Surface Roughness of Interior Fine Flow Channels in Selective Laser Melted Ti-6Al-4V Alloy Components

Abstract

A challenge remains in achieving adequate surface roughness of SLM fabricated interior channels, which is crucial for fuel delivery in the space industry. This study investigated the surface roughness of interior fine flow channels (1 mm diameter) embedded in SLM fabricated TC4 alloy space components. A machine learning approach identified layer thickness as a significant factor affecting interior channel surface roughness, with an importance score of 1.184, followed by scan speed and laser power with scores of 0.758 and 0.512, respectively. The roughness resulted from thin layer thickness of 20 µm, predominantly formed through powder adherence, while from thicker layer of 50 µm, the roughness was mainly due to the stair step effect. Slow scan speeds increased melt pools solidification time at roof overhangs, causing molten metal to sag under gravity. Higher laser power increased melt pools temperature and led to dross formation at roof overhangs. Smaller hatch spaces increased roughness due to overlapping of melt tracks, while larger hatch spaces reduced surface roughness but led to decreased part density. The surface roughness was recorded at 34 µm for roof areas and 26.15 µm for floor areas. These findings contribute to potential adoption of TC4 alloy components in the space industry.

Keywords: additive manufacturing; build orientation; confined geometry; heat penetration; interior channel; laser powder bed fusion; overhang; process parameters optimization; space industry; trapped powder.

Figure 1

SLM Sample Preparation and Experimental Design: (a) single scan tracks for hatch space estimation; (b) cubes produced for density and surface roughness measurements; (c) cuboids oriented at varying angles for surface roughness assessment; (d) channel cuboids designed for orientation dependent printing.

Figure 2

Building orientation effect on surface roughness of cuboid open surfaces and channel interior roof and floor.

Figure 3

An illustration of impact of build orientation on channel surface roughness: (a) layer slicing horizontal channels at 0° orientation resulting larger overhang areas and stair steps; (b) channels at 45° orientation with smaller overhang areas and smaller stair steps; (c) vertical channels at 90° orientation exhibits no overhangs and minimal stair steps.

Figure 4

Principal Component Statistical Analysis: (a) loading plot; (b) 2-D score plot of the first two principal components containing a cumulative 87.4% variance; (c) 3-D score plot of the first three principal components containing a cumulative 95% variance.

Figure 5

Permutation Importance of Process Parameters on Channel Surface Roughness.

Figure 6

Effects of layer thickness on the channel surface roughness and density of 1 mm fluidic channels at different scan speeds: (a) channel surface roughness variation at 60 W laser power; (b) density variation at 60 W laser power; (c) channel surface roughness variation at 120 W laser power; (d) density variation at 120 W laser power.

Figure 7

Impact of layer thickness on surface finish and overhangs: (a) small layer thickness leading to more pronounced channel surface roughness due to powder adherence, yielding a finer surface; (b) Large layer thickness resulting in increased channel surface roughness due to large stair steps and more significant overhang areas; (c) Optical microscopy image of a channel printed with a 20 µm layer thickness, showing a relatively finer inside surface; (d) Optical microscopy image of a channel printed with a 50 µm layer thickness, displaying poorer surface characteristics.

Figure 8

Effects of scan speed on the channel surface roughness and density of interior channels: (a) channel surface roughness variation at various scan speeds for 20 µm layer thickness; (b) density variation at various scan speeds for 20 µm layer thickness; (c) channel surface roughness variation at various scan speeds for 50 µm layer thickness; (d) density variation at various scan speeds for 50 µm layer thickness.

Figure 9

Factors affecting 1 mm diameter horizontally printed channel surface quality and overhang roof: (a) heat penetration into trapped powder increases powder adherence, especially at the channel roof; (b) FEA simulation of the channel’s manufacturing process showing a thermal gradient along the z-axis with elevated temperatures at the roof; (c) Optical microscopy image of a horizontal interior channel’s cross-sectional surface profile at scan speed 800 mm/s, laser power 60 W and layer thickness 20 µm, illustrating droplet phase formation at the roof due to sagging from longer consolidation times; (d) Optical microscopy image of a horizontal channel surface profile at a higher scan speed 1800 mm/s with same laser power and layer thickness, showing reduced roof sagging due to shorter consolidation time.

Figure 10

Effects of laser power on the channel surface roughness and density of interior channels: (a) Channel surface roughness variation at various laser powers for layer thickness of 20 µm; (b) Density variation at various laser powers for layer thickness of 20 µm; (c) Channel surface roughness variation at various laser powers for layer thickness of 50 µm; (d) Density variation at various laser powers for a layer thickness of 50 µm.

Figure 11

Factors affecting floor and roof formation in SLM printed horizontal channels: (a) laser scanning and welding process of scanning layers onto the preceding floor layer; (b) Overhang deviation during roof formation over the unhatched powder zone; (c) Optical microscopy image of the cross-sectional channel surface profile at a laser power of 60 W, scan speed of 1800 mm/s, and layer thickness of 20 µm; (d) Optical microscopy image at a higher laser power of 120 W printed with the same parameters.

Figure 12

Effect of hatch space on surface roughness and density of SLM 3D horizontally printed 1 mm diameter interior channel: (a) roof and floor roughness variation at various hatch spaces, for sample printed with laser power 60 W, scan speed 1800 mm/s and layer thickness 20 µm; (b) density variation at various hatch spaces.

Figure 13

Optical microscopy of horizontal interior channel’s cross-sectional surface, showing roof and floor profiles at laser power 60 W, scan speed 1800 mm/s and layer thickness 20 µm across hatch spaces from 30 µm to 60 µm (a–d). The red colored dotted lines represent the CAD-designed contours within the channel, the green dotted lines illustrate the upper boundary of the printed contour at the roof and the blue dotted lines show the lower boundary of the printed contour of the floor. This delineation highlights the variations in the surface profile attributable to changes in hatch space.