Precipitate-free Zones Research Articles

A comparative study on cryogenic formability and paint baking property of AA7075 alloy sheets tempered at fast retrogressed and pre-aged conditions

Forming at cryogenic temperatures provides a new approach to the formability improvement of 7000-series high-strength aluminum alloys. However, the cryogenic forming process based on solution-treated (W) or peak artificial aged (T6) alloys cannot meet the demands of autobody manufacturing for good formability, high post-formed strength and high efficiency simultaneously. The fast retrogression (FR) treatment of AA7075-T6 and pre-aging (PA) treatment of AA7075-W are elaborately designed to tailor a pre-hardened AA7075 alloy, which has good cryogenic formability and relatively high initial strength, and can be further strengthened by paint baking in high efficiency. The results show the optimum fast retrogression temperature for AA7075-FR is 400 °C, at which the sample can acquire a cryogenic elongation of 29.2% and obtain 90% of initial T6 sheet tensile strength by the combined effect of cryogenic pre-deformation and subsequent paint baking treatment. The optimum pre-aging temperature for AA7075-PA is 120 °C, at which the sample can acquire a cryogenic elongation of 31.0% and exceed 96% of tensile strength of AA7075-T6 after paint baking treatment. The comparative study demonstrates that the supersaturated solid solution matrix with an appropriate proportion of GP zones and η′ phases, but few Cr-containing dispersoids and precipitate free zones, is an ideal pre-hardened microstructure to get a good combination of cryogenic formability and final strength. The precipitation of GP zones and η′ phases is prior to the formation of precipitate free zones during pre-aging of AA7075-W, making the pre-aged hardening cryogenic forming process more suitable for automobile industry.

Asymmetric and anisotropic quench sensitivity of ZA81M magnesium alloy

The quench sensitivity of the ZA81M magnesium alloy is studied by combining experiment and crystal plasticity finite element simulation. The quench sensitivity is high in tension and low in compression making the single-valued quench sensitivity calculated from hardness insufficient. So, there is different loss rate of strength, along different test directions of the alloy, due to slow quenching, i.e., the quench sensitivity is asymmetric and anisotropic. The coarsening of the precipitate and widening of the precipitate-free zone are the origin of the quench sensitivity. Crystal plasticity finite element simulation is proposed to calculate the quench sensitivity. The phenomenological constitutive model is used to explicitly consider the strengthening effect of precipitates on different slip and twinning systems. So, the precipitate characteristics of the water-quenched and air-cooled alloy are easily captured. The anisotropy and asymmetry of the quench sensitivity in two perpendicular sample planes are determined by simulation. Such a method can be useful for determining the quench sensitivity of precipitation-hardening alloys with low crystal symmetry and with precipitates having different strengthening efficiencies on different slip or twinning systems.

Identifying multiple synergistic factors on the susceptibility to stress relaxation cracking in variously heat-treated weldments

The 347H austenitic stainless steel has been widely used for pressure vessels and pipeline (PVP) applications due to its excellent creep and corrosion resistance, which fit ideally to the harsh conditions in petrochemical industries, fossil fuel or nuclear power plants, and modern energy storages. However, a failure mode has been commonly observed with cracks emerging at the heat affected zone (HAZ) of weldments during post-weld heat treatment (PWHT) or under intermediate to high temperature service conditions. This phenomenon is termed as Stress Relaxation Cracking (SRC) since the purpose of PWHT is to relieve the welding-induced residual stress fields, or as Stress Age Cracking (SAC) if failure happens during service. A leading literature explanation of this failure suggests that the residual stress relaxation and the precipitation dissolution and/or re-precipitation occur in the same temperature range, which can lead to locally high strains and thus to crack at the grain boundaries. Since in situ spatial measurements of residual stress fields, microstructural evolution, and failure processes are nearly infeasible, this work recourses to a micromechanical finite element framework that models the high temperature failure as the nucleation and growth of grain boundary cavities, whereas various parameters such as thermomechanical loading history and its evolution, the competition of grain-interior dislocation creep and grain-boundary diffusion in failure lifetime, and microstructural heterogeneities (such as the precipitate free zone near grain boundaries) can be quantitatively incorporated. It can be concluded from these microstructure-explicit simulations that an accurate knowledge of residual stress evolution and a carefully calibrated set of material constitutive parameters are the essential prerequisites for lifetime predictions. The understanding of individual governing factors also leads to a mechanistic interpretation of the observed SRC susceptibility C-curves. These results suggest that the criticality of residual stress evolution, but not the precipitation-induced local strains, be the leading factor for SRC.

