AA7050 and AA7010 aluminium alloys in overaged tempers have been a vital material for aerospace applications since their development in the 1970 ' s. Latest generation materials including AA7085, AA7449 and AA7037 have higher levels of Zn and reduced Fe and Si and offered some potential advantages but have suffered from Environmentally Induced Cracking (EIC) in service [1] and now in many cases are being replaced with previous generation materials [2]. Holroyd and Scamans [3] detailed some of the major risk factors associated with the new range of compositions and work by Schwarzenbock et al. [4] showed the stark EIC performance differences however the mechanistic understanding of these differences was still not clear. In this work, in-situ monitoring of 4 point bending tests conducted in conditions of 70 degrees C and 50 % RH was used to compare thick plate AA7050T7651 and AA7085-T7651 as important alloys representative of the two generations of development. In both cases we find brittle intergranular cracks form, identified from the fracture surface, but in the case of AA7085 this is essentially the only mode of EIC whereas for AA7050 we see a mixed mode fracture surface which also includes intergranular modes of microvoid coalescence and transgranular cracking. It appears that cracking in AA7050 is only just viable. Short crack growth behaviour in the two materials allows us to identify an 'intrinsic' resistance of the grain boundaries to EIC and we find this is substantially greater in AA7050 compared to AA7085. This difference in performance is related to the ability of the grain boundary precipitate reactions to sustain hydrogen generation for continuous cracking. We find this process is very much disrupted in the case of AA7050 leading to a mixture of fracture modes and a commensurate reduction in crack growth rates by around an order of magnitude or more.

Environmentally Assisted Cracking (EAC) of 7xxx series aluminium alloys involves interactions between multiple physical phenomena, which ultimately influence the in-service life of critical components. In this work, we present a new model to study EAC in 7xxx series alloys, which is implemented in the multiphysics simulation framework, DAMASK. The chemo-mechanical model couples crack tip hydrogen generation, resulting from surface oxidation, and transport, with crystal-plasticity-governed intergranular crack propagation, through the microstructural trapping of hydrogen at dislocations, grain boundaries (GB), and crack tip stress fields. Large-scale simulations with realistic grain structures have been performed to provide novel insight into the dominant rate-controlling processes associated with intergranular EAC in 7xxx series aluminium alloys. The model was able to reproduce experimentally measured crack velocities under different loading conditions. Parametric studies indicate that, in addition to the GB network morphology, the crack growth rate was controlled by hydrogen generation at the crack tip with long-range diffusion having negligible influence. Additionally, the total hydrogen generated through crack tip oxidation appears to be more significant than the peak generation rate.

The mechanical and stress corrosion behaviour of 7xxx aluminium thick-plate alloys are strongly influenced by a dual distribution of η-phase quench-induced and age-induced grain boundary precipitates (Q-GBPs and A-GBPs) that nucleate independently during cooling from solution treatment and artificial ageing, respectively. A novel cold intergranular fracture technique has been used to allow the interaction between these two precipitate distributions, formed on GB faces, in two alloys with different quench sensitivities, AA7050 and AA7085, to be more rigorously compared than previously possible. After a full T76 heat treatment, large widely spaced Q-GBPs are observed, between which high-density patches of small A-GBPs nucleate during subsequent artificial ageing. The A-GBP patches are separated from the Q-GBPs by distinct GB precipitate-free areas. Their respective distributions vary in a non-intuitive way with cooling rate. As the rate increases, the A-GBPs and patch area fractions reduce to a minima before rapidly increasing at very high cooling rates as Q-GBP nucleation becomes fully suppressed. This occurs because, although the size of the Q-GBPS GBPs reduces with cooling rate, there is a more dramatic increase in their nucleation density, which leads to a minima in the GB area where sufficient solute supersaturation is retained for precipitation during ageing. The same relative effects are seen in both alloys, but the Q-GBPs formed in AA7085 are significantly smaller and the density higher due to the lower solvus temperature.

