Understanding internal defects in Mo fabricated by wire arc additive manufacturing through 3D computed tomography

Molybdenum (Mo) is an important high-temperature structural material but has poor processability. Additive manufacturing (AM) leads to a new possibility of fabricating Mo structural parts. However, a large number of internal defects appear during welding and AM processes in Mo and its alloys, which is far from well understood and has greatly limited their application. In this paper, the formation and evolution mechanisms of internal defects in Mo are systematically studied, based on the state-of-the-art high-resolution computed tomography. This study demonstrates three main types of defects in Mo: (1) small spherical pores; (2) inverted pear-shaped pores; and (3) cavities. The first type is similar to the observation in welded Mo, while the last two types are not reported before, which are associated with the heat cycling process during AM. The formation mechanism of different types of internal defects is proposed based on the experimental observations. The amount and morphology of internal defects are quantitively and statistically discussed to understand the effect of fabrication parameters, which are further explained according to their formation mechanisms.