% without strength loss, respectively. For the RRA treatment, the matrix precipitates were similar to the T6 state and the grain boundary precipitates were similar to the T74 state. The effect of heat treatment on the strength and fracture toughness could be explained by the cutting mechanism, the Orowan mechanism, and the strength difference between the matrix and grain boundaries.

Effect of retrogression temperature on the fracture toughness of 7136 aluminum alloy

7136 aluminum alloy was treated by retrogression and re-aging (RRA) at different retrogression temperatures in this study. The effect of retrogression temperature on the fracture toughness was investigated and the influence mechanism was discussed in detail. When the retrogression temperature is 175 °C, the KIC value of 7136 aluminum alloy reaches a peak of 47.3 MPa m1/2, increasing 59.2% compared to 171 °C. The fracture of the RRA-treated 7136 aluminum alloy is a combination of trans-granular crack and inter-granular crack, while the width of fatigue strips and the volume fraction of dimples are different. Al7Cu2Fe particles distributing along the grain boundary reduce the fracture toughness by weakening the interface bonding force and cracking themselves. TEM results show that retrogression temperature affects the fracture mode by changing the strength difference between the grain boundary and the grain. The grain boundary with a complex dislocation structure makes the dislocation movement more difficult, which is benefit to the fracture toughness of 7136 aluminum alloy.

Effect of heat treatment process on the micro machinability of 7075 aluminum alloy

This work aims to investigate the effect of heat treatment process on the micro machinability of 7075 aluminum alloy,7075 aluminum alloy was treated with T6, T73 and RRA.T6 single-stage aging process is 120 °C × 24 h; T73 two-stage aging process is 140 °C × 8 h + 180 × 12 h; retrogression and reaging (RRA) process is 120 °C × 16 h + 190 °C × 10min + 160 °C × 30 h. Finite element simulation and single-factor testing are used to examine how different heat treatment processes and different cutting parameters affect the cutting force, surface roughness, surface quality, and residual stress of 7075 aluminum alloy during micro machining.Under all three heat treatment processes, the cutting force of 7075 aluminum alloy increases with increasing cutting speed and cutting depth, but it is less affected by feed rate. Under the same cutting parameters, the cutting force of 7075 aluminum alloy under the three heat treatment processes is T73 > T6 > RRA. Under all three heat treatment processes, the surface roughness of 7075 aluminum alloy is negatively correlated with cutting speed and cutting depth, but positively correlated with feed rate. In the meantime, the micro-machined surface quality of 7075-RRA aluminum alloy is worse than that of 7075-T73 and 7075-T6 aluminum alloy. As cutting speed and cutting depth increase, the residual stress of 7075 aluminum alloy displays a typical “ladle” profile under all three heat treatment processes. When cutting speed changes, the machined surface residual tensile stress of 7075 aluminum alloy changes in much the same way under all three heat treatment processes. When cutting depth changes, the residual tensile stress of 7075 aluminum alloy under the three heat treatment processes is T73 > RRA > T6.

Effect of three-dimensional deformation at different temperatures on microstructure, strength, fracture toughness and corrosion resistance of 7A85 aluminum alloy

In this study, 7A85 aluminum alloy was subjected to 20% three-dimensional (3D) deformation at different temperatures (−196, 25, 250, and 480 °C), followed by solid solution, water quenching, 3% cold deformation, and retrogression and re-aging (RRA) treatment. The microstructural evolution and 3D mechanical properties, fracture toughness, electrical conductivity, and corrosion resistance of the alloy under different deformation processes were analyzed for comparison. The results indicated that conducting the 3D deformation process prior to solution treatment significantly improved the strength and fracture toughness of the alloy as compared to the undeformed (UD) specimen, and maintained its good corrosion resistance under the RRA treatment. In addition, with the decrease of the deformation temperature, the greater flow stress strongly sheared and crushed the coarse second-phase particles into fine particles, thus facilitating their full dissolution in the aluminum matrix during subsequent solution treatment and increasing the driving force for aging precipitation. Furthermore, more dislocations were introduced at low-temperature deformation, thus providing more nucleation sites for static recrystallization during solution treatment and refining grains. Ultimately, the densest precipitates and the finest equiaxed grains with the narrowest grain boundary precipitate-free zones (PFZs) were acquired for the cryogenic deformation (CD) sample, resulting in the highest ultimate tensile strength (UTS), elongation (EL) with the least anisotropy, and fracture toughness of the CD specimen. The optimal comprehensive performance was obtained for the CD specimen with the average 3D UTS, average 3D yield strength (YS), average 3D EL, and fracture toughness of 545 MPa, 476 MPa, 16%, and 42.49 MPa·m1/2, respectively, which were respectively 2.3%, 0.2%, 18.5%, and 21.4% higher than those of the UD specimen.

Tailoring precipitation/properties and related mechanisms for a high-strength aluminum alloy plate via low-temperature retrogression and re-aging processes

The retrogression and re-aging (RRA) processes, aimed mainly at tailoring intergranular precipitates, could significantly improve the corrosion resistance (i.e., stress corrosion cracking resistance) without considerably decreasing the strength, which signifies that an efficient control of the size, distribution and evolution of intergranular and intragranular precipitates becomes critical for the integrated properties of the (mid-)thick high-strength Al alloy plates. Compared to RRA process with retrogression at 200 °C (T77), this study investigated the impact of a modified RRA process (MT77) with lower retrogression temperatures (155-175 °C) and first-stage under-aging on the properties of a high-strength AA7050 Al alloy, in combination with detailed precipitate characterization. The study showed that the strength/microhardness of the RRA-treated alloys decreased with raising retrogression temperature and/or prolonging retrogression time, along with the increased electrical conductivity. The rapid responsiveness of microstructure/property typical of retrogression at 200 °C was obviously postponed or decreased by using MT77 process with longer retrogression time that was more suitable for treating the (mid-)thick plates. On the other hand, higher retrogression temperature facilitated more intragranular η precipitates, coarse intergranular precipitates and wide precipitate free zones, which prominently increased the electrical conductivity alongside a considerable strength loss as compared to the MT77-treated alloys. With the preferred MT77 process, the high strength approaching T6 level as well as good corrosion resistance was achieved. However, though a relatively homogeneous through-thickness strength was obtained, some small discrepancies of properties between the central and surface areas of an 86-mm thick 7050 Al alloy plate were observed, possibly related to the quenching sensitivity. The precipitate evolution and mechanistic connection to the properties were discussed and reviewed for high-strength Al alloys along with suggestions for further RRA optimization.