Surface Roughness in Selective Laser Melting Ti-6Al-4V alloy

Selective laser melting (SLM) is a powder bed additive manufacturing (AM) technique that can make complex-shaped metallic components by selectively melting metal powders. Surface finish is an important feature of engineering parts for fatigue performance, wear resistance and aerodynamic behaviour, and post-processing is often required to achieve the desired surface finish after SLM process. In order to better control the surface roughness of SLM Ti-6Al-4V products, an understanding of the physical mechanisms that give rise to the formation of various surface features is needed. As an SLM object is constructed track-by-track and layer-by-layer, the instabilities existing at each level of structural hierarchy are characterized and analysed. Samples with increasing geometrical complexity from single tracks, multi-layer thin walls to bulk samples with various build orientations were manufactured with SLM process. Considerable variations in the track width and height were found at high scanning speed regime for single track specimens.  The microstructure from the cross-section of single tracks shows a transition from key-hole mode to conduction mode of melting along with a decrease in melt pool size and heat affected zone when the scanning speed is increased from 770 mm/s to 1400 mm/s.  For thin wall samples, surface periodic wave-like structure appears at a scanning speed of 770 mm/s and becomes more severe at 1400 mm/s. Bulk samples with upward-facing and downward-facing surfaces inclined at various angles (40˚ to 90˚) to the substrate were manufactured and Ra values were measured. Apart from the stair-stepping effect, the roughness of inclined surfaces is also affected by wetting behaviour and overlapping of melt pools across deposition layers, the amount and size distribution of semi-melted powders attaching to the surface and the difference in heat transfer conditions in solid-supported zone and powder-supported zone.

Keywords:  selective laser melting, surface roughness, Ti-6Al-4V, single line instability