Overaging Treatment Research Articles

Influence of microstructure characteristics on the fatigue properties of 7075 aluminum alloy

Grain size and aging state are the two major microstructural factors affecting the mechanical properties of Al alloys. In this study, fatigue tests were conducted on 7075 Al alloy with varying grain sizes and aging states. The results indicate that both grain refinement and over aging (OA) treatment are beneficial for enhancing fatigue performance, with the 7075-Min alloy exhibiting the highest fatigue strength of 265.8MPa in the OA state. For grain size, a quantitative positive linear relationship was found between the reciprocal of the square root of the average grain size and the fatigue strength coefficient. Grain refinement optimizes the fatigue performance by increasing the fatigue strength coefficient. Compared to the under aging (UA) state, OA treatment promotes the growth and coarsening of precipitates at grain boundaries (GBs) and within the interior of grains. This change in precipitate morphology effectively facilitates dislocation cross slip, reducing fatigue damage caused by dislocation blocking. The impact of the precipitation-free zone (PFZ) on fatigue performance is negligible compared to that of the precipitates. This study clarifies the relationship between microstructure and fatigue performance, providing valuable insights for microstructure regulation to optimize the fatigue performance of Al alloys.

Ductility fracture mechanisms of Al-7Si-Mg casting alloys considering Si particles: A combined experimental and crystal plasticity study

Al-7Si-Mg casting alloys are widely utilized for their excellent formability, though they commonly exhibit poor ductility. This study comprehensively investigates the factors influencing the ductility of Al-7Si-Mg casting alloys, proposing a fracture mechanism and developing a model for ductile fracture of the alloy. Tensile tests were conducted on samples subjected to different aging times, with analysis of the alloy’s microstructure and tensile fracture surfaces. The findings reveal that fracture initiation occurs at clusters formed by the aggregation of Si particles, followed by the coalescence of cracks within these clusters, leading to macroscopic fracture. Different tensile strains were applied to specimens subjected to under-aging and over-aging treatments. The distribution of geometrically necessary dislocations (GNDs) in stretched samples indicates that the Si cluster area undergoes greater localized plastic deformation compared to the Al matrix, and this disparity increases with the applied strain. It has been observed that the reduced elongation observed in over-aged alloys is due to the coalescence of Si clusters under lower strain conditions. Representative volume elements with the characteristics of Si cluster is proposed, and the crystal plasticity finite element method is used for tensile simulations. The simulation results show that the proposed model is more consistent with experimental findings pertaining to elongation than with the results of a conventional void model. A longer aging time reduces ductility owing to a more intense strain localization under same tensile strain.

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.

Coarsening mechanism of over-aged δ-Ni2Si nanoscale precipitates in Cu-Ni-Si-Cr-Mg alloy

δ-Ni2Si precipitate is the crucial strengthening phase in Cu-Ni-Si alloy, and its coarsening behavior and mechanism impose dramatic effects on mechanical properties. In this paper, Cu-3Ni-0.7Si-0.1Cr-0.05 Mg (wt%) alloy was prepared to systematically investigate the coarsening behavior and elucidate the coarsening mechanism of δ-Ni2Si precipitate during over-aging treatment. Featured core-shell structure was observed via careful characterization during coarsening of δ-Ni2Si phase. Specifically, the core structure was stable δ-Ni2Si, while the shell structure was a metastable interface composed of Cu, Ni and Si elements. At later over-aging stage, the core-shell structure completely transformed into a stable and coarsened δ-Ni2Si phase being coherent with the Cu matrix. It is confirmed for the first time that the coarsening process of δ-Ni2Si precipitate can be interpreted by the trans-interface-diffusion-controlled (TIDC) coarsening theory. The core-shell structure was caused by the disparity between the diffusion activation energy and diffusion rate of Ni, Si, and Cu from the perspective of diffusion kinetics. Moreover, the presence of trace amounts of Cr and Mg components exhibited little effect on the coarsening process of δ-Ni2Si. During over-aging, tensile strength dramatically decreased owing to diminishing precipitation strengthening effect from coarsening of δ-Ni2Si precipitates.

