Powder-based 3D printing of titanium often leads to increased porosity of final products and therefore, increased risk of breakage. To investigate this problem, researchers from Carnegie Mellon University and the Argonne National Laboratory inspected Ti-6Al-4V titanium and learned that as the titanium powder heats, gases trapped in the material can create pores. This issue has been established and yet to be fixed.
Source: http://acceleratingscience.com/metals/new-research-on-3d-printing-with-titanium/