This study investigates the applicability of a novel laser-arc multi-energy deposition of Ti-6Al-4V with independent control of bead geometry and thermal input. A plasma transferred arc is used to generate an initial melt pool and melt wire feedstock, before controlled lateral elongation of the melt pool via a fiber laser and galvo scanner. The applicability to Ti-6Al-4V was first investigated using deposition parameters previously identified. Once successful bead geometry control was achieved, process parameters more conducive to wire directed energy deposition were investigated. This included investigation of the energy per unit area required to achieve accurate deposition of Ti-6Al-4V with minimal penetration and investigation into scanning strategy. In each case, optical microscopy was conducted and analysis of the bead geometry, penetration and heat-affected zone considered to determine the effect of each parameter change. The results demonstrated that independent control of bead geometry and thermal input could be achieved, allowing deposition of Ti-6Al-4V at a desired scan width and layer height and providing a framework for future multi-energy source directed energy deposition of Ti-6Al-4V.

This study investigates the applicability of a novel laser-arc multi-energy deposition of Ti-6Al-4V with independent control of bead geometry and thermal input. A plasma transferred arc is used to generate an initial melt pool and melt wire feedstock, before controlled lateral elongation of the melt pool via a fiber laser and galvo scanner. The applicability to Ti-6Al-4V was first investigated using deposition parameters previously identified. Once successful bead geometry control was achieved, process parameters more conducive to wire directed energy deposition were investigated. This included investigation of the energy per unit area required to achieve accurate deposition of Ti-6Al-4V with minimal penetration and investigation into scanning strategy. In each case, optical microscopy was conducted and analysis of the bead geometry, penetration and heat-affected zone considered to determine the effect of each parameter change. The results demonstrated that independent control of bead geometry and thermal input could be achieved, allowing deposition of Ti-6Al-4V at a desired scan width and layer height and providing a framework for future multi-energy source directed energy deposition of Ti-6Al-4V.

Additive manufacturing (AM) processes are known to produce anisotropic microstructures and thereby, anisotropic mechanical properties. However, fundamental understanding on the anisotropic mechanical behaviour of AM-built Ti6Al4V is limited, particularly for high-deposition rate wire feed directed energy deposition AM processes. The present study provides insights into the role of anisotropic microstructure and associated texture on the tensile deformation and damage accumulation in wire feed directed energy deposition Ti6Al4V. Materials were deposited using oscillation-pass and parallel-pass build strategies. In situ neutron diffraction studies were performed on samples with tensile loading applied parallel and perpendicular to the built layers. Dissimilar thermal histories experienced in the parallel-pass strategy resulted in thinner columnar (3 grains and finer transformed microstructure, the latter leading to higher yield strength compared to the oscillation strategy. The presence of strong columnar (3 fibre textures in both build strategies led to anisotropic deformation. When loaded perpendicular to the columnar grains, elastic strain accumulation is more crystallographically homogeneous and includes strain accumulation between basal, prismatic, and pyramidal planes in both build strategies. Conversely, when loaded parallel to the columnar (3 fibre texture, the majority of the pyramidal orientations preferentially aligned along the loading axis and were subjected to significant elastic strains. Similar anisotropy was inferred under plastic deformation where tensile strain appeared to be accommodated primarily by prismatic slip but was not detected when loaded parallel to the columnar grains.

Hydride precipitation in zirconium alloys leads to embrittlement, making it essential to understand their prevalence and stability in the microstructure. Dictionary indexing of Kikuchi patterns, along with orientation relationship analysis and x-ray diffraction, confirmed the presence of both delta and gamma hydride phases in Zircaloy-4. Both phases were found to be stable in recrystallised zirconium, with the gamma phase exhibiting a distinct orientation relationship with the matrix. Delta hydride morphology and orientation were influenced by local stresses, resulting in a change in orientation during precipitation. By analysing the orientation relationships, the evolution of hydride phases could be visualised, providing insights into the room temperature stability of both delta and gamma hydrides.

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.

Competitive continuous and discontinuous precipitation (CP and DP) have been studied in-situ for the first time in magnesium alloys, using the Mg-Al system as an exemplar. CP forms first, and strongly affects the subsequent migration of the high angle grain boundary (reaction front, RF) behind which DP occurs. It has been demonstrated that in contrast with expectations from classical DP theory, the RF does not migrate with a steady-state velocity, but instead proceeds in an irregular stop-start fashion. Furthermore, the RF velocity is not constant but varies from grain to grain, by a factor of 4 for the conditions investigated. This growth behaviour can be explained by the interaction of the RF with CP. Whilst a mean-field model has been demonstrated to correctly predict the overall CP and DP kinetics, it is shown that the irregular motion of the RF is due to local effects. A simple model has been developed that demonstrates how the locally depleted solute field around a CP leads to arrest of a segment of the RF until sufficient diffusion occurs along it to reactivate RF motion. Zener pinning and boundary curvature also play an important, but secondary role. The results have implications for controlling DP, which is usually considered undesirable.

