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Micrographs of metallographic sections show that additive manufacturing, with its local heat input and rapid cooling rates, brings about microstructures in metal components that differ from those generated in conventional manufacturing processes, such as casting or forging. The example of samples/components made from the material Alloy 718 and manufactured using a laser (Laser Powder Bed Fusion, abbr.: LPBF, designation according to DIN EN ISO ASTM 52900 [1]) shows a very low coarsely distributed porosity. The porosity is limited to the gas porosity already introduced during the atomization step of the powder manufacturing process. As opposed to the microstructure of conventionally produced Alloy 718, the precipitates formed are very small and can only be revealed in the light microscope when high magnifications are applied. Evenly distributed, cruciform γ” precipitates are formed. The grain structure is formed independent of the LPBF-typical melting zones. Isolated dendritic structures can be observed at the interfaces of the melting zones. In the samples manufactured by LPBF, grain structures in the longitudinal section differ from those in the cross section: It can be observed that the grains are elongated in the build direction, which explains the anisotropic behavior of the materials in the tensile test. The width of the melting zones and the laser track spacing can easily be measured in the cross section, while the depth of the melting zones is well recognizable in the longitudinal section. Mechanical properties such as those found in cast or forged Alloy 718 are already present in the stress relief annealed condition.

practical metallography

Specific Characteristics of Materials Produced by Additive Manufacturing as Compared to Those Produced by Established Manufacturing Methods taking the Example of Alloy 718