Effect of aging temperature on microstructure and corrosion behavior of 6082 aluminum alloy

The effect of artificial aging temperatures on microstructure precipitation characteristics and corrosion behavior of extruded 6082 aluminum alloy had been investigated. Electrochemical measurements and microstructure characterization techniques were conducted on the alloy with under-aging, peak-aging and over-aging. The results show that the alloy had the maximum corrosion current density and corrosion depth under peak-aging, and the over-aging treatment optimized the corrosion behavior. The fiber texture in extruded alloy was the weak site of corrosion, and the Si particles distributed along the plastic deformation flow lines acted as a bridge to the propagation of corrosion cracks. The coarsening and dissolution of precipitates β״- Mg2Si reduced the corrosion damage along the plastic deformation flow lines, leading to the change of corrosion model. The polarity change caused by β״- Mg2Si de-alloying behavior was the main driving force for the development of IGC.

Coarsening behavior of Al3Sc interface precipitates and related impact on mechanical properties of AlFeNi-Sc alloy at 400 °C

In the eutectic AlFeNi-0.2Sc alloy, microstructural features for two types of Al3Sc precipitates (i.e., matrix precipitates (Al3Sc(m)) and interface precipitates (Al3Sc(i)) were identified, and their effects on mechanical properties were analyzed and discussed. At 300 °C peak-aged state, both Al3Sc(m) and Al3Sc(i) precipitates exhibited isotropic strengthening behavior on Al/Al9FeNi eutectic structure. Through 400 °C over-aged treatment, Al3Sc(m) precipitates transformed to Al3Sc(i), making the excessive coarsening of Al3Sc(i) precipitates. Coupling with the morphological feature of Al9FeNi phase, the anisotropic strengthening behavior was induced on eutectic structure, as confirmed by hardness test. Based on these findings, the strengthening effects from both precipitates were modelled quantitatively, showing the transition from Orowan strengthening mechanism by Al3Sc(m) to load transfer effect by Al3Sc(i). As a result, AlFeNi-0.2Sc alloy exhibited superior 400 °C creep properties among available reports for Al alloys. Analyzing the creep constitution equation, the threshold stress from Al3Sc(m) was accounted for minor strengthening effect, and the load transfer effect from Al3Sc(i) worked as major strengthening effect to 400 °C creep of alloy. This study revealed that Al3Sc precipitates can effectively strengthen the eutectic structure in different forms. The excessive coarsening of Al3Sc(i) precipitates at higher temperatures was benefited to load transfer effect, which can be considered as an ideal strategy to improve the high temperature performance of Al alloys with multiphase structures.

Effects of direct aging treatment on microstructure, mechanical properties and residual stress of selective laser melted AlSi10Mg alloy

• The as-fabricated microstructure benefits short-circuit diffusion of Si. • Direct aging treatment promotes unique precipitation behaviors. • Direct aging removes roughly 40% of the residual stress. • Mechanisms of residual stress removal are explored. • Over-aging treatment produces better mechanical properties. Direct aging treatment is an important post-processing procedure, yet little research has been done on how it balances the mechanical properties and the stress removal for selective laser melted (SLMed) AlSi10Mg alloys. Here, we proposed a typical direct aging treatment on SLMed AlSi10Mg alloys, and studied the effects on their microstructure, properties and residual stress evolution. The results indicate that the as-built microstructure is mainly composed of fine cellular α-Al and reticulated Si phases, and some pre-existing precipitates and dislocations are found in these cells. The direct aging treatment promotes the precipitation of the nano-scaled Si phase and preserves the network-like Si structure. Therefore, the strength of the peak-aged alloy increases while the ductility decreases. As the aging temperature increases from 160 to 200°C, aging hardening behavior was accelerated significantly. Aging at 160 °C for 4-9 h removes 32.0%-43.0% of the residual stress, which is attributed to the decomposition of the supersaturated α-Al matrix, the precipitation of the nano-Si phase and the exposure of low-angle grain boundaries (LAGBs). Considering the overal alloy performance obtained, over-aging at 160°C for 4 h is the optimized heat treatment regime. Under this condition, the yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) of the alloy in the transverse and longitudinal direction are 309.5 MPa, 464.4 MPa and 8.3% and 286.4 MPa, 464.9 MPa and 5.1%, respectively.

