Two-stage Aging Research Articles

Enhanced intragranular dislocation capture ability in nano/ultrafine Al-Zn-Mg-Cu alloy by burnishing-assisted two-stage aging treatment-induced dynamic nucleation

The effects of burnishing-assisted two-stage aging (BATSA), including pre-aging, burnishing, and subsequent aging (SA) on the microstructural evolution, precipitation, and hardening behavior of Al-Zn-Mg-Cu alloy were systematically investigated. Transmission electron microscopy, X-ray diffraction, small-angle X-ray scattering, and Vickers microhardness tests were used to analyze the above-mentioned characteristics of the burnished-layer alloys and the matrix-layer alloys, respectively. The BATSA treatment promotes a concentrated distribution of precipitate sizes, delaying phase coalescence and maintaining a relatively dispersed precipitate distribution. Moreover, the persistent nucleation of Guinier-Preston (GP) zones in burnished alloys was observed during SA. These results are conducive to the enhancement of the dislocation storage capacity of nano/ultra fine-grained alloys (the decrease in dislocation density of pre-aged burnished alloy was about 65% that of non-pre-aged burnished alloy). Based on this, dynamic nucleation induced by pre-aging and burnishing resulted in a better synergy of multiple strengthening mechanisms, significantly shortening the time to peak hardness in pre-aged burnished alloys to about 1/3 that in non-pre-aged matrix alloys, and maintaining hardness stability. Moreover, it was found that the critical width of the in situ phase transformation from GPII zone to η' precipitate can be as low as 3–4 at. planes.

Effect of Final Thermomechanical Treatment on the Mechanical Properties and Microstructure of T Phase Hardened Al-5.8Mg-4.5Zn-0.5Cu Alloy.

The effect of final thermomechanical treatment (FTMT) on the mechanical properties and microstructure of a T-Mg32(Al Zn)49 phase precipitation hardened Al-5.8Mg-4.5Zn-0.5Cu alloy was studied. The as-cold rolled aluminum alloy samples were subjected sequentially to solid solution treatment, pre-deformation, and two-stage aging treatment. Vickers hardness was measured during the aging process under various parameters. Tensile tests were conducted on the representative samples based on the hardness results. Microstructural characteristics were analyzed via transmission electron microscopy and high-resolution transmission electron microscopy. The conventional T6 process was also carried out for comparison. The hardness and tensile strength are increased evidently by the FTMT process for the Al-Mg-Zn-Cu alloy, while the ductility is adversely affected to a small extent. The precipitation at the T6 state consists of a coherent Guinier-Preston zone and T″ phase in the form of intragranular, fine, and spherical particles, while a semi-coherent T' phase appears after the FTMT process as a new constituent. The distribution of dislocation tangles and isolated dislocations is another feature of FTMT samples. Enhanced precipitation hardening and dislocation strengthening account for the improved mechanical performance of FTMT samples.

Effect of deformation amount on microstructure and properties of AA2024-T8I4 with deep cryogenic treatment

To simultaneously improve the mechanical properties and corrosion resistance of AA2024 aluminum alloy, a T8I4 with deep cryogenic treatment (DCT) process is proposed, which includes solution heat-treatment, cold-drawing deformation, pre-aging, DCT and re-aging. The microstructure of AA2024 is observed by confocal laser scanning microscope, scanning electron microscopy and transmission electron microscopy. The properties of AA2024 are studied by hardness measurement, tensile test, friction and wear test, exfoliation corrosion test, intergranular corrosion test and electrochemical corrosion test. On comparison with T6, AA2024-T8I4 +DCT obviously increases its hardness, tensile strength and wear resistance. AA2024-T8I4 +DCT has similar corrosion resistance to T7X two-stage aging. According to comprehensive analysis, the MPts in AA2024-T8I4 +DCT with 3 % deformation amount have the maximum pinning force to the dislocations, and the alloy has the largest hardness, strength and the best wear resistance. The discontinuous grain boundary precipitates in AA2024-T8I4 +DCT with 1 % deformation amount are discontinuous and fine, and the precipitate free zone is not obvious, which makes the corrosion difficult to develop. The alloy has the shallowest intergranular corrosion depth, the smallest corrosion rate and the best corrosion resistance

Electrical current treatment of 6N01 Al alloy FSW joints

Studies successfully exploit electric current treatment including electropulsing solution and direct-current (DC) aging to thoroughly substitute post-weld heat treatment and realize artificial controlling of precipitation as well as the improvement of mechanical property in 6N01 Al alloy friction-stir-welding (FSW) joints. Utilizing electropulsing solution, β' can be dissolved completely within 3 s to form super-saturated solid solution. The subsequent DC two-stage aging promotes the formation of high-density βʺ within 5 h and increases the microhardness by 21.8% and strength by about 15%. This study extends perspectives on post-weld treatment of precipitation-hardening Al alloy joints and expand the potential function of artificial controlling of precipitation.

