Development of an Empirical Process Model for Adjusted Porosity in Laser-Based Powder Bed Fusion of Ti-6Al-4V

A promising approach to address the mismatch of bone and implant stiffness, leading to the stress-shielding phenomenon, is the application of functionally graded materials with adjusted porosity. Although defect formation and porosity in laser-based powder bed fusion of metals (PBF-LB/M) are already widely investigated, so far there is little research on the influences and parameter interactions regarding the pore characteristics. This work therefore aims to provide an empirical process model for the generation of gas porosity in the PBF-LB process of Ti-6Al-4V. For the first time, parts with closed locally adjusted porosity of ~ 6 % achieved through gaseous pores instead of lack-of-fusion defects or lattice structures were built by PBF-LB. Parameter variation and evaluation of relative density, pore size and sphericity was done in accordance with the design of experiments approach. A parameter set for maximum gas porosity (laser power of 189 W, scanning speed of 375 mm/s, hatch spacing of 150 µm) was determined for a constant layer thickness of 30 µm and a spot diameter of 35 µm. Tensile tests were conducted with specimens consisting of a core with maximum gas porosity or lack-of-fusion porosity, respectively, and a dense skin as well as fully dense specimens. Whereas lack of fusion defects can lead to significant reduction of stiffness, the elastic modulus remained unchanged when implementing spherical pores. Nevertheless, the found superior strength and ductility of specimens with gas porous core underline the advantages of adjusted porosity for the application in functionally graded materials and lightweight applications.