Variation of liquation cracking susceptibility for over–aged 247LC superalloy repair weld heat–affected zone

In this study, the liquation cracking temperature range (LCTR) of over-aged 247LC superalloy repair welds was quantitatively evaluated using spot-Varestraint testing, especially for application in the repair weld heat-affected zone (HAZ). The metallurgical mechanism of the evaluated liquation cracking susceptibility and variation of the LCTRs of the over-aged 247LC superalloys was clarified by analyzing the microstructural characteristics at the liquation cracking surface, and using thermodynamic calculations. From the spot-Varestraint tests of the over-aged materials, the LCTR was quantitatively evaluated to be 410 and 620 K for the 500 and 1000 h over-aged specimens, respectively. These results demonstrate that the over-aged 247LC superalloys face significant and serious liquation cracking susceptibility in repair weld HAZ. Based on local element distributions at the liquation cracking surface, analyzed by transmission electron microscopy and the Thermo-Calc calculations, the underlying metallurgical mechanism of high LCTRs of overaged materials could be attributed to the precipitation of fine MC and M6C carbides during the over-aging treatment. The carbides promote local liquation at the interface between the gamma phase at temperatures lower than the equilibrium solidus (1530 K) of the 247LC. The estimated liquation initiation temperatures at the MC/γ and γ/M6C interfaces were 1125 and 1420 K, respectively.

Effect of aging route on the precipitation behavior and thermal stability of Al–Cu–Mg–Ag alloy

In this study, the evolution of Ω plates during artificial aging and the effect of peak-aging treatment and over-aging treatment on the thermal stability of Al–Cu–Mg–Ag alloy were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was shown that the diameter distribution of Ω plates in the over-aged sample was narrower compared to the peak-aged one, which suppressed the Ostwald ripening in the over-aged sample. In addition, the Ω plates in the over-aged sample has a larger aspect ratio than that in the peak-aged sample, which makes it more likely for the soft impinge to inhibit the growth of the Ω plates. Quantitative analysis showed that after heat exposure, Ω plates in the over-aged sample contributed more to the yield strength compared to peak-aged sample. This study shows that better thermal stability can be obtained in exchange for a little loss of room temperature mechanical properties, improving the application potential of Al–Cu–Mg–Ag alloys.

Effect of Testing Conditions, Gauge, and Temper on Stress Corrosion Cracking of AA7xxx Aluminum Aerospace Plate Alloys

The European Aviation Safety Agency’s recent safety information bulletin (SIB 2018-04R2) describes a brittle intergranular cracking phenomenon in 7xxx aluminum alloys and recommends testing conditions. These testing conditions are severe compared with expectations regarding, temperature, humidity, and stress in aerostructures. In this study, strong increases in time to failure of smooth specimens tested in humid air were observed when reducing either temperature or stress to levels closer to what is representative of in-service conditions. The cracking morphologies after testing in hot, humid air were identical to those described in the literature for in-service occurrences of stress corrosion cracking (SCC), including the observation of crack arrest markings consistent with a mechanism of alternate local corrosion and crack propagation by hydrogen embrittlement. The differences between such fracture surfaces and those observed in the industry standard SCC screening test, which uses a chloride-containing solution to accelerate initiation and propagation, can be attributed to the pitting of the crack surface in a chloride-containing solution in the wake of the propagating crack. The performances of 7xxx-T7 aluminum plates SCC testing in either hot, humid air or a chloride-containing aqueous solution have been compared as a function of thickness, over-aging treatment, and composition. There is a strong effect of plate thickness in both tests, with substantial decreases in initiation time and lifetime for thinner plates. The effect of plate gauge is such that “legacy” materials (such as 7010-T7651, 7050-T7651, and 7050-T7451) passing the SIB 2018-04R2 test at high thicknesses, fail at lower gauges, yet have been flying for nearly 50 y. An increase in SCC performance with increased overaging is observed in both tests. As a consequence, comparisons of SCC performance of alloy compositions should be made wherever possible at identical plate thickness and for aging treatments that result in comparable strengths. When this is done, high Zn concentration 7449-T76++ (7449-T7651 with additional aging) shows a slightly worse strength-SCC balance than 7010-T7651 and 7050-T7651. The observed trends in SCC performance in the two tests are consistent, suggesting that similar repeated initiation and propagation mechanisms are responsible for cracking in humid air and in aqueous NaCl environments.

