# Femtosecond Laser Additive Manufacturing of Multi-Material Layered Structures
## 3 Results and Discussion  
### Fabrication of Multi-Layer and Multi-Material Structures  
The Ni-YSZ supporting layer was first made with the LAM. Similar to YSZ LAM [11], the process window is small and a perfect match between the laser power and scan speed is critical. The feasibility of melting Ni-YSZ powders was successfully demonstrated with 80 MHz PRR femtosecond laser AM and without mixing any binders with the Ni-YSZ powders. Figure 2 (left) shows the experimental result of porosity (or relative density) as a function of laser power at a fixed scan speed of 300 mm/s. A high density of Ni-YSZ can be achieved at 130 W laser power and 300 mm/s scan speed. Lower density (high porosity, specifications of the Ni-YSZ anode are in Table 1) can be achieved by either increasing the scan speed to 500 mm/s or reducing the laser power. Figure 2 (right) shows the microscopic photos of the melted Ni-YSZ layers under various scan speed conditions at a fixed power of 130 W. The results show that high density Ni-YSZ had a better surface smoothness (with much clearer image) than the low density Ni-YSZ. Moreover, we also demonstrated that it was possible to control the laser power (Figure 2) to obtain a support layer and functional layer in one process. Figure 3 (left) shows an example with the combined functional anode layer and support anode layer. It has 35 to 40 vol% of Ni from Energy Dispersed X-ray spectroscopy (EDX) measurement. Figure 3 (right) are examples of anode samples with excellent uniformity and repeatability. These findings lay a solid foundation for the next LAM process of YSZ, the electrolyte layer. Figure 3: (**Left**) Cross-section scanning electromagnetic (SEM) image of the interface between the functional layer and support layer of the anode. (**Right**) Ni–YSZ anode samples (the largest size was \(30\times 30\) mm). Figure 2: (**Left**) Density as a function of laser power at a scan speed of 300 mm/s. (**Right**) Microscopic images of melted layer with a target power of 130 W, hatching space of 30 μm, scanning speed at 300 mm/s (**a**,**b**), 400 mm/s (**c**,**d**), and 500 mm/s (**e**,**f**). (**Right**) column images (**b**,**d**,**f**) are magnified images of the left column (**a**,**c**,**e**). Second, a thin layer of YSZ, either in powder or ink, was spread on top of the Ni-YSZ anode support substrate and processed with the laser. Both can be melted and bonded on top of the anode surface, but the ink type of YSZ provides better uniformity. The scan patterning and hatching space play critical roles in the melting of YSZ on the Ni-YSZ anode. Two patterning types were systematically studied as shown in Figure 4, and the staggering pattern was better than the line pattern, with less thermally induced peeling off. YSZ was strongly bonded on Ni-YSZ. The YSZ layer was fabricated from 10 to 50 \(\upmu\)m in thickness. The melting effect of YSZ on Ni-YSZ as a function of hatching space was investigated and summarized in Figure 5. Figure 5a,b show the YSZ layers printed on the Ni-YSZ anode substrates with a hatching space of 15 \(\upmu\)m, scanning speed of 80 mm/s, and on-target power of 78 W. Figure 5c,d show the results with hatching space of 20 \(\upmu\)m, scanning speed of 100 mm/s and on-target power of 78 W. Figure 5e,f show the results with a hatching space of 20 \(\upmu\)m, scanning speed of 80 mm/s, and target power of 78 W. Figure 5g,h show the results with a hatching space of 25 \(\upmu\)m, scanning speed of 100 mm/s, and target power of 78 W. Images on the right column (b, d, f, h) differ only in the magnifying scale from their counterparts on the left column (a, c, e, g). When the hatching space is too small, continuous ripple lines in the narrow space can be found. This results in more cracks propagating in the cross direction. Consequently, the porosity is large (as shown in Figure 5a,b). When the hatching space is too large, melted lines cannot be formed continuously, resulting in unmelt regions and fractured structure with large porosity (as shown in Figure 5g,h).