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Effects of the two-step ageing treatment on the microstructure and properties of 7B04 alloy pre-stretched thick plates
Abstract
The effects of the two-step ageing parameters (temperature and time) on the mechanical properties and electrical conductivity of 7B04 (Al-Zn-Mg-Cu) pre-stretched thick plates were studied. The results reveal that the initial T1 ageing contributes a major increase of the tensile strength, and the 0.2% proof stress value reaches 482 MPa after ageing for 7 h at 115°C. Behavioral differences in the tensile properties of the alloy after the two-step ageing treatment were less with the first-step ageing at 115°C for different time periods (7, 14, and 21 h). The effects of the second ageing parameters on the properties and microstructure of the 7B04 alloy were remarkable. TEM analysis of the samples aged at Temper I (7 h at 115°C + 12 h at 160°C) and Temper II (7 h at 115°C + 16 h at 165°C) indicates that two kinds of phases, i.e. η' and η phases, precipitate from the matrix and efficiently improve the tensile strength of the alloy, and the grain boundary precipitates are coarse and discrete. There are obvious precipitate free zones (PFZs) along the grain boundary in the microstructure of the alloy after the two-step ageing treatment.
... The Cl − in the corrosive medium will also participate in the electrochemical reaction; the reaction process includes Al(OH) 3 + Cl − → Al(OH) 2 Cl + OH − , Al(OH) 2 Cl + Cl − → Al(OH)Cl 2 + OH − , and Al(OH)Cl 2 + Cl − → AlCl 3 + OH − . However, in the T74 thermal treatment, η and η phase particles at the grain boundary aggregate and spheroidise, resulting in a larger size of these intergranular precipitates [6,28,29]. In contrast to the fine intergranular precipitate of T6 thermal treatment, it effectively blocks the anode corrosion channel at the grain boundaries, preventing the corrosive medium from attacking the aluminium alloy matrix along the grain boundaries and significantly improving the intergranular corrosion resistance of 7B04 aluminium alloy. ...
... The Cl − in the corrosive medium will also participate in the electrochemical reaction; the reaction process includes Al(OH)3 + Cl − → Al(OH)2Cl + OH − , Al(OH)2Cl + Cl − → Al(OH)Cl2 + OH − , and Al(OH)Cl2 + Cl − → AlCl3 + OH − . However, in the T74 thermal treatment, η′ and η phase particles at the grain boundary aggregate and spheroidise, resulting in a larger size of these intergranular precipitates [6,28,29]. In contrast to the fine intergranular precipitate of T6 thermal treatment, it effectively blocks the anode corrosion channel at the grain boundaries, preventing the corrosive medium from attacking the aluminium alloy matrix along the grain boundaries and significantly improving the intergranular corrosion resistance of 7B04 aluminium alloy. ...
In this work, the effects of the tropical marine atmospheric environment on the corrosion behaviour of the 7B04-T74 aluminium alloy were systematically investigated by using accelerated testing, together with corrosion kinetic analysis, microstructure observation, product composition analysis, and potentiodynamic polarization curve tests. The weight loss method was used for the corrosion kinetics analysis. The surface morphology and corrosion products transformation law were investigated by OM, SEM, EDS, and XPS. The electrochemical characteristics were studied using potentiodynamic polarization curves. The research indicated that the 7B04-T74 aluminium alloy has eminent corrosion resistance in the tropical marine atmospheric environment. Localized pitting corrosion occurred rapidly in the tropical marine atmosphere. In the later stage of corrosion, the corrosion of aluminium alloy did not become serious. Specifically, no obvious intergranular corrosion was found, which is related to the thermal treatment method. Corrosion products included Al(OH)3, Al2O3, and AlCl3, of which Al(OH)3 is the most notable.
... The precipitation sequence for the ultra-high-strength 7xxx-series aluminum a has been ported as follows: supersaturated solid solution (SSS) → Guinier-Preston zones → η′ phase→η phase (MgZn2) [37]. After one-step peak aging (T6) heat treatm GP zones and η' phase are the main hardening phases precipitated in the matrix o 7xxx-series aluminum alloy, which makes the strength of the aluminum alloy reach a value [38]. However, under such a heat treatment, there will be continuous distribu of precipitated phases in the grain boundaries of the aluminum alloy. ...
