Fig. 1. Schematic of nitriding set up relative to gas flow and rake direction.

Fig. 2. Image analysis procedure images (a) Unfiltered secondary electron SEM image of Nitride layer (b) SEM image with median filtering and histogram equalization applied with Matlab? (c) Image threshold using Ostu’s method via ImageJ software (d) Selection of errors of Otsu’s method thresholding for manual filling of dendrites (e) Final image for analysis with filtered procedure, image threshold and manual correction.

Fig. 3. (a) Param 7 showing relatively dense matrix of dendrites in the Ti64 Matrix and (b) param 14 with slightly finer dendrite arm spacing and more matrix present. Highlighted region indicates secondary arms. The number of arms in the line are counted and divided by the length of the region.

Fig. 4. (a) X-ray diffraction results from l-PBF Ti64 processed entirely in argon with image of the Ti64 cube surface subjected to x-ray radiation and (b) X-ray diffraction results from surface nitrided Ti64 in l-PBF machine with image of golden cube surface subjected to x-ray radiation. Sample nitrided with parameter 7.

Fig. 5. Comparing energy input to secondary arm spacing using the more traditional terms of power over hatch times scan speed giving a fairly linear trend.

Fig. 6. Parameter set 1 showing the distinct martensitic α’ below the TiN dendrites.

Fig. 7. (a) Dendrite arm spacing based on average of 10 measurements from two SEM images at 2000×, (b) dendrite volume fraction in Ti64 matrix.

Fig. 8. (a) Embedded TiN hardness profile cube one built with parameter set 7 with Ti64 re-melt power of 300 W (b) same nitriding conditions with re-melt of Ti64 powder at 165 W for two layers before returning to 300 W It is important to control the laser scanning when re-melting on top of the TiN layer to maintain as much of the hard layer as possible while also creating a good bond with no porosity. The arrow in both images indicates the l-PBF build direction, and the TiN layer is contained between the dashed lines.

Fig. 9. 2000× mag from middle area of embedded TiN layer same sample area from Fig. 4(b) the border of re-melted TiN above the as deposited TIN dendrites. Nitriding laser parameter 7.

Fig. 10. (a) Linear hardness profile across embedded TiN layer for sample built with no reduced power on infill after the nitriding layer (b) hardness profile of sample built with reduced power infill for 2 layers after TiN layer. The arrow in the images indicates the l-PBF build direction.