The effect of retrogression and re‐aging treatment on mechanical and tribological features of AA7075 aluminium alloy

In this study, T651‐applied AA7075 alloy was subjected to retrogression and re‐aging (RRA) process. Various retrogression temperatures (180 °C, 280 °C, 370 °C) and times (15 min, 30 min, 90 min) were used to determine the effects of temperature and time on the mechanical and tribological properties of the AA7075 alloy. All re‐aging stages were performed at 120 °C for 24 hours. Retrogression and re‐aging‐applied specimens were characterized by scanning electron microscope, transmission electron microscope, x‐ray diffraction, Charpy V‐notch impact and tensile tests. Brinell hardness measurements and ball‐on‐disc type tribometer measurements by using AISI 316 ball as a counterpart have also been conducted. Grain boundary precipitates in the T651‐applied specimen was transformed from continuous to the discontinuous structure after retrogression and re‐aging process. Continuous MgZn₂ precipitates at grain boundaries were disintegrated and re‐precipitated along the grain boundaries. The sizes of intragranular precipitates have become coarsened by comparison with the T651 condition. Hardness, tensile strength and wear resistance were decreased whereas impact toughness values were increased with increasing retrogression temperature and time. The best wear resistance was obtained in the sample treated at 180 °C for 15 minutes.     

Effect of heat treatment on the microstructure and mechanical properties of the spray-deposited Al–10.8Zn–2.8Mg–1.9Cu alloy

In this study, effect of various aging tempers (T6, T73 and RRA treatment) on the microstructure and mechanical properties of the spray-deposited Al–10.8Zn–2.8Mg–1.9Cu alloy was studied using high-resolution electron microscopy, selected area diffraction, and tensile tests. The results indicate that the two types of GP zones, GPI and GPII, are major precipitates for the alloy under T6 condition. No clear precipitation free zone was observed, and the grain boundary precipitates were continuous. Under two-step aging condition, the GP zones and η′ are major precipitates for the alloy, the discontinuous grain boundary precipitates are favorable to SCC resistance in over-aged condition, which reduces its strength 58 MPa (about 7%) compared to the peak-aged condition. After retrogression and re-aging treatment, the grain boundary precipitates are discontinuous, which is closed to that resulting from T73 temper. RRA treatment decreased ultimate tensile strength 25 MPa (about 3%) in values compared with the alloy at T6 condition.     

Influence of repetitious-RRA treatment on the strength and SCC resistance of Al-Zn-Mg-Cu alloy

Influence of repetitious-retrogression and reaging (repetitious-RRA) treatment on the strength and SCC resistance of Al-Zn-Mg-Cu alloy was investigated by the slow strain rate technique (SSRT) and electrical conductivity test, using TEM, EDS and SEM. Additionally, the strength of alloy was characterized by Vickers hardness. The results show that within 3 times RRA treatments, SCC resistance increases with the increase of time, while maintaining strength as compared to RRA treatment, TEM observation along with EDS analysis indicates that within 3 times, matrix precipitates (MPs) in repetitious-RRA treatment are similar to those by RRA treatment and grain boundary precipitates (GBPs) become more discrete and coarser with the time. However, RRA treatment reaches 4 times, the SCC resistance gets worse because MPs are also coarse. Besides, the Cu content of GBPs increases with the time, inversely for Zn content, which is another reason for improving the SCC resistance.     

Two-stage double peaks ageing and its effect on stress corrosion cracking susceptibility of Al-Zn-Mg alloy

Different artificial two-stage ageing behaviors and their effect on stress corrosion cracking(SCC) susceptibility of Al-Zn-Mg alloy have been investigated. The experimental results show that two hardness peaks present on the second-stage ageing-hardening curve when the first-stage ageing is dealt with comparatively lower temperature than the conventional one. The first peak is caused by dispersive and evenly distributed G.P. zones, while η' phases and coarsened G.P. zones contribute to the second peak. Tensile strength of experimental alloy raises 9.6%(33.2 MPa) and SCC susceptibility decreases 38.9% by applying the second peak ageing regime instead of conventional T73. Al-Zn-Mg alloy obtains high strength and SCC resistance due to its finely dispersive matrix precipitates(MPts), coarsened and discontinuous grain boundary precipitates(GBPs), as well as the narrow precipitate free zone(PFZ) in the second peak ageing condition.     

