The purpose of this test was to test if changes of 1 μm in hatch distance had a real effect on the space between hatches and if inputs to hatch distance had a minimum value.
To test the limit and resolution of hatch distance, hatch files was made for 5 mm cubes with different hatch distances. A single layer of these cubes was scanned onto thermal sensitive paper, impart the laser scan path for that layer (as in 3.1.1). The point distance, exposure time, laser power and lens position were 50 μm, 50 μs, 20 W, and 15.00 mm, respectively. The lines created by these parameters are roughly 105 μm in width (104.8 ±0.6 μm measured from a single line at five points with imageJ), therefore hatch distances below this will have overlap and only distinguishable at the rounded ends of the line. Figure 92 shows two of the parameters used to create the hatched patterns. In all the hatched bocks, the hatches start at the top left with successive scan lines altering direction. Figure 92 show that the hatches start 300 μm away from the starting point, consistently across all hatched areas. During the transition between scan lines the laser switches off. The kink at the start of each line show the scan line starts before the y-axis mirror is at rest. This is caused by the mirrors being underdamped.
Rather than requiring a precise measurement between two lines and precision imaging, the approach was taken to measure across several lines with the reasonable assumption that hatch spacings are equal between them. The hatched blocks were 5 mm square, and the hatching software is designed to place as many hatched lines up to but not matching or exceeding the designed boundary and no offsets are included.
As such the number of hatch lines within the 5 mm boundary should change with hatch distance inputs.
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Figure 92 Hatched layer of 5mm square from hatch distance test; (a) 180 μm hatch distance sample showing separation of hatch lines, showing their width and direction, (b) 100 μm hatch distance sample used to measure real changes in hatch spacing and (c) close-up of 100 μm hatch distance
sample’s hatch lines, showing how they were counted to measure the change.
The number of hatches within the 5 mm square changes inversely to the hatch distance. Hatches distances of 98 μm, 99 μm and 100 μm had 51, 50 and 49 hatch lines respectively, within the 5 mm area. The area of these blocks measured 4.932mm, 4.870mm, and 4.808m, respectively, are shorter than what was expected but this could be from tilting in imaging. This confirms that the hatch distance resolution is at least equal to 1 μm.
The narrowest hatches caused overheating and melting of the build plate, with the greatest overheating occurring with hatch distance of 1 μm, the smallest value tested.
This value is smaller than would be needed as the nominal beam diameter is 44 μm and melt tracks are around 150~200 μm wide. This confirms that all hatch distances input as whole numbers in microns will have a real change on the machine.
Renishaw AM 125
The second SLM machine (Figure 93) used for this experimental work was a Renishaw AM125 (Renishaw, UK) with a R4 RedPOWER laser (SPI Lasers, UK), which is, similar to the Realizer’s YLR-200 laser, a 200W continuous wave ytterbium doped fibre laser.
Optical tracks for the two machines are identical in configuration but the Renishaw has a nominal beam diameter of 35 μm.
The build plate is a 125 mm square area with rounded corners (Φ = 20 mm) and the maximum build height is 125 mm. The build plates used in this research were 10-15 mm thick plates of aluminium supplied by Renishaw AMPD. The powder deposition system is similar to the SLM100, though the hopper is fixed in position and the slider to release a dose of powder is triggered by the wiper, which moves on to rails linearly across the build plate.
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The process performs in an overpressure argon atmosphere, with a similar gas recirculation system. Air is removed from the chamber by vacuuming the air from the chamber and refilling with argon.
Figure 93 Renishaw AM125 at the University of Liverpool
The intention of using this machine was as a replacement while the Realiser SLM100 was requiring repair. No density samples built in this machine are included in this thesis as a problem with focus was discovered. Initial tests observed that the top surface finish of parts showed variation with location on the build plate. (Figure 94).
This variation was radial and centred on the middle of the plate. It was expected, and proved by results shown in section 5.4, that the more reflective surface was an indication of lower surface oxides, and therefore the parts built by this machine were included in the study of oxides in SLM aluminium. Tests described in section 3.2.1 proved that the cause of the discrepancy was with the focus, which was not sufficiently altered to flat field focus. The problem is likely to have been with the f-theta and should the system be repaired, it would be expected that the range of spot sizes would still be achievable in the system, manufacturer permitting.
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Figure 94 Density optimisation parameter test for AlSi10Mg build in the Renishaw AM125
It was important to understand if the location, and the appearance, has an effect on the build quality. A build was made with 6x6 array of parts all with equal parameters.
The sample density was measured in the Archimedes method (Figure 95). The optimum parameters for this material in this machine are not known and the more porosity the greater the scatter in results would be, as porosity is a random occurrence. It is clear from the Figure 95 that the location has an effect on part density, similar in pattern to the surface appearance. The highest densities are in areas where the top surface appears darker and the lowest densities are in areas with more reflective surface.
Figure 95 Renishaw AM125 part consistency build, the gravimetric density colour gradient applied to aid identification of relative densities, with dense parts in darker colour.
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