  Laser metal additive manufacturing technology is capable of producing
components with complex geometries and compositions that cannot be realized by
conventional manufacturing methods. However, a large number of pores generated
during the additive manufacturing process greatly affect the mechanical
properties of the additively manufactured parts, and the mechanism of such pore
generation has not been revealed by direct observation clearly. Here, we report
the mechanism of pore generation in the laser direct energy deposition process
as revealed by {\it in-situ} high-speed high-resolution synchrotron X-ray
imaging. We found that dissolution and re-precipitation of external gases and
precipitation of metal vapors are the two main mechanisms of pore formation. We
further explored the effects of different process parameters on the generation
of pores and optimized the process to suppress pore generation. This work
provides important insights into the formation of porosity defects during laser
metal additive manufacturing, and can provide guidance for related process
optimization.
