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Cold rolling can be used in-process or post-process to improve microstructure, mechanical properties and residual stress in directed-energy-deposition techniques, such as the high deposition rate wire + arc additive manufacturing (WAAM) process. Finite element simulations of the rolling process are employed to investigate the effect of rolling parameters, in particular rolling load and roller profile radius on the residual stress field as well as plastic strain distribution for the profiled roller. The results show the response to rolling of commonly used structural metals in WAAM, i.e., AA2319, S335JR steel and Ti-6Al-4V, taking into account the presence of residual stresses. The rolling load leads to changes in the location and the maximum value of the compressive residual stresses, as well as the depth of the compressive residual stresses. However, the roller profile radius only changes the maximum value of these compressive residual stresses. Changing the rolling load influences the equivalent plastic strain close to the top surface of the wall as well as in deeper areas, whereas the influence of the roller profile radius is negligible. The plastic strain distribution is virtually unaffected by the initial residual stresses prior to rolling. Finally, design curves were generated from the simulations for different materials, suggesting ideal rolling load and roller profile combinations for microstructural improvement requiring certain plastic strains at a specific depth of the additive structure.

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Wire + arc additive manufacturing (WAAM) is an additive manufacturing (AM) process that employs wire as the feedstock and an arc as energy source, to construct near net-shape components at high build rates. Ti-6Al-4V deposits typically form large columnar prior β grains that can grow through the entire component height, leading to anisotropy and lower mechanical properties, compared to the equivalent wrought alloy. Cold-working techniques such as rolling can be used to promote grain refinement in Ti-6Al-4V WAAM parts, thus increasing strength and eliminating anisotropy concomitantly. Additionally, rolling can be beneficial in terms of reduction of residual stress and distortion. The aim of this study is to illustrate the effect of rolling process parameters on the plastic deformation characteristics in Ti-6Al-4V WAAM structures. To produce a certain refinement of the microstructure, a certain amount of strain is typically required; thus suitable design guidelines for practical applications are needed. The effect of different rolling process parameters, in particular, rolling load and roller profile radius on the plastic strain distribution is investigated based on the finite element method. From a numerical point of view, the effect of the stiffness of the roller is investigated, e.g. deformable vs. rigid roller. Results indicate that for an identical rolling load, the deformable roller produces lower equivalent plastic strains due to its own elastic deformation. Additionally, a lower friction coefficient produces higher equivalent plastic strains near the top surface but, it has an insignificant effect on the plastic deformation further away from the top surface. However, numerically the computation time significantly increased for a higher friction coefficient. Larger roller profile radii lead to lower plastic strain near the top surface, but simultaneously had nearly no noticeable effect on plastic strains at deeper depth. In addition, the effect of interspace between rollers on the uniformity of the plastic strain during multi-pass rolling was investigated for a selected example. The results show that a higher uniform plastic strain distribution is obtained when the interspace between two rollers is equal to the residual width of the groove produced by a single rolling pass.