Electrochemical Behavior of Additively Manufactured 7050 Aluminum Alloy in Chloride Environments

The benefits of additive manufacturing (AM) process combined with the high-strength, corrosion resistant and fracture toughness properties of the 7xxx series aluminum alloys make them promising candidates for the aerospace and defense industries. However, these alloys undergo unconventional thermal cycles during additive manufacturing process like laser powder bed fusion (LPBF) as well as during post-processing treatments like hot isostatic pressing, resulting in microstructures different from equivalent wrought alloys.In our work, we investigate how the effects of microstructural differences in a modified LPBF 7050 aluminum alloy compared to an equivalent wrought alloy, results in differences in electrochemical behavior. A series of AM 7050 aluminum alloy samples with different post-processing treatments and subsequent microstructures are tested for polarization and electrochemical impedance behavior in varying chloride ion concentrations. The effects of underaging and overaging treatment on the electrochemical behavior is also studied. Localized corrosion behavior of the alloys is investigated using immersion tests and scanning vibration electrode technique (SVET), which showed differences in pitting behavior and pit stability. The results indicate that AM 7050 alloy in general, showed less corrosion susceptibility compared to an equivalent wrought alloy.

Influence of Post-Processing Conditions on the Microstructure, Static, and Fatigue Resistance of Laser Powder Bed Fused Ti-6Al-4V Components

The microstructure, static, and fatigue mechanical properties of laser powder bed fused (LPBF) Ti-6Al-4V components subjected to three different post-processing treatments (PTs) are compared. The first treatment includes stress relief (SR) and beta-phase annealing (BA) heat treatments, the second one includes SR, beta-solution (BST) and over aging (OA) heat treatments, and the third procedure is a combination of hot isostatic pressing (HIP) and BST + OA. It was demonstrated that the three PTs led to the decomposition of α’ martensite inherited from the LPBF process and the formation of variable α + β structures. The SR + BA treatment forms a basket weave structure having an average α lamellae width of ~3 µm and surrounded by ~1 µm-sized zones of segregated β phase (4.6–5.2% β phase content) and globalized α phase (~10 µm in size) inside prior columnar β grains (~100 µm in width). The SR + BST + OA treatment forms semi-equiaxed α grains (~300 µm) containing colonies (~50 µm) of parallel-oriented α plates (~6 µm), and β phase (5.8–7.5%) in the interplate spacing. The HIP + BST + OA treatment leads to the formation of large grains (~500 µm) with both basket weave and colony (~40 µm) α structures containing α plates (1.1–4.2 µm) and β phase (5–7.1%). To compare the impact of these PTs on the mechanical properties of LPBF components, they were subjected to static and fatigue tensile testing at room temperature. The best combination of mechanical properties (yield strength ~920 MPa, ultimate strength ~1000 MPa, elongation to break ~22.5%, and fatigue strength ~600 MPa, 107 cycles) was obtained in the case of SR + BA specimens. These results demonstrate that an adequate thermal treatment, such as SR + BA, of the LPBF Ti64 components, could be a valuable and less expensive alternative to the established HIP + BST + OA treatment procedure when fatigue life is the main concern.