Effect of final rolling temperature on microstructure and intergranular corrosion resistance of an Al–Cu–Mg–Ag alloy

Connection of microstructure with corrosion properties of an Al-3.62Cu-0.9Mg-0.45Ag alloy in natural aging (NA) state and peak aging (PA) state at different final rolling temperatures (20 °C, 180 °C and 280 °C) are studied by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), X-ray diffraction (XRD), texture and electrochemical analysis. The results show that the deformation texture and recrystallization texture of T280-NA alloy account for the largest percentage with the largest degree of recrystallization. With the increase of final rolling temperature, the proportion of low angle grain boundaries (LAGBs) of NA alloy increases, the diameter, thickness and volume fraction of Ω phase in PA alloy decrease, and the grain boundary precipitates (GBPs) are gradually transformed from discontinuous distribution to continuous distribution. The grain boundary precipitates are also found to be consist of alternating S and θ phases and the width of precipitation free zones (PFZs) is essentially independent of final rolling temperature. The intergranular corrosion (IGC) resistance of T280-NA alloy and T20-PA alloy is the best, which is a result of the combined effect of GBPs, texture and grain boundary properties. In addition, this result is verified in this paper by studying the electrochemical properties of the alloy.

Influence of solid solution treatment on fatigue crack propagation behavior in the thickness direction of 2519A aluminum alloy thick plates

2519A aluminum alloy thick plates have excellent applications in the military industry due to their superior ballistic resistance and fatigue properties. However, the delamination cracking along the thickness direction of this alloy hinders its further utilization. To address this issue and enhance the fatigue properties of this alloy, the effect of single-stage solid solution treatment (SST) and two-stage solid solution treatment (DST) on the fatigue crack propagation (FCP) behavior in the thickness direction of the alloy was investigated. Compared to the optimum SST, the volume fraction of the secondary phase of the alloy was reduced by 28.6% after DST. The precipitated phases were finer with more uniform distribution, and the volume fraction and number density of the precipitation increased by 47% and 30%, respectively. The DST resulted in a lower fatigue crack growth rate (FCGR) and an increase of 2.92% in the critical stress intensity factor value (ΔKcr). The crack propagation mode of the alloy after SST was inter/transgranular crack propagation, while it was transgranular after DST. The aspect ratio of grain and the number of secondary phases in the alloy decrease after DST. Meanwhile, the grain boundary (GB) precipitation phase spacing, the precipitation-free zone (PFZ) width, and the spacing between the secondary phases increase. These changes result in decreased crack sources in the alloy and reduced the bridging effect of secondary phases on fatigue cracks, thus improving the fatigue performance of the alloy.

Simultaneously improved the strength and ductility of cast Al-3Li-2Cu alloy by Ti addition and heat treatment

This work systematically investigated the impact of minor Ti addition and heat treatment on grain structure, precipitates, precipitate-free zone (PFZ), and the performance of the Al-3Li-2Cu alloy. Minor Ti addition significantly refines the as-cast alloy, leading to a transition from columnar grains to equiaxed grains. The primary Al3Ti phases, with a low mismatch (0.99%) to the matrix, act as heterogeneous nucleation sites for α-Al, while the presence of Ti solutes restricts grain growth. As a result, the average grain size can be refined to 35.2 μm by adding 0.2 wt% Ti. Besides, minor Ti addition affects the corresponding precipitation behavior of the alloy. Detailed TEM observations reveal that the addition of Ti inhibits the growth of δ’(Al3Li) precipitates and the accompanying δ’-PFZ, which should be related to the interaction between Ti solutes and vacancies. Besides, reducing the aging temperature exerts a similar effect. Finally, the 0.2 wt% Ti-modified alloy exhibits excellent mechanical properties after aging at 175 °C for 8 h. It demonstrates a high yield strength (272 MPa), a high ultimate tensile strength (399 MPa), and an acceptable elongation (5.9%), which represent an increase of 8.8%, 34.8%, and 293.3%, respectively, compared to the alloy without Ti addition. The simultaneous improvement in strength and ductility is attributed to the presence of fine and equiaxed grains, high-density and homogenous δ’ precipitates, and a narrow δ’-PFZ. In-depth discussions are provided to elucidate the underlying mechanisms governing the microstructure and mechanical properties.

