# Applying Selective Laser Melting to Join Al and Fe: An Investigation of Dissimilar Materials
## 3 Results and Discussion
### Influence of Process Parameters on Join Quality of Dissimilar Materials  
In the SLM process, the intermetallic reaction layer plays a crucial role against the residual stress generated by the thermal gradient. The intermetallic layer is brittle in the area close to the side of the substrate [25], and the strength depends on its thickness. A thinner intermetallic layer has greater strength, and vice versa. Therefore, to prevent a low joining strength, the thickness of the reaction layer should be limited [26]. The flake behavior was investigated by observing the number of layers that could be printed before flaking occurred. Figure 9 shows the experimental results of the observed quantities of printed layers in the boundary area in which the flaking occurred. At the beginning of the process, when the printed section is not thick enough, residual stress from the thermal gradient in the X and Y directions is overwhelming; thus, an intermetallic layer can prevent cracking. Our experiment showed that flaking appeared around the 11 th layer but no sooner. When the printed section became increasingly thicker, the temperature difference between the top and intermetallic layers was greater. The intermetallic layer also suffers from greater residual stress created by the thermal gradient in the thickness direction. However, when the printed section achieved a certain thickness, the effect of the top layer on the intermetallic layer was reduced, and the risk of flaking decreased. It is reasonable to assume that further flaking did not occur after the 25 th layer. Figure 9a demonstrates the laser power effect at 1.5 m/s, 2 m/s, and 4 m/s presented by the triangle, circle, and rectangle makers, respectively. As shown, the printed layers decreased when the laser power increased. Increasing laser power results in a high peak temperature, increasing the residual stress by increasing the thermal transient in each layer. Increasing laser power results in a flaked happening sooner. Figure 9a also shows that, at a fixed laser power, flaking occurred also sooner at a lower laser scanning speed. At a certain printed thickness, a lower laser velocity creates a weaker thermal transient between the top layer and intermetallic layers, reducing residual stress. Additionally,  
Figure 8: Intermetallic layer thickness determined by EDS line mapping analysis: (**a**) a thin intermetallic layer [11] and (**b**) a thick reaction layer that creates a flake. a reduction in laser velocity raised the peak temperature. Higher temperatures and longer reaction times lead to an increase of reaction layer thickness and an easier flaked appearance. The influence of hatch distance on flaking is shown in Figure 9b. Laser power was investigated at 100 W, 150 W and 180 W displayed by the rectangle, circle and triangle shapes, respectively. Increasing the hatch distance increased the number of layers that could be printed. Increasing the hatch distance also reduced the heating time in certain printed areas, reducing the thermal transient in a printing layer and residual stress. The function of the intermetallic layer is to act as protection against the residual stress force. As mentioned in the previous section, high temperature tends to cause concave bending in the printed layers, which leads to the flake phenomenon. The initial residual stress force is not enough to disrupt the surface of the substrate because of the thin printed layers. During accumulative processes, an increase of printed layers causes the residual stress force to increase. With an unsuitable process parameter, the residual stress force overwhelms the opposing force existing in the intermetallic layers and leads to flaking. However, when the printed layers achieve a certain thickness, concave bending transforms into convex bending [27], which is an advantage of flaking. In our case, experimental results show that the 11 th and 25 th layers are the lower and upper boundaries of flaking behavior, respectively. Figure 9: Effects of laser power (**a**), and hatch distance (**b**) on flaking.