... I two-step high-temperature aging process, the GP zones will be transformed into η' p and the η' phase and η phase particles at the grain boundary will be aggregated The precipitation sequence for the ultra-high-strength 7xxx-series aluminum alloys has been ported as follows: supersaturated solid solution (SSS) → Guinier-Preston (GP) zones → η phase→η phase (MgZn 2 ) [37]. After one-step peak aging (T6) heat treatment, GP zones and η phase are the main hardening phases precipitated in the matrix of the 7xxx-series aluminum alloy, which makes the strength of the aluminum alloy reach a high value [38]. However, under such a heat treatment, there will be continuous distributions of precipitated phases in the grain boundaries of the aluminum alloy. ...
The 7xxx-series aluminum alloys are widely used in aircrafts due to their superior performance. The evolution of the mechanical properties of the aluminum alloys caused by marine atmospheric corrosion has become a research hotspot due to the increase in aircraft service time in the marine atmospheric environment. In this work, the evolution of the mechanical properties of the 7B04-T74 aluminum alloy was studied by an alternate immersion test. The surface microstructure was analyzed by SEM, EDS, XRD, and OM. The influence of the marine atmospheric corrosion on mechanical properties was studied by tensile and fatigue tests. The results show that the 7B04-T74 aluminum alloy has good corrosion resistance, as only pitting corrosion occurs in the marine atmospheric environment. The tensile properties of the 7B04-T74 aluminum alloy remained fundamentally the same before and after corrosion. The fatigue properties of the 7B04-T74 aluminum alloy were severely reduced, but the localized pitting corrosion only affected the initiation stage of the crack and had little effect on the crack propagation process.
... This is because, during the pre-strain process, the strain induces dislocation slip and rearrangement within the grains, thereby promoting grain boundary motion and grain refinement. The second phase particles are subjected to stretching, deformation, and even fracture as the deformation increases, leading to their reorientation and causing refinement of the second phase particles [5] . As a result, they become locally distributed more intensively. ...
The study focuses on the 7B04 alloy sheet. It investigates the effects of different pre-strain levels on the microstructure, mechanical properties, and fatigue behavior of the alloy through the use of optical microscopy, scanning electron microscopy, and DIC (Digital Image Correlation) system. The experimental results demonstrate that the yield strength and tensile strength of the alloy increase with the increase in pre-strain levels. At the same time, the elongation decreases with the increase in tensile deformation. In the non-uniform region, the plastic deformation capacity index of the material (denoted as ψ) follows the order ψ (non-uniform region (DIC value)) > ψ (original gauge length value) > ψ (uniform region (DIC value)). It is recommended to control the maximum local strain below ψ (uniform region (DIC value)) during production to avoid deformation instability or concentration. Furthermore, the thickness of the material decreases linearly with increasing tensile deformation, with each 4% increase in tensile deformation resulting in a 1.2% reduction in thickness. The fatigue life of the alloy increases gradually with the increase in pre-strain levels in the direction perpendicular to the fiber. At the same time, it decreases in the direction parallel to the fiber. Different deformation levels and loading directions exhibit similar fatigue fracture characteristics. Within the range of 12% pre-strain levels, the microstructure of the material remains continuous and intact without noticeable cracks at grain boundaries or phase boundaries. At the macroscopic level, when the pre-strain level exceeds 6%, distinct slip bands appear on the sheet, which is correlated with the micro-scale grain slip. This is accompanied by deformation bands and micro-cracks in the microstructure. The findings of this study are of significant importance for understanding the influence of pre-strain levels on the mechanical properties and fatigue behavior of the 7B04 alloy. The study provides valuable references for engineering practices in related fields, thereby ensuring the integrity and usability of manufactured components.
... Comparing the values in Table 2, it can be concluded that when the second aging temperature is different, the properties are significantly distinct: the strength of the specimens treated at 155Ԩ is much higher than that at 165Ԩ while the electric conductivity is lower. This is mainly because that the duplex aging is an over-aging treatment, the peak age can be achieved in a short time and the time decreases with the increase of aging temperature [2]. So when the temperature of the second age is higher, the time to peak age is shorter. ...