Additive manufacturing (AM) using recycled Ti-6Al-4V (Ti64) feedstock material from wrought waste streams is a novel process that can reduce the overall energy cost and carbon (CO2) footprint when compared to primary-production routes. The potential contamination of recycled feedstock material (e.g. C, O, N and Fe) can affect the microstructure and mechanical properties of the component. In this work, a Ti64 test wall built using wire arc AM (WAAM) was studied, where the top half only was contaminated by N through the shielding gas during deposition. This allowed a direct comparison of Ti64 WAAM material with high and low N content, deposited under otherwise identical conditions, to replicate the worst-case scenario of N contamination from using recycled swarf. The hardness of the N-contaminated section was found to be 25% higher than the uncontaminated section of the wall, demonstrating the N solid solution strengthening in Ti64. The room temperature transformed microstructure was found to have a 25% coarser alpha-lath thickness, which was proposed to be an effect of the AM cyclical heating and increasing of the beta-transus temperature due to a higher level of N. Additionally, the outer layer of the N-contaminated sample section was found to have a refined parent beta grain structure.

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.

Discontinuous precipitation (DP) is a commonly observed mechanism by which solid solutions decompose. It results from layers of precipitate and solute depleted matrix forming across a moving boundary. The morphology of the discontinuous regions can be highly complex, and the way dislocations or twins navigate such a structure during deformation is poorly understood. Only through 3-dimensional (3D) analysis can the true morphology of discontinuous precipitation be revealed. This work presents the first 3D study of DP using a novel PFIB based serial sectioning method. The material studied is AZ80, an industrially important class of Mg-Al-Zn alloy where DP is commonly observed. The structure is revealed to consist of an interconnected network of DP regions within an interconnected matrix. The width of the DP regions show strong local variations. This structure is expected to be readily penetrated by twins and dislocations.

Ti-6Al-4V components built with wire plus arc additive manufacturing (WAAM) generally have long columnar β grains that cause anisotropic behavior when the material undergoes static and cyclic failure. Recently, machine hammer peening (MHP) has been proved to induce prior-β grain refinement in WAAM resulting in isotropic properties and increased strength. In this study, MHP was investigated for WAAM walls to establish the dependency of the β grain refinement on peening parameters, such as energy, tool radius, and distance between impact steps. All combinations of parameters investigated resulted in grain-refined microstructures. The plastic strain theory failed to explain these results, as the microstructure refinement achieved did not match the strain distribution obtained. Thus, a new theory of accumulated energy was proposed in which the dynamic deformation of the MHP process should also be taken into consideration. The mechanical properties for the MHP conditions showed higher strength and decreased anisotropy as the energy per length increased. This was attributed to the reduction in texture in the WAAM walls. Thus, when applying MHP, the energy per unit length is controlling the grain size obtained and improved mechanical properties can be achieved.

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.

Magnesium–Aluminium alloys can decompose from a supersaturated solid solution by either continuous or discontinuous precipitation. Deformation prior to precipitation has been shown to strongly suppress the discontinuous precipitation mode and promote continuous precipitation. In this work, a model is used to explore the interaction between deformation and precipitation in the Mg–Al system. It has been shown that accelerated nucleation of continuous precipitates on dislocations is predicted to have the dominant effect in suppressing discontinuous precipitation by reducing the solute supersaturation. A secondary effect is the direct role played by twins in the deformed structure, which act as impenetrable barriers to discontinuous precipitate growth. However, even in the deformed case, small regions of discontinuous precipitation are still observed. It is proposed that this is due to the high level of strain concentration expected in the grain boundary regions, which provides a locally enhanced driving force for the migration of grain boundaries such that limited discontinuous precipitation occurs before continuous precipitation becomes established.

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.

The Mg-Sn-Y alloys exhibiting advanced elevated-temperature strength up to 300 degrees C were newly developed by tailoring precipitates through partially substituting and increasing yttrium (Y) (2, 3.5 wt%) for tin (Sn) in Mg-2.5Sn alloy. The effects of precipitates on the room/elevated-temperature mechanical properties, and the related dynamic precipitation behavior were investigated. The elevated-temperature strengthening mechanism of the alloy was revealed. The precipitates transformed from Mg2Sn in Mg-Sn alloy to Sn3Y5 in Mg-Sn-Y alloys. The abundant Sn3Y5 nanoparticles formed in the as-extruded Mg-0.5Sn-3.5Y alloy which exhibited significant higher peak strength as 223 MPa compared to that of Mg-2.5Sn as 53 MPa. The calculation of the critical nucleation energy for dynamic precipitation indicated that the Mg-Sn-Y alloys exhibited a smaller nucleation barrier for dynamic precipitation of dense nanoscale Sn3Y5 particles compared to the Mg-Sn alloy. This barrier was further decreased with increasing Y content, as exemplified by the increased area fraction of nanoparticles in the Mg-0.5Sn-3.5Y alloy. The abundant Sn3Y5 nanoparticles can inhibit the grain boundary crack propagation, and the formed fine grains (similar to 3.8 mu m) can effectively hinder the dislocation motion. Therefore, the present work demonstrated that coupling a high area fraction of thermally stable nanoparticles with grain refinement can provide an effective approach to acquire superior elevated-temperature strength for Mg alloys. (C) 2022 Elsevier B.V. All rights reserved.