Localized corrosion characteristics of Al-Mg-Si aluminum alloy with aging time and pre-delayed aging condition

The effects of single aging, delayed aging and pre-delayed aging treatment on the corrosion resistance of Al-Mg-Si alloy were studied by scanning Kelvin probe force microscopy (SKPFM), transmission electron microscopy (TEM), scanning electron microscopy (SEM), three-dimensional X-ray tomography (3D-XRT), confocal laser scanning microscopy (CLSM) and electrochemical measurements. The surface potentials of Al(MnFe)Si, Mg2Si, and Si-rich remnant phases are determined, identifying Al(MnFe)Si phase as the crucial cathodic site for corrosion initiation, and the increase in surface potential implies an accelerated corrosion rate after immersion. The intergranular corrosion susceptibility of the alloy is predominantly influenced by the microstructure caused by aging conditions. The factors affecting corrosion resistance are summarized as (i) adjustment of electrochemical reaction activity by controlling the area fraction of Al(MnFe)Si phase, (ii) the spacing of grain boundary precipitates (GBPs), and (iii) the width of the precipitation-free zone (PFZ). The delayed aging (DA) sample exhibits inferior corrosion resistance due to the continuous GBPs and extended sub-grain boundaries corrosion propagation paths. The pre-delayed aging treatment successfully mitigates the corrosion degree of DA by widening the GBPs spacing and narrowing the PFZ width. This suggests that pre-aging treatment is a potent strategy for enhancing corrosion resistance without compromising the alloy's strength.

Effect of Grain Size on High Temperature Oxidation Behavior of IN792 Superalloy

It is known that Ni-based superalloys possess superior mechanical properties in high-temperature and high-pressure environments. One of the drawbacks of these alloys is their tendency to oxidize at high temperatures. Improving their oxidation properties and obtaining a fundamental understanding of the underlying mechanism of their oxidation behavior are critical for high temperature applications. In this work, we manufactured IN792 Ni-based alloys with two different grain sizes to examine the effect of grain size on oxidation behavior at 850<sup>o</sup>C and 980<sup>o</sup>C. The oxidation rate became faster as the grain size became finer, and at 850<sup>o</sup>C, the oxide layer was composed of external oxides (TiO<sub>2</sub>/NiO, Ni-Co-Cr-O, Cr<sub>2</sub>O<sub>3</sub>), internal oxides (TiTaO<sub>4</sub>, non-oxidized layer, Al<sub>2</sub>O<sub>3</sub>) and a precipitate-free zone (PFZ). At 980<sup>o</sup>C, this structure became unstable due to the diffusion of Ni, and in particular, a lot of exfoliation of external oxides occurred in the coarse grain. Regardless of the structure of the oxide layers, the thickness of the PFZ increased significantly in the fine grains with a low  fraction. Compared to the hardness of external Cr-rich oxides of ~500 HV, the hardnesses of the internal oxides consisting of Al-rich and Ni-Al-O were approximately 1000 HV and 1500 HV, respectively. Therefore, the most vulnerable position is expected to be the interfacial boundary between the external oxide and internal oxide, and this resulted in the ease of exfoliation of the external oxide in this location, by exposing the Ni-Al-O oxide to the surface in the coarse grain at 980<sup>o</sup>C. The current work can be a useful method in the design of Ni-based superalloys by controlling the grain size of the components.

Effects of pre-strain deep cryogenic aging on the mechanical and corrosion properties of 2A97 aluminum–lithium alloy

In order to improve the mechanical properties and corrosion resistance of 2A97 aluminum-lithium alloy, this research develops a novel strain heat-treatment technology. Surprisingly, 2A97 alloy treated by 4% pre-strain and 48 h deep cryogenic aging exhibits excellent mechanical properties, whose yield strength, tensile strength, and elongation are 573.7 MPa, 593.6 MPa, and 14.25%, respectively. Furthermore, the corrosion resistance of samples treated by the above pre-strain deep cryogenic aging process is two times as much as traditional T6 aged. This study deeply investigates the influence of various precipitates in 2A97 alloy on its performance through microstructure analysis. The results show that the introduction of pre-strain inhibits the precipitates of the T1 and δ′ phases at the grain boundaries, and prevents the formation and widening of precipitate-free zones. At the same time, it reduces the potential difference between grain boundaries and matrix, and enhances the corrosion resistance of 2A97 alloy. Deep cryogenic can inhibit recovery and maintain deformation dislocation, so that a large number of defects produced by pre-strain promote the nucleation and diffusion of precipitates in the matrix, resulting in tiny and uniformly distributed precipitates. Therefore, the novel pre-strain deep cryogenic aging not only endues 2A97 alloy excellent performance, but also offers a new strategy for controlling precipitation.

