Additive manufacturing (AM) of metal alloys enables the creation of components with complex internal geometries that are often unachievable with traditional methods, offering reduced time and costs. A key application is the production of micro-channels for cooling high-temperature gas turbine components. However, the AM process introduces higher surface roughness compared to conventional methods, which significantly affects hydraulic and thermal performance, as well as material fatigue behavior. This experimental study aims to characterize the effect of relative roughness on these phenomena, focusing on two key performance parameters: the Darcy friction factor and the Nusselt number. Tests were conducted on aluminum and Inconel-939 specimens, featuring straight channels of various lengths and cross-sectional shapes, using air as the working fluid.