The Effect of Thermal History on Microstructural Evolution, Cold-Work Refinement and {\alpha}/\b{eta} Growth in Ti-6Al-4V Wire + Arc AM

Wire + arc additive manufacture (WAAM) is an attractive method for manufacturing large-scale aerospace components, however the microstructural changes that occur and the effect of interpass rolling are poorly understood. Therefore two fundamental studies were conducted: the first involved temperature measurement of a wrought dummy wall so that the microstructural changes in the heat affected zone (HAZ) could be related to the thermal cycle. This demonstrated that the white band in the microstructure corresponded to 825 C well below the beta-transus temperature and above this boundary the bi-modal substrate material was converted to lamellar. The second involved peening WAAM material along the side of a deposited wall before applying a typical WAAM thermal heat treatment. This showed that refinement occurred up to the first layer band in the microstructure and the smallest grains were observed just above this boundary at higher temperatures significant grain growth occurred. This study has provided the foundational understanding of microstructural changes that will facilitate future process developments.Comment: 15 Figures, 25 pages, Journal publicatio

Interpass rolling of Ti-6Al-4V wire + arc additively manufactured features for microstructural refinement

In-process deformation methods such as rolling can be used to refine the large columnar grains that form when wire + arc additively manufacturing (WAAM) titanium alloys. Due to the laterally restrained geometry, application to thick walls and intersecting features required the development of a new ‘inverted profile’ roller. A larger radii roller increased the extent of the recrystallised area, providing a more uniform grain size, and higher loads increased the amount of refinement. Electron backscatter diffraction showed that the majority of the strain is generated toward the edges of the rolled groove, up to 3 mm below the rolled surface. These results will help facilitate future optimisation of the rolling process and industrialisation of WAAM for large-scale titanium components

Study of residual stress and microstructural evolution in as-deposited and inter-pass rolled wire plus arc additively manufactured Inconel 718 alloy after ageing treatment

The manufacture of structural components made from nickel-based super alloys would benefit from the commercial advantages of Wire + Arc Additive Manufacturing (WAAM), as it is commonly expensive to process using other conventional techniques. The two major challenges of WAAM are process residual stress and undesired microstructure. Residual stress causes part distortion and build failures, while the as-deposited microstructure does not allow the common heat-treatment to be effective in achieving the desired mechanical properties. This paper focuses on understanding the microstructural features, phase formation and three-dimensional residual stress state variation in as-deposited and inter-pass rolled conditions and after solutionising, quenching and ageing. The thermal history from successive deposition and cold working were correlated to the phase formation and macro residual stress formation and subsequent evolution. The {311} family of crystallographic planes were used as atomic strain gauge to determine the macrostrain and analysis of three dimensional stress state in different processing conditions. The measured strain were corrected for the compositional variation by measuring EDM machined d0 specimens manufactured under similar processing conditions. While the as-deposited part show significant stress redistribution and distortion after removal from the main fixture, inter-pass rolling was found to reduce part distortion significantly, the residual stress profile after inter-pass rolling showed highest tensile magnitude near the substrate while near the top of the deposit it was compressive as can be expected from the rolling process. The other two beneficial effects of inter-pass rolling on the microstructure are mitigation of the formation of undesired Laves-phase, thereby improving the response to solution treatment and aging together with significantly reduced grain size and texture. The application of inter-pass rolling reduces the potential part complexity, which however does not prevent the manufacture of common candidate parts, which are typically 1-to-1 replacements of forged, cast or machined from soli

Compression behaviour of wire+ arc additive manufactured structures

Increasing demand for producing large-scale metal components via additive manufacturing requires relatively high building rate processes, such as wire + arc additive manufacturing (WAAM). For the industrial implementation of this technology, a throughout understanding of material behaviour is needed. In the present work, structures of Ti-6Al-4V, AA2319 and S355JR steel fabricated by means of WAAM were investigated and compared with respect to their mechanical and microstructural properties, in particular under compression loading. The microstructure of WAAM specimens is assessed by scanning electron microscopy, electron back-scatter diffraction, and optical microscopy. In Ti-6Al-4V, the results show that the presence of the basal and prismatic crystal planes in normal direction lead to an anisotropic behaviour under compression. Although AA2319 shows initially an isotropic plastic behaviour, the directional porosity distribution leads to an anisotropic behaviour at final stages of the compression tests before failure. In S355JR steel, isotropic mechanical behaviour is observed due to the presence of a relatively homogeneous microstructure. Microhardness is related to grain morphology variations, where higher hardness near the inter-layer grain boundaries for Ti-6Al-4V and AA2319 as well as within the refined regions in S355JR steel is observed. In summary, this study analyzes and compares the behaviour of three different materials fabricated by WAAM under compression loading, an important loading condition in mechanical post-processing techniques of WAAM structures, such as rolling. In this regard, the data can also be utilized for future modelling activities in this direction

Numerical analysis of heat transfer and fluid flow in multilayer deposition of PAW-based wire and arc additive manufacturing

