Interlayer plastic deformation in additive manufacturing: Is more deformation better?

Introducing plastic deformation via interlayer hammer peening (IHP) is an attractive method of inducing local grain refinement in alloys that are additively manufactured using techniques such as directed energy deposition (DED). In the present work, various IHP strategies were applied to DED Inconel 625. The high degree of deformation induced by IHP caused recrystallization to occur which resulted in the formation of a fine grain structure with an average grain size of ∼3 μm. The subsequent layer deposited on the deformed layer, also had a finer as-solidified grain structure. Results showed that a critical deformation level was required for the recrystallization to occur, below which a high dislocation density was maintained with no equiaxed grain formation. Quantitative analysis of the strengthening mechanisms revealed that dislocation strengthening was dominant relative to grain boundary strengthening. A moderate deformation energy input of 556.3 mJ/mm² was found to be more effective in increasing the hardness of the deposited material, by limiting recrystallization. In contrast, greater deformation energy input triggered extensive recrystallization, resulting in the formation of softer regions. These findings underscore the complex interplay between the interlayer deformation conditions and resulting mechanical properties and emphasize that more interlayer plastic strain is not universally beneficial.