Quench sensitivity in aluminium 7xxx alloy thick plates results from grain boundary precipitation (GBP) of the η-phase (Mg(ZnAlCu)2), which can significantly impact their fracture toughness and sensitivity to Environmentally Induced Cracking (EIC). A novel characterisation method, supported by phase-field simulations, has been employed to advance understanding of this phenomenon. To investigate the effect of cooling rate and alloy composition, Jominy end quench tests were performed on two commercial alloys, AA7050 and AA7085, with samples fractured under cryogenic conditions to expose grain boundary (GB) surfaces. This allowed the GB precipitate size distributions and morphologies to be quantified as a function of cooling rate. To understand the overall trends in the effect of composition and cooling rate on the size and density of the GBPs, ‘large’-scale CALPHAD-informed phase-field simulations were performed with a simplified particle geometry and nucleation model. The simulations replicated the observed greater quench sensitivity of lower Zn/Mg ratio alloys like AA7449 and AA7050 compared to AA7085. On increasing the cooling rate from 2.5 to 30 °C s−1, the average GB η-precipitate size and GB area coverage were decreased rapidly. Solute depletion adjacent to the GB, resulting from quench-induced precipitation, was also simulated, and shown to be largely determined by Zn and Mg diffusion, which reduced with increasing cooling rate. Using AA7050 as a baseline, the influence of alloy chemistry on GB η-precipitate was systematically investigated. Increasing Cu and Zn both results in a higher η solvus temperature, which promoted both the nucleation and growth of η precipitates and leads to an increased quench sensitivity, whereas reducing the Mg content reduces quench sensitivity.

As-built high-strength steel wire-arc additive manufactured (WAAM) components often contain coarse columnar prior austenite grain (PAG) structures. These microstructures can result in poor mechanical properties, hence post-build re-austenitisation treatments are of interest to improve microstructures. Here, the potential for engineering austenite refinement was investigated using high-temperature in-situ EBSD with high temporal resolution. It was found that 300M first austenitised with a memory effect, reforming the columnar PAG structures in both morphology and crystallographic orientation. This was observed to occur by the coalescence of acicular austenite and happened earlier in the depleted dendritic regions. On further heating, austenite underwent recrystallisation without the application of external deformation via a discontinuous mechanism, involving the bulging of the prior high angle boundaries. Recrystallisation twins were also seen to form and grow alongside regular recrystallised grains. The predominance of discontinuous recrystallisation meant that a coarse grain structure was still retained after austenitisation, owing to the influence of a grain ‘swapping’ effect.

The mechanical properties and stress corrosion cracking (SCC) resistance of 7xxx series aluminium alloys are significantly affected by the composition and distribution of precipitates formed during heat treatment. In particular, their quench sensitivity is related to the formation of η-phase precipitates that nucleate heterogeneously on grain boundaries at lower cooling rates after solution treatment, which has been a key factor restricting the gauge of hot rolled plates in the aerospace industry. To better understand the effects of slower cooling rates on the composition of quench-induced grain boundary precipitates (Q-GBPs) found in thick plate 7xxx alloys, plasma focused ion beam and high-resolution scanning transmission electron microscopy were used to obtain accurate composition data. The η-phase Q-GBPs have a complex- branched morphology, which develops higher aspect ratios and secondary arms as the cooling rate is reduced. Only a small change in average composition of Q-GBPs was found with cooling rate; but a large scatter was observed. This is caused by significant Zn/Cu/Al composition gradients developing along their principal growth directions in both AA7050 and AA7085 alloys. This concentration gradient did not reduce significantly after a T76 treatment. Simulations of Q-GBP growth with different cooling rates using a CALPHAD-informed phase-field model, with the η-phase represented by a two-sublattice model, gave results consistent with experimental observations. Chemical gradients were predicted to develop in the Q-GBPs due to the changing local equilibrium at the growth front during the cooling. The influence of this non-homogeneous microchemistry on the SCC behaviour of 7xxx alloys is briefly discussed.

