Effect of process parameters and electropolishing on the surface roughness of interior channels in additively manufactured nickel-titanium shape memory alloy actuators

Recently, laser-powder bed fusion (L-PBF) has been utilized to produce a NiTi shape memory alloy actuator with embedded channels for liquid metal forced fluid convection to increase actuator heat transfer rates. To enable further increases in performance, it is critical to characterize and control the surface quality of fully interior channels which have higher surface roughness compared to exterior top surfaces. This work utilizes a design of experiments methodology by varying laser power, scan speed, hatch space, scan pattern, channel orientation, and channel diameter on the as-fabricated surface roughness of the overhangs and walls of interior channels in NiTi. To enable post-process increases in surface quality, the channels are subjected to an electropolishing treatment and further characterized. Internal channel surfaces are characterized using optical profilometry and SEM imaging. It is concluded that channel orientation plays a prominent role in determining the surface roughness of as-fabricated interior channels, and a lower laser energy density results in the highest reduction in surface roughness after an electropolishing treatment.