# Applying Selective Laser Melting to Join Al and Fe: An Investigation of Dissimilar Materials
## 3 Results and Discussion
### Intermetallic Layer  
The printing process creates molten aluminum on the solid substrate. That aluminum melts on the solid surface of the substrate to create a diffusion area called an intermetallic layer, which exists as the interface between the printed section and the substrate. Formation of the intermetallic phases between the solid iron/steel and liquid aluminum depends on the chemical reaction and interdiffusion of the elemental constituents. Figure 5 demonstrates the SEM morphology and EDS mapping results of a sample, which indicate a distribution of Fe in Figure 5b and Al shown in Figure 5c elements. The results show that the Fe element diffused into molten aluminum to a larger extent than the Al element diffused into the stainless-steel substrate.  
Figure 4: Temperature gradient mechanism [11]: schematic of the SLM processing (**a**) created a new layer on other solidified layers (**b**), thermal expansion generated by heated new layer (**c**) and shrinkage process during cooling (**d**).  
Figure 5: (**a**) Energy dispersive spectroscopy (EDS) image, (**b**) Fe element distribution, and (**c**) Al element distribution.  
Figure 6 shows an EDS point analysis of a flaked sample with 31.11 at% iron and 68.89 at% aluminum. The analysis confirms that the Fe\({}_{2}\)Al\({}_{5}\) phase exists in the intermetallic layer. The results agree with previous research using other methods [14; 15; 16; 17]. However, the morphology of the reaction layer during the SLM process is different from the previous methods. Although previous studies reported tongue-like, needle-like, or wave-like [18; 19; 20; 21] morphology, the SLM process shows a more chaotic morphology of the reaction layer, as indicated in Figure 7. The chaotic morphology can be explained by the molten material flow during melting and evaporation during the SLM process [22]. Additionally, denudation [23], where a laser interacts with metal powder, results in non-repetitive formation of an intermetallic layer.  
Figure 6: Results of the EDS point analysis of the intermetallic layer of a flaked sample.  
Figure 7: Morphology of the intermetallic layers formed during the selective laser melting process.  
The intermetallic layer thickness was determined by detecting the Fe-Al change, as shown by the two dashed lines in Figure 8. The thickness is associated with the SLM process parameters of time and temperature. An increase in laser power increases the thickness of the reaction layer, whereas an increase in laser speed decreases the thickness of the metallic layer. An increase in laser power causes an increase in energy and thus an increase in peak temperature. Higher temperatures and longer reaction times increase the thickness of the reaction layer. Moreover, a higher laser scanning velocity results in a lower peak temperature and leads to a reduced thickness of the bimetallic layer. The EDS results show that the thickness of the metallic layer ranges from 3 \(\upmu\)m to 70 \(\upmu\)m. When the intermetallic layer is thin, the mechanical performance of the joint is not yet detrimental, as shown in Figure 8a. However, the joint will fail when the reaction layer thickens, as shown in Figure 8b. These results agree with previous studies [24].