Novel two-stage aging treatment to enhance hardening of Zn-containing SiCp/6xxxAl composites

Microalloying Zn in 6xxxAl (Al–Mg–Si) alloys and their composites can hardly form precipitates during artificial aging (AA) at 170 °C, so exhibits weak hardening abilities. In this study, a low-temperature AA at 120 °C was introduced after 170 °C AA to exploit the hardening potential of Zn. A 17 vol.% silicon carbide particles (SiCp) reinforced Al-1.2Mg-0.6Si-1.0Zn (wt.%) composite was used to inspect the effectiveness of the two-stage AA process. It is shown that AA at 120 °C can further improve the hardness based on the highest hardness during 170 °C AA. The increase in hardness during AA at 120 °C is mainly attributed to the precipitation of Zn. Interestingly, Zn neither independently forms precipitates nor forms new phases. The precipitation form of Zn is to segregate into dislocation-induced and discrete precipitates formed during AA at 170 °C. This study is expected to provide new ideas to harden SiCp/6xxxAl composites by composition design and AA process control.

Effect of heat treatment process parameters on the microstructure and properties of GH4720Li superalloy

The solution temperature and aging temperature of GH4720Li superalloy are the critical factors in determining its microstructure stability and mechanical properties. However, the matching of solution temperature, primary aging temperature, and second aging temperature greatly influences the final microstructure and mechanical properties, and the mechanism was rarely studied. In this paper, the effects of solution temperature, aging temperature, and other process parameters on the grain size, precipitated phase, and mechanical properties of GH4720Li superalloy were systematically studied by orthogonal experiment, and a set of optimal heat treatment process parameters were found. The results show that the sample was solution treated in the temperature range of 1060 °C ∼ 1120 °C. When the solution temperature was 1160 °C ∼ 1100 °C, although the primary γ ′ phase gradually dissolved, the secondary γ ′ phase gradually increased, and the primary γ ′ phase was pinned at the grain boundary to hinder the grain growth, and the hardness of the alloy gradually increases. When the solution temperature exceeded 1100 °C, the primary γ ′ phase dissolved in large quantities, the grains grew up rapidly, and the hardness of the alloy decreased. The sample was subjected to two-stage aging treatment in the temperature range of 650 °C ∼ 770 °C and 760 °C ∼ 880 °C. As the aging temperature increased, the primary γ ′ phase of the sample gradually grew, and the ability to pin the grain boundary weakened. The volume fraction of the secondary γ ′ phase of the sample gradually decreased and coarsened, resulting in grain growth and a gradual decrease in the hardness of the sample. When the samples were treated by 1100 °C/OC × 4 h + 650°C /AC × 8 h + 760 °C/AC × 8 h, the grain size of the sample was the smallest, the average grain size was 4.5 μm, the distribution of γ ′ phase was the most uniform, and the mechanical properties are the best, reaching 47 HRC.

Effect of aging treatment on Microstructure and properties of Al-Cu-Mg alloy

In order to meet the demand for high strength of Al alloy in aerospace, automobile, shipbuilding and other fields, the effects of aging treatment on the microstructure and properties of Al-4.5Cu-0.8Mg alloy were systematically studied. The results show that the main strengthening phase in the microstructure of the alloy after aging was θ’ And S’ phase, the comprehensive mechanical properties of single-stage aging after aging at 165 °C for 12h was better. In the alloy aged at 185 °C for 12 hours, the strengthening phase was coarsen obviously, the tensile strength and yield strength was decrease greatly and showed obvious over aging characteristics. The comprehensive mechanical properties of two-stage aging at 105 °C / 12h + 165 °C / 5h was the best. The tensile strength, yield strength, elongation and hardness are 543 MPa and 374 MPa, 17.5% and 170.6 HV respectively.

