A Comparative Study of the Modal Response of Additively and Subtractively Manufactured Thin Plates After Thermal Loading

Background: Additively-manufactured parts contain residual stresses induced by manufacturing. These residual stresses can be relaxed or redistributed by thermal loading. The presence of internal stress influences the dynamic response of parts, and this is of particular interest in thin plates subject to thermoacoustic loading in hypersonic vehicles and fusion reactors. Objective: To measure the changes in shape and modal frequencies caused by thermal loading of geometrically-reinforced thin plates that were additively manufactured in Inconel 625. Methods: Plates were additively-manufactured in landscape and portrait orientations using laser powder bed fusion. The plates were heated to a nominal temperature of 820 ̊C, which was expected to alleviate the residual stress from the build process. Pre- and post-heating, their modal frequencies were found experimentally and pulsed-laser stereo (3D) digital image correlation was used to evaluate their modal shapes. The resultant modal frequencies and shapes were compared with those from a subtractively-manufactured plate. Results: It was found that the heat cycle changed the shape of the plates relative to their as-manufactured state in addition to changing their natural frequencies and modal shapes. Conclusions: The change in shape induced by heating caused shifts in the natural frequencies and changes in the corresponding modal shapes. The results show quantitatively for the first time the important role that residual stresses can play in the dynamic response of geometrically-reinforced thin plates manufactured by additive and subtractive processes.