Influence of Heat Treatment on Corrosion and Electrochemical Behaviour of Al-Mg-Si-Ag Alloys with Different Mg/Si

The influences of Mg/Si and heat-treatment on the corrosion performance of Al-Mg-Si-Ag alloy were studied. The results show that when the Mg/Si<1, apart from the precipitation of β phase, there is a small amount of elemental silicon. In the corrosion process, the Si element acts as a cathode, which on the one hand leads to the decrease of the corrosion potential, and on the other hand, it increases the corrosion rate. When Si is excessive, the βʺ density increases significantly, and the continuity of the grain boundary phase becomes higher, which promotes electrochemical corrosion, resulting in severe intergranular corrosion (IGC). In under-aging state, the alloy exhibits the best corrosion resistance. The corrosion current density in the slightly over-aged state is slightly lower than that in the peak-aged state. The slightly over-aging treatment can effectively improve corrosion resistance of the alloy. After the alloy treated by severely over-ageing, the grain boundary coarsened and result in the increase of corrosion current density.

Ultrafine-grained dual-phase maraging steel with high strength and excellent cryogenic toughness

A novel heat treatment route consisting of a low-temperature solution followed by an over-aging treatment at 500 °C is proposed to develop a high-strength, high-cryogenic-toughness maraging steel by forming an ultrafine-grained martensite (α′) and austenite (γ) dual-phase microstructure. Compared to the same maraging steel subjected to the conventional heat treatment with a mostly martensitic microstructure, the present dual-phase microstructure offers a remarkable increase by 12 times of cryogenic impact energy (~140 J at 77 K), while the yield strength is not reduced obviously. A large amount of ultrafine-grained austenite (about 50% volume fraction) are formed in the present steel due to the reversed transformation of martensite to austenite during the over-aging process at 500 °C. It is surprised that the present steel with such a high fraction of austenite still possesses a high yield strength comparable to the conventional maraging steel with a mostly martensitic microstructure. Intensive nanoprecipitates are found not only in martensite but also in austenite phases, indicating both phases have high strength. This is confirmed by nanoindentation test, showing similar hardness values in both martensite and austenite phases. Such intensive nanoprecipitates in both phases ensure the high yield strength of the present steel. The excellent cryogenic toughness of the present steel mainly origins from: (i) the pronounced amount of reversed austenite that are intrinsically tough due to their face-centered cubic (fcc) structure; (ii) transformation-induced plasticity (TRIP) toughening as some austenite grains transforming to martensite during impact test; (iii) the ultrafine-grained structure of both martensite and austenite phases. The present heat treatment route offers a potential solution for processing large engineering components for cryogenic application that require a long heat treatment duration to achieving uniform mechanical properties in the components.

Effects of heat treatment on a novel continuous casting direct rolling 6056 aluminum alloy: cold rolling characteristics and tensile fracture properties

In this study, 6056 aluminum alloy produced through continuous casting direct rolling (CCDR) was investigated. This alloy is used in the automotive industry because of its high corrosion resistance and strength. However, the material may become embrittled as a result of excessive deformation during cold forging. Therefore, preheat treatment was employed to improve the formability of 6056 aluminum alloy. Five different preheating conditions were considered in this study: annealing (O), solution treatment (W), aging for 2 h at 195 °C after solution treatment (SPA195), aging for 2 h at 230 °C after solution treatment (SOA230), and aging for 2 h at 250 °C after solution treatment (SOA250). The results show that the over-aged specimen SOA250 exhibited the best formability and could compress 85% of the original diameter after cold rolling. However, the ductility decreased considerably after cold rolling. To increase ductility and strength, a T6 treatment was applied after cold rolling. The ductility and strength of the specimen were increased significantly, similar to the manner in which the ductility and strength of a raw material that has undergone T6 heat treatment. The CCDR 6056 aluminum alloys subjected to solution treatment and over-aging treatment exhibited good cold rolling formability, and with the subsequent T6 heat treatment, the alloys exhibited high strength and ductility. The materials can thus be used for the manufacture of automotive fasteners.