A three-dimensional numerical model has been developed to investigate the fluid flow and heat transfer behaviors in multilayer deposition of plasma arc welding (PAW) based wire and arc additive manufacture (WAAM). The volume of fluid (VOF) and porosity enthalpy methods are employed to track the molten pool free surface and solidification front, respectively. A modified double ellipsoidal heat source model is utilized to ensure constant arc heat input in calculation in the case that molten pool surface dynamically changes. Transient simulations were conducted for the 1st, 2nd and 21st layer depositions. The shape and size of deposited bead and weld pool were predicted and compared with experimental results. The results show that for each layer of deposition the Marangoni force plays the most important role in affecting fluid flow, conduction is the dominant method of heat dissipation compared to convection and radiation to the air. As the layer number increases, the length and width of molten pool and the width of deposited bead increase, whilst the layer height decreases. However these dimensions remain constant when the deposited part is sufficiently high. In high layer deposition, where side support is absent, the depth of the molten pool at the rear part is almost flat in the Y direction. The profile of the deposited bead is mainly determined by static pressure caused by gravity and surface tension pressure, therefore the bead profile is nearly circular. The simulated profiles and size dimensions of deposited bead and molten pool were validated with experimental weld appearance, cross-sectional images and process camera images. The simulated results are in good agreement with experimental results

Thermo-mechanical control of residual stress, distortion and microstructure in wire + arc additively manufactured Ti-6Al-4V.

Wire + arc additive manufacturing (WAAM), unlike most other additive techniques, targets the manufacture of near-net-shape parts for large-scale structural components with medium complexity. WAAM is of special interest for the aerospace industry for reducing lead-time, material and process costs. Ti−6Al−4V is one alloy that could potentially benefit most from the advantages of WAAM, due to high material and process costs, and is therefore the main scope of this research. The manufacture of critical-use components for civil aviation requires a high process control to provide consistently strong and isotropic mechanical properties, as well as the elimination of residual stresses. Cold work can manipulate and counteract residual stresses caused by the additive process. When applied between two layers (i.e. between the deposition passes → interpass) it was found to refine the microstructure and thereby significantly improve the mechanical properties. So far it was only understood that it can theoretically control both residual stress and microstructure, but the science behind the process and how different parameters influence the effectiveness was only proposed. The present research demonstrates how cold work can be used effectively to address both issues, by identifying the process-relevant mechanisms. Before manipulating residual stresses, their development needed to be investigated Behaviour that had only been predicted using numerical simulations was measured for the first time using neutron diffraction and contour method stress determination techniques. This behaviour includes the development of residual stress during the deposition of straight walls and intersections, stress redistribution upon distortion after unclamping and the potential of thermal stress relief. Analogies to previous findings on steel helped explain the findings. The knowledge of stress development finally helped the development of an analytical model to predict residual stress and distortion, as well as stress redistribution upon unclamping. The performance and parameters of plastic deformation strategies were investigated using various characterisation techniques. Those include hardness mapping, residual stress measurements using hole-drilling and the contour method, electron-backscatter-diffraction (EBSD) plastic strain mapping, heat treatment, as well as numerical simulations to compare against the respective measurement techniques. The methodology allowed the development of parameters that produce the required amount of plastic deformation into the required depth of the material, for different thermal histories. Even though 6 % to 8 % of plastic strain can allow reorientation and the development of finer grains, 12 % of plastic strain or more is probably required to achieve a desired grain size. This value is equivalent to 4° lattice misorientation using an EBSD strain mapping technique and it is equivalent to an increase of hardness by at least20 HV. Different rolling and one alternative cold working techniques were investigated to address both individual issues, residual stress and microstructure. Side rolling was found to be far more effective on controlling residual stress and distortion than vertical interpass rolling. Profiled vertical inter-pass rolling on the other hand is far more effective to refine the microstructure and improve mechanical properties than flat rolling. Machine Hammer Peening is an alternative cold working techniques that offers a much higher degree of freedom compared to rolling. The proof of concept to integrate peening into additive manufacturing was successful. However, available machine hammer peening tools do not supply the impact energy required to be at eye level with rolling. It is estimated that approximately 5000 mJ would be required to be as effective as rolling with 70 kN. The fast thermal cycle within the heat affected zone during the additive deposition was measured for the first time at different locations, which allowed conclusions regarding the respective and local development of the microstructure. It furthermore helped to better understand the grain refinement mechanism, and the influence of thermal cycles on subsequent undesired grain growth. The research findings can be applied to develop effective inter-pass cold work strategies for arbitrary thermal cycles and they are sufficient to validate numerical simulations to design better process parameters more efficiently.PhD in Manufacturin

Analytical model for distortion prediction in wire plus arc additive manufacturing

An analytical model was developed to predict bending distortion of the base-plate caused by residual stresses in additively manufactured metal deposits. This avoids timeconsuming numerical simulations for a fast estimation of the expected distortion. Distortion is the product of the geometry factor K, which is determined by the cross-section of substrate and deposit, and the material and process factor S, which is the quotient of residual stress and the Young’s Modulus. A critical wall height can be calculated for which the structure distorts the most. This critical height is typically less than 2.5 times the thickness of the substrate. Higher walls increase the stiffness of the cross-section and reduce the distortion with increasing height