An optimization strategy based on critical recrystallization strain to improve the recrystallization rate of ultrasonic impact treatment assisted laser directed energy deposition

Abstract

In laser directed energy deposition (LDED), the epitaxial growth of long columnar grains often results in performance anisotropy in the component. Severe plastic deformation is a method commonly employed to refine grains. However, current research tends to focus on applying larger pressure loads and strains, which increases the demand for auxiliary equipment and reduces the convenience of application. In this study, we design and optimize a hybrid manufacturing method that combines synchronous ultrasonic impact treatment (UIT) with LDED, leveraging the cumulative characteristics of ultrasonic impact micro-deformation and the principle of critical recrystallization strain. Through mechanical compression and heat treatment experiments, we establish the strain-recrystallization relationship of the material and identified the minimum plastic strain necessary for achieving the maximum recrystallization rate (as the critical recrystallization effective strain). We predicted the plastic strain transfer depth under the current impact parameters using a model and optimized the thickness of the single deposition layer to prevent the recrystallization zone from being covered by the remelted molten pool. Using 316L stainless steel as the verification material, we induced synchronous recovery recrystallization at an exceptionally low output pressure (about 600 N). This optimization led to a significant 56 % refinement in grain size. Compared to the unoptimized experiment, the recrystallization rate increased from 9.14 % to 25.75 %. Furthermore, the yield strength of the material improved by 34 % (compared to a 19 % increase without optimization), all while maintaining high ductility. Additionally, we elucidated the strain and temperature conditions achieved during synchronous recovery recrystallization through a finite element model, and verified the accuracy of this model, allowing for the method's extension to other materials. These findings significantly reduce the demand for high pressure loads on equipment in deformation strengthening methods and enhance practicality.