Surface Morphology
Skin scanning or selective remelting of the top layer of a SLM fabricated component is performed to increase the surface quality [93]. During skin scanning, the top/final layer is scanned twice. Figure 5-18 shows the surface morphology of a non-skin scanned and skin scanned part fabricated using SLM. Partial melting of particles to the build during SLM (side surface) can be observed. The non-skin scanned part had more particles sintered to the laser scanned surface than the skin scanned part. The surface morphology also clearly shows the laser scan track during the SLM process. An elevated ridge of the solidified material on the edges of both the NSK and SK parts can be observed from Figure 5-19. This is mainly because the remelted material is
Chapter 5: Surface Morphology and Surface Chemistry
partially pushed by the laser beam to the contour of the part. A similar effect has been reported by Kruth et al. [51] for SLM produced parts.
Figure 5-18 Surface morphology of non-skin scanned and skin scanned Ti6Al4V SLM surfaces depicting the laser scan tracts.
Figure 5-19 Surface morphology of a non-skin scanned (NSK) (a) and skin scanned (SK) (b) Ti6Al4V part fabricated by SLM.
Skin scanning or selective re-melting of layers during SLM is performed to increase the surface quality and density of the fabricated part [93]. This is achieved by remelting the laser-scanned layer to form an even surface. When the surface is even, the distribution of powder to build the next layer will be more homogenous and hence will reduce the entrapment of air. Hence, highly dense parts can be produced. In this study, both the NSK and SK parts fabricated using the AM 250 were observed to be rough (Ra = 3.4 µm ± 0.24 µm for NSK and Ra = 2.2 µm ± 0.16 µm for SK) due to the presence of partially melted particles and the laser scan pattern.
Apart from the partially sintered particles, laser scan tracks are clearly visible in the surface morphology of the NSK and especially in the SK surfaces (Figure 5-18). Since the SK surfaces are scanned by the laser twice (using a meander scan strategy i.e. rotating the laser scan to 67°), a relatively smooth
Chapter 5: Surface Morphology and Surface Chemistry
(Ra = 2.2 µm ± 0.2) surface compared to the NSK (Ra = 3.4 µm ± 0.2 µm) surface was obtained. However, it should also be noted that this re-melting leaves a laser scan track that might potentially affect the surface profile. Although optimising the hatch distance may potentially reduce the laser scan track, this is almost difficult to overcome.
Surface Chemistry
Figures 5-20 shows the high resolution C 1s, N 1s and O 1s spectral regions revealing the evolution of these constituent elements over the first few atomic layers (56 nm) of the NSK and SK components. On deconvoluting the C 1s region (Figure 3) of NSK and SK surfaces, peaks were observed at 285 ± 0.2 eV, 286.7 ± 0.1 eV and 288.2 ± 0.2 eV. These peaks were attributed to C-C, C- O/C-N and C=O [155]. For both NSK and SK surfaces, carbon observed in their outermost layer vanished on etching the surface for ~ 4 nm. As mentioned in the previous section, the disappearance of the C-C, O/C-N and C=O peaks after etching confirms that the carbon is mainly from contamination of the surface [132]. However a peak at 282 eV was observed for both NSK and SK sample after etching their outermost layer. This peak was attributed to metal carbide [183].
The N 1s region on deconvolution exhibited the possibility for peaks at 396.5 ± 0.2 eV and 400.2 ± 0.2 eV which corresponds to inorganically (metal-nitrogen) and organically (C-N) bound nitrogen to the surface [181]. On etching the surface for approximately 4 nm, the organically bound nitrogen disappeared. This reveals that this organic nitrogen is from nitrogen containing carbon contaminants (oil from pumps). The deconvoluted O 1s peak exhibited the possibility for presence of TiOx, C-O and adsorbed water molecules for both
NSK and SK samples. From the Figure 5-20, in addition to the formation of metal oxides, on depth profiling, the formation of metal carbide and metal nitride on both NSK and SK surfaces was observed. Although argon is used to make the chamber inert, there is the possibility for the presence of traces of C and N in addition to oxygen. AM 250 has a built-in sensor to quantify the amount of oxygen in the chamber; but not for C and N. Also, C and N are present in the atmosphere as organic contaminants and as gases (CO/CO2 and
Chapter 5: Surface Morphology and Surface Chemistry
N2). These elements were observed on the Ti6Al4V powders as contaminants
(discussed in section 5.2). Due to the high reactivity of Ti and Al towards carbon and nitrogen these metal carbides and nitrides are formed [183].