The parameters of duplex aging treatment of pre-stretched plates including both the temperature and the holding time of two aging stages have been systematically investigated by using electric conductivity and tensile properties tests, exfoliation corrosion experiments and TEM observations. The results show that both the first and the second age have effect on properties of plates after having been performed by different duplex aging treatments and the effect of the second age is much greater. It is found that such a duplex aging treatment is appropriate in which the temperature of the second age is 155°C and the holding time of the first and the second age is 4-6 and 28-32 hours, respectively. The ultimate strength of specimens treated with the duplex aging treatment given above can achieve 540MPa while electric conductivity is 40.5-42 %IACS.
This paper takes the recycled 7075 aluminum alloy as the object to study its smelting, extrusion and heat treatment processes. The composition, microstructure, mechanical properties and corrosion properties of recycled 7075 aluminum alloys were characterized by spectrometer, optical metallographic microscope, universal tensile testing machine and scanning electron microscope. The results showed that the composition distribution of recycled 7075 aluminum alloy ingots was more uniform and the microstructure was more optimized. The tensile strength, yield strength and elongation of recycled 7075 aluminum alloy plates were higher than ASTM standard. The recycled 7075-T6 aluminum alloy plates had good corrosion resistance to salt spray. This paper aims to provide guidance for the smelting, extrusion and heat treatment processes of scrap aircraft aluminum alloy regeneration process, and to lay the foundation of the recovery and reutilization of scrap aircraft aluminum alloys.
The influence of the two-step aging time on the mechanical properties and exfoliation corrosion susceptibility of a 7050 aluminum alloy plate was investigated by means of hardness tests, conductivity tests, transmission electron microscopy(TEM), electrochemical impedance spectroscopy(EIS). The results show that through the aging at low temperature for a short time, the main age-hardening precipitates in the matrix are GP zone and η' phase, then through the second step aging at elevated temperature the age-hardening precipitates are transformed to η phase gradually. And with the increase of the aging time, the η phase and precipitates on grain boundary become coarser gradually, PFZ becomes wider at the same time, which results in the increase of hardness, then decreases and continuous channels become discontinuously distributed, and the resistance to exfoliation corrosion of the 7050 aluminium alloy plate is improved. The exfoliation corrosion development depends on grain boundary microstructure and chemical composition, starts and expands with formation and development of pitting corrosion. With the increase of the aging time, the exfoliation corrosion degree becomes serious, then improves, and serious again.
The quenching and pre-stretching process of 7075 aluminum plates were simulated by MSC.Marc FEA program. The quenched residual stress was used as initial condition to apply on the pre-stretch model. The clamp and release of the clamp of pre-stretch machine were simulated using life and death element, and the pre-stretch mechanism was researched. The results show that the stress of the surface metal is changed from compress stress to tensile stress and the center metal stress suffers tensile stress all along in the process of the pre-stretch. When the clamp is released, the aluminum alloy thicken-plate is spring back, the strain is loosen and the stress diminishes greatly. The stress is diminished and the plastic deform is increased along with the increases of stretch value. The residual stress is increased with the increase of thickness of the aluminum alloy plate when the different plates have the same pre-stretching value. In order to diminish efficiently the residual stress, the pre-stretch value should increase with the increase of thickness of the aluminum alloy plate. The pre-stretch value should adopt the small value when the residual stress is diminished.
The mechanical and corrosion properties of 7B04 Al alloy pre-stretched plate with various heat treatments were investigated using mechanical properties testing, compact testing, conductivity testing, slow strain rate testing and exfoliation corrosion testing and so on. The results showed that the strength, toughness and corrosion properties of the alloy were greatly concerned with the aging tempers. Peak strength could be achieved, when T6 heat treatment was applied to the alloy, but with the lowest stress corrosion cracking (SCC) resistance and fracture toughness. The overaging T74 and T73 could improve SCC resistance and toughness of the alloy, but sacrifice its strength in great value. The retrogression and re-aging (RRA) processes provided a method for increasing the SCC resistance of the alloy without sacrificing its strength, and the fracture toughness could also be increased somewhat comparing with T6 temper.