The effect of cold rolling, double aging and overaging processes on the tensile property and precipitation of AA2024 alloy

In this investigation, the effect of the amount of cold rolling on the single stage and double stage aging responses of an AA2024 aluminum alloy was assessed. Two cold deformation amounts were applied on the solution heat treated AA2024 sheets. The effect of these deformation on a two-step short time aging performed at low, and subsequently at high temperatures, were studied in terms of precipitation and mechanical properties variations. As well, the effect of rolling percentage on the singe stage of overaging was assessed. Microstructural evolution was studied by optical microscope (OM) and Field Emission Scanning Electron Microscope (FE-SEM) and Transmission Electron Microscope (TEM). Mechanical properties were assessed through tensile testing and microhardness variation. Electrical conductivity (EC) measurements were also made to track the changes of precipitation condition. Results showed that applying cold rolling substantially increased the strength and slightly reduced ductility. In addition, performing the cold rolling created better condition, through dislocation generation, for the strengthening precipitates. Under such a condition, a short time aging through double aging process provided higher strength level compared with the long aging process of the overaging treatment. TEM studies showed the development of sub-structure through dislocation cells formation for the cold deformed cases. It also showed that very small size S′ hardening precipitates are present for the highest level of strength/hardness achieved in the double aging process. Also TEM studies suggested that no morphological changes occurred in the highest strength level achieved for the double aged conditions. Microhardness and electrical conductivity variation followed the same trend as the strength variation, for both the double aging process and the overaging condition.

Effects of Pre-stretching and Aging Treatments on Microstructure, Mechanical Properties, and Corrosion Behavior of Spray-Formed Al-Li Alloy 2195

In this study, different pre-stretching and aging processes were applied to spray-formed Al-Cu-Li alloy 2195. The effects of different processes on the microstructures, mechanical properties, and corrosion behaviors of the alloy were examined. The strengthening and corrosion mechanisms were investigated. The samples treated with pre-stretching and aging have higher strength than the only aged samples. However, the strength is not significantly improved with the increase in the amount of pre-stretching. The pre-stretching treatment can suppress the formations of grain boundary precipitates (GBPs) and precipitate-free zones (PFZ), and the intergranular corrosion (IGC) sensitivity of the alloy is therefore reduced. The increase in pre-stretching promotes massive precipitation of the T1 phase, which leads to the negative shift of open-circuit potential. The over-aging treatment after pre-stretching leads to the growth of the PFZ and GBPs as well as the coarsening of the T1 phase. These changes in microstructure cause a decrease in mechanical properties and the re-sensitivity to the IGC. Under the same pre-stretching deformation, the two-step aging treatment (110 °C/12 h + 160 °C/24 h) can result in more intensive precipitation of the T1 phase, which enhanced the strength to a higher level. Meanwhile, the two-step aged alloy was immune to the IGC. The evolution mechanism of IGC for the alloy with different pre-stretching and aging processes was proposed based on the TEM characterization and the observation of IGC morphology.

Effects of quenching rate and over-aging on microstructures, mechanical properties and corrosion resistance of an Al–Zn–Mg (7046A) alloy

To provide insight to the relationships between quenching rate and aging treatment on the microstructure and properties of Al–Zn–Mg alloys, 7046 A alloy, a low Cu-content commercial Al–Zn–Mg alloy, was selected in the present study. As-received hot-extruded 7046 A alloys were quenched in air and room temperature water and then aged at 120 °C for 0–120 h. The mechanical behavior, corrosion resistance and microstructures of the aged alloy were compared and analyzed. The experiment result shows that quenching rate exhibits a greater influence on the precipitates and precipitation free zones (PFZs) of aged alloys than over-aging treatment and the reasons have been discussed. The η equilibrium phases preferentially take the Al3Zr and Al6Mn dispersoids as the heterogeneous nucleation core and grow quickly inside the grains during the slow cooling process. Over-aging treatment would broaden the grain boundary PFZ and the space of grain boundary precipitates, thus improving the corrosion resistance of water quenched alloys, while it is harmful to air quenched (AQ) alloys. Pitting resistance can be summarized as: AQ24 < AQ120 < AQ60, WQ24 < WQ60 < WQ120. The yield strength, ultimate tensile strength, and elongation the T6 treated WQ alloys (∼120 °C/24 h) are ∼532 MPa, ∼568 MPa, ∼12.4%, respectively; and the corresponding values of T6 treated AQ alloys (∼120 °C/60 h) are ∼475 MPa, ∼521 MPa, ∼11.4%. Long-term over-aging (120 °C/120 h) has little effect on the hardness, while the tensile properties decrease by ∼5–6% compared to T6 treatment.