Significantly enhanced the ductility of the fine-grained Al–Zn–Mg–Cu alloy by strain-induced precipitation

A fine-grained structure of Al–Zn–Mg–Cu alloy was produced by a successive two-step deformation (STD) process based on strain-induced precipitation (SIP). The fine-grained alloy treated by the STD process exhibited significantly superior tensile ductility than the conventional hot-deformed (CHD) alloy. Effects of the STD process on microstructure and mechanical properties were investigated, in conjunction with fracture characterizations. Numerous spherical precipitates and dense dislocations were induced by the SIP at 300 °C. A fine lamellar structure was formed during subsequent heating and hot deformation of the STD process, finally contributing to the fine-grained T6-aged alloy. Due to the fine-grained structure, more dimples in the fracture surface of the STD treated alloy were produced than those of the CHD treated alloy. TEM in-situ testified that the grain boundary precipitates (GBPs) originated the initiation of micro-cracks, and the cracks propagated along the (sub)grain boundaries during the tensile loading. The initiation and propagation of micro-cracks were explained in terms of grain boundary precipitates (GBPs), precipitation free zones, and grain refinement. Although initiation and propagation of the cracks easily occur to coarse GBPs and grain boundaries, the fine-grained structure obtained by the STD treatment could effectively delay these behaviors and improve mechanical properties.     

Fatigue behaviour of friction stir welded AA2024‐T3 alloy: longitudinal and transverse crack growth

The fatigue crack growth properties of friction stir welded joints of 2024‐T3 aluminium alloy have been studied under constant load amplitude (increasing‐ΔK), with special emphasis on the residual stress (inverse weight function) effects on longitudinal and transverse crack growth rate predictions (Glinka's method). In general, welded joints were more resistant to longitudinally growing fatigue cracks than the parent material at threshold ΔK values, when beneficial thermal residual stresses decelerated crack growth rate, while the opposite behaviour was observed next to K_C instability, basically due to monotonic fracture modes intercepting fatigue crack growth in weld microstructures. As a result, fatigue crack growth rate (FCGR) predictions were conservative at lower propagation rates and non‐conservative for faster cracks. Regarding transverse cracks, intense compressive residual stresses rendered welded plates more fatigue resistant than neat parent plate. However, once the crack tip entered the more brittle weld region substantial acceleration of FCGR occurred due to operative monotonic tensile modes of fracture, leading to non‐conservative crack growth rate predictions next to K_C instability. At threshold ΔK values non‐conservative predictions values resulted from residual stress relaxation. Improvements on predicted FCGR values were strongly dependent on how the progressive plastic relaxation of the residual stress field was considered.     

Microstructures and properties evolution of Al-Zn-Mg-Cu alloy under electrical pulse assisted creep aging

Creep aging (CA) is a promising forming technology for integral panels with complex structures. However, how to balance excellent corrosion resistance and strength is still a challenge for CA parts. By synchronously applying electrical pulse (300 Hz, 15 A/mm²) in the middle of steady-state CA of Al-Zn-Mg-Cu alloy, the electrically assisted CA (ECA) is proposed to induce retrogression rapidly for realizing a three-step aging (retrogression and re-aging, RRA) in CA to tailor the needed properties balance. It is compared with conventional CA with one-step aging (peak aging, T6 or over aging, T7), two-step aging (over aging, T73) and RRA in hardness, intergranular corrosion and microstructures. For the ECA of pre-aging and re-aging for 20 h plus electropulsing for 10 min, the hardness is 10.5%, 20.5% and 18.9% higher than those of CA with T6, T7 and T73 processes, respectively, while the corrosion resistance is higher than T6 process and lower than T7 and T73 processes. Although the hardness and corrosion resistance are comparable to those of the CA with the RRA process, ECA takes one hour less due to the accelerated effect of the electropulsing and is not limited by the thick plates. The improved comprehensive performance of the ECA sample is due to both fine intragranular precipitates η′ and large discontinuous grain boundary precipitates η.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-022-00404-2     

Fatigue crack growth characteristics of inertia friction welded Ti17 alloy

Fatigue crack growth rates (FCGR) of the inertia friction welded Ti17 alloy joint was studied at room temperature (RT) and 400 °C at a stress ratio of 0.1. Microstructural analysis and mechanical tests were also carried out. The results show that at RT, FCGR of weld metal with recrystallisation microstructure was higher compared with the base metal and the thermo‐mechanically affected zone. At 400 °C, however, the difference of the FCGR became insignificant. Fractographic observation showed that the failure of the base metal was dominated by slip band in transgranular mode at both RT and 400 °C, whereas crack in weld metal grew in intergranular mode at RT but in transgranular mode at 400 °C. The combined effects of lower yield strength and bigger yield‐ultimate tensile strength difference may be responsible for the higher fatigue crack growth (FCG) resistance of inertia friction welded Ti17 alloy at elevated temperature.     