The heat treatment characteristic of the 7005 aluminum alloys was studied by mechanical properties measurement and TEM. The results show that, during the process of aging at 105°C after on-line quenching, the hardness(HB) of alloy increases quickly at first and reach about 100 HB after aging for 8h and 125 HB after 50 h, then the hardness increases slowly with increasing aging time, and the hardness does not reach the peak value even the alloy is aged for 200 h. After aging at (105°C, 8 h)+(155°C, 8 h), the mechanical properties of 7005 aluminum alloy are σb=395 MPa, σ0.2=330 MPa, δ=12%, 117 HB. If the aging of alloy at high temperature (155°C, 180°C) or the two-stage aging is too long, the precipitated phases of the alloy will grow and coarsen, and then the alloy strength decreases.
Through the hierarchical method, microstructure and mechanical properties in different areas of the weld nugget zone for 30 mm thick 7A05-T6 aluminum alloy friction stir welding join have been studied. The results show that, the size of recrystallization grain in the near area of surface is largest. When the rotation speed is 360 r/min and the welding speed is 100 mm/min, for the area of T1 to T5, the tensile strength separately is 326, 354, 342, 334 and 307 MPa, the yield strength and elongation are also increased gradually. The fracture micrographs show that, there are a lot of mesh dimples in the fracture surface, and they are deeper on slicing T2 especially. The hardness profile is not central symmetric, and hardness value of top area is larger than that of lower area. Friction heat will decrease with the increase of thickness, and that is the reason why the performance decline at the middle and bottom of workpiece joint.
The ageing precipitation and hardening behavior of 1973 high strength and high toughness aluminum alloy were studied by means of transmission electron microscopy (TEM), select area electron diffraction (SAED) and hardness measurements at different ageing temperatures. The results show that this alloy has obvious anti-over-aging capability at 120 and 140°C. And this alloy can keep the hardness of 195 HV in a long time after peak-ageing. Furthermore, the precipitation sequence of 1973 aluminum alloy can be described as follows: supersaturated solid solution (α) → GPI/GPII zone → metastable η'→ stable η→T phase. There are GPII zone and η' phase with the greatest strengthening effect on the 1973 aluminum alloy during the whole ageing process at 140°C.
The heat treatment process of 7AXX aluminum alloy was studied using orthogonal design experiment. The results indicate that after solid solution treatments of 470°C × 1 h, the precipitate particles of second phases decrease obviously and solute into the matrix alloy. The affecting hardness factors from strong to weak in the following order were found: second-step aging temperature, second-step aging time, first-step aging time and first-step aging temperature. The second step aging temperature is the key factor in double aging parameters, which mainly determine the final properties of 7AXX alloy. With the second-step aging temperature increasing, the alloy hardness is reduced. The reasonable heat treatment process of 7AXX alloy is two-stage aging, namely 110°C for 4 hours followed by 150°C for 8 h, and the ultimate tensile strength of 7AXX aluminum alloy is 750.27 MPa, the yield strength reaches 562.57 MPa, as well as the ductility reaches 26.43%.
The texture and recrystallization of 7B04-T351 alloy pre-stretched thick plates at different positions in thickness direction were investigated by EBSD technique, and the precipitates of 7B04-T6 alloy pre-stretched thick plates at the different positions were examined by TEM. The EBSD results show that the center of 7B04-T351 alloy plate keeps rolling orientation well. The main orientations of the center are copper, S and brass texture which are distributed in the β orientation line. R and cubic texture fraction becoames higher, and the copper, S and brass texture fraction decreases near the surface of plate. The recrystallization degree is weakened gradually from the surface to the center. The TEM observation results shows that the density of the precipitates in matrix near the surface of the plate is higher than that of the center of the plate, and there are more precipitates which are bigger than 10 nm at the center.