The coarse β-grain structures typically found in titanium alloys like Ti–6Al–4V (wt pct, Ti64) and Ti–6Al–2Sn–4Zr–2Mo–0.1Si (Ti6242), produced by high deposition rate additive manufacturing (AM) processes, are detrimental to mechanical performance. Certain modified processing conditions have been shown to lead to a more refined grain structure, which has generally been attributed to a change in the solidification conditions with respect to the experimental Hunt diagram proposed by Semiatin and Kobryn. It is shown that with Wire Arc AM (WAAM) increasing the wire feed speed (WFS) is effective in promoting a columnar-equiaxed transition (CET). Conversely, estimates of the dendrite-tip undercooling using the KGT model suggest that this will be too small for free nucleation without the addition of artificial nucleants, due to the very low solute partitioning in Ti alloys. It is also shown that it is difficult to promote a CET with plasma transferred arc WAAM as computational fluid dynamics (CFD) melt-pool simulations indicate that the solidification parameters remain within the columnar region on the Semiatin-Kobryn Hunt map, within the constraints of a stable process. However, a high fraction of twin boundaries was observed in the refined β-grain structures seen at high WFS. This has been attributed to departure of ⟨001⟩ _β alignment from the direction of maximum thermal gradient, caused by the curvature of the fusion boundary, stimulating dendrite twinning during solidification. In addition, it is shown that increasing the WFS leads to a change in melt-pool geometry and a reduction of remelt depth, which promoted dendrite twinning and grain refinement.

As-deposited Wire-Arc Additive Manufactured (WAAM) Ti-6Al-4V parts typically contain large columnar beta-grains on a centimetre scale, with a strong < 001 > fibre texture, leading to anisotropic mechanical properties and unacceptable scatter in damage tolerance. Inter-pass deformation, introduced by the application of Ultrasonic Impact Peening (UIP) across each added layer, has been shown to be effective in refining the beta-grain structure and achieving a weaker texture. The depth of deformation and the grain refinement mechanism induced by UIP have been investigated by combining advanced electron backscatter diffraction (EBSD) characterization with a 'stop action' observation technique. UIP facilitates a similar refinement mechanism and nearly the same depth of deformation as conventional machine hammer peening, with the advantages of a much higher strain rate, lower peak force, and two orders of magnitude lower impact energy, making it a faster and more economical process. beta recrystallization is seen within the deformation zone during re-heating through the alpha -> beta transition. Although new recrystallized beta-grains formed in the UIP surface-deformed layer to a shallower depth than that of remelting, recrystallization initiated ahead of the melt pool and the recrystallized grains grew downwards to a greater depth before remelting. These refined grains were thus able to survive and act as nucleation sites at the fusion boundary for epitaxial regrowth during solidification, greatly refining the grain structure.

Directed energy deposition via electric arc (DED-Arc) and wire-feed system can offer a low-cost, high deposition rate method to produce large-scale, near-net-shape structural components from Inconel 718 (IN718). In this work, DED-Arc with cold metal transfer (CMT) has been compared with pulsed-spray droplet transfer (GMAW-P) using different build strategies by correlating the local cyclic thermal histories and the position of deposition interruptions to the resulting tensile properties. The measured heat input (0.18 - 0.59 kJ/mm) and inter-pass temperature (80 - 650 degrees C) affected the thermal field and residence times in the 900 - 600 degrees C temperature range, where & gamma;'/& gamma;" phases are more likely to form, thereby impacting the yield strength along the build direction (370 - 460 MPa). Furthermore, CMT deposition with a lower travel speed (0.4 m/min vs. 1.0 m/min) prevented the onset of solidification defects, contributing to more consistent tensile ductility. Tensile properties of material surrounding build interruptions developed strain localisation and failed prematurely as compared to regions without, due to localised cracking and transient thermal fields generated once the deposition resumed.

Recent experience has shown that new-generation 7xxx-series alloys, that have a high Zn content and Zn/Mg ratios, have a greater susceptibility to hydrogen-environmental induced cracking (H-EIC) on exposure to humid air than more established materials, like AA7050. In this study, we report new evidence of the EIC initiation and crack growth behavior of two new-generation alloys, AA7085 and AA7449, when exposed to 50% humidity. In situ, time-lapse, optical imaging over large areas has enabled the exact initiation sites to be identified and investigated with high-resolution fractographic studies, providing evidence for the sequence and mechanisms of initiation and transition to sustained cracking. A consistent behavior was observed for both alloys. This has revealed that minute-scale corrosion reactions, involving highly localized condensed water, are necessary for initiation. The preferred initiation sites are metal ligaments between surface-connected pore clusters and/or intermetallic particles that are subjected to high-stress concentration and undergo mechanical damage with associated higher levels of local oxidation. The growth of short protocracks from these sites is a distinct stage and displays intermittent arrest markings evidenced by localized corrosion. In contrast, in humid air environments, long cracks in these alloys exhibited relatively constant, higher velocity, with extremely limited corrosion commensurate with oxidation of a free surface in this environment resulting in approximately 5 nm oxide layer.