Microstructure evolution of directionally solidified Ni–43Ti–7Al alloy during heat treatment

The microstructure change of directionally solidified Ni–43Ti–7Al alloy after heat treatment has been investigated by transmission electron microscopy and back scattered electron imagings in this paper. After solution and aging treatment, the NiTi + Ti₂Ni anomalous eutectic structure nearly cannot be observed and Ti₂Ni phases become spheroidized. The β′-Ni₂TiAl precipitates are nearly spherical at the early stage of aging at 800 °C (0.1–1 h); however, they become aligned along 〈100〉 directions and change to cubic shape after aging for 20 h. In the course of further aging, the coarsened semi-coherent β′ precipitates occur preferentially in intercellular regions. Then the coarsened β′ precipitates begin to occur in intracellular regions with the increasing aging time and aging temperature. These β′ precipitates change the shape to sphere and plate, accompanied with loss of their coherency by introducing interface dislocations surrounding them. Finally, the formation mechanism of β′ precipitates are discussed compared to other studies in the Ni–Ti–Al system.     

Effects of pre-stretching on corrosion resistance of a creep-aged Al–5.87Zn–2.07Mg–2.42Cu alloy

In this experiment, the corrosion resistance of an Al–5.87Zn–2.07 Mg–2.42Cu alloy with different pre-stretchings (0, 2, 4, 6%) after creep aging (CA) was investigated. It is found that the corrosion performance reaches the optimal value at 4% pre-stretched sample, which owns the largest stress corrosion sensitivity factor r_tf (93.8%) and the shallowest intergranular corrosion depth (24.7 µm). When the degree of pre-stretching before CA rises, coarser and sparser intragranular precipitates (IGPs) are observed after CA. The average size of IGPs rises from about 2.2 to 5.8 nm while the density declines from about 5.4?×?10¹⁶ to 3.5?×?10¹⁶ cm^?3, and thus diminish the electron scattering effect and release the strain field. Moreover, the grain boundary precipitates (GBPs) become larger and more discontinuous and the precipitate-free zones (PFZs) broaden. The average size of GBPs rises from about 9.3 to 23.9 nm, and the average distance between GBPs increases from about 11.8 to 39.4 nm. Additionally, the segregation degree of Mg reduces and the content of Cu of GBPs rises with the increase of pre-stretching. These influence factors reduce the possibility of anodic dissolution and hydrogen-induced cracking (HIC). However, the expanding PFZ increases the potential difference between matrix and PFZ, promoting the anodic dissolution. Therefore, the corrosion resistance of an Al–5.87Zn–2.07 Mg–2.42Cu alloy can be improved by applying 4% pre-stretching before CA.

     

Statistical modeling of fatigue crack growth rate in pre-strained 7475-T7351 aluminum alloy

In this study, 7475-T7351 aluminum strips were subjected to two tensile pre-strain levels of 3% and 5%. Using compact tension C(T) specimens, fatigue crack growth tests were conducted under constant amplitude loading at stress ratios of 0.1 and 0.5 in air and at room temperature. Three fatigue crack growth rate (FCGR) models, namely, Collipriest, Priddle, and NASGRO were examined. To handle the effect of stress ratio on FCGR, Walker equivalent stress intensity factor model was used. Consequently, generalized Collipriest (GC), generalized Priddle (GP), and generalized NASGRO (GN) models were developed and fitted to the FCGR data. It was shown that both GC and GP models fit the FCGR data in a similar fashion. However, the GP model provided a better fit than the GC model. The GN model was found to be the most appropriate model for the data. Therefore, this model may be suggested for use in critical applications, such as aeronautical structural design.     

Crack growth predictions using a strip‐yield model for variable‐amplitude and spectrum loading

Fatigue‐crack‐growth tests were conducted on compact, C(T), specimens made of D16Cz aluminum alloy. Constant‐amplitude tests were conducted over a range of stress ratios (R = P_min/P_max = 0.1 to 0.75). Comparisons were made between test data from middle‐crack tension, M(T), specimens from the literature and C(T) specimens. A crack‐closure analysis was used to collapse the rate data from both specimen types into a fairly narrow band over many orders of magnitude in rates using proper constraint factors. Constraint factors were established from single‐spike overload and constant‐amplitude tests. The life‐prediction code, FASTRAN, which is based on the strip‐yield‐model concept, was used to calculate the crack‐length‐against‐cycles under constant‐amplitude (CA) loading and the single‐spike overload (OL) tests; and to predict crack growth under variable‐amplitude (VA) loading on M(T) specimens and simulated aircraft loading spectrum tests on both specimen types. The calculated crack‐growth lives under CA and the OL tests were generally within ±20 % of the test results, the predicted crack‐growth lives for the VA and Mini‐Falstaff tests on the M(T) specimens were short by 30 to 45 %, while the Mini‐Falstaff+ results on the C(T) specimens were within 10 %. Issues on the crack‐starter notch effects under spectrum loading are discussed, and recommendations are suggested on avoiding these notch effects.     

