  Components manufactured via Wire + Arc Additive Manufacturing are usually
characterised by large columnar grains. This can be mitigated by introducing
in-process cold-rolling; in fact, the associated local plastic deformation
leads to a reduction of distortion and residual stresses, and to
microstructural refinement. In this research, inter-pass rolling was applied
with a load of 50 kN to a tantalum linear structure to assess its effectiveness
in changing the grain structure from columnar to equiaxed, as well as in
refining the grain size. An average grain size of 650 microns has been obtained
after five cycles of inter-pass rolling and deposition. When the deformed layer
was reheated during the subsequent deposition, recrystallisation occurred,
leading to the growth of new strain-free finer equiaxed grains. The depth of
the refined region has been characterised and correlated to the hardness
profile developed after rolling. A reduction of porosity was also registered.
Furthermore, a random texture was formed after rolling, which should result in
isotropic mechanical properties. Wire + Arc Additive Manufacturing process
demonstrated the ability to deposit sound refractory metal components and the
possibility to improve the microstructure when coupled with cold inter-pass
rolling.