Material Properties and Structure of Al-Mg-Si Alloy Thin-Walled Profiles with Different Alloy Compositions and Aging Processes

Thin-walled Al-Mg-Si alloy profiles with different compositions and aging states were prepared using the heating and extrusion process. The properties and structure of the profiles were then investigated using a metallographic microscope, scanning electron microscope, projection electron microscope, and universal testing machine. The results show that the yield strength and tensile strength of the profile increases with the increase in total Mg + Si content, and ductility is reduced. If the total Mg + Si content is too high or too low, the crush performance of the material would decrease. Compared with the under-aged and near-peak-aged states, the three types of AI-Mg-Si alloy thin-walled profiles at the over-aged state have better effective energy absorption during crushing and higher bending angle; however, the tensile strength of the profile is optimal at the near-peak-aged state. The effects of alloy composition and aging process on material strength and crushing energy absorption are mainly attributed to the grain structure and differences in precipitation. For coarse grain structures, the grain boundary precipitate free zones are wider, which decreases the profile ductility. Simultaneously, an increase in primary strengthening phases in the grains would increase the profile strength.

Effects of pre-deformation on microstructure, mechanical properties and corrosion performance of an Al-Cu-Mg-Ag alloys

The influence of pre-deformation on the microstructure, mechanical properties, and corrosion behavior of AA2029-T8 was investigated. The pre-deformation may potentially impede the formation of Mg-Ag clusters during the early stage of aging, altered the normal precipitation sequence of the alloy, and significantly increased dislocation density that facilitated non-uniform nucleation of S phase at dislocations, resulting in finer dispersion of precipitation. Moreover, it led to a decrease in Ω phase volume fraction and comparable aspect ratio(diameter/thickness). With increasing deformation, aging response was accelerated, leading to shorter time to peak strength but reduced plasticity. Additionally, the width of precipitate-free zone (PFZ) remained relatively unchanged with increasing pre-deformation; however, average size of grain boundary precipitates (GBPs) decreased while becoming more discontinuous. This resulted in narrowing intergranular corrosion (IGC) channels and decreased maximum corrosion depth as well as a transition from IGC to pitting corrosion behavior. These findings suggest that the mechanical properties and corrosion resistance of the alloy can be improved by increasing pre-deformation levels.

Yttrium effect on 475 °C brittleness of Fe–13Cr–6Al–2Mo-0.5Nb-0.15Zr alloy

The effect of yttrium on the 475 °C brittleness and microstructure stability of Fe–13Cr–6Al–2Mo-0.5Nb-0.15Zr alloy after long-term aging at 475 °C has been investigated. At the early stage of aging, the nanoscale metastable transition phase precipitates in large quantities. After aging for 10,000 h, the nanophase in the Y-free alloy has completely disappeared, and only a great deal of granular Laves phases are left, while a large number of nanoscale phases still exist in the grain of Y-containing alloys. This is because the addition of yttrium combines with vacancies, hinders the diffusion of Laves phase-forming elements, and has the result that the transformation of nanoscale phases Fe2(Mo0.3Nb0.7)Al to Laves phase is inhibited during the aging process of the Y-containing alloys. During aging, precipitation-free zones (PFZ) are formed in the surrounding area where the Laves phase and grain boundaries exist. The elongation of Y-containing alloys decreases with yttrium content, but it is still higher than that of Y-free alloy. The addition of yttrium improves the aging plasticity and weakens the degree of 475 °C brittleness of the alloy, and the effectiveness is diminished with the yttrium. This plays a key role in the further study of Fe–Cr–Al alloys for fuel cladding materials.

Influence of aging treatment on corrosion behavior of extruded 2195 Al−Li alloy

The 2195 Al-Li alloy was processed by under-aging (UA), peak-aging (PA), double-aging (DA) and retrogression re-aging (RRA) treatments, and the microstructure, intergranular corrosion (IGC), exfoliation corrosion (EXCO) and stress corrosion cracking (SCC) behaviors were observed and discussed. The results show that the sequence of IGC and EXCO resistance is: RRA > UA > DA > PA. The low corrosion resistance of PA and UA is mainly caused by the continuous precipitation of T1 phases at grain boundaries to form continuous corrosion channels, while the interruption of corrosion channels caused by coarse and discontinuous T1 phases and wider precipitate free zone is the reason for the best corrosion resistance of RRA. The RRA alloy exhibits the best SCC resistance, and the order of SCC resistance is: RRA > DA > UA > PA. The discontinuous precipitation of intergranular T1 phases promotes the propagation of corrosion cracks across grain boundaries, thereby improving the SCC resistance.