Fatigue crack growth in 7249‐T76511 aluminium alloy under constant‐amplitude and spectrum loading

Fatigue crack growth tests were conducted on compact, C(T), specimens made of 7249‐T76511 aluminium alloy. These tests were conducted to generate crack growth rate data from threshold to near fracture over a wide range of load ratios (R). Four methods were used to generate near threshold data: (1) ASTM E‐647 load reduction (LR), (2) compression pre‐cracking constant‐amplitude (CPCA), (3) compression pre‐cracking LR, and (4) constant crack mouth opening displacement LR method. A crack closure analysis was used to develop an effective stress‐intensity factor range against rate relation using a constraint factor (α = 1.85). Simulated aircraft wing spectrum tests were conducted on middle crack tension, M(T), specimens using a modified full‐scale fatigue test spectrum. The tests were used to develop the constraint‐loss regime (plane strain to plane stress; α = 1.85 to 1.15) behaviour. Comparisons were made between the spectrum tests and calculations made with the FASTRAN life prediction code; and the calculated crack growth lives were generally with ±10% of the test results.     

Vorhersagemodell für die Wachstumsraten kurzer Risse auf der Basis von Kmax‐konstant‐Versuchen an M(T)‐Proben

Prediction model for the growth rates of short cracks based on K_max‐constant tests with M(T) specimens The fatigue crack growth behaviour of short corner cracks in the Aluminium alloys Al 6013‐T6 and Al 2524‐T351 was investigated. The aim was to determine the crack growth rates of small corner cracks at stress ratios of R = 0.1, R = 0.7 and R = 0.8 and to develop a method to predict these crack growth rates from fatigue crack growth curves determined for long cracks. Corner cracks were introduced into short crack specimens, similar to M(T)‐specimens, at one side of a hole (Ø = 4.8 mm) by cyclic compression (R = 20). The pre‐cracks were smaller than 100 μm (notch + precrack). A completely new method was used to cut very small notches (10–50 μm) into the specimens with a Focussed Ion Beam. The results of the fatigue crack growth tests with short corner cracks were compared with long fatigue crack growth test data. The short cracks grew at ΔK‐values below the threshold for long cracks at the same stress ratio. They also grew faster than long cracks at the same ΔK‐values and the same stress ratios. A model was developed on the basis of K_max‐constant tests with long cracks that gives a good and conservative prediction of the short crack growth rates.     

Macromechanics study of stable fatigue crack growth in Al–Cu–Li–Mg–Ag alloy

This study was made on a fresh variety of Al–Li base alloy to investigate the role of ageing precipitates and microstructure dimensions in the fatigue crack growth resistance. The fatigue crack growth rate was measured in three different states of the material (i.e. base metal in T8 condition, friction stir weld and laser beam weld in full‐aged condition). Metallurgical analysis showed that the base metal in T8 temper is precipitation hardened by an equivalent amount of δ′ (AL₃Li), T₁ (AI₂CuLi) and θ′ (AI₂Cu) precipitates. The friction stir weld retained the morphology of strengthening precipitate; however, coarsening of Cu containing precipitates has occurred. On the other hand, laser beam weld showed a different type of CuAl phase morphology, which is characteristic of cast metal. The results of fatigue tests confirmed that fatigue crack growth resistance largely depends on microstructural features, specifically the strengthening phases. The fatigue crack resistance was in the order of base metal > laser beam weldment > friction stir weldment. The CuAl phase played a vital role in the crack closure of the laser beam weldment, thus enhancing the fatigue life as compared with the friction stir weldment, which was evident from the plot between log of da/dN (crack growth in each cycle) and log of ΔK (stress intensity range).     

The fatigue crack growth behaviour of 2524‐T3 aluminium alloy in an Al2O3 particle environment

The effect of the Al₂O₃ dust environment on the crack propagation behaviour of 2524‐T3 Al alloy was investigated. The results show that the Al₂O₃ dust environment reduces the fatigue crack growth rate (FCGR) of alloy especially at low ΔK. Many Al₂O₃ particles are deposited and stuck in the crack during fatigue loading which promotes crack closure, while this effect is gradually weakened with the increase of ΔK. The deposited Al₂O₃ particles induce the disorderly arranged slip bands (SBs) ahead of the crack tip which deflects the crack path making it more tortuous in the Al₂O₃ dust.     

