Analysis on Node Formation of the SLM Ti-6Al-4V BCC Micro-lattice Block

To analyze the node formation at the 45° angle diagonal plane, a metallurgical sample was prepared. Sectioning of sample using metallographic procedure was chosen in order to avoid the introduction of strain within the specimen. The as- received (180 W x 1000 µs) and the heat-treated (160 W x 1000 µs) SLM Ti-6Al-4V BCC micro-lattice blocks were resin mounted in order to firmly hold the struts and nodes position during the sectioning process. Sectioning process was done on the mounted blocks by grinding them with coarse emery paper (80 grits), starting at end points of the blocks in 45° angle, until the diagonal planes were reached. The specimens were then polished up to 50 nm surface finish and etched with hydrofluoric acid (HF) for 5 seconds in order to reveal the microstructure. Figure 3.8(a) to (d) show the images of the mounted block resin of the as-received material, before and after the metallographic sectioning.

(a) (b) (c) (d) Figure 3.8: Resin mounted block of the as-received SLM Ti-6Al-4V BCC micro-

lattice (180 W x 1000 µs) [B1-180-1000-AR-M] (a) from side view (b) at angle view; and (c) Diagonal plane from side view (d) at angle view

The sectioned diagonal plane of the as-received material (180 W x 1000 µs) revealed the details of node formation in a BCC micro-lattice block as shown in Figure 3.9. The x and y build directions were as shown in the figure, determined by the pattern of the globules of the struts. The black dots represented the estimated laser focus point at different times, and the thin lines represented the estimated globule boundaries. From the figure, the continuity of struts was considered good and there was little excess material. The quality of nodes depends on the laser scanning strategy [Tsopanos et al. (2010)]. It can be noted that only a single laser spot occurred at the centre of the node, in order to minimise material volume, and this agreed with that of laser points during manufacturing as shown in earlier Figure 3.3.

To summarise part of the manufacturing process, in Figure 3.9, the laser was switched on at point A for t=1000 µs. The laser was then moved to point B and

switched on for 1000 µs. The laser was then moved on to other nodes and blocks. A number of blocks or a mixture of specimens (micro-struts and blocks) can be manufactured at any one time [Tsopanos et al. (2010)]. The laser was returned to point C in 30-60 seconds, and by this time the globule at A has solidified. The

introduction of laser power at C melted the powder and partially melted the globule A. Note that the build direction was at an inclination of 45° angle. There was a

complex thermo-mechanical process within the area, and this was discussed in detail by Yadroitsev (2009) and Rehme (2010). The thermo-mechanical process affects the quality of micro-struts and nodes, as well as their geometry and properties.

Figure 3.9: Details of node formation in SLM Ti-6Al-4V BCC micro-lattice block (180 W X 1000 µs) [B1-180-1000-AR-M]

Figure 3.10: Approximation of dimensioned schematic drawing for globule formation (points A and C in Figure 3.9); layer thickness = 50 µm, laser beam spot

size = 90 µm, distance between melted surface = 75 µm, strut diameter = 300 µm

A B

Figure 3.10 shows the approximation of the dimensioned schematic drawing for the construction of the globule at point C. The globule was formed from the melting of

the selected Ti-6Al-4V powder with 50 µm thickness. As shown in the schematic, the heat conduction from the laser beam with 90 µm spot size dissipated into the powder and melted the surrounding powder for up to the diameter size of the strut, which was approximately 300 µm. The distance between the surface of the melted powder and the previously solidified surface was about 75 µm, which was the diagonal distance of the 50 µm powder thickness in a BCC configuration. It can be seen that a part of the solidified surface of globule A was also melted during the

formation of globule C. Therefore, there was an overlap between the two globules

which fused the globules together.

Figure 3.11 shows a sectioned node of the heat-treated SLM Ti-6Al-4V BCC micro lattice block with 160 W x 1000 µs parameters. The effects of heat-treatment can be seen along the outer surfaces where the fusion of discontinuities and bonded particles were observed. The microstructure of the heat-treated micro-lattice block was comparable to that of the heat-treated single strut as discussed in Chapter 2. However, a slightly different arrangement of β phase in the heat-treated micro-lattice block was observed as compared to that of the heat-treated single strut. This could be due to the effect of different initial manufacturing parameters of both the materials, which led to different cooling rates during the formation of materials. It is known that the microstructure formations of titanium alloys are very sensitive to cooling rates [Gilbert and Shannon (1998)]. Table 3.1 compares the differences in microstructures of the micro-lattice blocks and single struts.

Figure 3.11: Sectioned node of the heat-treated SLM Ti-6Al-4V BCC micro lattice block (160 W x 1000 µs) [B1-160-1000-HT-M]

Table 3.1: Microstructure comparison between the SLM Ti-6Al-4V BCC micro- lattice blocks and single manufactured SLM Ti-6Al-4V micro-struts from Chapter 2

BCC micro-lattice blocks Single manufactured struts

As-received (180 W X 1000 µs) [B1-180-1000-AR-M] As-received (200 W X 1000 µs) [S1-35-200-1000-AR-M] Heat-treated (160 W x 1000 µs) [B1-160-1000-HT-M] Heat-treated (200 W x 1000 µs) [S1-90-200-1000-HT(B)-M] Build direction  _200 µm 

Discontinuities and bonded particles were re-melted and fused along the surface of heat-treated material 

50 µm 

50 µm 

50 µm 

3.3.2 Ana