Evolution of grain and ripple structures on the surface of aluminum parts fabricated by laser foil printing process at preheat temperatures

In this study, the evolution of grain and ripple structures on the surface of 6061 aluminum alloy (AA6061) parts fabricated by a laser-foil-printing (LFP) additive manufacturing process at different preheated temperatures were characterized and compared. Results indicated that the preheated temperature of the substrate plays a crucial role in the formation of grain structures, the shape of ripples, and the surface quality of aluminum parts fabricated by LFP. As the preheated temperature of the substrate elevated from room temperature to 300 °C, the grain and ripple structures formed coarser and flatter, respectively, due to the reduction in the thermal gradient and cooling rate. The grain structure was coarsened with an increasing average grain size from 74.96 to 150.5 μm measured by the electron backscattered diffraction (EBSD) technique. Meanwhile, the ripple structure was stretched and transformed from short and curved shapes to smoother, which improved the surface roughness significantly and reduced the S_a value from 21.7 to 8.5 μm in three-dimensional (3D) surface morphologies. However, when the preheated temperature reached 500 °C in LFP, the grain structure experienced a recrystallization process, resulting in the formation of finer grains compared to those observed at 300 °C. Furthermore, the surface quality was improved to a S_a value of 5.2 μm because the flowability of the melt pool was enhanced from the extended solidification time by the smallest temperature gradient and cooling rate at the highest preheat temperature, which was estimated using the finite element method (FEM). In the X-ray diffraction (XRD) patterns, the ratio of the main peak (200) to the secondary peak was gradually increased when preheated temperature elevated; however, the ratio decreased at 500 °C due to the recrystallization.