Microstructure and Mechanical Properties of the Extruded Al-Cu-Mn-Sc-Zr Alloy during Single-Stage and Two-Stage Aging

Microstructure and Mechanical Properties of the Extruded Al-Cu-Mn-Sc-Zr Alloy during Single-Stage and Two-Stage Aging

Effect of α Phase on Dynamic Mechanical Properties and Failure of Ti-4Al-5Mo-5V-5Cr-1Nb Alloy after Two-Stage Aging

This work studied the high strain rate behavior of the Ti-4Al-5Mo-5V-5Cr-1Nb (Ti-45551) alloy after two-stage aging, with dynamic tension tests conducted using the split Hopkinson tensile bar. The results show that the microstructure, especially the size and distribution of the α phase, significantly affects the mechanical and failure behaviors of the Ti-45551 alloy under dynamic loading. As the strain rate increases, increasing trends can be observed in both ductility and strength, which is intimately related to the activation of the dislocation slip in the α phase. Moreover, obvious strain softening was found in the Ti-45551 alloy under dynamic loading. In this study, the microstructure observations suggest that dislocation slips are highly active in the α phase under dynamic loading. Fractographic characterization of the fracture surfaces under dynamic loading reveals a uniform distribution of ductile dimples, which indicates that a uniform distribution of the nano-scale α phase can effectively reduce brittle fracture tendency. Our studies provide a comprehensive picture of how strain rate drives dynamic plasticity and failure mode in the Ti alloy.

Strengthening effect of prefabrication (10–12) tensile twinning on AZ80+0.4%Ce magnesium alloy and inhibition mechanism of discontinuous precipitation

This paper provided an effective method to further improve the mechanical properties of the AZ80+0.4%Ce magnesium alloy wheel spoke. The effect of high strength and ductility was obtained with a yield strength of 295.36 MPa, an elongation of 10%, by the combination of pre-deformation (7% deformation) and two-stage aging treatment (120 °C/9 h + 175 °C/24 h). The evolution of the microstructure and properties of the alloy was explored under the coupling conditions of different pre-deformation degrees and multi-stage aging. The results show that, pre-deformation introduced a large number of (101¯2) tensile twinning and dislocations, which greatly promoted the probability of continuous precipitates (CPs) appearing. On the contrary, the discontinuous precipitates (DPs) were limited by the vertical and horizontal twin structure. As a result, the pre-nucleation method of two-stage aging increased the proportion of CPs by 34%-38%. Owing to the DPs was effectively suppressed, the alloy's yield strength has been greatly improved. Besides, under multi-stage aging, the twin boundaries induce protruding nucleation to form static recrystallization by hindering the migration of dislocations, and the matrix swallows the twins, then the texture gradually tilts from the two poles to the basal plane. As an important supplement, the grain refinement and oblique texture promoted the improvement of the yield strength of the component.

Microstructure and mechanical properties of spray formed 7055 aluminum alloy by fiber laser-arc hybrid welding

The spray formed 7055 aluminum alloy with 4.2mm thickness was welded by laser-arc hybrid heat source. The microstructures and properties of the welded joints are studied. The results show that the weld appearance is very full and the weld zone is primary composed of equiaxed crystals. Those equiaxed crystals morphology in different positions are also different, and only a small amount of columnar crystals are locally formed in the fusion zone. The average microhardness values of the weld zone and the heat affected zone are lower than that of the base metal, reaching 64% and 82% respectively. The average tensile strength of welded joints with as-welded state reaches 51% of the base metal. However, the tensile properties of the joints are significantly improved after the two-stage solution and aging heat treatment, and the average tensile strength can reach 515MPa, which was about 84% of the strength of the base metal.

Effect of ageing on precipitation evolution and corrosion behaviour of Al–Zn–Mg–Cu alloy

The Al–Zn–Mg–Cu alloy was subjected to 4 passes equal channel angular pressing (ECAP) at 400°C, followed by different solutions and ageing treatments. The size, type and distribution of precipitates in aluminium matrix are different in different ageing treatments. In single-stage ageing, a large number of η′ precipitates and Guinier-Preston-II (GP-II) zones are generated in the aluminium matrix. The precipitates are mainly MgZn2 (η) precipitates and η′ precipitates during the double-stage ageing. The size and type of precipitates in three-stage ageing are similar to those in two-stage ageing. Simultaneously, the single-stage ageing sample has poor corrosion resistance and good hardness. Two-stage ageing and three-stage ageing samples have better corrosion resistance, but their hardness is lower than that of single-stage ageing.