Effect of different initial tempers on creep aging behavior of 2195 aluminum-lithium alloy

Five kinds of 2195 aluminum-lithium alloys with different initial tempers were obtained through different heat treatment methods. The creep aging test with a constant stress of 140 MPa, at 180 °C, mechanical tensile test at room temperature and the scanning electron microscope (SEM) observation were used to study the dependence of microstructure and properties on the initial tempers of 2195 aluminum-lithium alloy under creep aging forming conditions. The results show that the creep deformation of the alloy with re-solution initial temper is the largest, followed by the solid solution, over-aging and peak aging tempers, while the aged alloy with the regression temper has the smallest creep deformation. The order of yield and tensile strength from high to low is re-solution > solid solution > peak aging > over-aging > regression treatment. The order of elongation to fraction from large to small is solid solution > re-solution > peak aging > over aging > regression treatment. Finally, the high-strength and ductility mechanism of the creep-aged alloy with the re-solution temper was revealed: uniformly distributed fine strengthening phase (T1 phase) within the grain and narrowed precipitation-free zone at grain boundaries.

Effect of aging treatment on high temperature mechanical properties of a cast Al–Li–Cu alloy

The effects of aging treatment on the high temperature mechanical properties of cast Al–2Li–2Cu-0.5Mg-0.15Sc-0.15Zr-0.05Ti alloy were investigated. Different test temperature (25 °C, 175 °C, 200 °C, 225 °C) and aging time (aging for 8 h, 32 h, 64 h at 175 °C) were designed for comparison. The results showed that at room temperature, the strength of the alloy improved with the prolongation of aging time. When the test temperature increased from 175 °C to 225 °C, the δ′(Al3Li)-Precipitation Free Zone (δ′-PFZ) widened from 31.25 nm to 110.83 nm, causing a decrease in strength. In addition, the presence of the dynamic recrystallization at 225 °C resulted in an increase in elongation. As the aging time prolonged from 8 h to 64 h, the high temperature strength of the alloy at 225 °C decreased first and then enhanced. The improvement of alloy strength aged for 64 h was mainly due to the increase of T1(Al2CuLi), S′(Al2CuMg) phases and the decrease of δ′(Al3Li) phases. The alloy aged for 64 h exhibiting excellent high-temperature performance at 225 °C, which the yield strength reached 302 MPa, the tensile strength reached 312 MPa and the elongation reached 4.4%. The results indicated that δ′-PFZ widening, precipitation strengthening and dynamic recrystallization were the main factors which influenced the high temperature mechanical properties of Al–Li–Cu alloy.

Comments on the Intermediate-Temperature Embrittlement of Metals and Alloys: The Conditions for Transgranular and Intergranular Failure

The intermediate-temperature embrittlement range was examined for Fe, Al, Cu, and Ni alloys. It was found that this embrittlement occurs in many alloys, although the causes are very diverse. Embrittlement can be due to fine matrix precipitation, precipitate free zones, melting of compounds at the grain boundaries, segregation of elements to the boundaries, and, additionally for steel, the presence of the soft ferrite film surrounding the harder austenite matrix. Grain boundary sliding and segregation to the boundaries seem to dominate the failure mode at the base of the trough when intergranular failure takes place. When cracking is due to the presence of hydrogen or liquid films at the boundary, then the dissociation along the boundaries is so easy, it is often independent of the strain rate and is always intergranular. In the other cases when failure occurs, if the deformation is carried out at a high strain rate, it is normally transgranular (e.g., hot rolling giving rise to edge cracking). However, when the strain rate is reduced to that of creep (e.g., bending during continuous casting of steel), failure can also take place by grain boundary sliding, and intergranular failure then becomes the favoured mode.