The quench sensitivity and their influential factors of 7,021, 7,085, and 7,050 alloys were investigated by the end quenching test method and the measurement of electrical conductivity, hardness, and microstructure after aging. The results indicate that 7,050 alloy has the largest changes with hardness decreasing from HV 199 to HV 167, and electrical conductivity increases from 16.6 to 18.2 MS·m−1 when the distance from quenched end increases from 2 to 100 mm. Alloys 7,085 and 7,021 have relatively smaller changes. According to the relationship between the hardness and electrical conductivity of a supersaturated solid solution, 7,050 alloy has higher quench sensitivity than 7,085 and 7,021 alloys. The microstructure of 7,050 alloy with higher major alloy element (Zn + Mg + Cu) addition and Cu element addition is mostly affected by the changes of distance from quenched end. In 7,050 alloy, the size of intragranular precipitates is from about 10–200 nm, and the (sub) grain boundary precipitates are about 20–300 nm. Alloy 7,085 with lower Cu content is moderately affected, while 7,021 is least affected. It is found that with the increase of distance from quenched end, quenched-induced precipitate preferentially nucleates and grows in the (sub) grain boundary and then on the pre-existing Al3Zr particles.
The tensile properties, electrical conductivity, and microstructure of the forged Al-7.1Zn-1.1Mg-1.6Cu-0.14Zr alloy were investigated
after a two-step ageing treatment at 120 and 170°C. The results indicate that the strength of the alloy reaches the peak value
at 170°C for 1 h during the second step ageing and then decreases sharply. However, the electrical conductivity value increases
continuously with the second ageing time increasing. The fracture mechanism of the alloy is intergranular fracture for 1 h
and then changes to dimple transgranular fracture later, and the toughness of the alloy is improved significantly. The phases
of η′ and η are major precipitates in the alloy under the two-step ageing condition. Discontinuous grain boundary precipitates
and precipitate-free zones along the grain boundary are clearly observed.
The microstructures after various ageing treatments and their relation to the strength, fracture toughness, and corrosion behavior of an Al-Zn-Mg-Cu alloy pre-stretched plate were investigated. The results show that retrogression and reaging (RRA) treatment led to a combination of high strength and stress corrosion cracking (SCC) resistance of the alloy. The TEM microstructure of the RRA-treated alloy is a distribution of very fine precipitates in the aluminum matrix grains, similar to that obtained under T6 condition, and the distribution of coarse η MgZn2 precipitates on the grain boundaries similar to that obtained by T7 temper. SEM observations revealed that most of the intergranular fracture characteristics were present on the fracture surface of both the T6 and RRA-treated specimens. On the contrary, the fractographs of the T7 treated specimens mainly consisted of dimple-type ductile transgranular fracture with minor intergranular cracking.
The early stages of pre-aging at near room temperature in an Al– Zn– Mg– Cu based commercial alloy were studied by electrical resistivity. The resistivity changes can be adequately described in the same terms of a Johnson– Mehl– Avrami (JMA) type function for the volume fraction growth of the Guinier– Preston zones or pre-precipitate solute clusters formed. For one specific case, resistivity results were correlated with those obtained using synchrotron-radiation small-angle X-ray scattering (SR-SAXS). Based on the analysis of the parameters of the JMA equation, the existence of a complex diffusion process during pre-precipita-tion is deduced. An apparent activation energy of 0.69 9 0.04 eV for the formation of the above mentioned particles is found. From this energy value, the migration of Mg-vacancy complexes is identified as the main microstructural mechanism responsible for the pre-precipitation.
Work has been conducted on commercial 7xxx series (Al-Zn-Mg-Cu) aluminium alloys to obtain an optimum combination of strength and fracture toughness. Two compositions with different zinc to magnesium ratios were investigated, with copper and total solute content held constant. Ageing studies revealed that, for overaged T73-type conditions, lower zinc to magnesium ratio appears to provide greater strength retention for a given heat treatment practice. Preliminary results indicate that this may in turn lead to superior strength to toughness ratios. These results have been interpreted using DSC and TEM observations in order to relate microstructure to property changes.
Secondary ageing may occur if age hardenable aluminium alloys are first underaged at an elevated temperature (eg. 150°C), quenched and then exposed to a lower temperature (eg. 25-65°C). At these lower temperatures, nucleation of fine precipitates occurs that further depletes the microstructure of solute elements and gives rise to additional strengthening. This treatment has been designated the T6I4 temper (I= interrupted) by the authors, and alloys normally develop tensile properties close to those for the respective T6 tempers. If these alloys are then aged again at an elevated temperature (T6I6 temper) further increases in tensile properties are possible (e.g.10-15%), usually with simultaneous increases in fracture toughness. Microstructural changes associated with these improved properties are discussed and examples are given of other tempers that have been developed to meet specific service requirements.