Improved creep forming efficiency and retained performance via a novel two-stage creep aging process of Al–Zn–Mg–Cu alloys

A novel two-stage creep aging forming (CAF) process with an elevated temperature followed by peak-aging temperature was developed to notably improve the creep forming efficiency and to achieve a favorable combination of strength and corrosion resistance in an Al–Zn–Mg–Cu alloy. Compared with traditional single-stage creep aging, the proposed two-stage creep aging (TSCA) halved the formation time while doubling the creep deformation without sacrificing strength. This specific aging procedure can manipulate precipitation evolution, thus effectively affecting the deformation and performance of the studied alloy. The creep responses, properties, microstructure evolution and their relationship of 7B50 aluminum alloy subjected to TSCA were investigated and discussed through creep and property tests, transmission electron microscopy, and differential scanning calorimetry. Both precipitate dissolution and high temperatures were beneficial for the exceptional creep response in the high-temperature creep aging process, whereas the re-precipitation of new Guinier–Preston (GP) zones in the low-temperature stage could practically counteract the strength loss at elevated temperatures. Moreover, the corrosion resistance was persistently enhanced throughout the process owing to coarse and discrete grain boundary precipitates. These results are of significance in the development of CAF technology for Al–Zn–Mg–Cu alloy components.

Creep behavior and microstructure evolution during two-stage creep aging of a 2195 Al–Li alloy

In this study, a two-stage creep aging (CA) is established by adding a new stage of low-temperature CA to the traditional single-stage CA. The results show that the two-stage CA can effectively increase the elongation (EL) of the material by about 39.2% without sacrificing strength and creep strain. In the first stage of two-stage CA, as temperature increases, EL improvement first increases and then decreases; when the temperature varies between 65 °C and 115 °C, total creep strain remains consistent. Observation of the microstructures of the specimens under different CA processes reveals that two-stage CA mainly affects the mechanical properties of the material by adjusting the properties of the GP zones inside the material in the first stage of two-stage CA, which further affects the number and size of the T1 phase in the subsequent CA process. Whether the GP zones evolve into a large number of T1 and θ′ phases in the first stage of two-stage CA can directly influence the creep strain of the entire two-stage CA process and changes in the EL of the material after CA.

Future Ageing Trajectory Prediction for Lithium-Ion Battery Considering the Knee Point Effect

Lithium-ion batteries have been widely applied in energy conversion sectors, where effective future ageing prediction is crucial to guarantee their safety and performance. Due to the highly nonlinear ageing behaviours, developing a reliable method that could not only consider the knee point effect but also predict the future ageing trajectory with uncertainty quantification poses a formidable task. This paper derives a machine learning solution, based on the migrated Gaussian process regression (GPR), for predicting future battery two-stage ageing trajectory. Specifically, a base model is first offline identified from the easier collected accelerated-speed ageing data, through which the long life ageing information can be effectively learned. With this base model, a migrated mean function is then designed and coupled within the GPR framework for battery ageing predictions. Experimental data from three different batteries are applied for model validation and performance evaluation. Results indicate that the proposed solution leads to effective improvements in prediction accuracy and uncertainty quantification for both cases of training before and after the knee point. This is the first time to couple migration concept within GPR, paving the way to reduce experimental cost and predict battery future two-stage ageing trajectory with only a few (first 30%) data available.

Two-stage aging trajectory prediction of LFP lithium-ion battery based on transfer learning with the cycle life prediction

With the wide application of the LFP lithium-ion batteries, more attention is paid to the battery life and future aging behaviors as the safety and performance of the battery are guaranteed by accurate battery aging monitoring. In recent years, long-term aging trajectory prediction of the lithium-ion battery is always a challenge due to its complex nonlinear aging behaviors especially the aging behaviors in the two aging stages are quite different when the battery experiences the two-stage aging process under fast-charging conditions. Thus, it is harder to achieve accurate long-term aging trajectory prediction of the LFP lithium-ion batteries on the condition of the two-stage aging process. To address it, a novel transfer learning strategy combined with the cycle life prediction technology is presented in this paper. Specifically, a new cycle life prediction method is proposed based on feature extraction and deep learning technology and achieves accurate cycle life prediction. The transfer learning is started by developing a base aging model offline to learn the information of the two-stage aging process. Then, taking the predicted cycle life as its prior information, the Bayesian model migration technology is employed to predict the aging trajectory accurately, and the uncertainty of the aging trajectory is quantified. Two batches of the battery datasets are used for performance evaluation and comparison with two benchmarks. It is novel to combine the cycle life prediction and transfer learning technique to achieve accurate two-stage aging trajectory prediction with only a few data available (first 30%).

