Laser additive manufacturing of Zn metal parts for biodegradable implants: Effect of gas flow on evaporation and formation quality

Zn based metals have exhibited promising applications for biodegradable medical implants. Although additive manufacturing has been widely investigated for many medical metals, only a handful of very recent reports can be found on laser powder bed fusion (L-PBF) of pure Zn cubes. Severe evaporation occurred during laser melting of Zn powders due to the low boiling point of Zn metal. The formation quality was either not satisfactory or the obtained processing window was too narrow. In this paper, a specially designed gas flow system was used in order to eliminate the negative effect of Zn metal evaporation on the processing of L-PBF. Numerical analysis was used to simulate the velocity distribution of shielding gas flow and the interaction between the gas flow and the evaporation fume based on computation fluid dynamics with considering different shielding flow designs and laser energy inputs. Pure Zn metal parts were obtained with density over 99.90% after the optimization of shielding gas flow and laser energy input. The areal surface roughness (Sa) of L-PBF parts was about 10 mm at side surfaces for as-melted status, and reduced to 4.83 mm after sandblasting. The effects of gas flow and laser energy input on evaporation and formation quality were discussed. An effective method was provided to deal with metal evaporation during L-PBF processing, which was not only beneficial to additive manufacturing of Zn based metals, but also to other active metals with high evaporation tendency like Mg and Al alloys.