Monolithic structures machined from thick aluminum plate are being employed with ever-increasing regularity in applications such as airframes. Understanding the "thin sheet" performance of the thick plate material is crucial for design. Recent improvements in the processing of plate alloys, including AA7050, have made plasticity and ductile processes much more relevant to properties such as fracture toughness. In this work, a set of tensile experiments was conducted using specimens of various thicknesses (0.508 mm and 3.175 mm) machined from 7050-T7451 plate in various gauges (50.8 mm, 101.6 mm, and 203.2 mm). It was found that there was no appreciable specimen thickness-dependence in the deformation behavior up to necking; however, differences in ductility were observed. Anisotropies in strength (slight) and ductility (significant) were observed, as were orientation-dependent differences in deformation-induced specimen surface roughening. In general, specimens with axes in the rolling direction (RD) were more ductile than those in the transverse direction (TD) and appeared to deform uniformly up to the point of failure. In the 50.8 mm and 101.6 mm gauges, "troughs" roughly aligned with the RD appeared at the onset of yielding in the TD specimens. These features seem to be linked with the underlying microstructure and indicate a general nonuniformity in the deformation when loaded in the TD.
The effect of one-step ageing temperature and time upon mechanical properties and electrical conductivity of 7B04 pre-stretched thick plate was studied, and the microstructures of the alloy aged at different temperatures were compared. The results show that the higher the ageing temperature is, the shorter the peak ageing time is. The strength and elongation of the peak-aged alloy will be lowered when the ageing temperature is higher. With increasing the ageing time, the electrical conductivity becomes large. The higher the ageing temperature is, the faster the speed rate of electrical conductivity grows. The microstructure of matrix in peak-aged alloy consists of fine dispersed precipitates and precipitates from grain are continuous. When the ageing temperature is lower than 120°C, there are no obvious microstructure difference between peak-aged and over-aged alloys; When the ageing temperature is higher than 125°C, there are many coarse precipitates in over-aged alloys and the precipitates from grain are discontinuous. There are no obvious precipitate free zones along the grain boundary in both peak-aged and over-aged alloys under different ageing temperatures.
Fine-scale precipitation of the metastable Zn- and Mg-rich η′ phase and its precursors is essential for the mechanical properties of Al–Zn–Mg alloys. However, at present neither the precipitation sequence nor the structure and composition of the intermediate precipitate phases are completely clear. This paper deals with an investigation of precipitation in an industrial Al–Zn–Mg alloy at various stages of a conventional two-step ageing treatment at 100° and 150°C. Studies were performed using both transmission electron microscopy and atom-probe field ion microscopy. Transmission electron microscopy (TEM) analysis revealed two parallel precipitation paths; one involving formation and dissolution of the ordered GP (I) zones, the other involving formation of clusters (type II), having a different atomic arrangement compared to the Al-matrix, which transform to the η′ phase. Atom-probe study of the material after short time ageing at 100°C did not show any observable distinction between GP (I) and type II precipitates. In the peak-aged material the best classification of precipitates was obtained using their morphology (the cigar-like and the plate-like) because there was significant overlap in the range of total solute contents of each type of precipitate. Generally the Zn:Mg ratio in all observed types of precipitates was close to 1:1 and the total solute atom content increased with ageing time. Distribution of alloying elements in the precipitates and in the surrounding matrix is discussed.
The susceptibility of aluminum alloy 7075 (AA7075 [UNS A97075]) to intergranular stress corrosion cracking (SCC) in the peak strength T6 temper is alleviated through the use of the T73, or over-aged temper, which provides improved SCC resistance with a 10% to 15% strength loss compared to the T6 temper. Previous research has indicated that retrogression and re-aging (RRA) heat treatments reduce the trade-off between T6 strength and T73 SCC resistance. The short-term heat treatment they used, however, limited the applicability of RRA to thin sections of material. The primary goals of this research effort were to determine if lower retrogression temperatures could be used in the RRA process to extend the applicability of this heat treatment to thick section aircraft components and to quantify any observed improvements. Alternate immersion (AI) and double-cantilever beam (DCB) tests were conducted in a 0.6-M sodium chloride (NaCl) solution to evaluate the SCC resistance of various tempers. Improvements in properties were demonstrated using RRA heat treatments at lower temperatures and longer times than those previously investigated. In general, the various RRA tempers <200°C produced strengths similar to that of T6 with improved SCC properties. The RRA temper with retrogression at 160°C for 660 min produced the greatest improvement in SCC resistance, with only a 4% reduction in strength below T6.