Constructing titanium aerospace parts by near-net-shape processing has the potential to greatly reduce cost and lead time, one method for this is Wire-Arc Additive Manufacturing (WAAM). Conventional WAAM processing with the most common Ti alloy, Ti-6Al-4V, results in solidification by epitaxial growth from previously deposited layers and a structure dominated by columnar β grains which are heavily <001> fibre textured and cm’s in scale. In order to prevent these large grains from forming, while maintaining deposition parameters, the solidification conditions were modified by the additions of particles to the melt; either using inoculant, TiN particles, or the solutal growth restrictor, Y, also added as elemental powder that dissolved in the melt. The powder particles were added by adhering them to the deposited tracks to avoid the costs of manufacturing new wires. With TiN inoculants the morphology of β grains was completely modified to equiaxed grains averaging 300 μm in diameter. Y additions narrowed the columnar grains from 1-2mm to 100-300 μm. Y also induced a change to equiaxed grains, late in solidification, in the region which was remelted by subsequent deposition. However, Yttria particles were found to have formed interdendritically with an interconnected skeletal morphology. High-resolution EBSD analysis showed both TiN and yttria particles exhibit specific orientation relationships with the solidified β grains, which were confirmed experimentally.

Large area, time resolved, in-situ optical scanning of constant displacement (four-point bend) tests has been used to monitor the environmentally assisted cracking (EAC) initiation and crack growth behaviour of AA7085 and AA7449 T7651 thick plate materials, exposed to warm-humid air (50 % RH & 70 degrees C). Subsequent fractography has revealed the dominant initiation sites originating at surface connected gas pore clusters, in combination with reactive Mg-containing constituent intermetallic compounds co-located on grain boundaries. Cracks were also shown to grow at stress intensities substantially below the KIEAC threshold measured via DCB testing, which should not therefore be taken as a limit below which EAC cannot occur.

Intergranular Environmentally-Assisted Cracking (EAC) has recently been reported to be an issue of concern in new-generation 7000 series aluminium alloys, such as AA7085, when exposed to humid air. The cracking process occurs in a highly brittle manner almost exclusively along grain boundaries (GB's) and has been attributed to hydrogen embrittlement, probably by GB decohesion within the stress field at the crack tip. Currently, how the highly heterogeneous grain structures found in these partially recrystallized materials impact the growth behaviour of microstructurally short cracks is poorly understood. In particular, there is expected to be a high sensitivity to the grain structure in the transition from initiation to sustained propagation, where the local mechanical driving force is very sensitive to the crack path. Volume Elements, VE's, with synthetic grain structures have been generated from real microstructure and texture data, so that the effects of important grain structure variables can be explored in crystal-plasticity simulations, to understand the extent to which typical grain-structural features affect the driving force for short-crack growth. Specifically, by considering the effect of different uncrystallised grain aspect ratios and embedding recrystallised grains in the model, the strain energy release rate has been calculated as a function of crack path. This has revealed large reductions and fluctuations in the driving force for short cracks in relation to the local grain structure encountered by the crack tip, which have been estimated by the model.