Improving the properties of aluminium alloys by retrogression and reageing

Retrogression and reageing heat treatments offer the potential of improved tensile properties in combination with greatly increased resistance to stress corrosion cracking. The potential of this technique is reviewed with respect to the current application in the European and North American Aerospace Industry. To illustrate the performance increase associated with RRA treatments, the stress corrosion cracking performance of the established aerospace plate and forging alloy 7010 has been evaluated using an alternate immersion constant load tensile type test (ECSS‐Q‐70‐37A) Specimens were cut from a large aerospace rectilinear forging and tested in three different tempers, T652, T7452 and a retrogressed and reaged condition (RRA). In the T652 condition the material has been shown to be highly susceptible to intergranular corrosion and stress corrosion cracking. In the T7452 and RRA conditions 7010 showed much improved resistance to SCC but pitting corrosion resulted in failure of some specimens within the 30day requirement of the test standard.     

7075T73510合金断裂韧性和疲劳裂纹扩展

研究了７０７５Ｔ７３５１０合金的断裂韧性和疲劳裂纹扩展行为，分析不同方向性能差异的原因。结果表明：合金的断裂韧性依赖于取向。Ｌ－Ｔ方向的断裂韧性值远大于Ｔ－Ｌ方向。而其疲劳裂纹扩展速率对应力比更敏感，无论Ｔ－Ｌ还是Ｌ－Ｔ方向，Ｒ＝０．６时的ＦＣＧＲ值是Ｒ＝０．１时的二倍左右。在相同应力比和较小的△Ｋ下，两个方向的ＦＣＧＲ值基本无差异。文中认为：呈带分或串状分布的杂质相对Ｔ－Ｌ向怕裂韧性影响较大。     

含Zr、Sc的Al-Zn-Mg-Cu合金的低周疲劳行为

利用透射电子显微镜和低周疲劳试验机研究了单级时效状态及回归再时效状态两种含Zr、Sc的Al-Zn-Mg-Cu合金的微观组织和低周疲劳性能。结果表明:单级时效基体析出相以η′相为主,晶界析出连续分布平衡相,并伴有晶间无析出带;回归再时效基体析出相略有长大,晶界析出相长大明显,无析出带变宽。低周疲劳加载条件下,合金在0.4%~0.7%外加总应变幅范围内表现出循环稳定性;在0.8%的应变幅下,呈现先软化后硬化。在0.4%~0.6%较低的外加总应变幅范围内,回归再时效合金表现出较高的低周疲劳寿命。两种状态合金的塑性应变幅和弹性应变幅与载荷反向周次之间均成直线关系,并可分别用Coffin-Manson公式和Basquin公式来描述。两种状态的合金的疲劳裂纹均萌生于试样表面,并以穿晶方式扩展。     

Improved test method for very low fatigue‐crack‐growth‐rate data

Currently, in North America, the threshold crack‐growth regime is experimentally defined by using ASTM Standard E647, which has been shown in many cases to exhibit anomalies due to the load‐reduction (LR) test method. The test method has been shown to induce remote closure, which prematurely slows down crack growth and produces an abnormally high threshold. In this paper, the fatigue‐crack growth rate properties in the threshold and near‐threshold regimes for a titanium alloy, Ti‐6Al‐4V (STOA), are determined by using the LR test method and an improved test method. The improved method uses ‘compression–compression’ precracking, as developed by Pippan, Topper and others, to provide fatigue‐crack‐growth rate data under constant‐amplitude loading in the near‐threshold regime, without load‐history effects. Tests were conducted over a wide range in stress ratios (R = 0.1–0.7) on compact C(T) specimens for three different widths (25, 51 and 76 mm). The slitting method was used on 51 mm C(T) specimens to confirm that the material did not contain significant levels of residual stresses from forming and/or machining. A crack‐mouth‐opening‐displacement gage was used to monitor crack growth. Data from the ASTM LR method gave near‐threshold values that were found to be dependent upon the specimen width. However, data from the compression precracking constant amplitude (CPCA) loading method gave near‐threshold data independent of specimen width. A crack‐closure analysis was performed for both the LR and CPCA data, to correlate data at the various stress ratios. The CPCA data correlated well with the effective stress‐intensity factor range against rate relation, whereas the LR data exhibited significant threshold fanning with both stress ratio and specimen width.