Effects of cryogenic deformation on the microstructure and mechanical properties of high-strength aluminum alloys

7A85 aluminum alloy forgings were compressed at 10% to 30% deformation at room and cryogenic temperatures, followed by solid-solution treatment, water quenching, 3% cold deformation, and two-stage aging. The results indicated that cryogenic deformation increased coarse second-phase particle fragmentation and led to massive dislocation proliferation, which facilitated particle dissolution due to the formation of more interfaces and dislocation-promoted dissolution. These increased the driving force for aging precipitation and produced denser matrix precipitates and narrower grain boundary precipitate-free zones. Meanwhile, numerous dislocations provided more nucleation sites for recrystallization and thus refined grains during solution treatment. The grains were refined from 500 μm in the undeformed state to an average size of 35 μm under 20% cryogenic deformation. The excessive dislocation energy and stored deformation energy at 30% deformation caused recrystallized grain growth. The optimal mechanical properties were obtained at 20% cryogenic deformation with an ultimate tensile strength, yield strength, and elongation of 499 MPa, 435 MPa, and 13.1%, which were respectively 1.6%, 2.1%, and 37.9% higher than those of the undeformed sample. • Uniformly fine equiaxed grains and the best elongation were obtained at 20% cryogenic deformation. • Excessive dislocation energy and stored deformation energy at 30% deformation led to recrystallized grain growth. • Increasing the deformation and decreasing the temperature resulted in denser matrix precipitates. • Increasing the deformation and decreasing the temperature resulted in narrower grain boundary PFZs.

Microstructural Impact on Fatigue Crack Growth Behavior of Alloy 718

Alloy 718 for forged parts can form a wide range of microstructures through a variety of thermo-mechanical processes, depending on the number of remelting processes, temperature and holding time of homogenization annealing, cogging and the number of forging steps depending on the forming characteristics. In industrial practice, these processing steps are tailored to achieve specific mechanical and microstructural properties in the final product. In the present work, we investigate the dependence of the threshold of stress intensity factor range ΔKth on associated microstructural elements, namely grain size and distribution. For this purpose, a series of tests with different starting microstructures were performed at the falling stress intensity factor range, ΔK, and a load ratio of R = 0.1 to evaluate the different threshold values. Fracture initiation and crack propagation were analyzed afterward using scanning electron microscopy of the resulting fracture surfaces. In order to obtain comparable initial conditions, all specimens were brought to the same strength level by means of a two-stage aging heat treatment. In the future, this knowledge shall be used in the context of simulation-aided product development for estimating local fatigue crack propagation properties of simulated microstructures obtained from forging and heat treatment modeling.

Influence of aging treatment on the microstructure, mechanical properties and corrosion behavior of Al-Zn-Mg-Sc-Zr alloy

The precipitation behaviour, mechanical properties and corrosion performance of an Al-Zn-Mg-Sc-Zr alloy under one-stage aging (T6), two-stage aging (T74) and retrogression and re-aging (RRA) treatment were mainly investigated. For the RRA treatment, with increasing temperature and duration of retrogression, the grain interior precipitates (GIPs) gradually grew and changed from GP zones + η′ phases to η′ phases + η phases. In addition, the discontinuity of coarsened grain boundary precipitates (GBPs) significantly increased, and the width of precipitate free zones (PFZs) became broader. Consequently, progressive decrease in strength and hardness, and improved corrosion resistance of the alloy can be observed. Compared to conventional T6 and T74 treatments, a good balance of mechanical properties (168.5 HV for hardness, 547 MPa for ultimate tensile strength) and corrosion resistance can be obtained with retrogression treatment at 160 °C for 30 min. Moreover, research on the localized corrosion of the Al-Zn-Mg-Sc-Zr alloy showed that, at the initial stage of pitting corrosion, the Al matrix around the cathodic α-Al(Fe,Mn)Si particles was uniformly dissolved, leading to the detachment of the α-Al(Fe,Mn)Si particles from the matrix and the formation of pitting pits.

Optimizing strength and electrical conductivity of Cu-Cr-Zr alloy by two-stage aging treatment

Cu-Cr-Zr alloy can be strengthened by modulating the aging process. In this work, the effect of two-stage aging on the properties of Cu-Cr-Zr solid solution was studied. Hardness, tensile properties and electrical properties were tested, and the microstructures were examined by transmission electron microscope. The results show that pre-aging can facilitate the formation of GP zones in the matrix, and thus accelerate the precipitation of high-density nano-sized phase in a shorter time as compared to that in the one-stage aging process. The two-stage aging route harmonizes the strength, ductility and electrical conductivity of the alloy, and provides an accessible strategy to ameliorate the comprehensive properties of the Cu-Cr-Zr alloy.