Precipitation evolution of Al-Zn-Mg-Cu-(Ag) alloys with a low Zn/Mg ratio

The influence of minor Ag on the precipitation evolution of the Al-4.2Zn-2.8Mg-1.0Cu (wt.%) alloy from early stages to over-aged stages at 150 °C was investigated. Surprisingly, co-precipitation of strengthening phases T′ and η′ are found in both Ag-free and Ag-added alloys. With Ag addition, precipitation of both T′ and η′ is refined and increased, such that the age-hardening capabilities and peak-aged tensile strength are improved. In addition, the quantitative proportion of η′ precipitates increases with the increase of Ag content due to the increase in the (Zn+Cu)/Mg ratio of nucleating particles. The narrowed precipitate-free zones (PFZs) are considered responsible for the undiminished fracture elongation in Ag-added alloys. Essentially, these effects of Ag are closely related to the strong Ag-vacancy and Ag-solute interactions. In over-aged stages, the Ag-added alloys still possess higher hardness values compared to the Ag-free alloy, which is related to precipitate coarsening mechanisms. The Ag-free alloy follows classical coarsening behavior by solid solution mediated diffusion, while the Ag-added alloy follows two possible coarsening mechanisms, coalescence of aggregates and diffusion of atoms. The smaller average size and higher residual number density of precipitates benefited from the slow diffusion-controlled coarsening behavior depending on the precipitate composition characteristics of the two-stage differentiation and the precipitate distribution characteristics of high-density dispersion in early-aged stages could explain why the hardness of Ag-added alloy keeps at a higher level than that of Ag-free alloy even after 1000 h ageing. Meanwhile, the transformation of metastable phases to stable phases is inhibited due to the addition of Ag, such that GP zones, T′, η′, η and T phases coexist even after 14 d of ageing. In terms of phase composition, the addition of Ag decreases the ratio of Mg/(Al+Zn) in T-type phase. For the Ag-added alloy, the sum concentration of Zn+Mg in η′ phase is about 10 at.% higher compared to T′ phase, and η phase continues to have a high sum concentration of Zn+Mg, besides, the Zn/Mg ratio and Cu concentration exhibit obvious differences from T phase.

Interplay between high-temperature creep deformation behavior and nanoscale precipitate evolution in a newly designed Fe–Ni-based superalloy

In an endeavor to achieve the critical goals of carbon neutrality, an explorative study was conducted to scrutinize the precipitate evolution and high-temperature creep behavior of an innovative and cost-effective Fe–Ni-based superalloy that was meticulously engineered for the next-generation ultra-supercritical (USC) power plants. The creep experiments conducted at temperatures and stresses of 650 °C/280 MPa (17,179 h), 675 °C/250 MPa (10,271 h), and 700 °C/250 MPa (2378 h), coupled with proficient data fitting were employed to accurately assess the long-term service performance of the superalloy under real-world operational environments. Though it can withstand theoretically an intense stress of 130 MPa at 700 °C for approximately 30 years as calculated by the Larson-Miller equation, its performance was not without a degree of perceived vulnerability. This was primarily traced to the conspicuous coarsening behavior of the precipitate-free zones (PFZs)/discontinuous coarsening zones (DCZs) formed in the grain boundaries (GBs) on a longer time scale. It had a significant diminishing impact on the overall strength of the GBs, resulting in a recorded creep life that was considerably shorter than initially projected. Moreover, a synergistic mechanism between PFZs/DCZs with TiC carbides was advanced to interpret the intergranular fracturing dynamic, which was postulated due to the evidence gathered from electron backscatter diffraction (EBSD), highlighting that the banded distribution of TiC carbides circumambient to GBs engendered considerable stress aggregation. Conversely, the plentiful supply of coincidence site lattice (CSL) boundaries disrupted the continuous network structure of random high-angle grain boundaries (HAGBs), thereby creating an effective barricade against the proliferation of cracks. These empirical findings paved the way for the proposition of an enhanced blueprint and the industrial utilization of such.

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.

In-situ rolling friction stir welding of aluminum alloys towards corrosion resistance

In-situ rolling friction stir welding (IRFSW) was proposed to improve corrosion resistance of 7075-T6 aluminum alloy joint via microstructural design and residual stress regulation. The residual stress at the weld edge was changed from +58.3 MPa to –25.7 MPa. The dissolution of precipitates and narrowing of precipitate-free zones reduced corrosion potential difference between the precipitates and the matrices. The active corrosion path was cut off by discontinuous distribution of fine precipitates in grain boundary. The ultimate tensile strength and elongation of IRFSW joints were increased by 24.1 % and 100.0 % compared to conventional FSW joints after constant load stress corrosion.