Driven by the increasing requirements from aircraft producers, Hoogovens Aluminium Rolled Products GmbH, together with Hoogovens Research & Development, has enhanced the property combinations of their aircraft materials. For these types of material, optimised processing routes as well as new alloy chemistries have been investigated. Whilst retaining the strength levels required by the aerospace industry, new processing routes offer major improvements in ductility, toughness, fatigue performance and in reduction of residual stress in large dimension plate and sheet products. A further goal of investigating new alloy chemistries is the trend towards new joining techniques such as welding and brazing for aircraft structures. These new joining techniques require different property combinations compared to the conventional aerospace alloys. In parallel to these improved processing routes and new alloy developments, new ultrasonic inspection techniques have been developed, which are able to predict fatigue performance and residual stress in thick plate products.
The structure of the η′ phase, one of the most important age-hardening precipitates in commercial Al–Zn–Mg alloys, has been studied at the atomic level by means of high-resolution electron microscopy (HREM). A structural model of the η′ phase has been constructed on the basis of the structural characteristics in the observed images and the structure of the η-MgZn2 phase. Image simulation of this model shows a good agreement between calculated and experimental images. Comparison of this model with the early existing model on the basis of the X-ray diffraction is also given.
Aluminum has long been the material of choice for commercial aircraft primary structure. Aluminum suppliers have steadfastly improved products to support performance needs. Selection of composites for Boeing's 7E7 challenges aluminum's predominant position. Composites must prove their abilities; but improvements in manufacturability, performance, and affordability supported their selection for the 7E7 program to enable operating efficiency improvements. To win future airframe applications, aluminum must compete with and/or be compatible with composites. Recently demonstrated improvements in aluminum product properties suggest further advances are feasible. Research supporting improved performance and cost reductions is needed to meet the composite challenge and keep aluminum flying.
The structure of GP-zones in an industrial, 7xxx-series Al–Zn–Mg alloy has been investigated by transmission electron microscopy methods: selected area diffraction, conventional and high-resolution imaging. Two types of GP-zones, GP(I) and (II) are characterized by their electron diffraction patterns. GP(I)-zones are formed over a wide temperature range, from room temperature to 140–150°C, independently of quenching temperature. The GP(I)-zones are coherent with the aluminum matrix, with internal ordering of Zn and Al/Mg on the matrix lattice, suggested to be based on AuCu(I)-type sub-unit, and anti-phase boundaries. GP(II) are formed after quenching from temperatures above 450°C, by aging at temperatures above 70°C. The GP(II)-zones are described as zinc-rich layers on {111}-planes, with internal order in the form of elongated <110> domains. The structural relation to the η′-precipitate is discussed.
The effect of process parameters such as quench rate and precipitation heat treatment on the compromise between the toughness and the yield strength of AA7050 aluminum alloy (AlZnMgCu) are investigated, as well as the anisotropy of this compromise in the rolling plane. Fracture toughness is experimentally approached by the Kahn tear test. The microstructure is studied quantitatively in detail by a combination of scanning electron microscopy, transmission electron microscopy and small-angle X-ray scattering, and the relative fractions of the various fracture modes as a function of microstructural state are quantitatively determined on scanning electron microscopy images. Toughness is confirmed to be minimum at peak strength, and lower for an overaged material than for an underaged material of the same yield strength. A lower quench rate is shown to result in an overall reduction of toughness, and in a reduced evolution of this toughness during the aging heat treatment. The overall toughness is also lowered when the main crack propagation direction is parallel to the preferential elongation direction of the coarse constituent particles (rolling direction). The competition between intergranular and transgranular fracture is explained in terms of the modifications of the work hardening rate, and of grain boundary precipitation. The evolution of fracture toughness is qualitatively explained in terms of evolution of yield stress, strain hardening rate, grain boundary precipitation and intragranular quench-induced precipitates.