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.

The electrochemical properties of high strength 7xxx aluminium alloys strongly depend on the substitutional occupancy of Zn by Cu and Al in the strengthening eta-phase with the two-sublattice structure, and its microstructural and compositional prediction is the key to design of new generation corrosion resistant alloys. In this work, we have developed a chemical-potential-based phase-field model capable of describing multi-component and two-sublattice ordered phases, during commercial multi-stage artificial ageing treatments, by directly incorporating the compound energy CALPHAD formalism. The model developed has been employed to explore the complex compositional pathway for the formation of the eta-phase in Al-Zn-Mg-Cu alloys during heat treatments. In particular, the influence of alloy composition, solute diffusivity, and heat treatment parameters on the microstructural and compositional evolution of eta-phase precipitates, was systematically investigated from a thermodynamic and kinetic perspective and compared to electron probe microanalysis validation data. The simulated eta-phase growth kinetics and the matrix residual solute evolution in the AA7050 alloy indicates that Zn depletion mainly controlled the eta-phase growth process during the early stage of ageing, resulting in fast eta-phase growth kinetics, enrichment of Zn in the eta-phase, and an excess in residual Cu in the matrix. The gradual substitution of Zn by Cu atoms in the eta-phase during the later ageing stage was in principle a kinetically controlled process, owing to the slower diffusivity of Cu relative to Zn in the matrix. It was also found that the higher nominal Zn content in alloys like the AA7085 alloy, compared to the AA7050 alloy, could significantly enhance the chemical potential of Zn, but this had a minor influence on Cu, which essentially led to the higher Zn content (and consequently lower Cu) seen in the eta-phase. Finally, substantial depletion of Zn and supersaturation of Cu in the matrix of the AA7050 alloy was predicted after 24 h ageing at 120 degrees C, whereas the second higher-temperature ageing stage at 180 degrees C markedly enhanced the diffusion of Cu from the supersaturated matrix into the eta-phase, while the matrix residual Zn content was only slightly affected. (c) 2021 The Author(s). Published by Elsevier Ltd on behalf of Acta Materialia Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

The as-deposited microstructure and mechanical properties of the near-beta titanium alloy Ti-5Al-5V-5Mo-3Cr (Ti-5553) produced by wire-arc additive manufacture (WAAM) were investigated, to understand its microstructural evolution under WAAM deposition conditions and to establish correlations between the microstructure features formed and the thermal cycles experienced during deposition. The 'as-deposited' Ti-5553 WAAM material exhibited higher tensile strengths than other as-deposited additively manufactured Ti-5553 deposits previously reported in the literature, but had significant anisotropy in elongation, as a consequence of the coarse and columnar beta-grain structure that formed on solidification, which exhibited a strong {001}(beta)< 001 >(beta) cube texture. The multiple reheating cycles, inherent to the WAAM process, were recorded using a novel 'harpoon' thermocouple technique, and the a precipitation evolution was related to the thermal history. Electron probe microanalysis chemical maps revealed significant solute microsegregation during solidification, which influenced the subsequent precipitation due to its effect on the local beta-phase stability. As each layer experienced more reheating cycles, the microstructure evolution could be 'time resolved' and the a laths were found to precipitate in a specific sequence of nucleation sites, starting at the beta-grain boundaries and then inter-dendritically, where there was lower matrix beta stability. However, after the reheating peak temperature was insufficiently high to have any further effect, the microstructure consisted of a relatively uniform distribution of a laths.

The factors controlling the evolution of abnormally coarse grain (ACG) structures during standard β annealing of titanium-6Al%-4V% (Ti64) hot-rolled plates, have been investigated in more detail than previously, using both ex-situ through-thickness large area, and in-situ EBSD micro-texture observation. Starting with a typical, through thickness, α+β deformation texture distribution that gives rise to ACGs at the plate mid-section, each stage of the annealing process has been sequentially characterised and phenomenologically linked to the spatial texture evolution that occurs during transient heating through the α→β phase transformation and the subsequent isothermal hold at the β-annealing temperature. It was found that during the ramp heating phase, the rotated cube texture component greatly expands from just below the β transus by broad front strain induced boundary migration (BF-SIBM), driven by a disparity in stored energy with neighbouring texture bands from the α and γ fibres. This subsequently sets up the necessary conditions for unstable coarsening of surviving grain clusters with predominately near α fibre orientations during the super-transus hold, in a process with similar characteristics to discontinuous recrystallisation. Humphreys’ mean field model of the stability of cellular microstructures, effectively explains the process by which these grain clusters, which have a lower misorientation range relative to the rotated cube component matrix, have a higher probability of entering a discontinuous growth regime, compared to other components from the α and γ fibres. Furthermore, the conditions that give rise to the development of ACG structures could be linked to the high strain rate and temperature experienced at the central mid-thickness of rolled plates.