alpha + beta titanium alloys, such as the commercially important alloy Ti-6Al-4V (Ti64), develop complex textures during thermomechanical processing due to the allotropic phase transformation between the beta and alpha phases. These phases are crystallographically related through the Burgers orientation relationship (BOR) and are often characterised by electron backscatter diffraction (EBSD) in the scanning electron microscope (SEM). However, the BOR can be destroyed for the primary alpha in wrought Ti64, and techniques that utilise the BOR to reconstruct the beta phase from the room temperature alpha-phase data cannot be used. Instead, the beta texture must be measured from the residual, nanoscale beta ligaments in the room temperature microstructure, which are challenging to index because of orientation and phase overlap in the EBSD Kikuchi patterns. In this work, the SEM-EBSD acquisition and processing parameters were systematically varied to determine how best to index the residual beta in Ti64, and an experimental methodology was thus developed to measure the beta-phase texture efficiently and reliably after thermomechanical processing. The best compromise for maximising indexing of the residual beta was achieved with a low current (-1 nA in this case), and additional indexing was achieved by increasing the sample stage tilt (to 75 degrees in this case) and by limited the scanning frame size. It was shown that the beta-phase texture could be reliably measured with the optimised beam and tilt settings using a relatively coarse step size (3 mu m), but this approach does not yield any morphological or spatially relevant microstructure data. Thus, it was proposed that numerous multi-scale scans be performed with different settings to characterise the residual beta phase in Ti64, each optimised to acquire either bulk texture analysis or microstructure spatial and morphological detail.

Predicting the performance of welds under blast loading is challenging, particularly in alloys where the weld region is associated with significant material property gradients. This situation arises for friction stir welds of aluminium alloy AA2139, which represents an example case of a high strength aluminium alloy. To address this problem, a novel multiscale model has been developed that captures the effect of welding on the microstructure and links this to the constitutive material behaviour. The properties of the local weld regions are determined by generating equivalent microstructures in specimens of sufficient size to perform representative tensile and high strain rate compression tests. In this way, the local material properties can be obtained based on the fundamental controlling microstructure, independent of the weld configuration. The constitutive behaviour informs a finite element model at the macro-scale in which material property gradients arising from microstructural changes are captured. The model has been demonstrated to accurately predict the local strain evolution across the weld zone and demonstrates that strain localization occurs in the heat affected zone region for both cross-weld tensile tests and air blast loading. It is shown that the gradual change in strength between weld zones must be correctly accounted for to predict the correct strain localization behaviour. The work highlights the importance of an accurate description of the variation in local material properties in determining the response of structures under blast loading.

Wire-Arc Additive Manufacturing (WAAM) of large near-net-shape titanium parts has the potential to reduce costs in aerospace applications. However, with titanium alloys, such as Ti-6Al-4V, conventional WAAM processing conditions generally result in epitaxial solidification from the melt pool fusion boundary, which over many layers can generate coarse cm-scale, <001> //ND fibre textured, columnar beta grain structures within the deposited metal. The mechanical anisotropy caused by this coarse primary grain structure cannot be eliminated by subsequent solid-state phase transformations. In order to attempt to refine the size of the solidified beta-grains and reduce their strong texture, the growth restriction efficiency of low addition levels of the strongly partitioning element (k = 0.1) yttrium (Y) has been investigated. Less than 0.8 wt.% Y was sufficient to reduce the widths of the solidified columnar beta grains from 1 to 2 mm to 100-300 mu m. Y was also found to induce a columnar-to-equiaxed transition (CET) in the latter stages of melt pool solidification, which benefits from a lower liquid thermal gradient and higher solidification velocity. Inter-dendritic segregation of Y was also found to be significant and oxygen scavenging led to the formation of Y2O3 particles in the inter-dendritic liquid, with a previously unreported irregular eutectic morphology. High resolution EBSD analysis showed these particles exhibited specific orientation relationships with the solidified beta grains, which were confirmed experimentally. (C) 2021 Elsevier B.V. All rights reserved.

The titanium alloy Ti–6Al–2Sn–4Zr–2Mo–0.1Si (Ti6242) has been deposited for the first time by a directed energy deposition process using a wire and arc system— i.e., wire-arc additive manufacturing (WAAM)—with and without inter-pass machine hammer peening, and its microstructure investigated and compared to the more commonly used alloy Ti–6Al–4V (Ti64). The application of inter-pass machine hammer peening—where each added layer was deformed before deposition—successfully refined the strongly textured, coarse, columnar β -grain structure that is commonly seen in α + β titanium alloys, producing a finer equiaxed grain structure with a near-random α texture. The average grain diameter and texture strength decreased with the peening pitch. When Ti6242 was deposited under identical conditions to Ti64, by switching the alloy feed wire in-situ, the refined β -grain size decreased across the alloy-to-alloy transition reaching on average 25 pct less in Ti6242 than in Ti64. A similar 25 pct scale reduction was also found in the Ti6242 α -lath transformation microstructure. This comparatively greater microstructure refinement in Ti6242 was attributed to the dissimilar alloying elements present in the two materials; specifically, molybdenum, which has a lower diffusivity than vanadium and led to slower β -grain growth during reheating as well as a finer transformation microstructure.