Effect of thermo-mechanical alloying and ageing in AA2014 aluminium alloys after synthesized using graphite

There is always a necessity to tailor new age materials for use in aircraft and automobile industries, and also in sophisticated fields at which resistance to elevated temperatures is of prime importance. To ensure the stability of high-strength levels of conventionally precipitation-hardened aluminium (Al) alloys, thermo-mechanical alloying and ageing treatment (TMAAT) was applied to AA2014 Al alloys. Essentially, AA2014 alloy was thoroughly decorated with fine Al dispersoids and precipitates by means of ball-milling and conventional ageing. Hence, using powder metallurgy methodology, powders of AA2014 alloy and graphite were mechanically alloyed (MA) for 7 h in a vertical attritor. The obtained powder mixture was then cold pressed into samples under the pressure of 800 MPa. To decorate the matrix with finely distributed $$\hbox {Al}_{4}\hbox {C}_{3}$$ phase particles (dispersoids), samples were systematically annealed (sintered) for a long time in a furnace system operating under a controlled atmosphere of argon at $$550{^{\circ }}\hbox {C}$$ . This temperature was found to form a ternary Al–Cu–Mg liquid phase in carbon-blended AA2014 alloy during sintering. However, the $$\hbox {Al}_{4}\hbox {C}_{3}$$ phase could not be observed. After subsequent homogenization and water quenching, all samples were age hardened at $$150{^{\circ }}\hbox {C}$$ for 46 h. Results confirmed that ordinary AA2014 alloy with $$\hbox {Al}_{2}\hbox {Cu}$$ precipitates over-aged normally as expected. In contrast, the material treated by MA had reinforcing $$\hbox {Al}_{4}\hbox {C}_{3}$$ particles together with $$\hbox {Al}_{2}\hbox {Cu}$$ precipitates. It was realized that this reinforced material did not soften and still retained its high peak hardness level even during prolonged over-ageing treatment. Consequently, it was apparent that TMAAT improved the strength, and therefore was promising for resistance to high-temperature exposure of the material.

Enlarged Zn, Mg Contents with a same Zn/Mg Ratio Improve Fatigue Crack Propagation Resistance of Al-Zn-Mg-Cu Alloys with T7651 State

The fatigue crack propagation of Al-Zn-Mg-Cu alloys could be influenced by the content of main alloying element. In the present work, two Al-Zn-Mg-Cu alloys with a same Zn/Mg ratio were treated by two stage over-aging aging treatment and typical T7651 states were extracted via mechanical properties. Fatigue crack propagation of the two alloys were tested and the related precipitation characteristics and fracture morphology were observed. The results showed that the alloy with higher Zn, Mg contents possessed a better fatigue crack propagation resistance compared with the alloy with lower Zn, Mg contents. The corresponding fracture morphology also showed the difference of fatigue striation, which provided an additional support. The precipitation observation demonstrated that the both alloys possessed GPII zone, η' phase and η phase while the alloy with higher Zn, Mg contents had a larger average precipitate size and a larger proportion of large size precipitates compared with the alloy with lower Zn, Mg contents. Cut and bypass mechanisms of dislocation-precipitate interactions were used to explain the difference of fatigue crack propagation between the two alloys.