Browsing by Author "Hönnige, Jan Roman"

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    Analytical model for distortion prediction in wire plus arc additive manufacturing
    (Materials Research Forum, 2018-10-05) Hönnige, Jan Roman; Colegrove, Paul A.; Williams, Stewart W.
    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.
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    Numerical analysis of heat transfer and fluid flow in multilayer deposition of PAW-based wire and arc additive manufacturing
    (Elsevier, 2018-04-05) Bai, Xingwang; Colegrove, Paul A.; Ding, Jialuo; Zhou, Xiangman; Diao, Chenglei; Bridgeman, Philippe; Hönnige, Jan Roman; Zhang, Haiou; Williams, Stewart W.
    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.
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    Thermo-mechanical control of residual stress, distortion and microstructure in wire + arc additively manufactured Ti-6Al-4V.
    (2018) Hönnige, Jan Roman; Colegrove, Paul A.; Williams, Stewart W.
    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.