Build orientation effects on the roughness of SLM channels

Increasingly advanced shapes and geometries are being manufactured using additive manufacturing and new characterization techniques must emerge in order to fully utilize the new possibilities given by freeform design. Cooling channels produced by the laser powder bed fusion process has been shown to have high roughness at overhanging areas due to powder particles being fused with the internal surface.

Classic techniques for characterizing profile roughness are falling short with respect to internal surfaces in freeform geometries. Hence, this work presents a methodology for characterizing internal surface roughness by extracting roughness profiles through the use of image analysis and X-ray CT. In order to fully describe the internal surface roughness, two orientations were defined, namely the global and local orientations α and β.

The internal profile roughness was evaluated in accordance with ISO 4287:1997. Seven selective laser melting manufactured straight channels made in 17-4 PH stainless steel were CT scanned and analyzed with the proposed methodology. Results showed that the Ra-values inside the channel were dependent on both α and β.

The average Ra-values and their standard deviations were found to be decreasing rapidly with increasing α. The highest average roughness was found for α = 0°, where an average Ra-value of 70.7 μm was found. The lowest average roughness was found at α = 90°, where an average Ra-value of 6.7 μm was found.

Furthermore, it was found that the surface texture and roughness changed dependent on the location along the length of the channel produced at α = 0°. These findings suggest the importance of characterizing the internal surface roughness of cooling channels with respect to both the global build orientation of a channel, the local orientation within a channel and the specific location along the length of a channel.