In order to better understand the mechanisms of environmentally-assisted cracking (EAC) of 7xxx aluminium aerospace alloys, large scale serial sectioning using the newly developed femtosecond laser plasma focused ion-beam (laser PFIB) has been performed on both AA7050 and AA7085 alloys containing environmentally-assisted cracks introduced during accelerated laboratory testing. The samples were firstly scanned using X-ray computed tomography in order to reveal the long-range crack morphology and allow for targeted lift out of the crack tip regions for serial sectioning. This paper describes the optimisation of the methodology for both the liftout procedure and subsequent serial sectioning using laser-PFIB. Electron backscatter diffraction (EBSD) mapping at each slice allowed for 3D reconstruction of statistically valid volumes of material, up to 0.5 mm(3) with a fine voxel size (1 mu m). High resolution secondary electron imaging at each slice allowed for 3D reconstruction of the crack volume, and the datasets were combined in order to investigate the interaction of the crack with the microstructure. Optimisation of both the liftout geometry and the slicing procedure resulted in significant reduction in laser-induced curtaining artefacts and higher quality data acquisition. For these alloys, laser slicing alone was sufficient to provide a deformation free surface allowing for EBSD indexing rates up to similar to 95% acquired at high speeds of similar to 2000 Hz leading to reasonable acquisition times of similar to 4-5 days for each dataset. The datasets have shown that the grains in these alloys are much larger and more complex than previously realised, and the interaction of these cracks with the microstructure have shown that complex-shaped recrystallised grains can lead to significant crack deflection in these alloys and so may help to explain the observed differences in EAC behaviour.

There are several facets of aluminum when it comes to sustainability. While it helps to save fuel due to its low density, producing it from ores is very energy-intensive. Recycling it shifts the balance towards higher sustainability, because the energy needed to melt aluminum from scrap is only about 5% of that consumed in ore reduction. The amount of aluminum available for recycling is estimated to double by 2050. This offers an opportunity to bring the metallurgical sector closer to a circular economy. A challenge is that large amounts of scrap are post-consumer scrap, containing high levels of elemental contamination. This has to be taken into account in more sustainable alloy design strategies. A "green aluminum " trend has already triggered a new trading platform for low-carbon aluminum at the London Metal Exchange (2020). The trend may lead to limits on the use of less-sustainable materials in future products. The shift from primary synthesis (ore reduction) to secondary synthesis (scrap melting) requires to gain better understanding of how multiple scrap-related contaminant elements act on aluminum alloys and how future alloys can be designed upfront to become scrap-compatible and composition-tolerant. The paper therefore discusses the influence of scrap-related impurities on the thermodynamics and kinetics of precipitation reactions and their mechanical and electrochemical effects; impurity effects on precipitation-free zones around grain boundaries; their effects on casting microstructures; and the possibilities presented by adjusting processing parameters and the associated mechanical, functional and chemical properties. The objective is to foster the design and production of aluminum alloys with the highest possible scrap fractions, using even low-quality scrap and scrap types which match only a few target alloys when recycled.

New generation (new-gen) 7xxx series alloys have recently been shown to have higher susceptibility to environmental assisted cracking (EAC) in humid air, compared to AA7050, in T7651 tempers. Hydrogen (H)-EAC fracture features of brittle intergranular (IG) and transgranular (TG) fracture modes were observed with humidity levels as low as 20%. Initiation was favoured at sites of intermetallic particle clusters with gas pores. The higher performance of AA7050 was attributed to the difficulty of initiation in this alloy, where the grain structure, combined with an intrinsic resistance of the grain boundaries to IG-EAC, made the attainment of sustained cracking difficult.