Figure 5-20 XPS spectra of C 1s, N 1s and O 1s regions for the non-skin scanned (NSK) and skin scanned (SK) Ti6Al4V surfaces fabricated by SLM. The dotted lines shows the peaks that
disappeared on etching the surface.
Figures 5-21 shows the high resolution Ti 2p, Al 2p and V 2p spectral regions on depth profiling the NSK and SK components. On depth profiling, a clear transition of the metal oxide to pure metal was observed (circled with dotted lines in Figure 5-21) for both NSK and SK surfaces. However, the
Chapter 5: Surface Morphology and Surface Chemistry
transformation of Al oxide to aluminium was more gradual for SK compared to NSK (circled in Figure 5-21 with dotted lines). This slow transition may be due to the formation of thick oxide of aluminium.
Figure 5-21XPS spectra of Ti 2p, Al 2p and V 2p regions for the non-skin scanned (NSK) and skin scanned (SK) Ti6Al4V surfaces fabricated by SLM. The dotted lines show the transition
of metal oxides from the outermost layer to pure metals.
Figure 5-22 shows the evolution of elements on depth profiling of NSK and SK Ti6Al4V samples using XPS. From the figure it can be observed that on skin scanning, the concentration of aluminium on the surface increases (nearly doubled). The thickness of the oxide layer was measured by two different
Chapter 5: Surface Morphology and Surface Chemistry
methods. As mentioned previously, in the first method, the thickness was measured based on the point at which the concentration of the major alloying element Ti was equal to that of oxygen. In the other method, the thickness was taken as the point at which the maximum observed concentration of oxygen has reduced to half. Using the initial method, the NSK showed a thickness of approximately 7 nm whereas the SK part showed approximately 14 nm which is nearly double that of the NSK’s oxide layer thickness. In method 2, for the NSK surface, the initially observed maximum O concentration of 46% reduced to 23% at nearly 35 nm; whereas for the SK surface, the initial maximum O concentration of nearly 47% did not reduce to half until the sampling depth of 56 nm. Thus the oxide layer thickness and the evolution of elements including Ti, C, N and O on the NSK and SK surfaces were observed to be significantly different.
Previous literature has suggested the use of skin scanning (re-melting) for improved surface quality and mechanical properties. However, the surface chemistry of such skin scanned surfaces has not been discussed [91]. During skin scanning or re-melting, the top/final layer is scanned twice. Hence during this scanning period, a high amount of energy will be transferred to re-melt and solidify the upper most surface. On re-melting, due to the increase in temperature, segregation of elements in an alloy is possible. For example, segregation of aluminium was reported due to rapid cooling/solidification [51].
This study observed a high concentration of aluminium in the SK surface compared to the NSK surface. Although the Ti6Al4V alloy has only 6% Al, more than 10% on the surface and over 15% into the SK component was observed; whereas for the NSK component, Al was relatively less abundant near the surface. Since the solubility of Al in Ti is very low, precipitation of a Ti3Al phase is possible as the temperature reaches 500-600 C. On re-melting
the previously scanned layer, the material will remain at a high temperature for a long time leading to precipitation [51]. However, the XPS determined surface concentration did not show an increase in the metallic titanium concentration.
Chapter 5: Surface Morphology and Surface Chemistry
Both NSK and SK surfaces mainly consisted of oxides Ti and Al; however, their concentrations were significantly different when compared to each other. Ti6Al4V alloy is preferred for various biomedical applications because of the high cytocompatibility offered by the presence of corrosion resistant TiO2 film.
Also, SLM is considered to be one of the viable processes for making customised metallic implants with complex structures. The application of SLM to fabricate implants will be limited if the process affects the surface chemical composition. Small changes in the chemical composition may cause catastrophic loss of ductility, corrosion resistance, toxicity and cytocompatibility [3,51].
Figure 5-22 Evolution of the surface chemistry of non-skin scanned (a) and skin scanned (b) SLM fabricated surface.
Chapter 5: Surface Morphology and Surface Chemistry
Thus selective remelting of the final layer may be advantageous in terms of rendering a better surface finish. However, it should be noted that it may also alter the surface chemical composition and surface oxide layer. The presence of TiO2 as the predominant surface oxide layer on Ti6Al4V surface is
responsible for its corrosion resistance and cytocompatibility. Since selective remelting alters this composition by increasing aluminium oxide concentration on the surface, cytocompatibility of these components may be reduced and should be properly investigated.